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PUBLICATIONS

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nati INST CiF STANDARDS & TECH R.I.C. NBSIR 86-3403 All 102542260

«r i ruiic n /Final evaluation of a co

Final Evaluation of a Color

Calibrator for a Radar Remote

Weather Display System

Louis G. Porter

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Physics Division

Gaithersburg, MD 20899

July 1986

QC

100

• U56

r;aiation Administration

rtment of Transportation

n, DC 20591

86-3403

1986

C. 2

NBSIR 86-3403

N3S

RESEARCH

INFORMATION CENTER

FINAL EVALUATION OF A COLOR

CALIBRATOR FOR A RADAR REMOTE

WEATHER DISPLAY SYSTEM

Louis G. Porter

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Physics Division

Gaithersburg, MD 20899

July 1986

Sponsored by:

Federal Aviation Administration

U.S. Department of Transportation

Washington, DC 20591

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This report is the final report on the development and field

testing of an inexpensive color calibrator for the

standardization of the Weather Intensity Level (WIL) colors used

in the FAA's Radar Remote Weather Display System or RRWDS. The

report covers the field validation of the color calibrator and,

as an end product, the construction of a tentative look-up table

that identifies whether the six WIL colors are within acceptable

limits* The look-up table is tentative due to the small (5)

number of field sites visited. The development of a color lookup

table finalizes the validation of the color calibrator as a

useful tool for calibrating the WIL colors used in RRWDS.

Moreover, the color calibrator can be used to calibrate the

colors of other tricolor display systems, provided an appropriate

lookup table is developed.

In addition, this report includes a general review of significant

literature on color-coding, since RRWDS color codes weather

information. The report presents f irst-of-its-kind objective

data on the effects of ambient room lighting on colors used in a

self-luminous display. The effects were chromaticity shifts due

to the presence of veiling reflections or glare from the face of

the CRT console. This report may, accordingly, have significant

impact on the design of building environments where color-coded

CRT's are to be used.

Keywords:

building lighting environments; cathode ray tube (CRT)

;

chromaticity shifts; color calibrator; color coding; room

illuminant reflections; self-luminous displays; specular glare

from CRT's; veiling luminances; video display terminals (VDT)

ill
FOREWORD

This report is the final report of the FAA/NBS Agreement No.

DTFAO 1-83-Y-2059 2 , including the agreements attached thereto.

This report is a continuation of material discussed in report

NBSIR 84-2924: Color Calibrator for RRWDS Radar Remote Weather

Display System prepared by J. L. Heldenbrand and L. G. Porter for

the FAA, U.S. Department of Transportation [9]. NBSIR 84-2924

proved the feasibility of an inexpensive color calibrator under

laboratory conditions only. The present report summarizes

research conducted from January 1, 1985 to October 30, 1985 to

verify that the color calibrator - with lookup table - can

calibrate the WIL colors of the RRWDS as originally envisioned.

In addition, this report represents some original information on

the effect of a building's lighting environment on colors used in

a CRT display.

ACKNOWLEDGEMENTS

The author wishes to acknowledge with thanks the technical advice

and consultations provided by Drs. Belinda Collins, Gerald

Howett, and James Worthey of NBS. Special appreciation goes to

Mrs. Donna Shoemaker for her perseverance in the typing and

editing of the several drafts of this report. The final

preparation of the graphics in this report by Mr. William Remmert

and Ms. Belinda Kuo was much appreciated. Finally but not least,

the author acknowledges with appreciation the active

encouragement and support provided by the FAA technical

representatives, Mr. Armond Maillet, Mr. Robert Benson and

various FAA field personnel, who provided assistance for the

collection of the field data for this report.

iv
DISCLAIMER

Certain commercial equipment, instruments, or materials are

identified in this report to specify the experimental procedure.

Such identification does not imply recommendation or endorsement

by the National Bureau of Standards nor does it imply that the

materials or equipment identified are necessarily the best

available for the purpose.

v
TABLE OF CONTENTS

Page

ABSTRACT iii

FOREWORD iv

ACKNOWLEDGEMENTS iv

DISCLAIMER v TABLE OF CONTENTS vi-vii

LIST OF TABLES viii

LIST OF FIGURES ix

1.0 INTRODUCTION 1

1.1 Background 1

1.2 Literature Review - Color Coding 2

1.2.1 Review by Christ 2

1.2.2 Post-Christ Review 3

1.3 Review Summary 4

2 . 0 BACKGROUND 6

2.1 Weather Intensity Levels (WIL's) Chromaticit ies . 6

2.2 WIL Luminances 6

2.3 Suggested WIL Chromaticity Changes 10

2.4 WIL Variabilities 10

3.0 FIELD TEST OF COLOR CALIBRATOR FOR RRWDS 11

3.1 Field Observations 11

3.2 Tri-color System Effects 11

3.3 Grid #2 Voltages 12

3.4 WIL Chromat icities 14

3.5 Field Test Procedure and Results 14

3.6 WIL Chromaticity Changes Over Time 22

4.0 ROOM ILLUMINATION EFFECTS 24

4.1 Laboratory Observations on Room

Illuminant Reflections 24

4.2 Laboratory Illumination Level Effects 24

4.3 Illuminant Reflections in the Field 25

4.4 Analysis of Color Shifts 25

4.4.1 Background Studies 25

4.4.2 Effect of Room Illuminants on WIL

Chromat icities 26

4. 4. 2.1 Laboratory Description

and Procedure 26

4. 4. 2.

2

Tabulated Results 28

4. 4. 2.

3

Interpretation of Table 10 36

vi
4. 4. 2.

4

Fluorescence/Phosphorescence of

CRT Display 37

4. 4. 2.

5

RRWDS Variability 39

4. 4. 2.

6

Color Vision Theory - CIELAB ... 41

4.4.3 Comparison of Various Illuminants 43

4.4.4 Results 43

4.5 CIE Chromaticity Results 57

4.6 Summary 7 0

5.0 REFERENCES 73

vii
LIST OF TABLES

Page

Table 1 . NBS's original proposed color code for use in

color weather radar plus target chromaticity

coordinates for color weather radar; Figure 1 7,8

Table 2. Summary measurements taken at Electrodynamics

(April 15, 1981) representing NBS's final

proposed chromaticities for the WIL's. 9

Table 3. VDC readings of grid #2 voltages for each RGB

channel

.

13

Table 4. WIL chromaticities as a function of field sites 15

Table 5. Color calibrator DC voltage readings as a function of field site - Leesburg #1. 17

Table 6. Color calibrator DC voltage readings as a function of field site - Leesburg #2 and #3. 19

Table 7. Color calibrator DC voltage readings as a function of field site - Leesburg #2/Teterboro

,

NJ. 20

Table 8. Tentative look-up table for the WIL's using the

color calibrator. 21

Table 9. Illuminance values for laboratory evaluation 27

Table 10. Measured and calculated chromaticity and

luminance values for the 6 WILs and White under

selected light sources.

29-34

viii
List of Figures

Page

Figure 1. Target chromaticity coordinates for color weather

radar. 8

Figure 2. Effect of LPS illuminant on the measured WILs. 44

Figure 3. Effect of LPS illuminant (low illuminance of 41 fc)

on WIL colors (triangles) as compared with their

appearance under dark conditions (squares) . 46

Figure 4. Effect of LPS illuminant (high illuminance of 86 fc)

on WIL colors as compared to their appearance under

dark conditions. 47

Figure 5. Effect of HPS illuminant (low illuminance of 3 fc)

on WIL colors compared to their appearance under

dark conditions. 49

Figure 6. Effect of HPS illuminant (high illuminance of 76 fc)

on WIL colors compared to their appearance under

dark conditions. 50

Figure 7. Effect of cool-white fluorescent illuminant (10 fc)

on WIL colors as compared to their appearance under

dark conditions. 51

Figure 8. Effect of color-classer fluorescent illuminant (14 fc)

on WIL colors compared to their appearance under

dark conditions. 52

Figure 9. Effect of tungsten illuminant (3 fc) on WIL colors as

compared to their appearance under dark conditions. 54

Figure 10. Effect of metal halide illuminant (8 fc) on WIL colors

as compared to their appearance under dark conditions. 55

Figure 11. Effect of mercury illuminant (4 fc) on WIL colors as

compared to their appearance under dark conditions. 56

Figure 12. WIL chromaticit ies as a function of a low

illuminance LPS source (approximately 41 fc at

the CRT display) . 59

Figure 13. WIL chromaticities as a function of a high

illuminance LPS source (approximately 86 fc at

the CRT display)

.

60
Figure 14. WIL chromaticities as a function of a low

illuminance HPS source (approximately 1 fc at

CRT display)

.

61

Figure 15. WIL chromaticities as a function of a high

illuminance HPS source (approximately 76 fc at

the CRT display)

.

63

Figure 16. WIL chromaticities as a function of a cool-white

fluorescent source (approximately 14 fc at the

CRT display)

.

64

Figure 17. WIL chromaticities as a function of a colorclasser fluorescent source (approximately 10 fc

at the CRT display)

.

66

Figure 18. WIL chromaticities as a function of an incandescent

source (approximately 3 fc at the CRT display)

.

67

Figure 19. WIL chromaticities as a function of a metal halide

source (approximately 8 fc at the CRT display)

.

68

Figure 20. WIL chromaticities as a function of a mercury source

(approximately 4 fc)

.

69

X
1. INTRODUCTION

1.1 BACKGROUND

The Federal Aviation Administration ( FAA) commissioned the

National Bureau of Standards (NBS) to develop, test, and field

evaluate an inexpensive ($100.00) color calibrator for its Radar

Remote Weather Display System (RRWDS) . An RRWDS receives radar

echoes from moisture in the air (i.e., in liquid or solid forms

such as mist, rain, snow, sleet, etc.), processes the echoes into

six Weather Intensity Levels (WIL's) , and displays the resulting

weather patterns on a tri-color (red, green, blue or RGB or P22

phosphors) CRT display console. The six WIL's represent

increasing amounts of air moisture. Starting from the least

amount of air moisture, the colors used are described as: (1)

light green, (2) dark green, (3) yellow, (4) tan/gold, (5) light

red, and (6) dark red. This assignment of weather information to

colors is called color coding. U.S. Weather Service

meteorologists use this color-coded information as one source in

making their local area weather forecasts.

A first report (see reference No. 9) was prepared for the FAA to

document the design, development, and laboratory testing required

to produce an inexpensive color calibrator for RRWDS. The

recommended color calibrator consists of three silicon

photodiodes, each with an R, G, or B encapsulated narrow-band

Wratten filter, and an AC-DC power supply. To keep costs down,

the output of the color measuring head was designed to be used

with most digital volt-meters (DVM's) currently available." The

sensitivity of the color calibrator plus DVM was at least

equivalent to one test digital level (TDL) as entered into the

RRWDS display console keyboard using special erasable, programmable read only memory (EPROM) chips or integrated circuits (IC)

.

Each of the RGB colors can be changed over 16 TDL's, i.e. TDL=00

to 15, for a total of 4096 color combinations. Since the spectra

of the RGB phosphors are nominally the same for all RRWDSs ' CRTs,

it is possible for the color calibrator to provide the basis for

a true CRT colorimeter employing a microprocessor and using

appropriate matrix algebra algorithms. However, using such a

colorimeter with any other CRT requires that the spectral power

distribution (SPD) of the CRT's phosphor be known and entered-in

in advance of any measurement.

The present report documents the results of the field-testing

phase of this project as well as a laboratory study of the

effects of different building lighting environments on WIL

chromat icities as measured by a rapid-scan spectroradiometer

.

The basic reasons for this continuation study were to: (1)

develop an RGB look-up table of the acceptable range of color

calibrator outputs for each WIL; (2) assess ease of use and

accuracy of color adjustments by field technicians ; and (3)

assess the effect of various building lighting environments on

color d isc r iminabi 1 ity under real-time field use of RRWDS.

Although only a limited number of field sites were visited of the

1
30 field sites proposed, the obtained intersite differences

clearly highlight the need for more data on such differences as

well as on the effects of different lighting environments on

color disc riminability . The last part of this report contains

data on a substantive, first of its kind, laboratory study of the

effect of various room illuminants on WIL chromaticit ies . The

effect investigated was the determination of the net result of

superimposing room illuminant reflections from the CRT display on

each of the WIL colors. Logically, both the luminance and

spectral composition of these reflections should simply add to

the luminance and spectral composition of the WILs to produce

some net effect. That this is not necessarily true will be

discussed later.

1.2 LITERATURE REVIEW - COLOR CODING

1.2.1 Review by Christ

The most recent and most substantive review of the use of color

as a coding medium was performed by Christ [3] . Color coding has

been used primarily for two kinds of tasks: search and

identification. A search task consists simply of searching,

locating and counting categories of items known in advance. An

identification task consists of finding a given target/location

and identifying its category when the category is not known in

advance. Christ concluded that color coding, either redundant 1

or non-redundant , for search tasks is the most effective form of

coding, e.g. reducing search time by up to 70%. Christ also

concluded that nonredundant color coding is superior to size,

brightness, and shape coding for identification tasks although it

is inferior to alphanumeric coding. The use of redundant color

coding for identification tasks remains questionable due to a scarcity of relevant experiments.

