AlllDD TTDEEl

NATL INST OF STANDARDS & TECH R.I.C.

A1 1100990221

Douglas, C. A/Visual range : concepts, I QC100 .U556 V159;1977 C.I NBS-PUB-C 1977

NBS MONOGRAPH 159

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Visual Range:

Concepts, Instrumental Determination,

and Aviation Applications
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Visual Range:

Concepts, Instrumental Determination,

and Aviation Applications

OF STANijri ..^ti {BOABT

3 0 1977

Qc

C. A. Douglas and R. L. Booker

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

Sponsored by;

Federal Aviation Administration Systems Research and Development Service Weather Data Section Washington, D.C. 20553

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

US NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued June 1977
Library of Congress Cataloging in Publication Data

Douglas, C A

Visual range. (National Bureau of Standards monograph ; 159) Bibliography: p. Supt. of Docs, no.: €13.44:159

1. Airports--visibility. 2. Transmissometer. 3. Ceilometer.

I. Booker, R. L., joint author. II, Title. III. Series: United States. National Bureau of Standards. Monograph ; 159. QC100.U556 no. 159 [TL557.V5] 602'.ls [629.135'2] 77-7343

National Bureau of Standards Monograph 159

Nat. Bur. Stand. (U.S.), Monogr. 159, 362 pages (June 1977) CODEN: NBSMA6

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $5.00

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-A- U.S. GOVERNMENT PRINTING OFFICE ; 1977 O—240-848
ABSTRACT

This document is a review of the principles, procedures, and

instruments used in the measurement of visual range. The fundamental

concepts of the visual range of objects and lights are discussed.

The principles of operation of the several classes of atmospheric

attenuation meters are reviewed and representative instruments are

described. The course of development of the NBS transmi ssometer, its

validation and application to aviation operations is reported. An

error analysis is made of the effects of instrument errors and of

differences in observer thresholds on visibility measurements. A

chronological review of the development and application of the runway

visual range concept is included together with a discussion of cloud

height measurements.

KEY WORDS: Ceilometer, runway visual range, scattering, transmi ssometer,

visibility, visual range.

i i i
TABLE OF CONTENTS

Page

CHAPTER 1 INTRODUCTION 1-1

CHAPTER 2 DEFINITION OF TERMS 2-1

2.1 General photometric terms 2-2

2.2 Terms relating to the atmosphere 2-11

2.3 Terms related to vision 2-15

2.4 Terms related to aircraft landing operations 2-22

2.5 Miscellaneous terms 2-26

2.6 Index of terms 2-27

CHAPTER 3 CHRONOLOGICAL RESUME 3-1

3.1 Introduction 3-1

3.2 Development of the transmi ssometer, initial phase 3-1

3.3 Development of the transmissometer, 1941-1945 3-3

3.4 Development at the Landing Aids Experiment Station 3-6

3.5 Air Force application of the transmissometer, 1949-1953 3-14

3.6 Application of transmi ssometers to civil use, initial phase 3-20

3.6.1 Initial applications 3-20

3.6.2 Initial studies by the Weather Bureau 3-21

3.6.3 Transmi ssometer-ceilometer program at Washington

National Airport 3-22

3.7 ANDB tests at Washington National Airport 1951-1953 3-23

3.7.1 Application of television 3-23

3.7.2 Application of the transmi ssometer-ceilometer

system 3-25

3.8 ANDB tests at MacArthur Field (1953) and at Idlewild (1954) 3-28

3.9 Development of the RVR system 3-33

3.9.1 Inauguration of runway visual range readout 3-33

iv
3.9.2 Reconsideration of thresholds 3-35

3.9.3 Early operational use 3-38

3.9.4 Approach visibility project at Newark 3-38

3.9.5 Development and application of the RVR computer 3-38

3.9.6 Further application 3-40

3.10 The development and application of the RVR concept outside

the United States 3-43

3.10.1 European practice 3-43

3.10.2 Development of RVR in the United Kingdom 3-44

3.10.3 Advantages and disadvantages of the visual method

of assessing RVR 3-46

3.10.4 Development of instrumental methods 3-47

3.11 ICAO actions on RVR 3-47

3.11.1 The initial phase 3-48

3.11 .2 Definition of RVR 3-48

3.11.3 Standardization of the operational application

of the RVR concept 3-51

3.11.3.1 Location 3-51

3.11.3.2 Observational techniques 3-53

3.11.3.3 Reporting of RVR 3-53

Reporting increments 3-53

The upper limit of RVR 3-55

Rounding down of RVR values 3-56

Pilot's eye height 3-56

Choice of light targets 3-57

Sampling period 3-58

Standardization of intensities and

thresholds used in the assessment of RVR 3-60

Intensities 3-60

Thresholds 3-62

v
CHAPTER 4 THEORY OF VISUAL RANGE 4-1

4.1 Visual range of objects 4-1

4.1.1 Historical 4-1

4.1.2 Objects with sky background 4-1

4.1.3 Objects with terrestrial background 4-6

4.2 Contrast thresholds 4-9

4.2.1 Daytime thresholds in practice 4-9

4.2.2 Contrast threshold criteria 4-11

4.3 Visual range of lights 4-15

4.3.1 Historical 4-15

4.3.2 Analysis of Allard's Law 4-16

4.4 Illuminance threshold 4-18

4.4.1 Illuminance threshold for "White" point sources 4-18

4.4.2 Effects of source size 4-20

4.4.3 Illuminance threshold for composite light sources 4-23

4.4.4 Illuminance threshold for flashing lights 4-24

4.4.5 Illuminance threshold for colored light sources 4-26

4.4.6 Curves relating atmospheric transmissivi ty, intensity,

and illuminance threshold 4-26

4.4.7 Illuminance threshold criteria 4-29

4.4.8 Thresholds applicable to the aircraft pilot 4-31

4.4.8.1 Nighttime thresholds for pilots 4-31

4.4.8.2 Daytime thresholds for pilots 4-34

4.4.8.3 Ground observations of NAFEC 4-34

4.5 Relation between visual range of objects and lights 4-35

4.6 Effects of the moving observer 4-37

vi
CHAPTER 5 MEASUREMENTS OF ATMOSPHERIC ATTENUATION 5-1

5.1 Transmissometers 5-3

5.1.1 Principles of operation 5-3

5.1.2 Error analysis - transmissometers 5-5

5.1.2.1 Relative error in the visual range of

objects 5-5

5.1.2.2 Relative error in the visual range of

lights 5-9

5.1.2.3 Choice of baseline 5-12

5.1.2.4 Instrumental determination of transmi ssometer 100% setting 5-14

5.1.2.5 Visual estimate calibration 5-15

5.1.2.6 Errors in the visual estimate calibration 5-16

5.1.2.7 Analysis of the effects of errors in the

visual estimate calibration 5-17

5.1.3 Scattered light error in transmissometers 5-19

5.1.4 Background illuminance errors 5-36

5.2 Scattering coefficient meters 5-37

5.2.1 Backscatter meters 5-38

5.2.2 Forward scatter meters 5-46

5.2.3 Side-scatter meters 5-49

5.2.4 Error analysis of scattering coefficient meters 5-52

5.2.4.1 Relative error in the visual range of

objects 5-52

5.2.4.2 Relative error in the visual range of

lights 5-53

5.2.4.3 lOO/o calibration errors in scattering

coefficient meters 5-54

5.2.4.4 Methods of calibrating scattering

coefficient meters 5-56

5.3 Miscellaneous transmi ttance meters 5-56

vi i
5.4 Comparison of transmi ssometer and scattering coefficient

meter accuracy 5-57

5.5 Effects of errors in assumed threshold on the accuracy of

visibility meters 5-58

CHAPTER 6 DEVELOPMENT AND APPLICATION OF THE NBS TRANSMISSOMETER 6-1

6.1 Principles of operation 6-1

6.1.1 General 6-1

6.1.2 Projector 6-3

6.1.3 Receiver and receiver amplifier-power supply 6-5

6.1.3.1 Optical systems 6-5

6.1.3.2 Pulse amplifier 6-9

6.1.3.3 Amplifier-power supply 6-13

6.1.4 Indicator and recorder 6-13

6.1.4.1 Amplifier 6-13

6.1.4.2 Pulse integrator 6-15

6.1.4.3 Voltmeter section 6-15

6.1.4.4 Calibrator 6-15

6.1.4.5 Recorder 6-15

6.1.4.6 Indicator linearity and stability 6-15

6.1.5 Illuminometer 6-16

6.1.6 Runway visual range converter 6-16

6.2 Theory of operation 6-16

6.2.1 General 6-16

6.2.2 System calibration 6-20

6.2.3 Effect of changes in baseline 6-25

6.2.4 Methods of modifying the transmi ssometer system to

permit its use during periods of very low runway

visual ranges 6-27

6.3 Errors in the photometric system and their effects on the

indicated visibility 6-33

viii
6.3.1 General 6-33

6.3.2 Constant incremental errors 6-35

6.3.2.1 Errors produced by background illumination 6-38

6.3.2.2 Errors produced by dark and leakage currents 6-38

6.3.2.3 Reading out errors 6-40

6.3.3 Constant relative errors 6-42

6.3.3.1 Projector 6-42

6.3.3.2 Receiver 6-42

6.3.3.3 Indicator 6-43

6.3.3.4 II 1 uminometer 6-43

6.3.3.5 System 6-43

6.3.4 Summary of effects of errors 6-43

6.4 Considerations other than instrumental errors 6-49

6.4.1 Scattered light errors 6-49

6.4.2 Sampling errors 6-50

6.4.3 Spectral sensitivity of phototubes 6-61

CHAPTER 7 PERFORMANCE VALIDATION OF THE NBS TRANSMISSOMETER 7-1

7.1 Introduction 7-1

7.2 Description of field experiments 7-2

7.2.1 Nantucket Island ' 7-2

7.2.1.1 Visibility calibration procedure 7-4

7.2.2 Areata 7-6

7.2.3 Washington National Airport 7-6

7.2.3.1 Validation procedures 7-7

7.3 Discussion of results - daytime observations 7-8

7.3.1 Nantucket Island 7-8

7.3.2 Areata 7-13

ix
7.3.3 Washington National Airport 7-15

7.3.4 Recapitulation of daytime visibility calibration 7-15

7.3.5 Effect of threshold on day visibility 7-15

7.4 Discussion of results - nighttime observations 7-17

7.4.1 Nantucket Island 7-17

7.4.1.1 Rationale for choice of intensity of lights 7-19

7.4.2 Areata 7-20

7.4.3 Washington National Airport 7-21

7.4.4 Effects of background luminance 7-24

7.4.5 Effects of threshold on night visibility 7-24

7.4.6 Comparison of the nighttime illuminance thresholds

applicable to meteorological visibility and to runway

visual range 7-24

7.5 Comparisons between observed prevailing visibility and

transmissometer data - day and night 7-26

7.6 Twilight transmissometer visibility calibration 7-28

CHAPTER 8 MEASUREMENT OF CLOUD HEIGHT 8-1

8.1 Introduction 8-1

8.2 Photoelectric detection of cloud heights 8-3

8.2.1 Selection of baseline length 8-4

8.3 Fixed-beam ceilometer 8-7

8.4 Rotating-beam ceilometer 8-9

8.5 Comparison of rotating-beam with fixed-beam ceilometer 8-12

8.6 Flashtube pulsed-light ceilometers 8-19

8.7 Laser pulsed-light ceilometers 8-19

8.7.1 Sperry lidar ceilometer 8-19

8.7.2 Erbium and ruby laser ceilometers 8-20

X
REFERENCES

Appendix A. Topical outline of the first Weather Bureau meeting on runway visual range A-1

Appendix B. Notes on the absorption of light by air and the range

of different lights B-1

Appendix C. Nomographs for computing the visual range of lights C-1

Appendix D. NBS transmi ssometer performance check list D-1

xi
LIST OF TABLES

Page

2.1 Relations between units of illuminance. 2-5

2.2 Relations between units of luminance. 2-7

3.1 Visibility conditions, LAES Flight Test No. 49-23. 3-12

3.2 Missed approaches at Washington National Airport. 3-28

3.3 Comparison of observations made from standard station, endof-runway station, and aircraft. 3-31

4.1 Luminances of backgrounds against which light signals are viewed. 4-20

4.2 Size factors for obtaining thresholds for sources other than

point sources. 4-23

4.3 Values of the "Blondel-Rey Constant" obtained at Areata. 4-25

4.4 Illuminance thresholds (km candles) 4-31

5.1 Transmi ssometer error factor for a 500 foot baseline

transmissometer, nighttime conditions. 5-11

5.2 Transmissometer error factor for a 500 foot baseline

transmissometer, daytime conditions. 5-11

5.3 Maximum permissible transmi ttance. 5-18

5.4 "Scattered light" error in transmi ttance measurements (for

500 foot baseline transmissometer). 5-28

5.5 Error in indicated visibility (day) resulting from "scattered

light" error in transmissometer measurements (for a 500 foot

baseline transmissometer). 5-29

5.6 Correlation of extinction coefficients from forward scatter

visibility meter against transmissometer (based on logarithms). 5-49

6.1 Reflector apertures required for several baseline lengths. 6-25

6.2 Minimum measurable RVR of present NBS transmissometer and RVR

converter as a function of lengths of the baseline. 6-31

6.3 Summary of estimated maximum uncertainties. 6-44

6.4 Errors in transmittance (At) required to produce a change in RVR

(aRVR) which will produce a shift to the next reportable value. 6-47

xi i
6.5 Average scattered light errors for a 500 foot baseline

transmi ssometer . 6-49

6.6 Relation between transmi ttance over a 250 foot baseline and

RVR (Step 5, day). 6-56

7.1 Data used to determine one of the Grade A points for the daytime visibility calibration curve. 7-11

8.1 Projection angle e corresponding to selected ceiling heights

h for a 1500 foot baseline ceilometer. 8-5

xi i i
LIST OF ILLUSTRATIONS

Figure Page

2.1 Units of luminous intensity, luminous flux, illumination, and

luminance. 2-7

2.2 Geometry of the visual segment. 2-19

3.1 Elementary block diagram of first NBS transmissometer. 3-2

3.2 Transmissometer test bed on Nantucket Island, 1941. 3-4

3.3 Transmissometer calibration curve developed at Nantucket. 3-5

3.4 Meteorological installations at the Landing Aids Experiment

Station, 1948. 3-7

3.5 The Landing Aids Experiment Station in 1948. 3-8

3.6 Central transmissometer recording position at the Landing Aids

Experiment Station. 3-9

3.7 Automatic runway-light intensity control system at the Landing

Aids Experiment Station. 3-11

3.8 Locations of transmissometers and ceilometer proposed by

Mr. G. H. Stocker of the Landing Aids Experiment Station. 3-15

3.9 Transmissometer set AN/GMQ-10. 3-17

3.10 Transmissometer set AN/GMQ-10, schematic arrangement. 3-18

3.11 Transmi ttance - visibility conversion scale for chart recorders. 3-19

3.12 Instrument sites at Washington National Airport, 1951. 3-24

3.13 Runway visibility (RVV) scale used in early installation, for

750 foot baseline. 3-27

3.14 Transmissometer receiver (left) and light source (right)

installed at MacArthur Field. 3-30

3.15 RVR meter scale used in control tower at Newark, 1955. 3-36

3.16 RVR calibration based upon three intensity settings and object

visibility. 3-41

4.T Effects of inherent contrast upon visibility factor. 4-5

xi v
List of Illustrations (continued)

4.2 Effects of ground/horizon sky luminance ratio and contrast

on visibility factor for objects with a terrestrial background,

based upon a contrast threshold of 0.05. 4-8

4.3 Effect of change in contrast threshold upon visibility factor. 4-12

4.4 Minimum illuminance at the eye from an achromatic (white) point

source for about 98 percent probability of detection as a function of the background. 4-19

4.5 Maximum angular diameter of a circular source which can be

considered strictly as a point source. 4-22

4.6 The intensity required to produce unit illuminance as a function

of distance for several atmospheric transmi ssi vities . 4-28

4.7 The relation between the day and night visibilities indicated

by the transmi ssometer. 4-36

4.8 Intensity, I, as a function of the threshold illuminance, E,

required to produce a visual range K times the daytime

visibility, Vd- 4-38

4.9 Effect of speed of moving observer upon his apparent threshold. 4-40

4.10 Effects of search time on visibility factor. 4-41

5.1 Optical diagram of Eltro dual beam transmi ssometer. 5-6

5.2 Arrangement of two transmi ssometers installed at Areata on parallel 500 foot baselines 20 feet apart. 5-22

5.3 Some of the curves used in determining correction values due

to disc baffle using the transmissometer projector as a light

source. 5-23

5.4 Corrections for effects of disc baffle, which were applied

to the scattered light error, At/t, as a function of the

transmittance, t. 5-25

5.5 The scattered light error, At/t, corrected for effects of

disc baffle, plotted as a function of true transmittance, t,

for different receiver half-angle fields of view. The source

is a transmissometer projector. 5-26

5.6 The scattered light error, At/t, corrected for effects of

disc baffle, plotted as a function of true transmittance, t,

for different receiver half-angle fields of view. The source

is a partially shielded bare lamp. 5-27

XV
List of Illustrations (continued)

5.7 Scattered light increment, At, plotted as a function of

transmittance, t, for red-sensitive S-1 and blue-sensitive

S-4 phototubes.

5.8 Scattered light error, At/t, as a function of transmittance,

t, for red-sensitive S-1 and blue-sensitive S-4 phototubes.

5.9 Scattered light error, At/t, predicted by Middleton as a function of receiver aperture half-angle y for source aperture half-angle e.

5.10 Experimental values of the scattered light error, At/t, as determined by Middleton at two periods in a single fog.

5.11 A comparison of scattered light errors obtained by Middleton

with those from Areata for different source (e) and receiver

(m) half-angle beam spreads.

5.12 Simplified schematic arrangement of a backscatter meter.

5.13a Relative backscatter signal in fog as a function of

meteorological range obtained by Curcio and Knestrick.

5.13b Composite of Areata and NRL data normalized at 1.5 miles to

give continuous straight line curve.

5.14 Backscatter signal as a function of meteorological range for

white light.

5.15 Backscatter signal as a function of meteorological range for

monochromatic light.

5.16 Schematic diagram of the Air Force Forward Scatter Meter.

5.17 Comparison of extinction coefficient indicated by forward

scatter meter, ap, with extinction coefficient computed

from transmi ssometer measurements, o^, and transmittance

per 75 meters (250 feet), ty^.

5.18 Schematic diagram of a sidescatter visibility meter.

6.1 Circuit diagram of first NBS transmi ssometer.

6.2 Block diagram of transmi ssometer projector and projector

power supply.

6.3 Block diagram of transmissometer receiver and receiver

amplifier-power supply.

xvi
List of Illustrations (continued)

6.4 Alignment of the receiver telescope. 6-8

6.5 Schematic diagram of pulse amplifier and associated

components

.

6-10

6.6 Block diagram of transmissometer indicator. 6-14

6.7 Optical path of the transmissometer. 6-18

6.8 Transmissometer calibrator. 6-22

6.9 Transmi ttance as a function of RVR (night-step 5) for

several lengths of baseline. 6-28

6.10 Pulse period as a function of RVR (night-step 5) for

several lengths of baseline. 6-29

6.11 Effects of constant incremental errors in transmi ttance

measurements on the indicated daytime visibility. 6-36

6.12 Locations of some of the transmi ssometers at Areata,

California. 6-51

6.13 Aerial view of Areata, California, airport showing the

transmissometer test site. 6-52

6.14 Fog of October 24, 1966, an example of rapid variations in

fog density with time. 6-53

6.15 Fog of October 22-23, 1966, an example of records showing

variations in fog density at the locations of the

transmissometers . 6-54

6.16 Fog of November 4, 1966, an example showing variations in

fog density at the locations of the transmissometers and also

rapid changes with time. 6-55

6.17 Simultaneous records of two back-to-back, 250 foot baseline

transmissometers at Areata, California, October 31, 1965. 6-57

6.18 Simultaneous RVR determinations from two back-to-back

transmissometers at Areata, California, October 31, 1965. 6-58

6.19 An example of extreme variations of transmittanee with time. 6-60

7.1 Location of visibility calibration range on Nantucket Island. 7-3

7.2 Daytime transmissometer visibility calibration curve derived

from data obtained at Nantucket Island. 7-10

xvi i
List of Illustrations (continued)

7.3 Visual range observations at Areata of a single fixed dark

object obtained while varying the position of the observer. 7-14

7.4 Cumulative frequency distribution of contrast threshold determined during Washington National Airport investigations. 7-16

7.5 Nighttime transmissometer visibility calibration curves derived

from data obtained at Nantucket Island. 7-18

7.6 Visual range observations at Areata of a single fixed 25 candela

light obtained in 1947 while varying the position of the

observer. 7-22

7.7 Cumulative frequency distribution of Sy obtained during

Washington National Airport investigations. 7-23

7.8 Cumulative frequency of various differences between visibility

computed from transmissometer and visibility estimated by an end-of-runway observer during Washington National Airport

observations. 7-27

7.9 Day and night visibility calibration curves for a 500 foot

baseline transmissometer. 7-29

7.10 Illuminance thresholds based upon Washington National Airport

measurements of horizon sky luminance during twilight. 7-31

7.11 Illuminance thresholds computed from Washington National Airport

twilight observations plotted against time. 7-32

8.1 Schematic representation of cloud height measurement. 8-2

8.2 Ceilometer operation-angles of scan vs. equivalent ceilings for

various baseline lengths. 8-6

8.3 Geometry of fixed-beam ceilometer. 8-8

8.4 Some relationships between observed balloon heights and

significant ceilometer signals from tests at Areata. 8-10

8.5 Schematic diagram of the rotating-beam ceilometer. 8-11

8.6 Ceilometer layout at Silver Hill Observatory, Silver Hill,

Maryland. ' 8-13

8.7 Comparison of rotating-beam and fixed-beam ceilometers. 8-15

8.8 Pictoral comparison of ceilometers. 8-17

xvi i i
List of Illustrations (continued)

8.9 Sampling rates and corresponding instrumental presentations

of rotating-beam and fixed-beam ceilometers. 8-18

8.10 Comparisons of both fixed-beam and rotating-beam ceilometers

with highest visible tethered balloon. 8-21

8.11 Comparison of rotating-beam ceilometer and lowest partially

obscured balloon. 8-22

C-1 Generalized nomographs for the solution of Al lard's Law for

distances of 0.30 to 500. C-4

C-2 Generalized nomographs for the solution of Al lard's Law for

distances of 0.01 to 10.0. C-5

C-3 Generalized nomographs for the solution of Allard's Law for

distances of 0.004 to 1.0. C-6

xix

1. nmUDUCTlON

The Federal Aviation Administration of the Department of Transportation requested that the National Bureau of Standards, by InterAgency

Agreement No. D0T-FA72WAI-267; "provide analysis, experimentation, and

report preparation work in the areas of ceiling, visibility, and related

techniques and instrumentation." The study was to culminate in the

preparation of "an encyclopedia of principles, procedures, and equipments

utilized in the measiarement of visual range in the United States". This

report was prepared in an effort to fulfill the stated requirement.

The development and application of the visual range concept in the

United States is not well documented. Very little work was reported in

the formal literature . Intergovernmental reports were not given wide

circulation and at this time are not usually available. Often decisions

were made at meetings of ad hoc committees, by memoranda, or by letters.

In preparing this report we have reviewed the formal literature (restricting

our efforts almost entirely to papers in English) and the collection of

reports, progress reports, memoranda, correspondence, information copies,

etc. which has collected during some 35 years of work on problems of

visual range as applied to aviation. In our preparation of the text we have referenced formal papers and reports as applicable, but we have not

referenced correspondence, memoranda and the like. There have been a few instances in which we could find no documentation relating to a significant step. In those instances we have relied on the memory of

the senior author.

At the risk of being slightly repetitive, we have, in so far as practicable, made each chapter substantially independent of the others.

In Chapter 2 we define and explain the photometric, meteorological, and

aeronautical terms used in the report. Chapter 3 is a chronological

resume of the development and application of the runway visual range

1-1
concept, presented without a detailed technical analysis in order to

maintain a historical perspective. Chapters 4 and 5 are detailed discussions

of the theory of the visual range of objects and lights and of instrioments

used to measure atmospheric clarity, respectively.

Chapters 6 and 7 comprise a review of the work of NBS and other related

work in the development of the transmissometer , gathering together reports

of work which has been previously presented in progress reports to sponsors,

working and discussion papers, and internal reports. Chapter 8 is a review

of the development of the ceilometer and of studies in the measurement of

cloud height. The report closes with a bibliography of the books, papers

and reports consulted during its preparation and found to be pertinent.

The scope of this report does not include a discussion of the parallel

paths of development of visual range meters in other countries or the application of the NBS transmissometer to other purposes such as air pollution,

fog dispersal and camouflage studies. Nor does it include an evaluation of

the effectiveness of approach and runway lights; only the visual range of

these lights is considered.

In preparing this report, extensive use has been made of extracts from

early National Bureau of Standards, Civil Aeronautics Administration, and

Weather Bureau reports.

English photometric unites and units of length have been used in most

of this report, since the original choices for values of such items as distances and illuminance were rounded values when expressed in English

units. Where the original work was in metric units, as in the work of the

International Civil Aviation Organization, metric units have been retained.

Parenthetical equivalents are given only when considered essential since

it is expected that most readers will be "bilingual". However, because of

the complexity of photometric units, tables of equivalents are given in

Sections 2.1.6 and 2.1.9.

At this point we want to pay tribute to the many persons who contributed

significantly to the development and application of the RVR concept in the

United States. At the risk of omitting some deserving persons, we express

our appreciation to our former colleagues at NBS: to F. C. Breckenridge,

1-2
who with H. J. C. Pearson of the CAA, initiated the ceilometer and transmissometer development; to M. K. Laufer and L. L. Young who designed the

initial model of the transmissometer ; to A. N. Hill for his assistance in the

design and testing of the first commercial transmissometers; to J. W. Simeroth,

who for twenty years was in charge of the NBS Visual Landing Aids Field

Laboratory at Areata, and to his associates, especially J. E. Davis and J. C.

Wilkerson; to D. H. Hutchison and G. H. Stocker, Meteorologists at the

Landing Aids Experiment Station 1946-1950; to R. E. Crossley of the Grouse

Hinds Company, for his mechanical design of the commercial transmissometer;

to our associates L. W. Foskett, R. H. Guenthner, Joseph N. Cooper, and

Dale Harris, of the Weather Bureau, who implemented the operation of the

civil runway range systems; to J. W. Connolly, who when with the Air Force,

conducted the operational suitability test of the transmissometer; to E. F.

Corwin, A. L. Lewis, R. D. Hartz of the Navy for their continued support of

visibility studies at NBS; to W. E. Eggert, E. W. Estelle, C. G. Knutson,

Mathew Lefkowitz, and E. E. Schlatter of the Weather Bureau, who conducted

the flight tests at Newark and Atlantic City; and to the many people of

the CAA and FAA for technical and administrative support during the "testing"

period, especially to B. J. Vincent. To those whose names we have omitted,

we apologize and express our sincere appreciation of their contributions.

1-3

2. DEFINITIONS OF TERHS I

This section contains the definitions and, where required, the

explanations of technical terms as used in this report. Definitions

are based upon current usage, and, when there has been a change in

usage, an explanation is given.

The following publications have been used in the preparation of

these definitions

:

ASTM Special Technical Publication 475, Nomenclature

and Definitions Applicable to Radiometric and

Photometric Properties of Matter. [6]^

CIE International Lighting Vocabulary. [71]

Federal Meteorological Handbook No. 1. Surface Observations

Circular N [36]

Holmes, Terminology for Flashing Light Signals. [4-6]

lES Lighting Handbook [51]

International Civil Aviation Annex 14 [53; 54]

International Dictionary of Aids to Marine

Navigation, Chapter 2, Visual Aids. [52]

Middleton, Vision Through the Atmosphere. [95]

USA Standard Z7. 1-1967, Nomenclature and Definitions

for Illuminating Engineering. [5]

World Meteorological Organization Guide, Chapter 10. [128]

In this section the terms are arranged so that, insofar as possible,

technical terms used in a definition have been defined before they are used in defining other terms. As an aid in locating specific terms an alphabetical index of terms is given at the end of this Chapter.

For clarity, some of the definitions are in colloquial language

rather than in the precise general terminology of the publications

referenced above.

^Numbers in brackets refer to references listed at the end of the report.

2-1
2.1 GENERAL PHOTOMETRIC TERMS

The symbols used in these definitions, and throughout the report, are consistent with those recommended by the International Commission

on Illumination.

2.1.1 Light:-

For the purpose of this report light may be considered as visually

evaluated radiant energy. Strictly speaking the term light should be

applied only to that part of the electromagnetic radiation spectrum which

is capable of causing a visual sensation directly. However, it is

common engineering practice to apply the term to radiation which contains

some ultraviolet and infrared radiation in addition to the visible

radiation which is of primary concern; for example, "the light from a source incident on a photoelectric receiver"

.

The term light is also to mean the fitting or fixture which

emits a light signal; for example, approach light.

2.1.2 Radiant Flux (Flux):-

The time rate of flow of radiant energy

Symbol: ({)^ unit: watt

2.1.3 Luminous Flux:-

The time rate of flow of luminous energy (light).

Symbol: cj)^ unit: lumen

When the context is clear the term f I ux is often used instead of

luminous flux.

Unless otherwise indicated the luminous flux in question relates to

photopic (cone) vision.

^The symbols for photometric quantities are the same as those for the

corresponding radiometric quantities. When it is necessary to differentiate

them the subscripts v and e should be used, e.g., cj)-^ and <^q.

2-2
2.1.4 Luminous Intensity (Intensity) :-

The luminous flux per unit solid angle.

Symbol: I unit: candela

older \mits: candle,

candlepower

.

For engineering purposes the terms candela» candle and candlepower

can be considered synonomous

.

The defining equation of luminous intensity is

I = d(f)/dfl.

The intensity of a source is a function of the direction in which

the light is emitted. Thus the direction of emission with respect to

a known frame of reference should be stated. The luminous intensity is the

luminous flux on a small surface normal to that direction, divided by the

solid angle (fi) (in steradians ) that the surface subtends at the source. Light fittings are frequently described in terms of their peak intensity or by their average intensity.

2.1.5 Average Intensity (Representative Intensity) :-

The concept of using an average or representative intensity in computing

the effective visual range of airfield lights originated in the U.S. during

the early development of the runway visual range program. See Sections

3.9.1 and 3-11.3.3, and reference [40].

During its Fifth Meeting, the Visual Aids Panel stated that the

intensity to be used in the assessment of RVR should be the intensity

of a typical new light averaged over the specified beam spreads, with

all intensities greater than three times the minimum intensity within

the specified region being considered as three times the minimum, multiplied by an appropriate reduction factor [67] . The beam pattern is

described as an ellipse with the lengths of its axes equal to the specified

vertical and horizontal beam spreads. (A Cartesian system with vertical and

horizontal angles as the coordinates is assumed).

2-3
If detailed Iso-candela curves, or numerous intensity distribution

curves are available, the average intensity within the ellipse is com- puted from these curves . Frequently, detailed information is not

available. In such cases an approximation to the average intensity is

computed from the horizontal and vertical intensity distributions through

the beam axis. (The beam axis is defined as lying midway between the ^0%

points of these two distributions. ) The approximate average intensity is

taken as the average of the average intensity in the horizontal plane

between the specified horizontal angles and the average intensity in the

vertical plane between the specified vertical angles.

In all cases, the restriction that any intensity more than three

times the minimum be treated as being three times the minimum must be

observed. This restriction is needed to avoid the undue influence on

the "average" intensity which would be produced by a narrow peak having

an intensity much greater than the intensity in other parts of the

specified region.

2.1.6 niuminance (Illumination) :-

Luminous flux per unit area.

Symbol : E Unit : SI - lux ( one lumen per square meter )

.

English - footcandle (one lumen per

square foot. )

The defining equation is

E = d(j)/dA.

Note: The term il lumination is frequently used instead of the

preferred term i I I uminance although this usage conflicts with the

recommended practice. In present usage the ending tion is reserved for

terms designating processes; that is, the process of reflection, transmission,

illumination, etc. The ending ance is used for the designating of measurable

quantities

.

Other units of il luminance which have been used in relation to

vision through the atmosphere are

2-4
mile candle:- (one lumen per square mile) and kilometer candle

(one lumen per square kilometer).

A mile candle is the illuminance which would be produced by a source having an intensity of one candela at a distance of one mile in a perfectly

transmitting atmosphere. The terms footcandle, meter candle, mile candle,

and sea mile candle are similarly defined with the unit of distance

being changed as appropriate

.

The il luminance produced on a surface a distance x from a point

source and perpendicular to the line of sight in a perfectly clear

atmosphere is given by the equation

E = I/x^ (2.01)

where I is the intensity in the direction of the line of sight .

Relations between the several units of i I I uminance computed from

this relation are given in table 2.1.

Table 2.1

Relations between Units of Illuminance

6 6

One. lux = 10 kilometer candles = 2.59 x 10 mile candles^

= 0.0929 footcandles.

7

One footcandle = 10.76 lux = 1.076 x 10 kilometer candles

= 2.79 X lo'^ mile candles^.

One mile candle^ = 3.59 x 10"^ footcandles = 0.386 kilometer candle

= 3.86 X 10-'7 lux.

One kilometer candle = 10"^ lux (one microlux)

=2.59 mile candles^ = 9.29 x 10"^ footcandles.

^When English units are used the usual practice is to use the term

mile candle when the unit of length is a statute mile and sea-mile candle

when the unit of length is a nautical mile. One mile candle is equal to

1.326 sea-mile candles.

2-5
2.1.7 Normal 111 umi nance :-

The il luminance measured at a point in a plane perpendicular to the

incident rays.

In signal lighting the modifier normal is usually omitted. Normal

illuminance is referred to as illuminance.

2.1.8 Point Brilliance:-

The normal illuminance produced by a (distant) source on a plane

at the observer's eye. It is the quantity involved in the visual

observation of a source of light when viewed directly from such a distance that the apparent diameter is not appreciable. The point

brilliance is measured by the illuminance produced by the source on a plane at the observer's eye normal to the direction of the source.

Symbol: SI Unit: lux (ix). Frequently expressed in

microlux (ylx).

Non-metric Unit: Footcandle

Customary Units: Mile candle, sea-mile candle, kilometer

candle.

2.1.9 Lumi nance :-

This is the most difficult concept to define. For purposes of this

report the term can be best defined by a figure illustrating photometric

concepts. Such an illustration, prepared by L. E. Barbrow of the National

Bureau of Standards, is shown as figure 2.1. The following definition

is that given in reference [5]

.

"Luminance (Photometric Brightness),

L = d2<i)/dw (dA cos 6) = dI/( dA cos 8).

Luminance (photometric brightness) in a direction, at a point of the surface of a source, of a receiver, or of any other

real or virtual surface is the quotient of the luminous flux

leaving, passing through, or arriving at an element

of the surface surrounding the point, and propagated

in directions defined by an elementary cone containing

2-6
oc

T3a

\_>%

T3O^uoaoo

0)

k.o

CM

T3 'd-

(U

O

O CM

ro E Q 13 O

•r—

+->

CO fO Eu Q.

"o o c O o

CD oo o D >^ o

'in

</> c H—

<u o t-c w

10o o c w

<u E Q. 3 E

(U

<L) < O

_i cn

UJ c Q o

k. < a>a.o

J=

T3O

Ol

«1

O)coo

Olo>coo

"o

COcoo

O) 3* c ni ° E o — < — c Q o Q

Q.

O

T3 o —uUu Z 0)

cm LU o rom

Cu

O

_j c

<u T5

'c o> c 3 o m

(J

O 3

any

OJE (f) CO

res cE < o o

1- co 3 om

(A Z O < Z) X o M

CD Q. o 00 c CO <i> E X

OQ o oa x: 3 3 CO Q. m m cc

T3C

(T3

!->

COOc

>>

COe

O)c

COO

CO

+->

CM

cn < 3 -o

2-7
the given direction, by the product of the solid angle

of the cone and the area of the orthogonal projection

of the element of the surface on a plane perpendicular

to the given direction; or it is the luminous intensity

of any surface in a given direction per unit of projected

area of the surface as viewed from that direction.

In many of the earlier papers the term brightness, with the

symbol B, is used instead of luminance.

Units of Luminance

:

The endorsed method of expressing luminance is candelas per

unit area, the defining equation being

L = I/A.

The SI unit of luminance is one candela per square meter. The

I uminance of lamp filaments has usually been expressed in candelas

per square millimeter, or per square centimeter. In the English system

both the inch and the foot have been used as the unit of length.

There is also a group of units contrived for the sake of numerical

convenience, the lambert, or millilambert, in metric units and the

footlambert in English units. The luminance of a uniform, perfect

diffuser is one footlambert when the illuminance on it is one lumen per

square foot ( one footcandle )

.

Relations between several of the units of I uminance in common use

are given .in table 2.2.

Table 2.2

Relations between Units of Luminance

One lambert = 10-^ millilamberts = 3-183 x 103 candelas per square meter = 9.290 X 10^ footlamberts = 2.957 x lo2 candelas per square foot.

One candela per square foot = 3.382 x 10"-^ lamberts = 3.382 millilamberts = 10.76 candelas per square meter = 3.142 footlamberts.

2-8
One footlambert = 0.3183 candelas per square foot = 1.076 x 10 lamberts

1.076 millilamberts = 3.426 candelas per square meter.

_2 One candela per square meter = 0.2919 footlamberts - 9.290 x 10 candelas

per square foot = 3.1-42 x 10 lamberts = 0.314-2 millilamberts.

One millilambert - 3 -1^3 candelas per square meter = 0.9290 footlamberts = -3

0.2957 candelas per square foot =10 lamberts.

2.1.10 Brightness, Subjective Brightness, or Luminosity:

These terms are used to describe the visual sensation. In practice^

luminance is the descriptive term used for light energy effective at the

eye. It is the physical stimulus. The brightness of a surface depends on various elements of the visual image as perceived by the eye and the

brain. It is the sensation.

2.1.11 Contrast:-

As used in this report, contrast is defined by the following

equation:

The luminances referred to above are the inherent luminances; that

is, the luminances as measured from a position, on the line of sight,

sufficiently close to the object so that the measurements are not

affected by atmospheric losses.

When an object is darker than its background, is less than

and C is negative.

The inherent luminance of a truly black object is zero. Hence,

for such an object, L is zero and the contrast is -1* for all backo grounds. This is the lowest contrast an object may have. Theoretically,

there is no limit to the contrast for an object which is lighter than

(2.02)

where

is the luminance of the object, and

L, is the luminance of the background

^See 2.3.6

2-9
its background. However, under natural conditions, the maximum

contrast which a sun-lighted white object may have is about 5 for a sky

background and 20 for a terrestrial background.

2.1.2 Apparent Contrast:-

The apparent contrast, C^, of an object at a distance, x, from the

object is defined by the following equation:

respectively. These .luminances are the luminances measured from the

distance x.

Additional Material : For a m.ore detailed explanation of photometric

concepts see references [95] , [51] and [5]

.

(2.03)

2-10
2.2 TERMS RELATING TO THE ATMOSPHERE

2.2.1 Transmission :-

Transmission is now defined as the passage of radiation thro\igh a medium without changes in the frequency of the monochromatic omponents

of which the radiation is composed. In the past the term transmission

has been used synonomously with transmittance. See the Note

following definition 2.1.6.

2.2.2 Transmittance:-

The ratio of the transmitted radiant or luminous flux to the incident

flux. That is

Transmittance may he considered as the ratio of the flux from a source received incident on a receptor (which may be the eye) after

passage through a medium, without refraction, to that which would be

received if the medium were removed. Since the aerosols of the atmosphere

both scatter and absorb light, the transmittance is mixed, that is, part

of the flux from the source incident on the receptor has been transmitted

from the source without scattering, and some after having been scattered

in the direction of the' receptor.

(2.04)

Thus

t = t + t r d (2.05)

where

t is the (total) transmittance

»

t is the regular transmittance*

r the transmittance based upon

unscattered flux, and

is the diffuse transmittance*

the transmittance based upon

scattered flux.

2-11
Note: The symbol T is the internationally recommended symbol for

transmittance. This symbol has not been used in this report since so many of the earlier reports and papers used t^.

The distance one can see is a function of the regular transmittance.'

In most of the literature pertaining to visual range, as in this report,

the term transmittance is used without a modifier when regular transmittance is meant. The definition of transmittance given by WMO, "the

fraction of luminous flux which remains in the beam after traversing

an optical path of a given length in the atmosphere" implies regular

transmittance.

Although transmittance is dimenslonless, when the term transmittance

is used to describe a state of the atmosphere the distance to which

the transmittance applies must be stated, e.g., the transmittance over a baseline of 500 feet is 0.01.

Note: If the (regular) transmittance, t^, of a uniform atmosphere

over a path of length b is known, the (regular) transmittance, t , over a

a path of length a^ is given by the relation

t = (t^)^^^ (2.06)

a b

Equation (2.06) is valid only when the transmittance is independent

of the wavelength( s ) of the incident flux.

2.2.3 Transmissivity :-

Transmittance for unit distance within a light transmitting medium.

Symbol : T

The unit of length must be stated although the term is dlmensionless.

Note: In current practice the endings -tion and -sion are used for

the designation of processes, as in, transmission; the endings -ance and -ancy are used in reference to measurable quantities such as transmittance; and the ending -itv to the properties of materials or media.

Tha term coefficient also refers to properties of materials or media.

2-12
The relation betvreen the t ransmissivity, T, and the transmittance

over a path of a given length, b, may be found from equation (2.06) by

setting a equal to one. Since t is then the transmissivity, 1,

a

T = (t^)^^^ (2.07)

or

t^ = T^. (2.08)

Equation (2.08) has caused difficulty to many who use dimensional

analysis because the exponent of T does not appear to be dimensionless

.

It should be remembered that the exponent is in reality the ratio of two

lengths, where the magnitude of the length in the denominator of the

exponent is unity.

Equations (2.06), (2.07) and (2.08) and those following apply strictly

only to monochromatic light or to an atmosphere which transmits light of all

wavelengths equally. They are, however, sufficiently accurate in approximations for most work in atmospheric optics. See Section 6.4- 3.

2.2.4 Transmissometer , Atmospheric :-

An instrument for measuring the regular transmittance of the atmosphere between two points in space. The term is usually used without the

modifier atmospheric.

It is not possible to construct an instrument which will accept only

the regularly transmitted light . The instrument will always accept some radiation which, though emitted by the source, would not be accepted by

the receiver, had its durection not been changed by scattering enroute.

In a well designed transmissometer the amount of scattered radiation

accepted is kept as low as is feasible.

2.2.5 Absorption Coefficient, Atmospheric:

The absorption coefficient, a, may be defined by the equation

a = -d(f) /(})dx

2-13
where

d(j) is the flux absorbed as light passes through a lamina of thickness dx perpendicular to the line of sight and (j) is the flux entering the

lamina. The unit of length, must be stated.

The negative sign indicates that an increase in x is accompanied

by a decrease in (j), that is, dc}) and dx are of opposite signs, and a

is, therefore, positive.

Note: The absorption coefficient is negligible in clean fog.

2.2.6 Scattering Coefficient, Atmospheric:

The defining equation for the scattering coefficient, 3, is

where d(}) is the flux scattered as light passes through a lamina of thickness

s dx and is the flux entering the lamina. The unit of length must be

stated.

2.2.7 Extinction Coefficient:-

The extinction coefficient, a, is defined by

a = -d(j)/(j)dx (2.09)

where d<p is the flux absorbed and scattered as light passes through

a lamina of thickness dx and (f) is the incident flux. It is apparent that

a = a + 3 (2.10)

The unit of legnth must be stated as a per meter or a per foot, etc.

Integration of equation (2.09) and applying the boundary condition

that when x is equal to zero, (}) is equal to <p^, the incident flux yields

the following equation for the flux in the beam at a distance x: V^i = e-^^ (2.11)

Equation (2.11) is strictly applicable only to monochromatic light

or to a non-selective atmosphere. See Section 6.4.3-

The quantity cf) /(}). is the transmittance over the distance x.

2-14
Hence

t = e"^"" (2.12)

and since from (2.07)

T = t 1/^,

or

T = e~^ (2.13)

a = -In T. (2.14)

Note that the integration as performed is valid only if the atmosphere is spatially uniform and is not spectrally selective.

2.2.8 Scattering Coefficient Meter :-

An instrument for determining the scattering coefficient by measurements of the flux scattered from a light beam.

These meters are frequently called extinction coefficient meters.

Since their response is not significantly affected by atmospheric absorption, their output is a function only of the scattering coefficient, 3;

not of the extinction coefficient, a.

2.3 TERMS RELATED TO VISION

2.3.1 Threshold:-

The value of a physical stimulus (such as size, luminance, contrast

or time) that permits an object to be seen a specific percentage of the

time or at a specific accuracy level. Often thresho'lds are presented in

terms of 50 percent, or 95 or 99 per cent, detection. However, the

threshold also is expressed as the value of the physical variable that

permits the object to be just barely seen. The threshold may be

determined by merely detecting the presence of an object or it may be

determined by discriminating certain details of the object, designated

as detection and recognition thresholds respectively. Detection thresholds

are applicable to laboratory, but usually not to practical, problems.

See definition 2.3.4-.

2-15
2.3.2 Illuminance Threshold (Visual Threshold, Threshold Illuminance):-

The minimum illuminance at the eye required to make a light source

visible. This threshold is a function of the angle subtended at the eye

by the source, the luminance of the background, the observer's knowledge

of the location of the light, and the criteria used in determining

whether the light is "visible". Usually it is not greatly affected by the

color of the light. See Section 4.4.7 for a discussion of illuminance

thresholds applicable to the meteorological observer and to the pilots.

2.3.3 Luminance Contrast Threshold:-

The minimum luminance contrast at which an object is visible against

its background under stated conditions: The contrast threshold is not a constant but is a function of the angular size of the object, the

luminance of the background and the criteria which are used to determine

if the object is "visible", and the observer's knowledge of the location

of the object. See Section 4-2.1 for a discussion of contrast thresholds

applicable to the meteorological observer and to the pilot.

Symbol: e (epsilon)

2.3.4 Field Factor:-

The ratio of the threshold applicable to operational conditions with

unstructured viewing to the threshold obtained under laboratory conditions

using a forced choice response, a simple background, with the observer

knowirjg where and when to look for the target is frequently designated as

the field factor. Field factors are usually of the order of 2 to 20,

depending upon the criteria applied [51, 118]

.

2.3.5 Allard's Law:-

An equation relating the illuminance produced by a source of

intensity 1 on a plane normal to the line of sight at a distance x from

the source and the atmospheric transmissivity [3, 4] . The equation

relating these parameters is

:

E = ItVx^ (2.15)

A uniform atmosphere which is not spectrally selective is implied.

2-16
2.3.6 Koschmieder' s Law:-

An equation relating the apparent contrast, C^, of an object viewed

against a sky or fog background, its inherent contrast, C^, and the atmospheric transmissivity [78, 79] • The equation relating these

parameters is

C = C T^. ' (2.16)

X o ^

^

Note that, since the transmissivity, T, is never less than zero,

C and C will always have the same sign. Hence in mathematical manipuX o lation of equation (2.] 6) all contrasts may be considered as being positive.

This is necessary when logarithms of both sides of the equation are taken

.

2.3.7 Visual Range (V):-

The maximum distance, usually horizontal, at which a given object

or light is visible under particular conditions of atmospheric transmission and background luminance. Photometric data describing the object

or light in question, and the viewing conditions must be stated. Thus

"the visual ranee of a light source (producing an intensity) of 10,000

candelas (in the direction of view) is x miles when the (atmospheric)

transmissivity is 0.5 (per mile) by day (when the background luminance

is 1000 footlamberts )" . The parts of the sentence enclosed in parenthesis

are frequently omitted.

Note: In maritime practice the term visual ranfie is applied only to

objects. The term "luminous range" is applied to lights.

Note: In the 1940's the term visual range was frequently used

synonomously as or in place of the term visibility as defined in 2,3.13.

2.3.8 Runway Visual Range (RVR):-

As defined in Annex 14 [531 runway visual range is "the maxim\im

distance in the direction of take-off or landing at which the runway or the specified lights or markers delineating it can be seen from a position

above a specified point on its center line at a height corresponding to

the average eye-level of pilots at touchdown.

"Note 1. A height of approximately 5 m (16 ft) is regarded as corresponding to the average eye-level of pilots at touchdown.

2-17 .
"Note 2. In practice, runway visual range cannot be measured directly

from the position specified in the definition but is an assessment of what

a pilot would see from that position.

"Note 3. For the purposes of the specifications in Annex 14 the specified lights are considered to be high intensity lights of the order of

10,000 candelas. Markers are not taken into account."

The U.S. definition of runway visual range in Federal Meteorological

Handbook No. 1, Surface Observations, is given as "A value normally

determined by instruments located alongside and about 14 feet higher than

the centerllne of the runway and calibrated with reference to the sighting

of high Intensity riinway lights or the visual contrast of other targets - whichever yields the greater visual range."

2.3.9 Slant Visual Range (SVR):-

Fundamentally, slant visual range is the visual range of a specified

object or light along a line of sight which differs significantly from the

horizontal. If the transmlssivlty does not vary with height, then the

visual range of a light or object along any slant path will be equal to

the horizontal visual range of the light or object providing changes in

background conditions do not have a significant effect. If the transmittance

changes with height, the slant visual range is also a function of height

and the height to which the stated slant visual range applies must be

given.

Since the maximum depression of the line of sight over the nose of

a typical aircraft is about 15°, the difference between the length of

the slant path to the most distant object or light visible and the length

of the projection of this path on the horizontal is usually not significant.

