NBSIR 74-603

A Standard Procedure for Setting

of Photoelectric Controls Prior

to Installation on Lighted Obstructions

I. Nimeroff

Heat Division, Optical Radiation Section

Institute for Basic Standards

National Bureau of Standards

Washington. D. C. 20234

October, 1974

Final Report

Prepared for Federal Aviation Administration

Washington, D. C. 20590

NBSIR 74-603

A STANDARD PROCEDURE FOR SETTING

OF PHOTOELECTRIC CONTROLS PRIOR

TO INSTALLATION ON LIGHTED OBSTRUCTIONS

I. Nimeroff

Heat Division, Optical Radiation Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

October, 1974

Final Report

Prepared for Federal Aviation Administration

Washington, D. C. 20 590

u. TU. S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

Preface

The Federal Aviation Administration of the U. S. Department

of Transportation, requested the National Bureau of Standards to

prepare a standard procedure by which photoelectric controls could

turn on and off, or turn on and control the intensity of obstruction lights in accordance with the specifications of FAA Advisory

Circular AC 70/74601C. The work included a study of the spectral

properties of available materials to simulate twilight conditions

that prevail when the photoelectric controls operate.

The opinions, findings, and conclusions in this report are

those of the author and are not those of the Federal Aviation

Administration. Certain comnercially available equipment, instruments, or materials are identified in this report to specify

adequately the procedures used. In no case should such identification be inferred as either recommendation or nonendorsement by

the National Bureau of Standards, nor does the identification imply

that the materials are necessarily the best available for the

purpose

.
Contents

Page

Preface i

Abstract iii

1. Introduction 1

2. Requirements of the Light Control Device 1

3. Calibrating Device Requirements 2

4. Design of Calibrating Device 2

5. Selection of Distances 5

6. Recommended Calibrating Procedure 10

6.1 Photometric Equipment 10

6.2 Setting Procedure 11

7. Discussion 11

- ii -
Abstract

A calibration device was designed and a procedure was developed by which photoelectric controls of obstruction lights

could operate in accordance with specification requirements of FAA

Advisory Circular AC 70/74601C. The device, consisting of a source

and a diffuser-filter , was designed primarily to provide the

photocell of the control with diffuse illumination of proper

spectral distribution to approximate normal twilight conditions.

Illumination at the photocell is varied by changing the distance

between the source and the diffuser-filter . Because there may be

wide variation from noxrmal twilight conditions, causes of such

variation and their possible effects are also discussed.

A Standard Procedure for Setting of Photoelectric Controls

Prior to Installation on Lighted Obstructions

1. Introduction

In a letter dated March 29, 1974, the Federal Aviation

Administration requested the National Bureau of Standards to

prepare, under Inter-Agency Agreement, DOT-FA72WAI-299, a standard

procedure by which photoelectric controls could turn on and off, or turn on and control the intensity of obstruction lights in

accordance with the specification requirements of FAA Advisory

Circular AC 70/74501C. This report is in response to that request.

2 . Requirements of the Light Control Device

FAA Standard on Obstruction Marking and Lighting, AC 70/7460-

IC, requires that a light-sensitive control device be installed so that it faces the north sky. The device is used to control the on

and off condition of aviation red obstruction lighting and flashing

red hazard beacons, and to control the daytime, twilight, and

nighttime conditions of light intensity emitted by high- intensity

flashing white obstruction lights.

The light control is required to operate the red lights when

the illiaminance on the photodetec ting surface is 30 footcandles or less and to turn off the lights when the illuminance is 60 footcandles or more.

The control is required to change the setting of the highintensity flashing white obstruction lights when the illuminance

level on the photodetecting surface of the device meets the

following conditions:

1. The night setting shall be switched to the

twilight setting, and the twilight setting to the

night setting, when the illumination is in the range

2 to 5 footcandles inclusive.

2. The twilight setting shall be switched to

the day setting, and the day setting to the twilight

setting, when the illumination is in the range 30 to

60 footcandles, inclusive.

- 1

-
3. Calibrating Device Requirements

A calibrating device is required that will produce at the

entrance window of the control device a diffuse illumination that

simulates in spectral distribution and magnitude the ambient

north sky characteristics at twilight conditions. Diffuse

illumination at the photoelectric control may be obtained by

placing the entrance window of the control against one surface of

a diffuser and illuminating the other surface, as shown in Figure

1.

The proper spectral distribution, estimated to have a correlated color temperature of about 5000 K, can be achieved by

mounting an optical filter onto the diffuser, between the source

and the diffuser, as in Figure 1. An alternate diffuser-filter

may be a simple, properly colored diffusing material as is shown

in Figure 2.