On the other hand, the presence of irrelevant color may adversely

affect or interfere with the processing of the achromatic

attributes of a multidimensional display. Smith and Thomas [15]

found that the presence of irrelevant color decreased counting

speed for geometric shapes whereas Egeth and Pachella [4] found

that it reduced the accuracy of shape and size identification.

When a color code has been applied systematically but not made

known, performance on a color-coded display is impaired relative

to that on a monochromatic display [5]

.

This interference effect

increases as the number of colors in the display is increased.

The reason(s) for the interference effect of irrelevant color is

unknown, although the human visual system appears to assign color

high priority in viewing a global visual scene.

1 Redundant coding refers to the use of two (or more) coding

techniques, e.g. a deck of cards uses color, form, numeric, and

positional (one-eyed jack) coding. Redundant coding lets the

same deck of cards to be used in a variety of card games.

2
The interference effect of irrelevant color may be due to

different visual system mechanisms for processing color than for

processing achromatic target attributes. A colordetection/letter-recall study by Gummerman [8] suggested that

letters are processed serially, left to right or top to bottom,

whereas color is processed in parallel by operating over the

entire display simultaneously. Further support of different

visual mechanisms for shape and color-processing was reported by

Williams [19] who found evidence that search can be guided

extrafoveally on the basis of color but not of shape. The global

nature of color processing was supported by Luria and Strauss

[13] who found that fewer fixations are needed to locate a color-coded target than a shape-coded one. Although large

multivariate experiments are extremely difficult to accomplish,

the findings cited above suggest that color processing and shape

processing may be performed by different visual system

mechanisms. Whether or not color is always processed in

parallel and shape is always processed serially is a point of

conjecture contingent on the type of experiment performed.

Independently of the type of processing involved, color coding is

still beneficial in many multi-task situations.

1.2.2 Post-Christ Review

A post-Christ literature search found only four studies

[11,14,16,17] that used a multi-task paradigm instead of the

usual single-task paradigm. These four studies investigated

redundant color-coding for search tasks. Color-coding seemed to

have a slight advantage over, other forms of coding, although this

effect was not clear-cut. In three of these studies [11,14,17], a significant improvement in performance of the non-color-coded

task was found, whereas in two of these studies [11,14], a significant improvement on the color-coded task was also found.

The fourth study [16] found no advantage of color-coding for

non-color-coded tasks. With increasing use of color CRT's aboard

aircraft and other military weapons systems, these studies

indicate an increasing need for multi-task studies employing both

search and identification task paradigms.

The review of the post-Christ literature found no studies using

color coding on dynamic (unfixed) or cluttered displays such as

the RRWDS . A single study using a fixed-format, redundant

color-coding for airborne CRT displays was performed by Luder and

Barber [12]. They found that: "the availability of redundant

color coding on the secondary task resulted in a global

improvement in tracking performance (primary task) that applied

at all times, even when subjects were not attending to the

systems-management display. With regard to the response-time

data for the color-coded display, redundant color coding improved

search performance, but there was no benefit for performance on

the identification task."

Another type of fixed-format task is the static system maps often

used in process control systems (e.g. nuclear power plants,

3
oil/gas refineries, gas transmission control operations).

Dynamic system information, depicting the state of the system,

occurs at specific map locations. For such identification tasks,

the operator is position-cued as to where to look (given a particular status of the display) for cues for appropriate

further responses. In some systems additional visual cues

(flashing lights) and auditory cues (gongs) are used to reduce an

operator's "alert", "search", "identify", and "take appropriate

action" times for high criticality conditions.

1.3 REVIEW SUMMARY

In summary, the behavioral research data indicate that

color-coding is very effective in reducing search time but much

less so in reducing identification time. The research data also

indicate that as the number of colors in the code increases, the

various response times increase accordingly. If the number of

colors becomes too large (the critical number remains

undetermined and appears to be task dependent) , the color codes

may become irrelevant and result in a serious decrement in human

performance. With the increasing use of color CRT's to display

vital cockpit information, already on the Boeing 757 and 767 and

airbus A310 and with 7-9 color CRT's proposed for certain

military aircraft, careful attention must be paid to the number

and kind of colors selected to insure aircraft safety.

The literature clearly indicated a critical need for more data

about systems which, incorporate self-luminous CRT displays. The

International Standards Organization (ISO) has set up a special

committee to develop standards for various visual attributes of

CRT's, including some aspects of color, e.g. figure/background

relationships. Undoubtedly, these standards will be useful but

will not address the need for more information (data) on

chromaticity maintenance of self-luminous displays. These data

are needed for total systems (including the CRT) that use

color-coding techniques, i.e. where various colors represent

specific information content in contrast to, say, TV's where

color is not only symbolic but aesthetically pleasing. Some of

the general problems of color-coded self-luminous displays that

might be addressed are:

1. Chromaticity changes and/or loss of color contrasts as

a function of system aging and/or maintenance/replacement of parts.

2. Impact of the building lighting environment on color

and color contrasts.

3. Enhancement of color contrasts for the mildly color

defective, e.g. the anomalous observer.

4. Optimization of the number of colors used in colorcoded CRT displays to achieve clearly detectable color

contrasts

.

4
The above list of problems covering color-coded CRT-type displays

is meant as an introduction to the body of this report. It is

not an exhaustive listing of the many problems associated with

CRT-generated displays in general. For an interested reader,

reference #1 is suggested as an overview.

5
2.0 BACKGROUND FOR RRWDS EVALUATION

2.1 WEATHER INTENSITY LEVELS (WIL'S) CHROMATICITIES

The first step in the evaluation of the RRWDS was to determine

specifications established for the chromat icit ies used to

indicate different weather intensity levels. A review of the

documentation for contract DOT-FA79WA1-097 did not find any

official FAA chromaticity specifications that applied to the

WIL's for RRWDS. The first NBS color code proposal [10],

however, contained chromaticity coordinates and suggested

relevant luminances for the 6 WIL's (see Table 1 and Figure 1) . The six chromaticit ies were chosen to lie on the edge of the

triangle formed by the RCA RGB P22 phosphor chromat ic it ies

.

These colors were chosen on the triangle edges to obtain maximum

saturation. The relative luminances were chosen to achieve

maximum available light/dark contrast ratios. The six WIL colors

recommended would allow people with moderate red-green color

deficiencies (protanomalous or deuteranomalous) to discriminate

on the basis of color and/or light/dark contrasts.

The next NBS color code proposal for RRWDS was contained in a letter report on May 8, 1981 by Robert A. Glass [6] (see Table

2) . The proposed chromaticit ies were based on a summary of

spect roradiometer measurements taken at the RRWDS system manufacturer's premises. By- this time a decision had been made to

use three colors; namely red, yellow, and green with high/low

luminances to provide the six WIL's. Further, in the NBS letter

report, the brighter WIL was the more intense weather condition.

Later, the National Weather Service recommended that the darker

color should represent the more intense weather because of

experience with black-white displays where the darker levels

represented more extreme conditions.

2.2 WIL LUMINANCES

Except for the first NBS proposal, no other specification for the

luminances of the six WIL's was found. The rationale for specifying the luminances of the WIL's is to insure optimum light/dark

contrast ratios. There are, in fact, two light/dark contrast

ratios involved, namely: (a) the contrast between the light and

dark color for a single hue, e.g. light versus dark green; and

(b) the contrast ratio between a dark color and the display's

background. To illustrate, a more intense weather condition

(e.g. the dark green of WIL #2) may be embedded in a large area

of less intense weather (e.g. the light green of WIL #1).

Depending on the ambient lighting system as well as the luminance

of WIL #1, the actually more intense WIL #2 cells may be misinterpreted as dark background. This is due to a visual effect

known as "simultaneous contrast", in which a bright area makes a dark area appear even darker. Recourse to the display console's

menu, which permits any combination of WIL's to be displayed, is

a built-in solution to this problem, i.e. a single dark WIL can

be displayed individually from the dynamic weather display.

6
Table 1. Nbs's original proposed color code for use in color

weather radar (August 14,

Chromaticity^-

Color Coordinates

Level Name X Y

6 Red .575 .315

5 (Orange)

Brown

.530 .425

4 Yellow .490 .460

3 (Yellow-green)

Olive

.435 .505

2 Green .270 .390

1 Blue .195 .190

1980) .

Munsell

Hue

Relative

Luminance

%

Equivalent

Munsell

Value

5R 100 10

5YR 30 6

2.5Y 80 9

10Y 30 6

5G 60 8

5PB 30 6

^ The chromaticities of all six colors were chosen to lie

on the edge of the triangle formed by the RCA phosphor chromaticites. These colors were chosen on the triangle edges to

obtain maximum saturation. We have not attached tolerances,

and these exact coordinates may not be achievable. However,

these coordinates should be viewed as target coordinates.

The manufacturer should try to get as far out from the white

point towards the chromat icites as possible, to maximize the

color saturation.

7
VALUES

OFy

Figure 1 TARGET CHROMATICITY COORDINATES FOR

COLOR WEATHER RADAR

8
Table 2. Summary measurements taken at Electrodynamics (April

15, 1981), representing NBS's final proposed

chromaticities for the WIL's.

Intensity Color Name Average Chromaticity

Number Coordinates

6 RED X = .6656

y .3340

5 *DARK RED X .6812

y .3186

4 YELLOW X .4021

y

— .5292

3 * TAN/GOLD X- .4064

y

““ .5538

2 GREEN X , .3454

y

— .5793

1 *DARK GREEN X .3553

y

ss .5885

* These NBS color pairs for light and dark were reversed at the

request of the NWS of NOAA (DOC) per letter request of May 16,

1981.
2.3 SUGGESTED WIL CHROMATICITY CHANGES

The chromaticiti.es of the current 6 WIL's, when plotted on a CIE

chromaticity diagram, lie almost in a straight line (see Figure 1

or Figures 12-20) . This line is very close to the line of

maximum RG color confusion. NBS has suggested that a new set of

chromaticities could be developed which would allow people with

minor RG color deficiencies to make appropriate discriminations

among the 6 WIL's. The changes are relatively small but appear

to be effective. People with normal color vision will still see

high contrasts between pairs of the proposed new chromaticities

although some may aesthetically prefer the original six

chromaticities. On the other hand, people with slight color

deficiencies should be able to discriminate between all six WIL's

using the proposed new set of chromaticities. These people would

generally pass the usual color-blindness tests. It would take

somewhat more sophisticated color testing techniques to determine

their true color deficiencies.

The effect of making slight color changes in some of the

recommended WIL's may allow more people to use RRWDS and avoid

possible discrimination against hiring new people who may have

only minor color defects. Two possible retrofits to the RRWDS

are available: (1) change the firmware color table in the

EPROM's on the display CPU/interface circuit card assembly (i.e.

CCA N3NBA1) or (2) replace the same EPROM's with color test

EPROM's. These color test EPROM's permit entry through the

console keyboard to the WIL color and background RAMs . The

programs written into the EPROM's allow each color gun to be

changed by 16 test digital levels (TDL) . Although this latter

procedure would allow individual color-deficients to personally

minimize WIL color confusions, the need to check and/or change

the chromaticities every shift may be irritating and/or

unacceptable to some meteorologists.

2.4 WIL VARIABILITIES

In NBSIR 84-2924 the variability of both luminance and

chromaticity for a single RRWDS was stressed. Some sources of

variability were: (a) warm-up effect; (b) system drift; (c)

change in the power supplies (e.g. replacing a new tripler and

high voltage junction box); and (d) a hysteresis effect. The

latter effect was seen when the TDL's were plotted as a function

of mV DC (the output of the color calibrator) for an ascending

series (00-15) and for a descending series (15-00) . This effect

was measured for each of the RGB primary colors. Variability in

both chromaticity and luminance was also observed in the data

taken for the present report.

10
3.0 FIELD TEST OF COLOR CALIBRATOR FOR RRWDS

The original purpose for the development of the color calibrator

was to produce a cheap device ($100.00) that would objectively

discriminate the six WIL colors. NBSIR 84-2924 provided

supporting data which verified that the color calibrator can

detect minor changes in the chromaticities of the WIL's. If a technician receives a call that one or more of the WIL's do not

look right, he/she could try tweaking the grid controls

(especially the G-2s) to obtain the best WIL color. If the

technician were using the color calibrator, he/she would obtain

the color wedges from a digitizer. Using available console

menus, each WIL can be centered one at a time. The technician

would check each RGB channel to see if the color calibrator

readout was within the limits specified in the look-up table. If

by tweaking the G-2s, the WIL colors are brought within those set

in the look-up table, then the system is within specifications.