2.3.10 Visual Segment:-

The distance between the most distant light or object which is

visible and the nearest light, or object which is not obstructed by the nose of the aircraft. The geometry is shown in figure 2.2.

Frequently, the concepts of visual segment and slant visual range

are confused and a short visual segment is incorrectly interpreted as

2-18
d

VISUAL SEGMENT

d = SVR cos <^ - h CSC 0

d- SVR -h CSC ^

Figure 2.2 Geometry of the visual segment.

2-19
indicating that there is a large difference between the slant visual

range and the horizontal visual range. In dense fog, a change in height can make a very significant difference in the length of the guidance

segment. For example, in a normal approach, with a 15° cockpit cut-off

and a uniform fog in which the slant and horizontal visual ranges are I4OO

feet, at a height of 200 feet, the length of the visual segment is 627 feet

and 6 or 7 lights of an approach light system (with lights spaced at 100-foot

intervals) would be visible. However, at a height of 260 feet, the length

of the visual segment is 395 feet and only 3 lights would be visible. The

effects of change in height are even more pronounced if the fog intensity

increases with height, as it often does.

2.3.11 Contact Height (Vertical Contact Height) :-

The height at which visual reference with recognized lights or objects

on the surface can be established sufficiently to permit visual determination of the ground plane and position.

2.3.12 Approach Light Contact Height (ALCH):-

The concept of approach light contact height is defined by Eggert

[34] as,

"The height above ground at which a pilot making an

ILS or GCA approach can expect to see at least a 500-

foot segment of the approach light system, with certain

probabilities"

.

2.3.13 Visibility or Meteorological Visibility:-

The term visibility is used for two concepts in describing atmospheric conditions:

a. As a qualitative term to describe the clarity of the atmosphere,

as "in periods of good visibility".

b. As a quantitative term to express the clarity of the atmosphere

in units of distance.

In Federal Meteorological Handbook No. 1 [36], visibility is

defined as, "the greatest distance at which selected objects can be seen

and identified." Dark or nearly dark objects viewed against

2-20
the horizon sky should be used by day and unfocussed lights of moderate

intensity (25 cd) should be used by night. The Fourth (1971) Edition

of the WMO "Guide to Meteorological Instrument and Observing Practices" [128]

gives essentially the same definition. This is, of course, U.S. practice

and the NBS transmissometer was calibrated using these criteria.

However, inmost countries visibility and meteorological visibility

at night have been reported as the distance at which a black object viewed

against a sky background would be seen if it were day. This usage satisfies the requirements of the meteorologist since it yields a one-to-one

correlation with atmospheric transmittance and a change from day to

night does not produce, by itself, a change in the visibility. However,

it is operationally unsound. A prominent British lighting engineer once stated that the reaction of pilots when this meaning of nighttime meteorological visibility is explained to them, is one of incredulity mixed with

resentment. One must constantly keep in mind this difference in usage

when interpreting reports and discussions of those who are not residents

of North America.

As stated in definition 2.3.7, during the 194-0 's there was a move,

led by Middleton, to use the term visual range for the concept of visibility

as defined above, and many papers and reports prepared during that period,

including those of NBS, use the term visual range in this context.

2.3.14 Meteorological Optical Range (MOR):-

The length of the path in the atmosphere required to reduce the

luminous flux in a collimated beam from an incandescent lamp at a color

temperature of 2700 K to 0.05 of its original value, that is, the length

of the path in the atmosphere for which the regular transmittance is

0.05.

2.3.15 Meteorological Range (MR):-

The length of the path in the atmosphere required to reduce apparent

contrast to 0.02 of the inherent contrast.

This definition is based upon the assumption of a contrast threshold,

first made by Koschmieder, of 0.02. This contrast threshold was based

upon laboratory conditions . Experience has shown that the use of this

2-21
value was rather optimistic; however, it is still in frequent use.

2.3.16 Prevailing Visibility:-

The greatest visibility equal or exceeded through at least half the

horizon circle, which need not he continuous, that is, the median visibility

around the horizon circle.

2.3.17 Runway Visibility (RVV):-

The meteorological visibility along an identified runway. When a transmissometer is used for the assessment, the instrument is calibrated in

terms of sighting a dark object against a fog or horizon sky background

by day and lights of moderate intensity, about 25 candelas, by night. The use of runway visibility was superceded by the use of runway visual range

in the early 1960's as the RVR digital readouts ("computers") were

obtained.

It is now used when observations are made by a human observer

from a position near the runway during transmissometer outages.

2.4 TERMS RELATED TO AIRCRAFT LANDING OPERATIONS

2.4.1 Runway :-

A defined rectangular area, on a land aerodrome, prepared for landing

and takeoff of aircraft along its length.

2.4.2 Runway Threshold (Threshol d) :

The beginning of the runway usable for landing.

2.4.3 Touchdown Zone (TDZ):-

The part of the runway immediately beyond the runway threshold where

aircraft usually touch down; usually considered as being 900 meters ( 3000

feet ) in length.

2.4.4 Approach Zone:-

The area immediately before the runway threshold over which the

aircraft passes when making a landing.

2.4.5 Approach Lights:-

A configuration of lights located in extension of a runway before

the threshold to provide visual approach and landing guidance to pilots.

2-22
2.4.6 Threshold and Runway End Lights:-

Lights placed to indicate the beginning and end of that portion of

a runway usable for landing, respectively.

2.4.7 Runway-edge Lights:-

Lights installed along the edges of a runway marking its lateral

limits and indicating its direction.

2.4.8 Runway Centerline Lights:-

Lights installed in the surface of the runway along the centerline

indicating the location and direction of the runway centerline; of

particular value in conditions of very poor visibility.

2.4.9 Touchdown Zone Lights:-

Barettes of runway lights installed in the surface of the runway

between the runway edge lights and the runway centerline lights to provide

additional guidance during the touchdown phase of a landing in conditions

of very poor visibility.

2.4.10 Critical Height (C.H.):-

The minimum height above the ground at which an aircraft can execute

a missed approach. Until mid-1964 the term critical height was used with

the meaning now identified with decision height. At that time there was an abrupt change in usage presumably because of the realization that an aircraft should not descend as low as the critical height (as now

defined) without visual reference.

2.4.11 Decision Height (D.H.):-

The minimum height above the ground to which a pilot making an instrument approach may descend without reference to lights or objects

on the ground before executing a missed approach.

2.4.12 Operational Categories :-

Operational Performance categories as defined in Annex 10 [541 are

2-23
Operational Performance Category I: Operation dovm to 60 meters

(200 feet) decision height and with a runway visual range not less than

a value of the order of 800 meters (2,600 feet) with a high probability

of approach success.

Note: In the U.S.A., FAA Order 6560.10 allows some Category I operations with a minimum RVR of 1800 feet if touchdown zone and centerline lights

are available

.

Operational Performance Category II: Operation down to 30 meters

(100 feet) decision height and with a runway visual range not less than

a value of the order of 400 meters (1200 feet) with a high probability

of approach success.

Operational Performance Category IIIA: Operation, with no decision

height limitation, to and along the surface of the runway with external

visual reference during the final phase of the landing and with a runway

visual range not less than a value of the order of 200 meters (700 feet).

Operational Performance Category IIIB: Operation, with no decision

height limitation, to and along the surface of the runway without

reliance on external visual reference; and, subsequently, taxiiing with

external visual reference in a visibility corresponding to a runway visual

range not less than a value of the order of 50 meters (150 feet).

Operational Performance Category IIIC: Operation, with no decision

height limitation, to and along the surface of the runway and taxiways

without reliance on external visual reference.

Note 1. — The values given in feet are approximate rather than exact

equivalents for those given in meters and they have been chosen on the

basis of their operational significance in establishing runway visual

range values

.

Note 2 — The term "decision height" is defined in the ICAO PANS-OPS

and the term "runway visual range" is defined in Annex 14.

The terms CAT I, CAT II, etc. are frequently used to describe weather

in which the runway visual range is within the following limits:

2-24
CATEGORY Runway Visual Range

Limits

Meters ( Feet

)

II

I 400-800

800+ (2600 + ) (1200-2600)

IIIC

IIIA

IIIB

200-400

50-200

-50

(700-1200)

(150-700)

( -150)

Note: A runway light intensity of 10,000 candelas is assumed

unless otherwise stated.

2.4.13 Instrument Runway :-

A runway intended for the operation of aircraft using nonvisual aids

and comprising:

2.4.13.1 Instrument Approach Runway. An instrment runway served by a non- visual aid providing at least directional guidance adequate for a straightin approach.

2.4.13.2 Precision Approach Runway, Category I. An instrument runway

served by ILS or GCA approach aids and visual aids intended for operation

down to 60 m ( 200 ft ) decision height and down to an RVR of the order

of 800 m (2600 ft).

2.4.13.3 Precision Approach Runway, Category II. An instrument runway

served by ILS and visual aids intended for operations down to 30 m (100

ft) decision height and down to an RVR of the order of 4OO m (1200 ft).

2.4.13.4 Precision Approach Runway, Category III. An instrument runway

served by ILS (no decision height being applicable) and:

a. —by visual aids intended for operations down to an RVR of the

order of 200 m (700 ft);

b. —by visual aids intended for operations down to an RVR of the

order of 50 m (150 ft);

c. —intended for operations without reliance on external visual

reference.

2-25
Note: The figures given in feet are approximate equivalents for meters

(rather than exact equivalents) . They were chosen on the basis of their

operational significance in establishing runway visual range values.

2.5 MISCELLANEOUS TERMS

2.5.1 Areata (Cal ifornia) :-

This place name appears frequently in this report and in many other

reports concerning tests in fog. The name refers specifically to what

Is now the Eureka - Areata Airport. The airport is about seven miles north

of the town of Areata on the shore of the Pacific. It is reputedly the

foggiest airport in the continental United States. During World War II it was first an Army Air Corps Airfield, later a Naval Auxiliary Air Station.

In 194-5 the Navy selected this site for conducting thermal fog

dispersal experiments . During the period 194-6-1950 the Landing Aids

Experiment Station - operated under Air Force, Navy, CAA sponsorship - was located on the site. The Station was converted to a commercial

airport in 1950 and has continued as a County Airport. In 1953 the National

Bureau of Standards established a Visual Landing Aids Laboratory at the

airport. This operation was terminated in 1972. However, a small

staff is now maintained at the airport by a commercial engineering firm

under FAA contract. In addition many organizations, both government and

private, have used the site for short periods to conduct experiments in

fog.

2.5.2 Cei lometer, Rotating Beam or Fixed Beam:-

An instrument system used for determining cloud height by solving

the triangle formed by a light source, the illuminated spot produced on

the cloud by the projector and a photoelectric receiver which detects

the angular elevation of the spot. See Section 8.2. The term ceilometer

is often used without modification for either a fixed beam or a rotating

beam ceilometer.

2.5.3 Laser Cei lometer :-

An Instrument which determines cloud height by measuring the elapsed

time of a light pulse reflected off a cloud base. The receiver and laser

source are usually positioned next to each other.

2-26
2.6 INDEX OF TERMS

Term Paragraph

Absorption coefficient 2.2.5

Allard's law 2.3.5

Apparent contrast 2.1.12

Approach lights 2.4.5

Approach light contact height (ALCH) 2.3.12

Approach zone 2.4.4

Areata 2.5.1

Average intensity 2.1.5

Brightness 2.1.10

Categories I, II, III 2.4.12

Ceilometer 2.5.2

Contact height 2.3.11

Contrast 2.1.11

Contrast threshold 2.3.3

Critical height (CH) 2.4.10

Decision height (DH) 2.4.11

Extinction coefficient 2.2.7

Field factor 2.3.4

Fixed beam ceilometer 2.5.2

Flux 2.1.2

Illuminance 2.1.6

Illuminance threshold 2.3.2

Illumination 2.1.6

Instrument runway 2.4.13

Intensity 2.1.4

Koschmieder 's law 2.3.6

Laser ceilometer 2.5.3

Light 2.1.1

Luminance 2.1.9

Luminance contrast threshold 2.3.3

Luminosity 2.1.10

Luminous flux 2.1.3

Luminous intensity 2.1.4

Meteorological optical range (MOR) 2.3.14

Meteorological range (MR) ' 2.3.15

Meteorological visibility 2.3.13

Normal illuminance 2.1.7

Operational categories 2.4.12

Point brilliance 2.1.8

Precision approach runway 2.4.13

Prevailing visibility 2.3.16

Radiant flux 2.1.2

Representative intensity 2.1.5

Rotating beam ceilometer (RBC) 2.5.2

Runway 2.4.1

Runway centerline lights 2.4.8

2-27
Runway edge lights 2.4.7

Runway end lights 2.4.6

Runway threshold 2.4.2

Runway visibility (RVV) 2.3.17

Runway visual range (RVR) 2.3.8

Scattering coefficient 2.2.6

Scattering coefficient meter 2.2.8

Slant visual range (SVR) 2.3.9

Subjective brightness 2.1.10

Threshol

d

2.3.1, 2.4.2

Threshold illuminance 2.3.2

Threshold lights 2.4.6

Touchdown zone (TDZ) 2.4.3

Touchdown zone lights 2.4.9

Transmission 2.2.1

Transmissivi ty 2.2.3

Transmissometer 2.2.4

Transmi ttance 2.2.2

Vertical contact height 2.3.11

Visibility 2.3.13

Visual range 2.3.7

Visual segment 2.3.10

Visual threshold 2.3.2

2-28
3. CHKOiaOGlCAL RESUE

3.1 INTRODUCTION

The purpose of this section of the report is to review briefly

the history of the development and application of the runway visual

range concept. This section is essentially a flow chart given without

extensive detail. A detailed discussion of the pertinent parameters

is given later in the report.*

The distance at which one can see and recognize objects and lights

has been a very important factor in determining the safety and regularity

of travel since ancient times, particularly in the operation of aircraft.

Reports of the prevailing visibility have been made by Weather Service

since the early days of cross-country flight. At that time, and even

today, these reports have been based upon the observations of human

observers. From the beginning, there has been a desire to replace these

subjective observations with quantitative measurements, and by 1940

several types of visibility meters had been designed.

3.2 DEVELOPMENT OF THE TRANSMISSOMETER

INITIAL PHASE

In 194-0 the National Bureau of Standards was requested by the Civil

Aeronautics Administration to develop a visibility meter suitable for

routine use at airports. At that time the National Bureau of Standards

was completing its development of the prototype ceilometer .-^^ The first model of the transmissometer was constructed. Then,

as now, the transmissometer consisted of three units, an unmodulated

light source operating at a fixed intensity, a receiver with an output

in the form of pulses with the pulse frequency proportional to the

illuminance on the receiver, and an indicator consisting of a counting

rate meter. Figure 3.1 is an elementary block diagram of the 1941

instrument. It was field tested on Nantucket Island, Massachusetts

'^A glossary of terms is given in Chapter 2.

'^'^The development of the oeilometer is described in detail in Chapter 8.

3-1
o

3-2
diiring the summer of 1941. '^^^ Views of the field installation are shown

in figure 3. 2. During these tests numerous observations were made corre- lating the visual range of black objects by day and of lights by night

with the transmissometer readings.

These observations confirmed the validity of Koschmieder ' s law

for object visual range except that the value for the contrast applicable

to weather observers was found to be 0.055 instead of 0.02, the accepted

value at that time. The observations showed that at night the observer's

illuminance threshold was increased by glow from the source being

observed so that the threshold increased as the visual range decreased.

An empirical relation between the visual range of lights by night and

transmissometer reading was developed.

The transmissometer calibration curves for a 500-foot baseline

instrument developed from the Nantucket study are shown in figure 3.3.

The relations illustrated by these curves have been used since then in

the United States to convert transmissometer readings to visibility and

are the basis of tables A3 - SA, B, and C of the present issue of

Federal Meteorological Handbook No. 1, Surface Observations [36].

The Nantucket work also showed that spatial non-uniformity in

atmospheric transmittance severely limited the applicability of a short baseline instrument in the assessment of prevailing visibility

but that such instruments would be useful in the determination of

atmospheric transmittance in restricted areas, for example, an approach

zone.

3.3 DEVELOPMENT OF THE TRANSMISSOMETER 1941 - 1945

In the fall of 19'41j the instrument was modified to provide internal

calibration of the indicator, to permit operation of a recorder, and to

allow the indicator to be located at a distance of several thousand feet

from the field site. The instrument was then installed in the approach

zone of runway 31 of the Indianapolis Municipal Airport where it was used by the C.A.A. Experimental Station in their tests of approach lights.

The instrument remained at Indianapolis unitl it was turned over to the

Navy for other work in 1943.

The Development of the NBS transmissometer is desorihed in detail in

Chapter 6. A detailed discussion of field calibration of the instrument

is given in Chapter 7. 3-3
Receiver V:3ibi'sty MgrHs

FIELD INSTALLATION

Figure 3.2 Transmissometer test bed on Nantucket Island, 1941.

3-4
I

.2 .3 ,4 .6 .8 I. 2. 3. 4. 6. 8. 10. VISIBILITY (MILES)

Figure 3.3 Transmi ssometer calibration curve developed at Nantucket. The visual range , of objects is computed from contrast threshold of 0.055. The visual ! range of lights is computed from Allard's law assuming a light intensity i of 25 candelas and an illuminance threshold of 0.084/V where V is the

j vi si bi 1 i ty i n mi 1 es .

'

3-5 i
Progress during the war years was slow. During this period a transmissometer was installed at the Naval Air Test Center, Patuxent

River and was used in conjunction with tests of airfield lighting equipment. In addition, three transmissometers with baselines of 2267, 3280,

and -4000 feet were installed at Washington National Airport, the CAA

Experimental Station, and the Naval Air Test Center, Patuxent River

for correlation studies in visibilities in the 0.5 to 10 mile range.

Very little quantitative information was obtained from these studies.

The data indicated that changes in prevailing visibility were frequently

apparent from the transmissometer records before they were recorded by

the observers, and that spatial differences in fog density frequently

produced large differences between the visibility indicated by the transmissometer and the observer's estimates of prevailing visibility.

3.4 DEVELOPMENT AT THE LANDING AIDS EXPERIMENT STATION

During the period 1946-1950, the Landing Aids Experiment Station

(lags) was operated at the Areata, California, Airport, under the joint

sponsorship of the Air Force, Navy, and Civil Aeronautics Administration

to study methods of fog dispersal and approach light system configurations.

For a detailed account of the meteorological instrumentation program see references [83, 84, 85 and 86]. All existing NBS-type transmissometers (6)

were moved to LAES and, except for one, were used on 500-foot baselines along the instrument runway and in the approach zone to measure

fog density in specific areas during tests. The other transmissometer

was installed on a 3000-foot baseline to provide a measure of the prevailing transmissivity . The arrangement of instruments which evolved

during the tests is shown in figure 3.4. Figure 3.5 is a montage showing

the station and some of the equipment. The central transmissometer

recording station is shown in figure 3.6.

Althoiigh its primary purpose was to provide test data, this

assembly of instruments provided a unique opportunity to study the

problems encountered in the operation and maintenance of the instruments.

Throughout the period of operation of the station, refinements were made

to improve the performance and maintainability of the instruments, and

operating and maintenance techniques were developed.

3-6

Figure 3.5 The Landing Aids Experiment Station in 1948. Shown

also are: A ceilometer receiver, a transmi ssometer

light source and a transmi ssometer receiver.

3-8
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3-9
These instruments were provided with automatic sensitivity and

pulse-counting controls so that they could provide automatically, with

satisfactory accuracy, continuous records of transmittance ranging,

at night, from 0.00002 to 1.00.

In addition to the regular operation of transmissometers, a dualbaseline transmissometer T-D2 of figure 3-4 was operated in the touchdown

zone during part of this period and transmissometer equipment was adopted

to automatically control the intensity of lighting systems, satisfactorily

controlling the intensity of the runway edge lights during the 1949 test

season. A pictorial block diagram of the automatic intensity control

system is shown in figure 3-7.

Some of these transmissometers were still in operation at Areata

when the NBS Visibility Laboratory was closed in 1972.

The array of instruments at the station was an excellent source of

data regarding the temporal and spatial variations in fog density. Large

random and systematic differences were found, confirming the data taken

at Nantucket and elsewhere. An example of the spatial and temporal

variations in fog density is shown in table 3.1. In this table the columns

headed T-A, etc., show the visibilities computed from transmittance measurements of the indicated transmissometer. Locations of the transmissometers

and the observers are shown in figure 3.4.

Although the purpose of the installation of transmissometers at LAES

was not to test their use as visibility meters at airports, during the

flight tests observers reported the horizontal visual range of selected

objects or lights periodically, and pilots reported their visual contact

height and the visual segment of the approach and runway (edge) lights

during an approach and touchdown. These data formed an extensive data

base correlating visual observations with transmissometer measurements.

The blockade of Berlin began in the summer of 1948 and the renowned

airlift was started. The very high flight frequency required that after

a missed approach an aircraft return to its base without making a second

approach. This procedure imposed high demands on the accuracy of

weather observations, and the existing routine procedures using visual

3-10
•TTER CONTROL

LAMP

9

400- - „J

Figure 3.7 Automatic runway-light intensity control system at the

Landing Aids Experiment Station.

3-11
TABLE 3.1

VISIBILITY CONDITIONS

LAES Flight Test No. 49-23

Time Visibility in Feet

Outer

ADDToa h-' J—' ohN-f J. -L Threshold

ObservPT* k_/ !>-/ ^ -L V -1- T-A T-B T-C Observer T-D T-E

Vn s n h 1 1 i tv Visibility

North South North

0829 5000 3900 3600 3900 4500 6000 5600 3400

0838 6000 6500 3900 4600 5000 5000 11000 2500

0846 5000 8400 5900 4600 5500 4000 2100 1200

0856 3000 8400 3000 1300 2500 2000 1500 1200

0906 2500 2600 1800 1500 2300 1800 1400 2000

0916 2200 2100 2100 1800 3000 3500 5700 850

0926 2000 1800 1700 2000 2500 3200 3000 600

0934 2000 2100 2100 1300 1800 2000 960 710

0944 1500 1000 1200 760 1400 1000 520 630

095^ 1500 850 1200 840 1500 1000 680 360

Time Visibility Relative to T-C

Outer

ADDToa ph Threshold

Ql-) qp-ri-u-pT' ^ X V ^ X T-A T-B Observer T-D T-E

Vi R 1 Tri 1 1 "f,

V

V X. o j_ L.y _u j_ u^y Visibility

KfnTt.h South North

0829 1.3 1.0 1.0 1.2 1.5 1.4 0.9

0838 1.4 1.4 0.8 1.1 1.1 2.4 0.5

0846 1.1 1.8 1.3 1.2 0.9 0.5 0.3

0856 2.3 6.5 2.3 1.9 1.5 1.2 0.9

0906 1.7 1.7 1.2 1.5 1.2 0.9 1.3

0916 1.2 1.2 1.2 1.7 1.9 3.2 0.5

0926 1.0 0.9 0.8 1.2 1.6 1.5 0.3

0934 1.5 1.6 1.6 1.4 1.5 0.7 0.6

0944 2.0 1.3 1.6 1.8 1.3 0.7 0.8

0954 1.8 1.0 1.4 1.8 1.2 0.8 0.4

3-12
observations were not adequate. Efforts to improve the situation were initiated immediately.

In November of 1948, Mr. G. H. Stocker, Meteorologist of LAES, suggested

to the Chief of the Air Weather Service that transmissometers located

in the touchdown and approach zones of the instrument runway be used in

conjunction with a ceilometer in the approach zone as a standard operational weather reporting procedure. The following reasons were cited [85];

"Observations at LAES, as well as at other airports, have indicated

that in weather conditions at or below ceilings of 200 feet and visibilities

of 1/2 mile, the irregularity and variability of the respective weather

elements requires continuous, automatic, objective meteorological measurements that are actually representative of "pilot's weather" in the instrument approach zone.

"The meteorological instrumentation available at this time, with a few changes in placement and in operational methods, appears to be

readily adaptable as a basis for the aforesaid development. The fundamental requirements in any such instrumentation appear to be as follows:

"a. Airport weather observations should be made along the actual

approach (or take-off) path of the aircraft.

"b. Automatic, objective instrumentation should be utilized in

making weather reports in order to eliminate inconsistencies resulting

from individual variations and errors among human observers

.

"c. Airport station weather reports should be revised in form to

include the operational (or "pilot's") weather in the flight path of

aircraft at least in the "Remarks" section of the report.

"With reference to "weather" resulting from the presence

of an extremely low cloud deck^, it would appear that an installation consisting of one shortbase ceilometer and two shortbase transmissometers should be adequate to indicate and record

the effective operational "weather" along a normal instrument

approach zone and runway.

"mother conditions, such as ground fog or heavy precipitation,

may require addition consideration."

3-13
(The suggested configuration of meteorological instruments is shown

in figure 3-8).

"The instrument is intended to measure:

"The ceiling and meteorological visibility in the area where the

pilots of approaching aircraft first establish visual contact with

approach lights or the ground; and the meteorological visibility in the

touchdown zone along the runway.

"These are the two operationally critical areas in adverse weather

conditions, since local ceiling and visibility conditions in these areas determine the amount of guidance available to the pilot from the ground

plane, the approach lights, the runway-marker lights, and surface

markings .

Although it had been tacitly assumed during their development that

transmissometers would eventually be so used, this was the first explicit

proposal for their use.

3.5 AIR FORCE APPLICATION OF THE TRANSMISSOMETER 1949-1953

The Air Weather Service accepted the LAGS recommendation and requested

that the transmissometers be moved from LAES to Berlin in the spring of

1949. However, further consideration showed that the Air Force was not prepared to provide logistic support and maintenance. Human

- observers were used instead.

Study of the concept continued, and its adoption as a standard

weather reporting procedure was accepted by the Air Force in the

early summer of 194-9 although by that time the airlift had been

discontinued. A program to obtain commercially manufactured instriiments

and to train Air Force personnel to maintain and operate the equipment

was initiated, and during the 1949 fog season two groups of Air Force

personnel were trained at LAES. However, it was not until the spring

of 1950 that the procurement program was completed. At that time the

National Bureau of Standards was requested to provide the Air Force

with 25 instruments, spare parts, and an instruction book.

In June of 1950, a contract was awarded to the Grouse Hinds

Company, Syracuse, New York, to construct 25 transmissometers with

3-14
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spare parts. The first instruments were delivered in June of 1951;

tested at the National Bureau of Standards; minor modifications made;

and type approval obtained in August, 1951. Although the instruments

differed considerably in construction details (figures 3.9 and 3-10),

the principles of mechanical and optical design, and of electronic circuitry

were the same as those of the earlier instruments [1031 . Concurrently,

kits for modifying ceilometers to permit remote indication and to

improve their response during periods of low visibility were being

procured through other channels.

One transmissometer and a modified ceilometer were then given

operational suitability tests at Eglin Air Force Base [2] . The primary

conclusions of these tests were:

a. "Transmissometer-cellometer equipment is operationally

suitable for measuring cloud height (ceiling) and visibility

in the approach and touchdown zones of instrument runways

under temperate and extreme climatic conditions."

b. "The inclusion of two transmissometers in the system

is necessary because of visibility variations between

approach and touchdown zones .

c. "There is no appreciable difference in visibility

observations made at the runway edge and as far as 400

feet from the runway edge .

Conversion from the transmissometer transmittance measurements to

visibilities was done by means of the equations and threshold constants

developed at Nantucket and verified by subsequent testings. In

addition to conversion tables relating transmittance and day and night

visibility, converters were supplied to be attached to the recorders,

so that the charts could be read through them, and for desk use. A

drawing of the desk converter is shown in figure 3.11.

Following these tests, the Air Force proceeded with the installation

of modified ceilometers and the transmissometers procured by contract with

the National Bureau of Standards . In designing these installations for

3-16
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16 of the bases, the Air Force followed the recommendations of the Weather

Bureau (see section 3.7.2) and only one transmissometer was installed at

each airport, near the touchdown zone. However, at two of the bases, the

recommendations of LAES and Eglin AFB were followed and an additional transmissometer was planned for the approach zone [7] . By the spring of 1954-,

transmissometer installations had been made or were scheduled for the

following bases:

Andrews AFB, Maryland McGuire AFB, New Jersey

Brookley AFB, Alabama Mitchel AFB, New York

Dover AFB, Delaware Otis AFB, Massachusetts

Dow AFB, Maine Selfridge AFB, Michigan

Ernest Harmon AFB, Newfoundland Shaw AFB, South Carolina

Fairchild AFB, Washington Westover AFB, Massachusetts (Two Sets)

Langley AFB, Virginia Wright-Patterson AFB, Ohio

March AFB, California Sewart AFB, Tennessee or McChord AFB, Washington Moody AFB, Georgia (Alternate Site)

McClellan AFB, California ( Two Sets

)

In August of 1954, the National Bureau of Standards conducted a twoweek training course for Air Force operations and maintenance personnel.

By the end of 1954, the Navy had ordered 10 transmissometers.

3.6 APPLICATION OF TRANSMISSOMETERS TO

CIVIL USE, INITIAL PHASE

3.6.1 Initial Appl ications :

The application of transmissometers to operational use in civil aviation

was, with two exceptions, more deliberate than in military aviation.

One of the exceptions was the authorization in 1949 of the use of

minima of 1/4-mile visibility and 100-foot ceiling by Southwest Airways

at Areata provided transmissometers and a ceilometer, installed, as shown

in figure 3.4, were in operation and the prescribed electronic and visual

aids were available. Operations at these minima were terminated with

the closing of LAES and the removal of the transmissometers and ceilometer

in the summer of 1950. Transmissometers were reinstalled in 1951, but

routine operations at the low minima were not reestablished.

3-20
The other exception was the use of transmissometers in conjunction

with the fog dispersal (FIDO) system at Los Angeles Municipal Airport. Five

transmissometers were purchased hy the City of Los Angeles for this purpose,

from the Crouse-Hinds Company, using a specification prepared by the National

Bioreau of Standards . This specification predated the specification used

for the Air Force procurement. These transmissometers were installed

along the runway on which the fog dispersal system was installed and used

to measure the fog clearance produced.

3.6.2 Initial Studies by the Weather Bureau:-

The systematic study of the feasibility of civil use of the visibilitymeter system proposed by Mr. Stocker as a replacement for the conven- tional visibility observations started with a Symposium called by the

Weather Bureau on May 5 and 6, 1949. Because of its significance, the

operational requirements for low visibility and ceiling measurements

developed for the meeting are given below.

"1. To measure the visibility and base of low clouds in the

approach zone between the inner^ marker and one-half to three-fourths

the distance down the ILS runway. This distance may vary from two to

six miles depending upon airport size, surrounding topography, and

obstructions

.

2. Visibility measurements between zero and 1 mile along a straight

line

.

3. Base of low cloud measurement between zero and 500 to 1000

feet above the runway.

4. These observations will be relayed to the pilot on his final

approach

.

5. It is necessary that the observations be on a continuing

basis and transmitted automatically via ground wire or micro-wave for

a distance of one to six miles (to the Control Tower or other points

of control).

^The outer marker was meant.

3-21
6. The values of base of low cloud and visibility should be

recorded on a single dial indicator calibrated to indicate combinations

of the two elements . These combinations will be determined from

operational requirements and necessarily tempered by the limitations

of equipment ".

The meeting agenda is given in Appendix A. Some believed that there was a need for more data on the spatial variations of visibility and

ceilings than that which had been obtained at the Landing Aids Experiment Station. Therefore, during the last half of 1949 the Weather

Bureau, with the cooperation of the Civil Aeronautics Board, the Civil

Aviation Administration, and the Air Transport Association, conducted

a study at Washington National Airport of the differences in meteorological

visibility reported by an observer from the Weather Observatory on the

terminal building and visibility along runway 36 reported by an observer

near the threshold. The two observation points were about 3000 feet

apart. The observer at the terminal was about 70 feet above the ground,

the observer at the runway site 10 feet above the ground. Ceiling

measurements were made using a dual-projector ceilometer at the terminal

and balloons or a ceiling projector at the runway site. The study

confirmed the general opinion that the differences in visibility and

ceiling were rather large and highly significant. Although the

average ceiling was approximately the same for the two locations and

the average visibility was slightly lower at the runway site that at

the terminal, there were a number of instances when there were important

differences. For example, although 21% of the visibility observations were

identical, \% of the runway visibility observations were 1/4- mile lower

than the mirador observations and 10^ were 1/4- mile higher.

Late in 1949 the organizations concerned determined that it was not economically feasible to solve the problem of spatial variations in

visibility by placing an observer at the end of the runway during periods

of low visibility and that instruments would be necessary.

3.6.3 Transmissometer - Ceilometer Program at Washington National

Airport

During the period of the visibility correlation tests at Washington

3-22
National Airport, plans were developed for the installation of a transmissometer and a ceilometer there. The ceilometer was to be installed at the middle marker and a location near the glide path

shelter was selected for the transmissometer. The indicators for the

equipment were to be located in the Weather Bureau Marador Office. In

their review of these plans some representatives of the Civil Aeronautics Administration and the Weather Bureau felt that the transmissometer should be located as close as practicable to the ceilometer.

Others favored the touchdown zone site. The group was unable to reach

agreement as to the most desirable location and plans were made to

install a second transmissometer near the middle marker if one could be

obtained. The equipment was procured and installed as shown in figure

3.12 in 1951. Note that the transmissometers used had 750 foot baselines.

A second test site was established to test the rotating-beam and pulsedlight ceilometers at the Weather Bureau's Silver Hill Observatory.

In the fall of 1951 the Air Navigation Development Board (ANDB)

agreed to sponsor a Weather Bureau project for "research and development

work in methods of determining ceiling and visibility which affect the

operation and control of aircraft during final approach and landing,

particularly under low-ceiling, low-visibility conditions".

3.7 ANDB TESTS AT WASHINGTON NATIONAL AIPORT 1951-1953

Under ANDB sponsorship the Weather Bureau then established a project at Washington National Airport to study the meteorological aspects

of the problem. At the same time a contract was let with the Sperry

Gyroscope Company, with the Weather Buearu as a monitor, to conduct a program of flight-landing operations under low ceiling-low visibility

conditions.

The work was given added impetus by Congressional hearings held in

February and March of 1952 on aviation safety [41] .

3.7.1 Application of Television:-

At these hearings there was considerable discussion of the feasibility

of using television techniques to secure data to fulfill the operational

3-23
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need for better information at airports on visibility and ceiling

conditions near the flight path, and an exploratory study was conducted

as a part of the project at Washington National Airport. The camera

was located at the approach end of the instrument runway (runway 36).

The video signal was transmitted by microwave to the observatory in the

terminal building where the receiver was located. During the tests all

equipment was continuously monitored and adjusted by skilled operators

for the "best picture" . The experiment was designed so that comparisons

could be made between the visibility determined by an observer in the

observatory viewing the picture and an observer near the camera at

the end of the runway viewing the scene directly. The results of the

tests were negative. Although daytime visibility, using objects, could

be determined "fairly well", the rendition of light sources near the

limit of visibility was not representative of the visual scene. There

were many engineering problems to be solved before the system was capable of continuous automatic operation [126] . At this point the

study of the use of television systems was dropped.

3.7.2 Application of the Transmissometer-Cei lometer System:-

Of more importance than the tests of the application of television

were the detailed studies made of the application of the transmissometer

and ceilometer. A study was made of the relation between observed

(meteorological) visibility and transmissometer data. From this study

it was concluded that "the readings of the transmissometer, as calibrated

by Douglas, show reasonably good agreement with (meteorological) visibilities reported by a nearby observer. There appears to be little

difference between performance by day and night" [126] . The data

obtained during stable and uniform low visibility conditions were

analyzed to determine the contrast and illuminance thresholds of the

persons making the visual observations . Both thresholds were in sufficiently

good agreement with the transmissometer calibration thresholds that changes

in the transmissometer calibration were not warranted. In addition it was found that operational measurement of the nighttime background

luminance and consequent adjustment of illuminance threshold was

not necessary. No twilight calibration was developed. See Section 4-4

3-25
for a more complete treatment of thresholds.

A study was made of the height of cloud base measurements using

the rotating beam ceilometer. In this study it was found that an average

of four measurements was required to yield an indication of the ceiling

an approaching pilot would encounter. It was found that cloud "bases

were ragged and that cloud height used as an approach forecast must be

considered as a zone, not a plane surface, and that the thickness of this

zone may often be of the order of several hundred feet.

Tests of a French pulsed-light cloud height meter indicated that

the instrument was not satisfactory because the pulse length made

measurements of cloud height below 500 feet unsatisfactory.

The general conclusion of the study was that the transmissometer and

rotating beam ceilometer were suitable for routine operational use. Operational use of the instruments was started December 15, 1952, using

ceiling indications from the rotating beam ceilometer at the middle

marker and visibility indications from the touchdown zone transmissometer

fox regulatory or control purposes whenever the touchdown zone visibility was IJ miles or less. When the visibility indication of the

middle-marker transmissometer was lower, the visibility indicated by this

instrument was reported also. Experience soon demonstrated that changes

in visibility occurred so rapidly that they could not be handled

promptly by the regular weather observer and a direct indication was required in the control tower. Accordingly a five inch, 250° scale

meter, calibrated as shown in figure 3-13? was installed in the tower for

use by the air traffic controller. Note that a visibility of I5 miles,

which is about 10 times the length of the baseline, is the maximum

visibility indicated on the scale, making the full 250° scale available

for the operationally useful visibilities.

The day scale was used during twilight periods until the background

luminance was so low that lights were clearly visible. When the visibility

was varying rapidly, the mean value and the extremes were reported.

During these tests it was concluded that the additional transmissometer

installed near the middle marker was not cost effective and it was

3-26
Figure 3.13 Runway visibility (RVV) scale used in

I early installation, for 750-foot baseline.

i

3-27
removed in September of 1953 for installation at Idlewild. (No documentation concerning the decision to terminate the use of the middle marker

transmissometer has been located. )

The criterion used to judge the suitability of instrument program

was approach success. Records of missed approaches at Washington National

Airport during inclement weather were examined to determine if the

operational use of the transmissometers and ceilometer had produced an improvement. Only approaches during periods where the visibility was

less than one mile or the ceiling was below 500 feet were used in the

analysis. Results of this analysis are shown in table 3-2.

Table 3-2

MISSED APPROACHES AT

WASHINGTON NATIONAL AIRPORT [126]

Ceiling Less than 500 ft. and/or

Visibility less than 1 mile

# of Approaches

0) -pft X3 S (U

(D CO -P CQ Periods of Operation ^ g

Approaches

Missed

Jan. 1, 1952-Dec. 15, 1952 (before 983 63 6.3^

use of runway observations

)

Dec. 16, 1952-Mar. 31, 1954 (after 838 36 ^.3%

use of runway observations

)

From this analysis it was concluded that the data indicated that the

low-weather instrument-approach success had been improved; the inference

being that runway observations are more nearly representative of

conditions experienced by the pilot in landing. Although some or all

of the improvement might have been due to other causes, the results were

encouraging.

3.8 ANDB TESTS AT MacARTHUR FIELD (1953) AND AT IDLEWILD (1954)

Except for the analysis of missed approach data, the Washington

National Airport Studies were limited to observations from near ground

level. The study at MacArthur Field, conducted by the Sperry Gyroscope

Company and monitored by the Weather Bureau, was designed to complete the

3-28
program. The objective of this study was to evaluate the transmissometerceilometer system in relation to the operational requirements of the

instrument approach by correlating the measurements obtained from the

instruments with what the pilot saw simultaneously from the cockpit

during ILS approaches.

A commercially produced transmissometer was installed near the

touchdown zone mounted at a height of 15 feet on stands of recommended

design. See figure 3.14. Two ceilometers were installed, one at the

middle marker site and the other in the approach zone near the threshold.

A terrain illuminometer, thirteen visibility targets and lights for

obtaining meteorological visibility, and 28 "pup tents" (see Section 3.10.2)

were installed as visibility targets to obtain supplementary data. The

installation in the aircraft consisted of two photometers, a motion

picture camera aimed to photograph the pilots view, a mapping camera directed downward to photograph the terrain, a NASA type cloud detector,

and a photo-panel to document aircraft instrument readings.

The pilot or copilot reported a) vertical contact, b) approach

light contact, and c) threshold contact. The approach light was the

earliest system consisting of 14 neon bars each having an intensity of

about 1000 candelas.

Because the approach lights at MacArthur Field were low- intensity

lights and the Sperry pilots were very familiar with the field and

surrounding terrain, the flights at MacArthur Field were supplemented

by flights at Idlewild, where a high-intensity approach-light system

was installed. A total of 468 instrument approaches, 409 at MacArthur

and 59 at Idlewild, were made in low ceiling and/or low visibility con- ditions .

The results of the tests are summarized in table 3.3 [126].

3-29
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Conclusions drawn by the Weather Bureau were, in part, that the

transmlssometer-ceilometer combination provided a sound method for

remotely measuring weather in the approach zone, but that optimum

interpretation of the data required supplementary photometric measurements .

The single recommendation made in the report on the project was that

a program of field tests at regular airline terminals with airline aircraft

and airline pilots should be implemented to evaluate a method of reporting

which incorporates photometric measurements in addition to the usual

meteorological parameters.

The basic data obtained in the project provided important information regarding the manner in which conventional ceiling and visibility

observations compare with pilot experience during an approach. The

following general conclusions were drawn from a study of the data from the

standard station and the in-flight data:

1. Ceiling is usually a conservative estimate of vertical

contact height. In 89^ of the cases vertical contact

height was greater than reported ceiling. Vertical contact height averaged 105 feet higher than reported ceiling.

2. When low clouds are present, reported visibility is usually

greater than the contact range of the runway threshold. In

91% of the cases this condition prevailed. The average

difference was 2,100 feet (approximately 0.4 mile) for the day cases

and 5,4-00 feet (approximately one mile) for the night cases.

3- When radiation fog is present, the contact range of the

runway threshold and reported visibility agree fairly well

on the average. Threshold contact range averaged 1,500

feet (about 1/4 mile) less than reported visibility

in daytime and 700 feet ( about 1/8 mile ) greater than

reported visibility at night. Although the average

differences were not great, the extreme differences,

both day and night, were about four times the average.

3-32
The comparisons showed that there was ample Justification for

the rather common "belief that meteorological observations^ as routinely

made at present, do not accurately indicate conditions the pilot will

experience if, as is frequently done, ceiling is interpreted as vertical

contact height and visibility is interpreted as contact range of the

runway threshold, or other slant visibility.

When the end-of-runway station data were compared with the in-flight

data, the overall nature of the comparison was not greatly changed, although

there were, of coirrse, variations in the details.

The results of the MacArthur Field tests and those at Washington

National Airport were sufficiently convincing that by April 1, 1954

transmissometer systems were in, or near, operational use at Idlewild,

Newark, and Washington and scheduled for installation at the following

airports

:

La Guardia, N.Y. Seattle-Tacoma, Wash.

Boston, Mass. Portland, Oregon

Philadelphia, Pa. San Francisco, Calif,

Pittsburgh, Pa. Los Angeles, Calif.

Cleveland, Ohio Fort Worth, Tex.

Chicago, 111. Anchorage, Alaska

Detroit, Willow Run, Mich.

Kansas City, Mo.

3.9 DEVELOPMENT OF THE RVR SYSTEM

3.9.1 Inauguration of Runway Visual Range Readout:-

Even as the runway visibility systems were being placed into operational

use, plans were being made to convert to a system which indicated runway

visual range instead of meteorological visibility. The request for

further development was motivated by several factors : a ) European

practice in reporting RVR, b ) a desire to report visibility conditions

in units which were more representative of what the pilot saw during an approach and landing, and c ) the desire to take into account the

increased visual range obtained with high intensity approach and runway

edge lights and to obtain authority to land in more dense fogs without

3-33
lowering the visibility minimums. (The relative importance of these

factors is uncertain. )

By inid-1955 plans had been made for an RVR installation at Newark,

and the values of the parameters to be used in converting transmissometer

readings to RVR had been fixed.

An intensity of 10,000 candelas was chosen as being representative of

the in-service intensity of a high-intensity runway-edge light in the

directions from which it would be viewed during a flare and landing.

This intensity was later chosen by the Visual Aid Panel in their amendment

of the definition of RVR. See Section 3.11.2. The method of

determining the intensity to be used in assessing RVR was later adopted

by the International Civil Aviation Organization. See Section 3.11.3.

No consideration was given to the, changes in intensity which result

from dimming the lighting systems in conditions of less dense fogs.

No special tests v/ere made to determine the night and day illuminance

thresholds to be used in the conversion to RVR. Laboratory data were not

applicable. The spread of the illuminance threshold values obtained

from flight test data in fog, for example, the Landing Aids Experiment

Station data, was so large - several orders of magnitude - because of

the effects of non-uniformities in the fog density and reporting errors,

that use of the mean or the median threshold values would have been of

doubtful validity. Hence, the thresholds were based upon engineering

Judgements considering past experience and practices.

A value of 2 mile candles was chosen for the nighttime illuminance

threshold. In the early days of aviation, an illuminance threshold of

0.5 mile candle was used [117]. In the 1940' s, an illuminance threshold

of 1 mile candle was used by some engineers both in the United States

and in Great Britain. The increase was made in consideration of the

increased losses in sloped, multi -element, "bird proof" windscreens,

the increased number of lighted instruments in the cockpit, and the

increased complexity of flying. A further increase was made to 2 mile

candles for use in the RVR conversion to obtain a value which was con- servative in nature.

3-34
It is interesting to note that in 1955 the Aviation Committee

of the International Commission on Illumination (CIE) was considering

the nighttime values of illuminance threshold applicable to aviation

[72]. This choice was completely independent of the work in the U. S. on illuminance thresholds applicable to RVR. The CIE recommended a value

of one microlux (2.6 mile candles) for the nighttime illuminance threshold.

It should be noted that neither the value of illuminanae threshold

ohosenj 2 mile candles, nor the CIE value^ was intended to

be applied to the high liminanoe conditions whiah now exist over

a runway with high intensity edge^ touchdown zone^ and centerline lights^ operating at full intensity as has been implied by some recent ICAO documents [55] . An analysis of thresholds applicable to RVR is given in Section 4.4.8.

A value of 1000 mile candles was chosen for the daytime threshold in

a manner similar to that used in choosing the nighttime threshold. The

1955 meeting of the CIE recommended a value of 300 microlux (780 mile

candles) for dull overcast conditions and 1000 microlux (2600 mile

candles ) for bright sunlight conditions

.

The relative agreement between these two independent

evaluations of illuminance thresholds is gratifying.

An RVR scale, shown in figure 3-15, was prepared as a replacement for

the RW of figure 3.13 scale used at prior installations. Note that the

scale is graduated in feet instead of fractions of a mile. This scale was based upon an intensity of 10,000 candelas, and the two thresholds,

2 and 1000 mile candles, discussed above. No consideration was given to

the effects of dimming the lights or to the visual range of objects.

An 810-foot baseline was used at Newark because the location of

taxiways prevented the use of a 750 foot baseline.

3.9.2 Reconsideration of Thresholds:-

The landing minimum was set at an RVR of 2600 feet with no statement

of minimum ceiling. Experience with the RVR and the lighting system

was so favorable that, in early 1957, the operators requested that

a study be made of the feasibility of modifying the transmissometer

3-35
Figure 3.15 RVR meter scale used in control tower at Newark,

1955. This calibration is based upon 10,000

candela lights only.

3-36
RVR calibration stating that the calibration was too conservative.

A small working group considered the problems and reviewed the factors

considered in the choice of intensity and values of threshold illuminance.

The group concluded that the value of 10,000 candelas was representative of the intensity of the beam of the runway edge light in the

direction of the pilot.

The group took into account the following factors in their discussion of illumination thresholds.

1) Thresholds obtained during the MacArthur Field tests.

2) Threshold obtained at Newark Airport, based on the

sighting of the green threshold lights.

3) Internationally recommended values.

4) Reports that many pilots felt the present calibration to be too

conservative

.

The group found that for daytime thresholds

:

1) The illuminance threshold for daylight of 1000 mile candles was more conservative than the 95^ probability value observed

at MacArthur Field.

2 ) It was very close to the 75^ probability value observed at

Newark. (Higher illuminance thresholds at Newark were ex- pected because of the effect of the high intensity approach

lights, which were not present at MacArthur Field. ) 3) The 1% probability value had been suggested by pilots

as an appropriate one. 4) The International Illumination Commission, meeting

in Zurich in June 1955, recommended a value of approximately 780 mile candles for the illuminance threshold

for a dull day.

The group found that for nighttime operations

:

l) The night illuminance threshold of 2 mile candles corresponded

to about the 40^ probability level for MacArthur Field thresholds

and to about the 20^ probability level for thresholds at Newark

based on sighting of the green threshold lights

.

3-37
2) This indicated that the night illuminance threshold was somewhat optimistic. However, the group was reluctant to

accept that concept in view of pilot reports to the contrary.

3) In support of the present value was the value of approximately

2.6 mile candles recommended by the International Illumination

Commission for nighttime use. The 2.5 mile candle value was somewhat more conservative than the U.S. value of 2 mile

candles, but the resulting difference in calibration was of no practical significance.

The group concluded that the evidence available was somewhat

contradictory in nature and did not warrant a change in the present

calibration of the transmissometer

.

At this meeting it was suggested, informally, that, if the primary

motive in suggesting a change in the RVR calibration was to permit

landings in more dense fog, this should be accomplished directly by

lowering the RVR minimum. Subsequently the RVR mlnimiim was lowered to

2000 feet for airports having a "Configuration A" approach light system

with sequenced flashing lights, and a high intensity runway edge light

system with lights spaced at 100-foot intervals. (At that time

Newark was the only airport meeting these requirements.