The magnitude of the illuminance E on the photoelectric

control is determined by the illuminance 8 on the diffuser-fliter

and its luminous transmittance t , thus

E = Et (1)

p

The illuminance on the diffuser-filter depends on the intensity I of the source and the distance d between the filter and source. The

illuminance from the source may be varied by changing the distance

because

E = W

(The illuminance should not be varied by changing the voltage

applied to the source to change its intensity, as this procedure

also changes the spectral distribut ion of the emitted flux.) By

combining equations (1) and (2) the illuminance on the photosensitive surface of the photoelectric control is found as

E = Tl/d^ (3)

4 . Design of a Calibrating Device

To alter the spectral distribution of the light emitted by

an incandescent tungsten-filament lamp from a color temperature

of 2855 K to approximately 5000 K requires an optical filter

that has high transmittance in the blue (short wave) end of the

visible spectrum and low transmittance in the red (long wave)

end. The ideal spectral transmittance of such a filter is shown

- 2 -
UJ

u

UJ

IL. o Ooz

UJ

un 1

vuQ

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Z3 a

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- 4 -
in Figure 3. This filter has the spectral distribution needed to

simulate color temperature 4800 K when used with a source operating

at a color temperature of 2856 K.

Two materials were found that could alter the spectral

distribution of a lamp operating at color temperature 2856 K to

approximate a source of color temperature 5000 K.

Optical filtering materials are made in nominal stock

thickness d with corresponding spectral transmittance i for a specific co?orant concentration. The spectral transmittance can

be varied to transmittance by varying the thickness to a relative thickness d^ = a d , where a is the fraction d^^ /d^ of

stock thickness. The fraction of stock thickness and transmittance

are related by the equation,

T. = (T )^ (k^)^"^ (4)

1 o where k is unity minus specular reflectance. By varying the

thickness of the materials in order to vary the spectral transmittance, then computing the chroma ticity coordinates at several

thicknesses and from these data evaluating the correlated color

temperature, a plot of color temperature against relative thickness

can be drawn. See Figure 4. Such a plot can be used as an aid

in choosing the proper thickness of material to obtain the

desired color temperature change.

From these curves it can be seen that Corning glass, type

5900, of 0.83 d (d = nominal stock thickness) will alter color

temperature 285? K ?o 5000 K, as will Rohm and Haas Plexiglass,

type 2329, of 0.49 d^.

The type 5900 glass filter at a relative thickness of 2/3

d yields a color temperature of approximately 4630 K with

luminous transmittance of 0.36. See Figure 5. The type 2329

plexiglass filter at a relative thickness of 1/2 d yields a color temperature of about 5130 K with luminous transmittance of

0.29. The spectral transmittance of these filters is shown in

Figure 6 for two thicknesses.

5 . Selection of Distances

Since the illuminance requirements for turning on or increasing the light output, and turning off or decreasing the

light output of obstruction lighting are specified, there remains

only the problem of setting the proper distance between the

calibrating source and the calibrating diffuser-fliter to obtain

the required illuminance. To determine the distances to obtain

- 5 -
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- 9

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the required illuminance on the photoelectric control, equation

(3) should be solved. Thus, as an example, with a 500-watt

projection lamp of 700 candelas and a diffuser-filter combination

of transmittance 0.30, a distance of 0.57 meters will yield an illumination of 60 footcandles at the photoelectric control.

6. Recommended Calibrating Procedure

The following procedure is recommended for presetting

photoelectric controls for lighted obstructions.

6.1 Photometric Equipment

The photometric equipment shall consist of a source, source power supply, source mount, optical bench, baffles, filter and

diffuser (or diffuser-filter combination) and mount.

6.1.1 Source

The source shall be a 500-watt, or larger, incandescent

filament projection lamp of known luminous intensity when

operated at the proper voltage to obtain a color temperature

of 2856 K. It shall be positioned in its mount so that the

axis of its filament is perpendicular to the optical axis.

The optical axis is an axis that intersects the center of

the lamp filament axis and the center of the diffuser-filter

.

In a lamp that has a single C8 or CCS filament its axis

passes through the gross coils, while in a lamp that has a C13 or CC13 filament configuration its axis passes through

the plane of the configuration in a plane parallel to the

long dimension of the individual filciments.

The plane of the diffuser-filter combination shall be

perpendicular to the optical axis. Baffles shall be placed

between the source and the filter diffuser to eliminate

stray light.

6.1.2 Diffuser-Filter

The diffuser-f ilter shall be either a single-component

plate or a multiple-component plate that diffuses and

selectively absorbs the light from the source to alter the

spectral distribution from a color temperature of 2856 K to

5000 K ± 500 K. The luminous transmittance of the

diffuser-filter shall be known.

6.1.3 Arrangement of Components

The components of the calibrating devices shall be

arranged as shown in either Figures 1 or 2 of this report.

- 10 -
Care shall be taken to mount the source, the diffuser-filter

,

and the photoelectric control of the control device so that they

are properly aligned.

6.2 Setting Procedure

(1) From the known intensity of the source I and the

known luminous transmittance of the diffuser-filter x, determine

the distances between source and diffuser-f ilter to yield the

specified illumination E on the photoelectric control

devices. Use the following equation to compute these

distances d,

E = I T/d^

P

or d = (IT/E )^^^ (5)

P

(2) When the source is operating and placed at each

of the computed distances, set the photoelectric control

devices so that it turns on or increases, or turns off or decreases the light output of the obstruction lights as prescribed in FAA Standard AC 70/7460-lC.