3.1 FIELD OBSERVATIONS

Casual observation of a few field RRWDS indicated that some WIL

colors did not appear to have the same chromaticities as the test

system at NBS . To check for correct adjustment of the processor,

the processor input signal to the control console was monitored

for each RGB channel. An oscilloscope was attached to J3, J4, or

J5 on the back of the console (using Tee BNC connectors) . Using

the vertical bars from the processor's Built In Test Equipment

(BITE) , the contribution of each primary color to each WIL color

could be measured by means of the oscilloscope. If the

oscilloscope was connected to J3, then the display showed the

amount of red present in each of the vertical bars (white, black,

plus the 6 WILS) , in a form similar to an eight item histogram.

According to site acceptance specifications, the voltage reading

on the oscilloscope should be 3.2 mVDC for those bars having the

maximum level (TDL=15) for the color channel being monitored.

Under field conditions, a voltage range of 3. 0-3. 6 mVDC was found

for some RGB channels. The RGB video amplifiers on the video

color generator board in the processor were adjusted to meet site

acceptance specifications before the field color calibrator tests

were conducted. Further, at some sites the brightness control

did not "track" accurately, i.e. at high brightness settings the

alphanumer ics on a live display tended to "bloom" or become

blurred rather than simply become brighter.

3.2 TRI-COLOR SYSTEM EFFECTS

It has been established by the television tube industry (i.e.

Joint Electron Device Engineering Council or JEDEC, see

Publication No. 16-C, 1975) that each phosphor has its own unique

spectral power distribution (SPD). Usually variations across

different batches are very small. However, when phosphors are

selected for a tri-color system, the basic primaries (RGB) may

show slight changes in the phosphor chromaticities. In the NBSIR

84-2924, it was noted that decreasing the TDL of a single RGB

11
cathode of the RRWDS's CRT resulted in changes in chromaticity as

the TDL was adjusted to a low level. On the other hand, if a similar CRT is placed in a test jig, then decreasing the current

to a single cathode does not result in chromaticity shifts for

an-y of the RGB primary colors. The distinction is that in RRWDS

some current is always applied to the RGB cathodes whereas in a test jig current can be applied to a single RGB cathode only.

This differential effect may be interpreted as a system's

effect

.

3.3 GRID #2 VOLTAGES

Table 3 presents the DC voltage readings taken at test points

TP4, TP5, and TP6 for each of the sites visited. The test points

are located on the dynamic focus/screen balance Circuit Card

Assembly (CCA N3NC1) in the CRT console. At each site visited,

the local technician was asked to check that the system was

operating "normally". By normal, it was meant that: 1) the

CRT's background was barely visible as a neutral gray; and, 2)

the WIL color wedges obtained from a digitizer were visually

"acceptable". At this point, the display CPU/Interface CCA

(N3NB3A1) was removed and replaced by the CCA containing the

color test EPROM's.

With the Brightness Control set to maximum and the contrast

control to a minimum, a white (R=15, G=15, and B=13) target area

was keyed in to determine the CRT's specification luminance (i.e.

77 fL as specified by RCA) . Only two systems (NBS and Teterboro)

could achieve this luminance level. For the two Leesburg

consoles the target area would "snap in/out", thereby indicating

trouble with the triplers and/or high voltage junction block.

Thus, the Leesburg CRT console tests were made at a lower

Brightness Control setting. Theoretically, running the system at

lower luminance levels should not seriously affect the chromaticities of the WIL colors.

A question may arise as to why NBS preferred to use the color

test EPROM's in the field tests. The answer is: a.) ease of

use? b.) the ability to fill the whole target area with a single

color; and c.) the ability to change a WIL color component one or

two TDL's. Previous laboratory testing had shown no significant

chromaticity differences between: a.) the vertical bars from the

processor's BITE; b.) the WIL wedges from a digitizer's BITE; or

c.) the WIL's produced by using the color test EPROM's.

Therefore, a major reason for using the color test EPROM's was to

determine if the color calibrator would still detect a one TDL

change in any of the RGB color components over all six WIL

colors. The results of this test were positive except when an

RGB component was 2 TDL's or less as in WIL #2 (R=01, G=06,

B=02) . As cited in NBSIR 84-2924, low TDL's can not be detected

by the spectroradiometer

.

12
Table 3. VDC Readings of Grid #2 Voltages for Each RGB Channel

RED GREEN BLUE

TP4 TP5 TP6

NBS 394 460 395

Leesburg #1 319 471 404

Leesburg #2 543 393 542

A. Godfrey 343 501 328

Leesburg #3 475 485 403

Teterboro 392 323 321

Mean 411.0 438.8 398.8

Standard Deviation 83.97 67.82 79.54

13
3.4 WIL CHROMATI CITIES

Table 4 presents the six WIL chromat ic ities , measured by the

spect roradiometer , in CIE (x,y) coordinates, for each of the

field sites visited. The data in Table 4 allow some interesting

comparisons to be made. First, the standard deviation (S.D.)

data indicate fairly large (and visually detectable) inter-site

differences for five of the six WIL's. The smallest S.D. is for

WIL #5 (red). WIL #5 is the only WIL composed of a single

primary (R) color. This suggests that, although the RGB

phosphors may vary slightly between different CRT's, there is a larger variation in the mixing of RGB primaries which occurs in

the electronics of the RRWDS - a so-called systems effect. This

effect appears to be supported by the smaller differences in

chromaticity between the two Leesburg RRWDS. Re-examination of

Table 3 indicates that these smaller WIL color differences were

probably obtained by changes in the G-2 DC voltages. Since the

two consoles are side-by-side, the maintenance technician can

adjust one system and then use it to aid in adjusting the second

system. If this is true, then using a standard color measuring

device, i.e. the color calibrator, may prove to be useful in

maintaining color constancy across all RRWDS' s. Comparison of the WIL color data for the two NBS measurements

indicates that the WIL colors change over time for a given system

(approximately 2 years for the NBS RRWDS) . This change over time

may be due to a number of factors, e.g. erosion of the

electrodes, warping of the color shadow mask, replacement of the

triplers/high-voltage junction block, or -other changes. However,

if the WIL colors are too different from their desired colors,

new CCA's or the CRT may have to be replaced before the colors

can be adjusted to the color look-up table values.

3.5 FIELD TEST PROCEDURE AND RESULTS

The initial procedure used to obtain the data to develop a color

calibrator look-up table consumed a great deal of time. The

first step was to obtain the WIL color wedges from a digitizer.

Each WIL wedge was then displayed independently and spectroradiometric data recorded. The second step was to switch to the

color test EPROM's and repeat step one. The two sets of data

were then scanned for major discrepancies. As expected, no

significant differences were found. The third step was to record

the DC voltages for each RGB component of each WIL using the

color calibrator. These data formed the basis for the look-up

table and are presented in Tables 5-8. The fourth step involved

incrementing/decrementing each RGB component of a WIL by one TDL

.

The data obtained during this step were compared to the data from

step 3. The results from this comparison were consistent with

laboratory findings - the color calibrator can detect a one TDL

\

14
Table

4.

WIL

chromat

icities

asa

function

of

field

sites c

'a o U *H 00 r- o r- in rH CO rH 00 oo vo

fO -M oo t-* rH CN o in r-~ oo o ^ OO CO

'o

a CN CN CN VO CO UO N* rH

O CN rH c -h o o O O o o o o o o O O

(0 >

4J <u W Q • • • • • • • • • • • • c oo oo 00 1H

rH CO

O CN o uo 00 CN

(0 rH o r- o COO rH N* VO CO CO

O

a> VO VO CO VO CN rH

N1 OO

O N' OO UO s CO IT) co in N1 U0 •sj* N1 VO CO UO CO • • • • • • • • • • • •

rH X2

(0 <d CN CO VO 00 VO UO VO oo

N1 VO •H J r-~ cn o in CN CN in vo r-~ oo 00 OO

-U in r- OO CN rH co uo rH CO rH CO •H CO co in co in UO N' N* VO CO VO CO C CQ • • • • • • • • • • • • M Zouo 00 VO VO VO CN CO 00 o r— cn

.a oo co VO uo rH N* CO CO CO co co

(H rH ^ rH

O

N1 O rH rH N<

a) in CO VO UO

N1 uo ^ VO CO VO CO

4J • • • • • • • • • • • • o

Eh

>i

u a) rH r- cn CO rH ^ r- UO 1H

r>- 3 in CO 00 O OO r-» UO CO 00 rH 00 OO rH XI 4-1 r» in vo O CN OO CN OO •N' co r4J co in CO uo N* UO CO uo UO CO uo co U O < u t • • • • • • • • • • •

O'

4l3 vo ^ OO o •'3* CN ^ o OO CO 00

Xi CN cn r-- vo r-» r- r- N’ r* OO rH OO rH

CQ ^ r-~ OO CN CN

OO N* vo

<U co uo CO UO co uo N* UO VO CO UO CO

<u • • • • • • • • • • • • i cn

413 VO N’ cn in oo vo CO r—t CN rH UO rH

X! rH cn in 00 CN r-» n- oo VO

O o rCQ =** N* 00 VO 00 N1 uo vo OO UO rH •N* CD co in CO uo co in CO uo UO CO VO CO

CD • • • • • • • • • • • •

CO CQ UO CO rH Oo o 1H

O OO CN CN OO

CQ < O CO -*r oo CN UO r- o f" 00 rH r-~ Z J VO CO OO CO CN O l-> vo O CO O CO

co in CN N' ^ UO N’ ^ VO CO VO CO • • • • • • • • • • • •

4t= iJ

X >1

X >iX >i

X tHX >1X >i

M>=» •H CN CO N* in vo

15
change in any of the RGB components of a WIL (except when TDL's

of two or less) . As a result, step one was deleted and step 4

was reduced to several checks using the color test EPROM's.

For the remainder of the tests, spect roradiometric data were

always taken first to determine if stray fluorescent light was

detectable. (Fluorescent light has characteristic peaks at 404,

436, 546, and 578 nanometers). Since this characteristic

fluorescent signature was noted at all field sites, a canopy with

black sides was used thereafter. In addition, a black cardboard

tube was used as a light conduit between the CRT's protective

screen and the spect roradiometer lens. Color calibrator data

were obtained with the face of the color calibrator directly

against the protective screen.

Table 5 presents data, in DC voltage form, from the main console

used at Leesburg. In order to reduce congestion at its original

use position, this console was moved to the basement. The

console was connected to the processor with 10 ft. of coaxial

cable and readjusted to yield EPROM-produced chromat ic it ies

closer to those of the NBS lab console. Comparison of the

relative luminances of the RGB primaries when using the EPROM's

vs the digitizer's wedges indicated that the wedges had the

higher relative luminances, e.g. WIL #3 had almost twice the

voltage readout for the wedges than for the EPROM's. During

rather extensive laboratory testing, when all system controls

were kept constant, the wedges generally yielded lower relative

luminances than the EPROM's, although both sets of luminances

varied over time.

The higher relative luminances for the digitizer's wedges used

for the Leesburg #1 console may be partially due to the console's

close proximity to the processor. However, the RRWDS

manufacturer claims there should be no "significant" voltage drop

in the coaxial cable up to a distance of 200 feet. NBS

laboratory tests suggested there is a voltage drop of 0.1 to 0.2

mVDC per 100 feet of coiled coaxial cable. Additionally, "noise"

may be induced by the electromagnetic fields surrounding such

computer peripherals as magnetic disks. Other NBS laboratory

tests revealed small luminance differences in the WIL wedges

obtained from different digitizers although the WIL wedges never

produced higher luminances than the WIL luminances obtained using

the EPROM's. On the other hand, the proportions of the RGB

components are in the right direction. For example, WIL #3 is

generated by using 15 TDL's of red and green and no blue (i.e.

R=15 , G=15 , B=00) . Therefore, the color calibrator readout

should be near some maximum value for both R and G. Wil #5,

which is a pure red (R=15, G=00, B=00) , is a good indicator of

the variability seen across all five field sites. Since the blue

components of WIL's 1 & 2 contain TDLs of only 2 units, which are

not detectable by the human eye, differences in the color

calibrator readouts for the blue phosphor across the five field

sites appear to be random in nature.

16
Table

5.

Color

calibrator

DC

voltage

readings

asa

function

of

field

site

Leesburg

#1 i

a; rH CN pH *3* in o D pH O CN O o o

CO r—1 O o OO o o

<uO' 'O

<v

CQ • • • • • • & C CN o CN VO 00 ro a <y\ r- o o o M o CN o 1—1 o o

0)N

•H

4J•H

uu • • • 9 9 «

O' 'O pH iH o o co •H a Os i—i

rH ["• VO iH

Q

a

O• o• ID• o• CO9 O• ozJMs

iH CN ro ^ in Id

(1) O'

(C

4-1 CO iH 4-> O rH > O> U -H D rH o

rH

O

l-l -H rH 00 rH CN ro CO o O S 0Q o O rH o o o

4J o O

O o o o

ra in • • • • 9 • c

•H CO 0) rH O' 0) VO 00

O CN CO CO

to c i4 00 CN C" r- o o U *H u rH o CN o o o

H3 • t • • 9 •

5-j

ro O <D rH K HD o <U

O VO O' r- VO

u OS VO rH r- CO O' o o O CN o rH o • • • • • •

rH4*

O' • V4 CO o D -

-Q S z

co

O

J rH CN co in VO

CD OSM

<1) &4 J w s o CN oo9 in

in CO o O' o CO •11 >-l O

CN CO 4J o (CJ o in 14 • A 4J •H c a» rH

<D O' ttj u * to U u • 4-1 3 O rH u V • O O o > rH 4C o U VM CN o

to o 1 Q U *

17
Table 6 presents color calibrator data from the backup console in

the Leesburg Air Traffic Control (Leesburg #2) room which was

functioning properly at the time. The RRWDS of Leesburg #3,

actually Arthur Godfrey airport, had just been installed but not

checked out. With the overhead fluorescent lights off, the

maximum luminance of a white target circle was 24.25 FL or

approximately one-third the site acceptance specification value

of 77 FL. With all fluorescent lights on, the dark WIL’s were

barely detectable. Leesburg #3 appeared to represent an

unadjusted console, perhaps because of its recent installation.