)

3.9.3 Early Operational Use:-

By 1958 RVR systems with a meter readout were in operational use

at Washington National, Idlewlld and Boston in addition to Newark.

3.9.4 Approach Visibility Project at Newark:-

The approach visibility studies conducted at Newark during the period

1956 to 1958 by the Weather Bureau [34] and the Air Force [40] and at

NAFEC during the period 1959 to 1962 [89] except as they relate to

illiiminance thresholds, are considered beyond the scope of this report and

are not summarized.

3.9.5 Development and Application of the RVR "Computer" :-

Even before the first RVR system with a meter readout was placed

into service, plans were being made for the replacement of the meter

3-38
readout with a digital display. The original request for proposal

indicated that the same parameters were to be used for the digital display as were used for the meter calibration. The display was to be updated every

thirty seconds, but there was not a requirement for averaging the

transmittance over a period of time . The reporting increment was

to be 100 feet from 2000 to 6000 feet.

These plans were considered by a working group comprised of representatives of the Civil Aviation Administration, the Weather Bureau, the

Air Force, the airline operators, the airline pilots, and the National

Bureau of Standards. The following design features were recommended by

the group.

1) The nighttime and daytime thresholds then in use should not

be changed. After considerable thought, adjustment of the

daytime threshold for changes in background luminance and

for twilight was rejected as not being cost beneficial.

2) An intensity of 10,000 candelas should be used as representative of the runway edge lights operated at full intensity but

2000 and 4-00 candelas should be used when the lights were operated at intensity steps 4- and 3 respectively. The applicable intensity should be determined automatically the

position of the intensity setting switch in the control tower.

3) The use of 100 foot increments was not practical because of

the great variability of fog density with time. Studies of

the temporal variation of RVR computed from NBS transmissometer

records indicated that a 200 foot increment was suitable for

RVR's below 4000 feet and 500 foot intervals were suitable for

greater RVR's.

4) An averaging period of 45 to 60 seconds should be used. The

averaging periods used at the Landing Aids Experiment Station

and in the MacArthur Field tests of 4 and 3 minutes were considered too long to permit adequate representation of

sudden changes in RVR, and periods less than 45 seconds were

considered too short to permit adequate representation of RVR

obtained by measurements of transmittance over a relatively

short baseline.

3-39
5 ) The minimum RVR to be displayed should be considerably lower

than 2000 feet. To accomplish this the length of the transmissometer baseline should be reduced from 750 to 500 feet.

6) Since in daylight, the meteorological range exceeded the RVR

at high transmittances and the minimum visibility requirement for

the jet aircraft then being introduced was in this transmittance

region (^000 feet RVR or 3/4 mile meteorological visibility),

the indicated RVR should be based upon the visual range of

black objects whenever it exceeded the RVR. Otherwise the

fog would be less dense under minimum conditions at RVR

equipped airports than at airports using RVR or weather station

observations

.

7) The contrast threshold to be used in the computation of the

visual range of black objects should be 0.055, the same as used for the RW calibrations.

These criteria were accepted and procurement of computers was initiated.

The "computers" designed to these requirements were essentially

memory banks of six sets of tables based upon the two thresholds and

the three light intensities, with selection of the proper value of the

appropriate table to be displayed controlled by the number of pulses

generated by the transmissometer in a period of 55 seconds, an illuminance

meter to select day or night scales and the position of the runwayedge light intensity-control switch. A graphical representation of the

six scales used in the calibration of the computer is presented in

figure 3.16. As many as five RVR readouts could be used with the computer.

3.9.6 Further Appl ication :

In mid-1962, RVR systems were in use at the following locations:

Computer Commissioned

3. Los Angeles, Calif.

4. Montgomery, Ala.

5. New York (Idlewild), N.Y.

1.

2. Dallas, Texas

Baltimore, Md. 6. Newark, N.J.

7. Philadelphia, Pa.

8. Pittsburgh, Pa.

9. Portland, Ore.

10. Washington, D.C.

Runway 4R

3-40

Meter Commissioned

1. Anchorage, Alaska

2. Atlanta, Ga.

3. Birmingham, Ala.

4. Boston, Mass.

5. Charleston, S.C.

6. Chicago (O'Hare), 111.

7. Cleveland, Ohio

8. Columbus, Ohio

9. Denver, Colo.

10. Detroit (Met.), Mich

11. Detroit (Willow Run), Mich.

12. Fort Worth, Texas

13. Houston, Texas

14. Indianapolis, Ind.

15. Louisville, Ky.

16 . Memphi s , T enn

.

17. Milwaukee, Wise.

18. Minneapolis, Minn.

19. Oakland, Calif.

20. New York ( Idlewild), N.Y.

Runway 31L

21. St. Louis, Mo.

22. Salt Lake City, Utah

23. San Francisco, Calif.

24. Seattle-Tacoma, Wash.

25 . Spokane , Wash

.

26. Tampa, Fla.

27. Tulsa, Okla.

At that time 4 computers were being installed and an additional

156 were on order.

As instriment landing systems, high-intensity approach-light

systems with sequenced-flashing lights, and high- intensity runwayedge lights were installed, there was an increasing demand for RVR systems

with a goal of installing an RVR system on every full instrumented runway.

The years following these developments have been evolutionary with

no significant changes in operational principles. The RVR minimums were lowered as confidence in the RVR system increased with experience and

as improvements were made in the electronic aids and lighting systems.

The transmissometer baseline was shortened to 250 feet on runways intended

for Category III service to permit measurements of RVR down to 600 feet

[I23, 26] . The computer was redesigned to provide for displaying RVR

as low as 600 feet and modernized by using modern solid-state techniques.

At some airports, the computers were replaced with AMOSV (automatic

3-42
meteorological observation station, Mark V) which could free four

computers. As the RVR minimum was reduced, better information of

visibility conditions along the runway beyond the touchdown zone became

necessary, and transmissometers were installed at the midpoint of some runways [113a] . Calibrators designed to replace the visual estimates used

in adjusting the full scale (perfectly clear weather) reading of the

transmissometer were developed and are now coming into service [29, 75]

.

However, the basic transmissometer, the contrast and illuminance

thresholds, the illuminance level for transition from day to night scales,

and the reporting increments have not been changed since the first use

of the RVR system, nearly 20 years ago.

3.10 THE DEVELOPMENT AND APPLICATION OF THE RVR

CONCEPT OUTSIDE THE UNITED STATES

3.10.1 European Practice:-

In Europe, the development and application of the runway visual

range concept was quite different from that in the United States, in

that the assessment of RVR was based entirely upon visual observations

and the test period was very short. By 1953^ France, Italy, the Netherlands, and the United Kingdom were using RVR operationally when the

meteorological visibility was below 1200 meters (1200 yards in the

United Kingdom) and Ireland was using RVR experimentally.

The praatiae of reporting meteorological visibility at

night as the distance a black object would be seen by day^ see definition 2.2.12^ was an important factor in accelerating

the use of RVR in Europe. At night the RVR would be roughly

three times the meteorological visibility. Rence, reported

visibility had little meaning.

In this period the distinction between the concepts of runway visual

range and runway visibility was not clear. For example, at LeBourget

the meteorological office was located 1000 feet from the end of the

instrument runway and visibility was determined by observing available

lights along the runway and airport boundaries. (Specific data are

3-43
not available, but it is believed that the intensity of these lights

in the direction of the meteorological observer was considerably lower

than the intensity in the direction of the pilot. )

3.10.2 Development of RVR in the United Kingdom:-

In the United Kingdom, development of the RVR concept was stimulated

by the advent of the Comet Airliner and, as in the United States, the

Berlin airlift. Operational use of RVR was started at London airport

following an accident in fog and the subsequent issuance of the Brabazon

Report

.

Operational use of RVR assessments started at Heathrow Airport in

the fall of 1951. Observations were made by an observer located near

the approach end of the runway in a "runway control caravan" . The

observer's eye height was approximately 15 feet, and he was about 120

feet from the runway edge. The observers were selected from members

of the rescue and fire-fighting services and changed at hoiorly intervals

to maintain the efficiency of observations.

Frequently it has been stated that this pvooedure involves

no labor costs. Although there is no direct costj there is a very important hidden cost in that during periods of lew

visibility the number of persons immediately available for

fire and rescue service is reduced by two^ one person

observing and one in transit.

Instrument runways were equipped with frangible pup-tent targets

for day use. The targets were located beyond the far side of the

runway and spaced so that the increment in distance between the ob- server and the targets was 100 yeards . The targets were six feet long and

three feet high painted half black and half white. (Thus by day the U.K.

RVR was equivalent to the U.S. RW. )

Special gooseneck flares having an intensity of about 800 candelas were installed near the line of targets for use at night as reference

lights. A rather circuitous calibration procedure was used to determine

the proper distance between the observer and successive lights. These

distances were chosen so that when light number "n" was at the

3-44
limit of visibility, the RVR was n hundred years. In making this

calibration, the average, or representative, intensity of the runway

lights was not used. Instead the intensity of the runway light in the

direction at which a pilot about 15 feet above the runway centerline

would view the light when it was at a distance of n hundred yards

was used.

The use of these end-of-runway assessments of RVR so reduced

pilot complaints that the Meteorological Office felt that the need for

slant visibility assessments no longer existed. (However, as will be

discussed later, others in the U.K. did not agree.

)

Use of this method of assessing RVR was extended rapidly to other

airports. Often lights other than the "gooseneck" flares" were used as reference lights. When the runway edge lights could be seen and

counted by the observer, they were used. In all cases, the observed

vis\ial range of the lights was converted to an RVR based upon runway

centerline lights when such lights are installed, otherwise it was based upon the edge lights

.

By 1964- the United Kingdom had found it necessary to develop a form of automatic data-transfer equipment to pass changes in RVR

instantaneously as they occur and to provide air traffic controllers,

notably the precision approach controller, with an illuminated visual

presentation of up-to-date RVR values so that pilots may have the

latest RVR value down to touchdown. The RVR value, as determined by

the observer from the conversion table, was dialed to the approach

and aerodrome controller's positions where the required figures appeared

on an illumination indicator. Only a few seconds elapsed between the

time the observation was made to the time the RVR was passed to the

pilot. A simpler form of this equipment was used at some airports.

The observer telephoned his count of lights to air traffic control,

where the observation was converted into RVR, and the value was indicated mechanically in a master recorder and automatically relayed to

slave repeaters at the various controllers positions . The time

delay with this system did not exceed 30 seconds. In both systems

a flashing red light indicated changes in RVR.

3-45
At the present time, the U.K. is in a state of transition with

regard to the method of RVR assessment. Instrumented RVR systems

have replaced or are scheduled to replace visual RVR methods at Heathrow,

Gatwick, Glasgow, Liverpool, Belfast, and Edinburgh.

3.10.3 Advantages and Disadvantages of the Visual Method of Assessing RVR:

Among the advantages of the visual method, as stated by its proponents

are:

a. Automatic compensation for the change in illuminance

threshold produced by the change in background luminance,

particularly during twilight.

b. A baseline longer than that of a transmissometer

.

However, there are several serious disadvantages which the proponents

of the visual method may have overlooked. Among these are the following:

a. The absence of continuous observations in the

touchdown zone area. Hence, especially at night,

it is possible for an undetected fog patch to move

in over the approach and touchdown zone. The

increased activity at meteorological stations during

periods of low visibility, when RVR observations are most critical, precludes the use of trained personnel

for runway duty as observers unless the meteorological

staff is increased. The resulting use of non-meteorological

personnel as observers has a number of problems associated

with it. Delays are incurred in transporting people to the

- runway observing sites.

b. There are problems in making the observations

themselves. R\mway lights are difficult to count at

night because they appear to merge at distances more

than about 3000 feet. The intensity of the runway edge

lights in the direction of the observer is frequently

very different from that in the direction of the pilot,

particularly at the shorter viewing distances. Hence,

installation of special lights may be required.

c. The visual method tacitly assumes that the observer's

threshold is equal to the pilot's threshold.

( See Section ^4.6).

3-46
d. Usually no allowance is made for light losses in the

windscreen.

e. There are no checks on the reliability and accuracy

of the data supplied by an isolated unsupervised observer.

f. Experience has indicated that changes and trends in the

output of an instrument are detected earlier than from

visual observations

.

g. Cost of special observers and installations.

All of the factors listed above were considered by the United

States in making their decision to use an instrumental method. However,

despite these disadvantages, the use of visual observations is still

extensive.

3.10.4 Development of Instrumental Methods:-

Despite the extensive use of visual observations, studies of the use of instruments have been conducted in many European countries including

Great Britain. The feasibility of using television, automatic

light -coimting instruments and transmissometers has been investigated.

By 1962, the United States, Canada, and the Union of South Africa were using transmissometers operationally. Australia, Denmark, France,

Netherlands, Norway, and Switzerland were using them experimentally;

Belgium was experimenting with television; and Germany was testing the

feasibility of using a photoelectric light-counting device.

A detailed report of the status in 1962 of the application of the

runway visual range and the slant visual range concepts throughout the

world is given in the report of the 1963 MET/OPS meeting [70]

.

3.11 ICAO ACTIONS ON RVR

Note: Only those actions of the ICAO Confevenoe^ Divisions

and Panels which were oonsideved to have produaed significant forward steps in the development and application of

the runway visual range concept are included in this

Section of the report. General discussions and reports

of current practices3 details of reporting procedures^ etc.y

3-47
have not been included. In this Section the ICAO (English)

spelling of such terms such as meter^ center^ and color has been

retained in direct quotations in which these spellings

are used.

3.11.1 The Initial Phase:-

The Brabazon Report, issued in the United Kingdom in early 1951,

not only advocated operational use of the runway visual range concept

in the United Kingdom but also suggested that Contracting States to

ICAO should be "invited to take parallel action" . This was done and

the application of runway visual range measurements was considered

at the First Air Navigation Conference (1954).

The First Air Navigation Conference (1954) developed a statement

of operational requirements for more detailed information on meteorological

phenomena which included the concepts of runway visual range and slant

visual range. It was noted that some States were currently making and

reporting runway visual range operationally or on an experimental basis

but that there were no existing facilities or procedures for making slant

visual range measurements. The Conference recognized that proposals

for standardization were premature and recommended that States submit

information on methods for measuring runway visual range currently

being used or being investigated [60]

.

The Conference recommended continued investigation on methods

of measuring slant visual range. The Conference also recommended that

States which are not already doing so should provide, as soon as practicable, runway visual range observations and reports, at least

for instrument runways, at international aerodromes when low visibility

conditions exist and where justified by economic, meteorological,

operational and other factors.

3.11.2 Definition of RVR:-

The following definition of runway visual range was proposed by

the First Air Navigation Conference [60] for further consideration:

"Runway Visual Range - The maximum distance along a runway

3-48
or landing strip, measiired in the direction of landing or take-off from the end from which an aircraft will

approach or from which it will commence its take-off

rim, at which the runway, landing strip or the markers

or lights delineating it, are visible.

" Note 1 . The use of the word "end" in this definition is not

intended to limit the location of the point from which the

observations will be made to the physical end (or beginning)

of a runway, but observations may be made from the touchdown

point, the ILS reference point, or such other points as may be agreed as most suitable to provide the most

representative observations.

" Note 2 . Information given on any particular occasion

should be related to the objects which will be used by

pilots on that occasion visually to determine their position

relative to the runway."

The Second Air Navigation Conference (1955), in response to a recommendation of the International Federal of Airline Pilots

Associations, agreed that RVR reports should be transmitted to aircraft

en route when there were indications that the horizontal visibility

along the runway was of the order of 1500 meters or less.

This definition of runway visual range, with the exception of Note 2, was included in PANS-MET 1961.

At the 1964 Meteorology and Operations Division Meeting [69], the

definition of runway visual range was modified to take into account the

experience which had been gained in the reporting of RVB. as follows:

" Runway visual range - The maximum distance in the

direction of take-off or landing at which the runway or the specified lights or markers delineating it can be

seen from a specified point above its centre line from a height corresponding to the average eye-level of pilots

at touchdown.

Note . - A height of 5 metres is regarded as a satisfactory

approximation to the average eye-level of pilots at touchdown.

3-49
At its third meeting (1964-)^ the Visual Aids Panel [65] found that

when the fog densities corresponding to the Operational Categories (I, II,

Ilia, b and c) are defined by runway visual ranges, it is not possible

to specify light intensities and distributions according to category

since changing the intensity changes the visual range. A note was added to the definition of runway visual range stating "For the

purposes of the specifications in Annex 14 the specified lights are considered to be high intensity lights of the order of 10,000 candelas.

Markers are not taken into account".

Later, at the third meeting of the All Weather Operations Panel,

[57] a Note was added stating "In practice runway visual range cannot be

measured directly from the position specified in the definition but is an assessment of what a pilot would see from that position"

.

The definition of runway visual range was revised at the Eighth

Air Navigation Conference [62] to read:

"Runway visual range is the range over which the pilot

of an aircraft on the centre line of a runway can see

the runway surface markings or the lights delineating the

runway or identifying its centre line.

"Note 1. - The height of approximately 5 metres (16 feet)

is regarded as corresponding to the average eye level of

a pilot in an aircraft on the centre line of a runway.

"Note 2. - In practice, runway visual range cannot be

measured directly from the position specified in the definition

but is an assessment of what a pilot would see from that

position.

Note: Until the time of the 8th ANCj as the

definition of runway visual range was "refined"

,

it heoame a detailed description of the procedure

developed by the United Kingdom. Little or no attention was given to the operational application

of the RVR concept. The definition recommended by

the 8th ANC resolves the question as to whether

RVR related to the pilot or the observer.

3-50
3.11.3 Standardization of the Operational Application of the

RVR Concept:

At its first meeting (i960) the Visual Aids Panel [63] agreed

that an international standard on the assessment of runway visual range was required to ensure the comparability of RVR reports and that a statement of operational requirements was required to cover such factors

as the location and length of section of the runway over which RVR

should be measured, whether lights, objects, or both should be used as references, and the degree of misalignment from the runway centerline

that should be allowed, etc.

3.11.3.1 Location:-

Accordingly, "Runway Visual Range Measurement" was an item on

the agenda of the Second Meeting of the Visual Aids Panel [64] • The

panel recommended:

1. "That runway visual range (RVR) reports be made available for

operational use:

a) for landing purposes on all precision approach runways

intended for use in poor visibility;

b ) for take-off purposes on all runways having high intensity

edge lighting and/or runway centreline lighting, and

intended for use in poor visibility;

c) for such other runways as may be agreed locally."

2. "That, pending establishment of standard locations for RVR

observations. States should:

a) for landing purposes select a location adjacent to the

runway in the first 300 metres from the threshold and,

if practicable, arrange for supplementary observations from

other points so as to extend, to at least 1000 metres from

the threshold, the sector of the runway over which the RVR

can be assessed when in the lower operational ranges;

b) for take-off purposes, if practicable, establish observation sites different from those provided for landing

purposes, by selecting one or more locations which will

provide an indication of the RVR over the last 1000 metres

3-51
of the average take-off role of the more critical of the

civil transport aircraft regularly using the runway."

3. "That states working on RVR observation procedures he invited:

a) to modify their programs where practicable, with a view

to providing information on aspects of RVR observing and

reporting that need further study before the subject is

considered at the MET/OPS Meeting (1963) ."

Using the material obtained as a consequence of recommendation 3>

the 1964 Meteorology and Operational Divisional Meeting [69]

implemented the first recommendations with the following,

"Runway visual range observations shall be made at aerodromes,

for inclusion in reports issued in accordance with 2.2.1, 2.2.2

and 2.2.3, throughout any operationally significant period during

which the horizontal visibility is equal to or less than a value - not below 1,000 m - specified by regional air navigation agreement,

on runways intended for use dirring such periods of reduced visibility

and selected as follows:

a) precision approach runways;

b ) runways used for take-offs and having high intensity

edge lighting and/or centre line lighting;

NOTE : Local arrangements should be made to allow the runway

visual range reporting system to be brought into operation quickly,

especially when a rapid deterioration in visibility occurs or is ex- pected. "

(The NOTE is indicative of the difficulties encountered

in staffing an RVR observation post when RVR is based upon

reports of a human observer. In the U.S.j transmissometers

have been operated 24 hours a day^ 7 days a week from the

beginning of their use . )

In response to the recommendation of the Visual Aids Panel con- cerning location of the RVR observations, the MET/OPS meeting (1964)

"agreed that the requirement for landing was for a report

on the visual range from a location 300 m along the runway

from the threshold supplemented by Information up to 1,500 m

3-52
from the threshold. For take-off, there was a requirement

for additional information for the latter parts of the takeoff role. It appeared that these requirements would be

satisfied hy observations along the runway about 300 m from

the threshold together with observations at about the midpoint of the runway or at a location about 300 m from the

other threshold of the runway. However, it was decided that

it would be best to be specified only regarding the need

for observations at about 300 m from the threshold, leaving

the location of other observation points to be determined

locally in a way that would best suit local conditions, e.g.,

if there were a swamp near one part of the runway a special

observation point in this area might be found desirable. It was also considered necessary to provide pilots with an indication of the significant variations with time."

3.11.3.2 Observational Techniques

None of the ICAO Panels, Divisions, and Conferences has considered

it advisable to introduce complete standardization into the methods of

assessing runway visual range, considering that both visual observations and instrument measurements had been in use from the start of

operational application of the RVR concept. It was recognized that

either method, carefully employed, was adequate to meet the operational

needs. In the early 1950 's there was a strong bias for visual

observations but recently the trend has been toward the use of instruments.

3.11.3.3 Reporting of RVR

Reporting Increments

The 1964 MET/OPS Divisional Meeting recommended that observed values

of RVR up to 500 meters (1600 feet) be reported insofar as possible in

steps of not greater than 50 meters (160 feet), those from 500 to

1000 meters (1600 to 3200 feet) in ^5teps of not greater than 100 meters C320 feet)

and those above 1000 meters in steps not greater than 200 meters (700 feet).

However, later in the year, the All Weather Operations Panel at its

First Meeting (1964) [55] did not agree and recommended instead that

1) 3-53
RVR should be reported in increments of 30 meters without stating the

range of RVR measurements to which the recommendation was applicable.

At its Second Meeting (1967) [56] the Panel clarified the recommendation

stating that it applied to Categories II and III only and recommended

that RVR values be reported in increments as follows:

OPERATIONS REPORTING INCREMENTS

Category I Specifications recommended by the MET/OPS

Meeting (1964) given in preceding paragraph

satisfactory.

Category II 30 m (100 ft)

Category III 30 m (100 ft)

NOTE: Until improved techniques are available to allow this, reporting

increments of the order of 50 m (160 ft) or 60 m (200 ft) would be

acceptable for the RVR in CAT II operations. To achieve the

limit of CAT IIIB, additional refinement in RVR or visibility

measurements will be needed.

The Panel also considered the procedure for issuing Special Reports

and found that for Category II and III operations "the system of routine

and special reports was not suitable as a means of keeping the appropriate

ATS unit informed regarding the current RVR, and that the service should

be virtually continuous and subject to negligible delay - at least while

the aircraft was on final approach. The Panel also believed that it would

be most helpful to supplement the reports by very short period forecasts

if these could be given with sufficient accuracy."

The Panel accordingly recommended "that for aircraft for which the

applicable RVR minimum for landing on the runway to be used is 800 meters

(2600 ft) or less, the following procedure be used for keeping the appropriate ATS unit informed of the current RVR value and its expected changes

from the time final approach is commenced until the landing is completed:

a) the runway visual range should be reported to the

appropriate ATS unit within 15 seconds whenever

there is a change in the value to be reported in

3-54
accordance with the reporting scale in use^ provided

that the RVR before or after the change is 800 metres

(2600 ft) or less; the integrity of the communication

lirQc should be assTjred;

b) when practicable a forecast of RVR for the ensuing 10

minutes, expressed as a trend (e.g., decreasing rapidly),

should be added to each report and should be corrected

whenever appropriate .

This recommendation was further modified by the All Weather

Operations Panel at its Third Meeting (1967) [57] to authorize the use of 60 meter increments for observed runway visual ranges in the

upper region of Category II operations (500 meters to 800 meters),

and at the Eighth Air Navigation Conference provision was made for

reporting RVR in increments of 25 to 60 meters at the low end of the

scale and 100 meters was recommended for RVRs above 800 meters [62]

.

The Upper Limit of RVR

The First Air Navigation Conference (1953) was of the opinion

that reporting of runway visual range should commence when the horizontal

(meteorological) visibility fell, or was expected soon to fall, below

1000 meters or a higher figure as agreed locally [60] . This figure was formally recommended by the I\/[ET/OPS Divisonal Meeting in 1964 although

the International Federation of Airline Pilot Associations had recom- mended that observations start when the meteorological visibility fell

below 1500 meters.

The Fifth Air Navigation Conference (1967) [61] accepted the

1500 meter limit in order to provide more adequately for operations in

the lower visibility ranges of Category I. It was recognized that under

some conditions in which the visibility was 1500 meters, or somewhat

lower, the RVR would exceed the upper limit of the system in use, whether

instrumental or visual, recommending that when the visibility is below

1500 meters and the RVR is above 2000 meters, RVR may be reported as RVR above 2000 meters . The upper limit of 2000 meters was reduced

to 1500 meters by the Eighth Air Navigation Conference [62]

.

This latter change is in acoovd with U. S. Practice of reporting RVR values above 6000 feet

(1800 meters) as 6000+.

3-55
Rounding Down of RVR Values

The Fifth Air Navigation Conference also recoimnended that RYR

values should be rounded down to the nearest incremental value of the

reporting scale in use.

HJkis Teeommendation is in oonfliot with U. S. Praotioe

in that when tvansmittanee measurements are converted

to RVR values the value reported is the RVR corres- ponding to the transmittance at the midpoint of the

transmittance increment. Thus^, at nighty transmittances

,

over a 250-foot baseline^ in the range 0.030 (RVR of

500 feet) to 0. 104 (RVR of 700 feet) ai>e reported as an RVR of 600 feet.

Pilot Eye Height

Throughout the course of the development of the operational

application of the RVR concept in ICAO, a pilot eye height of 5 meters

has been assumed as being representative of the average eye height

at touchdown. This is in accord with the technique used in 1951 by

the United Kingdom and others using visual observations. In the

United States and in other States using the U. S. transmissometer, the

receiver and light source are installed at a height of approximately

15 feet thus representing the average height, at touchdown, of the line

of sight from an aircraft with a pilot eye height of 30 feet.

The advent of "jumbo" and supersonic aircraft with pilot eye heights

at touchdown of 30 feet, or more, has reopened the question of instrument mounting height. Sufficient data are not available to date to

resolve the question as to whether:

a) the present RVR values, related to an average eye level

of 5 meters, are sufficient; or_ b ) RVR values relating to a compromise pilot eye level for all

aircraft could or should be provided; or

c ) two values of RVR, one related to an eye level of 5 meters

and one to some higher eye level, say 15 meters, could or should be provided.

3-56
Choice of Light Targets

Under some visibility conditions the runway edge lights provide

better guidance than do the runway centerline lights. In very low

visibility conditions the centerline lights provide the primary

guidance. The ideal system should allow for usage of either. However

in the beginning, some States, including the United Kingdom, used an RVR system based upon centerline lights only, and others, including

the United States, based their RVR system on edge lights only. The

question of the proper selection was complicated by such factors as the

low intensity of many centerline lighting systems, the additive effects •

of the centerline lights under many conditions of view, and the narrow

horizontal beam spread of many edge lighting systems. Several solutions

were proposed over the years. Then, in 1974, the 8th Air Navigation

Conference [62] resolved the issue by accepting the recommendation of

the Fourth Meeting of the All Weather Operations Panel [58] and

prepared the following guidance material.

"GUIDANCE MATERIAL ON LIGHTS TO BE USED FOR ASSESSMENT OF RVR

"The following guidance material is necessarily general in nature,

since it is recognized that there can be a wide variance in individual

lighting configurations and light characteristics. The major concern is

that there be the greatest degree of standardization possible, so that

RVR readings will give uniform results worldwide and it is the function

of States to ensure that RVR values are as representative as possible of

actual visibility conditions:

a) For runways on which the centre line and edge lights conform

to the specifications for Precision Approach Runways, given in

Annex 14, Table V-1, RVR measurements should be based on the edge

lights, since these lights give a representative indication of the

visual guidance provided by the whole system.

b ) For runways on which the centre line and edge lights do not

conform to the specification s referred to in a) above, RVR

reports should be based on one or both sets of lights over certain ranges of values determined as follows, where Rp is the

3-57
RVR based on centre line lights and is that based on edge lights:

1 ) RVR should normally be determined as R for values of

R^ up to about 350 metres (1,150 feet).

NOTE: An upper limit for between 300 and 350 metres (1,000 and

1,150 feet) may be satisfactory where the guidance provided by

the edge lights is abnormally high compared with that provided

by the center line lights (e.g., because of unusual relative intensites or beam spreads). An upper limit above 350 metres (1,150

feet) may be satisfactory where the opposite is the case (e.g.,

where the edge lights have unusually wide lateral spacing or the

center line lights have unusually favourable characteristics)

.

2) RVR should be determined as R^ for values of R^ more than

about 250 metres (820 feet) above the upper limit for RC.

Note: The lower limit for R may need to be raised in a few

E

^ cases to avoid having in the transition zone, too rapid a change

in RVR with meteorological visibility or with the atmospheric

transmission factor for the transmissometer baseline.

3 ) The transition from the upper limit of R^^ to the lower limit

of should be approximately linear between the corresponding

points on graphs of R^ and R^ versus meteorological visibility

or atmospheric transmission factor for the transmissometer

baseline."

Sampling Period

The problem of the length of the sampling period ( the length of

time over which transmittance is averaged for the determination of an RVR value

)

did not arise until the trend to replace the visual observation with an instrumental measurement was established. At its Third Meeting, the All

Weather Operations Panel [57] considered the problem and noted that in order

to obtain a representative RVR reading based upon the transmittance over a transmissometer baseline, the sample had to be taken over a sufficient

period. However, in order to obtain an indication of sudden changes, the

sampling period should be kept as short as possible and in no case should

exceed 60 seconds.

3-58
Throughout the development of instrumental methods of assessing RVR,

there were many who believed that the requirement for a sampling period

of the order of 60 seconds was an indication of a deficiency in the instrument

and did not realize that the requirement was not a limitation of the instrument

but was based primarily upon meteorological considerations. Hence, a minimum length of sampling period was not stated until, in 1974, the

8th Air Navigation Conference recommended that the sampling period be not

less than 30 seconds.

3-59
standardization of Intensities and Thresholds Used in the

Assessment of RVR

a. Intensities

The All Weather Operations Panel at its Third Meeting (1967)

found that "since RVR is the distance at which a particular light

may he seen in given circumstances, the same opacity of the atmosphere

may represent different values of runway visual range due to different

light intensities. The Panel, however, recognized that there was some relationship between RVR and the probability of being contact at a given height. They therefore saw an advantage in standardized runway

and approach lighting intensities so that the same numerical values of

RVR would mean the same horizontal thickness of fog and in many cases

the same chances of approach success. Such standardization would also

improve the comparability of RVR observations made on different run- ways and at different aerodromes, - for example, by reducing variations

in the allowance to be made for exposure to approach lighting in determining

the value of pilot visual threshold to be used in computing RVR." The

Panel recommended that the Visual Aids Panel consider "the advantages,

in regard to both the observation and the interpretation of RVR, that

would result from standardization of the relative intensities of

runway and approach lighting .

These factors were considered by the Visual Aids Panel at its Fifth

Meeting (1970) [67]. The Panel noted that "RVR is a function not

only of the intensity of the lights used to determine RVR but also of

the setting of the intensity control of the system and of the visual

threshold assumed for the pilot (and for the observer when visual observations are used for the determination of RVR)." The Panel agreed that

RVR values should be based upon an intensity representative of the performance of a light in service and prepared the following statement:

"RVR intensity: The intensity to be used in the assessment

of RVR is the intensity of a typical new light averaged over the

beam spread specified in Table V-1 with all intensities greater than

three times the minimum intensity within this region considered

3-60
as being only three times the minimum. The RYR intensity is equal

to the average intensity so computed multiplied by an appropriate

reduction factor. The reduction factor should account for the

decrease in intensity caused by lamp blackening

and contamination of the optical surfaces. This factor is

dependent upon such details as the type of light, elevated

or inset; the location of the lights; and the established

maintenance programme. Factors ranging from 0.8 for runway

edge lights to 0.5 for runway center line lights were

suggested.

"The Panel considered specification of minimum and maximum

intensities for approach and runway lights undesirable since

uniformity within the beam of individual lights had been

specified; since compatibility between approach and runway

lights had been obtained by specifying the ratios of their

intensities; and since there can be no fixed relation between

fog density and RVR as presently defined if the intensity of the

lights is adjustable. However, a relation between RVR and the

probability of being contact at a given height had been obtained.

Further study of contamination reduction factors and of the

additive intensity effect of closely spaced runway centre line

lights was required."

Throughout the course of its development RVR has been considered

in two ways: a) As defined, it is the maximum distance at which a pilot

in a specified location would be expected to see the particular lights

used in a specified system; and b ) as a measure of the fog density,

similar to meteorological visibility, expressed in a distance approximating the visual range of approach and runway edge lights.

Both conaepts are somet-imes expressed in the same paragraph^

as in the AWOP recommendations quoted above.

If the first concept is accepted, the intensity used

in assessing RVR would he the intensity emitted by a typical

light in the direction of the pilot. Since the intensity

3-61
changes with the distance from which the tight can be viewed^

the intensity in the direction of the pilot is different from

each runway light. Hence, a different intensity would be

required for each value of RVR. The conversion charts used by

the U.K. to obtain RVR from visual observations followed this

'practice. 'Hhe Fifth Air Navigation Conference agreed with the

first concept and recommended that account be taken of the

Variation of intensity with angle of view and also of the intensity

setting in use for the lights.

If the second concept is accepted, , intensity representative of the intensity of approach and runway lights

over an angular region covering the expected approach path

of the aircraft would be used. This concept is most

effective when the representative intensities of approach

and runway lights do not differ significantly. This second

concept is the one used by the Visual Aids Ranel in preparing

its material and is in accord with U.S. practice.

Since the intensities of both the runway and approach lights are adjusted, often independently, to accommodate the systems to fog density

and background luminance, there will not be a fixed relation between

RATR and fog density, or approach success as desired by the All Weather

Operations Panel, unless intensity control as well as design intensity

is standardized.

b. Thresholds

When visual observations are used in assessing RVR the question of

illuminance threshold does not arise since it is assumed that the pilot's

and observer's thresholds are equal. However, knowledge of illuminance

threshold is required when RVR is assessed from instrumental measurements, as is evident from equation (2.15). In order to obtain a fixed

relation between RVR and horizontal fog density, the values of illuminance

threshold used in obtaining RVR from transmittance measurements must

be standardized.

3-62
This question was considered at the Third Meeting of the All

Weather Operations Panel [57] which recommended, on an interim basis,

the use of the illuminance thresholds used by the U.S., 1000 mile

candles (3.9 x 10 lux) by day and 2 mile candles (7.7 x 10 lux)

by night with the change between day and night occurring at a horizontal

background illuminance of 2 footcandles (22 lux). The Panel also believed

that 0.05 was a suitable value for the contrast threshold when markings

rather than lights were used for guidance.

However, at the time the recommendation for illuminance thresholds

was made, one State was using a four-step illuminance threshold system

and another was making measurements of background luminance at the time of

each transmittance measurement and using a continuous adjustment of illuminance

threshold. At its Fourth Meeting, the All Weather Operations Panel [58]

revised its opinion and prepared the following guidance material:

"The following constants are given for guidance for use when converting transmissometer indications into runway visual

range

:

Pilot contrast threshold - 0.05 ( dimensionless

)

Background

Illumination Threshold Luminance

(lux) (Mile Candles) (cd/m^)

Night

or

8 X

10"'^

,0-6.1

2 4-50

Intermediate Value 10-5 26 51-999

Normal day 10"'^ 260 1000-12000

Bright day 10-^ 2600 more than 12000

(e.g., sunlit fog)

"The above values are given in the interest of obtaining

standardization of RVR readings. The four illuminance thresholds

are equally spaced and are convenient in converting values of

transmission factor (transmittance) to RVR. However, other

intermediate and normal day threshold values may be used provided

3-63
they give more conservative (lower) values of RVR which are proven to be operationally acceptable.

"The above illumination threshold/background luminance

relationship is shown diagrairaiatically below^ in the form of a step function. When, in the computation of RVR, the illumination

threshold is adjusted continuously in accordance with the output

of a background luminance sensor values derived from the continuous

threshold/background luminance relation curve shown in the diagram

should be used.

"The number of illumination threshold values to be used

at any location will depend on the frequency of occurrence and

duration of various levels of background luminance. For example,

in some areas two values may be found adequate."

In addition States were invited to continue their studies of

illumination thresholds

.

The authors are convinced that the threshold values

proposed above are not appropriate. See Sections 4.4.8 and

7.4.6.

*Not included in this report.

3-64
^. THEORY OF VISUAL RAl^GE

4.1 VISUAL RANGE OF OBJECTS

An object will be seen against a sky or terrestrial background if

the apparent contrast (see Section 2.1.12) between the object and its background

is above some minimum value. This apparent contrast is reduced by the

scattering action of the air molecules and aerosols in the line of sight

between the observer and the object. The purpose of this section is to

study the effects of the atmosphere on the visual range of objects.

4.1.1 Historical

Although the development of the visual range of objects is usually

attributed to Koschmieder, Middleton [931 has shown that Bouguer in

1758 developed a so\ind theory of the reduction in contrast between a dark object and a sky background. The principles were rediscovered

by Lambert a few years later. Since there was no immediate application

for this knowledge^ the study of the visibility of objects was essentially dormant until the advent of aviation.

4.1.2 Objects with Sky Background

In 192/+, on the assumption of a uniform atmosphere having a scattering

coefficient 6, and illumination by the sun and a uniform sky (overcast or cloudless), Koschmieder [78, 79] showed that the light scattered into the

line of sight by the air molecules and aerosols is such that the apparent

luminance, L^, of a black object at a distance x, viewed against the horizon

sky is

^x ^ Vl - ^"^""^^ (^-01)

where is the luminance of the horizon sky in the direction of view,

and 3 is the scattering coefficient.

The apparent contrast C of the object and the horizon sky is then

4-1
Thus,

^-e-^"". (4.03)

If an observer recedes from a black object with the sky behind it,

the apparent contrast C decreases according to equation (4-03), until at some distance R it becomes numerically equal to e, the contrast threshold.

This distance R is the visual range, V. Therefore,

e = e"^^ . (4.04)

Koschmieder also showed that if the inherent luminance of the object

is not zero, (4-01) may be generalized to

= e"^^ + L (1 - e"^^) . (4.05)

h

This is the basic relation in the theory of the visual range of

objects and all later work evolves from it.

The first term of (4.05) represents the direct attenuation of the

intrinsic luminance of the object by scattering due to the fog droplets.

The second term represents the additional contribution to the apparent

luminance of the object due to "airlight" or external illumination from

all directions which is scattered into the observers eye by the fog

droplets

.

The foregoing equations contain no mention of absorption, which

in reality is sometimes as important as scattering. Absorption could

easily be accounted for by simply substituting a for 3 in the foregoing

equations, although it is not immediately obvious how such an extension

can be justified. Later work by Duntley [31] and others has justified

this change and (4.05) may be written as

L

' +^L^i"''' + K (1 - e""""") . (4.06) X o h

\-f /

Similarly, (4. 04) may be written

e = e-''^ (4.07)

and, since the transmissivity T is equal to e (see Section 2.2.7)

e = T^. (4.08)

4-2
Equations (4-. 07) and (4.08) are generally referred to as "Koschmieder 's Law"

.

Koschmieder assiamed a value of 0.02 for e. Later

work indicates that the contrast threshold is higher

than this, and the World Meteorological Organization

recommends a value of 0.05. Contrast thresholds are discussed in detail in Section 4.2.

Combining (4-02) and (4.06) yields

-ax

Since

C = (L - L )/L , (4.09a)

C = C e X o

•ox (4.09b)

or

C = C T

X o (4.09c)

If the object is just visible,

-aV

e = C e

o or

e = C T

o Solving for V,

(C > e),

(C > e),

^ o

^

V = (In - In e)/a ,

or V = -(In C - In e)/ln T,

(4.10a)

(4.10b)

(4.11a)

(4.11b)

Note that in equations (4.11)? and in subsequent equations using the

logarithms of contrast, apparent contrast, and contrast threshold, the

convention of considering these terms negative when the object is darker

than its background is abandoned and these terms are considered positive,

4-3
This is permissible as these terms are always of the same sign, for

atmospheric scattering never reverses contrast.

It is apparent from equations (4.11) that the visual range of an

object is determined by the inherent contrast of an object, C^, the

extinction coefficient, a, (or the transmissivity, T) and the contrast

threshold, e. Methods of measuring T and a will be discussed later

(Sections 5.1 and 5.2), as will the choice of an applicable value

of e (Section 4.2). There is, however, no feasible way of determining

C except by direct measurement of L and L, . Moreover, except for black

O oh ^ Jr- objects, is not constant but will vary with the extent of cloud cover and with the position of the sun with respect to the object.

It is for these reasons that black, or very dark, objects are chosen as marks for the Weather Observer. Under this condition the

value of is -1, or nearly so, the logarithm of the absolute value

of is very close to zero, and (4. 11) reduces to

V = (-In e)/a, (4.12a)

or

V = (In £;)/ln T . (4.12b)

It is obvious that (4.12a and b) may also be obtained directly

from (4.07) and (4.O8)-

Note that the visual range V of a black object is, as indicated

by (4.12), independent of luminance of the background sky and the

direction of view with respect to the sun. Although variation in contrast is not a significant factor in

determining the accuracy of routine meteorological observations, the

effect of contrast on visual range is of interest because the visual

range of mariy objects which are not black, for example, tall buildings

and lighthouses, is of interest. The effect of contrast on visual

range is illustrated in figure 4.1. The visibility factor, K, is the

ratio of the visual range of a large object of inherent contrast

to the visual range of a large black object. The equation for the

visibility factor is obtained as follows. Let V be the visual range

of an object of contrast C^. Then from (4.11a),

V = (In - Ln e) /o (4.13)

4-4
4-5
Dividing (4.13) by (4.12a) yields

K = V'/V = 1 - In cyin e. (4.14)

For reasons which will be apparent later (Section 4.2), a value

of 0.05 was assumed for e.

Values of range from -1 (+1 on figure 4.1 for reasons stated above),

through zero for a grey object so lighted by daylight that it blends

with the sky, to as high as 5 for a white object in direct sunlight.

A extensive discussion of the luminances found in nature has been

prepared by Gordon [39]

.

4.1.3 Objects with Terrestrial Background

The apparent contrast, and hence the visual range, of an object viewed,

horizontally, against a background other than the sky may be computed by

applying (4.05). This has been done by Duntley [31] in a generalized

treatment of slant visibility. For objects viewed against an immediate

terrestrial background, such as painted stripes on a runway, the following

relation applies

:

e = C^[l + (L^/I^)(e^^' - l)]-\ (4.15)

In the foregoing relation, is the inherent contrast between the mark

and its background, V is the visual range of the object, is the

inherent luminance of the background of the object, and is the luminance

of the horizon sky in the direction making the same angle at the object

with the line of sight as it does with a line from the object to the sun. In

this situation the apparent contrast, and therefore the distance a mark can

be seen, is a function of the direction of view, the luminance of a particular section of the sky, and the inherent luminance of the background

as well as the contrast between the mark and its background. When the line

of sight is horizontal, is the luminance of the portion of the sky directly

behind the mark and its background. In the case of a landing aircraft, the

line of sight is within a few degrees of the horizontal projection of the

line of sight.

The visibility factor, K, obtained by combining (4.15) and (4.12a)

is given by

4-6
K = ln[l - (L^Ah^^^ " 1/e). (4.16)

Note that K is independent of the visibility or transmittance of the

atmosphere

.

The results of computations of the visibility factor by means of

this equation are shown in figure 4.2. A contrast threshold value of

0.05 was used in making the computations.

In considering the curves, the following information should be

helpful. The ratio L^/L^ varies from about 5 or more on a sunny day

with a snow backgroimd down to less than 0.01 in directions near a low sun shining through a haze with a grass background. In the case of a grass background on an overcast day, the ratio will be about 0.2 to

0.3. On hazy days, with the sun visible through the haze, the ratio

may vary over a range of more than 10 to 1 around the horizon.

The contrast between a runway and its marks or its background is

usually in the range 0.5 to 2.0. Contrasts between natural objects and

their backgrounds may be as low as 0.2 [20, 39] . The conditions at the

upper righthand part of the figure are not common and usually require,

in order to simultaneously produce both a high contrast and a high

value for the ratio L^/L^, that sunlight be specularly reflected from

the object.

In general, therefore, the visibility factor will be less than

one, ranging from 0.3 to 0.6 under overcast daylight conditions to

less than 0.1 with a low sun shining through haze.

The development of the theory of the visibility . of objects given

above has been simplified in the interest of brevity and clarity. A

uniform atmosphere with a constant extinction coefficient, or transmissivity, has been assumed. The restrictions stated by those who developed

the theory have not been listed. However, field experience has indicated

that the equations developed above are sufficiently general to represent

the visual range of objects in practical applications.

For more complete treatments see Middleton [95] and Duntley [32, 33]

and the references listed therein.

I

4-7
Figure 4.2 Effects of ground/horizon sky luminance ratio and contrast on visibility

factor for objects with a terrestrial background, based upon a contrast

threshold of 0.05.

4-8
4.2 CONTRAST THRESHOLDS

Light falling on the eye acts as a stimulus. If the stimulus is

intense enough, an individual will experience a sensation; e.g., a sensation of brightness. Brightness, as a psychological concept, cannot

be measured in the physical sense. ^ However, it is valid to make

judgements as to the equality or inequality of two or more sensations.

The least stimulus that will produce a sensation is known as the absolute

threshold for that stimulus. Similarly, the smallest difference between

two stimuli which will make their corresponding sensations distinguishable

is known as a difference lumen. If two adjacent objects are just

distinguishable, and the luminance of one is L.^^ and the other is L^, then

(L^ - L^)/!^ = AL/L^ = e. (4.17)

which defines e, the contrast threshold.

4.2.1 Daytime Thresholds in Practice

Koschmieder used the value of 0.02 for the threshold of contrast

based upon the work of Helmholtz in photometry. There is no indication

that this choice was based on any field experiment. The use of this

particular value has continued for some uses and is regarded by some

as the standard value for meteorological uses. Its usage gives the

equation 4.12a the form

MR = 3.912/0. (4.18)

Also from (4.08),

T^ = c-^^ - 0.02.

The quantity MR is defined as the meteorological range, an unfortunate

choice of words as will be seen later. The natural logarithm of e is

traditionally given to four significant figures, implying that the contrast

threshold, e, is known to an accuracy of one part in ten thousand. A

value of 4 would be more appropriate

.

Among the more recent laboratory studies to determine contrast

threshold, those by Blackwell [9] , and Lamar et al [82] are noteworthy. These researchers determined the luminance contrast under laboratory

^Brightness is not the same as luminance. Luminance :s a psychophysical

concept and can indeed be physically quantified (measured).

4-9
conditions while varying size and shape of stimuli, with stimuli both

lighter and darker than the background, with varying exposures.

The thresholds obtained by Blackwell and others in their laboratory

experiments were lower than those obtained in field experiments . The

field experiments of Houghton [49] and Douglas and Young [30]

suggested a threshold of about 0.05 and led Blackwell to undertake an

extensive outdoor experiment to validate the application of laboratory

results fo field situations . These tests were conducted in the forest

country of northern Michigan and are generally known as the Roscommon

tests [10] . Blackwell checked parts of his earlier results by direct

observation in the field, using distances up to 30 miles, with some of

the observations with binoculars, some with the naked eye, and by night

as well as in the daytime. These observations showed that the results

of laboratory experiments could be applied to field observations, with

a tolerance of ±25 percent, if the conditions of observation were the

same, i.e., "forced-choice" response by an observer with knowledge of

the exact location of the target.

These conditions are, however, not those of the meteorologicalobserver. Although he knows the general location of the visibility

marks, he needs to see them sufficiently well so that he knows that he is

observing the desired mark. Hence he uses a recognition, not a forcedchoice detection, threshold. Therefore the contrast-threshold chosen

for the meteorological observer, and later for the pilot was based upon

field experience such as that of Houghton [<49], Douglas and Young [30],

and MDB [126], who used criteria of recognition, not detection. Later,

Middleton [95] used routine meteorological observations of "the visibility"

to determine a relevant contrast threshold.

In view of these studies, the World Meteorological Organization (WMO)

recommends that the value of 0.05 be used for the contrast-threshold in

computing visual range from measurements of fog density. Thus (4-. 12a) may

be written

MOR = 3/a. (4.19)

The quantity MOR is defined as the meteorological optical range.

4-10
A value of 0.055 is used for transmissometer-based computations

of visual range in the U.S., based upon work described in Sections 7.2 and

7.3.

As shown by (4.12) the visual range is a function of the logarithm

of the contrast threshold and thus is not very sensitive to a change in

the value of e. For example, the ratio of the meteorological range (MR)

to the meteorological optical range (MOR) is 4/3 although the ratio of the

two thresholds is 2.5. The effects of changes in contrast threshold on

the visibility factor are shown in Figure 4.3. The general relation is

V^/V2 = In e-j_/ln (4-20)

where Y-^ is the visual range computed from (4.12) using e-^ as a threshold

and is the visual range obtained using e^.

It should be noted that the above discussion tacitly assumes that

the contrast-threshold, e, is a constant when, in fact, it is a function

of the angular size of the object and upon the background luminance,

increasing as these parameters decrease. However, under field conditions

with objects subtending 0.1 degree or more and under normal daylight

conditions, the recognition contrast-threhold is sufficiently stable

that assuming it to be a constant does not Introduce significant errors.