7. Discussion

The procedure described in this report primarily considers

the approximation of the spectral characteristics of the light

incident on the photoelectric control during calibration to that

during operation. Employment of a diffusing component in the

calibrating system is an attempt to approximate the typical

illuminating conditions of use. Because in use the photosensitive

surface of the photoelectric control is directed at the north

sky, a diffuse luminance standard is a better approximation to

typical conditions than a bare lamp standard of horizontal

intensity. The output of these detectors is a function of the

field of view of the receiver when the detector views the sky, or

a diffuse luminance standard, but, it is essentially independent

of the field of view when illuminated by a bare lamp. Thus two

detectors calibrated with a bare lamp might have quite different outputs when illuminated by the sky or a diffuse standard.

It must, however, be noted that the ambient illiamination

conditions under which the controls do operate may vary considerably from the predominating illuminating conditions. One of the

conditions that may cause too high an illumination on the photoelectric control is snow-covered ground. In this condition the

control would set the high intensity white lights for nighttime

later than required and would set the lights for daytime too

early in the morning. The red lights would be turned on later,

and off earlier, when the ground is snow covered.

- 11 -
This effect would also prevail in certain conditions of

fog and haze. In other conditions, that is when the fog and haze

are dense, skylight may be attenuated sufficiently to cause the

control to set for nighttime earlier and for daytime later than

required.

There are, in addition to the causes discussed above,

others that will affect the illumination on the photoelectric

control device. Some of these are:

1. Location of photocell on tower

2. Height of tower

3. Elevation of site of tower

4. Variation of sky brightness

5. Presence of low clouds

6. Duration of twilight which determines period of

uncertainty

i

Because of the complex nature of these sources of variation

of ambient lighting conditions and the unpredictability of their

occurence, either singly or in combination, the best that can be

done at the outset is to set the calibration according to the

present requirements of AC 70/74601C. Then, after some trial

period, the calibration system is evaluated on the basis of its

effectiveness at a particular location. If it is found to be

adequate no further work will be needed. If, however, it is

found wanting for certain locations where one or more sources of

variation predominate, adjustments in setting can be made to

accommodate the ambient illuminating conditions.

After this report was drafted, we received a copy of the

October, 1974, issue of the Journal of the Illuminating Engineering

Society which contained, on pages 69-71, the lES Guide for

Calibration of Photoelectric Control Devices. The specific

procedures recommended in this report are consistent with the

general procedures given in the lES Guide for Calibration of

Photoelectric Control Devices.

USCOMM NHS f), - 12 -
NIS-IMA (REV. 7.73)

U.S. OCPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUni.K ATION OR REPORT NO

NBSIR 74-603

2. Gov't Accession

No.

3. Rci Ipient's Accession No.

4. TITLE AND SUBTITLE

A Standiurd Procedure for Setting of Photoelectric Controls

Prior to Installation on Lighted Obstructions

5. Publication Datt- October 1974

6. Performing Organization Codf

7. AUTHOR(S)

I. Mimeroff

8. Performing Organ. Keport No. NBSIR 74-603

9, PERFORMING ORGANIZAI ION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OP COMMERCE

WASHINGTON, D.C. 20234

10. Proii ct /Task /Work Un m No 2211500

11. f-ontr«ct /Grant No. Inter-Agency Agreement

I»T-rA72WAI-292

12. Sponsoring Organization Name and Complete Address (Street, City. State, ZIP)

Federal Aviation Administration

Washington, D. C. 20590

13. Type of Report & Period Covered

Final, Oct. 1974

^ponsorin^ A^cnt y ^-odc

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or lea* (actual summary ot moat aignilic^ni iniormation. If document includes a si^iticant

biblioitaphy or literature aurvey, mention it here.)

A calibration device was designed and a procedure was developed by which

photoelectric controls of obstruction lights could operate in accordance with

specification requirements of FAA Advisory Circular AC 70/7460-lC. The device,

consisting of a source and a diffuser filter, was designed primarily to provide the

photocell of the control with diffuse illumination of proper spectral distribution

to approximate normal twilight conditions. Illumination at the photocell is varied

by chamging the distance between the source and the diffuser filter. Because there

may be wide variation from normal twilight conditions, causes of such variation and

their possible effects are also discussed.

17. KEY WORDS (six to twelve entries; alphabetic al order; capitalize only the lirst letter o( the first key word unless a proper name; aeparated by aemicolona)

caastruction lights; photoelectric calibration device; photoelectric controls;

illumination controls; television tower obstruction lighting; tower obstruction

liahtina

18. AVAILABILITY Unlimited

[_J>|f or Official Distribution. Do Not Release to NTIS

Order h rom Sup. of Doc, U.S. Government Printing Office Washineton. D.C. 20402. SD ( at. No. <:i3

1 1 Order I'rom National Technical Information Service (N I LS) Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UN( L ASSIKIFD

21. NO. OF PAGES

17

20. SI ( URITY ( LASS

( mis PAGE)

UNCLASSIFIED

22. Price

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