Table 7 presents a second set of color calibrator data from

Leesburg #2 as well as from Teterboro, N.J. Approximately three

months elapsed between the data for Leesburg #2 presented in

Table 6 and in Table 7. Local Leesburg maintenance people

thought the differences in color calibrator voltage readouts were

due to a need for new triplers. On the other hand, the Teterboro

system had recently installed new triplers and high voltage

junction block. Visual inspection of the Teterboro WIL's

suggested that they were green-deficient. Spectroradiometr ic and

color calibrator data support the green deficiency visual

observation. Since local maintenance personnel were not

available, no changes in the G-2's were made. The Teterboro

data, however, strongly suggest the need for an objective

measuring device to bring each RRWDS more in line with

chromaticity standards for each WIL.

Table 8 presents a tentative look-up table for using the color

calibrator. Examination of this table illustrates some obvious

problems. First, the small size (N=6) of the sample of sites on

which the values in the table are based is grossly inadequate to

obtain a stable distribution of WIL chromaticit ies . A single

misaligned system, e.g. Teterboro, will greatly distort both mean

and standard deviation values. Comparison of the Leesburg versus

the Teterboro data indicates, that in the absence of some

objective color measuring device, such as the color calibrator,

individual technicians vary greatly in obtaining the "standard"

WIL colors. Some technicians may add too much blue or others too

much red or green. To reduce inter-system variances in the WIL

chromaticities , a twofold approach may be required. First, by

sampling a large number of RRWDS (e.g. N > 30), a better

distribution of WIL chromaticities could be obtained to set

"standards" for the WIL chromaticities. Second, the color

calibrator should be used to adjust the WIL chromaticities,

thereby eliminating "eyeballing" differences. These procedures

may not eliminate inter-system differences but will tend to

reduce them to a less noticeable level.

Examination of the B component in Table 8 indicates that the

blue-filtered photodiode apparently was responding to green

wavelengths. For example, the maximum response of the B

photodiode was to WIL #3 (G=15) , then WIL #1 (G-12) and WIL #4

(G=8)

.

This suggested problems in assuring the proper

sensitivity setting for the B photodiode. The color calibrator

18
Table

6.

Color

calibrator

EXT

voltage

readings

asa

function

of

field

site

Leesburg

#2

and

#3 i

<u

4J CO iH 4J aaU -H o s 4J2-S

"9 CO

r-H C>

nj £ u 5T3

l-l (0 ou

<D3

rH

03

<D

<u

s-i O

<US'

4J 0} iH -U n

«aU -H 5 s

2.5

CO o3

iH

CQ

rH CJ1 3-S

<uuu

u !§ O 0) rH 0h ou

Red

COoo

COoCO

rHo

inoo*=r iH o o o o • •

iH <N co

N1 in vo

co in CN rH O O • •

co

CO rH O• O•

r-~ cn

rH

O o o • •

co in rrH oo

co o ^ • • •

ID VO

CN

O

O

O • •

cn

m cn in rH O10

N*

in

rr* rH 00

CO ID CO O • •

rH CN co in in

19
0

S'

4-J 0}

33> g

as 03

rH

CO

r*o o

CO CO VO o o o o H -H O 2 4-> § ro rH vo CO

<0

C 0 o CO CN o rH rH

5-1 »H 5-1 iH

O rH rH

O

O

•"9 01

U • • • . • . <h cn 0 q V -H 3

ro CN 00

O ro CO

'O vo rH O' VO rH ov

5-1 (0 QS rH

O in ID CO

O 0 • • « • • • rH « OU

(0 o a

01 • - O

01 .1 0

4-»

g 2 S J £ as s0u

oi0

S'

4-J I—I £

5Ho . •St 0o

S-4 ou o

5-4 o

fi0

4-> £

r-0

rHg 0

S'

(0

4-1 01 rH 4-J

5-1 -H s s

2-5

•"3 oi rH C7>

5-1 O

rH8 03

CQ0

5-1 O

CN co

[— co co rH O CN O o o o o O 00 o in vo vo

CN

O

N1 CN

CN ro

in

in

vo

VO CN o o o o

iH VO

rH

O

O

'O O' <3* o o in 05

as 0 00 CN 00 05

st

PS

O• O. O'• rH• CN• O.

VO

20

Data

are

from a

second

trip

taken 3

months

later.
Table 8 Tentative look-up table for the WIL's using the color

calibrator

.

WIL No. Red Green Blue

1 U.L. 2 .142 .283 .023

Mean .098 .202 .014

L.L .054 .121 .005

2 U.L. .042 .076 .010

Mean .025 .052 .005

L.L. .007 .027 .000

3 U.L. .553 .463 .030

Mean .418 .308 .020

L.L. .282 .153 .010

4 U.L. .352 .134 .015

Mean .190 .104 .009

L.L. .028 .075 .003

5 U.L. .448 .022 .013

Mean .313 .013 .009

L.L. .179 .004 .002

6 U.L. .277 .011 .004

Mean .123 .006 .002

L.L. .000 .002 .000

o The Upper and Lower Limits (U.L. and L.L.) represent 1.0

standard deviation, i.e. the middle 68.26% of a normal distribution.

21
head was redesigned to place adjustable sensitivity

potentiometers in the head for each photodiode. The result is a cleaner engineering design which also allows the dark current and

sensitivity both to be pre-set at the factory. This new design

was not thoroughly tested but was easier to use even though the

head was considerably larger. Further minor refinements could be

made but should probably be performed by the eventual

manufacturer

.

One of the early questions concerning the RRWDS was: do the

WIL's shift in chromaticity and luminance and, if so, over what

time frame? The previous NBS report discussed some of these

problems. However, for the laboratory study, weekly minor

adjustments were often required to keep the WIL chromaticities

within approximately one standard deviation of initial values.

Field data is scanty but suggest that checks should be made at

least monthly. The color calibrator data should be recorded by

local personnel for possible use in detecting and correcting

problems. Drops in luminance, for example, were generally a precursor of future problems in the high voltage power supply

(e.g. tripler or HV junction block). Adjustment of the Green G-2

appeared to be critical in maintaining a neutral gray background,

whereas the Blue G-2 was not very critical, except in adjusting a white circle for an equivalent temperature of 9300°K (RCA

specification). RGB phosphor dots were examined, using 10X

magnification to check for misregistration and required

adjustments to the CRT yoke and RGB control magnets. Although

the status of the color shadow mask could also be checked for

demagnetization and possible local warping, these latter tests

and checks require laboratory equipment. They are given here as

an indication that the variability of the WIL chromaticities and

luminances can be kept to a minimum.

During laboratory testing, one of the problems investigated was

whether the source of inter-system variability could be

determined. Discussion with the CRT manufacturer indicated that

their quality control program was designed to minimize

variability between CRT's. Similar discussions with TV

manufacturers indicated that their quality control programs

permit somewhat more variability in RGB chromaticities.

Manufacturers of special military systems indicated they had more

of a problem in minimizing variability for color-coded display

systems

.

The conclusion drawn from these discussions is the more complex

the system, the more the information content of the colors, and

the more controls to be set, the greater the difficulty of

maintaining inter-system constancy.

3.6 WIL CHROMATICITY CHANGES OVER TIME

Although not part of the laboratory testing of RRWDS, data were

obtained on the chromaticities of the RGB primaries and the WIL's

over approximately 6 months when no major system changes were

22
made. The CIE plots for each of the RGB primaries displayed

tightly packed measurements. On the other hand, the CIE plots of

the WIL's suggested there was a slow, long term trend towards the

center of the color gamut, i.e. toward white.

Since the above data confirmed the usual presumption that the

color of radiation from each phosphor is stable, the above trend

in the WILs must arise somewhere in the color mixing electronic

circuitry. This trend raises some questions about the long-term

stability of the 16 reference colors to be used in NEXRAD

,

depending on how the reference colors are produced. For example,

if highly-stable impregnated glass filters, backlighted by

halogen bulbs to produce more blue, are used, then the long-term

stability problem would be greatly reduced. On the other hand,

if the reference colors are produced through electronic mixing,

there may be a problem with adjacent colors losing color contrast

through desaturation (whitening) , depending on the electronics

used in NEXRAD. Detection of such changes by the color

calibrator may be difficult without the development of a color

look-up table based on a large number of NEXRAD systems. Since

the sensitivity of the detector head would remain the same, the

advantage of the suggested CRT colorimeter is that it would

provide a readout in the well known CIE (x,y) color system.

23
4.0 ROOM ILLUMINATION EFFECTS

The final stage of the RRWDS evaluation was a laboratory

assessment of the effects of room illumination. Three major

types of effects from room illuminations have been observed:

chromaticity shifts, luminance variations, and bright reflections

from the protective glass screen of the RRWDS CRT display

console.

CRT workers have frequently complained of eyestrain and fatigue

from working with video display terminals or VDT's. To date, the

source of eyestrain and fatigue has not been firmly established,

although experiments have ranged from using pebbled CRT face

surfaces (i.e. to reduce reflections) to studies of self- versus

machine-pacing. The present study was designed to determine if

reflections from room illuminants produce chromaticity shifts in

the WIL's used in RRWDS. If chromaticity shifts did occur, they

would be expected to be towards the spectral power distribution

of the room illuminant. If the room illuminant was a white light

source, then all the WIL colors would shift towards white,

i.e. the WIL's would become more desaturated and color contrasts

would be reduced. The result would be to make the discrimination

of the WIL colors more difficult. The term screen reflections is

used hereafter to define the total of the five reflecting

surfaces involved, namely: (1) the front and back surfaces of the

protective screen, (2) the front and back surfaces of the CRT

glass tube, and (3) the phosphor layer itself.

4.1 LABORATORY OBSERVATIONS ON ROOM ILLUMINANT REFLECTIONS

One effect noted early in the laboratory work was the large

amount of specular reflections or glare from the overhead lights

reflecting from the protective screen. Due to the 18° backward

tilt of the CRT, these reflections appeared greatest when a person was sitting at the console such that the plane of the

protective screen was perpendicular to the typical line of

vision. Bright, diffuse overhead lighting produced the most

specular reflections or glare? with side lighting and reflections

from other surfaces having a somewhat lesser effect. In the

laboratory, illumination coming from behind the seated operator

also produced severe specular reflections while direct

illuminance to the face of the operator produced strong veiling

glare. Both conditions should be avoided. Placement of the

RRWDS console, therefore, should be made only after consideration

of the various illumination sources, e.g. overhead lights,

windows, and even desk lamps.

4.2 LABORATORY ILLUMINATION LEVEL EFFECTS

With an illumination of 100 fc from a fluorescent light source,

as measured at the face of the protective screen, the spectral

power distribution (SPD) of the WIL's was dominated by the SPD of

the illuminant. With an illumination level of approximately 1 f c , the SPD of the WIL's did not show the four spectral peaks

24
associated with the laboratory's fluorescent lighting system.

Evidence of similar reflections at high illuminance was also

detected at all field sites visited.

4.3 ILLUMINANT REFLECTIONS IN THE FIELD

The role of RRWDS in the forecasts of meteorologists appears to

be useful, especially for monitoring moving storm systems. The

meteorologists have a multiplicity of data sources for making

weather predictions. When questioned, these meteorologists

indicated that they were generally not affected by the screen

reflections present. On the other hand, at some flight services

facilities a private pilot could misinterpret certain weather

conditions along his intended flight path due to the obscuring of

reflections from an RRWDS's display.

4.4 ANALYSIS OF COLOR SHIFTS

4.4.1 Background Studies

As cited earlier, no studies were found in the literature that

investigated the impact of building lighting environments in

producing specular reflections or chromaticity shifts, on

dynamic, multi-color (i.e. color-coded) visual display systems as

a function of the type of ambient lighting available. A

considerable amount of research has been performed on various

ergonomic parameters of video display terminals (VDT's) , e.g. see

textbook references [2] and [7] . Some of the research cited is

of unknown validity and has been seriously questioned [1] , as

have the differences between the various European national

standards for VDT's. Unfortunately, obtaining a consensus on

optimum values for the many parameters of coded self-luminous

displays (e.g. via International Standards Organization

standards) may be long and time-consuming. Among the more

difficult parameters to be resolved are: (a) optimum number of

colors to be used in color codes; (b) optimum chromaticities and

brightnesses to allow for minor color deficiencies; and (c) the

impact of various lighting systems and their optimum lighting

geometries

.