4.2.2 Contrast Threshold Criteria

The difference between the contrast threshold value of 0.055 used in the

transmissometer calibration and the value of 0.02 which had been

traditionally used to relate meteorological observations of

visibility to transmittance is due to several factors Including threshold

criteria. Instructions used by weather observers stated that "visibility

in a definite direction is the greatest horizontal distance at which

outlines of visibility markers can be distinguished against the horizon

sky" [128] . This criterion was used in the initial transmissometer

calibration^. The distance at which the most distant mark appeared as

a square was called the visual range. Beyond this mark one, and sometimes

^These field experiments were conducted on Nantucket Island, Mass., and

are covered extensively in Chapter 7.

4-11
4-12
two, additional marks appeared as shapeless smudges but could not be seen well enough to be identified. It is unlikely that these marks

could have been located had it not been for the guidance furnished by

the visible marks. Usually, contrast threshold values lower than 0.055

have been based upon experiments where the criterion of visibility was any detection of a mark whose location is known.

The recognition threshold derived for objects by day during the

Nantucket field tests was determined using observers knowing the approximate location of the marks. The search time was nearly imperceptible

when the marks, both objects and lights, were at the recognition

threshold. Once the mark was initially located, there was no doubt

on the observer's part as to its location.

During the 1940 's, a qualitative study of the effect of the

differences in criteria of visibility was made during the course of

observations in fog at the Landing Aids Experiment Station by evaluating

the distances at which an observer, knowing the location of a mark,

could just detect it and the distance at which the shape of the mark

could be distinguished (recognized). If e-^^ and x-^^ are the contrast

threshold and distance values for the detection case and and x^

the corresponding values for the recognition case, then

x-j^/x^ = In e-|^/ln

which shows that the ratio of the distance is independent of the

prevailing transmittance. Since the line of sight is nearly the same

in both cases, the effect of any non-uniformity in the fog is considerably

reduced. Therefore, the number of observations needed to determine

this ratio is considerably less than the number required in determining

the actual contrast threshold. The ratio, based upon 23 observations

made in fogs varying in visual range from 0.06 to 1 mile, was found to

be 1.09 with a coefficient of variance of 4 percent. On the basis of

4-13
0.055 as the recognition threshold and 1.09 as the ratio of the two

distances, the detection threshold computed from (4-20) is 0.041.

Another factor which may account for some of the difference between

laboratory and field values of contrast threshold is the difference in

uniformity of background luminance. Byram [19] suggested that the

detection threshold of 0.02 should be increased to a value on the order

of 0.03 to 0.04 due to local variations in retinal adaptation resulting

from the large point-to-point variations in background luminance of

outdoor scenes.

Another factor to be considered is the size of the marks. The

contrast threshold 0.02 was based upon objects subtending one degree or more. The marks used in calibrating the transmissometer were somewhat

smaller than this. They were, however, about the same angular size as marks often used by meteorological observers in determining visibility,

since marks even as large as 0.5 degree frequently are not available.

For instance, a one degree target 5 miles away would have to be 460 feet

square while a typical two story house would provide a one degree mark at

a distance of less than one-half mile. Moreover, the density of fog

and haze frequently varies so much from point to point that, with a mark as large as one degree, parts of it may be below the limit of

visibility while other parts are distinctly visible.

Using Blackwell's data [9], the contrast threshold, using 50^

probability of detection, is approximately 0.01 for a mark of 30 minutes

diameter and 0.02 for a mark of 10 minutes diameter. If a correction

factor of 2 is used to obtain thresholds more nearly corresponding to

the usual criteria of detection, threshold values of 0.02 and 0.04 are obtained for marks 30 and 10 minutes in diameter.

Contrast threshold values obtained during the period of the transmissometer

calibration varied but are all greater than 0.02. A computation of e based

on Hulburt's data [50], in which the visual range was described as the

distance at which an object could just not be seen^ yields a value of

0.027. Houghton [48] reported a yield of 0.065 for clouds and about

half that for fog. Duntley [31] reported that a comparison of "visibility", as determined by the staff meteorologist at the Tiffany Foundation,

4-14
and measurements of beam transmittance was in agreement with what is now

the transmissometer calibration. Muench, et al [100], in their calibration

of a forward scatter visibility meter, report a correlation between routine

weather observations and the extinction coefficient which are in agreement

with the transmissometer calibration.

From the foregoing, it appears that the use of a recognition

contrast threshold value of about 0.05 when measurements of transmittance are used for determining the visual range equivalent of that

reported by meteorologists and vice-versa and the use of a value of e in the region 0.035 to 0.04- for a detection contrast threshold under

field conditions will give reliable results

.

Recognition contrast threshold values, such as those used in the

transmissometer calibration are applicable to situations in which the

observer is deliberately searching for a particular mark of known

approximate location with adequate search time. When the attention of

the observer must be attracted, when the approximate location of the

mark is not known, or when the search time is limited, the values of

contrast threshold are higher. When the location of the object is known

precisely and recognition is not a criterion, the values of contrast

threshold are lower.

4.3 VISUAL RANGE OF LIGHTS

4.3.1 Historical

Work of Boufiuer and A I lard. The following equation, which gives

the illumination from a point source of light at any distance in a partly

transparent medium, is known as Allard's Law:

E = I e~^Vx^ , (4.21)

where E is the illumination at distance x from a source of luminous

intensity I in a medium of extinction coefficient a.

If the illuminance threshold E^ is substituted for E in equation 4.21,

the corresponding value of x will be the visual range V, as follows:

E^ = I e"^W (4.22)

which can then be solved for V, yielding

4-15
V = [ln(l/E^) - 2 In V]/a . (4.23)

Although the foregoing law was indeed published by Allard in 1876

[3], Middleton [93] > in his role as a historian of science, has

found that Bouguer had stated this law much earlier, in 1729.

Work of French Li£[hthouse Service. The French Lighthouse Service

developed an early interest in the visual range of lights —an interest

which began in the mid-seventeenth century. An 1864 work by Reynaud [111]

documents this work^, much of it under the leadership of Allard and Fresnel.

The French conducted experiments in the mid 1800 's to determine appropriate values of the transmissivity, T, under a number of different

atmospheric conditions. Reynaud 's remarks indicate that the work of

Bouguer provided the basis for the French Lighthouse Service experiments.

Bouguer had assigned to T a value of 0.973/kilometer for a "clear calm

atmosphere"

.

In 1876, Allard published his famous Memoire reviewing his work on

the visual range of lights. Reynaud 's manual does not specifically

credit Allard with the law. The French Lighthouse Service, when questioned

as to the originator of the law, responded with a photocopy of an unsigned

memorandum which could be attributed "only to Allard". See Appendix B.

Allard expressed equation (4.22) in terms of transmissivity, thus

= I T^/V^ . (4.24)

This is the most common expression of Allard 's law.

4.3.2 Analysis of Allard's Law:-

Using this terminology (4.23) may be written as

V = [ln( I/E^ ) - 2 In V] /In T . ( 4 • 25 )

Equations (4.23) and (4.25) can not be solved directly for the visual

range V. Graphical and iterative approximations must be used. See

Section 4.4.6.

^This work was of sufficient importance that it was translated into

English, in 1876, by Peter C. Haines.

4-16
The equations given above are strictly applicable only when the

luminance of the background is small compared to the average luminance

of the light. Otherwise Equation (4-22) becomes:

= [I - (L - L' ) A] e'^^^/V^ (4.26)

where L is the luminance of the background of the light, L' is the

average luminance of the unlighted projector, both in candelas per unit

area, and A is the area of the entire projector projected on a plane

normal to the line of sight.

Both L and L' are measured in the direction of the line of sight

from a position near the light.

Since the intensity of a light source is the product of its area and its average luminance, equation (4-26) may be written

E^ = [L^ - (L - L' )] A e"^^/V^ (4.27)

where is the average l\miinance of the light source.

The quantity (L - L' )A is the intensity required of the light to

make its average luminance equal to that of the background. The visual

range of the light is determined by the net intensity, that is, the

difference between the measured intensity of the light and this intensity.

Typically the term (L - L' )A has a significant effect on the visual range

of a signal light only under daylight conditions when the light is dimmed

or when the light has a low average luminance in the direction of view.

For example, an approach light has a luminance, L^, in excess of

100,000 candelas per square foot when operated at full intensity without

filters. The inherent luminance of the background, L, will exceed

1000 candelas per square foot only on very rare occasions. Hence, L

is less than 1% of L^. On the other hand, when these lights are dimmed

to Step 1, 0.2^ of full intensity, the luminance of the light is reduced

to 200 candelas per square foot, and the effect of background luminance

in (4.27) may be very significant. Filters will reduce the luminance of

the light to about 15% of the unfiltered intensity for red and green,

and to about 2% for blue. Omnidirectional lights, such as taxiway lights

and obstruction lights have a luminance, when lighted, of less than

500 candelas per square foot and the effect of background luminance in

4-17
(4.27) is highly significant.

4.4 ILLUMINANCE THRESHOLD

The value of Illuminance threshold, E_j_, to be applied in Allard's

law, (4.22 and 4.24) in transmissivity-visual range computations has been

of concern since the days of Allard and, in the early days of night flying,

Langmuir and Westendorp made a comprehensive study of illuminance thresholds

applicable to aviation [87]

.

The illuminance threshold is not a constant. It is a function of the

luminance of the background of the light, the position of the light in the

field of view, the angular size and shape of the light, its color and, if

not steady burning, its flash characteristics. In addition, the observer's

knowledge of the position of the light and his time for search have a significant influence on the threshold.

4.4.1 Illuminance Threshold for "White" Point Sources

Figure 4.4 shows the relation between illuminance threshold and

background luminance for steady burning, white, point sources for about

98 per cent probability of detection. The illuminance threshold values

shown are applicable only when the observer knows precisely where to

look for the light. Even if the illuminance is twice the values shown

the light will be hard to find. The illuminance values must be increased

by a factor of 5 to 10 if the light is to be easy to find [118]

.

These increases in threshold illuminance are applicable only when the

observer is looking for the light signal. Much greater increases are needed

if the light signal is to attract the attention of an observer who is not

searching for it. Factors of 100 and 1000 are not excessive [51]

.

The break in the curve (the knee of the curve) in figure 4.4 represents the change from cone to rod vision. At low background luminances,

the illuminance threshold for cone vision remains essentially unchanged

as indicated by the dashed line. The dashed horizontal portion of the curve represents most night seeing conditions since a light used as a signal

is usually observed by looking directly at it; hence cone, not rod

vision, is used. Moreover, it is doubtful whether those engaged in

transport, even lookouts on ships, even reach the state of dark adaptation

4-18
1 1 1 1

1

1 1 1 1 i\ 1

CO

C/1

<

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C9u

<

(9

o

&-3 CU o

CO +->

+-> 4-

c: O •»~o

o

^ +-> o

+-> c

4- S

fO u CO

fO

+-> c

E o

o "r— i- +-> -c: O

o O)

fO +-> CU a

M- E o

o

S- >>

M— -M

•1— <U 1

•r— CU J3

fO

O) JDO +-> S-Q.

+-> n3 +-> C

O) CU a o

(T3 i-cuQ.

E

00

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•TD

+-> e

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:3E rO

•1

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I SaiQNVO BUM 'aiOHS3IIHl 33NVNIIMni1l 901

4-19
required for rod vision [11]

.

Representative background luminances are given in table 4.1. It

should be noted that the luminance of the night sky in the vicinity of

cities and airports seldom falls below 0.001 footlambert because of the

effects of man-made sources [114] • Note also that^ unless there are glare sources in the field of view, it is probably necessary to consider

only the background in the immediate vicinity of the light [77]

.

TABLE 4.1

Liuninances of Backgrounds Against

Which Light Signals are Viev/ed

Background

Background Luminance

( footlamberts

)

Candelas per

square meter

Horizon sky

Overcast, no moon

Clear, no moon

Overcast, moon

Clear, moonlight

Deep twilight

Twilight

Very dark day

Overcast day

Clear day

Clouds, sun-lighted

0.00001

.0001

.001

.01

.1

1

10

100

1000

10000

0.000034

.00034

.0034

.034

.34

3.4

34

340

3400

34000

Daylight fog

Dull

Typical

Bright

100-300

300-1000

1000-5000

300-1000

1000-3000

3000-16000

Ground

On sunny day

On overcast day

Snow, full sunlight

100

10-30

5000

300

30-100

17,000

4.4.2 Effects of Source Size

As stated earlier, the values of illuminance threshold shown in

figure 4.4 are applicable only to lights which are, in effect, point

sources. A light source behaves as a point source if the angle it subtends

at the eye is below a certain size called the critical angle. For sources

4-20
subtending angles below the critical angle, the eye responds to the

intensity of the source, that is to the product of the luminance of

the source and the solid angle subtended by the source. This relation

was reported by Ricco in 1877 and is sometimes called Ricco's Law.

Critical visual angle is plotted as a function of background luminance

in figure 4.5 and shows the critical angle below which Ricco's Law is

valid. This critical angle increases as background luminance decreases.

When the critical visual angle is plotted as a function of backgro\md luminance, as in figure 4-5, it provides a practical definition

of a point source —a stimulus which affects the eye only in proportion

to its intensity.

Most aviation signal lights behave as point sources at distances

near the visual range of the lights, as is evident from the following

example. From figure 4.5 it is seen that at a typical nighttime background luminance of 0.01 footlambert, the critical angle is of the order

of 2.5 minutes. The diameter of U.S. approach lights is 7 inches.

A source of this diameter subtends an angle of 2.5 minutes or less for

all distances greater than 700 ft.

When a light source subtends an angle larger than about 1° its

visibility is determined not by its intensity, but its luminance

(intensity per unit area).. Thus, increasing the size of a large source

at threshold visibility, without increasing the luminance, increases

the intensity, and the corresponding illuminance at the observer's eye,

but does not increase the visual range. However, if the size of a point

source at threshold is increased, maintaining the luminance constant, and

if, after the increase in size, it is still a point source, the visual

range is increased.

There is a transitional range of angular sizes in which the light

can be considered neither as a point source or as a large source. This

region has been studied by deBoer [25] . Correction factors called

"size factors" have been computed from his study. Approximate thresholds

for sources which are too large to be considered point sources may be

obtained by multiplying the thresholds obtained from figure 4.4 by these

size factors which ^are given in table 4-2.

4-21
4-22
TABLE 4.2

Size Factors for Obtaining Thresholds

for Sources Other Than Point Sources

Ratio of Source Diameter

to Viewing Distance

SIZE FACTOR

Night Day

0.0005

0.001

0.003

0.005

0.01

1.0

1.0

1.1

1.4

2.5

1.0

1.2

2.5

4.9

20.0

Figure 4-4 and table 4.2 apply only to threshold and near threshold

viewing, with the observer looking directly at the light. In an investigation of optimum intensity of road traffic signal light intensity, Cole

and Brown [21] found that the intensity of a red traffic signal light

required to produce optimum recognition under bright daylight conditions

and peripheral viewing is independent of source size for sources subtending

up to 16.5 minutes of arc. These findings extend the effects of Ricco's

Law over a much greater field of view.

4.4.3. Illuminance Threshold for 'Composite' Light Sources

In aviation lighting the approach light units are barettes of several

sealed-reflector lights spaced a few feet apart. Likewise the VASI light

box is comprised of 3 separate but closely spaced lights.

It is well known that, when observed at sufficiently great distances,

a light unit of this type may be considered as a point source with the

intensity being the sum of the intensities in the direction of view or all of the lamps of the light. When a light of this type is observed

at sufficiently short distances, the effective intensity of the unit will

be approximately that of a single lamp of the unit. Either of these

extreme distances may be outside the range for which the approach-light

system is most useful.

The Importance of considering this factor has been studied in the

laboratory for deBoer [25] and observed experimentally at Areata [101]

.

DeBoer developed a "row" factor, which is a measure of the "mutual

assistance" of the lights in the row, to apply to composite sources in

which the distance between lamps is so great that the light cannot be

4-23
considered as a simple rectangular unit of uniform brightness. If the

composite unit is considered as a group of individual sources, each

assisted by the adjacent lamps, the intensity of a single lamp plus the

"assistance" of the adjacent lamp may be computed by means of deBoer's

"row" factor.

The Areata testa verified the general principles of deBoer's "row"

factor. However, no application has been made of this concept and visual

range computations are usually based upon the Intensity of a single

lamp. Further study is needed.

4.4.4. Illumindnce Thresholds for Flashing Lights

Blondel and Rey [12] found that the illuminance threshold for

a square-wave flash (a flash producing a relatively constant illuminance

throughout its duration) is given by

E = E^(a + t)/t ,

where is the illuminance threshold for a steady light, t is the flash

duration and 'a' is a constant. The value used for 'a' is usually 0.2 for

lights viewed at threshold at night.

However, in the computation of the visual range of flashing lights,

it is more convenient to use the illuminance threshold for a steadyburning light and to apply the concept of effective intensity in determining the intensity to be used in Allard's law (4-22) and (4-24).

• The effective intensity, 1^, is defined as

-: 1=1 E,/E , e t or

Ig = 1 t/(a + t) , . (4.28a)

where 1^ is the effective intensity, and I is the instantaneous intensity

producing the illuminance E.

The flash from most lights used in aviation service, such as airway

beacons and anti-collision lights, is not abrupt. The instantaneous

intensity often rises and falls gradually and may vary appreciably during

the flash.

4-24
In a subsequent paper, Blondel and Rey [13] proposed the following

modification of equation (4.28a).

(4.28b)

for flashes which were not abrupt. The limits t-j^ and t^ are the times

at the beginning and end of flash respectively. This proposal was based

on intuitive grounds

.

There has been little experimental verification of equation (4- 28b)

However, in a field test conducted in fog at Areata by the Visual Landing

Aids Laboratory of the National Bureau of Standards [102], the following values

of 'a' were obtained:

TABLE 4.3

Values of the "Blondel-Rey Constant" Obtained at Areata

DAY NIGHT

Flash # of # of

Duration 'a' Observations 'a' Observations

20 microsecond 0.24 (317) 0.44 (258)

200 microsecond 0.13 (407) 0.33 (253)

0.3 second 0.23 (175) 0.40 (152)

0.5 second 0.18 (50) 0.39 (92)

Weighted Average* 0.19 (949) 0.39 (755)

^Weighted in accordance with the number of effective intensity observations used in the determination of the value of 'a'.

The general agreement of the values of 'a' obtained for different

flash lengths indicates the usefulness of equation (4.28b).

The effective intensity of most flashing lights is now in terms

of equation (4.28b) using 0.2 as the value of 'a'.

For a more extended discussion of effective intensity see Douglas [27]

and Projector [109]

.

4-25
4.4.5 Illuminance Thresholds for Colored Light Sources

Illuminance thresholds for colored lights fall into two categories

:

a ) the achromatic illuminance threshold which is based upon the criterion

that the light is visible but that the color need not be recognized and

b) the chromatic illuminance threshold which is based upon the criterion

that the color must be recognized, resulting in somewhat higher illuminance

thresholds. As visual range in practice is based upon achromatic

thresholds, only they will be discussed here.

There are many laboratory studies of color recognition but only a few use sources of an angular size sufficiently small to simulate

signal lights. Among these are those of Hill [431 ^ Middleton and

Gottfried [97], Middleton and Wyszecki [96], and Jainski and

Schmidt-Glaus en [741 . The results of these studies are in reasonably

good agreement. For sources which may be considered as point (or near- point) sources, the order of increasing illuminance threshold is red,

yellow, white, green, and blue.

Typically, the illuminance threshold for red is about half of that

of white, and the illuminance threshold for blue is about twice that of

white. The differences between yellow, green, and white thresholds are small

.

The fact that a red light is more visible than its "photometric"

intensity indicates it would be, has been independently discovered mariy

times; for example, by the French Lighthouse Service in 186-4 [111]^

deBoer [25], Middleton and Gottfried [97], and Projector [110].

4.4.6 Curves Relating Atmospheric Transmissivi ty, Intensity,

and Illuminance Threshold

As stated earlier, Allard's Law, (4.22) and (4.24), can not be

solved directly for the visual range, V. Hence, it is not possible

to compute simple equations giving visibility factors showing the

effects of intensity, illuminance threshold, or transmissivity upon

visual range. In this section, several graphs are presented illustrating the effects of these parameters and making possible graphical

solutions of Allard's Law for V.

4-26
It will be apparent from these curves that in fog the law of

diminishing returns takes effect at relatively short distances. For

example, if the transmissivity is 0.01 per mile (light fog), a light

with an intensity of 100 candelas will produce an illuminance of one mile candle at a distance of one mile; an intensity of 40,000 candelas

is required to produce that illumination at 2 miles; and 9,000,000 candelas

is required at 3 miles.

In figure 4-6 the intensity required to produce an illuminance

of one lumen per unit area has been plotted as a function of distance

for several values of atmospheric transmissivity. The curves of this

figure demonstrate the importance of the fact that the intensity is

attenuated exponentially with distance and can be used to graphically

solve the various forms of Allard's law. The curves can be used with

any consistent set of units; for instance, I in candelas, D in miles,

E in mile candles (lumens per square mile), and T per mile.

The figure can be used for thresholds other than one lumen per unit

area by simply dividing the ordinate scale by the illuminance threshold.

For example, if a daytime threshold of 1000 mile candles is assumed,

D would be stated in miles and each intensity shown on the ordinate

6 3 scale would be divided by 1000. Thus 10 candelas would become 10

candelas

.

The curve T = 0.70 (per mile) is representative of average

visibility conditions, while that for T = 0.90 (per mile) represents

an unusually clear condition. Note that at a distance of 10 miles,

the intensity required to produce unit illimination for average conditions (T = 0.70), is approximately 12 times that required for conditions

when T = 0.90, assuming an illuminance threshold on the order of one mile candle. At a distance of 20 miles, it is approximately 150 times

greater. It is also evident from a comparison of the curves T = 0.90

and T = 1.00 that even for a very clear atmosphere the transmissivity

caimot be disregarded when the distance to the source is large.

A series of nomographs which permit a precise solution of Allard's

Law for V, or any of the other parameters, and instructions for their

use is given in Appendix C.

4-27
<4-O •4-> C

c (U o +J

4-> U (/) c

o

t- o

10 >>

I/) cC

03

0)O (/) c (U

res -(J i >

1/1 i

I/) e ta

S-

+-> O)o o

13 -a s- o O)

S- ^

Q-

(/) O O +J E+-> T3 fO <U

S- T3

(O <U

=s s- (/> O" <U r3 (U >

s- cu <u

(/) J3

s- >) o 03 in M- E c

cu Ol 10

-•-> O +->

fO +J

Ol (/) x:1— -o O

svnaoNVD 'aiisnbini

4-28
4.4.7 Illuminance Threshold Criteria

The illuminance threshold is a function of both background luminance

(see Section 4.4-.1) and the type of vision used by the observer. In the

Nantucket observations, foveal vision was used to determine whether the

light was seen steadily or not, although parafoveal vision may have

been used to locate the light. Background luminance was usually on the

-3 -2

order of 10 to 10 footlambert. In this region, there is little

change in foveal sensitivity with the background luminance. These

conditions are believed typical of those for most meteorological observations. The combined effect of lights from surrounding areas and from

-3

an airport itself make background luminances of less than 10 footlambert

imlikely.

The effects of observers' criteria (i.e., recognition versus detection) upon the determination of contrast threshold (objects by day)

were discussed in Section 4.2.2. There are similar effects, according to

observers' criteria, when determining the illuminance threshold (lights

by night).

The effect of differences in threshold criteria was investigated

using the same method as for objects by day. The ratio of the distance

at which an observer, knowing the approximate location of the light, can

just detect it to the maximum distance at which an observer can see the

light steadily was found to be 1.06 for visual ranges in the region of

1000 feet. The spread of the values obtained for this ratio was small,

indicating again that the principal cause of the spread of the points of

the measurements is the "sampling error", resulting from the measurement of a different path in the atmosphere than that through which the

lights were viewed.

If E-j^ is the recognition threshold when a light at distance D

can be seen steadily and is the detection threshold at distance

(1 + k)D,

then

Ini'K^/'E.^) = kDlnT-2 1n(l + k)'

or, since k is small.

4-29
In (E2/E-^) = k D In T - 2k

Using this equation, the detection threshold was found to be 0.6

times the recognition threshold. The recognition threshold for night

observation at Nantucket was found to be 0.052 kilometer candle when the

light was at a distance of 1 kilometer. The corresponding detection

threshold is 0.031 candle. The values of both these thresholds are in

good agreement with the small amount of data obtained in clear weather

at Areata.

A very few data were taken at Areata to obtain the 50^ detection

threshold. A value of 0.02 km candle was obtained with a background

luminance of approximately 0.005 footlambert.

A recognition threshold of about 150 km candles was obtained at

Areata in daylight with background luminances from 500 to 700 footlamberts

.

The corresponding detection threshold is 100 km candles . Again the

data are few.

Kevern's report [76] can be used in comparing laboratory values

with the field values discussed thus far. Kevern computed from

Blackwell's [9] data the illminance threshold in mile candles for an effective point source, for a variety of background luminances, using

a threshold criteria of 50^ detection. The illuminance threshold

-3 -2

obtained for a background luminance of 10 to 10 footlambert is

0.032 mile candle (0.012 km candle). For daylight conditions, with

a background of 700 lamberts, the detection threshold computed from

Kevern gives a value of 64 mile^ candles (25 km candles). If these

values are increased by a factor of 2, to obtain values more nearly

corresponding to the usual criteria of detection thresholds, then

values of 0.025 and 50 km candles are obtained for background

luminances of 0.005 and 700 footlamberts . Detection thresholds of

0.034- and 24 km candles were obtained from data reported by Knoll,

-2

Tousey and Hulburt [77] for background luminances of 10 and 700

footlamberts

.

The results of these determinations are si:immarized in table 4.4.

4-30
Table 4-4

Illuminance thresholds (km candles)

Night Day

Detection Recognition Detection Recognition

Nantucket 0.031* 0.052*

Areata 0.02 100 150

Kevern 0.025 50

NRL 0.034 24

*At a distance of one kilometer.

Thus the values for illuminance threshold obtained in field

experiments during periods of fog do not differ greatly from those obtained

in the laboratory when the same criteria of threshold are applied.

4.4.8 Thresholds Applicable to the Aircraft Pilot

NOTE: In analyzing flight test data and in computing

visual range from aircraft, it is convenient and conventional to consider the illiaminance incident on the windscreen, not on the pilot's eye as the illuminance used

in Allard's law. Thus a correction must be applied to

laboratory threshold data and to field threshold data

obtained from outside the cockpit when applying it to

aircraft situations. The correction factor for a modern air carrier aircraft is estimated to be 2. This

factor has been often ignored in applying laboratory

data to flight situations.

4.4.8.1 Nighttime Thresholds for Pilots

It should be apparent that the illuminance thresholds given in

figure 4.4 are not directly applicable to the pilot.

An illuminance threshold of 0.5 mile candle (0.2 microlux) was chosen as the illuminance threshold applicable to pilots in the 1930's

[117] . This value was also accepted as applicable to shipboard

lookouts at the International Technical Conference on Lighthouse

4-31
Authorities, Paris, 1933. Because of the increase in the complexity

of the flying task, the great increase in the number of lighted instruments, and the increased losses in aircraft windscreens, a number of

workers in the field of aviation lighting used an increased threshold

illuminance of 1 mile candle (0.4 microlux) in the 1940' s. Later this

value was increased to 2 mile candles (0.8 microlux) in the U.S. at

the start of the runway visual range program. At the same time, but

independently, a value of 1 microlux (2.5 mile candles) was recommended

by the International Commission on Illumination [72] . A value of

of 0.87 microlux (2 mile candles) has been recommended to the International Civil Aviation Organization by the All Weather Operations Panel [58]

.

The choices of illuminance threshold were based primarily upon

engineering judgment with little, or no, hard evidence. Note that a threshold of 2 mile candles ia about 40 times the illuminance threshold

shown for a background luminance of 10 footlambert in figure 4-4.

An illuminance of 2 mile candles is about the value of the minimum

useful signal found by Breckenridge and Douglas [16] . In 1966,

Lefkowitz and Schlatter [90] analyzed the flight test data of the

Landing Aids Experiment Station and of the MacArthur Field tests and

obtained values for illuminance threshold, based upon a 50^ probability

of sighting, of 1.6 and 5 mile candles respectively for the pilot's

sighting of runway edge lights. The flight test data obtained at

Newark, based upon the pilot's sighting of the green threshold lights, was also analyzed. These data showed illuminance thresholds of 0.9,

2.5, and 30 mile candles for rimway light settings of 4? 20, and 100^

respectively. Although no statement of the approach light intensity is

made, these lights were presumably operated at the same intensities as the runway lights. These Newark data were the first flight test data

obtained with a high intensity Configuration A approach light system.

They provide a clear indication of the effects of light intensity setting

on the pilots nighttime illuminance threshold. The data are in general

agreement with the analysis made by Simeroth [1141 of the self-defeating

effects of increasing approach and runway light intensity on the pilot's

illuminance threshold.

4-32
The nighttime data taken at Atlantic City in 1959 and I960 [90] appear

to be strongly influenced by the effects of a dense cloud cover over- lying the fog. A nighttime threshold of 1000 mile candles was obtained.

The spread of the data was very large. Whereas in the laboratory the

ratio of the illuminance threshold for 95^ probability of seeing to

the threshold for 5^ probability of seeing is about four, in the flight

test data analyzed the ratio was in the range 1000 to 10,000. Hence the

uncertainty in thresholds obtained from data of this type is very large.

It should be noted that when the decision was made (see Section 3.9.1)

to use a fixed nighttime illuminance threshold of 2 mile candles, the nighttime

luminances of fogs, as seen from the glide path or runway centerline, were

about one tenth the luminance of fogs of the same density today. The in- crease in luminance results from the addition of high intensity, wide beam

centerline and touchdown zone lights and the use of Configuration A approach

light systems with wide beam lights. Not only have the number of lights and

the lumen output per light both increased, but in addition, these lights are located closer to the center of the visual field. Thus, the effects on the

illuminance threshold of the changes in fog luminance with the intensity

setting of the lights are greater today than they were at the time the

system was designed. Theoretical analyses [107, 114] > ground observations

[90], and flight tests [34-> 90] confirm the need for a consideration of

these effects in assessing RVR. See also Section 3.9.2.

A complete analysis of the many parameters which influence the RVR

nighttime illuminance threshold is beyond the scope of this document.

There is, however, one misconception which should be noted here. In the

considerations of the All Weather Operations Panel, an' illuminance threshold

of 2 mile candles has been justified for background luminances typical of

fog luminances with present lighting systems (l to 15 footlamberts, 4- to

50 candelas/meter ) because this choice is in agreement with Blackwell's

data [9] and other ttireshold data. However, in reaching this conclusion,

no consideration was given to the conditions under which Blackwell's data

are applicable, namely a 50^ detection probability by an observer whose

sole task is to look for a light of known location and occurrence. For a 9S% detection probability, the illuminance threshold must be doubled.

1^

4-33
(See figure 4.4). Additional increases of from two to ten are required

if the pilot does not know exactly where to look [51] . A further increase

of as much as two is required to compensate for losses in the aircraft

windscreen. Thus, an increase on the order of ten to twenty in the nighttime RVR illuminance threshold is appropriate for conditions in which the

fog luminance is on the order of 1 to 15 footlamherts

.

4.. 4. 8. 2 Daytime TTiresholds for Pilots

Lefkowitz and Schlatter, in their study, computed the daytime illuminance

thresholds obtained from flight test data. Daytime illuminance

thresholds based upon 50^ probability of seeing were as follows:

During daylight the effects of the intensity setting of the lights

are expected to have little effect on the illuminance threshold. The

ratio of 95% to 5% probability of seeing was about an order of

magnitude lower than that found at night, being in the range 100 to

1000.

4.4.8.3 Ground Observations at NAFEC

In the study referred to above, Lefkowitz and Schlatter conducted

a field study to determine illuminance thresholds applicable to the

assessment of runway visual range. Observations were made from a height

of 15 ft above the centerline of a runway equipped with runway edge

lights (type L-819) and centerline lights (type L-84-5, improved). The

average intensity (see 2.1.5) of the edge lights was approximately

10,000 candelas; the average intensity of the centerline lights, 4500

candelas. Peak intensities were approximately 25,000 candelas and

7,000 candelas for the edge and centerline lights respectively. Edge

lights were spaced at 200 foot intervals; centerline lights at 25 or

50 ft intervals as desired.

The illuminance thresholds for 50^ probability of seeing were as follows

:

Landing Aids Experiment Station

MacArthur Field

Newark

Atlantic City 20^ intensity

1000 mile candles

20

600

1600

100^ intensity 9000

4-34
NIGHT 100^ intensity

20^ intensity

4^ intensity

2.0 mile candles

0.8

0.4

DAY 100^ intensity 110

The night observations show again the effects of the fog luminance

produced by the lights themselves on the illuminance threshold.

To be applicable to the pilot, all values listed above should be

multiplied by a factor of about 2 to compensate for the light losses

in the aircraft windscreen and by an unknown factor to compensate for

the forward motion of the aircraft. See Section 4-6.

Since pilots use both natural marks and lights for guidance it is

desirable to know the relation between the day and night meteorological

visibilities for equal transmissivities . Figure 4-7 shows the relation

between the day and night visibilities, and V^, obtained from the

transmissometer calibration curves, figure 3.3. The intensity of

lights is taken as 25 candelas. A value of 0.055 is used for contrast

threshold. When the atmospheric transmissivity, and consequently the

visibility, is very low, a 25-candela light can be seen at night more

than three times as far as a large black object would be seen during

the day in the same atmosphere. This difference decreases as the

visibility inaproves, becoming two to one when objects can be seen one half mile and equal when the visibility is about 16 miles. When the

visibility is greater than 16 miles, objects will be seen further than

25-candela lights.

The relation between the visual range of approach and runway lights

and the daytime visibility is important in designing airport runways

and approach light systems. If the visual range of lights' of practicable

intensity is greater than the daytime visibility, the visual range of

objects, then lights are of considerable assistance to a pilot.

The intensity required for a light to be seen a distance d by day

can be computed from Allard's law, by choosing a value of E suitable for

4.5 RELATION BETWEEN VISUAL RANGE OF OBJECTS

AND OF LIGHTS

4-35

the prevailing conditions of background luminance. The value of T for

the visual range of any given object may be found from Koschmieder ' s law.

The intensities required of lights so that they may be seen at distances

proportional to the visual range of black objects can be found on figure -4.8.

The value chosen for the background brightness, 1000 footlamberts, is

representative of bright daylight fogs. The corresponding value of threshold illumination is 1000 mile candles . This figure shows that in thick

fogs lights of even moderate intensity can be seen at distances further

than objects.

The use of lights by day is of considerable advantage for the range

of visibilities of most concern in the landing of aircraft. A system

using approach and runway lights on the order of 25,000 and 10,000

candelas intensity will be seen 1.5 times as far as the object (daylight)

visual range during periods of fog.

4.6 EFFECTS OF THE MOVING OBSERVER

The threshold constants applicable to aircraft pilots are con- siderably higher than those of ground observers because of the longer

search times involved. This is due to a number of factors including

the pilot's preoccupation with controlling the aircraft, windshield

distortion, differences in adaptation, and the absence of cues. It is

extremely difficult to find marks or lights which are at or near threshold when their exact position is not known.

An important factor affecting a pilot's search time is the rapid

change in transmittance between the pilot and a given mark from the

time the pilot is at a point where he would detect it if he were stationary and the time until he actually locates it.

Consider an observer approaching an object at the rate of S feet

per second. If is the inherent contrast between the mark and a sky

background, 0^ the apparent contrast between the mark and its background

at a distance x, and is the apparent contrast after x seconds, then

0 = C T^, X o and, where T is the transmissivity (per foot).

4-37
OBJECT VISIBILITY (V^)

Figure 4.8 Intensity, I, as a function of the threshold illuminance, E, required

to produce a visual range K times the daytime visibility V,.

4-38
Thus

C = C

T O

Note that the ratio C^/C^ is independent of the distance to the object

and is equal to the transmittance of the path traversed during time t .

Equation (4-. 29) may also be written as

e'/e = T"^"^ (^4.30)

where e' is the apparent contrast threshold of the moving observer, that

is, the contrast threshold which would be obtained if his reported range

were used with transmissivity measurements in Koschmieder 's law to

compute his threshold.

In figure 4.9, the ratio e^/e has been computed using a value of

200 feet per second for S and a time interval of one second. The figure

shows that, even for a search period as short as 1 second, the contrast

can increase rapidly when the visual range is low. Thus, by the time a moving pilot has located an object, its contrast, and consequently his

apparent threshold, may be much higher than the contrast threshold of a stationary observer.^

Another approach illustrating the effect of the time required to

find an object is through the use of the ratio of the apparent visual

range to the visual range which would be obtained by a stationary

observer. In figrire 4.10, this ratio was computed from the relation

^ ^ V^s ^ ^^s " ^ ""^/^s ^'^•^^^

where V is the distance at which the pilot will find the object, V

p ^ ' s is the distance at which it could be found if the pilot were stationary,

T is the time in seconds required to find the mark, and S is the speed

of the aircraft in feet per second.

*In view of the work of Lamar [81] and others, the assumption of a search time as low as one second may be optimistic.

4-39
NIGHT RVR, feet

.001 .01 .1 ,0316 ,1 .316

TRANSMITTANCE FOR 500 ft (upper scale)

AND FOR 250 ft (lower scale)

FIGURE 4.9 Effect of speed of moving observer upon his apparent

threshold. An aircraft speed of 200 feet per second

and a search time of one second have been assumed.

4-40
4-41
Similarly the change in illuminance from a light can be shown

to be

E^E = T"^V(1 - S/x^) (4.32)

where E is the illuminance at a distance and E is the illuminance

T after t in seconds. The denomination of (4.32) represents the change

in E due to the variation in illuminance with the square of the distance

and is always less than 1. Thus the increase in the ratio E_^/E will

be always greater than the increase in the ratio e^/e. Thus figure 4.9

may be considered as representing the lower limit of the ratio E'/E.^.

where E' is the apparent illuminance threshold of a moving observer and

E, is the illuminance threshold of the stationary observer.

All of the foregoing ratios, and consequently the apparent threshold

of a moving observer, are dependent upon both speed and fog density.

The apparent spread in the threshold data obtained from flight tests is

increased significantly by the effects of search time.

4-42
5. MEASUREMENTS OF ATMOSPHERIC ATTENUATION

Note: In this Chapter, primary attention is

given to horizontal lines of sight in a uniform

atmosphere. Methods of measuring "slant visibility" are beyond the scope of this document.

As shown in Chapter L,, the visual range of objects and lights

is directly related to the attenuation of the atmosphere by Koschmieder's and Allard's laws. The transmission of light through the

atmosphere is determined by the absorption and scattering which

occurs in the light path. Absorption by the fixed gases of the

atmosphere is negligible in the visible and near Infrared region of

the spectrum. Smoke and dust absorb to some extent in the visible

region of the spectrum, and, in areas of heavy industrial pollution,

the absorption of these particles become significant [122]

.

Water vapor has absorption bands In the near Infrared but absorption In

the visible region is not significant. Scattering, therefore, is the

principal cause of atmospheric attenuation.

Scattering is a complex function of the ratio of the diameter of

the atmospheric particles to the wavelength of light. The air

molecules are very small compared to the wavelength of the visible

region, and, under these conditions the scattering coefficient varies

inversely as the fourth power of the wavelength (Raylelgh scattering).

For example, the sky is blue because of the greater scattering of

short wavelength (blue) light. The scattering properties of the

smaller dust and smoke (Aitken size) particles approach that of Raylelgh

scattering. When the diameter of the aerosols is large in comparison to

the wavelength of light, the scattering coefficient Is essentially independent of wavelength. As the diameter of fog droplets Is typically

in the range of 2 to 5u^, and the wavelength limits of the visible

^ly = 1 micrometer (micron) = 1000 nanometers (nm).

5-1
region of the spectrum are about 0.4 and 0.7 y, the scattering co- efficients of fogs and clouds are usually independent of wavelength.

Hence the sun usually appears "white" when barely visible through

cloud or fog. When the diameter of the aerosols is roughly equal

to the wavelength of the light being transmitted, the situation is

complex and will not be treated here: For a detailed discussion see Middleton [95], Van de Hulst [120], and Zuev [130].

Many different instruments and methods have been designed to

assess the clarity of the atmosphere; each has been chosen to meet

the requirements of a particular problem. This report will discuss

only those photoelectric instruments which measure the light from

a source, transmitted or scattered by the atmosphere. For a discussion

of visual instruments and photoelectric instruments which measure

the luminance of surfaces at a distance, see Middleton [95], Chapter

Nine.

In this report, these instruments discussed will be grouped into

two types, as follows:

1. Those which determine the transmittance of a path

of known length using a light source and a telephotometer

.

The transmissivity is then computed from the relationship

T = (t^)^/^, (2.07)

where is the measured transmittance over a path of

length b. A block diagram of the NBS transmissometer,

a type 1 instrument, is given in figure 3.1. Several types

of transmissometers have been described by Oddie [106]

.

'2. Those which measure the flux scattered, from a source,

into a receiver by the aerosols present. The output of

these instruments is assumed to be proportional to 3, the

scattering coefficient. However, their output is somewhat

dependent upon the size distribution of the aerosols.

These instruments can be used to obtain a, and hence T,

,

5-2
assuming that a, the absorption coefficient;, is negligible.

This assumption is valid for clean fogs since from (2.10) and

(2.13)

T =^ e-^ = e-(^

"

Backscatter meters, side-scatter meters, and forwardscatter meters are examples of type 2 instruments which

will be covered in this Chapter.

5.1 TRANSMISSOMETERS

5.1.1 Principles of Operation

As previously stated, a transmissometer determines the transmittance of a path of known length using a light source and a telephotometer. The transmissivity is then computed from the relation

T = t^^"^^. (2.07)

where t-j^ is the transmittance over path length b. The transmissivity

T is correlated to the visual range of objects using Koschmieder ' s law,

e = T^, (4.08)

where e is the contrast threshold and V is the visual range. Allard's

law,

= ItW, (4.24)

is used to correlate transmissivity with the visual range of lights

at night.

It is essential to note that in both laws distance enters as an

5-3
exponential. This fact, together with the visual ranges to be determined,

governs the choice of distance between the light source and receiver

of a transmissometer

.

Combining (2.07) and (<4.08) yields

£ = t. V/b

(5.01)

'b

which is the basis of the calibration of a transmissometer based on

the visual range of black objects.

Similarly, combining (2.07) and (4-. 24) yields

which is the basis of the calibration of a transmissometer based on the

visual range of lights

.

NOTE: Equations (5.01) and (5.02) strictly apply only to

monochromatic light or to an atmosphere which uniformly transmits

light of all wavelengths in the wavelength band to which the

telephotometer responds or the visible wavelength region,

whichever is larger. (See Section 5.4.3.)

Transmissometers may be subdivided according to the geometrical

arrangements of source(s) and receiver( s ) as follows:

a) The simplest and most direct arrangement is a straightline one which uses a single source and a single receiver

(figure 3-10). A variation of the straight-line approach

uses a single source and two receivers at different distances

or a single receiver and two sources positioned at different

distances from the receiver. A multiplicity of sources or receivers creates more than one baseline which extends the

(5.02)

5-4
range of transmissivities which can be determined from the

instrument readings. The first transmissometer as tested

at Nantucket ( See Chapter 6 ) was installed with two sources

.

However, such an arrangement is usually not satisfactory

because an increase in the field of view of the receiver

is required. Therefore, when a dual-baseline transmissometer

was installed at the Landing Aids Experiment Station, two

receivers were used, , positioned 500 and 1000 feet from the

single soiirce at the T-D location.

b) Another arrangement uses folded optics and includes one or more reflectors which "fold" the light from the source back

to the receiver. Since the source and receiver can be located

in close proximity to each other when using folded optics, this

arrangement is frequently used where long straight-line

arrangements are not feasible.

To aid in calibration or to compensate for changes in the

intensity of the source, a feedback loop, consisting of immediately

reflecting a small portion of the light from the source directly to the receiver, or to an auxiliary receiver,

is frequently a design feature of both straight-line and

folded optics arrangements. Such an arrangement is shown

in figure 5.1.

5.1.2 Error Analysis - Transmissometers

Note: In the error analyses of this chapter, an atmosphere in

which the transmittance does not change with position has been

tacitly assumed.

5.1.2.1 Relative error in the visual range of objects

The effect of small instrumental errors in a transmissometer on

the indicated visual range of objects by day may be found as follows:

5-5
: C *tJ objective twdi

^

BEAM J C(T)

I SPLITTER U i W ' b

EYEPIECE i /A.B

OBJECTIVE TRIPLE

LENS MIRROR

1/

Tpcolour

(I^^FILTER

PHOTOCELL

MODU LATOR 250 Hz

I

I AS

MODULATOR 3000Hz

LI6» I6HT

SOURCE

Figure 5.1 Optical diagram of Eltro dual beam

transmissometer.

5-6
From Koschmieder ' s law (4-08) and the relation between transmissivity and transmittance (2.07)

^ - \ • (5.01)

If t^ ' is the erroneous transmittance indicated by the instrument, then the erroneous visual range determined from this transmittance is given by

. = t^'"'^. (5.03)

Let

and

V = (1 + n) V,

where m is the relative error in t-, , and n is the relative error in V. b

Then

V/b_ , >,V7b

^b ^ ^b ^

or

t//^ = Id . m) * (5.04)

and

(V/b) In t.^ = (1 + n) (V/b) [In (l + m) + In t.^] . (5.05)

Since, from (5.01),

In t^ = (b/V) In e,

it is possible to substitute in (5.05) and obtain

5-7
In e = (1 + n)(V/b)ln(l + m)+(l + n) In e. (5.06)

Equation (5.06) can then be solved, obtaining

n/(l + n) = -Vlr(l + m)/b In e . (5.07)

If m is sufficiently small, In (l + m) = m, and (5.07) may be

written as

n/(l + n) = -mV/b In e, (5.08)

and if n also is small, (5.08) reduces to

n = -mV/b In e. (5.09a)

Since n is defined as the relative error in V and m as the relative

error in t^, (5.09a) can be rewritten as

AV/V = -(V/b In e) ^\/\, (5.09b)

When n and m are sufficiently small, the preceding analysis can also

be made through the use of calculus.

Thus, from (5.01),

(V/b) In t^ = In e. (5.10)

Differentiating, assuming e is constant,

(In t^) dV/b + V dt^Ab t^) = 0,

or

dV/V = -d/ln t^) ^\/\. (5.11)

Substituting for In t^ from (5.10)

dV/V = -(V/b In e) dt^/t^ (5.12)

b b

or, for small finite errors,

AV/V = -(VA In e) At^/t^

which is (5.09b).

Note also that In e is negative. Thus a positive error in the

transmittance measurement will cause a positive error in the indicated

visual range.

5-8
If a value of 0.05 is assumed for e, (5.09b) becomes

AV/V = (V/3b) At^/t^ (5.13)

which shows that if the visual range is less than three times the

length of the transmissometer baseline, the relative error in indicated

visual range will be less than the error in the transmittance

measurement

.

5.1.2.2 Relative error in the visual rarige of lights

From Allard's law,

= I t^^//. (5.02)

t b

Rearranging terms and taking natural logarithms,

(V/b) In t^ = In (E^/I) + 2 In V. (5.14)

Differentiating, holding and I constant,

[(In t.^)/b] dV + (V/bt^) dt^ = (2/V) dV.

Rearranging terms,

dV/V = (V/b)(dt^/t^)/[2 - (V In t^)/b] (5.15)

Note that the second term in the denominator contains both V

and In t.^^ and that these quantities are not independent. Equation

(5.15) can be put into a more useful form by substituting the righthand

side of (5.1-4) for (V/b) In t^, obtaining

b

dV/V = (V/b) [1/(2 - ln(E^/l)- 2 In V)] dt^/t^. (5.16)

Then, for small finite errors

AV/V = (V/b) [1/(2 - ln(E,/l) - 2 In V)] At /t, , (5.17)

X D D where AV/V is the error in the indicated visual range corresponding to

a relative error in the transmissometer At, /t, . b b

5-9
Comparing (5.17) with (5.12) shows that the two equations are

the same except that the term -1/ln e in (5.12) has been replaced by

1/(2 - In(Eyi) - 2 In V)]. The effect of this change is illustrated

by the following.

Note that if T is equal to one (perfectly clear weather),

(5.02) reduces to

2

E^/I = 1/V (inverse square law),

and

In (E^/I) = -2 In V. (5.18)

The maximum possible value of the visual range, V, can be

determined by combining (5.18) and (5.17). When V is near this value

AV/V = (V/2b) At^/t^. (5.19)

Comparing (5.19) and (5.13) shows that, in clear weather, the

effects of instrumental errors on the visual range of lights is

somewhat greater than on the visual range of objects.

As an example of the effect of instr\imental errors during

conditions of restricted visibility assume:

Illuminance threshold, E , of 2 mile

candles (7.17 x 10 footcandle );

(nighttime conditions),

source intensity, I, of 10,000 candelas, and

transmissometer baseline of 500 feet.

Then from (5.17), since, when the foot is used as the unit of length,

In E/I = -25.66

AV/V = (V/500) [1/(27.66 - 2 In V)] At^/t^.

Let F, the error factor, be defined as

F = 1/(27.66 - 2 In V). (5.20)

Then

AV/V = (FV/b) At^/t^. (5.21)

Table 5.1 has been computed from (5.20) using the assumptions

given above.

5-10
TABLE 5.1

Transmissometer Error Factor for a 500 Foot

Baseline Transmissometer,

Nighttime Conditions

Visual Range, V

( feet

)

Error Factor, F FV/b

500 0.066 0.066

1000 .072 .u

2000 .080 .32

5000 .094 .94

10000 .108 2.2

Table 5.2 has been computed, similarly, for daytime conditions

assuming a threshold illuminance of 1000 mile candles (3.59 x 10

footcandles )

.

TABLE 5.2

Transmissometer Error Factor for a 500 Foot

Baseline Transmissometer,

Daytime Conditions

Visual Range, V

( feet

)

Error Factor, F FV/b

500 0.11 0.11

1000 .13 .26

2000 .16 .64

5000 .23 2.3

10000 .33 6.6

These tables illustrate the very small effect that transmissometer

instrument errors have on the visual range of high intensity lights

during conditions of restricted visibility, especially at night.

5-n
Note that for objects the error factor is 1/ln e, and if a contrast threshold of 0.05 is assumed, the error factor for objects

becomes 0.3 for all visual ranges.

5.1.2.3 Choice of Baseline

The preceding analysis shows that the percentage error in the

visual range indicated by the instrimient is directly proportional to

the relative Instrumental error and to the true visual range and

inversely proportional to the distance between the light source and

the receiver. It is evident that if the error in the indicated

visual range is to be kept small for large visual ranges, either the

instrument must be very accurate, so that At^/t^ is small, or the

distance b, between the source and the receiver, must be an appreciable fraction of V, the visual range. Thus, error analysis also

plays a role in the choice of a baseline for a transmissometer

system, i.e., the distance traversed by the beam from the lamp to

the telephotometer . Equations (5.13) and (5.21) indicate the desirability of making the length of the baseline as long as possible.

The limiting factor is the minimum transmittance, t-^, which is to

be measured.

It can be easily shown from (2.08) that the maximum length of a transmissometer baseline is given by

b - in t /In T (5.22)

m m where

b is the length of the baseline,

is the minimum transmissivity to be measured, and

t is the minimum transmittance which can be measured

m

by the instrument.

As an example, assume that a transmissometer is to be used to

indicate runway visual ranges down to 1000 feet; that these ranges

are based upon an intensity of 10,000 candelas; that the illuminance

threshold is 2 mile candles (since nighttime is the limiting case);

and that the instrument can operate down to a transmittance of 0.001.

5-12
Using the assiimptions given above, converting b to miles to

keep the units consistent, and rewriting (5.14)7 using common

logarithms, a value of -27.2 is obtained for log T (per mile).

Note that common logarithms are used here because of their con- venience when using nomograms of Appendix C to compute visual ranges,

Thus

-3/-27.2 = 0.11 miles,

or

b = 580 feet.

Similarly for an RVR of 500 feet, (0.095 mile)

log T = -61,

b = 0.049 mile = 260 feet.

In keeping with this principle, the baselines of the first

transmissometers installed for operational use at civil airports used

a 750 foot baseline. This was later reduced to 500 feet when Category

II operations, having an RVR minimum of 1200 feet, were started. Now,

with the advent of Category III operations, a 250 foot baseline is

being used.

Errors in reported RVR will result if the transmissometer ' s actual

baseline length differs from the specified baseline distance used by the

"computer" in RVR computations. On some occasions, a transmissometer

system must be installed with a baseline differing slightly from the

specified baseline because physical obstructions prevent the baseline

from being laid out to its exact nominal length. The errors in daytime

RVR can be found from Koschmieder ' s Law,

e = t//\ (5.01)

If b' is the baseline used, then V', the computed visual range,

is given by

. = t^7'/'' (5.31)

Combining (5.01) and (5.31)

+ V/b _ , V/b

5-13
and

V/b In t, = V'/b In t.

' D (5.32)

Since

(5.32) reduces to

V Vb/b', (5.33)

and

AV

V

-Ab

b' (5.34)

Therefore the relative error in the indicated visual range by day

is equal to the relative error in the length of the baseline, but

opposite in sign.

The errors by night will be ±ess, due to the compensating effects of

the inverse square aspects of Allard's Law.

5.1.2.4 Instrumental Determination of Transmissometer 100^ Setting.

The most significant source of error in a well designed transmissometer is in the adjustment of the sensitivity of the instrument.

In transmissometers, this error is apparent as an error in the 100^

setting, that is, the transmittance which would be indicated were the

air perfectly clear. There are four basic instrumental methods of

calibrating a transmissometer to obtain the 100^ setting.

a. By separate measurement of the intensity of the source and the sensitivity of the receiver and adjustment of the

intensity or sensitivity to obtain an instrument reading of

1.00 with no attenuation losses. Experience on the outdoor

photometric range at the National Bureau of Standards indicates

that using this method, even with careful photometry, errors

in the 100^ setting may be as large as 5^. This method is not

readily applicable to field conditions.

b. By measiirement of the illuminance produced by the transmissometer light source with a receptor having a stable, but

\inknown, sensitivity at the receiver and at a position near the source and computing the transmittance from these measurements.

A uniform and stable transmittance over the transmissometer

baseline is assumed. Field tests by NBS at Areata indicate

5-14
that calibrations can be made in relatively clear weather

(meteorological visibility above 5 miles) with expected errors

of about 2% [29]. See Section 6.2.2.

Note: The distance between the transmissometer source and the

nearer position of the receptor must be sufficiently large to

insure that the illuminance from the source follows the inverse

square law.

'C A variant of method (b) utilizes the very narrow-beam properties of a laser [75] . If a laser is used as a source, it

is possible to construct a portable receptor which, when

located at the transmissometer receiver, will intercept the

entire beam of a laser located at the transmissometer light

source. The prevailing transmittance is then the ratio of

the flux incident on the portable receptor when it is at the

transmissometer receiver to the flux incident on the receptor

when it is immediately in front of the laser source. Such

calibrators have been constructed, but their accuracy is

not known. This method is, in principle, independent of the

visibility prevailing at the time of calibration. However,

its accuracy under conditions of poor visibility is limited

by the accuracy with which the output meters of the calibrator

and the transmissometer can be read. Moving pointer indicators,

of the type used in the transmissometer, can be read to about

one-quarter of a scale division or, with a typical meter having

50 scale divisions, to about 0.005 units. Thus, if the prevailing

transmittance were 0.20, the uncertainty in reading the transmittance would be assuming no linearity or zero correction errors.

d. Use of a stable portable instrument which can be installed

on the transmissometer stands and aligned easily under conditions

of poor, as well as of good, visibility. The problems of setting

accuracy discussed in (c) apply here also.

.2.5 Visual Estimate Calibration.

Another method of obtaining a 100^ setting is by the visual estimation

5-15
of meteorological visibility and subsequent adjustment of the transmissometer to obtain a reading equal to the transmittance corresponding

to the estimated object visibility. The accuracy of the setting depends

upon the length of the baseline and the prevailing meteorological visibility

as well as upon the uniformity of the atmosphere. This is the method

most frequently used.

5.1.2.6 Errors in the Visual Estimte Calibration.

The relative error in transmittance resulting from the visibility

estimate of this method may be computed as follows:

Let be the "true" visibility, that is, the visibility representative of the transmittance over the transmissometer baseline and

let be the visibility estimated by the observer. Then the

"true" transmittance, t^*, is given by the relation

V /b

t^ ° = e, (5.23a)

and the estimated transmittance, t , is given by the relation

V /b

®

tg ® = e. (5.23b)

If 'a' is the relative error in transmittance,

tg = (l'+ a) t^. (5.2^)

Combining equation (5.23a) and 5.23b)

(b/V - b/V ) t /t = £ ^ °

, e o'

and from ( 5 . 24 )

a = £

If the quantity (b/V^ - b/V^ ) is small, less than about 0.1,

(5.25) may be expressed as

a = (b/V^ - b/V^) In £. (5.26)

The error in transmittance will be not more than 3% with a 500-foot baseline transmissometer (1.5^ for a 250-foot baseline

^The subscript b is omitted throughout this development.

5-16
instrument) if the transmittance over the transmissometer baseline

is that corresponding to a visibility of 5 miles or more, and the

transmissometer is adjusted as though the transmittance were representative of a visibility of 10 miles. The error would be 6^ for a 500-foot instrument ( % for a 250-foot instrument ) if for all

visibilities above 2.5 miles the transmissometer were adjusted as though the transmittance over its baseline corresponded to a visibility

of 5 miles.

5.1.2.7 Analysis of the Effects of Errors in the Visual Estimate

Calibration

.

The effects of errors in the 100^ setting on other visibilities

indicated by the transmissometer as a function of transmittance

and visibility may be computed on the basis of the visual range of

objects as follows. (Because of the effects of the inverse square

law, the errors in the indicated visual range of lights will be less

than those for objects. See Section 5.1.2.2.)

Let V be the visibility which would be obtained if the 100^

setting of the transmissometer were correct and let t-^ be the

corresponding transmittance over the baseline b, and let V be the

indicated visibility as a result of an incorrect 100^ setting and

t'^ be the corresponding transmittance over the baseline b. Then,

if from (5.03)

\' = (1 - a) t^,

(V/b) In t^ = (V'A)) [In t^ + In (1 + a)]. (5.27)

The relative error in visibility is then given by

(V - V)/V = AVA = -ln(l + a)/[ln t^ + ln(l + a)]. (5.28)

If 'a' is small,

ln(l + a) = a, and

AV/V = -a/( In t^ + a ) . ( 5 . 29

)

By using either of equations (5.28) or (5.29), it is possible

to compute the maximum transmittance for which the error in indicated visibility will be less than a stated tolerable error as a

5-17
function of the error in the 100^ setting. The results of such a conrputation using equation (5.28) are given in table 5.3. If equation

(5.29) had been used the maximum transmittance shown would have been

slightly (less than 0.01 ) lower. Note that AV/V and 'a' have the same

sign.

TABI.F 5 ?

Maximum Permissible Transmittance

Tolerable Relative Error Present Maximum

Relative Error in Transmittance Permissible

in Visibility Reading Transmittance ( 1

)

0 05 - 0 01 0 81

.02 .66

.05 .36

.10 .14

0.10 0.01 0.90

.02 .80

.05 .58

.10 .35

0.20 0.01 0.94

.02 .89

.05 .75

.10 .56

^''"^At which the error in visibility produced by the error in 100^

setting shown in Column 2 is less than the tolerable error shown

in Column 1.

For typical values of 'a' and for values of V less than of the

order of ten times the baseline, equation (5.12) applies. It may

be written

AV/V = -aV/(b In e), (5.30a)

since

a = At, /t, . b b

If an error in transmittance of 2% and a contrast threshold of

0.05 are assumed,

AV/V = 0.007 V/b (5.30b)

5-18
It is apparent from the figures given above, that, if the

transmittance over the transmissometer path is representative of the

prevailing transmittance, errors in the estimation of visibility

during a transmissometer calibration will have a small effect on the

accuracy of the indicated visibility by the transmissometer during

periods of low visibility. Experience has shown that spatial nonuniformity in atmospheric transmittance is the greatest source of

error in the 100^ setting. The effects of non-uniformity can be

reduced significantly by using records of transmittance and visibility

reports extending over a period of hours, or days if possible.

Note that discussion has been based upon the visual

range of objects . The relative errors in the visual range of lights,

as shown earlier, will usually be less than that for objects.

5.1.3 Scattered Light Error in Transmissometers.

A very important consideration in the accuracy of a transmissometer

is the extent to which it measures the regular transmittance of the

atmosphere

.

In a fog, a telephotometer such as the transmissomieter receiver

will receive, in addition to the light which passes directly from

the source to the receiver, some additional light which, although

radiated from the source, would not reach the receiver in clear

weather. This scattered light constitutes an error in the transmittance measurement.^ In dense fogs this error may become large,

especially when the size of the fog droplets is small. In the NBS

transmissometer, the error is minimized by keeping the field of view

of the receiver and the beam spread of the projector as small as possible

.

*For an extensive treatment of theoretical aspects of the effects of

scatter on measurements of atmospheric attenuation, see the study by

Hoijer [-45] .

5-19
In the 1950 's, NBS conducted extensive experiments at Areata,

under the direction of John W. Simeroth, to determine the errors introduced in transmissometer measurements by scattered light, the

results of which are previously unpublished. Two transmissometers

were installed on parallel 500-foot baselines 20 feet apart. In the

initial setup, baffles were installed along the line-of-sight of one instrument to restrict the light beam and further restrict the

field of view of the receiver, thereby reducing the effects of

scattered light on this instrument. This configuration would allow

the direct comparison of transmittance measurements consisting,

essentially, only of directly transmitted light with those con- sisting of directly transmitted light plus any increase in the

measurement due to scattered light. This method requires very

accurate measurements of transmittance, since the scattered light

component of the measurement may be small, and is dependent on both

instruments measuring portions of the atmosphere which are identical

in density and homogeneity. However, the terrain in the vicinity of

this instrument, although without large or sudden changes in the

contour, was such that even slight winds had a considerable effect

on the homogeneity of the fog with the result that the measurements

were unreliable and sometimes the baffled instrument actually

indicated higher transmittance than the unbaffled instrument ( it

should always be lower). An indirect method was therefore adopted.

One transmissometer, well-baffled, was used to measure as nearly

as possible the direct transmittance. The baffling of this

instrument consisted of five 4--foot square baffles located at

distances of 6, 8, 54; 162, and 486 feet from the projector having'

aperture diameters of 9, 9, 9, 7, and 4-5 inches. The other transmissometer, unbaffled, with a 6-inch disc midway between the projector and receiver to occlude all directly transmitted light,

measured only the scattered light, i.e., the error that is introduced into measurements made with an unbaffled transmissometer.

5-20
The disc was removable to check the 100^ calibration. The

arrangement is shown in figure 5.2. Measurements of the scattered

light and regularly transmitted light were made simultaneously.

The standard transmissometer lamp is a 120-watt, 6-volt, PAR-64

type having a C-6 filament. Initial output is approximately

180,000 candelas peak beam candlepower with a 2.5° horizontal, 5°

vertical beam spread, measured at 50^ of peak intensity. The filament

was so shielded that no direct light from it was emitted by the lamp.

During some of the measurements, a partially shielded bare

lamp was used. This lamp was a 5-kilowatt, 120-volt, T-6/4 type

having a C-13 planar filament. The shield consisted of a cylinder,

24 inches in diameter, placed over the lamp with its upper edge

level with the lower edge of the filament. The lamp was completely

unshielded in the upper hemisphere and had an approximately

hemispherical distribution.

The disc used on the unbaffled transmissometer unavoidably

obsciirred that small portion of scattered light due to scattering

which takes place near the line of sight between the source and

the edge of the baffle. Also, the scattered light measurement

contained a component resulting from diffraction around the disc

baffle. To determine which corrections should be made for the

effects of the disc baffle, the ratio of scattered to regularly

transmitted light, At/t, was plotted as a function of receiver

half-angle field' of view for six different atmospheric conditions.

According to geometrical optics, if only first order scattering

is considered, the ratio At/t should approach zero as the angular field

of the receiver is decreased to equal that angle subtended at

the receiver by the disc. As shown on figure 5.3^ the ratio becomes zero at a half-angle field of view of 1 milliradian. If

it were possible to measure scattered light error without discs

or baffles, the lines on figure 5.3 would all intersect at the

point 'i' = 0, At/t - 0. Therefore the y intercept shown is the

correction that must be applied for the indicated transmittance.

5-21
oo

in

to "O

+-> OJ o fO +-> o S- fO M— +-> oo O +-> o

Ln O) o

S- to

<D o -a 1

' " to -o

n3 CD

S- S- =3 fO 13 O

Q. to

fO ro

OJ o E -o

<U

fO S-

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fO +-> +-> o CD 4-> S- E TCS =c o U

to to

-l-> to fO 4-> 'e

"O to

dJ I— fO I s- fD +-> lO s-

-l-> OJ T3 s- >

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to

n3 o

(/) ^- CD OJ

S- E SO) J3

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dJ s- O) E <u +-> o Q. CD to CL E s-

(/) =D

O o

•1— to 4-> E to (_) to CD +j *E '<—>

(T3 S- to o

fO c s-

+-> Q. fO Ql

fC S- o 4-> O)

+->

+-> OJ s- +J

QJ O)

i»- M- c o o O) O _l O)

+-> CM c -t-> O) to E CD

O)

a

C7) •1

c >> c fO

fO O) -M

s_ to 4-> -O

s- fO •r—E 'e

CVJ Ln

s-

=3cn

5-22
i HALF-ANGLE FIELD OF VIEW, ^, MILLIRADIANS

Figure 5.3 Some of the curves used in determining correction i values due to disc baffle using the transmissometer I projector as a light source. Note that 6" baffle

i subtends angle of 1 milliradian at receiver.

5-23
Figure 5.4- shows the correction, as a function of transmittance,

that must be applied in correcting for the effects of the disc baffle.

The curve in figure 5.4 was obtained by plotting the adjustments

(increases) in At/t, for each transmittance, required to make all

of the lines in figure 5.3 pass through the point ^ = 0, At/t = 0.

The curve in figure 5.4- shows the amount that At/t must be increased,

for a particular transmittance, to correct the scattered light

measurements for the effects of the disc baffle, when the source is

the regular transmissometer lamp. These corrections apply regardless

of the field of view of the receiver, since the phenomena is due

solely to the disc baffle.

Figures 5.5 and 5.6 show the ratio, At/t, plotted as a function

of transmittance for seven different half-angle fields of view.

The source for the data shown in figure 5.5 was the transmissometer

projector while the source for the data shown in figure 5.6 was the

partially shielded bare lamp. The data show that the errors due to

scattered light are dependent upon both the beam spread of the light

source and the field of view of the receiver. A comparison of the curves on figures 5.5 and 5.6 shows that over the transmittance range

0.0001 to 0.01, and with a half angle field of view of 6.2 milliradians,

the ratio At/t is approximately 67% greater with the partially shielded

lamp than with the standard transmissometer lamp. When the half angle

field of view is reduced to 1.2 milliradians, the standard opening for

the 500 foot transmissometer receiver, the increase in At/t drops to

36% at a transmittance of 0.0001 and /^2% at 0.01.

A few measurements were made with the shield removed from the

bare lamp. Under these conditions, and at a transmittance of about

0.00025, the error for the bare lamp was about 4- times that obtained

using the standard transmissometer lamp. Not many unshielded bare

lamp measurements were taken due to the "ground factor" - an unknown

quantity of light is reflected from the ground surface toward the

receiver. The unknown ground factor was considered too high to make

continuation of these measurements useful.

5-24
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o

o

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0) S-

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CD

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QJ -a

s- s- Ol

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-o

QJ +-> o

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S_ OJ

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1/1

l/l rO

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OJ ^- c

M- o

OJ

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4- 3

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s_ +J

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5-25
ft3 5

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5-26
CO

+-> u

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The errors in relative transmittance shown on figure 5.5 are summarized in table 5.4.

Table 5.4

"Scattered Light" Error in Transmittance Measurements

(For 500-foot baseline transmissometer

)

Receiver Field

of View (Milliradians )^ 0.92 1.2 1.8 2.5 3.1 3.7 6.2

"True" Transmittance Percent Error in Transmittance (lOO At/t

)

0.5 0.6 0.8 1.1 1.4 1.9 2.3 3

0.1 3.7 4.6 6.0 7.5 11 14 21

0.01 10 12 16 22 29 35 56

0.001 19 24 32 43 57 70 110

0.0001 38 47 64 85 110 140 220

^Half-cone angle

The errors shown are average values. Frequently there was considerable

variation in the relative transmittance error from fog to fog with

these errors ranging from about half to twice the average errors given

in table 5.4- The half-angular field of view of the in-service 500-foot

baseline transmissometer with a 2.0 mm field stop in the receiver is 1.2

milliradians . The half-angular field of view of an in-service 250- foot

baseline transmissometer with a 3.5 mm field stop is 2.2 milliradians. The errors given in table 5.4^ column 5 (2.5 milliradians) are representative of

the errors which would be obtained with a 250-foot baseline transmissometer where the transmittances given in column 1 are considered

as the transmittance over 250 feet.

The effects of the errors shown in table 5.4 on the day visibility

computed using Koschmieder ' s law are shown in table 5.5.

5-28
Table 5.5

Error in Indicated Visibility (Day) Resulting from "Scattered

Light" Error in Transmissometer Measurements

(For a 500-foot baseline transmissometer)

Receiver Field

of View (Milliradians)^ 0.92 1.2 1.8 2.5 3»1 3-7 6.2

"True"

Transmittance Visibility (V) Percent Error in Indicated Visibility (100 AV/V)

0.5 2092 0.9 1.2 1.6 2.0 2.8 3.4 5.4

0.1 630 1.6 2.0 2.5 3.2 4.7 6.0 9.0

0.01 315 2.1 2.5 3.3 4-5 5.8 7.0 10.7

0.001 210 2.6 3.2 4.2 5.5 7.0 8.3 12.0

0.0001 157 3.6 4.4 5.7 7.2 8.8 10.5 14-4

^Half-cone angle

The errors shown in the column headed 1.2 milliradians are applicable to a 500-foot baseline transmissometer with a 2.0 mm

field stop in the receiver. Representative errors for a 250-foot

baseline transmissometer may be obtained from the column headed 2.5

milliradians if the visibilities shown in the second column are halved. The transmittances in the first column are then the transmittances over a 250-foot baseline.

Note that for a given visibility (not transmittance) the relative

errors in visibility for 500- and 250-foot baseline transmissometers

are not significantly different. Note also that at low transmittances,

although the relative errors in transmittance become large even for

small angular fields of vieW;, the relative errors in visibility for

small angular fields of view are acceptable. For a given transmittance

the relative error in night visibility or in day or night RVR is less than,

or equal to, the relative error in daylight visibility.

Two different detectors were used during the scattered-light

measurements. The phototube normally used in the transmissometer

receiver has an S-1 photocathode, which is red-sensitive.

5-29
Measurements were also made with a phototube having an S-4 surface,

which is blue-sensitive, to determine if any effects are due to

spectral sensitivity. These results are shown in figures 5.7 and

5.8 plotted as a function of transmittance . There is negligible

difference produced by the difference in spectral sensitivity.

As shown by these results, the amount of scattered light increased

as transmittance decreased, reached a maximum, and then decreased

as transmittance continued to decrease whereas At/t increases

continuously as the transmittance decreases.

Middleton also investigated the effects of scattered light,

both theoretically and experimentally, for systems involving a projector and receiver [91] . Making certain assumptions, he calculated the ratio of measured illuminance for four different sets of

conditions, depending on droplet radii, droplet density, and distance

between source and receiver (baseline). His theoretical results are shown in figure 5.9 as a function of receiver aperture half-angle ^.

The conditions for the three cases illustrated differ only in baseline length. 6 is the source beam spread half-angle.

The predicted errors were so large that Middleton checked his

results experimentally. Measurements were made at two periods during

a single dense fog with a visual range of about 125 meters. The

results, figure 5.10, show error as a function of receiver field

of view. The error is about 5 times that calculated for Case I,

probably because of a preponderence of smaller droplets. Middleton

suggests that a series of baffles would be necessary in any

permanent installation of a transmissometer to eliminate error due

to scattered light.

A comparison of the scattered light errors obtained by Middleton

with those obtained by NBS at Areata revsals significant differences,

as illustrated on figure 5.11. The higher values obtained by

Middleton can be partially explained as follows:

a) There is no Indication that Middleton corrected for background luminance. NBS turned off the projector and took direct

5-30
OJ> O

-o%c

O) U_ cu

1/11 u

S- -D 13 O) o

s_ 0) CO

s_ 1+- </) o M- ra M- ra JD -o

OJ

+->

U

l/l

OJ T3 S- O o

H- +-) to O o

+-> O)

+-> l/l •'—)

+-> o E O Si/i O) Q.

c:fO M- SOJ CD +J +-> s_ O) 4- o E O 14- OS c T3 CO o 0)

-l-> E

+->

O CO o QJ

S- ra

13 S- s_ 4- o +->

(/I o c c fO -o "O

O) CO

1-1 CD s-

-!-> o tu CL to E o

+-> un

<1 CL

C\J

•=d-1 CO oo

OlE OJ O) > 9-

S-u +-> C\J c

COc s-

+-1 tu tu

CO >

cn 1 OJ

OJ OJ CO 3 <_) 0) T3 S- OJ +-1 S- C|- o

O) o o

+-> rT3 +-> s 1 fO O) o u 1 oo > LT)

<3

5-31
03 s_

0)

1

+->

1 OJEo

0) 01 > 00

•1—

+-1 E

•r— 00 ^n c

c n3

Ol S- 01 +->

1 TD . <^ 0) C/) S- c

rO o T3

fO

S_

*^ •f—

-(-> 1

1

•f— O) E

o

LD

-M CNJ

+-> •1— II E

LO .—

n3

S- OJ

+-> 4-O OJ 0)c >

o

•T— <4- CD +J O 00 u n3 c XJ ^

:3 M- Ol -i-J O

ro Ll_ O

(/I 1 rO Ooo o * 0) LO

-!-> -\ 13

+-> o O Ol

-(-> u O^ 3

o o

s- oo

s_Ol 1 (/I fO

+-> O) "O

cn > OJoo3

-a Ol S_ OJ o

S- O) +-> O) 00 o

+-> i Ol

+-> re ^ o

o S- oo

CX3 on

OJ

S-
Figure 5.9

1.0

.9

.8

.7

.6

.5

.4

.3

2

.1

0

B = 1.00

8= 0.02

\

TT / Ji/

-/L /

1/

/

/

/

/

/

/

/ /

/

/

f Vf—r- 1 /

f /

/

/

/ /

1 r

/ 1. .-^"^

vl <

0 10 20 30 40 50

Scattered light error, At/t, predicted

by Middleton as a function of receiver

aperture half-angle ^ for source aperture

half-angle 6 where ^ is expressed in

mi Hi radians and e is in radians. Cases

I, II, and III differ only in baseline

lengths which are 1000, 500, and 200

meters. After Middleton [91].

5-33
Figure 5.10 Experimental values of the scattered light

error, At/t, as determined by Middleton at

two periods in a single fog. e = 0.30 radians.

^ in milliradians. After Middleton [91],

5-34
\

<

GCO

CD

O

UJ

cc

LU

<U

CO

1 2 3 4 5 6

HALF-ANGLE FIELD OF VIEW>, MILLIRADIANS

Figure 5.11 A comparison of scattered light errors obtained by

Middleton with those from Areata for different

source (e) and receiver (t|;) half-angle beam spreads

8 in radians, t^ = 0.03.

5-35
measurements of background luminance.

"b ) NBS obtained a higher number of data points

.

Stewart and Curcio carried out measurements of scattered and

directly transmitted light at distances of 2 miles along the shore

of Chesapeake Bay and 9 miles across the open Bay [116] . Halfangle fields of view for the shorter distance were varied between

43.6 and 218 milliradians . The half-angle field of view at the

larger distance was fixed at 0.118 radian. They investigated

scattering at various regions of the visible spectrum by mounting

Wratten filters in front of their detector. Data over the 2-mile

range showed a gradual increase in At/t toward the shorter wavelengths and a sharp increase at 360 nm over the 9 mile range. However, the baseline distances and receiver fields of view are so much greater than those of the NBS transmissometer that no conclusions can be drawn relative to transmissometer performance.

Additionally, their measurements were carried out at transmittances

ranging from 30 to 8^% which are much higher than those found in

fog.

The experiments at Areata showed that errors due to scattered

light are not as large as had been predicted. They are not

significant in present-day operations of the NBS transmissometer

which do not include, RVR conditions below about 600 feet. Extensive

baffling to reduce scattered light errors is unnecessary for today's

operations

.

Although the scattered light error is not significant for the

fog densities under which the tests were conducted, it will become

significant at very high fog densities. The effect of the scatteredlight error on the indicated visual range is independent of the

transmissometer baseline for a given transmissivity although the

scattered light error for a short baseline instrument will be lower

because of the higher transmittance over the short baseline [108]

.

5.1.4 Background Illuminance Errors

Some light from the background of the transmissometer projector

5-36
will enter the receiver and impinge upon the photoreceiver . In addition,

during periods of low transmittance, aerosols in the light path between the

projector and the receiver will scatter light from the surround into the

receiver. Usually the effect of this light on the accuracy of the transmission measurement is of concern only in daylight.

The effects of background illuminance are greatest in instruments which

use unmodulated light because then the output of the receiver is proportional

to the sum of the illuminance from the projector and the background

illuminance. (This problem in the NBS transmissometer is analyzed in

detail in Section 6.3.2.1). The effects of background illuminance can be

reduced by:

a. Restricting the field of view of the receiver to the smallest

angle compatible with the size of the projector and maintenance of receiver

alignment

.

b. Orienting the transmissometer receiver in a northerly direction

(in the northern hemisphere).

c. Using baffles and blackened surfaces in the receiver to reduce

interreflections. (See figure 6.4).

d. Using a projector of high intensity.

Other methods of reducing the effects of background illminance are the use of a modulated light beam and a tuned receiver, e.g., use of a projector

which emits short flashes of very high intensity and a receiver designed to

respond to these flashes. It should be noted that the use of these methods

does not eliminate the need to restrict the field of view of the receiver

since a restricted field of view is required to reduce the scattered light

error.

5.2 SCATTERING COEFFICIENT METERS

In many meteorological situations, almost all of the extinction

of a beam of light is due to scattering. When this is true, as with "clean" fogs, absorption may be neglected completely and the

scattering coefficient 6 may be substituted for the extinction

coefficient a. This being the case, equation (2.13) may be written

as follows:

5-37
T = e ^ (5.31)

where 3 is the (total) scattering coefficient. Thus, under these conditions visual range may be determined by measuring the scattering ( and

hence the extinction coefficient ) and applying these measurements to

Koschmeider ' s and Allard's laws, which may be expressed as:

e = e-^^ (4.07)

and

= I e """W. (4.22)

Since the concept of obtaining visual range through measurements

of scattering assumes that extinction of a light beam is due solely

to scattering, the use of such instruments should be avoided in

industrial regions or regions where significant absorption is probable

because of the presence of environmental pollution. Scattering co- efficient meters are more appropriate for use at sea, for example on aircraft carriers, because a) absorption is usually negligible over the open

sea, b) the uniformity of the atmosphere minimizes the consequences

of the small sample, and c ) the compact design permits use of an instrument of this type under conditions where a transmissometer with

the required baseline could not be installed.

Extinction coefficient meters may be grouped into three general

types: a) backscatter meters, b) sidescatter meters, and c) forward

scatter meters. These three groups will be discussed separately in

the Sections following.

5.2.1 Backscatter Meters

A backscatter meter consists of a light source and receiver

located nearly adjacent to one another. Both are oriented in the same direction and slightly inclined towards each other, which allows

the optical axes to intersect at a known distance. Typically, the

light source emits either modulated light or short pulses of light,

a portion of which is scattered back toward the receiver. The strength

of this backscattered signal is then correlated to atmospheric transmittance through a knowledge of the instrioment ' s calibration. A

5-38
simplified arrangement is shown in figure 5.12. The darkened portion

of the diagram represents the volume of air that is capable of scattering

light rays back into the receiver.

Since the basic components of a backscatter meter are the same

as those of a transmissometer , backscatter meters are sometimes erroneously

referred to as single-ended transmissometers

.

The ultimate usefulness of backscatter measurements depends on

the existence of some relationship between backscattered light and

atmospheric transmittance for actual atmospheric conditions. As previously

mentioned, the attenuation of light by the atmosphere is due to both

scattering and absorption, with scattering predominating in the visible spectrum.

The volume scattering coefficient is defined as the total amount of light

scattered by unit volume of air for unit volume of incident illumination.

Backscattered light is only a fraction of the total amount of light

scattered. The relation between light scattered in a given direction,

as is the case with the backscatter meter, and the total scattering co- efficient is not constant but depends upon the particle size distribution

of the scattering medium. An excellent analysis of this problem is

given by Twomey and Howell [119] who compared white and monochromatic

light (700 nm) as sources for measurement of backscatter in determining

visibility. They computed the ratio of reflectivity (backscatter

signal ) to extinction coefficient for the four possible separate combinations

of monochromatic and heterochromatic sources incident upon scattering media

both homogeneous and heterogeneous in drop-size. These computations

showed that increasing the wavelength band of the illuminating light

reduces the excursions in this ratio caused by the sharp maxima and

minima foimd in the Mie scattering coefficient and intensity function

for a single particle size and wavelength. Thus the use of a broadband source, such as a xenon lamp, is preferable to the use of a monochromatic source, such as a laser.

There have been several field studies of the relation between

atmospheric transmissivity and the response of backscatter meters in a real atmosphere. An early study was conducted by Born and Franz [14]

who, in the late 1920' s, made simultaneous measurements of atmospheric

transmittance and scattered light.

5-39
. n

O) O)

+-> c

0)E OJ

sS- o

<U E

t->

4-> 0) O

>) ^ I— a I—

fO =3 4- -O O r—

-M o

c S

OJE c

CD S

CD O

E -e

fO CO

S- to

fC -£= -M O fO

•I- Q.

4-5n3 O

E 5

<D +J JZa cu

I/) ^

-o

cu "

•1- cu

CU 1— 4-> Q- U fO E fO S-

•.- s- (C CO Q- cx

Lf)

CU

S-:3

5-40
The study by Curcio and Knestrick [23, 24] provided a practical

basis upon which backscatter visibility instruments depend for

useful measurements. Their experiments provided data from which the

relationship

MR = k S~^'^ (5.32)

was derived, where

MR is the meteorological range (obtained from transmittance

measurements assuming e is 0.02),

k is a constant, and

S is the backscatter signal.

Measurements were made correlating backscatter measurements with

transmittance measurements for a variety of conditions where the

meteorological range varied from less than 0.10 mile to more than

40 miles in atmospheres which were free of industrial pollution.

Their results are summarized in figures 5.13a and 5.13b. Curcio and

Knestrick concluded that the point spread about the curves indicates

that visibility can be determined from the backscattered signal with

an accuracy of 20^ for all visibilitities in the ranges studied.

This favorable result can be explained theoretically only on the

assunoption that the sizes of the scattering particles are so irregularly distributed in the atmosphere that their total effect

is to produce backscattering almost independent of the type of fog.

First order theory indicates that, for a given particle size distribution, the exponent of S should be -1. Hence, the value of -1.5

may be considered as an empirical correction for a' quasi systematic

change in particle size distribution with atmospheric clarity.

However, it should be noted that in their analysis the data were normalized to force a match at a visibility of 1.5 miles of

data taken at Areata, in meteorological ranges of 0.1 to 1.5 miles, and

of data in the range 1 to 40 miles, taken at the Naval Research

Laboratory. This normalizing would conceal any systematic differences

in the two sets of data. Close visual examination of the data in

figure 5.13b indicates an exponent in (5.32) somewhat less than

5-41
o

•r— -o

4JUC

Z2 o <+- •r—O

ns s-

(/) o n3cnO

^- -o

O)

c: c

fO '

+->

c: 0

cn

<u

to cnc

s- ro

OJ S- -M

+-> n3 (T3 O U • CO •1— 1—1

CDOO 0 00

ra 1— 1—1 0

S- -i^ 0 0 > O) -r- •r— +-> S-

-l-> <u +-> n3 E ^

CD

cu •4-C 0 :^

no

5-42
I/) O)

•r—E

tn

-M

fC "O

OiN

•p—

fOE

o

c

fO 1—

1

+-> CO

C\J -a 1 1 _J >

S-

:z T3 u

03 O)c

fO

-)-> fOO -M

S- cC cn

•r— M- (T3 O S-

+-> (U to +J

CO > O

Q. CT) E

o o o

CO

S-=3

nvN9is a3iivos>i3va 3Aiivn3a

5-43
-1.5 (about -1.7) for meteorological ranges above 1.5 miles and

somewhat greater (about -1.1 ) for the lower meteorological ranges.

Nevertheless linearity of the log-log curve over the wide range of

0.10 to 4-0 miles is surprisingly good.

Vogt conducted tests in Germany which demonstrated that backscatter devices indicate visibility equally well as other types of

visibility meters with an accuracy of about ±20^, provided that no completely abnormal distribution of aerosol particles occurs [121]

.

(Note that Vogt's estimate of accuracy coincides with that reported

by Curcio and Knestrick. ) Vogt used a commercially available backscatter device and compared its measurements with estimates of

visibility carried out by experienced observers. When the visibility

was estimated simultaneously by eight observers in eight different

directions, the average of these values seldom deviated from backscatter recorder determinations by more than ±25^ and the standard

deviation was only 12^

.

It should be noted, however, that a requirement for different

calibration curves for snow conditions and for rain is evident in a recent study of the Sterling Research and Evaluation Center [104]

•

Sweden has found it necessary to use three sets of calibration

curves—fog, rain, and snow—in their application of backscatter

meters to airfield operations [1131

•

Twomey and Howell [119] compared their computations of the

ratio of backscatter to extinction coefficient for one of their

combinations (white light and heteorogeneous scatters) with field

measurements made by Curcio, et al. Figure 5 . 14 shows the computed

relative backscattered signal R plotted against meteorological range.

Included in this figure are points (triangles) from a similar curve by Curcio. When this is compared to figure 5.15, which represents similar

conditions but with monochromatic illumination, it is apparent that

the correlation between backscatter and meteorological range is very

much lower when the smoothing effect of the spread in wavelength is

absent. Based on these findings, it is concluded that narrow band

light sources are inferior to heterochromatic sources.

5-44
0,5

0.4

0,3

R

0.2

0.1

A

T T

0.5 IjO Vm 1.5 2.0 kmmi

Figure 5.14 Backscatter signal as a function of

meteorological range for white light

(0.4y £ A ^ 0.7y). Key: 0, result

of simulation study; A data from

Curcio, Krestrick, and Cosden. From

Twoomey and Howell [119],

0.5

0.4

0.3

R

0.2

0.1

T T

0.5 1,0 Vm 1.5 20

km

mi

Figure 5.15 Backscatter signal as a function of

meteorological range for monochromatic

light (a = 0.7y). From Twoomey and

Howell [119].

5-45
The work reported above may not apply to all real particle

size distributions. Hence, when backscatter instruments are calibrated

by means of parallel transmittance measurements, calibrations should

be performed in a region where the mean particle size distribution

is similar to that in the intended area of operation.

Frequently the statement is made that inclining a backscatter

meter of the integrating type shown in figure 5.12 along a runway

glide path will yield a measure of the slant visibility. This concept

is very much in error. In fog scattering, nearly all the response of a backscatter meter is produced by scattering very near the instrument.

Curcio and Knestrick reported 90^ of the backscattered signal from the

first 100 feet in fog conditions [23] . In instruments which have been

analyzed at NBS, more than 80% of the response comes from scattering

within the two hundred feet nearest the instrument when the transmissivity

is such that the day visibility is less than one mile. If the instrument

was inclined at a glide slope angle of 3° the outer end of this

200 foot region would be only 10 feet above the instrument. Compare

this with a transmissometer baseline height of 15 feet.

It should be noted that Lidar or gated instruments, which measure the

backscattered signal as a function of distance from the instrument,

sample at a considerably greater distance than do integrating type

instruments. However, these instruments must be inclined at angles

considerably greater than the glide slope to obtain adequate signals up

to the maximum desired height.

5.2.2 Forward Scatter Meters

In forward scatter meters the receiver accepts light from a source which has been scattered in a near-forward direction as illustrated in

figure 5.16. In his classic study, Waldram [122] found that the

scattering coefficient in the direction of an angle of about 150°

between the axis of the beam and the axis of the receiver was nearly

independent of the particle size distribution of the atmosphere. Hence,

forward scatter meters are expected to be less sensitive to particle size

distribution than are backscatter meters. As stated by Waldram in discussing

his polar scattering coefficient curves; "A consideration of the curves

5-46
5-47
shows that in most cases the polar scatter index, for a given

scattering coefficient, is fairly constant in the region of 150

deg., but is very much more variable at the tail of the curve representing back scatter. This is simply because in most curves the

forward scatter is much greater; consequently the flux in a (say)

5-deg. zone is a maximum near 150 deg. The total flux represented

by the polar distribution, and consequently the scattering coefficient,

is therefore usually governed by the polar scatter index in this

region. There are however, some interesting consequences. It

follows that back scatter for a given scattering coefficient, may

vary widely. ..."

Waldram then reports that experiments which depended upon backscatter as a measure of the volume scattering coefficient were un- successful, but that later experiments based upon forward scatter

were more successful.

The current state-of-the-art is represented by the forward

scatter meter developed for the Air Force [4-2].^ (Shown in figure

5.16). The light from a halogen-cycle projector lamp is mechanically

chopped at 290 Hz before entering an optical system that projects a cone of light, with an inner cone masked out. A photodiode detector

is located 120 cm from the projector and receives light from a similar cone-shaped volume. The intersection of the projection

3 and viewing cones forms a sampling volume of 0.05 m ; projected

light scattered by particulates within this volume at angles between

20 and 50 degrees is accepted by the detector. Synchronous modulation is used in detecting the scattered light. The detected energy

is proportional to the extinction coefficient a assuming that

(a) forward scattering is proportional to total volume scattering,

and (b) atmospheric absorption is negligible relative to attenuation

by scattering.

This instrument has been thoroughly evaluated through comparisons

with both transmissometers and human observers. The feasibility models

were tested primarily at the U.S. Naval Radio Station, Cutler,

Maine in August, 1970 [100] . The prototjrpe and production models

^Manufactured by EG & G.

5-48
were tested primarily at three military installations in the fall of

1972 by comparison with transmissometers . Comparisons with visual

determinations was also conducted at one of the sites.

Results obtained at these three sites are shown in fig\ire 5.17

and table 5.6. Note that effects of inhomogeneities in fog were

minimized by time-averaging the data over five-minute periods and

data with RMS variations greater than ±15 percent were excluded

from the sample

.

Note also that the relation between meteorological range and

the forward scatter signal of this instrument is of the form

MR = kS'-^,

whereas Curcio and Knestrick (Section 5.2.1) obtained an exponent of

-1.5 in their study of back scatter. In addition, one straight line fits

the data of figure 5.17 very well. These considerations demonstrate

that forward scatter meters are less affected by changes in particle

size distribution than are back scatter meters.

Table 5.6

Correlation of Extinction Coefficients from Forward Scatter

Visibility Meter Against Transmissometer (based on logarithms)^

RMS

Station Station Number of Correlation Difference

Number Location Comparisons Coefficient {%) Slope

07 N. Ipswich 200 0.93 ± 20 0.92

10 Boston Hill 435 0.99 ±16 0.99

23 Nike Site 84 0.96 ±22 1.00

All combined 729 0.98 ±19 0.98

^From Muench, Moroz, and Jacobs [100]

.

5.2.3 Side-Scatter Meters

The design of side-scatter meters is based upon the work of

Beuttell and Brewer [8] . The principles of the design of instruments of this type are illustrated by figiire 5.18. The light source

is a perfect diffuser and the axis of the photometer is parallel to

the plane of the source. Under these circumstances the response of

the photometer is proportional to the total scattering coefficient, 3,

5-49
5-50
5-51
and hence, is independent of the particle size distribution of the

aerosols. Ideally, the rays limiting the scattering volume should

form an angle of 180°. However, this can only be approximated in

practice

.

Instruments of this type have been used in the study of air

pollution [1] , on board ships at sea [112] and an instrument has

been designed for airport and fog detector use; but apparently no instruments of this type have been adopted for operational use.

5.2.4 Error Analysis of Scattering-Coefficient Meters

5.2.4.1 Relative error in the visual range of objects

The following error analysis applies to all types of scatteringcoefficient meters, backscatter-meters, side-scattering meters, and

forward-scatter meters

.

From Koschmleder ' s law,

e = 0^6-""^, (^.lOa)

and

In C - In e = aV. (4.11a)

o Differentiating,

adV = -V da.

Then for small finite errors,

aAV = -VAa,

and

AV/V = -Aa/a. (5.33)

If atmospheric absorption is negligible, the extinction co- efficient, a, and the volume scattering coefficient, 3, are equal.

If the particle size distribution of the aerosols does not significantly

affect the response of the scattering meter, the instrument measures a. Hence under these conditions, the relative error in the visual range

of objects obtained from scattering type meters measurements is equal

to the relative error in the meter indication.

5-52
The negative sign indicates that a negative error in a produces

a positive error in V, the visual range. Note that the relative

error in the indicated visual range is independent of the visual range

itself and of the inherent contrast of the object viewed.

Note that equation (5.33) does not take into account the effects

of a non-uniform atmosphere, absorption;, and the effects of particle

size distribution.

5.2.4.2 Relative error in the visual range of lights

The analysis of the relative error in the visual range of lights

as a function of the relative error for scattering coefficient meters

is very similar to the corresponding analysis for transmissometers

.

From Allard's law

= Ie"^V^. (4.22)

Taking natural logarithms and transposing,

-aV = [2 In V - In I/E^] . (4-23)

Differentiating, holding I and constant, and rearranging terms,

adV + V da =-2 dV/V,

or

dV/V =-da/(a + 2A), (5.3 4)

or, for small finite errors

AV/V =-Aa/(a + 2/1). (5.3^0

As with the error analysis of transmissometers (Section 5.1.2.2),

an explicit expression for AV/V cannot be obtained. However, the

term (a + 2/V ) is always greater than a alone, since V is always

positive. Therefore for lights

AV/V<( -Aa/a ) . (5.36)

Thus for a given instrument error, AV/V, the error in the indicated

visual range of lights, is less than the error in the visual range of

objects and (5.36) may be taken as the limiting equation.

5-53
5.2.4.3 100% calibration errors In scattering coefficient meters

The problems involved in the 100% calibration of scattering

coefficient meters are quite different from those involved in the

100% calibration of transmissometers

:

a. Calibration cannot be accomplished by separate measurements of the output of the source and the sensitivity of the

receptor since the scattering volume common to the light beam

and the receiver field of view is not known with sufficient

accuracy. (However, see 5.2.4.'4b).

b. Satisfactory calibration cannot be obtained from observations of the prevailing meterological visibility for, as shown

below, the error in indicated visibility which is produced by

a calibration error is the same for all visibilities, and does not

decrease with decreasing visibility as is the case for a transmissometer, as shown by the following.

Let

be the "true" prevailing object visibility during

calibration and

c^ be the corresponding extinction-^ coefficient; computed

from Koschmieder 's Law, (4.07), and

be the estimated object visibility at the time of calibration, and

be the corresponding extinction coefficient.

Then, from (4.07),

o /a = V /V . (5.37) o' e e o \ ' ^ u

To determine the effect of this error in 100% calibration, let

a' be the extinction coefficient indicated by the

instrument during operational use and

a be the true extinction coefficient.

^ The term "extinction" is used in place of "scattering" in this discussion

as the principles are applicable to extinction as well as scattering meters.

5-54
Then,

a = a ' a /a , o e or

a = a' V /V . (5.38)

e o w /

If V is the object visibility indicated by the instrument, and

V is the "true" object visibility, then since

a' V = aV, .

from (5.38)

V'/V = Vg/V^. (5.39)

Thus, if during 100^ calibration by visual estimates, the visibility is estimated as 20 miles when the atmosphere at the

instrument site is representative of a 10 mile visibility, a visibility of 500 feet will be reported as a visibility of

1000 feet.

Note that the above equations are independent of the contrast

threshold, e. This means that the accuracy of the indication

is independent of the value used for e if the same value is

used for both calibration and service . ( Conversely, com- parisons of visual range observations with measurements of the

extinction coefficient by an instrument calibrated by visual

estimates cannot be used to determine the value of e. Apparently, this factor has been frequently overlooJked .

)

c. Once a stable reference scattering coefficient meter is

calibrated, it can, of course, be used to calibrate similar

instruments by comparison' in the laboratory or in the field.

d. It also is possible to calibrate scattering coefficient

meters using calibrated standard samples of a scattering

material, such as Teflon, mounted so that they may be

repositioned accurately. These calibration samples would

be calibrated on the reference instrument.

5-55
5.2.4.^ Methods of calibrating scattering coefficient meters

It is apparent that the primary problem in the 100^ calibration

of scattering coefficient meters is the 100^ calibration of the first, or reference, instrument. At present there are two methods which are feasible

:

a. Direct comparison with a transmissometer . Comparisons

should be made when the transmittance is low enough so that

the relative uncertainties in the visibility indicated by

the two instruments are roughly equal. On this basis the

optimum transmittance is 0.37 as shown in Section 5.4.

However, a transmittance this low may cause difficulties

because of non-uniformities in the fog as the scattering

coefficient meter would sample only a small part of the

transmissometer path.

b. If the instrument is sufficiently sensitive, it may be

calibrated by placing it in a chamber which contains an atmosphere (gas) whose scattering coefficient can be

computed from theory [1]. One such gas is clean dry carbon dioxide.

5.3 MISCELLANEOUS TRANSMITTANCE METERS

In addition to the two general classes of meters described

above there are some transmittance meters under development which

can be classed as hybrids. These instruments are single-ended

devices, intended for use as slant visibility meters, in which

light scattered from a small section of the light beam projected by

the light source of the instrument is viewed by the receiver.

One such instrument uses Raman scattering from the nitrogen • in the air induced by a pulsed ultra-violet (nitrogen) laser. The

intensity of the Raman scattering from the scattering volujne can be

computed from the characteristics of the laser source and the

characteristics of Raman scattering [90a]

.

Another type is- the "Lidar" type described by Herman [47]

,

Collis [22] and Brown [18] . In this type of instrument

5-56
the source is a pulsed laser, the receiver looks out along the beam

and is time-gated so that signals are accepted from scattering volumes

at different distances . Transmittance is determined from the change

in receiver output as a function of distance. These instruments

may be used as slant visibility meters, and fall outside the scope

of this report. See Section 5.2.1 for comments on the use of integrating type

baekscatter meters to indicate slant visibility.