Subjective responses and logic suggests that darker colors should

appear more "washed-out" or become more desaturated (loss of

chroma) than brighter colors in the presence of screen

reflections. For dark colors, the ambient light added by the

room illumination represents a much higher percentage dilution of

the emitted phosphor light than is true for bright colors. Also,

no experimental evidence was found in the literature concerning

an optimum ratio of various self-luminous bright colors to

various dark colors, e.g. bright green vs dark green. Similarly,

there was no evidence on the optimum ratios between these colors

and a neutral gray, slightly luminous background. In a "realtime" weather display, the amount and distribution of the weather

display's background (no weather condition) varies continuously

as a function of each radar sweep. Thus, under some weather

25
conditions a dark color may become confused with or become

indistinguishable from the background. However, when the WIL

color wedges (i.e. from a digitizer's Test #10) were used, each

of the three dark WIL wedges appeared quite bright in contrast to

the background in a darkened room. Filling the remainder of the

screen with the (three) bright wedges, however, changed

subjective contrast ratios so dark colors appeared dark and

desaturated. Since no physical changes were involved, the

apparent change must occur in the human visual system.

4.4.2 Effect of Room Illuminants on WIL Chromaticit ies

4. 4. 2.1 Laboratory Description and Procedure

The experiment on the effects of various room illuminants on WIL

c h r oma t ic it ies was conducted in the NBS Illumination Color

Laboratory with the plane of the CRT console's screen aligned

along the room's long axis. A Spectra (Model PR 702AM) rapid

scan spect roradiometer (SR) was used to make luminance and

chromaticity measurements. The NBS Illumination Color Laboratory

permits such variations as: (a) diffuse vs direct lighting; (b)

changing three of the walls for reflectivity and/or color; and

(c) a selection of various illuminants. Some control of the

brightness level of the illuminants can be accomplished by using

shutters, or disconnecting some lamps to obtain a given

illuminance. Although it was desirable to equalize maximum and

minimum illuminance levels across all illuminants (at the face of

the CRT display), this could only be approximated.

Three illuminance levels were used in the room illuminant

reflections study, namely: (a) dark or no light; (b) minimum;

and (c) maximum illuminance (see Table 9). Each WIL was

presented serially (1 through 6) by keyboard entry of the

appropriate digital code. Thus, each WIL completely filled the

circular weather target area (14.7 in. diameter) . This approach

eliminates interacting color relationships found in dynamic or

unfixed, color-coded self-luminous displays. In order to

evaluate the figures presented in this section, a number of

factors must be taken into consideration: (1) a full screen

display of a WIL has significantly higher luminance than a small

area of the same WIL isolated from a regular weather display; and

(2) the human visual response system is a non-linear system and

maintains some color constancy. [However, color constancy is not

operative under all possible conditions. Color constancy may be

demonstrated by looking at a color sample under two different

illuminants having different SPD's. For some pairs of

illuminants, the sample will be seen as having very nearly the

same color under the two illuminants, even though the spectrum of

light reflected from the sample may be quite different in the two

cases;] (3) the chromat ic it ies of the three primaries (RGB)

change at low TDLs ; and (4) finally, as an extension of (1)

above, a large static display appears to produce reduced screen

reflections relative to a dynamic display (under the same

lighting conditions) because, in the latter situation one is

26
Table 9. Illuminance Values for Laboratory Evaluation

Illumination Type Illuminance

Dark Room 0

LPS Low (minimum) 41 fc

LPS High (maximum) 86 fc

HPS High (maximum) 76 fc

HPS Low (minimum) 1 fc

Cool White Fluorescent 10 fc

Color Classer Fluorescent 14 fc

Incandescent (minimum) 3 fc

Clear Mercury (minimum) 4 fc

Metal Halide (minimum) 8 fc

27
looking for fine details associated with rather small weather

cells. Specular reflections tend to have more visual effect on

fine details than on gross images.

A preliminary study was conducted in the NBS laboratory to

determine the conditions most representative of the building

lighting environments observed at FAA field sites. The results

of this preliminary study suggested that: (a) diffuse overhead

lighting (obtained by using two separate plastic diffusing

panels) ; and (b) neutral gray walls of low reflectivity would

best simulate field site lighting environments. Since

illuminance could not be precisely controlled, a Weston

foot-candle meter (Model 614-60) was placed at the center of the

CRT face to measure the actual illuminance at the start of each

set of data measurements.

4. 4. 2. 2 Tabulated Results

Table 10 presents, in summary form, the results of many

measurements of the WIL chromaticit ies and luminances taken under

the different room illuminants (currently) available in the NBS

lighting laboratory. For many readers, it may be difficult to

translate or visualize the various shifts in WIL chromaticit ies

from the numerical data presented in Table 10. To assist the

reader in visualizing these shifts in WIL chromaticities , as they

occur under different room illuminants, the next section presents

computerized graphics, based on Table 10. The computerized

graphics are presented in two forms, namely: (1) CIELAB plots

and (2) the more familiar CIE chromaticity diagrams.

The human visual system responds to the total radiation,

regardless of its original source, reaching the eye from every

area of a visual scene. In this RRWDS experiment, the total

luminance of the circular area (14.7 in diameter) emitting a particular WIL color may be analyzed into three components,

namely: (1) the emitted CRT display luminances; (2) the

luminances resulting from the reflection (s) of the ambient light

from the CRT display; and (3) the luminance resulting from the

scattering within the eye of light entering the eye from the room

illuminant. Component number 2 above presents a very complex

problem because of the five reflecting surfaces cited earlier.

For each of the four glass surfaces there is approximately a four

percent loss in the reflectance of the light energy. The effect

of the ambient light striking the phosphor layer presents a separate problem, to be discussed later.

When a room illuminant is a very bright light source, then the

room illuminant's contribution to the total luminance measured

becomes proportionally very large. This results in the CRT

colors being driven towards the color of the room illuminant,

i.e. in the case of a white light all the CRT colors are driven

towards the white of the room illuminant which is to say the CRT

colors become more desaturated. As the intensity of the light

source is reduced, the light source's contribution to total

28
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34
luminance becomes proportionally less, and has less effect on

shifting the chromaticit ies of the WIL's of the CRT display.

Obviously, the extreme condition is total darkness where the only

luminances available are those of the WIL's themselves. Thus, in

this report, the reference conditions are always the WIL's as

measured in complete darkness.

Since lighting engineers can eliminate direct light from reaching

the human eye, e.g. by employing recessed lighting, this study

used two layers of overhead plastic diffusing panels to reduce or

eliminate scattered direct light as a component (component #3

above) of the total luminances involved. The walls of the

lighting laboratory were a neutral, medium grey having low

reflectance in the range of 20-30%. Since the walls are diffuse,

extended sources of reflected light which are considerably dimmer

then the ceiling, the wall's reflected light was considered

relatively insignificant compared to that of the ceiling for the

purposes of this study. In addition, the lens shield of the SR

is designed to reduce or prevent direct light from entering the

lens of the SR during measurement taking. Thus, any shift in WIL

chromaticity could be attributed to the room illuminant's

reflected light from the CRT console display.

Table 10 contains sets of luminance and chromaticity data

labelled "measured" and "calculated". The "measured" data are

spectroradiometric measurements taken with the room illuminant (s)

ON and the WILs displayed to .fill the 14.7 diameter circular

radar sweep area. These data are the net result of the CRT

display values plus the supe r impos it ion of the veiling

luminance (s) of the room light. The "calculated" data represent

the results of a computerized calculation (discussed later) which

utilizes the luminance and chromaticity data of the WILs, as

measured under dark room conditions, plus the spectroradiometric

measurements of the room light reflections, as measured with the

CRT console OFF. Theoretically, the measured and calculated data

should be in close agreement. That they were not is discussed in

the next paragraph.

The measured values presented in Table 10 contain certain

anomalies which required further experimentation and evaluation

to understand what occurred in the room light ON measurements.

Of the two anomalies in the data of Table 10, one had a small

effect and the other a much larger one. The small effect usually

occurred under maximum luminance conditions. This small anomaly

occurred when the measured luminance was slightly larger than the

calculated luminance. This increase in luminance was thought to

be due to a slight fluorescing of the phosphor layer of the CRT

and is discussed in the next section. The second and larger

anomaly occurred when the measured luminance (i.e. with the added

room light) was less than the luminance of the WIL(s), as

measured under dark room conditions. Theoretically this result

should not but did occur. The answer appears to lie in the

electronics of the RRWDS . As cited in NBSIR 84-2924 there are

day-to-day fluctuations in the WIL luminances. However in the

35
present study these larger fluctuations appeared to be caused by

variations in the anode voltage due to malfunctioning triplers in

the HV sections of the power supplies (PS #1 and #2) . The

question of concern regarding these anomalies is whether any

change in WIL luminance will result in a change in WIL

chromaticity . Of particular concern is whether a degraded RRWDS

produces changes in WIL chromaticity sufficient to cause errors

in interpreting live weather displays.

4. 4. 2. 3 Interpretation of Table 10

The WIL luminances and chromaticities presented in the first row

were measured under dark (i.e. no light) conditions during

October 1983. These measurements were taken after the RRWDS

manufacturer's technical representative (TR) had "peaked" the NBS

RRWDS in compliance with FAA ' s Field Site Acceptance

Specifications,, Note that White had a luminance of 80 fL whereas

the specification requires a minimum luminance for White of 77

fL. In the judgement of the manufacturer's TR, the "peaked"

systems WIL colors were approximately similar to those originally

agreed upon [6] . During the three day alignment procedure,

various boards and parts (i.e. HV function block and PS #1 and #2

triplers) were replaced. The new triplers were improved parts

designed to insure long-term reliability of the 30 kv to the CRT

anode. Critical system parameters were recorded for maintenance

purposes, e.g. critical waveforms were photographed and test

point (TP) /potent iometer voltages recorded for various system

controls. Thus, the data presented in NBSIR 84-2924 was based on

a "peaked" system.

The measured values in Table 10 reflect a "non-peak" system after

two years of continuous operation of and experimentation with the

RRWDS. The low luminance value of 27 fL for White indicates a drop in anode voltage produced by deteriorating triplers. In

order to keep the image from jumping in and out, the Brightness

Control was reduced slightly for the room illuminant experiment.

Comparison of critical waveforms and some of the test point

voltages between the Leesburg systems (#1 and #2) and the NBS

RRWDS (at the time of the room illuminant experiment) indicated

that all three systems were performing properly except for the

tripler problem. This finding suggests that the shifts in WIL

chromaticity found in the laboratory, as a function of the room

illuminants tested, may be generalized to field RRWDS under

equivalent lighting geometries and conditions.

The column labeled "Screen Reflections, with No Signal" refers to

the light energy reflected from the screen of the CRT surfaces,

(the five areas of reflectance cited earlier) , with no power

applied to the CRT console. If the screen were a "perfect" or

simply a nonselective reflector then the spectral power

distribution (SPD) of this reflection should be similar to the

SPD of the room illuminant itself. However, based on

experimental data, the SPD's of the room illuminant ' s reflections

and the SPD's of the room illuminant measured directly were not

36
exactly the same. The SPD of the room illuminant was measured

using a pressed polytetraf luorethylene (PTFE) powder plaque [18]

which is a near perfect reflector. The PTFE plaque was attached

to a large piece of black felt so it could be placed over the

console face and located in the same measurement position used to

measure the CRT screen reflections.

4. 4. 2. 4 Fluorescence/Phosphorescence of CRT Display

Since many of the lamps in the NBS lighting laboratory have many

hours of use, the SPD's of each room illuminant were checked

prior to the study. Under high illumination of some lamps, e.g.

metal halide, the measured WIL values under the room illuminant

were slightly higher than the calculated values of the WIL, as

measured in the dark, plus the reflected light measured with the

console power off. The question posed was: "Where did this extra

spectral power come from?" This question was answered by the

following experiments:

Experiment #1: Four (400 watt) metal halide lamps were energized

and warmed up for 15 minutes with the (closeable) shutters wide

open. A black felt cloth, with the PTFE plaque attached, was

placed over the face of the CRT console. The spectral radiance

of the PTFE plaque was measured by the SR, resulting in a SPD

plot and/or print-out of the spectral power, from 390 to 730

nanometers (nm) at 2 nm intervals. Five to 15 minutes after the

black felt was removed, the same measurement procedure was

repeated for the power-off CRT display. Comparison of the two

SPD's indicated many small but a few large increases in spectral

power over the 2 nm intervals, particularly for the green

wavelengths

.