A third type instrument which has been proposed for the

determination of RVR is a photoelectric instrument which scans a row of runway lights and counts the number visible [17] . No

reports of operational use of instruments of this type are available.

5.4 COMPARISON OF TRANSMISSOMETER AND SCATTERING

COEFFICIENT METER ACCURACY

It is useful to compare the accuracy of visibilities indicated

by scattering coefficient meters with the accuracy of visibilities

indicated by transmissometers

.

For scattering coefficient meters

AV/V = -Aa/a, (5.33)

and for transmissometers

AV/V - -(V/b In e) At^/t^. (5.09b)

If the instrumental errors are equal, the errors in the visibilities

indicated by the two types of instruments will be equal when

V = b In e. (5.40)

If a value of 0.05 is assumed for e,

V = 3b. (5.41)

When the visibility is three times the baseline, the transmittance over the baseline, b, is given by

t = e^^

Since b/V is 0.333,

0.37.

5-57
Thus, by day, if the visibility is less than three times the

length of the transmissometer baseline (the transmittance is less

than 0.37), the instrumental errors of the transmissometer have less

effect on the determination of visibility than do the instrumental

errors of scattering coefficient meters and for higher transmittances

the instrumental errors of scattering coefficient meters have less effect,

5.5 EFFECTS OF ERRORS IN ASSUMED THRESHOLD ON THE ACCURACY

OF VISIBILITY METERS

The effects of assuming a threshold inappropriate to the circumstances on the relation of the indicated visual range to what is

seen may be analyzed in a manner similar to that used in analyzing

the effects of instr\imental errors.

Let

Since

and

then

e be the applicable contrast threshold,

V be the corresponding object visibility,

e' be the inappropriate contrast threshold, and

V be the indicated visibility obtained using this

threshold

.

^ - b

V'/V = In e'/ln e. (5.^2)

If a value of 0.02 is chosen for e' and a value of 0.055 for e,

^'/^ ^ ^.02^^.055 ^ ^/^.055 ^ ^'^^ ^^'^^^

Similarly, if e' is 0.05 and e is 0.055

^.0/^.055

^

Also,

5-58
MR/MOR = 1.30. (5.45)

From this it is apparent that the indicated visual range of

objects is not overly sensitive to the value used for the contrast

threshold.

As before, when lights are involved, the analysis is more

complex because of the effects of the inverse square law and because

the effects are dependent upon the intensity of the light and the

fog density transmissivity as well. Analysis, shows that, at night, if

the illuminance threshold is changed by a factor f_, the percentage

change is visual range of nearly all lights is less than it would be

for an object by day if the contrast threshold were changed by the same factor. The change in the visual range of a lighM by day would be le

than the change in the visual range of an object only if the intensity of

the light is greater than about 20,000 candelas.

5-59
1
6. DEVELOTOfT AI^ APPLICATION OF THE 1^ TRAIffllSSQ^ETER

This chapter contains a brief description of the NBS transmissometer, an analysis of the theory of operation of the photometric

system, a discussion of the sources of error in the photometric

system, and an analysis of the effects of these errors on indicated

visual range.

6.1 PRINCIPLES OF OPERATION

NOTE: A detailed theory of operation of the electronics of

the NBS transmissometer is contained in the NBS-prepared Instruction

Book for Transmissometer Set AN/GMQ-10 [103] and in the manuals which

have followed it, and it will not be repeated here. However, a brief review will be given as an aid to understanding the analysis

of the photometric system.

6.1.1 General

The transmissometer is essentially a remote-indicating photoelectric telephotometer . It consists of three major elements:

(l) a projector operating at a fixed Intensity; (2) a receiver

consisting of a telescope of adjustable aperture and a photoelectric

unit which provides an output signal in the form of pulses; and

(3) an indicator with a dc output proportional to the pulse rate.

An elementary block diagram of the first transmissometer (19-41) is

shown in figure 3.1. The complete circuit diagram of this instrument

is shown in figure 6.1. As development preceded, circuits were

added or modified to permit transmission of the signal to a remote location, to calibrate the metering circuits, to improve

stability, linearity, sensitivity, and to provide an RYR readout.

However, the basic principles of operation have not been changed.

Figures 3.9 and 3.10 show the present equipment. A brief

description of this instrumentation is given below.

6-1
6-2
6.1.2 Projector (See figure 6.2)

The projector consists of a reflector lamp mounted in a castaluminum housing, the alignment of which is adjustable.

The lamp is designed for use in the Transmissometer Set. It

is a type PAR-64j sealed-reflector, lamp having a 6-volt, 120-watt,

single-coil filament and is designed to operate for 3000 hours. The

parabolic reflector of the lamp produces an essentially parallel

beam with an intensity in the peak of the beam of approximately

140,000 candelas. The beam spread at 95^ of peak intensity is

0.5° horizontal by 1° vertical and at 10^ of peak intensity it

is 5° horizontal by 10° vertical. A hemisphericjal shield within

the lamp prevents direct ( uncollimated ) light from the filament

leaving the lamp. A low-voltage, high-current filament is used to

obtain the maximum peak intensity consistent with long life.

The housing supporting the lamp can be rotated through a horizontal angle of 6° and through vertical angles of plus or minus

3° from the horizontal plane and then locked in place. These adjustments permit alignment of the lamp to within 0.02°. In service the

lamp is aligned so that the peak of the beam falls on the receiver.

This minimizes the effects of small shifts in lamp alignment and the

effects of background illumination.

A protective hood attached to the lamp housing extends beyond

the lamp to protect its cover from dirt and debris.

The lamp may be turned off manually from the indicator or periodically from the hourly-check timer in the projector power

supply to obtain measurements of the effect of background illumination

upon the transmittance measurement.

Power for the lamp is regulated by a resonant-circuit voltageregulating transformer which is frequency compensated when required. This tranformer maintains the lamp voltage constant to within

1% of supplied voltage when the input voltage to the projector is

within the range 95 to 130 volts.

6-3
o

I-o

UJo

Q_

(rUJ 1 t- inUJ

IRAN

ISSOM

CL ~ S O 2 o

<_>

U 2 I- <

to cc

-11 ^

Q- IT Q. O ^ \3 in UJ cr o %'

(TO

I-o _

UJ

-Jo

cr

Q- 5

tn

I?

Pox O o I- o > UJ =

to 5 UJ I'

Q.

Q.

$-O)o

Q.

S_o

+-> u

O)

•<—) o

Q.

(T3 o

4-> a

•-3 o

Qs-O)

+-> OJEo

I/) COc

4-> 4-O

(T3 s-cooo

6-4
A tapped stepdown transformer is used to reduce the output

voltage of the voltage -regulating transformer from 115 volts to the

low voltage required by the lamp. Five taps on this transformer

permit a selection of the voltage applied to the projector lamp.

These taps are in the primary of the step-down transformer so that

the effects of contact resistance in the voltage-selector switch

are minimized.

The resistances of the transformers and of the wiring in the

projector power supply change slightly with the operating temperature

of these parts. The projector lamp also requires some time to reach

stability. These effects combine to produce a downward drift in the

lamp intensity during the warmup period. The warmup period after the

hourly cutoff is one to two minutes in length.

If an unseasoned lamp is placed in operation, the intensity will

frequently increase for several days. The total increase in intensity

may be as great as 15%. To eliminate this increase after the lamp has

been placed in service, the lamp is seasoned at a current higher than

that used in operation. Seasoning for eight hours at 22.5 amperes or 48 hours at 20 amperes will stabilize the lamp.

After the lamp has stabilized there is usually a slow drift

upward in intensity until near the end of the useful life of the

lamp. The lamp then becomes unstable with erratic changes in intensity.

This instability is first evident as a small shift in transmissometer

reading after the lamp has been turned off for a background check.

The instability then increases within a few days until the transmissometer reading varies erratically over a range of about 10 percent of reading.

6.1.3 Receiver and Receiver Amplifier-Power Supply (See figure 6.3)

6.1.3.1 Optical System

The receiver is composed of a telescope which collects light from

the projector and directs it onto a photoelectric receiver (the pulse

amplifier) which produces a pulse signal with a pulse rate proportional

6-5
6-6
to the light falling on the photoelectric cell.

The receiver telescope consists of an objective lens 4 inches in

diameter with a focal length of 32 inches. Immediately behind the

objective is an iris diaphragm which provides a means of varying the

aperture of the system and hence controlling the light flux incident

on the photoelectric cell. The light from the projector is focused

on the opening in the field stop at the rear of the baffle system ( section

AA, figure 6.4-a). The field stop restricts the field of view of the

receiver to approximately 1.2 milliradians (0.07 degree) (half-cone angle) for a 500 foot baseline transmissometer and 2.2 milliradians

(0.13 degree) for a 250 foot baseline instrument. This restricts

the size of the field at the projector to an area only slightly

larger than that of the projector lamp. This reduces the Illumination

received from the background and from scattered light from the

projector itself. A secondary lens is placed immediately behind the

field stop. This lens forms an image of the objective lens on the

sensitive surface of the photoelectric cell. This is done so that the

position of the spot of light on the photoelectronic cell does not change

with changes in receiver alignment. This lens also provides a seal

between the pulse-amplifier unit and the telescope tube.

The set of baffles mounted ahead of the field stop in the tube at

the front of the pulse amplifier reduces further the stray light in

the system. The baffles are so arranged that light entering the

telescope from most directions other than axially must be reflected

several times in order to reach the photoelectric cell. The baffles

and the interior of the telescope are finished with a dull black paint

so that a high percentage of the light is absorbed at each reflection.

A protective hood is mounted in front of the lens to protect the lens

from dirt and to reduce the amount of stray light entering the system.

Two heaters and a blower are provided to keep the objective lens

free of condensation and the hood free of snow.

The image of the projector lamp will not fall in the center of

the opening in the field stop of the receiver telescope system as shown in figure 6.4-a unless the center of the projector lamp, the

center of the objective lens, and the center of the field stop all lie

6-7
IRIS DIAPHRAGM

ASSEMBLY

BAFFLE TUBE

FIELD STOP

a. ALIGNED

Sec. AA

ENLARGED

b. COMPLETELY MISALIGNED

c. PARTIALLY MISALIGNED

d. EFFECTS OF BACKGROUND

Figure 6.4 Alignment of the receiver telescope,

6-8
on the same straight line. This condition is obtained by shifting the

position of the field stop with respect to the line through the centers

of the lamp and the lens by means of alignment screws at the rear of the

telescope. When the image of the projector lamp falls in the center

of the opening of the field stop as shown in figiire 6.4-a, small changes

in alignment with the resulting small changes in the position of the

image will not affect the light falling on the photoelectric cell as long as the edge of the image does not touch the edge of the opening

in the field stop. When the field stop cuts the image of the lamp

as shown in figure 6.4-c, any slight change in alignment and, hence,

in the position of the image, will change the light falling on the

photoelectric cell and consequently will alter the transmittance

reading. No stand can be made completely stable, because of the

effects of small ground movements with changes in soil conditions

and the effects of non-uniformities in stand temperature produced

by solar heating. Hence, the opening in the field stop is made sufficiently

larger than the image of the projector lamp so that the ordinary small

shifts in alignment do not cause the field stop to cut the image.

6.1.3.2 Pulse Anpllfier

THEORY OF OPERATION. A stable light source and a sensitive, linear,

and stable pulse amplifier are the prime requirements of the NBS

transmissometer . Therefore the theory of operation of the pulse amplifier and its interaction with the projector will be analyzed in detail.

The pulses in the receiver are generated in the following way

(see figure 6.5). The current from the phototube, VlOl, charges capacitor

ClOl raising the potential of the starter anode of the trigger tube,

V102, to about 160 volts where an arc discharge is initiated in this

tube. During the discharge the grid potential decreases to about 100

volts, partially discharging capacitor ClOl. When the discharge through

the tube is extinguished, by the action of R102, the current from the

phototube again starts charging ClOl and the process is repeated. The

time of discharge is very small, 10 microseconds, in comparison to the

time of charge, 15 milliseconds, or more.

The constancy of the voltage change across capacitor ClOl for

6-9
6-10
each discharge is determined by the trigger tube V102. At pulse rates

slower than about 5 pulses per minute, the voltage change is usually

constant within the limits of measurement. At higher rates there is

sufficient variation in this voltage drop from discharge to discharge

to produce a noticeably erratic pulse rate in some tubes. It has been

found, however, that the voltage change when averaged over a number of

pulses is constant and does not change significantly with the pulse rate.

The grid voltage necessary to initiate a discharge in the trigger tube

is a function of the plate voltage of the tube. There is a range of

plate voltage in the region of 255 volts in which the grid voltage

required to initiate the discharge is independent of the plate voltage.

The plate voltage of the trigger tube is therefore regulated at 255

volts by voltage-regulating tubes in the receiver amplifier.

To reduce the leakage currents over capacitor ClOl, the capacitor

is molded of a high-resistance red bakelite. To reduce the surface

leakage over the bakelite, it is coated with ceresin wax. In addition,

the low side of the capacitor, which would ordinarly be connected to

ground, is kept at a potential of 150 volts. Thus the average voltage

difference across the capacitor is approximately -20 volts instead of

130 volts. The envelopes of the phototube and of the trigger tube are also treated with ceresin wax to reduce the effects of humidity on the

leakage over the glass surfaces. The connection between the anode of

the phototube, the starter anode of the trigger tube, and the

high side of the capacitor ClOl is supported by the tubes themselves

so that leakage to ground Gan occur only over the envelopes of these tubes

and the case of the capacitor.

A second type of leakage is the leakage within the envelopes of

the tubes themselves. In photoelectric cells this leakage generally

produces a current across the tube even when the tube is dark and less

frequently produces a leakage of the photoelectric current to ground.

In the first case, the pulse amplifier will continue to pulse at a low rate ( one pulse in 10 seconds or slower ) when the receiver is

dark. In the second case the pulse amplifier may stop pulsing when

the light on the photoelectric cell is reduced to that level for

which the pulse rate should be one pulse per second or less. Leakage

6-11
within the trigger tube generally produces the same effect as described for the second case above. In the transmissometer

set, tubes VlOl and V102 are so selected that the "dark current" of

the photoelectric cell will not produce a pulse rate faster than

one pulse per minute and the internal leakage of the two tubes

is small enough that pulse rates as low as one pulse per minute can be obtained as the light on the photoelectric cell is reduced

sufficiently. Using especially selected tubes, well cleaned, a receiver can be operated with photoelectric currents of the order of

-12

2 X 10 ampere corresponding to a pulse rate of one pulse in

15 minutes when the capacitance of ClOl is 20 picofarads.

TEMPERATURE CONTROL. To reduce the effects of temperature upon

the sensitivity of the tubes, a thermostatic switch and a heater operating

on line voltage are installed in the receiver.

LINEARITY. The time required to produce a given change in the voltage

across capacitor ClOl varies inversely with the current through the

phototube. Therefore the degree to which the pulse rate is proportional

to the flux incident on the phototube depends upon the following factors:

(l) the degree of proportionality between the phototube current and the

flux incident upon its cathode, (2) the leakage currents, (3) the constancy

of the change in voltage across capacitor ClOl for each discharge, and

(4) the degree to which the time of discharging is negligible as compared

to the time of charging capacitor ClOl. The phototube is of the highvacuum type and is operated at a voltage sufficiently above the saturation

voltage (the minimum voltage across the tube being approximately 75 volts)

that the photocurrent is proportional to the incident flux and is substantially independent of the applied voltage. The treatment with

ceresin wax and the selection of tubes reduces the leakage current to

a low value. The time for discharge of capacitor ClOl is less than one thousandth of the time of charge, and Trigger tube V102 is operated at a voltage which produces stable operation. (A detailed discussion of errors

in the photometric system is given in Section 6.3.

)

STABILITY. It will be noted that the circuitry of the pulse

amplifier of figure 6.5 differs from that of figure 6.1. The type 929

phototube of the first transmissometer was replaced with a type 919

5-12
phototube to obtain greater stability and lower leakage and dark

currents. (The type 919 phototube was later superceded by a special

tube designed for the transmissometer which is a variant of the

type 919.) The neon lamp, type Ne-2, which was the pulse generator

of the first transmissometers was replaced by the type WL-759 for

several reasons. Careful and tedious selection was required to obtain

neon lamps of sufficiently high leakage resistance. The drift in sensitivity of the neon lamp was greater. Illumination of the electrodes of

the neon lamp was required in order to obtain a stable pulse rate.

Very careful adjustment of this illumination was required to obtain the

stable pulse rate without producing an excessive leakage current. The

type WL-759 uses a trace of radioactive material to obtain a stable

pulse rate. This tube had previously been used as a photoelectric integrator at slow pulse rates by Nottingham [105] and by Kuper, Brackett,

and Eicher [80] , but not at the low photoelectric currents or the pulse

rates of the transmissometer.

6.1.3.3 Amplifier-Power Supply.

The amplifier-power supply (figure 6.3) for the receiver provides

regulated B+ for the pulse amplifier, and amplifies the pulse signal for

transmission to the indicator. A metering circuit which measures the pulse

rate is included to facilitate alignment and calibration adjustments of the

receiver and the projector.

6.1.4 Indicator and Recorder (See Figure 6.6)

6.1.4.1 Arrpllfier

The indicator contains a two-stage amplifier, a pulse-rate measuring

system, and a calibrator providing pulse rates of line frequency and

one-fifth line ' frequency . Before being sent to the metering stages, the

pulse signal is amplified by a two-stage, resistance-capacitance-coupled

voltage amplifier. This amplifier is designed to improve the signal-tonoise ratio of the pulse signal by providing greater amplification of

strong signals than of weak signals and greater amplification of high

frequencies than of low frequencies . Thus the voltage amplifier gain

for a 60 Hz signal is O.O4 and the gain for a 20 KHz signal is 9.0.

6-13
COo

S-o

+-> u

•r- -o

s-cu

+-> 0)Eo

CO

00

to

s-

+-> M-o

(_) o

6-14
6.1.4.2 Pulse Integrator

The pulse integrator consists of a thyratron used as an electronic

switch discharging a charged capacitor once, and only once, for each

pulse inpressed upon the control grid. This charge is transferred to a smoothing capacitor discharging through a fixed resistor. When the system

is in equilibrium, the voltage drop across this resistor is proportional

to the input pulse rate and to the value of this resistor. Range changing

of the indicator is accomplished by switching resistors.

6.1.4.3 Voltmeter Section

In order to maintain linearity, the capacitance of the capacitor

which discharges through the electronic switch is kept small. (Sec. 6.1. 4.6).

Hence, the average current flowing through the electronic switch is too

small to operate a rugged meter or recorder directly and direct-current

amplification is necessary. This is accomplished by the voltmeter section

which operates from the voltage signal produced by the current through the

electronic switch and produces an output current varying between zero and one milliampere . Since this current is proportional to the average

current through the electronic switch, it is proportional to the input

pulse rate.

6.1.4.4 Calibrator

A calibrator stage is used to provide a pulse signal of a rate equal

to the line frequency, (or one fifth line frequency, as desired), for use

in correcting sensitivity drifts of the indicator. With an indicator

scale graduated 0 to 100, the sensitivity is adjusted so that when a pulse frequency of 60 Hz is applied, a meter reading of 90 is obtained.

Since the receiver is adjusted to produce a pulse rate of 60 Hz

when the transmittance is 0.90, the indicator meter is direct reading,

indicating transmittance in percent.

6.1.4.5 Recorder

A recorder provides a continuous record of the output of the indicator.

6.1.4.6 Indicator linearity and stability

The linearity of the indicator depends primarily upon the degree

to which the charge which passes through the electronic switch for each

6-15
incoming pulse Is Independent of the pulse rate and upon the degree to

which the response of the voltmeter section is proportional to the

average current through the electronic switch, and the linearity of the

output meter, or recorder. To obtain linearity, the time constant in the

electronic switch is kept small in comparison to the minimum time

interval between pulses, their ratio being about 0.1, and the voltmeter

section is essentially a cathode follower.

6.1.5 II luminometer

An illuminometer, installed to measure the ambient horizontal

illuminance, is used to determine whether the day or night RVR, or RW,

scales should be used. Typically the scale change occurs at an illumin- ance of 2 footcandles. The illuminometer is usually a simple

photoelectrically actuated relay having an accuracy of about 10% in the

switching level.

6.1.6 Runway Visual Range Converter

In runway visual range systems the indicator described above is

used only as a monitor and a special unit is used to indicate the run- way visual range directly. This unit accepts the transmissometer pulse

signal either directly from the signal line to the receiver, or,

preferably, after the signal has been filtered and amplified by the

indicator amplifier, and counts the number of pulses in a fixed time

interval in the range 4-5 to 55 seconds.

The units are essentially memory banks from which the RVR appropriate to: a) a given number of pulses, b) the day or night position

of the Illuminometer switch, c ) the brightness setting of the runway edg

lights, and d) the transmissometer baseline, is hooked up and displayed.

6.2 THEORY OF OPERATION

6.2.1 General

The pulse rate of this system is

r = i/c(E^ - E^), (6.01)

or

6-16
r (6.02)

where

r is the pulse rate in pulses per second.

i^ is the average current, in microamperes;, charging

capacitor ClOl (and the distributed capacitance in

parallel with ClOl ) during the charging part of the

cycle,

c is the capacitance, in microfarads, of ClOl plus

the distributed capacitance in parallel with it,

is the potential, in volts, of the starter anode of

the trigger tube, V102, at the time a discharge in

the tube is initiated,

is the potential, in volts, of the starter anode at

the time the discharge is extinguished, and

is the sensitivity of the trigger tube.

Consider first the ideal condition in which the charging current

i^ is either equal to i, the current generated in the phototube by the

light from the projector, or in which any extraneous currents in the

charging circuit are very small in comparison to i. (This is frequently

the condition on clear, dark nights.) The photoelectric current is

given by the following relation (see figure 6.7).

I is the intensity, in candles, of the projector,

A is the area of the receiver objective, in square

feet, exposed to the flux from the projector,

Sp is the sensitivity of the phototube, in microamperes

per lumen, for light of the color temperature of the

projector,

T is the transmittance of the optical system of the

receiver,

i (lASpX/b ) t^ (6.03)

where

6-17
6-18
b is the length of the path, in feet, between the

projector and the receiver, and

is the transmittance of the path between the projector

and the receiver.

Since for the conditions now under consideration i^ is equal to i,

t-^ = r(cb^/IASpS^T), (6.04a)

or

t^ = rS (6.04b)

where S, the sensitivity of the photometric system is given by

S = cb^/IASpS^T. (6.05)

It is, of course, desirable that the sensitivity S be adjusted

so that the pulse rate of all transraissometers will be the same for

any given transmittance. For reasons which will be evident later, a pulse rate of 60 pulses per second has been selected for a transmittance

of 0.90 (equivalent to 4000 pulses per minute for a transmittance of

1.00).

Then

or

0.015, (6.06)

cb^/IASpS^T =0.015, (6.07)

and

t^ = 0.015r. ' (6.08)

Consider now the factors affecting S. Except for minor changes

resulting from cleaning the dirt from the components of the optical

system, the transmittance of the optical system, x, is beyond the

control of the operator. The sensitivities of the tubes, and

Srp, are characteristics determined during the manufacturer of the

tubes and cannot be controlled by the operator except by the

selection of tubes. The intensity of the projector, I, is determined

largely by the design of the unit but may be varied somewhat by

6-19
changing the voltage applied to the lamp. The value of the charging

capacitor can he varied by changing ClOl. (Because of the very high

leakage resistance which is required, it is not practicable to use a variable capacitor for ClOl. ) The exposed area of the receiver

objective, A, can be varied continuously by means of the iris diaphragm.

With the exception of the length of the baseline (and possibly the area of the lens opening), it is not feasible to determine the values

of the terms on the right hand side of (6.05) to permit the use of this

equation in the adjusting of the sensitivity.

Instead S is adjusted empirically by changing the value of one

or more of the parameters in (6.07) to obtain the desired pulse rate

at a measured or estimated transmittance. Coarse adjustments may be

made by changing the lamp intensity, I, or the capacitance of the

changing capacitor. Fine adjustments are made by adjusting the exposed

area of the objective lens by means of the iris diaphragm.

6.2.2 System Calibration

To facilitate the interpretation of the transmissoraeter readings,

it is desirable that the instrument be adjusted so that, with the

indicator on the low sensitivity range, a reading of 100 would be

obtained if the air were perfectly clear. Since the air is never

perfectly clear, this adjustment can not be done directly. The most

frequently used method of making this "100^ setting adjustment" is

the method of Section 5.1.2.5. With this method, an estimate is made

of the visibility at a time when the air is relatively clear and uniform

the transmittance over the transmissometer baseline corresponding to

this visibility is determined from appropriate tables or curves based

on equation (2.07) using, in the U.S., a value of 0.055 for the

contrast threshold. (See Section 3-2.)

In order to overcome the inaccuracies which are inherent in the

extrapolation method, the National Bureau of Standards developed (in

1961) a calibrator based upon the principles of Section 5.1.2.4.b [291.

This instrument is a portable photometer consisting of a barrier-layer

6-20
photocell with a stray-light shield, a telescoping stand for supporting

the cell, and battery operated indicating unit using a zero resistance

circuit. See figure 6.8.

The design of the instrument is based upon the fact that the

atmospheric transmittance can be obtained from the measurement of

the illuminance from a reference projector at two different distances

if the atmosphere is uniform over the range of distances used. The

results are independent of the sensitivity of the receiver and of the

intensity of the projector; these quantities need not be known if

they remain constant during the calibration.

The transmissometer projector is used as the reference light.

A portable photoelectric photometer is used to determine the illuminance at the two distances. The greater of these distances, b, is

the transmissometer baseline. The shorter distance, d, is made one- fifth of this distance. These two positions will be referred to as the FAR and the NEAR positions, respectively. The distance to the

NEAR position is sufficiently large that the illuminance from the projector

follows the inverse square law. The two points of measurement and the

projector are colinear. Therefore,

and

where E is the illuminance, I the intensity, and t^ and t^ are the

transmittances over the paths indicated.

These equations can be solved simultaneously for t^, the transmittance of the transmissometer path, using the relation

t t- 0.2

d 'b

Therefore,

[(E^b^)/(E^d'')]

1.25

(6.09)

In operation the receiver was positioned and aligned at the NEAR

6-21
a

z:<i

I—<^ 2

CD 2 2 — O 3 o+->

S-U

S-<D

O)Eo

(/) CO

n3 S 3

3: S

00

cn

6-22
position and the indicator sensitivity adjusted to obtain a fixed reading.

The receiver was then moved to the FAR position and the transmittance

read directly from a specially calibrated potentiometer scale. (See

figure 6.8). With careful operation, the transmissometer 100^ setting

could be made with an expected uncertainty of about 1.5^.

Calibrators of this type were purchased by the Air Force from a commercial source, but none were purchased for civil use.

The NBS calibrator had some significant deficiencies.

a. The field of view of the receiver was made large so that there

would be no alignment difficulties when it was used on a telescoping rod at the NEAR position. As a result the effects of

illuminance from the background were so large that the

calibrator could not be used during daylight.

b. The illuminance at the FAR position was less than one- twenty-fifth of the illuminance at the NEAR position. This

placed severe requirements on the linearity of the instrument

and careful laboratory adjustment was required to compensate

for any non-linearity.

c. The method requires a stable, uniform atmosphere. Hence it

could not be used in fog where temporal and spatial changes were

significant. (It could, however, be used in visibilities of the

order of one mile (night)).

All of the deficiencies of the NBS calibrator are overcome in the

second generation of calibrators. In these calibrators, a laser (developed

after the NBS calibrator) is used as a source (Method c of Section

5.1.2.4 )• The aperture of the receiver is made large enough to accept

the entire beam of the laser at the FAR, or receiver, position, as well

as at the NEAR, or source, position.

If the receiver is placed at the source and adjusted to read 1.00,

it will then directly indicate the transmittance over the transmissometer

baseline when it is moved to the transmissometer receiver for

6-23
(6.10)

where

is the laser flux incident on the calibrator

o when it is at the source, and

is the flux incident on the calibrator when

it is at the transmissometer receiver.

Then if the instrument in linear,

where R and R, are the calibrator readings at the source and receiver, respectively.

When the sensitivity of the calibrator is adjusted so that R^ is

equal to 1.00,

The iris diaphragm of the transmissometer receiver is then adjusted

to obtain a reading on the transmissometer the same as that of the calibrator .

In principle, a calibrator of this type could be used under all

visibility conditions. However, at high fog densities, accuracy is

limited by the meter readouts of the Calibrator and the Amplifier Power

Supply. When conventional moving-coil meters are used, these errors may

be as large as 0.01. See Section 6.3.3. The relative error in subsequent

transmittance measurements is given approximately by

(6.11)

where

AR

a

is the error in reading the Calibrator meter,

is the error in reading the meter of the Amplifier

Power Supply, and

is the calibrator reading, the transmittance over the

baseline, at the time of calibration.

6-24
6.2.3 Effect of Changes in Baseline

As seen from equation (6.07) the required sensitivity of the transmissometer varies as the square of the length of the baseline. The NBS

transmissometer, which uses a PAR 64 type sealed-reflector lamp having

a peak intensity of about 140,000 candles when operated at design voltage, was originally designed for use on a 500-foot baseline.

If the baseline is increased, the over-all sensitivity, S, must

also be increased in order to maintain a reading of 100 when t.^ is 1.00.

(For a 750-foot baseline, the sensitivity must be more than double that

required for a 500-foot baseline.) This increase in sensitivity can be

accomplished by decreasing the capacitance of ClOl, by operating the

lamp at somewhat above design voltage, by using tubes of higher than

average sensitivity in the pulse amplifier, or by changing the indicator

sensitivity so that a greater reading is obtained for a given pulse rate.

If the baseline is not longer than 750 feet, changes in the capacitance

of ClOl are usually sufficient. However, the increase in sensitivity

which can be obtained by this means is limited. Therefore, if the

baseline is longer than 750 feet, other methods of increasing the

system sensitivity must be used. Since, as indicated by equation (6.19),

increasing the length of the baseline results in an increase in the

background reading unless the intensity of the projector is increased, the change in system sensitivity is usually accomplished

by increasing the intensity of the projector. This requires the use

of projectors of larger aperture than the PAR 64 lamp used in the

regular projector. Projectors using precision parabolic reflectors and

250-watt, 12.5-volt, C-8 filament, projection lamps have been satisfactory.

The lamp is usually operated at a voltage which will give a lamp life

of about 1000 hours . The recommended maximum lengths of baseline for

projectors of various apertures are given in table 6.1.

Table 6.1

Reflector Aperture Required for Several Baseline Lengths

Reflector Aperture Recommended Maximum Baseline

( inches ) , ( feet

)

12

18

24

1500

2500

4000

6-25
Baselines longer than those listed in table 6-1 may be used if the

indicator sensitivity or the receiver aperture is increased and careful

attention is given to the background correction. The sensitivity of the

indicator may be readily increased by simple circuit changes so that,

instead of obtaining a reading of 1.5 when the pulse rate is one per

second, a reading of 15 or more is obtained. On the other hand, increasing the aperture of the receiver would require rebuilding of the

receiver telescope.

Conversely, when baselines shorter than 500 feet are used, the

sensitivity requirements of the instrument are less severe. The re- duction in sensitivity can be obtained by using any of the following

singly or in combination:

1. An increase in the value of ClOl, thereby decreasing

the sensitivity of the receiver. This is the preferred

method since it also reduces the effects of both background

illumination and leakage currents

.

2. Reduction of light source intensity by operation at a lower

voltage. Such a reduction will increase lamp life significantly

and may be used in combination with method 1. Redesign of the

instrument to use a smaller lamp is not desirable since the effects

of background and leakage would not be reduced.

3. Reduction of the receiver aperture either by closing the

diaphragm or redesigning the optical system of the receiver.

This method would reduce the effects of background illumination

but not the effects of leakage currents, and, hence, is not as desirable as method 1.

4. Use of phototubes (VlOl) and trigger-tubes (V102) of

lower sensitivity. This method has the same effects as method

3, and, hence, does not appear desirable. However, some tubes

of the types now in service, not satisfactory for 500 or 750

foot baselines, would be acceptable when using baselines shorter

than 500 feet.

6-26
6.2.4 Methods of Modifying the Transmissometer System to Permit

Its Use During Periods of Very Low Runway Visual Ranges

This section is a brief updating of reference [26]

.

As aircraft landing minimums are reduced, the transmissometer pulse

rate becomes a limiting factor during periods of low visibility. For

example, the minimum pulse rate with a 250 foot baseline transmissometer

for the range of transmittances reported as a nighttime RVR of 600 feet

is about one pulse in 5 seconds, assuming a reporting interval of 1£)0

feet and, therefore, an actual RVR of 550 feet.

Therefore, if RVR's lower than 600 feet are to be reported, some modification of the transmissometer-RVR converter system is required.

The following methods of modifying the system are analyzed in this report.

1. Increase in length of time interval in which pulses

are counted by the converter.

2. Modifications of the RVR converter to measure the time

interval between pulses.

3. Increase in the pulse rate of the transmissometer by

increasing its sensitivity.

4-. Reduction of the length of the baseline.

Figures 6.9 and 6.10 have been prepared to assist in the interpretation

of the effects of these modifications. Figure 6.9 shows the relation

between transmittance and RVR as a function of the length of the baseline.

In preparing this figure an intensity of 10,000 candles and a threshold

illuminance of 2 mile candles have been assumed in computing the RVR.

These curves represent nighttime operation with the lighting system at

full intensity. Figure 6.10 shows the relation between seconds per pulse

and RVR as a function of length of the baseline. It was prepared from the

data of figure 6.9 assuming a pulse rate of 4000 pulses per minute for

a transmittance of 1.00.

It is apparent from the figure that the reduction in minimum RVR^

*The RVR's referred to in this discussion are the actual minimum RVR's

which could be indicated. Thus they are the lower limit, not the center,

of a reportable interval.

6-27
RUNWAY VISUAL RANGE (Feet)

Figure 6.9 Transmi ttance as a function of RVR (night-step 5) for several

lengths of baseline.

6-28
RUNWAY VISUAL RANGE (Feet)

Figure 6.10 Pulse period as a function of RVR (night-step 5) for several

lengths of basel ine.

6-29

I
which can be accomplished by increasing the length of the time interval

during which pulses are counted is very limited. For example, with a 250 foot baseline transmissometer, changing the counting time from 1 to

5 minutes would reduce the minimum measurable RVR from 480 to 400 feet.

Similarly modifying the RVR converter to measure the time interval

between pulses would be only a little better. A pulse interval of

100 seconds would be required to obtain an RVR of 400 feet . Increasing

transmissometer sensitivity would be more satisfactory.

An increase in sensitivity of the present 250 foot baseline

transmissometer by a factor of 10 is feasible. This increase would reduce

the minimum RVR to 350 feet. The pulse rate corresponding to a transmittance of 1.00 would then be 40,000 pulses per minute.

Greater increases in sensitivity could, of course, be obtained with

a complete redesign of the instrument. However, to obtain a minimum

-9

RVR of 200 feet, corresponding to a transmittance of 6.6 x 10 over a 250 foot baseline, a pulse rate of 1.52 x 10 pulses per minute would

be required, assuming a minimum permissible pulse rate of one pulse

per minute. Transmission of pulses at this rate would require coaxial

transmission lines.

Shortening the baseline will produce a significant increase in the

pulse rate in very dense fogs as is demonstrated by figures 6.9 and 6.10.

The lower limit of operation as a function of the length of the baseline

is summarized in table 6.2.

6-30
Table 6.2

Minimum Measurable RVR of Present NBS Transmissometer

and RVR Converter as a Function of Length of the Baseline

Length of Baseline Minimum Measurable RVR in ^

the Reportable Value

( feet } (feet

)

1000 1700^ ^

750 1300^ ^ ^

500

'

250 500^-^^

100 220^-^^

50 120^-^^

(2) 250 360^ ^

(2) 100 170^ ^

(2)

50 90^ ^

^"'"^ Based on a pulse rate of 4-000 pulses per minute when t^ = 1.00

(2) Based on a, pulse rate of 40,000 pulses per minute when t-j^ = 1.00

Note: In computing the lowest reportable RVR, a 200 foot increment is

assumed for RVR's of 1000 feet and higher. A 100 foot increment is

assumed for RVR's below 1000 feet. Thus, RVR's in the interval 900

to 1100 feet would be reported as an RVR of 1000 feet.

The primary difficulties involved in the use of a shorter baseline

are the reduction in the length of the sample measured and the decrease

in the maximum RVR which can be indicated with satisfactory accuracy.

There are several possible methods of compensating, at least in part,

for these difficulties; Among these are: (l) the use of two transmissometers "end-to-end", (2) the use of a dual-baseline transmissometer,

and (3) the use of a composite baseline transmissometer.

6-31
If two transmissometers are installed with their baselines end-to-end

and their outputs combined so that the product of the two transmittances

is indicated, the problem of decreased length of sample will be eliminated.

It should be noted that the two transmissometers need not be located so the two baselines fom a continuous line. The two baselines need not,

and should not, be precisely colinear. A deviation of several degrees

in the orientation of baselines is desirable to reduce the effects of

the light source of one on the receiver of the other. Nor are

the two baselines required to be so located that one starts where the

other ends. An interval as long as one or two baselines may be left

between the end of the first baseline and the start of the second. This

procedure would require a more complex computer than is now used.

Another means of improving the sampling is the use of a dualbaseline transmissometer, that is, two receivers at different distances

from a projector. A switch-over device would be included so that the

longer baseline is used whenever the fog density is such that operation

with this baseline is feasible . ( Such instruments were used in the

initial transmissometer installation at Nantucket and also at the Landing

Aids Experiment Station. ) Because of the limited field of view of the

receiver, the use of two receivers with a single projector is preferable

to the converse. Although this method requires less instrumentation and

would be less costly than the "end-to-end" transmissometers, it is

considered less desirable for the following reasons

:

(a) During periods when use of the short-baseline section is required,

a less adequate sample of fog is used with this method.

(b) This method provides a less satisfactory means of indicating the

variations in a non-homogeneous fog.

( c ) There is no redundancy in the instrumentation during periods when

the use of the short baseline is required. The only redundancy in

good weather (when the long baseline should be used) is the

possibility of using the short baseline should the long-baseline

receiver fail.

6-32
This method would, however, provide more accurate RVR Indications

for the higher RVR's than would the "end-to-end" transmissometers . If

the length of the "long" baseline is made more than 250 feet, this

method would provide higher accuracy during periods of high RVR's than

does the present 250 foot instrument. It should he noted that, if the

"long" baseline is increased to more than 750 feet, a projector of

higher intensity than the present projector would be required unless a more sensitive receiver is used. \

A composite baseline transmissometer is similar to a double-baseline

instrument using two projectors, except that in this instrument the two

projectors are operated simultaneously. The primary advantage of this

system is that no switch-over between baselines is required. However, its

accuracy would be inferior to that of a double-baseline instrument for

all RVR's.

Note: A careful study should be made of the frequency of very low

RVR's before a choice is made of the method of modification. NBS experience

at Areata indicates that very dense fogs are surprisingly infrequent.

During a three year period, no RVR's below 300 feet were observed and

there were only 25 minutes during which the RVR was in the 300-400 foot

range [25a]

.

6.3 ERRORS IN THE PHOTOMETRIC SYSTEM

AND THEIR EFFECTS ON THE INDICATED

VISIBILITY

6.3.1 General

Consideration will now be given to the errors in the photometric

system and to the effect of these errors on the measurements of transmittance and on the indicated visibility.

As previously shown, by day the indicated visibility, V, is obtained

from Koschmieder ' s Law, which may be written as

6-33
(V/b) In t = In (5.10)

where e is the contrast threshold of the observer;

and from this it follows that, if At is small,

AVA = -(V/b) (1/ln e) (At/t). (5.09b)

Therefore, the relative error in the indicated visibility is proportional

to the ratio of the indicated visibility to the length of the baseline.

Hence, if reasonable accuracy is to be obtained in the indicated visibility

in clear weather, the requirements of instrumental accuracy for short

baseline transmissometers are very stringent. For example, when the day

visibility is about 10 miles, instrumental errors of 0.15^ and 0.3% with

250 foot and 500 foot baseline transmissometers, respectively, will

produce an error of 10^ in the indicated visibility. On the other hand,

when the day visibility is about one quarter mile, instrumental errors of

5.5^ and 11^ are required to produce an error of 10^ in the indicated

visibility.

The analysis which follows is based upon NBS tests and analyses [28]

and performance tests conducted at NAFEC by Hochreiter and McCann [<44] •

Whenever possible, quantitative statements of errors of the latter

study are used as the instruments used in this study are considered more

representative of instruments now in service than those used in the earlier

NBS studies, since some of the electronic components and circuitry have been

modified over the years. There are, however, no significant, or unexplainable, differences between the NAFEC and NBS data.

The analysis is based upon the perfomance of a transmissometer

whose components are operating properly and which meets the performance

criteria of Section 6.5 of reference [103] > given in Appendix D. Note

that the requirements stated are minimum requirements (maximum errors),

not representative values.

The errors in the transmissometer may be conveniently divided into

two general classes: (l) those in which At is constant and independent

^ For the sake of clarity, the subscript b is omitted through this Section.

6-34
of transmittance; and (2) those in which the error, At, in the transmittance measurement is proportional to the transmittance. Since the sources of these errors and their effects on the indicated visibility

differ, the two classes will be considered separately.

6.3.2 Constant Incremental Errors.

Consider now the effect of errors which are independent of the

transmittance of V, that is, At is constant. Equation (5.09b) may be

written as

The quantity t Int approaches zero as t approaches zero and as In t approaches zero ( t approaches one ) . Hence the relative error

in the indicated visibility produced by a constant At will become large

in either very foggy weather or in very clear weather. This effect is

illustrated in figure 6.11 where the indicated visibility corresponding

to transmittances equal to t ± At is plotted against the indicated

visibility corresponding to the transmittance, t. Figure 6.11 is directly

applicable to a 500 foot baseline transmissometer . It may be

applied to transmissometers of other baselines by multiplying the

coordinates of the visibility scales by 0.002 times the length of the new

baseline

.

It may be easily shown, by differentiation of (6.12) with respect to

t, that the effect of constant errors is a minimum when the transmittance

of the path b is 37 percent. The corresponding daylight visibility is

0.3 mile when a 500 foot baseline is used. See Section 5.4-

Sources of constant errors are (a) extraneous currents in the charging

circuit of the pulse amplifier, resulting from dark current, leakage

current and from illumination of the phototube by light from the background, and (b) reading out errors. These will now be considered in detail.

The extraneous currents are independent of the atmospheric transmittance and produce relatively constant errors

.

The charging constant in the pulse amplifier, i^, is the sum of

the following:

AV/V = At/(t Int). (6.12)

(6.13)

6-35
0.1

1

1.0 " TRUE VISIBILITY (MILES )

1 1 1 1

10

1

IC

1000

I 1

5000

RVR-OAY (Feet) - STEP 5

1 1

1

10000

1000 III i5000 10000 RVR-NIGHT(Feet)-STEP 5

1 1 1 1 1 1 1 1 1 0.0001 t 0.01 0.1 0.001 0.5 0.9 0.95 TRANSMITTANCE - tgQQ

0.99

Figure 6.11 Effects of constant incremental errors in transmi ttance mea- surements on the indicated daytime visibility. Also shown

are associated daytime RVR and transmi ttance scales. Although

shown only on the abscissa, these scales also apply to the

ordinate. All computations are based on a 500 foot baseline.

6-36
Where i is again the current of the phototube generated by the

light from the projector,

i is the current of the phototube produced by the light

s from the terrain or the sky behind the projector,

i^ is the dark current of the phototube (Note that i^ will

be negative if the leakage between the cathode of the

tube and ground exceeds the leakage current between the

cathode and the anode. ),

i-^ is the leakage current across ClOl and from the grid of

the trigger tube V102 to ground.

If i is i^ when t is unity, then

t = i/i . ' o Let t' be the apparent transmittance.

Then

t' = i^/i^, (6.14)

and, since

t' - t = At,

= (^s " ^d - ^L^/V (^-l^a)

or At = At + At, - At^ (6.15b)

s d L

where

and

At = i /i , s s o

At-, = i-,/i , d d o

At-r = iy/i

L L o

If the sum i and 1 is greater than i-^ , the receiver will generate

S Q J-i pulses at a rate proportional to At when the projector is turned off. The

incremental error. At, is then numerically equal to the transmittance

corresponding to this reading. No pulses will be generated with the

projector off when At is negative. Instead, the instrument will indicate

a transmittance of zero for all values of t which are numerically less than

At.

6-37
6. 3. 2.1 Errors prcduced by background illumination.

During daylight hours i is usually large in comparison to either

i, or Ij . From equation (6.03) we have

d J-j

ior= lAS^T/b^. (6.16)

Hence

At^^ = i^b^/IASpT. (6.17)

The photoelectric current generated by the light from the background

is given by

i^ = LASpT (6.18)

where ij; is the half angle of the field of view of the receiver

in radians and

L is the average luminance of the background within this

field, in candelas per square foot.

Therefore

At = i)^ Lb^/I. (6.19)'

s

The performance requirements of Appendix D limit the pulse rate caused

by background illumination to one pulse per second, corresponding to a At of 0.015. To obtain this low value, is made as small (about

s 0.0022 and 0.0013 radian {half-cone angle) for 250 foot baseline and

500 foot baseline transmissometers respectively) as is consistent with

the stability of the receiver mounting; wherever possible the receiver

is so oriented that L will be as low as possible; and a projector having

a suitably high intensity is used. Note that when the length of the

baseline is increased, the projector intensity must be increased as the

square of the length of the baseline, if At is to be kept constant.

s 6.3-2.2 Errors produced by dark and leakage currents.

At night the phototube dark current, i^, and the leakage current, i^^,

-sually gres

( 6 . 16 )

, we have

are usually greater than the background current, i . Substituting from

s

6-38
At^ = i^b^/IASpT , (6.20)

and

At^ = i^b^/IASpT. (6.21)

These equations show again the desirability of using projectors of

high intensity and phototubes of high sensitivity. Note that keeping the

receiving aperture large will reduce At^ and At^ but will not effect

At . In order to be able to use the iris diaphragm at nearly its

s maximum aperture, it is often desirable to adjust the sensitivity of

the receiver by means of ClOl so that the diaphragm must be nearly fully

open before equation (6.07) is satisfied.

The performance requirements of Appendix D state that the sum of

i^ and i^ produce a pulse rate no greater than one pulse in 30 seconds,

corresponding to a At^ plus At-j^ of 0.0005.

Since the effects of the phototube dark current and the triggertube leakage current are opposite, at first thought matching of tubes appears

to be advantageous. However, unless the dark current of the phototube and

the leakage current of the trigger tube are very nearly equal, the

incremental error, At, is not affected significantly. Since both the

dark currents and the leakage currents of the tubes of a group extend

over wide ranges, it is improbable that tubes with nearly equal extraneous

currents can be found. Experience has indicated that the net error

resulting from these currents can seldom be made less than one-fifth the error produced by the current of either tube. Therefore, for critical

applications, it is essential that the extraneous currents of both tubes

be low.

From figure 6.11 it is evident that when the visual range is low,

these errors, At^, At^, and At-j^ will not have a significant effect on

the indicated visual range until t approaches At in value.

When the visibility is high, only At is sufficiently large to

produce a significant error, and this error may be reduced significantly,

if desired, by measuring At with the projector off and subtracting At

s s from t '

.

6-39
6.3.2.3 Reading Out Errors

Reading out of the transmittance determined by the transmissometer

may be accomplished in two ways: by producing meter deflection which

is proportional to the pulse rate, as is done by the indicator; or by

timing the pulses . The errors in the use of the indicator will be

considered first.

These errors may be conveniently divided into three groups:

(l) those resulting from deviations from linearity in the conversion of

pulse rate to output current in the pulse-rate integrator and voltmeter

stages; (2) errors in the indicating meter or recorder; and (3) errors

in the reading of the indication of the meter or the recorder.

Repeated tests with signals whose pulse rate is known accurately show

that the electronic circuitry is sufficiently linear so that the linearity

of the output meter, or recorder, is the limiting factor. These meters

usually have a rated accuracy of 0.5% of full scale at any place on

the scale. Since the indicator is adjusted to give a reading of 0.90 when

the pulse rate is 60 pulses per second, absolute accuracy is not a significant factor, but linearity and repeatability are.

The meter and recorder scales are 50-division scales which can be read

easily to one-fourth of a division or 0.5% of full scale. The expected

error resulting from both meter inaccuracies and reading errors is, then,

about 1% of the full-scale meter reading. Therefore, At is about 0.01

when the indicator is on the low sensitivity range and about 0.002 when

it is on the high sensitivity range. However, since in making the 100^

setting, the over-all sensitivity of the instrument is adjusted so that

a full-scale meter reading would be obtained if the air were perfectly

clear, errors due to the meter are reduced for deflections near full

scale so that the expected error in this region is about 0.005 when

the low sensitivity range is used. Since the high sensitivity range

is adjusted to five times the sensitivity of the low range when the

readings are near full scale, the error in this region is about 0.001

when the high range is used. The curves of figure 6.11 indicate that

these errors do not produce significant errors in the indicated visibility except when the visibility is greater than about 5 miles or less

than about 0.1 mile, for a 500 foot baseline instrument.

6-40
When an RVR converter is used, the counting interval is timed by

reference to line frequency. Thus the accuracy of the timing interval

is determined by the accuracy to which line frequency is known. The

frequency of commercial power sources typically does not differ from

line frequency, 60 Hz, by more than 0.05 Hz, and the frequency of

engine generators typically is within 1 Hz of line frequency. Thus the errors in the timing interval for commercial power and engine generators

are expected not to exceed 0.1 and 1.6 per cent respectively.

There is also a possibility of an error of as much as one count in

the number of pulses counted as the number of pulses in a fixed

interval is counted not the time for a fixed number of pulses. From

equation (6.08) it follows that when pulses are counted for a fixed

time interval

t = 0.015 N/Z.

where N is the number of pulses counted in the time interval Z, in

seconds

.

Then

At = 0.015 A N/Z (6.23)

As stated earlier AN is equal to one. If the couating period, Z, is

50 seconds. At is 0.0003.

It is apparent from figure 6.11 that the readout error of the

converter is significant only at very low visibilities, where the

readout error is, indeed, the factor which determines the lower limit

of RVR measurements at night.

Note that if an integral number of pulses is counted, there need

be no error in N. With appropriate electronics, the time interval, Z, can be measured easily to an accuracy much better than required. When

this method is used, a sufficient number of pulses should be counted so that the time interval is long enough to average out the random

fluctuations in the pulse rate of the trigger tube. Thus, by using

this method, very low transmittances can be determined with the same precision as can the higher transmittances.

6-41
5.3.3 Constant Relative Errors

Relative errors are errors which are proportional to the transmittance. Thus, the relative error, At/t, is equal to a constant k.

Equation (5.12) may then be written as

AV/V = -k(V/b)(l/lii£). (6.24)

Note that In e is negative. Therefore, AV and At are of the same sign.

At any given instant the relative error in the transmittance measurement is equal to the relative error in the system sensitivity and

approximately equal to the sum of the relative errors in the parameters

of equation (6.24). Thus, as these parameters are independent of

atmospheric conditions, changes in _ their values are sources of constant

relative errors. (Note that this is based upon the assinnption that

there is no significant difference between the charging current, i^, and

the photoelectric current, i.)

6.3.3.1 Projector

Tests at NAFEC, using a voltage regulating transformer, of the

type currently in use in the projector power supply, showed that a change in pulse rate of about 2.4^ resulted from an input voltage change

of lOOV to 125V. Drift in the intensity of a seasoned lamp during its

service life is low, usually less than Q>.% per week, and is usually

upward. The effects of the collection of dirt on the projector face

is highly dependent upon the condition of the grass cover of the airfield, the proximity of jet blast, and the weather. It is, however,

seldom greater than 2% per week.

6.3.3.2 Receiver

The sensitivity of the receiver is independent of the input voltage

over the range lOOV to 125V. The sensitivity of the pulse-amplifier

unit gradually decreases in use, primarily due to decay of sensitivity

in the phototube. Initial drift in phototubes has been sufficiently

large so that seasoning is required. The rate of change is expected

6-42
to be exponential and after 1000 hours of seasoning and service to be

less than 1% per month. The change in sensitivity of a thermostatically

controlled pulse-amplifier unit is of the order of 0.2% per 10°F. The

effects of dirt on the receiver lens will seldom reduce the sensitivity

by more than 1% per week.

6.3.3.3 Indicator

Drifts in the indicator sensitivity are not a significant factor

since the indicator may be calibrated by reference to line frequency

at the operator's discretion. The drift seldom exceeds 1% per day.

6 . 3 . 3 • ^ Illuminometer

The only effect the illuminometer has is in the selection of day

and night scales. The estimated accuracy in the calibration of these

instruments is 10^. An error of this magnitude will typically produce

a shift in the switch over time of the order of only two minutes at

30° latitude and a shift of five minutes at 50* latitude.

6.3.3.5 System

Errors in the 100^ setting resulting from errors in the estimate

of visibility used to make this setting have been discussed in detail

in Section 5.1.2.4. Experience indicates that the error in the 100^

setting will be less than 3% with a well maintained transmissometer,

with the 100^ setting based upon past records, but may be much larger

if the 100^ setting is based upon a single estimate during a period

of restricted, -non-uniform visibility conditions

.

6.3.4 Summary of Effects of Errors

The estimated limits of imcertalnty in transmittance measurements

produced by the several parameters of the transmissometer photometric

system are given in table 6.3.

In order to compare the relative errors with the constant increment errors, the relative errors are converted to incremental errors by rearranging equation (6.24) so that

At = kt. (6.25)

6-43
Table 6.3

Summary of Estimated Maximum Uncertainties

Estimated^

]\/laximum At

Constant increment errors due to:

Background

Day

Night

Leakage plus dark current

Read-out

Indicator

Converter

Relative errors due to:

Dirt on lenses

Projector intensity

Phototube sensitivity

Temperature effect on pulse amplifier

100^ setting

+ 0.015

+< 0.00001

± 0.0005

± 0.01

± 0.0003

- 0.03t

± 0.005t

- 0.002t

± O.Olt

± 0.03t

^Based upon the assumption that routine maintenance is performed

weekly.

6-44
When t is near unity, the relative error in the indicated visibility

can be shown to be approximately

AVA = At/(l-t). (6.26)

by expanding the denominator of equation (6.12).

If (AV/V) is the maximum permissible relative error in the ^ max

indicated visibility, the maximum transmittance, t , at which the

max

transmissometer is useful is

t = 1 - At/(AV/V) . (6.27)

max max

Since At is usually less than 0.03 when t is near one, after cor- rection has been made for At , the relative error in the indicated

s visibility will become greater than 10^ when t becomes greater than

0.70 and the day visibility becomes greater than 10 times the baseline.

Similarly, from equation (6.12) it is obvious that the minimum

transmittance, t . , at which the transmissometer is useful is given by ' mm

t . = - (1/lnt) (At/(AV/V) ). (6.28) min max ^

'

Therefore, in this region the relative error in indicated

visibility will not exceed 10^ until t becomes less than about 3At.

The over-all effect of all instrumental errors is that the errors in indicated visibility will be significant only at the extremes

of visibility; clear air in which the visibility is more than 10 times

the baseline, and in very dense fog, in which the visibility (by day) is

less than about one-half baseline. This is clearly indicated in figure

6.11.

It should be noted that the errors tabulated in table 6.3 are maximum errors and that it is highly improbable that they would all be

in the same direction. Note also that in restricted visibilities,

when the transmittance is 0.1, or less, the effect of the relative

errors is small in comparison with the background error. Thus by day,

the background illuminance is the factor which determines the minimum

transmittance at which the system will operate.

6-45
By night, the minimum transmittance, t . , based upon the readout mm

error of the converter and the leakage and dark currents is about 3

times 0.0006 or 0.002. With this transmittance, the effect of all other

errors is insignificant.

In the discussion above, a tolerance of ±10^ has been arbitrarily

chosen for the maximum permissible error in the indicated visibility.

When the transmissometer is applied to RVR measurements, the accepted

tolerance is usually taken as one reportable value. Table 6.4 has been

prepared to show the error in transmittance ( At ) required to produce

a shift to a higher or lower reportable value for several indicated

RVR's. The increments between reportable values are those given in

Tables A3-11B and A3-11C, Federal Meteorological Handbook No. 1, Surface

Observations (1971) [36]. They are: for 500 foot baseline instruments - 200 feet for RVR's in the range, 1,000 to 4,000 feet and 500 feet for

RVR's in the range 4,000 to 6,000 feet; and for 250 foot baseline instruments - 200 feet for RVR's in the range 600 to 3,000 feet and 500 feet for

RVR's in the range 3,000 to 6,000 feet. The values in the second

column, t , are the transmittance values corresponding to the RVR

values listed in the first column. The colimnn headed ARVR lists the

changes in RVR, and the corresponding errors in transmittance At,

required to produce a change to the next higher or lower reportable

value for the RVR's listed in column 1. In calculating the transmittances,

t , corresponding to the RVR's listed in column 1, a source

exact,

intensity of 10,000 candelas (step 5), a day threshold illuminance of

1,000 mile candles, and a night threshold illuminance of 2 mile candles

were used.

6-46
Table 6.4

Errors in Transmittance (At) Required to Produce a Change in RVR (ARVR) Which Will Produce a Shift to the

Next Reportable Value

250 Ft. Baseline

Night

RVR exact ARVR At At/t( % )

600 0.00470 - NA -0.00336 71

+ 100 + .00658 140

1000 .0518 - 100 - .0166 32

+ 100 + .0190 37

2000 .271 - 100 - .0214 7.9

+ 100 + .0207 7.7

3000 .448 - 100 - .0148 3.3

+ 250 + .0345 7.7

4000 .567 - 250 - .0257 4.5

+ 250 + .0234 4.1

Day

RVR exact ARVR At At/t(^)

600 0.0626 - NA -0.0326 52

+ 100 + .0412 66

1000 .245 - 100 - .0473 19

+ 100 + .0457 19

2000 .588 - 100 - .0239 4.1

+ 100 + .'0221 3.8

3000 .751 - 100 - .0117 1.6

+ 250 + .0262 3.5

4000 .836 - 250 - .0170 2.0

+ 250 + .0149 1.8

6-47
Table 6.-4 (Continued)

500 Ft. Baseline

Night

RVR

1000

2000

3000

4000

t

exact

0.00268

.0732

.200

.322

ARVR

- NA

+ 100

- 100

+ 100

- 100

+ 100

- 100

+ 250

At

-0.00145

+ .00232

- .0111

+ .0117

- .0130

+ .0129

- .0112

+ .0271

At/t( % )

54

87

15

16

6.5

6.4

3.5

8.4

Day

RVR

t

exact ARVR At At/t(^)

1000 0.0599 - NA -0.0209 35

+ 100 + .0245 41

2000 .346 - 100 - .0275 7.9

+ 100 + .0265 7.7

3000 .564 - 100 - .0175 3.1

+ 100 + .0166 2.9

4000 .700 - 100 - .0109 1.6

+ 250 + .0251 3.6

6-48
6.4 CONSIDERATIONS OTHER THAN INSTRUMENTAL ERRORS

6.4.1 Scattered Light Errors

In a fog, a telephotoraeter such as the transmissometer receiver

will receive, in addition to the light which passes directly from the source to the receiver, some light which, though radiated by the source,

would not reach the receiver in clear weather. This scattered light

constitutes an error in the transmittance measurement as shown in Section

5.1.3- Under conditions of dense fogs this error may become

large. The error is minimized by keeping the field of view of the

receiver and the beam spread of the projector small. Measurements made

at Areata indicate that when the field of view of the receiver and the

beam spread of the projector are no larger than those in the NBS

transmissometer, these errors do not cause a significant error

in the indicated visibility under present operating conditions as shown in table 6.5.

Table 6.5

Average Scattered Light Errors

for a 500 foot Baseline Transmissometer

"True" Percent Error Percent Error

Transmittance in Transmittance in Day Visibility

0.5 0.8 1.2

.1 4.6 2.0

.01 12 2.5

.001 24 3.2

.0001 47 4.4

Note the very small error in day visibility corresponding to the

rather large scattered light error in the transmittance measurements at

low transmittances . Errors in the visual range of lights would be

slightly less.

Although the scattered light error is not significant for the fog

densities under which the tests were conducted, it will become significant

at very high fog densities. The effect of the scattered-light error on

6-49
the indicated visual range is independent of the transmissometer "baseline

for a given transmissivity although the scattered light error for a short

baseline instrument will be lower because of the higher transmittance over the short baseline [108]

.

6.4.2 Sampling Errors

The foregoing analysis is based upon the performance of the transmissometer alone. No consideration has been given to the variation in

transmittance in time and location nor to whether the sample measured

by the transmissometer is representative of the atmosphere in general.

Experience indicates that, in general, the "sampling error" is the

limiting factor and that the differences between indicated and observed

visibilities resulting from sampling differences are larger, and often

much larger, than those resulting from instrumental errors. The extent

of variations in transmittance with time and place is illustrated in

the following examples of transmissometer records. These records were obtained by NBS from transmissometers located at Areata, California.

The fogs there are predominantly advection fogs. The locations of some

of the instruments on the field are shown in figure 6.12. Figure 6.13

is an aerial view of the field showing the terrain around the visibility

test area.

Figures 6.14, 6.15 and 6.16 are examples of simultaneous chart

records obtained with 250 foot baseline transmissometers T-Ll, T-L2,

and T-L3. Time is shown along the bottom of the charts, running from

right to left, and the transmittance over the transmissometer baseline

is shown across the chart with each major division representing an increment

in transmittance of 0.10. Runway visiial ranges, based upon intensity

step 5 day operation, as a function of the transmittance over a 250 foot

baseline are given in table 6.6 as an aid in studying the significance of

temporal and spatial variations in transmittance.

6-50
6-51
O CO

to

•I- ra E S

(/) £=

n3 SS-

+-> M-O

O)cr 4->

6-52
6-53
-a

>, o

+-> c

•I- Z3

COO

CDO

-a s-CL O M4- O 3 3 3 I I ! I ! I

•r- 4->

to -I— c o o a.

to

•r- "r— S- +J

fO c: > O) cn-i- c

•r- to O OJ ^ -l-> to +->

to I

T3

S- <U o jt: O f—

<u

S- 3 3 3 O to

sO) d) I— +J Q. O) E E

fO o X c/1 OJ (/) ! I I 1 I <c to

to +-> I—I O) ! 1 - +-> CO

C\J M- I O

CM

c\J

m

S- o

<u -I- ^ XJ +-> • O (C to -M U OJ o o o O I— c O 3 3 5

MO +-> -1- U. n3 "O I I OJ

6-54
I * ' i [" 1 i

-I I ! | ! 1 I 3 2 J I - . I 3 i 1 1 CZ o

fO

(_) o' c

O)

+-> -a

4-> CO

S- >> OJ

4-> +J

•p— +-> to O) c

O)O

M-c E

CO •r— £1 +-> O

•r-

+-> +-> rc 5

CO >

CD 1

^S O o

-C T3

CO CX

O) (O

s_ Q.E o

fO CO X

OJ

(13

CO

«JD S_

CT> <D I—I +-)OJ " E ^ O

CO

S- I/) a; -1-E^ E w • CU c +-> > 13 to O O) ^ +-> M- OJ +J o -c cr -M -i- cnO <+- 4-

U- o o

6-55
Table 6.6

Relation between Transmittance over a 250 foot

Baseline and RVR (Step 5, Day)

^250

0.02

.05

.10

.20

.30

.40

.50

.60

.70

.80

Simultaneous measurements were also made at Areata with two

end-to-end 250 foot baseline transmissometers ( TD-1 and TD-2 ) located

near the touchdown zone of runway 31, as shown in figure 6.12.

The records of these two instruments during a five hour fog are shown in figure 6.17. For ease in comparison the records of the two

instruments have been placed on a single chart. In obtaining these

records, the transmissometer recorders were operated with a chart

speed of 12 inches per hour, instead of the usual speed of three inches

per hour. Again time runs from right to left.

Runway visual ranges have been computed from these transmittance

records using, for convenience, a 75-second integrating period. The

results of these computations are given in figure 6.18. A comparison of

figures 6.17 and 6.18 demonstrates the value of a suitable integrating

period.

An example of a record of a fog of extreme variability obtained

with a 500 foot baseline transmissometer installed in the touchdown

zone is shown in figure 6.19. Such fogs are not rare at Areata.

RVR

( feet

)

500

550

700

900

1100

1400

1700

2000

2600

3500

6-56
Figure 6.17 Simultaneous records of two back-to-back, 250 foot baseline

transmi ssometers at Areata, California, October 31, 1965.

The solid line corresponds to transmi ssometer TD-1 and the

dashed line to TD-2 as shown on figure 6.12. Times, which

run from right to left, are shown at the bottom of charts,

RVR scales for LS-5 day at each end. Distance between

parallel "vertical" arcs on chart represents 75 seconds.

6-57
+J

O)

O) <D MfC

+->

u o cu

ea CO

O AO

to >^'^ j3

S- fO

CU

+-> > 1/1 CD " o E LO ^ > o 03

(/) Q.

t/) O) X>

•I- +J CU O) E CO -)-> u

to •4-> E <U O +-> a

%- 4-> Q. fO

+-> S- C to Q. ^ o o > E

fT3 -O •r- 1 to +-> O fT3 to

-)-> JD 1 th ea ^ to S- O -1- OJ cu

+-> sz o >o cv: «\ -M

to

4-> .,

to 1

E LD -o cn • 1

O U3 jn

s- cri o •r— 4- .—t

(J to

QJ M- .(

to to

O > o un CU -M cn U

•1-> S- E CU

n3 O) to fO ••—) c: fO S- jCi

•1- o O E +J

SU T3 (O E

CD

O o 13

O 4-J •1— to

cu •> -o

X3 fO cu > CU Q. to q; c <u > s-o c +-> cu

to -t— +J E S- 3 I— fO o fO O fO s_ O) o cn-a +-> E" cu cu cu fO •> -i-j to cu

•)-> ro E fO tt- 1— +-> JD ^ fO o E O CU CU

•I- x: s_ oo =t 1—

00

cu

C7^

6-58

<+- (T3 CO O) o

O)

sOJ E

to T3 -M

S_ E

o o 13 O) a O) -M

-M cu 10 n3 d; fO O

JD

fO -o

+-> +-> > me o

o

ca

CD •(

M- -l-> E -M =£ s- <U o s- x: o +->

-)-> +-> LO 03 X

O) s rO

er of

<+- O) E +-> CO o O o <u E

E o

(V M- o +J o

CO -M

Q.+-> o CO O

E OJ JU 'e 'i

X I/) CO cr> OJ (T3 c:

res +-> 03 O

s- Cl. < -M O -M 03

CTi

OJ

S-CD

3Nn3SV8 J.OOJ 005 3nvOS MOT-AVa

(S31IIN ivoiinvN) AiniaisiA

6-60
6.4.3 Spectral Sensitivity of Phototubes

Note that the pulse amplifier contains no filter to modify the

spectral response of the phototube. This apparent omission has caused

concern to some, for example Middleton [95] . At first thought,

it would appear that such a filter should be provided so that the phototube-filter combination has a spectral response similar to that of the

eye, that is, the CIE standard observer luminous efficiency function.

However, it should be noted that if the transmissivity of the atmosphere

does not change with wavelength throughout the waveband in which the

eye and the transmissometer receiver respond, the spectral sensitivity

of the transmissometer is immaterial. Moreover, when Koschmieder ' s and

Allard's laws, in their usual form, are applied to situations where the

visual range differs significantly from the length of the transmissometer

baseline, an atmosphere which is nonselective is tacitly assumed. Otherwise Allard's law would take the form

760

(6.29)

or 760

(6.29b)

where

is the spectral intensity, at wavelength A,

of the source being viewed.

A is the spectral sensitivity of the eye

at wavelength A,

is the transmissivity at v/avelength A,

is the transmittance over the baseline b at wavelength A,

and

X is the distance between the observer and the light.

Similarly Koschmieder 's law would take the form

6-61
760

C

=J

, /t,V dX, (6.30a)

380

or

760

C =^ 380

dA (6.30b)

where

C , is the contrast "between the object and its

o, A sky background at wavelength A.

Moreover, an instrument which uses a single measurement of the

entire visible spectrum is not suitable. Instead this transmittance

must be obtained from measurements of the spectral transmittance,

wavelength by wavelength, to obtain t-, , over the spectral region 380 to

D , A 760 nanometers

.

Note: Current recommended practice is to denote wavelength A in

parentheses when the quantity is a function of wavelength,

e.g., V(A), and to use a subscript A to denote spectral concentration of a quantity to a narrow wavelength band, e.g., P . A However, in the interest of clarity in equations, subscripts

of A have been used for both types of quantities in this

document.

As stated by Zuev in his comprehensive review of atmosphere

transparency [129] , the transmissivity of all fogs, clouds, and rain

is virtually independent of wavelength in the visible spectrum

(this accounts for the white color of clouds). However, Zuev also

shows that in haze the transmissivity for red light is greater than

for blue, but that the difference decreases as the haze density decreases,

Since haze atmospheres are somewhat selective, it is appropriate

to re-examine the spectral sensitivity of the transmissometer receiver.

As mentioned, correction of the detector's spectral response to duplicate

that of the human eye would initially seem to be both desirable and

6-62
necessary. Consider, however, an extreme case of atmospheric sensitivity. Suppose that, in dense fogs, blue light at wavelengths shorter

than nm was totally attenuated but wavelengths above 4^0 nm were attenuated only slightly. In this hypothetical situation, a transmissometer system with a receiver corrected to the spectral sensitivity

of the eye and with an incandescent lamp, or other sources which are not

predominantly blue, as the light source would indicate an RVR far in excess

of the visual range of blue lights. From this example we can see that

correction of only the receiver's response is not sufficient and that

consideration must also be given to the spectral power distribution of

the transmissometer light source and to the signalling light or object to

be viewed as well as to the spectral sensitivity of the eye and of the

transmissometer receiver. The most desirable situation would be to make

the product of the spectral sensitivity of the transmissometer receiver and

the spectral power distribution of the transmissometer source proportional

to the product of the spectral sensitivity of the eye and the spectral power

distribution of the light or object being viewed, that is

760 760

380 380

where

is the spectral power distribution of the

transmissometer source,

is- the spectral sensitivity of the transmissometer

receiver,

is the spectral sensitivity of the eye,

is the spectral power distribution of the

the light or object being viewed, and

k is a proportionality constant.

Thus, a filter which modifies the spectral sensitivity of the receiver

to that of the eye, as proposed by some, would be suitable when un- dimmed white lights are the visual signal but would not be suitable if sources with different spectral distributions (such as xenon flashers.

6-63
red approach lights, green threshold lights, blue taxiway lights or runway markings ) are the visual signal since the transmissometer light

source is a clear incandescent light.

The receiver of the NBS transmissometer uses a phototube with a type S-1 (red-sensitive) cathode. The double-peaked spectral sensitivity

curve of this tube does not approximate that of the eye. However, its

response range of 300-1080 nm encompasses that of the human eye.

The phototube used in the early receivers (figure 6.1) had an

S-4 (blue-sensitive) cathode which is a better, although poor,

approximation of the eye's response. However, these tubes had un- desirably high dark and leakage currents and were not sufficiently

stable. They were, therefore, replaced with the type 919, or C-75, tubes

with an S-1 surface. Use of a correcting filter is not feasible

with these latter tubes because of the extreme loss in sensitivity which

would be incurred. Over the years all efforts to obtain phototubes with

a more appropriate spectral sensitivity have been futile.

In considering the need for a tube with improved response, it

should be noted that transmissometers having S-1 phototubes, have been

operated simultaneously at Areata with transmissometers having S-4

phototubes, thereby bracketing the spectral sensitivity desired. No

systematic differences were detected. Presumably, such differences

as exist were concealed by variations in fog density with time and

location.

6-64
7. PERFOKNAICE VALIDATION OF THE 1^ TRAfeilSSQ-eER

7.1 INTRODUCTION

In 1945, Douglas and Young reported on the development of a transmissometer for measuring visual range [30] . The principles of

operation of this instrument and a discussion of errors are contained

in Chapter 5 of this report, and its development and application are discussed in Chapter 6. This chapter is a discussion of the performance

validation of the instrument with particular emphasis on the field ex- periments conducted by NBS on Nantucket Island and at Areata and those

sponsored by the Air Navigation Development Board (ANDB) at Washington

National Airport [124] • The general purpose of all of these field

experiments was to determine the correlation between the visual range

of selected objects and lights, as determined by hman observers, and

the transmittance measurements obtained with the transmissometer. Both

Koschmieder ' s and Allard's laws (see Sections 4.1.2 and 4.3.1) involve

threshold constants which must be used in determining visual range from

transmittance data. Values for the contrast threshold and the illuminance threshold were determined from data obtained during these field

experiments. The actual procedures and techniques employed during these

experiments are described in a fair amount of detail to enable direct

comparisons with any future work in the determination of threshold

calibrations. As a result of these studies, it has been shown that

the correlation between the transmittance measurements and the visual

range of non-luminous objects by day follows the theory developed by

Koschmeider, whereas that between transmittance measurements and the

visual range of lights at night shows systematic deviations from Allard's

Law.

7-1
7.2 DESCRIPTION OF FIELD EXPERIMENTS

7.2.1 Nantucket Island

The NBS transraissometer was field tested and its performance

initially validated on Nantucket Island, Mass., during the summer of

1941. Since this location had already been chosen for fog observations

in the development of approach light systems, it was an obvious choice

for the transmissometer performance validation because personnel and

equipment could be applied to both projects. The equipment was installed about one-quarter mile frcmi the south shore of the island

near Surfside where periods of fog were frequent. An aerial photograph

of the locality is shown in figure 7.1.

The light source and the receiver were mounted on 3 inch pipes

set about 2 feet into the ground and rigidly guyed by wires . The

units were located about 6 feet above the surface of the ground. The

output meter was installed in the control house located about 175 feet

from the receiver. Diaphragms with 10 inch holes were installed in

front of the light source at distances of 12 and 50 feet to restrict

the cross-section of the beam because the projector used gave a broader

beam than that considered desirable for service use.

The row of daytime visibility marks shown in figure 3.2 consisted

of ten 4-foot-square pieces of plywood painted flat-black and spaced

at 100 foot intervals. The lower edges were about 8 feet above the

surface of the ground so that the marks would be observed against a background of fog rather than against the terrain. The line of the

marks was parallel to and approximately 250 feet from the light path

of the transmissometer.

A light was mounted on top of each of the visibility marks for

observations at night. These lights consisted of clear-bulb streetseries lamps and were operated at an intensity of 25 candelas. The

variation of the intensity of the lamps with horizontal angle was negligible

.

7-2
3 Transriiissonieter source

R Transmissoaieter receiver

0 Observation area

M Visibility marks

- Small marks

Figure 7.1 Location of visibility calibration range on Nantucket Islan'd.
Observation stations were laid off at 100 foot intervals throughout

the length of the observation area. Telephone terminals were provided

at each of the observation stations

.

Because of the flat terrain and lack of a sufficient number of

suitable marks and lights, it was necessary to provide a mark and

light which could be observed for distances between 1000 and 3500

meters. Figure 3-2 also shows this visibility mark which consisted of

two 4 foot by 8 foot pieces of plywood painted flat-black and mounted

about 100 feet from the control house. A 25-candela light and a signalling

lamp were outside the areas shown in figure 7.1. The distances to these

various stations were approximately 1000, 1300, 1500, 1800, 2400,

and 3500 meters respectively. The distances were established by

surveying. The stations were located, as nearly as possible, the same distance from the shore line as the mark and in a direction

approximately normal to the plane of the mark.

7.2.1.1 Visibility Calibration Procedure

The visibility marks and lights were observed from the observation area indicated in figure 7.1 for visual ranges less than 1000

meters. The observer would locate himself, if possible, at that

observation station from which approximately only one half of the

visibility marks or lights were visible. Then, every minute, he would

report by telephone to an associate in the control house the number

of marks or lights that could be seen. The visual range was con- sidered to be the distance to the last mark or light which could be

seen steadily. The associate would list this visual range information,

together with the transmissometer reading taken at approximately the same time. When the visual range changed so no marks or lights were

visible or all were visible, the observer, if possible, would proceed to another station from which about half of the marks or lights

could again be seen and the observations continued. For twilight

conditions, observations were made on both the marks and the lights.

7-4
When the visual range of the marks or lights was greater than

1000 meters, recourse was made to the large mark or its associated

light. The observer would go to the station from which the mark

appeared to be approximately at the limit of visibility and would

record the periods, if any occurred, during which this mark was at

the limit of visibility. When the visibility of the mark changed

sufficiently so the observer believed that the mark might be at the

limit of visibility from some other station, he would proceed to that

station so that he could again determine the periods during which the

mark was at the limit of visibility. Throughout the period of these

observations the associate would list the transmissometer readings

at intervals of 1 minute . When this type of observation was made at

night the signal light of about 75 candelas intensity was flashed

from time to time to identify the visibility light.

In addition to the observations on the large mark or associated

lamp, the appearance of other objects and lights at known locations,

which were at the limit of visibility for either the observer or the

recorder, was listed. For visual ranges somewhat greater than 3500

meters, natural objects such as church steeples, water towers, and

lights of the villages of Nantucket and Siasconset located on the

island were observed.

During the periods of daytime observations the light source of

the transmissometer was turned off from time to time to determine the

transmissometer reading due to background illuminance. In the re- duction of the data the background reading was subtracted from the

reading with the light on to obtain the true transmittance reading.

Also, transmissometer readings were obtained from time to time when

the atmosphere was especially clear. These values were used in estimating the transmissometer reading for 100 percent transmittance.

Since the visual range data were to be used primarily to calibrate

the transmissometer, the calibration observations were not made during

7-5
periods when the fog or haze was obviously non-homogeneous or when the

visual range was fluctuating rapidly.

In addition to the above observations, the transmissometer reading

was frequently recorded at the time during which the Nantucket Weather

Bureau Station, about 2-1/2 miles away, made its periodic observations.

7.2.2 Areata

Comparative visual range observations were made by NBS at Areata

during the 194V, 1948, and 1954 fog seasons.

The 1947 observations by day used a single fixed target while

varying the position of the observer. Subsequent observations in 1948

used a path of sight which nearly duplicated that of the transmissometer.

The 1947 observations by night attempted to investigate the effects

of unseen lamp glow upon the illuminance threshold. Comparative observations were made in 1954 of two lamps, one shielded and one unshielded,

to further investigate the effects of lamp glow.

7.2.3 Washington National Airport

In the early 1950' s, the Weather Bureau, at the request of the

Air Navigation Development Board, conducted a series of investigations

at Washington National Airport. The objectives were two-fold:

a) to independently validate the performance of the NBS

transmissometer in the field and

b ) to compare the prevailing visibility reported by a stationary

'observer at the end of the runway with the visual range

determined through the use of a nearby transmissometer.

Comparisons were made between a transmissometer located near

the touchdown point on runway 36 at Washington National Airport and

the visibility reported by an observer stationed at the end of the

runway. Figure 3-12 shows the airport and the location of the various

instrumental and observational sites.

7-6
7.2.3.1 Validation Procedures

A mobile observatory was constructed to house the observer and

some of the instruments necessary for taking observations at the end

of the runway. The indicator for the transmissometer was installed in

the main observatory, so that all visual observations made by the runway

observer were without reference to instrumental indications.

Targets used to determine daytime visibility were ordinary buildings

trees, etc., but were selected with as much care as possible, consistent

with the need for obtaining a sufficient number for the purpose.

Although it was impossible to do so completely, an attempt was made to

select dark objects against a horizon sky background and to select those

having one dimension which subtended an angle to the observer's eye of

not less than 1/2 degree and not more than 5 degrees. Point-source

lights were selected for night targets. Red and white lights with

effective intensity of from 25 to 75 candelas predominated. Focused

lights were not used. These criteria reduced the number of available

targets less than three miles away to 29 for daytime and to 21 for

nighttime. In any quadrant there were only 5 or 6 targets within the

three mile range, resulting in very few observations being made when

a target was exactly at the observer's visual range. In order to

provide as much precision as possible mder these circumstances, a target clarity code was devised whereby the runway observer would grade

the clarity with which any visible target was observed. Five code

numbers were used, with the following definitions:

1. Target barely detectable: appears to fade in and out,

visible not more than 75^ of the time.

2. Detection threshold: target appears to fade in and out but

is visible more than 75^ of the time.

3. Recognition threshold: target in view 100^ of the time and

is defined just clearly enough to be recognized.

4. Just better than recognition threshold: good definition with

some texture and color realized in daytime targets.
5. Prominent: sharp outline, details stand out, colors of

close targets recognized.

In addition to estimating the quadrantal visibility, in standard

visibility increments, the runway observer noted the targets used and

the clarity with which they were observed, expressing results on the

basis of the above code. In accordance with standard meteorological

practice, visibility was determined by using the so-called "recognition

threshold" [73]

.

During the latter portion of the program measurements of background

luminance were made during periods of twilight and darkness.

7.3 DISCUSSION OF RESULTS - DAYTIME OBSERVATIONS

7.3.1 Nantucket Island

Between two and three thousand simultaneous observations of the

visual range and transmissometer reading were made. Approximately

one third of these data were rejected as unsuitable for determining

the visibility calibration curves. The data were rejected because the

transmissometer reading of visual range did not remain sufficiently

constant to give a reliable calibration point . For the acceptable

data, the average values of the transmissometer readings and of the

visual ranges during each period when both remained reasonably constant

were used to determine a calibration point. The points were graded in

accordance with their relative reliability, using as a measure of

reliability the constancy of the transmissometer readings and visualrange observations and the length of the period of constancy.

Grade A points were obtained in general from periods of 10 to

30 minutes or more duration during which the transmissometer readings

and visual range varied relatively little . As an illustration, the

individual values from which one of the better grade A points was obtained are listed in table 7.1.

Grade B points were obtained in general from periods 5 to 10

minutes in length and occasionally with slightly more variation in the

data than for grade A points

.

7-8
Grade C points were obtained primarily from two types of data,

(l) periods of less than 5 minutes in d\iration, and (2) single visualrange observations with reasonably steady atmospheric conditions as indicated by the transmissometer readings

.

The grade A, B, and C points include only the visual range observations made on the special marks and lights installed for the calibration.

The calibration points obtained from observations on other objects

and lights and from the Nantucket Weather Bureau observations have

not been graded. Weather Bureau data listing prevailing visibilities

less than the distance (2.5 miles) from the station to the test location

were not used for calibration points

.

Figure 7.2 is a plot of the daytime visibility calibration points

for observations made on marks . The visual range is plotted on a log

scale and the transmittance on a log-log scale. With these scales

Koschmieder ' s law (4.08) is represented by a family of straight lines

having a slope of one, with the contrast threshold, e, as a parameter.

7-9
VISUAL RANGE IN KILOMETERS

(LOG SCALE)

W 50 60 80 160

Figure 7.2 Daytime transmissometer visibility calibration curve derived from data obtained at Nantucket Island. [30]

7-10
TABLE 7.1

DATA USED TO DETERMINE ONE OF THE GRADE A POINTS

FOR THE DAYTIME VISIBILITY CALIBRATION CURVE

Time Transmittance Visual Range

7-30-41 Per 250 meters Feet Kilometers

4:47 A.M. 0.15 1300 0.40

48 0.13 1200 0.37

49 1300 0.40^

50 1300 0.40*

51 0.15 1300 0.40

52 0.15 1300 0.40

53 0.16 1400 0.43

54 1400 0.43*

55 1500 0.46*

56 0.17 1400 0.43

57 0.17 1400 0.43

58 0.11 1400 0.43

59 0.10 1500 0.46

5:00 0.10 1400 0.43

01 0.10 1400 0.43

02 0.09 1300 0.40

03

04 0.10 1300 0.40

05 0.10 1200 0.37

06 0.10 1300 0.40

07 0.11 1300 0.40

08 0.11 1200 0.37

Average 0.124 0.409

*Not used in computing average visual range.

7-11
The value of the contrast threshold, e, determined from these

observations was 0.055. This value corresponds to the center line on

figure 7.2 which is the minimum deviation for the points obtained

exclusive of Weather Bureau and twilight points. The value 0.055 was considerably greater than the values from 0.01 to 0.02 generally accepted

at that time. This value did, however, fall between the value 0.065

reported by Houghton for clouds and his value for fogs which is about

one-half that for clouds [4-B] .

The line for e equal to 0.055 is a reasonable representation of

the calibration points for the shorter visual ranges. It was con- sidered as the daytime and twilight calibration curve of the transmissometer. There appears to be, however, a somewhat systematic departure of the points from this line with increasing visual range.

The lines e = 0.031 and e = 0.098 were chosen so that for any

given transmittance the corresponding visual ranges determined from

these two lines differ from that determined by the calibration line

by plus and minus 20 percent, respectively. Similarly, the lines

e = 0.234 and e = 0.003 give visual ranges equal to one-half and twice

that of the calibration line. These lines are of assistance in studying

the departure of the points from the calibration curve.

A classification of the calibration points according to the

general illuminance, as determined by the transmissometer background

readings, showed no systematic differences of visual range with

illuminance. This apparent independence of the visual range of objects

with illuminance is in agreement with previous investigations and theory.

Even the twilight points in figure 7.2 show no systematic differences.

Some of these points result from observations when it was so dark that

the 25-candela lights could be seen farther than the marks . The

computed average value of e for the twilight calibration points was

0.04-4. This value of e is less than that for the daytime value (0.055),

whereas laboratory studies show e increasing at low illuminances.

Because of the spread of the values for the comparatively small number

of twilight points, however, the difference between e for daylight and

e for twilight cannot be considered significant.

7-12
The 4-foot-square marks were observed from distances between 0,15

and 1.0 kilometer. The corresponding angles subtended at the eye by

the marks varied from 0.4-6° to 0.07°. Similarly;, the large mark when

observed subtended angles from 0.15° to 0.05°. IVliddleton [94]

suggests that the least angular dimension of objects should not be

less than 1° and that the maximum size of the nearer objects should

not exceed 5°. However, it was frequently noted that the visibility of

other objects, both larger and somewhat smaller than the marks, gave

no indication that their size had any effect on their visual range.

For example, observations on a power line pole to one side of the row

of marks frequently indicated the same visual range as that given by the

marks

.

7.3.2 Areata

Some observations were made by NBS^ at Areata in 194-7 using a single fixed target and varying the position of the observer. These

results are shown as the crosses on figure 7.3. Each of these points

represents the average of data in a manner similar to that used at

Nantucket Island.

This method of obtaining thresholds requires the assumption that

the fog density is uniform throughout the space through which both the

target is observed and the transmittance is measured. The greater

part of the point spread in figure 7.3 was believed to be caused by

the transmittance of the measured path being not representative of

the path for observations; there is the possibility of both random

and systematic sampling errors . To reduce the magnitude of these effects

observations were made diiring the 1948 fog season at Areata using,

when possible, a path of sight nearly parallel and very close to that of one of the transmissometers . When the visual range was 500 feet, the

two paths were nearly identical. These particular observations are shown as circles on figure 7.3- The variations that can occur in

visibility observations using human observers are quantitatively discussed in Section 4.2.2.

*Note: These observations were made by Douglas, who made more than

half of the Nantucket observations

.

7-13
0001 « ' — ' ' " 100 200 500 1000 2000 5000

VISUAL RANGE IN FEET

(Log Scale)

Figure 7.3 Visual range observations at Areata of a single fixed dark

object obtained while varying the position of the observer.

1947 results are shown as crosses. Results in 1948, shown

as circles, were obtained using a path of sight nearly

parallel and very close to the transmissometer baseline.

The straight line shown represents the transmissometer calibration curve obtained at Nantucket.

7-14
7.3.3 Washington National Airport

Only those data which met the following criteria were used in

threshold computations

:

1. The visibility of the runway observer, looking north, was 3 miles or less. (All visibility determinations used

in the determination of e were for the north quadrant. )

2. The visibility in all quadrants did not vary more than 1/8

mile from that of the north quadrant.

3. The observer's visibility for the preceding observation

and the following observation in the north quadrant did

not differ by more than 1/4 mile from the observation used.

4. Transmittance as indicated by the transmissometer remained

within ± % for the 15-minute period embracing the observation.

(The value of transmittance used was based on a 4-niinute

average, the same as used by the Landing Aids Experiment

Station in their application of transmissometer measurements

to flight tests of approach and runway lights).

The cumulative frequency distribution of the contrast threshold

values is shown on figiire 7.4. The median of the observed values was

0.05.

7.3.4 Recapitulation of Daytime Visibility Calibration

Based on the observations obtained at Areata, figure 1 .'}, and

by the Weather Bureau, it would seem that a contrast threshold value

of 0.05 is a better choice than the presently used value of 0.055.

The value 0.05 is currently recommended by WMO [127].

7.3.5 Effect of Threshold on Day Visibility

In an atmosphere of transmissivity such that an observer, having

a contrast threshold e of 0.055, could just see a black object at a distance of 1/2 mile, under existing instructions [36] all observers

having effective thresholds of from 0.039 to 0.080 would also report

a visibility of 1/2 mile. At one mile, the tolerance would be from

0.037 to 0.064, and at 1/8 mile from 0.027 to O.II4. As evident from

7-15
1 o , j:z +->

t/) to

CD

U- >

s~

+->

+-> 4-)

fO o

s_ o

+J

c:o 5

u

14-o

o

o 4-> n3

o

+->

-)-> en

CO -o j::CO >,o

c

cu cn

cr

-o

1—

1

CO

C\J > 1 1 CO

rC 'e

S- o

13 O) E +-> +-> 13 O)

C_) -o cn

(%)A0N3n03«d 3Aiivnnwno

7-16
figure 7.2, e can vary from 0.031 to 0.098 while affecting visibility

computed on the basis of e = 0.055 by only ± 20%. Similarly,

£ = 0.234 and e = 0.003 give visibilities equal to one-half and

twice the calibration line e = 0.055. These examples show that the

relationship between contrast threshold and computed visibility is

not linear and are not intended to imply that the variations are un- important .

7.4 DISCUSSION OF RESULTS - NIGHTTIME OBSERVATIONS

7.4.1 Nantucket Island

Figiire 7.5 shows the results obtained from observations on 25 candela

lights. The scales and symbols are similar to those used for objects

by day, figure 7.2. Three curves are shown on figure 7.5, corres- ponding to:

Allard's law which states that the visual range of a light is

the distance at which the light will produce a fixed illuminance

threshold,

E = ItVv^; (4.24)

an assumption that the illuminance threshold varies inversely as the visual range,

S = ItVv; (7.01)

and, for comparison, an equation where ly/I is equal to the transmittance over the distance V,

ly = IT^. . (7.02)

Equation (7.02) may be considered as a form of Koschmieder ' s law in

which contrast and contrast threshold are replaced by intensity and

intensity threshold.

For all three curves, I = 25 candelas and E, S, and 1-^. have been

set numerically equal to 0.052; E has the dimensions of illiiminance and

7-17
I/

/

/

1 f

1/—

/

1 /

/

1 /

/

/

/ 7^7^

/ + +

/

/ /,+

/

i-

/

/ / /

- +

/

/

/

/

>

t:

o —-/- / / / /' /

<> —8- /

—o1 A A yo

>

•

• A

O

O

n

Orede A O Orede B A Orede C WlBcellsneous

[ rrom Weather Bureau data • A Twlllprht

: ^

/ A

A / / /

\ 7^

VISUAL RANGE IN KILOMETERS

(LOG SCALE)

Figure 7.5 Nighttime transmissometer visibility calibration curves, based

upon a light intensity of 25 candles, derived from data obtained

at Nantucket Island [30].

7-18
is expressed in kilometer candles*, S has the dimensions of intensity

per unit distance and is expressed in candelas per kilometer, and

has the dimensions of intensity and is expressed in candelas.

Equation (7.02), like Koschmieder ' s law, is represented by a straight line. The value for ly/I was obtained in the same manner as

e was obtained for the daytime visibility calibration with 1^ = 0.052

candelas

.

The S curve provides the best fit for the calibration points on

figure 7.5. This relation was first derived from observations made at

Nantucket in 1940 on a calibrated lamp by two observers at different

distances.

It should not be inferred from this statement that the illuminance

at a point is not given by (4-. 24) but rather that the minimum perceptible

illuminance is not a constant and is such a function of T and V that

(7.01) represents the data more satisfactorily. From these data it

appears that the illijminance threshold varies inversely as V, the

visual range. Hence, if E/V is equal to E^/V where E^ is the illuminance

threshold at unit distance, and is set equal to 0.052 where V

is in kilometers, then (4-24) is equivalent to (7.01). Note that when

the mile is the unit of distance, S is equal to

^(mi) 0-052 cd/km x 1.61 km/mi = 0.084 candela/mile (7.03)

7.4.1.1 Rationale for Choice of Intensity of Lights

In obtaining a transmissometer visibility calibration curve for

determining visual ranges at night, it is evident that consideration must

be given to the intensity of the lights which will be used as marks.

In this respect the visual range by night differs from that by day,

for by day the visual range is substantially independent of the

objects observed whereas the visual range of a light at night depends

on the intensity of the light. Thus, for any given distance it is

possible to choose the intensity of a light so that it will be at the

*The kilometer candle and the mile oandle oj'e early units of illiminance

traditionally used in signal lighting. In modern practice^ one kilometer

candley which is one lumen -per square kilometer, should be designated as 10~^ lux or I microlux. See Para. 2.1.6. Metric units were used by NBS

and ANDB in their reports in the 1940 's and 1950 's with the expectation

that the U.S. would soon adopt these units. However, the English unit

mile candle (I lumen/mi^) continues to be used.

7-19
limit of visibility at night when the transmissivity is such than an

object at this distance would be at the limit of visibility by day.

(See section 2.3.13 and reference [9A] > for further discussion

of the equivalence of day and night visibility. ) If a transmissometer

were calibrated by means of a system of such lights, the visibility

calibration curve for the lights should be the same as that obtained

for objects by day. The intensities of the lights required by this

system for visual range in the region 20 to 40 kilometers are of the same order of magnitude as the intensities of airport boundary lights

and street lights. The required intensities for the lights for visual

ranges lower than 10 kilometers are less, however, and the intensities

for visual ranges lower than 1 kilometer are much less than the intensity

of any ordinary light used as a landmark and thus these landmark lights can be seen at greater distances than the indicated visual range. For

example, when the transmittance is such that object visibility by day

is 1 kilometer or less, the visual range of a 25-candela light by

night is about twice this distance. This is evident from a comparison

of figures 7.2 and 7.5, or from figure 7.9. It seems preferable, therefore, that in practice nighttime visual ranges should be determined from a visibility calibration curve which is based on lights having intensities

comparable to those usually used as landmarks . Twenty-five candelas is

about the minimum intensity of such lights . Frequently lights of greater

intensity are used, especially for the larger visual ranges. The visibility calibration curve to be used to determine visibility at night

should therefore be based upon the intensity of the lights which a pilot

uses and is expected to see from various distances.

It should be noted that the Intensity of these lights is such that

the observers used foveal (cone) vision in determining their visual range.

Foveal (cone) vision is also used by aviators, even at low light levels.

7.4.2 Areata

In the preceding discussion, it was shown that assuming the illuminance

threi3hold varies inversely as the visual range provides the best fit

to the data on figure 7.5. From this it appears that an observer's

7-20
threshold is affected by changes in the background luminance in the

iiranediate vicinity of the lamp which are produced by the lamp itself.

Under conditions of low background luminance, when an unscreened lamp

is definitely above threshold, an area of illuminated fog surrounding

the lamp is visible. As the apparent intensity of the lamp approaches

threshold, this glow is generally no longer apparent and only the point

source itself is visible. Nevertheless, there appears to be some sort

of an effect from this unseen glow.

A few observations made by NBS at Areata in 194-7 are shown in

figure 7.6 together with the transmissometer visibility calibration

curve based on the Nantucket data ( over the ranges shown, the calibration curve is nearly a straight line). The range of distances here

is too small and the spread of points too large to determine whether

Allard's law or the calibration curve provides the best fit. However,

the data do show that the threshold used is the right order of magnitude

This variation of the illuminance threshold with visual range can be considered as approximate only.

In 1954? NBS made comparative visual range measurements in fog

at Areata of two 25-candle lamps, one unshielded and one shielded so that only the light emitted in the direction of the observer was allowed to escape. There were times at night when the glow surrounding

the bare lamp could be seen further than the direct light from either

lamp. However, the distances at which the direct light from the two

lamps could be seen was essentially the same.

7.4.3 Washington National Airport

In an attempt to be reasonably certain of a homogeneous atmosphere

in the vicinity of the airport, the Weather Bureau subjected their night

time observations to the same restrictions applied to the daytime observations; see Section 7.3.3. The median value of E determined from 94

observations was 0.045 kilometer candle. Figure 7.7 shows a cumulative

frequency curve for values of S for these observations. S was computed

from the relation S = EV. The median value obtained was 0.052 candelas/

kilometer which is identical with the value obtained at Nantucket and

7-21
Figure 7.6 Visual range observations at Areata of a single fixed 25 candela light obtained in 1947 while varying the position of

the observer. The solid line is the transmissometer visibility

calibration curve based on Nantucket data. As in Fig. 7.3,

the abscissa is a log scale and the ordinate is log-log.

7-22
-a

O) o

c:

•r- +J

fO

4-> CD

JDO -M > OJ oo >

C

M-O

4-> O o

Q.

+-> S- Z3 •1

^ cC

S_

+-> ea

(/) o

-o

>^o

E

<D c

13 o

cr+J

O) CD

s- M- •1

jC

(1) to > (O

•1

3:

+-> fO cn E

Z3

cu

(%) A0N3n03dd

O

3Aiivnnwno

o

CM

7-23
now used in the night visibility calibration for the transmissometer

.

7.4.4 Effects of Background Luminance

The illuminance threshold varies with background luminance, as shown

in figure /+.4. The Weather Bureau measured the background luminance

from the end of the runway location. Their measurements indicated

-2

background luminances of the order of 10 mL. This agrees roughly

with unpublished measurements made by Douglas. At a later date, Simeroth

[114] reportr~d ajTibient horizon-sky luminances in the region of the

airport (approach and runway lights off or at low intensity) which were

-2

also of the order of 10 mL.

7.4.5 Effect of Threshold on Night Visibility

Figure 7.5 shows three different nighttime transmissometer visibility

calibration curves derived from data obtained at Nantucket Island. The

three curves all yield identical values at a visibility of about 1 kilometer.

Different values of threshold produce a family of curves of increasing

ordinate scale with increasing threshold values. It is evident from

figure 7.5 that the exact shape of the visibility calibration cirrve

has little effect in the region 0.-4 to 3 kilometers (0.25 to 2 miles)

visibility. The curves on figure 7.5 are divergent and yield appreciably different results for greater visibilities.

7.4.6 Comparison of the Nighttime Illuminance Threshold Applicable to

Meteorological Visibility and to Runway Visual Range

It is evident from Sections 7.4.1^ 7.4.2, and 7.4.3 that different

criteria were used in the establishment of the illuminance threshold

applicable to meteorological visibility than to runway visual range.