Comparison of the total measured luminance with a calculated

luminance indicated the measured luminance was slightly larger

than the corresponding calculated value. Since the ordinary

reflectance of the CRT could not exceed that of the PTFE, the

spectral power increases can only be explained by assuming that

the CRT's RGB phosphors were fluorescing due to the high

intensity of the light energy from the metal halide lamps. At

low light intensities of the metal halide lamps, achieved by

closing the shutters, this phenomenon did not occur. Experiments

in which the black felt was used to control exposure time to the

metal halide lamps, indicated that exposure times of ten to

fifteen minutes produced the maximum fluorescence of the RGB

phosphors

.

Experiment #2: This experiment replicated #1 except that after

the fifteen minute warm-up, the shutters were closed. With the

shutters closed, 2-8 fc of illumination were present due to light

leakage around the shutters. SR measurements were taken every

'five minutes for up to thirty minutes. This latter time interval

was a direct function of the initial fifteen minutes exposure

time. For exposure times of less than ten minutes, the period of

phosphorescence was reduced.

37
The first five minute SPD plot indicated significant amounts of

added spectral energy over the green wavelengths with lesser

amounts over the blue and red wavelengths. From visual

inspection, the first five minute plot was W shaped. During the

next five (at five minute intervals) measurements, the overall

added spectral energy or phosphorescence decreased. After the

thirty minutes, the black felt, with the PTFE plaque attached,

was placed over the console face and another SR measurement

taken. Comparison of the PTFE SPD plot against the last test (30

minute) plot indicated the shapes of the two plots were similar.

The major difference was less spectral energy in the SPD plot

taken from the CRT console display. This loss of spectral energy

from the CRT console display may be explained by light losses at

the four glass display surfaces, i.e. at lease a 4% loss per each

glass surface plus an additional loss at the phosphor layer.

The results of both experiments 1 and 2 indicated that high light

intensities produce both fluorescence and phosphorescence of the

CRT's RGB phosphors. Low light intensities, e.g. 2-8 fc after

the metal halide shutters were closed, were not sufficient to

produce either fluorescence or phosphorescence. Further

experiments with other room illuminants suggest that fluorescence

and phosphorescence were a direct function of the initial light

intensity. The length of initial exposure time tended to effect

the period of phosphorescence but appeared to be less important

than the initial light intensity itself.

Experiment #3: A "black light" fluorescent lamp (F15 T8) was set

up vertically with a rectangular piece of aluminum roughly bent

to act as a parabolic reflector. Since the "black light"

fluorescent tube was only rated at 15 watts, it was desirable to

increase the ultra-violet irradiance on the CRT display. The

"black light" was not a pure UV source since it produced some

light that was visually detectable.

The crude "black light" set-up precluded the measurement of the

amount of the fluorescence of the RGB phosphors. As judged

visually, the RGB phosphors fluoresced fairly strongly, but not

as much as regular fluorescent color samples. In contrast, the

PTFE plaque visually appeared almost black when held up against

the fluorescing CRT display. The appearance of the PTFE plaque

indicated that the amount of light emitted in the visible range

by the "black light" source was relatively small.

The duration of the phosphorescent phase, after the crude set-up

was removed, was very short, approximately five minutes. This

was probably due to the low intensity level of the UV light

source. The SPD of the phosphorescence showed maximum spectral

power at the blue and red wavelengths of the visible spectrum.

The relative proportion of spectral energy in the green

wavelengths was considerably less than that found for the metal

halide lamps. Thus, the SPD of the phosphorescence produced by

the "black light" was roughly U shaped.

38
Further experimentation with all the room illuminants used in

this study indicated they all produced fluorescence and

phosphorescence at the maximum illuminance obtainable in the

lighting laboratory. At the minimum illuminance, less than 10

fc, none of the room illuminants produced phosphorescence. If

any of the room illuminants produced fluorescence, it was below

the detection ability of the measurement equipment used. At

maximum illuminance, each room illuminant had its own

characteristic effect related to its SPD, as measured by the SR

using the PTFE plaque as a reference. As one would expect, since

the low pressure sodium illuminant has little energy in the short

wavelengths, it produced the least amount of fluorescence and

phosphorescence whereas UV- and violet rich mercury and metal

halide lamps produced the most. These latter results on

fluorescence and phosphorescence were in agreement with the

Stokes radiation theory wherein a photon of a given wavelength

will ordinarily only produce a re-emitted photon of a longer

wavelength. There is also an anti-Stokes theory which states

that high energy photons (UV) may produce re-emitted photons at

shorter wavelengths but the amount is very small.

It should be noted that the NBS lighting laboratory was not

designed to equalize the maximum or minimum illuminance across

all room illuminants. Since the amount of fluorescence and

phosphorescence appears to be a direct function of the intensity

of the illumination, the relative rankings of the room

illuminants cited in the preceding paragraph should be considered

as tentative only. From a knowledge of the SPDs of the room

illuminants used in this experiment, the rankings appear

appropriate but NBS does not have equal-illuminance data to

support the rankings unequivocally.

4. 4. 2. 5 RRWDS Variability

Examination of Table 10 reveals a small number of occasions where

the luminance of a WIL, as measured under a room illuminant, is

less than the luminance of the same WIL, as measured under dark

conditions. This decrease in total luminance is not constant for

all WIL's under one illuminant nor is it constant for a single

WIL under all room illuminants. The lack of constancy in the

measured WIL luminances raises three separate but related issues

about the anomalies seen in Figure 10. The first issue concerns

the anomaly that some of the WIL luminances, when measured under

the room illuminants, were less than when measured under dark

conditions. Physically, the superimposition of reflected room

light should simply add to the WIL luminance(s) to produce a larger total luminance. If the room light can be considered a constant, then it must be true that the WIL luminance ( s) , if they

had been measured in the dark at the start of each day's

measurements, should have been less than the "dark" values given

in Table 10. The in-the-dark measurements were taken only at the

start of the three month's long experiment. Thus, the answer to

this issue lies in the day-to-day variability of the electronics

of the RRWDS's display console, and in particular the HV power

39
supplies' tripler. The malfunctioning of the triplers led to

unusual and (at the time) unsuspected differences in the total

luminance measurements under the various light sources

investigated. Following completion of this study, the potting

material used in the tripler (s) failed completely with the result

that the *HV arcs to ground. This failure precluded postexperiment measurements of the WILs, again under the dark room

conditions

.

The second issue is whether the day-to-day variations in the

RRWDS's performance also affected the WIL ch romat ic it ies

.

Theoretical chromaticities were first calculated on the basis of

a simple additive model. In the model the light emitted by the

WILs, originally measured in the dark, was assumed to combine

with the ambient room light reflected from the CRT screen,

measured with the CRT off, to produce a total additive

chromaticity . These calculations, of course, are not influenced

by day-to-day variations in the WIL outputs. When the measured

and calculated chromaticities were plotted and compared for the

anomalies in Table 10, only small and apparently random

differences in chromaticities were found. This finding indicates

that changes in WIL luminance (issue one above) did not

necessarily result in changes in WIL chromaticity. However, this

finding should not be generalized to all malfunctions in the HV

section for the CRT display.

The third issue, which involves both luminance and chromaticity,

is: Does the reflected room light combine with the emitted WIL

light (s) in a simple additive way, as would be expected on the

basis of the physics of light, or is there a more complex

interactive effect involved? Such an interaction might occur if

the glass surfaces of the CRT or the phosphor layer, were not

non-select ive reflectors; or if the room light excited strong

fluorescence in the phosphor layer. Again, comparison of the

measured values versus the calculated values for luminance and

for chromaticity indicates that the combination of the two light

sources, at low levels of the room light, produces net results

that are in good agreement with the concept of simple additivity.

At moderate to high levels of this room light, some fluorescence

of the phosphor layer does occur as mentioned earlier. Examination of Table 10 and Figure 12-20 indicates that the white

target areas luminance and chromaticity was more affected by this

fluorescence than the WILs. For example, the measured luminances

of the white target were greater than the calculated luminances

under high illumination levels for the LPS and HPS room light

sources. The chromaticities of the white target were also

shifted in unexpected directions as can be seen in Figures 12-20.

The basis for the unusual shift in the white target area as

compared to the WILs should be investigated further; and

especially the role of photon stimulation of the CRT's phosphor

layer and the resulting fluorescence.

40
In summary, the above discussion of some issues raised by

apparent anomalies in Table 10 establishes the fact most can be

explained by erratic variations in the CRT display. These

variations were due to (at the time) unknown malfunctioning of

the triplers in the CRT HV power supply (s) . Thus, the anomalies

are not due to errors in the measurement instrument or in the

measurement procedure. The effect of fluorescence by photon

stimulation on the chromat icities of CRT colors should be

investigated further to explain the remaining anomalies.

4. 4. 2. 6 CIELAB Color Space

CIELAB color space is designed to compare object colors in a uniform color space, based on the relationship of the object

colors to some predetermined reference white. The reference

white may be any available white, e.g. the white of a piece of

paper on which there are colors. Thus, to calculate the effect

of the various room illuminants' reflections on the CIELAB

coordinates of each WIL, two assumptions had to be made. The

first assumption concerns the selection of a reference white.

The reference white selected was the White column of Table 10, as

measured under dark conditions. Since this reference white had

the same RGB digital levels as the white used in the built-in

RRWDS geographical maps of a live weather display, it was an

appropriate reference white. The other assumption used in the

CIELAB equations involved the reflections of the room illuminant

from the CRT display, as measured with the power off. For the

purposes of the CIELAB calculations, it was assumed that the

reflections (plus some fluorescence at high illuminance levels)

obtained with the power off would be constant across all WIL's

for any given illuminant, and would combine in a simple additive

way with the emitted WIL light. In other words, the fluorescence

of the RGB phosphors, as stimulated by photon emissions, simply

adds to the emitted light of the WIL's, as stimulated by electron

emissions, i.e. the two types of emissions do not interact in

some strange way. It is similarly assumed that there is no

interaction between the reflected room light and electron-driven

emissions from the phosphor layer.

The reflection values used in the CIELAB calculations are

presented in Table 10 under the heading "Screen Reflections, with

No Signal." A special computer program was written to perform

the desired calculation of additive light mixtures. This program

incorporated two CIELAB subroutines. The program changes the

(x,y) values of the reference white, the measured WIL's, and the

display screen reflections to their tristimulus values, (i.e.

X , Y , Z

)

in order to make the appropriate calculations. For the

reference condition the CIELAB calculations compared the measured

WIL color, measured in the dark, to the reference white, measured

in the dark. Then to calculate the CIELAB co-ordinates of a WIL

plus display screen reflections, the tristimulus values of the

display screen reflections, as measured with the CRT off, were

added to both the reference white and the WIL, both measured in

the dark. In these calculations, as in real life, the display

41
screen reflection tends to shift all colors toward itself in

color. In CIELAB the reference white always maps to L*=100,

a*=0, and b*=0 as can be seen in Table 10. The final conversion

of the tristimulus values (X,Y,Z) back to the x,y coordinate

system are presented in Table 10 as "calculated" values. It

should be noted that the calculated luminance values are always

the sum of the WIL luminances, as measured in the dark, plus the

luminance of the display screen due to the room lights, as

measured with the CRT console power off.

CIELAB plots of the calculated values along with the measured

values were made for all room illuminants. Comparison of the

calculated versus the measured values showed no significant

chromaticity differences between these two sets of values for any

of the room illuminants. These findings may be interpreted in

two ways. First, the decreases in total luminance, i.e. where

the WIL luminance plus the luminance due to display screen

reflections is less than the WIL luminance measured in the dark,

is due to a drop in the anode voltage of the RRWDS CRT. More

importantly this decrease in total luminance apparently affected

all three about equally, an so did not produce a shift in WIL

chromaticity. Second, the presence of display screen

reflections, due to ambient lighting, did produce significant WIL

chromaticity shifts. The amount of WIL chromaticity shifts is a direct function of the intensity of the room illuminant that is

being reflected off the display screen face.

The significant findings of this part of the report may be

summarized as follows:

(1) The luminance, produced by the ambient lighting systems

reflections off the display screen, can and does produce

significant shifts in WIL chromaticity. The direction of the WIL

chromaticity shifts is dependent on the type of lighting used and

the magnitude of the shifts depends on the intensity of the

reflections, as measured by the total luminance of the CRT

display. Much additional experimental information is required to

determine such parameters as: (a) the best lighting source, e.g.

LPS versus metal halide, for CRT viewing; (b) the maximum light

intensity that will not produce fluorescence or phosphorescence

of the RGB layer of the CRT; and (c) the most appropriate

lighting geometry for CRT viewing. One recommendation would be

to eliminate massive, diffuse overhead lighting in favor of

overhead point source lighting to highlight light/dark contrast

ratios along with point source lighting (as appropriate) for work

station lighting. Although initially this recommendation may

require extensive and expensive lighting evaluations, the end

result would increase worker efficiency as well as reduce

long-term lighting energy costs. The fluorescence and phosphorescence of the phosphor layer of the CRT display presumably may

reduce a viewer's ability to discriminate between colors of a color-coded CRT display, but much less than the reflected light.

The overall loss of information may be significant under certain

lighting conditions, e.g. when a CRT display is located where

42
relatively strong lighting from an outdoor environment may

provide obscuring light such as in some airport control towers or

field service units. Potentially there are short term fixes for

this problem, e.g. canopies over the CRT display. However,

additional experimentation would be required to delineate and/or

standardize the best lighting sources and the best lighting

geometries required to improve worker efficiency and reduce longterm lighting energy costs.

4.4.3 Comparison of Various Illuminants

Figures 3 through 12 show the effect of illumination from various

overhead illuminants on the chromaticities of the six WIL's as

measured by the spectroradiometer (SR) . The effect on the six

WIL's of each illuminant (triangles) is compared against the six

WIL chromaticities taken under complete dark conditions

(squares) . The CIE x,y coordinates obtained by the SR were the

data inputs to a CIELAB computer program used to make the

plots/figures showing color differences between the dark and

illuminated conditions. CIELAB, more formally known as the CIE

1976 L*a*b* color space, is an accepted formula for the amount of

color difference perceived between any two colors, and the name

also refers to the geometric representation of the color spacing

implied by the formula. The idea of a so-called uniform color

space is a space in which the distance between the points

representing any two colors is proportional to the amount of

difference perceived between those colors.

Several realistic features incorporated in the CIELAB space are:

(a)’ recognition of the non-linear relationship between stimulus

and response characterizing color vision, as with most other

sensory processes; (b) an allowance, of a simple sort, for the

chromatic adaptation process that contributes to the maintenance

of color constancy; and (c) an adjustment for the fact that if a color of fixed chromaticity is varied only in brightness or

lightness, its (perceived) saturation (vividness) decreases as it

gets darker.

The L* variable of CIELAB corresponds to lightness or brightness.

The chromatic variables are symbolized by a*, the red (positive)

-green (negative) dimension, and b*, the yellow (positive) -blue

(negative) dimension (see figure 2) . Although the L* dimension

does not appear in Figures 3-12, the quantities a* and b* are

influenced by the luminance of the light, in accordance with (c)

of the preceding paragraph. These a*b* diagrams are intended to

predict color appearance and color difference, but the quantities

plotted are calculated exclusively from the physical spectroradiometric data.

4.4.4 Results

There are several different computer graphics programs to make

CIELAB plots according to opponent color theory. Both programs

depict the same information. The first program produces diagrams

43
b*

Values

LPS 41 FC, MODERATE/ RRWDS

iinnn • ! MW 1®.®

9C K

80.®

yn ry\

00.®

rn «r

wLi.WW 40.®

X.®

®OT

lu.u; Q.®

- 10.®

-20.®

-X.®

-40.00

-X®

-90.®

-70.®

-50.®

-90.®

4 fin nn -1 vA/.:JW-1® -80 -60 -40 -20 0 20 40 00 80 1®

Yellow

3

1 o 0 n r 24 . “

Green Red

wt- I7F

Blue

T—l—l—l—I—l—l—l—l—I—l—I—I—i—I—I—I—i—i—

r

a* Values

Figure 2. Effect of LPS illuminant on the measured WILs.

44
wherein white is placed close to the center of the diagram while

displaying four nearly equal quadrants. These diagrams point up

the opponent-colors structure of a*b* space, even if (as here)

some of the quadrants are empty. The resulting diagrams (Figure

2) show major relationships between all colors according to

opponent color theory. The second program scales and centers the

diagrams so as to encompass just the data points entered; i.e.

regardless of whether entire quadrants are eliminated (as they

are in Figures 3-11) . These diagrams display finer details since

they incorporate only the colors of interest. Therefore, these

diagrams are useful where colors are nearly alike as for WILs #5

and #6.

Figure 3 shows the effect of illumination from a Low Pressure

Sodium (LPS) lamp, at minimum luminance, on the six WIL

chromaticities (triangles) . The SPD of LPS lamps shows that the

bulk of the radiant energy occurs in two very narrow peaks at ("D

lines") 589.0 and 589.6 nanometers, i.e. the resonance

wavelengths of the electrically excited sodium atom. Examination

of Figure 3 shows, that under low LPS illuminance, the WIL's tend

to be grouped into two sets of nearby colors, i.e. WIL's (1 and

3) and (2, 4, 5, 6) . Within each set, the WIL's appear difficult to

distinguish. Under LPS the CIELAB predicted perceived WIL colors

should have a strongly yellowish tinge, as indicated by the high

b* values for all six WILs. WIL #3 (a bright yellow is shifted

very little under the LPS light whereas WIL #2 (a dark green) is

shifted the most (it changes quadrants) to become a perceived

reddish yellow. WILs #2, #4, #5 and #6 are all shifted to a reddish-yellow in contrast to their color in the dark. This

occurs even though the WIL's are self-luminous. In contrast,

when LPS light is reflected from almost all colored opaque

surfaces, they are visually described as black, tan, or yellow,

(except red or orange fluorescent colors) . Since LPS has a

uniquely low color rendering index of -44, it is used almost

solely for outdoor application where high brightness and

efficiency are more important than color rendition.

Figure 4 shows the CIELAB predicted perceived colors for the

WIL's from a high luminance LPS lighting source. Here the effect

of high LPS illuminance is to group the WIL's into two internally

indistinguishable groups; one originally having a green component

in the dark (1,2,3) and one having a red component (4,5,6).

Since the two groups are relatively close together, a deuteranope

or protanope (i.e. one who strongly confuses red and green)

should see only yellowish shades varying only in brightness level

and saturation. Even most normal vision individuals would have

trouble distinguishing the six WIL's because of the shift of all

the colors toward orange/yellow - the color of the wavelengths

characteristic of LPS. It is important to remember that the

input data to the. CIELAB program was objectively obtained by

means of the SR. Although spec t ro rad iomet ry indicates the

presence of trace amounts of power in the blue or short

wavelengths of the LPS SPD, this amount is usually insufficient

to allow accurate recognition of blue colors. As a result, when

45
120.00

EFFECT OF LOW LPS LIGHT ON WILS

1 00 . 00 -

80.00 -

ft 60 . 00 "

40 . 00 -

20.00 -

0. 00 “I - 40 . 00

1 1 - 20 . 00 0 . 00

SOURCE: DARK

T T 20 . 00 40 . 00 60 . 00

A*

A SOURCE: LOW LUM. LPS

80.00

Figure 3. Effect of LPS illuminant (low illuminance of 41 fc)

on WIL colors (triangles) as compared with their

appearance under dark conditions (squares)

.

46
;u *i 1 1 1 1 1 -40.00 -20.00 0.00 20.00 40.00 60.00 80.00

A*

SOURCE: DARK A SOURCE: HIGH LUM. LPS

Figure 4. Effect of LPS illuminant (high illuminance of 86 fc)

on WIL colors as compared to their appearance under

dark conditions.

47
LPS sources are used occasionally for indoor purposes (e.g. high

ceilinged warehouses or heavy manufacturing facilities) , they

should be interspersed with sufficient metal halide or mercury

lamps to provide the extra energy needed in the blue part of the

spectrum.

High Pressure Sodium (HPS) lamps are one type of high intensity

discharge (HID) lamps - some of the others being metal halide and

mercury vapor lamps - that are becoming more frequently used,

especially in street lighting. HPS and LPS lamps differ

significantly in their construction and in their operation. For

example, a 400 watt LPS lamp is approximately four feet long,

whereas HPS lamps are less than a foot long and have a screw-inbase. Operationally, a LPS lamp operates most efficiently at a sodium vapor pressure of the order of 0.7 pascal (5xl0 -3 torr)

whereas HPS lamps operate most efficiently at 27 kilo pascals

(200 torr) . To achieve the higher sodium vapor pressure needed

for HPS lamps, a small quantity of sodium-mercury amalgam is used

so that the mercury vapor acts as a buffer gas to raise the

sodium vapor pressure and increase the operating voltage of the

HPS lamp. The higher sodium pressure causes the sodium ("D n line

frequency) radiation to be self-absorbed and re-radiated as a continuation on both sides of this n D" line frequency. Thus, HPS

lamps typically radiate energy across the visible spectrumexcept for a "dark" region around the "D" line frequency or 589

nanometers. The golden-white light of the HPS lamp has a better

color rendering index (CRI=21) than that of LPS. However, the

efficacy of HPS is less than that of LPS - i.e. efficacies of 60-

140 lumens per watt, depending on size for HPS whereas LPS has

efficacies that may exceed 180 lumens per watt.

Figures 5 and 6 show the effect of low and high HPS illuminance,

respectively, on the predicted perceived colors of the six WlL's.

Figure 5 shows that the predicted perceived chromaticit ies of the

six WIL's changed very little from the chromat ic it ies measured

under a dark (no light) condition. On the other hand, Figure 6

indicates that high levels of HPS induced a significant shift of

all six WIL's in the yellow (high b*) direction, resulting in two

groupings of WIL's 1 and 3 and WIL's 2,4,5 and 6. Within each

grouping, the WIL's will be difficult to distinguish, except

possibly by brightness differences. Although brightness ( L* ) was

calculated and used, it was not plotted separately for each WIL.

If the L*s had been plotted they would have presented better

evidence as to which WIL's could be distinguished by brightness

differences alone.

Figure 7 presents the effect of illumination from cool-white

fluorescent lamps supplying medium illuminance (approximately 10

fc) . Again, although the relationships between the WIL's remains

approximately the same as for dark conditions, there is a trend

for the reddish WIL's (4,5,6) to lose a significant portion of

their redness, i.e. they tend to become desaturated and move

towards the white or neutral condition (i.e. where a* and b* - 0) . Under the color classer fluorescent illuminant (Figure 8),

48
120.00

EFFECT OF LOW LUM. HPS ON WILS

100.00 -

80.00 -

£ 60.00 -

40.00 -

20.00 -

0.00 H

-40.00

1 1 -20.00 0.00

SOURCE: DARK

1 1 1—

20.00 40.00 60.00

A*

A SOURCE: HPS AT MIN LUM.

80.00

Figure 5. Effect of HPS illuminant (low illuminance of 3 fc)

on WIL colors compared to their appearance under

dark conditions.

49
EFFECT OF HPS AT MAX LUM. ON WILS

A*

SOURCE: DARK A SOURCE: HPS AT MAX LUM

6. Effect of HPS illuminant (high illuminance of 76 fc)

on WIL colors compared to their appearance under

dark conditions.

50
120.00

EFFECT ON WILS, FLUOR-COOL WHITE

100.00

80.00

60.00

40.00

20.00

0.00 H

-40.00

1 1—

-20.00 0.00

SOURCE: DARK

1 1 1—

20.00 40.00 60.00

A*

A SOURCE: FLUOR-COOL WHITE

80.00

Figure 7. Effect of cool-white fluorescent illuminant (10 fc)

on WIL colors as compared to their appearance under

dark conditions.

51
120.00

EFFECT OF A FLUOR. LAMP ON WILS

100.00

80.00

& 60.00

4

- 0.00

20.00

0.00 -} -40.00 -20.00 0.00

SOURCE: DARK

1 1 1—

20.00 40.00 60.00

A*

A SOURCE: FLUOR—DAYLIGHT

80.00

Figure 8. Effect of color-classer fluorescent illuminant (14 fc

on WIL colors compared to their appearance under

dark conditions.

52
the same shift of WIL's 4, 5 and 6 occurs but to a slightly

greater extent, i.e. these WIL's tend to shift even further

towards the equal energy point thereby losing more of their

redness. Since all the field sites visited used fluorescent

lamps, these results suggest that cool white fluorescent lamps

may be preferable to color classer fluorescent lamps. However,

since the amount of chromaticity shift between the dark condition

and the two illumination conditions (i.e. from cool-white and

color classer fluorescent lamps) were not statistically evaluated

for significant differences, the choice for fluorescent lamping

may be one of preference which could include other factors than

their effect on the RRWDS 's.

During field testing of the color calibrator, one advantage of

ambient fluorescent lighting overcame the disadvantage of not

being able to make certain color calibrator tests in complete

darkness. By the judicious use of a black felt canopy over and

around the RRWDS display console, spect roradiomet ric measurements

could be taken so the spect roradiomet r ic data could be examined

for any trace of spectral power at the four characteristic

fluorescent peaks cited earlier. If these peaks were

undetectable by the SR, then it was assumed that any residual

fluorescent illumination was negligible and the color calibrator

tests could be conducted as if the room were in darkness.

The effect of a tungsten (incandescent) illuminant. Figure S, was

a tendency to reduce the brightness contrasts between the six

WIL 1 s - (common to the effect produced by most illuminants)

with the greatest effect on WIL #2 or dark green. The red

component WIL's 4, 5, and 6 again appeared somewhat desaturated

or washed out. As judged by the CIELAB results, illumination

from the yellowish white tungsten illuminant tended to drive the

self-luminous WIL colors toward the illuminant point i.e. they

desaturated all the WIL colors.