Combining equations (4.24), (7.01) and (7.03) yields

= S/V = 0.084/V (7.04)

where is the illuminance threshold in mile candles and V is the visibility

in miles. This equation provides for a variable, not fixed, threshold.

7-24
Thus, for a meteorological visibility of 1/8 mile, the illuminance

threshold is 0.67 mile candle and for a visibility of 10 miles, the

illuminance threshold is 0.0084 mile candle. This range of illuminance

thresholds is considerably greater than would be expected from the changes

in background luminance with visibility reported by Simeroth and the Weather

-3

Bureau. They found that background luminances ranged from about 10

footlambert in clear weather to about 10 ^ footlambert in fog. The curve of

figure 4-4 indicates that such a change in background luminance would be

expected to change the illuminance threshold by a factor of 3, not 80 as shown in the example cited above. This large change in illuminance threshold

is presumably produced by a change in the glow surrounding the lights used as visibility marks due to changes in atmospheric transmittance . This glow

is primarily produced by the light itself, as discussed in Section 7.4.2,

and not by other lights in the surround.

In the computation of nighttime RVR, a fixed ill\iminance threshold

of 2 mile candles was chosen in 1955 by the United States. This value

has since been accepted internationally. As already stated, in Sections

3.9.1 and 4.4.8, this value was intended to be a conservative value

applicable to typical ambient background luminances of less than 0.1 footlambert. The illuminance threshold of 2 mile candles is 4 times the

classical value for mariners and pilots, 0.5 mile candle, and is 10 to 20

times the illuminance thresholds shown in figure 4.4.

At the time this threshold was chose, RVR was considered to be a meteorological quantity reporting conditions of atmospheric clarity to the

pilot in terms more meaningful than either atmospheric transmissivity or meteorological visibility. No adjustments were made for changes in intensity

of the approach and runway lights when changing the operating intensity step

of the lights nor from changes in illuminance threshold with background

luminance. Allard's law (4.24) was used instead of the transmissometer

calibration equation (7.01) in the computation of visual range since there were no data to demonstrate the applicability of an equation similar to

the transmissometer calibration equation to the RVR computation. Instead,

there were several reasons to question its applicability:

*ln this context the term "background luminance" refers to the luminance of

the background around the airport, as seen by the meteorological observer, not

the luminance of the approach zone and runway as seen by the pilot.

7-25
1. It was considered unsuitable for day and twilight conditions

since the luminance of the glow produced by the approach and

runway lights is small in comparison with the ambient background luminance,

2. Under nighttime conditions, the apparent luminance of the

background of the lights upon which meteorological visibility

observations are based is significantly lower than the

luminance of the fog over approach or runway light systems,

and

3. The illuminance threshold value chosen for RVR computations

was significantly higher than that chosen for meteorological

visibility computations and hence the two thresholds might

be affected differently by changes in visual range and fog

density.

At the time the RVR computers were designed (Section 3-9.5), the

effects of a change in intensity of the lights with intensity setting were included in the RVR computation, but the effects of changes in fog

luminance, with intensity setting, on the illuminance threshold were not.

7.5 COMPARISONS BETWEEN OBSERVED PREVAILING VISIBILITY

AND TRANSMISSOMETER DATA - DAY AND NIGHT

The Weather Bureau compared their observations of the prevailing

visibility with visibility determinations based on transmissometer

measurements using the Nantucket visibility calibration curves. The

results of the comparison are shown graphically in figure 7.8. The

ordinate of each diagram shows the percentage of occurrence of cases wherein the differences between the visibilities computed from the

transmissometer indication and the visibility observed by runway observer

looking north was equal to or less than the given abscissa value. Graphs

are shown for several ranges of visibility for day and for night.

In assuming that the runway observer reported the correct visibility,

the data on figure 7.8 indicate that for visibilities of 1 1/2 miles or less the transmissometer correlates reasonably well with the Weather

Bureau observations and there is no significant difference between day

and night performance.

7-26
100

in

90-

80-

^ 70

cc

i}j 60

m

o

o

\- 40

z

UJo 30

UJ

°- 20^

10-

o — Q Z

o

— C\J

II II z z

I re i i I 2 f

DIFFERENCE (miles)

VISIBILITY { MILE AND LESS

lor more

100

90

z 80

o

5 70 >

UJ 60

tom

o

u.o

50

H 40

z

UJo 30

(TUJ

°- 20

I 0-

to Tj16 8 I lor more

100

90

80

70-1

60

50

40

30

20-

10-

e z

ro — O (O CJ CM

n II Z Z

0

I f6 113 8 4 7

DIFFERENCE (miles)

VISIBILITY -| MILES AND LESS

I lor more

100 r- 90

80-1

70

60-

50-

40

30

20

10

>«oo

CM U>

fO

0

3 5 4

VISIBILITY 1^ DIFFERENCE (miles) DIFFERENCE (miles)

MILES AND LESS VISIBILITY 3 MILES AND LESS

I I Day Observations (objects)

tsassgiftg::;} Night Observations (lights)

lor more

Figure 7.8 Cumulative frequency of various differences between

visibility computed from transmissometer and visibility estimated by an end-of-runway observer during

Washington National Airport investigations [126].

7-27
However, there are a number of limitations in comparisons of this

sort that must be considered. Because of the subjective nature of the

estimate, there is no guarantee that the observer is reporting the

correct visibility. In addition to the recognized differences in threshold which are present among different observers, an individual observer'

threshold can vary from time to time. However, as shown in Section 4- .2.

2

the effects of variations in observer's threshold are much too small to

explain the differences of figure 7.8.

7.6 TWILIGHT TRANSMISSOMETER VISIBILITY CALIBRATION

Figure 7.9 shows both the day and night visibility calibration

curves for a 500 foot baseline transmissometer . It is apparent from

this figure that over the range of most concern, the night visibility is

approximately twice the day visibility for a given transmissivity . The

problem of transition from one curve to the other during twilight is of concern.

During both the NBS and Weather Bureau tests it was observed that,

in the absence of appreciable changes in atmospheric transmissivity

during the evening twilight, object visibility would hold relatively

constant until sometime after which the lights could be seen further than

the daytime markers, and then would decrease rapidly. Visibility of the

lights gradually increased, then held relatively constant. In the

morning twilight, the reverse sequence occurred. This process is

discussed in more detail in reference [85] . During twilight conditions

at Nantucket Island, NBS made simultaneous observations on both marks

and lights. In figure 7.5, it is seen that the visibility of the

25-candela lights during twilight does not agree with any of the three

curves, the lights being, as expected, less visible in twilight than

at night. However, the daytime visibility calibration curve, figure

7.2, does agree approximately with the visibility of these lights

during twilight, in addition to giving a satisfactory determination of

the visibility of objects.

The Weather Bureau considered that one possible solution to the

problem would be to take photometric measurements of the luminance of

7-28

the horizon sky and then use a visibility calibration curve based on

the appropriate illuminance threshold [126] . The results of such

measurements are shown on figure 7.10. A curve based upon data of Knoll,

Tousey, and Hulburt [77] is also shown for comparison. There is only

fair agreement between the points and the curve.

The possibility that the transition from daylight to darkness

would follow a repetitious pattern from day to day such that an appropriate calibration curve could be chosen on the basis of time, without

direct reference to photometric measurements, was also explored by the

Weather Bureau [126] . Figure 7.11 shows the illuminance thresholds

for all twilight observations plotted against time. As shown on the

figure, there is no correlation.

In the U.S., the choice of which transmissometer visibility

calibration curve to use during twilight has been approached pragmatically and conservatively. Consequently, the day visibility curve is

always used during twilight. The night visibility calibration curve is

used only during periods of darkness [103] . The problem of twilight

visibility obviously increases with increasing latitude and the corresponding increase in the length of the twilight period. The solution

adopted for the continental U.S., i.e., use the day visibility curve during twilight, is not applicable to latitudes somewhat greater than

50^.

7-30
i

1

.01' '

' —

.01 log scale 0.1 1.0 10.0

BACKGROUND LUMINANCE

(mL)

Figure 7.10 Illuminance thresholds based

upon Washington National Airport

measurements of horizon sky

luminance during twilight. [126]

7-31
I0»

E

10*

inc«

E

10'

o

X

CO

UJ

a:X

UJo

<

o

o

w

O

10"

10"

4» •

I0-'

10' 10 20 30 40 50 PERCENT OF TIME BETWEEN SUNSET AND ASTRONOMICAL DARKNESS

60

Figure 7.11 Illuminance thresholds computed from

Washington National Airport twilight

observations plotted against time

[126].
8. NEASURBW OF CLOUD HEIGHT

8.1 INTRODUCTION

A beam of light incident on the base of a cloud is scattered

to such an extent that at night the spot produced by a narrow beam

of light is usually visible. This scattering provides a means for easily

measuring cloud height. The simplest method consists of an observer

measuring the angular elevation of such a spot of light at a fixed

distance from the projector and in the vertical plane through the beam.

The geometry of this arrangement is shown in figure 8.1. The angular

elevation of the spot of light can be determined by the use of various

types of clinometers such as alidades

.

Let h be the ceiling height, L the baseline, Q-^ the elevation angle

of the beam, and 6^ the angular elevation of the spot.

Then

L = h Cot 0^ + h Cot e^, (8.01)

and the ceiling is

h = L/(Cot 9^ + Cot 62). (8.02)

If a fixed beam is projected vertically, as is U. S. practice.

Cot 9^ = 0,

and

h = L tan 9^ (8.03)

The foregoing assumes that the projector and the clinometer lie

in the same horizontal plane. Simple corrections can be made when they

8-1
p L A

Figure 8.1. Schematic representation of cloud height measurement. Projector

is located at P, observer/alidade is at A. In U.S. practice,

the beam is projected vertically and 61 = 90°.

8-2
are not horizontal [92]

.

Middleton discussed the theory of ceiling projectors, and their

use for night measurements in 1939 [92] . He favored a baseline of

1000 feet and a projector angle of e = tan"^ 3.0 = 71° 34' for

maximum accuracy to ceilings in the neighborhood of 3000 feet.

Vertical projection of the beam gives somewhat greater accuracy to

measurements of very high ceilings.

Since the underside of a cloud cannot be considered as a plane,

diffusely reflecting surface, Middleton analyzed the luminance of the

spot of light on the cloud for different ceilings. In discussing the

results of his theoretical calculations for projection angles of 71° 34-' and 90° under three different conditions of the atmosphere, he noted

the following:

(1) When the visibility is low, 0.5 mile, the luminance of the spot

falls off rapidly for heights greater than 1000 feet. The ceiling height

which can be measured is thus severely limited by the obscuring power

of the air beneath the cloud.

(2) For visibilities of 5 miles and greater there is a maximum

luminance at about 1000 ft for the 71° 34' angle of projection and at

about 700 ft for a 90° angle of projection.

(3) For visibilities of 5 miles and greater, the luminance does

not vary over a range of more than 10 to 1 between 200 and 2500 feet.

Within this range the .90° angle of projection produces higher luminances.

8.2 PHOTOELECTRIC DETECTION OF CLOUD HEIGHTS

The visual ceiling projector is limited to use -at night. During

the 1930 's, Middleton initiated the first work on a photoelectric

detector which would extend the use of the ceiling projector to the

daytime measurement of cloud heights through the utilization of a modulated beam. Starting with this basic principle, La^ofer and Roskett

[88] developed the first working model of what is now referred to as

the fixed-beam ceilometer. This instrument successfully detected dark

8-3
overcast clouds at an elevation of 9000 feet. A subsequent improved

version [37] was successful in detecting clouds at 20,000 feet during

the daytime.

8.2.1 Selection of Baseline Length

The length of the "baseline" i.e., the horizontal distance

between the projector and the detector, is selected with reference

to the range of greatest accuracy desired in a given ceilometer

installation. The principles on which this selection is based were

outlined by the Landing Aids Experimental Station [85]

.

As previously stated,

h = L tan e (8.03)

where h is the ceiling height, L is the baseline, and 9 is the angular

elevation. If dh is the error in h, then the ratio dh/h is the relative

error;

dh = L sec^e de , (8.04)

dh ^ de (f^n^)

h sine cose'

ko.kj^j

dh ^ 2de (8.06)

h sin 2e '

From (8.06) it can be seen that the greatest accuracy in the measurement of ceiling height (minimum relative error) occurs when the denominator

of the right hand side of the equation is at a maximum. Since this occurs when 2e = 90 degrees, the greatest accuracy of measurement is achieved at

the 45 degree angle of scan, i.e., where the ceiling elevation is equal

to the length of the baseline.

In installations with a long baseline, a small change in angle

of scan at low elevations is equivalent to a relatively large change

in the reported ceiling height. Therefore, extremely low ceiling measurements cannot be accurate if an excessively long baseline is employed.

Conversely, when short baselines are used, the accuracy of low-ceiling

measurements is improved at a sacrifice in the precision of measurements

of high ceilings.

8-4
Figure 8.2 shows the relationship between angles of scan and the

equivalent ceiling with various lengths of baselines as determined using

equation (8.03).

An argument is sometimes made that, since the ceilometer indicator

can be read to an accuracy of about 1 degree, a ceilometer with, say, a 1500 foot baseline is adequate for the measurement of low ceiling height,

e.g., in the range of 300 feet. From a mathematical viewpoint this seems

correct, since reporting increments in this range are 100 feet. An

examination of the projection angles listed in table 8.1, corresponding

to cloud heights at the changeover points to the next reportable increment, reveals that the difference between these angles, about 1.8 degrees,

is indeed greater than 1 degree.

TABLE 8.1

PROJECTION ANGLE 0 CORRESPONDING TO

SELECTED CEILING HEIGHTS h FOR A

1500 FOOT BASELINE CEILOMETER

h ( feet

)

6 (degrees)

250 9.5

300 11.3

350 13.1

However, under conditions of low visibility, which often accompany

conditions of low ceiling, the ceilometer indicator can no longer be

read to an accuracy of 1 degree because:

a. The irradiance at the detector, produced by light scattered

from the cloud base, can be greatly attenuated during

periods of reduced visibility, thereby reducing the signal

to noise ratio, and

b. Scattered light effects broaden the angle of return, making

it more difficult to determine the angle of maximum response.

c. When the sub-ceiling fog is sufficiently dense, the upper portion

of the projected beam is attenuated to a level where the

detector receives no meaningful signal from the cloud base.

However, in dense fog, the ceilometer response is an indication

of vertical visibility.

8-5
/ / 1

10 20 30 40 50 60

ANGLE OF SCAN IN DEGREES - (0)

70 80 90

Figure 8.2 Ceilometer operation - angles of scan vs. equivalent

ceilings for various baseline lengths [85].

8-6
The length of a ceilometer baseline must be chosen with reference

to the objective of the intended installation, i.e., whether the

ceilometer installation should serve as a general observational tool for

large-scale synoptic analysis or as an operational aid in determining the

precise values of extremely low ceilings in the approach zones of

instr\iment runways.

The length of the baseline for purely operational installations

should be selected to afford maximum accuracy at the critical cloud

heights to be measured, usually near the minimum ceiling established by

the regulatory authorities for aircraft operations at the given airport,

and with reference to the density of fog or other obstructions to vision

that may reduce visibility to the established local minimum for authorized

aircraft operations.

8.3 FIXED-BEAM CEILOMETER

The advent of the fixed-beam ceilometer in 1941 added much to

the precision and utility of cloud-measuring devices . It provided a means of automatically measuring and recording cloud heights, both night

and day. The essential components are a vertical projector, a photoelectric detector which scans the modulated vertical light beam, an

amplifier, and a recorder that provides a permanent record of the signal

scattered from the light beam by the cloud particles and picked up by

the detector. The height from which maximum response is obtained is used

as the cloud height. The geometry of this fixed-beam ceilometer is shown

in figure 8.3. The projector (vertical beam) consisted of a 1000-watt,

water-cooled mercury-arc lamp located at the focus of a 24-lnch parabolic

mirror having a focal length of 10 inches. The ac operation of this lamp

provided 95% modulation. An electronic synchronous switch was used to

eliminate the effect of the varying background luminance of the clouds.

The Weather Bureau began using the ceilometer for operational

purposes in 1943 and by 1953 had 140 in operation at civil airports

throughout the United States and its possessions. An additional

comparable number were also in use at military airports.

8-7

During the 1940 's, activity at LAES included the testing and

development of ceilometer models produced by various companies.

(See figure 3.4 showing their locations at Areata). The 1947 season

included tests to compare the ceilometer indications with balloon height

observations [841 . In relating balloon heights to significant ceilometer

readings, mean values of similar observations taken during a given

scanning cycle were used. Some of the results are shown in figure 8.4.

Subsequent tests of the 700-foot baseline Grouse-Hinds instrument,

furnished by the U.S. Weather Bureau, and a 250-foot baseline General

Electric instrument, furnished by the U.S. Navy disclosed minor design

deficiencies which were corrected as the evolution of the ceilometer

continued [85]

.

8.4 ROTATING-BEAM CEILOMETER

In the early 1950 's the Instrument Division of the Weather Bureau

developed a new type of ceilometer based on the same general principles

as the fixed-beam model — that is, after the angle of maximum response

from a light beam has been measured, the cloud height is computed by

triangulation. The new instrument was different, however, in that the

detector was directed vertically and the light beam swept the receiveracceptance cone. A schematic diagram of the rotating-beam ceilometer

is shown in figure 8.5.

In general, a rotating-beam ceilometer offers the following advantage

over a fixed-beam [125]

.

(1) It is faster. The fixed-beam ceilometer was designed to

give two indications per 12 minutes, whereas the rotating-beam ceilometer

was designed to give an indication every 24 seconds. .

(2) Because the electronic circuits are less complicated, the unit

is less expensive, requires less maintenance, and is more easily installed.

(3) It is more economical to operate, due to its lower power

consumption.

(4) Lamp life is much greater.

8-9
700'

600'

500' ^ 400'

< 300' O

200'

UJ>

100

T — lOO^ "OHO A

100 200' 300' 400 500' 600' 700'

700'

^600'

J 500'

=! 400'

CD

tn>^ 300

Z 200'

<

i 100' >-

Y =-335)+0.94X —

100 200 300' 400'

X4-X2

500 600 700'

ure 8.4. Some relationships between observed balloon heights and

significant ceilometer signals from tests at Areata. X2

is the height corresponding to maximum ceilometer signal, X3

corresponds to a beginning constant signal after decreasing

from a maximum signal and X4 corresponds to a height

above X3 at which the signal disappears. The slant visibility

observer was stationed 700 feet from the balloon anchor point.

The least squares equations and standard errors of estimate

are also shown [84].

8-10

8.5 COMPARISON OF ROTATING-BEAM WITH FIXED-BEAM CEILOMETER

Much of the material in this section has been extracted from

references [125^ 126] . The information obtained during these

comparisons is illustrative of the problems due to the inherent variability in cloud base height.

The measuring methods of the rotating-beam and fixed-beam ceilometers

are fundamentally the same. The measurements are made by simple triangulation in both cases so that, other things being equal, the cloud

heights measured by both instruments should be identical. However, the

spectral characteristics of the lamps and the photocells employed in

the two systems differ. The fixed-beam projector utilizes a mercuryvapor quartz-tube lamp with a detector photocell whose sensitivity is

at a maximum in the visible spectrum near 0.^5 micron (450 nm). The

rotating beam utilizes an incandescent lamp rich in infrared, with a detector photocell whose maximum sensitivity with filters is beyond

the visible spectrum in the near infrared (1.5 to 2 microns). In the

past, all cloud-height or ceiling measurements, such as with ceiling

balloons, ceiling lights, and fixed-beam ceilometers, have been made by

methods employing some form of visible light transmission or scatter.

The fact that the rotating-beam system utilizes light beyond the

visible spectrum brought up the question of the validity of its

measurements

.

Comparisons of the ceilometers in the 1950 's were designed to

determine whether cloud-height measurements made by the rotating beam,

using an infrared photocell, were compatible with those made by the

fixed beam using a visible light cell. Some of these tests were conducted as part of the transmissometer-ceilometer program at

Washington National Airport. Figure 3.12 shows the equipment layout.

The primary experiments, however, were conducted in 1952 at

the Silver Hill Observatory in Maryland. Figure 8.6 is a perspective

diagram of the ceilometer layout. R refers to a rotating-beam and

F to a fixed-beam ceilometer. It can be seen that the ceilometers

do not measure the same point on the clouds. However, due to

8-12
PROJECTOR^

" b tan 9^

b ton df

DETECTORS

'ClZlVzir Observatory, Silver Hil, ,

8-13
difficulties in preventing interference between the two units, it was not feasible to have the components so situated that they could

make independent measurements of the same point of the clouds. It was thought the separation did not materially affect the results of the

comparisons, because the distance of separation was relatively small.

The fixed-beam ceilometer used in the tests was a conventional

instrument of the type then in common use in the United States,

with the exception that its scanning cycle was twice as fast as normal.

It gave two measurements every six minutes and was equipped with a reversing switch, so that any scan could be stopped and reversed at

v/ill by the observer. During these comparisons, the reversing switch

was used to provide as many discrete measurements as possible, so that the frequency of measuring shown on the following diagram is

not representative of its normal use.

Figure 8.7 shows a comparison of heights corresponding to the

maximum reactions of the two ceilometers. The comparisons indicate

that although the rotating-beam ceilometer gives greater detail, the

two systems are compatible in that there is no systematic difference

between them. The diagram points up the large variations in lowlevel cloud base structure.

The diagram also shows the need for frequent spot checks, if

measurements are to be made of all significant points of the contour

of a cloud base as it passes over the instrument. The line connecting

the seven spot measurements of the rotating-beam ceilometer following

minute 76 probably gives a reasonably accurate representation of the cloudbase contour passing over the ceilometer at that time. On the other hand,

it is very unlikely that the line connecting the ten spot measurements of

the rotating-beam ceilometer following minute 86 gives a true representation.

The spot indication of the instruments were connected on this diagram to

facilitate comparison of the heights measured by the respective

instruments

.

8-14

Figure 8.8 is an expanded pictorial comparison of the two systems.

The bottom picture shows possible undulations in the cloud base as it

passes over the fixed-beam system during one cloud-height measurement.

Most of the fluctuations are smoothed out and integrated into one measurement. During this time, the top picture shows the rotating-beam ceilometer

making two discrete measurements. The horizontal extent of clouds passing

during one indication of the rotating beam is represented by the separation

in the detector cone-of-acceptance shafts shown between 0 and 1 second and

23 and 24 seconds

.

Fig\rre 8.9 is a pictorial comparison of the sampling rates and cor- responding instrumental presentation of a rotating-beam ceilometer, which

gives an indication every 24 seconds, and a fixed-beam ceilometer, which

yields two indications every 12 minutes, both operating on a 438 foot

baseline. The areas AA' and BB' represent the portions of the cloud base

measured by the fixed-beam ceilometer. The drawing shows that the smallscale fluctuations in the cloud base are integrated into one indication of

this instrument; while each indication of the rotating-beam instrument,

represented by the black areas, includes only a small horizontal extent of

a moving cloud.

The points of first reactions of the rotating-beam ceilometer refer to

the lowest height at which a reaction occurs, and are represented by the

bottoms of the "blips" shown on the oscilloscope presentations. The maximum

reactions are represented by the widest portions of the traces, and the

final reactions refer to the greatest height at which a reaction occurs, as represented by the tops of the "blips". A rate of one measurement every 24

seconds gives a rather complete base-contour picture, and leads to the belief

that an instrument that will yield indications every six seconds will, for

practical purposes, give a continuous cloud-base measurement.

In order to accurately describe the cloud base or ceiling, it is

essential to know as much as possible about the variations or fluctuations

in cloud base height that a pilot may expect to encounter during an approach.

For this reason, the Weather Bureau changed their specifications for rotatingbeam ceilometers in 1953 to require double -project or instruments rotating once every 12 seconds, which gives a cloud measurement every six seconds.

8-16

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8-18
8.6 FLASHTUBE PULSED-LIGHT CEILOMETERS

Early model pulsed-light ceilometers were also tested in the

United States in the 1950 's [125, 126]. The projector and detector

are positioned side by side. A narrow beam of short (100 y sec),

intense pulses of light from the xenon source is directed at the

cloud and a small portion is reflected back to the receiving equipment.

The time interval required for each pulse of light to travel to and

from the cloud is measured as an indication of range.

The results obtained with the pulsed-light ceilometer were discouraging. The intensity of the light and/or the sensitivity of the

receiver was insufficient to allow cloud base measurements except in

ideal atmospheric conditions. Moreover the length of the tail of the

light pulse was so long that cloud heights below 500 feet could not be

measured.

8.7 LASER PULSED-LIGHT CEILOMETERSThe laser beam ceilometers developed to date operate on the same time measuring principles as the pulsed-light ceilometer previously

described. They are rangefinders, i.e., the time elapsed between the

transmission of the laser pulse and the return of the laser energy,

scattered by the cloud is directly proportional to the cloud base

height. Laser sources are capable of emitting pulses that are much

shorter and more powerful than those of xenon sources.

8.7.1 Sperry Lidar Ceilometer

The National Weather Service conducted an evaluation of a Sperry lidar ceilometer (SLC) in 1973 [15, 38]. When possible, evaluations

were made relative to the rotating beam ceilometer (RBC).

The projector of the SLC consists of a stack of 17 gallium arsenide (GaAs)

diodes in a fiber coupled array. The projector emits 350 watt, 80

nanosecond pulses at 906 nanometers wavelength with a beam diameter of

10 inches (25 cm). This is much shorter than the 100 microsecond

pulse length of the xenon pulsed-light ceilometer.

8-19
In comparison with an 800 ft baseline RBC, the relationship of

SLC to RBC results was nearly linear. As SLC height changed 200 ft,

the RBC changed about 300 ft. In evaluating the data, George stated

there was no definitive method for determining which of the two

ceilometers was more correct.

It should be pointed out that the response of the RBC can be

interpreted many ways — one does not have to necessarily use the

point of maximum response, although maximum response has been the standard

method.

In a sense, the SLC evaluation is consistent with the MDB tethered

ballon results [126] . Figure 8.10, both left and right, shows that the

maximum reaction of both fixed and rotating-beam ceilometers is 200

feet when most highest visible balloon observation data plot around

300 feet. This is consistent with the 2/3 linear relationship between

the SLC and RBC found by the National Weather Service. However, when

first reaction of the RBC is compared with lowest partially obscured

balloon observed, figure 8.11, very good agreement is obtained.

8.7.2 Erbium and Ruby Laser Ceilometers

Air Force Cambridge Research Laboratories recently reported on

the results of the test and evaluation of two laser ceilometers in

comparison with the RBC [98]. One was a ruby laser (694.3 nm) system

manufactured by ASM of Sweden and the other was a prototype "eyesafe" erbium laser (1.54 um) system developed by American Optical Company.

Both ceilometers are high powered, single pulse systems. The 2 megawatt

ruby laser pulse had a 30 nanosecond width. The 1 megawatt erbium laser

had a 35 nanosecond pulse width.

Data comparisons showed that these lidars produced the same type

of cloud height information as the RBC. They have a significantly

greater ranging capability than the RBC and provide more cloud

structure information. AFCRL concluded that lidars are potentially

superior to the RBC as cloud-height measuring devices

.

Additional comparisons between the ruby lidar ceilometer and the RBC

8-20
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8-21
z

o

)-

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<

LjJ 00

700

600

1^500

Lu UJ

I- o ^

O400

O -J Q O

z

2|300

UJ UJ

cr 03

en ' o ^

o z

r2oo

UJ o

ui cr 100

X

UJX

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0 100 200 300 400

HEIGHT (FEET) OF LOWEST BALLOON

500 600

IN CLOUDS

700

Figure 8.11 Comparison of rota ting-beam ceilometer

and lowest partially obscured balloon

[126].

8-22
[99] show the lldar Indicates an accurate presentation of cloud structure

and that the RBC cloud return is affected hy its geometry and by multiple

scatter in the cloud. These effects bias the RBC to indicate somewhat

higher cloud heights. However, the difference in cloud heights as measured by the two systems is not significant.

8-23

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R-1
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.

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R-2
29. Douglas, C.A. and Simeroth, J.W., Development of a Transmissometer

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44. Hochreiter, F.C. and McCann, R.J., Transmissometer Calibration

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R-3
45. Hoijer, S., Atmospheric Attenuation of a Light Beam Due to

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48. Houghton, H.G., On the Relation Between Visibility and the

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49. Houghton, H.G., The Transmission of Light in the Atmosphere

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.

57. International Civil Aviation Organization, All-Weather Operations

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, Doc. 8685 AWOP/III-1.

58. International Civil Aviation Organization, All Weather Operations

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R-4
59. International Civil Aviation Organization, All Weather Operations

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.

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AN-CONF/5

.

62. International Civil Aviation Organization, Eighth Air Navigation

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First Meeting, Montreal, 5 December 1950.

64. International Civil Aviation Organization, Visual Aids Panel,

Second Meeting, Montreal, 28 June to 20 July, 1962.

65. International Civil Aviation Organization, Visual Aids Panel, Third

Meeting, Montreal, 14-30 October 1954, Doc. 8483-AGA/594

.

66. International Civil Aviation Organization, Visual Aids Panel,

Fourth Meeting, Montreal, 30 August - 16 September 1956. Doc.

8631-AGA/595.

67. International Civil Aviation Organization, Visual Aids Panel,

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68. International Civil Aviation Organization, Visual Aids Panel, Sixth

Meeting, Montreal, 5-17 March 1972, Doc. 9005, VAP/VI

.

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1955.

73. International Meteorological Organization, Conference of Directors,

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R-5
74. Jainski, P. and Schmidt-Clausen, H.J., Uber den Einfluz der

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of Colored Signal Lights) , Lichttechnik (Lighting Technology)

19(1) : lA - 6A, 1967.

75. Keller, R.A., Problems Encountered in the Design of a Portable

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76. Kevern, G.M., Effect of Source Size Upon Approach Light Performance,

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Atmos. 12^, 33-55, 171, 1925.

79. Koschmieder, H., Measurements of Visibility at Danzig, Monthly Weather

Review, 58, 439, 1930.

80. Kuper, J.B.H., Brackett, F.S., Eicher, M., Integrating Photoelectric

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1960, Washington, D. C.

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Contrast in Detection of Targets by Daylight Vision. I. Data and

Analytical Description, J. Opt. Soc. Am. 37 , 531, 1947.

83. Landing Aids Experiment Station, Meteorology, 1946 Test Season,

Areata, Calif.

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Areata, Calif.

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Areata, Calif.

86. Landing Aids Experiment Station, Meteorology Section 1949 Final

Reports, Areata, Calif.

87. Langmuir, I. and Westendorp, W.F., A Study of Light Signals in

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R-6
89. Lefkowitz, M. , et. al., Studies in the Field of Approach Visibility

Measurement and Instrumentation, FAA, Final Report, Project No.

202-2-IX, April 1962.

90. Lefkowitz, M., and Schlatter, E.E., An Analysis of Runway Visual Range,

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90a. Leonard, D.A., An Experimental Feasibility Study of a Laser Raman

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(Everett, Mass.) June 1972.

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95. Middleton, W.E.K., Vision Through the Atmosphere, University of

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96. Middleton, W.E.K. and Wyszecki, G.W., Visual Thresholds in the Retinal

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54, 1961.

97. Middleton, W.E.K. and Gottfried, H.S.T., Apparent Intensities of

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98. Moroz, E.Y., Lawrance, C.L., and Travers, G.A. , Results of Comparison

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100. Muench, H.S., Moroz, E.Y., and Jacobs, L.P., Development and Calibration of the Forward Scatter Visibility Meter, AFCRL-TR-74-0145,

March 1974.

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the Effective Intensity of Composite Light Units in Restricted

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the Effective Intensity and the Visual Range of Flashing Lights in

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R-7
103. Nat. Bur. Stand., Instruction Book for Transmissometer Set AN/GMQ-10

NBS Report 2588, Revised January 1955.

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a Radioactive Cathode, Rev. Sci. Instr. 11 , 2, 1940.

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1968.

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of France (1864) , Translated, for the Use of the Light-House Board

of the United States by Major Peter C. Hains, USA, Washington:

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FOA-2 Rapt. A-2486-93, May 1967.

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Systems, Final Report, FAA Report RD-68-49, August 1968.

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Lighting, NBS Report 8019, June 1963.

115. Sperry Gyroscope, Visibility Measurements Technique Program, Report

SGD-4221-0649, Aug. 1973.

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801, 1952.

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May 1937.

R-8
118. Tousey, R. , and Koomen, M.J., Visibility of Stars and Planets During

Twilight, J. Opt. Soc . Am. 43_, 177, 1953.

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Appl. Opt. 4, 501, 1965.

120. Van de Hulst, H.C., Light Scattering by Small Particles, John Wiley

and Sons, Inc., New York, 1957.

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Upper Atmosphere, Trans. lES (London) 10, 147, 1945.

123. Waters, R.H., and Reynolds, R.R., Analysis and Modification of RVR

Equipment for RVR Values 500 Feet and Above, FAA, Final Report

No. RD-66-34, May 1966.

124. Weather Bureau/Air Navigation Development Board, Final Approach

Visibility Studies, Fiscal Year 1952 Progress Report, Part I.,

Washington, June 1952.

125. Weather Bureau/Air Navigation Development Board, Final Approach Visibility Studies, Fiscal Year 1952 Progress Report, Part II., Washington,

March 1953.

126. Weather Bureau/Air Navigation Development Board, Final Approach

Visibility Studies, Final Report, Washington, April 1955.

127. World Meteorological Organization, Guide to Meteorological Instrument

and Observing Practice (1950) with supplements (Feb. 1955), Chapter 8 - Measurement of Visibility.

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and Observing Practices, Chapter 10 - Measurement of Visibility, 1971.

129. Zuev, V.E., Atmospheric Transparency in the Visible and the Infrared,

translated from Russian, NTIS-TT-69-55102, 1970.

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in the Atmosphere, translated from Russian, NTIS-N72-30336, 1972.

R-9
!

!
APPENDIX A

Topical Outline of the First Weather Bureau

Meeting on Runway Visual Range

Washington, D. C.

May 5 & 6, 1949

I. Statement of Problem:

1. Requirements for Ceiling and Visibility Data for:

a. Civilian Aviation Operation

(by Representatives of CAA)

b. Military Aviation Operation

(by Representatives of the Military Services)

c. FIDO Operation

(by Representatives familiar with Landing Aids Experiment

Station work)

2. Summary of Discussions of the Problem.

II. Plan of Attack:

1. Discussion of solution to problem by:

a. Employing ceilometer and transmissometer at selected points

to give in effect glide path visibility.

b. Employing pulsed light range meter to obtain required

information.

c. Employing 1 cm radar to obtain required information.

d . Other means

.

a. To be discussed by person familiar with Landing

Aids Experiments

.

b. To be discussed by Signal Corps and Weather Bureau.

c. To be discussed by Signal Corps.

2. General discussion of proposed solutions.

III. Present Engineering Assignments having Bearing on Solution of Problem:

A-1
1. Outline of projects bearing on problem and being carried out by

a. Signal Corps

b . Weather Bureau

c . Other Agencies

2. Summary Discussion

Fut\ire Plans:

1. Plans covering development and testing of equipment by:

a. Weather Bureau

b. Signal Corps

c . Other Agencies

A-2
APPENDIX B

NOTES ON THE ABSORPTION OF LIGHT BY AIR AND THE

RANGE OF DIFFERENT LIGHTS*

-France

/Following is a translation of an article by a Staff

Engineer, Central Lighthouse Service, in the French

language periodical Archives, Visibilite'' des Feux

(divers

)

(Archives , Visibility of Lights (Varied) j,

Paris, April 1S60, 4 pages^

Light, in traversing any medium, is subjected to loss or

partial absorption independently of intensity loss due to distance increase. The absorption effected by each infinitely

thin surface cross section is evidently proportional to the

density of this cross section and to the intensity of the light

which traverses it, so that if v/e call y the luminous intensity

and dx the density of a surface cross section, the loss due to

absorption is expressed by

dy = -ky dx.

If the rays are divergent, the intensity will also diminish inversely as the square of the distance, and we then

have : o \ o rt^ =/Lx_.r - A+dx = i.2dx

y \ x+dx / \ X / X

and

The total attenuation of intensity is the sum of these

two expressions so that we finally have a . differential equation

dx = .2

^

-k dx

y X

and we obtain t v L a-^

y = —X'

*An unsigned Note was sent to Douglas "by Monsieur P. Blaise, of the French

Lighthouse Service, who stated that the author "could be none other than

Allard."

B-1
The quantity a = e~^ varies from 0 to 1 depending on

whether the medium absorbs completely or allows all the light

to pass; this is the coefficient of transparence of the medium

under consideration. L represents the intensity of the luminous

source of light at unit distance in vacuum or for a = 1.

V/e will indicate the results that can be obtained by

applying this formula to the passage of light in the atmosphere.

From experiments performed by Bouger and reported by

Lacaille in his optics, it follows that a horizontal section of

1S9 fathoms or 368.4 feet of air causes a loss of one hundredth

of the light and that 7469 fathoms, or 14,557.4 feet dissipates

a tiiird. Taking the kilometer^as the unit of distance, we should

therefore obtain: 0.99 = a^*-^^^^ and 2/3 = a-'-^»5574. The first

of these equations gives a = 0.9731 and the second gives a = 0.9725. Ve can take a = 0.973 as representing the coefficient

of transparence of air during Bouger *s experiments.

In order to determine the range of a light L, i.e. the

farthest distance at which this light can be seen, it is necessary to define another coefficient X vfhich represents the smallest quantity of light perceptible to an observer. Thus:

This equation, solved with respect to x, will give the range of

a light L in an atmosphere whose transparence is a, for an observer whose eye cannot perceive a luminous intensity smaller

than X . Only one observation is necessary to determine A . Thus, for example, we haye recognized at the lighthouse workshop that one pilot light representing 1/200 carcel could just

be perceived when judged at 500 meters. If we assume that the

transparence of the atmosphere during this observation was the

same as that during Bouger* s experiments - 0.973 - 'we will have:

T • 1/200 (0. 973)-^/^

/ = —i ^ ^.^00^ 0.019728.

(1/2)2

This value of A should be checked by new experiments. If

we accept it provisionally a s valid, only the second equation

M = 0.019728

x2 will be required to obtain the intensity L of the smallest light

that could be perceived at a given distance x, or likewise the

greatest distance x at which a given light L could be seen, in

B-2
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<r\ ^CM <A CM

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c^ ^ vr\ • •» «k NO CJN CM

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-r^- NO NO

CM CN- CM

CM 5 ^ o vr\ cA

rH NO

00 C7N rH ^

0^' H

CN- cn

O NO »A rH 00

CJN 00 iH

CM U>(O

»A CM ^ CN- rH VA NO

NO

CTn n2) ^ >A Cvl (NO

CA CjN CM

CM

O NO

CM NO o u^

Ci- CM CM

CJN On VA NO

CM00

NO NO

CA

VA lA

rH CM

C3N jii- CJN CA

O

H

NO

rH

OO

H >A

SCA ON CM ^ CM

JN- NO

cn

CM

O CM NO CA

CJN rH ON O ^

CA C3N

H Cs- On

On cj- 00

CM ^

NO

CM NO

rH NO

rH ^

<A NO

C7N NO

rH (A

CM CM

NO>A

NO

NO NO

CNJ ^ 00 O

rH CA

NO

O CO NO Cvl 00 CM o

NO• ^

ON 00 NO 00

CM rH

CN- CM CO rH

O rHO >A

O

rH

H CM

CMrH rH

VA

O

CM CA

CA

CJN rH NO NO CJN (v- CJN CM VA ON

t-< r-i r-\

tA

O

0^ .^^

O

O

>A NO

OO

CO o o ON O

fH

CA ON ^ CO o o VA O

rH CM

B-3
LTnainous Valuex x of the greatest distance at which light L can be perceived in

Intensity an atmosphere whose coefficient of transparence is a = L

1.00 0.973 0.95 0.93 0.925 0.90 0.85 0.75 0.50 0.25

1 7.12 6.48 6.08 5.75 5.67 5.33 4.78 4.00 2.62 1.78

5 15.9 13.2 11.8 10.8 10.5 9.6 8.2 6.3 4.0 2.4

10 22.5 17.5 15.3 13.7 13.3 12.0 10.0 7.5 4.5 2.9

14 26.6 21.8 16.9 15.3 14.0 13.1 10.9 8.2 4.9 3.1

90 67.5 39.4 30.7 20.2 25.3 21.3 16.4 12.0 6.6 4.1

280 119.1 55.1 41.0 34.3 32.7 27.3 21.5 14.4 7.8 4.5

600 17^ A 68.2 49.0 49.4 38.4 31.8 24.0 15.7 8.6 5.1

5000 503 A 105. 75.0 59.0 55 44 33 21 10.7 6.2

20000 1006.9 136 93 72.0 69 54 40 26 12.3 7.1

00000 2251.4 175 153 89.0 84 66 46 31 14 8.0

These tables allow us to appreciate the effect of atmospheric state on the

range of lights. This effect is all the more stronger the more intense the

light.

Thus, a mist which reduces the usual range of a carcel in clear air by

1/4 will reduce the range of a forirth order light by half, that of a second

order light by 3/5 and that of a first order light by nearly 2/3. Likewise,

if we compare two lights placed successively in different atmospheres, we see that the ratio of their ranges diminishes as the air becomes less transparent.

For example, if a fixed first order light were replaced by an electric light

8 to 9 times stronger, 5OOO btimers to 6OO burners, the range would be increased

by half in ordinary air, by 1/4, I/5 and even less in less transparent atmospheres.

The distances taken by navigators as representing the ranges of different

lighthouses should be calculated on the basis of an atmosphere i^ich is neither

too clear nor too misty. We are using numbers irfiich pretty nearly correspond

to the coefficient a = 0.93. Here, moreover, are the ranges of different

lights for this coefficient and for two similar ones O.9O and 0.95» expressed

in miles from I852 meters. We also indicate the ranges which an electric

lighthouse of 5OOO burners will have.

B-4
Range, calc\ilated in miles Accepted

Designations of lights Intensity according to formula for a range

0^ 0.93 0.90

Lamp burning 40 g

(caroel lamp) 1 3.3

9.1

16.6

3.1

8.3

14.2

18.5

21.8

31.9

2.9

7.1

11.5

14.8

17.2

23.7

3

Fourth order light

Third order light

Second order light

First order light

Electric lamp

14 10

90

280

600

22.1

26.4

40.5

15

18

20

5000 11

If we want to adjust the numbers for the ranges so that they

exactly correspond to identical atmospheric states, it would be necessary

to decrease those of fo\irth and third orders a little and increase those

of second and first orders a little. We could adopt either the numbers

8, 14, 18, 21, neglecting fractions, or 9. 15t 19. 22 using the next

higher article number. Under the same circumstances, the range of a first

order electric light would be 31 or 32 miles.

6718

CSC: R-7133-D

B-5

APPENDIX C

Nomographs for Computing the Visual Range

of Lights

1.1 Allard's Law

E = IT /D (1)

can not be solved directly for D and can not be solved for I or T

with an ordinary desk calculator unless logarithms are used. For

these reasons nomographs are very useful in visual range computations.

Such nomographs have been prepared in many different forms. Most

of these nomographs lack flexibility in application in that they

are based upon particular thresholds and \mits of distance. The

nomographs used at the National Bureau of Standards for many years

are shown in figures C-1, C-2, and C-3. They are based on a

design developed by the late M. K. Laufer, and provide the desired

flexibility.

The basic nomogram is an alignment diagram which can be used

to solve equation (l) for one of the variables D, T, or I/E if the

other two are known.

These nomographs have been prepared with the ratio I/E as one

of the three variables. Hence the nomographs are applicable for

any value of E. Since the value of E is usually the product of an

integer and a power of 10, the relation

I/E = K

can usually be solved mentally. Moreover, any convenient unit of

distance may be used provided that E, D, and T are in consistent \mits.

C-1
Four nomographs have been prepared, instead of a single

nomograph, in order to cover the desired range of distance and

transmittance without unduly compressing the scales and compromising accuracy. Transmissivities down to 0.10 are covered

by figure C-1; to 10""^° by C-2; to 10""^°° by C-3 (case III)

and to 10"^°°° by C-3 (case IV).

Auxiliary scales have been drawn to the right of the basic

alignment diagram for easy conversion between transmissivity and

related quantities. The defining equation is given for each

scale. The DAY and NIGHT visibility scales are those used in the

United States to relate transmissometer measurements to human

visibility observations (except that the WMO contrast threshold of

0.05 have been used instead of the conventional 0.055. (See

Section 5.5).

Example 1 .

How far can a runway light with an intensity of 50,000 candelas

be seen in daylight when the meteorological visibility is 0.035 miles.

Note: For daylight, E = 1000 lumens per square mile (lOOO mile candles).

The solution: From 0.035 on VISIBILITY, DAY, draw a horizontal line

to TRANSMISSIVITY scale. Since 0.035 is on CASE III scale, the transmissivity is read on the CASE III TRANSMISSIVITY scale. Thus,

T = 10"^'^

Since I = 50,000 and E = 1000,

I/E = 50.

-37 1 From 50 on the I/E scale, draw a line to T = 10 . Read D on

CASE III DISTANCE scale as 0.10 mile.

C-2
Example 2 .

How far can the same light be seen at night.

The solution: In this case E is 2 lumens per square mile (2 mile

candles), and I/E is 25,000. From 0.035 on the VISIBILITY, NIGHT

scale, draw a horizontal line to the TRANSMISSIVITY SCALE

-112

obtaining a T of 10 . Draw a line from this point to I/E =

25,000, obtaining a D of 0.06 mile. Note that the CASE IV scales

are used.

C-3
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C-6
Appendix D

NBS Transmissometer Performance Check List

(Extracted from Reference [103].

)

D-1 The following are the miniminn performance requirements for a

transmissometer properly adjusted and operating. The paragraph

references indicate the paragraphs describing the corrective

procedures

.

D-2 PROJECTOR

Hourly Cutoff

Manual Cutoff

Alignment

Stability

D-3 RECEIVER

Alignment

Stability of

Pulse Rate

Minimum

Pulse Rate

Duration 45 to 75 seconds

.

Background switch to TEST turns off

lamp.

All small changes in alignment produce

either no change or a decrease in the

transmissometer reading.

No systematic changes in reading after

lamp cutoff are observable

.

All small changes in alignment produce

either no change or a decrease in the

transmissometer reading.

Width of the line of the clear-weather

trace on the recorder does not exceed one

chart division,

a. With receiver lens blocked, pulse rate

is not faster than one pulse in thirty

seconds

.

D-1
Calibration

Background

D-^ INDICATOR

Zero Adjustment

Calibration

Voliune and

Bias

Meters

b. As the light falling on the receiver is

decreased, the receiver will not stop

pulsing until the pulse rate becomes

slower than a pulse in thirty seconds

.

With the CALIBRATOR Switch ON, the •

TRANSMISSION meter reading differs from

the CALIBRATION SETTING reading by not

more than 1%.

The pulse rate with the light off is less

than 1 per second.

a. With POWER switch OFF the TRANSMISSION

meter and the recorder read zero.

b. The electrical zero of the indicator

is within + 1% of zero.

With the CALIBRATE switch to TEST, the

TRANSMISSION meter and the recorder differ

from the indication of the CALIBRATION

SETTING meter by not more than + 1% for

both positions of the RANGE switch.

Connecting a load of 0.1 megohm across

the OUTPUT TEST jack causes no change in

meter reading when CALIBRATE switch is

in either position.

TRANSMISSION meter reading at the indicator and recorder reading differ by not

more than 1%.

D-2
stability

Chart Time

D-5 SYSTEM

Signal Line

100^ Setting

Stability

Zero and calibration settings do not

drift more than + 2% in 24 hours.

The gain or loss of time in the chart

drive does not exceed one minute per

day.

Connecting a load of 1000 ohms across

the INPUT TEST jack of the indicator

causes no change in the meter reading.

On a clear day, the 100^-setting factor

differs from 1.00 by not more than +_ 0.03.

100^ setting does not change more than

3^ per week.

D-3
; NBS-114A (REV. 7-73)

"U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS MN 159

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Visual Range: Concepts, Instrumental Determination,

and Aviation Applications

5. Publication Date June 1977

6. Performing Organization Code

7. AUTHOR{S) Douglas and R. L. Booker 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Prnierr /Taslc/Worlf tinir Nn

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Federal Aviation Administration

Systems Research and Development Service

Weather Data Section

Washington, D. C. 20553

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card. Number: 77-7343

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This document is a review of the principles, procedures, and instruments used

in the measurement of visual range. The fundamental concepts of the visual range

of objects and lights are discussed. The principles of operation of the several

classes of atmospheric attenuation meters are reviewed and representative

instruments are described. The course of development of the NBS transmissometer

,

its validation and application to aviation operations is reported. An error

analysis is made of the effects of instrument errors and of differences in observer

thresholds on visibility measurements. A chronological review of the development

and application of the runway visual range concept is included together with a discussion of cloud height measurements.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Ceilometer; runway visual range; scattering; transmissometer; visibility; visual

range.

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Systems Analysis; Product Engineering — Center for Building Technology: Structures, Materials, and Safety; Building

Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment: and serves as the principal focus wthin the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer

languages. The Institute consist of the following divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process

to facilitate technological change in the private sector by examining and experimenting with Government policies and practices in order to identify and remove Government-related barriers and to correct inherent market imperfections that impede

the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units:

Office of Standard Reference Data — Office of Information Activities — Office of Technical Publications — Library —

Office of International Standards — Office of International Relations.

'Headquarters and Laboratories at Gaithersbuxg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

" Located at Boulder, Colorado 80302.
U.S. OEPARTMENT OF COMMERCE

National Bureau of Standards

Washington.D C. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM-21S

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