Figure 10 (metal halide) and Figure 11 (mercury) show the effects

of these two illuminants. Both of these illuminants had more of

an effect on WIL's 4, 5, and 6 than did the incandescent

illuminant. These colors shifted away from their chromat ic it ies

under the dark condition toward the origin, indicating a decrease

in the amount of redness.

The results of the CIELAB analysis shows that illumination from

different light sources produced different light shifts in the

predicted perceived chromat icities of the WIL colors. However,

additional research is needed to determine if the color shifts

from the white-colored sources differ from one another with

statistical significance. Further research is also needed to

determine a "best" light source for a building lighting

environment where a number of activities take place as well as

just viewing the colors of a RRWDS display. According to the

CIELAB analysis, the "best" light source was low-level HPS since

it produced the least shift in the WIL colors. This is probably

not an artifact since the color of HPS light is closer to the

53
1 20.00

EFFECT OF A TUNGSTEN LAMP ON WILS

100.00

80.00

60.00

40.00

20.00

0.00 H - 40.00

1 1 -20.00 0.00

SOURCE: DARK

20.00 40.00

A*

A SOURCE: TUNGSTEN

1—

60.00 80.00

Figure 9. Effect of tungsten illuminant (3 fc) on WIL colors as

compared to their appearance under dark conditions.
120.00

EFFECT OF MET. HAUDE LAMP ON WILS

100.00

80.00

60.00

40.00

20.00

0.00 H -40.00

1 1 - 20.00 0.00

SOURCE: DARK

1 1 1—

20.00 40.00 60.00

A*

A SOURCE: METAL HAUDE

80.00

Figure lo. Effect of metal halide illuminant (8 fc) on WIL colors

as compared to their appearance under dark conditions.

55
1 20.00

EFFECT OF A MERCURY U\MP ON WILS

100.00 -

*

eo

80.00 -

60.00 -

40.00

20.00 -

0.00- 40.00 - 20.00 0.00 20.00 40.00 60.00

A*

SOURCE: DARK A SOURCE: MERCURY

Figure 11. Effect of mercury illuminant (4 fc) on WIL color:-.

compared to their appearance under dark condition:

80.00

as

56
average of the six WIL colors than white is, and hence HPS

changes the WIL colors the least when mixed with the WIL colors.

In addition, at 1 fc it was much closer to the darkened room

condition. For higher levels of illuminance, all sources produce

distinct shifts in chromaticity in the RRWDS. However, whether

low-level HPS is equally good for other activities and is

acceptable by people remains to be determine.

4.5 CIE CHROMATICITY RESULTS

This section presents the basic data obtained by using the SR to

measure the SPD's of various light sources. In addition, this

instrument also provides other data of interest, and in

particular, the CIE x,y chromaticity values. These CIE values

were plotted on a standard chromaticity diagram as presented in

Figures 12-20. The information presented in these figures is

similar to that presented in Figures 3-12 (except for the

influence of luminance in the latter) , but readers more familiar

with the x,y diagram than with the a*,b* diagram may prefer the

presentations in Figures 12-20. In these figures, the squares

connected by lines represent the WILs measured under dark

conditions and is a constant for all room illuminants. The

triangles represent the WILs as measured with the various room

lights on. The CIE x,y values, recorded with the room lights

off, were also used as the input to the CIELAB program discussed

in the preceding section.

Readers not familiar with chromaticity diagrams might find it

helpful to obtain a full-color poster of the diagram, e.g. as

distributed by Photo Research, RCA Solid State Division or

Hoffman Engineering. Reference to such a poster will provide

guidance about the colors represented at various points in the

diagram. The colors represented on the posters are only

approximately correct as the saturations are too low toward the

outer edges of the region of real colors, i.e. near the horseshoe

curve which represents the purity limits of all physically

realizable colors. The low saturation is due to the printing

inks, which cannot reach theoretical maximum saturations

achievable only by monochromatic (single-wavelength) lights from

lasers or from white light dispersed into a spectrum by a prism

or grating. The curved edge of the horseshoe represents the

"spectrum locus", the chromaticities of all pure monochromatic

lights. Thus, the blue/violet (shortwave) end of the visible

spectrum is at the left end of the curve while the red (longwave)

end of the spectrum is at the right. The straight line at the

bottom of the horseshoe is the purple boundary, i.e. the loci of

purples of maximum saturation obtained by mixing various

proportions of red and blue lights. The interior of the closed

horseshoe region contains the chromaticities of all colors of

lesser saturation, with pure white in the middle. [Note that in

Figure 1, page 11, that the gamut of colors that can be produced

by a color CRT (i.e. the triangle shown) is limited to less than

full saturation by the light emitted by the RRWDS CRT phosphorsa limitation analogous to the poster inks.]

57
Figure 12 presents the WIL chromat icities as measured under dark

conditions (squares) together with their chromat ic it ies as

measured with illumination produced by a low level LPS illuminant

(i.e. a single 400 watt LPS lamp) (triangles). When illuminated

by the LPS source, all the WIL's are yellowish and tend to form

two groupings, namely: (1) a greenish-yellow group for WIL's 1

and 3 ; (2) a reddish-orange group for WIL's 2,4,5, and 6. In

comparison with the dark condition, the six WIL's tend to be more

alike with the members of each color grouping being virtually

indistinguishable, except for possible brightness differences.

Note that WIL's 5 and 6 have nearly the same measured

chromat icities , although in the RRWDS display they are designed

to vary considerably in brightness level.

The expression "virtually indistinguishable" needs further

clarification since it refers to chromatic differences only. For

example, if the chromaticities of WIL's 5 and 6 were reproduced

on large color chips having equal brightnesses and then shown

side by side, the human color vision system would probably detect

the chromatic difference. However, if the WIL's 5 and 6 test

samples were then shown to the same subject sequentially (i.e.

with a temporal interval between viewings), the subject's visual

memory system may not detect these small chromatic differences.

Therefore, if two colors lie close together on the chromatic

diagrams, due to the effects of the SPD of the light source, then

the effect of the light source may be interpreted as reducing

color contrasts, leaving only brightness contrast as a means of

distinguishing between WIL's. In the case of WILs #5 and 6,

since the chromaticities are nearly the same in the dark, the

similarity is not a function of the light source. Obviously, as

the level of illumination is increased, the screen reflections

are increased with the result that brightness contrasts are

correspondingly reduced.

At high illuminance (86 fc) , the LPS illuminant produced two

groupings (see Figure 13), namely: (1) a greenish-yellow group

(WIL's 1 and 3) and (2) an orange-yellow group (WIL's 2, 4, 5, 6).

Members of the orange-yellow group appear to be indistinguishable

from each other, except for brightness differences, and barely

distinguishable from the greenish-yellow WIL's. Visually, a white appeared to the author to be tinged with yellow-green at

low illuminance but orange-pink under high LPS illuminance. The

actual color names may vary from experimenter to experimenter,

and therefore should be interpreted on a subjective rather than

an objective basis. The shifts shown in Figures 12 and 13,

however, depict objectively measured changes in physical

chromaticity

.

The effects of HPS lamps, which have a broader SPD than LPS, are

shown in Figures 14 and 15. As might be expected on the basis of

light intensity only, the low illuminance (1 fc) HPS light source

(Figure 14) had little effect on the chromaticities of five of

the six WILs, as compared to their chromaticity under dark

conditions. The simple addition of the reflected HPS light did

58
EFFECT OF LOU LPS LIGHT ON UILS

SOURCE: DARK ASOURCE: LOU LUM. LPS XUHITE : DARK + WHITE : LOU LUM. lps

Figure 12. WIL chromaticities as a function of a low

illuminance LPS source (approximately 41 fc at

the CRT display)

59
0.90

EFFECT OF HIGH LPS LIGHT ON LIILS

0.80 -

0.70 -

0.60 -

0.50 -

0.40 -

0.30 -

0.20 -

0 . 10 -

0. 00

0 . 00

SOURCE: DARK ASOURE: HIGH LUM. LPS XUHITE : DARK + WH I TE : HIGH i M. I P< Figure 13. WIL chromat ic it ies as a function of a high

illuminance LPS source (approximately 86 fc at

the CRT display)

.

60
EFFECT OF HPS AT MIN LUM. ON NILS

Figure 14. WIL chromaticit ies as a function of a low

illuminance HPS source (approximately 1 fc at

CRT display) .

61
produce a fairly sizeable shift in the chromaticity of WIL 2,

i.e. from a dark green to a dark yellowish-green. WIL 2 had the

lowest luminance (0.28 fc at the time of measurement) of the six

WILs. Since WIL 2 contributed proportionally little to it's

total measured luminance under HPS, it shifted more than the

other WILs towards the color of the room illuminant. Another

consequence of the low luminance of WIL 2, is that if it is

embedded in or adjacent to a bright WIL it will appear even

darker due to the simultaneous contrast phenomenon. The amount

of blue in the white target is curious and not easily explained.

At its maximum illumination of 76 fc, the HPS light source

(Figure 15) produced chromaticity shifts in all six WILs. The

dark WILs were shifted more than the bright WILs. Figure 15

indicates the WILs were shifted towards the yellow of the HPS

light source, however, the HPS induced shifts were less than the

corresponding shifts under an LPS light source. Similar to the

LPS source, the superimposition of the HPS reflections resulted

in two groups of WILs; namely one group formed by WILs 1,2, and

3, and the other formed by WILs 4,5, and 6. Figures 13 (high

LPS) and 14 (high HPS) indicate that the superimposit ion of these

lights sources results in more yellow saturated WIL colors rather

than in the greater saturation of the WIL's original colors, as

measured under dark conditions.

Comparison of Figures 13-20 versus Figures 3-12 should be done

with care. Figures 13-20 are x,y or chromaticity plots of the

WIL chromat icities objectively measured by a spect roradiometer

.

Figures 3-12 are a*,b* plots of the calculated perceived WIL

colors. As discussed earlier, these calculations include the

objective data in Figures 13-20, plus SR obtained luminance data,

and plus known human visual system responses, i.e. a*,b* is

chromaticity weighted by luminance. The a*,b* plots are closer

to what a human may actually see. To illustrate, the

chromat icities of WILs 5 and 6, as shown in Figures 13-20 for the

dark condition, lie sufficiently close together so as to be

indiscr iminable on the basis of color alone, i.e. there is a lack

of color contrast. On the other hand, Figures 2-12 indicate WILs

5 and 6 are discr iminable on the basis of luminance differences,

i.e. they can be discriminated on the basis of brightness

contrasts as originally intended.

The overall effect of the cool-white fluorescent source, shown in

Figure 16, tends to shift all the WIL colors towards the

chromaticity of this light source which is not far from the equal

energy white point (i.e. x=.3333, y=.3333). This shift towards

white is greatest for the dark WIL's because the SR detects

proportionately more of the spectral energy from the white

illuminant than from the spectral energy of the dark WIL's. At

low illuminance levels of cool-white fluorescent, WIL's 4, 5 and

6 should still be distinguishable by people with norma] vision,

although WIL's 4 and 6 may occasionally be confused. The optirnun

illuminance level for viewing the RRWDS CRT is, of course, full

darkness. Since this would not allow safe movement of peoj le,

62
0.90

EFFECT OF HPS AT MAX LUM. ON NILS

0.80 -

0.70 -

0.60 -

0.50 -

0.40 -

0.30 -

0.20 -

0.10 -

0.00

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

SOURCE: DARK ASOURCE: UPS AT MAX LUM. XUHITE : DARK + WHITE : HPS AT MAX

Figure 15. WIL chromaticit ies as a function of a high

illuminance HPS source (approximately 76 fc at

the CRT display)

.

63
0.90

0.80

0.70

0.60

0.50

0.40

0. 30

0.20

0.10

0. 00

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0 . R0

SOURCE t DARK ASOURCE: FLUOR-COOL WHITE XUHITE : DARK -f WHITE : FLUOR- COX

Figure 16. WIL chromaticities as a function of a cool-white

fluorescent source (approximately 14 fc at the

CRT display)

.

EFFECT ON WILS, FLUOR-COOL WHITE

64
the preferred illuminance at the CRT's face appeared (to the

author) to be around 1 to 5 fc . Five fc is about the average

illumination level for dimly lit hallways. If higher

illuminances are necessary for other operations, then a dark

canopy (with sides) may be required or preferably, changes in the

building lighting environment as recommended earlier.

Different types of fluorescent lamps will produce different color

shifts in the WIL's as indicated by comparing the Color Classer

fluorescent lamp (Figure 17) with the cool-white lamp (Figure

16) . The cool-white lamp tended to shift all the WIL's more

toward the equal energy point, than did the Color Classer lamp.

The chromaticity of WIL 2 shifted almost to white under the coolwhite lamp. As expected, 1 the remaining dark WIL's 4 and 6 are

shifted more to the white than the bright WIL's 1,3,5. Visual

inspection of various weather conditions displayed on the RRWDS

CRT indicates that the dark WIL's are often hard to discriminate

when embedded in the bright member of their color pair. NBS has

previously demonstrated minor modifications in the WIL

chromaticities to enhance certain color contrasts, as well as the

brightness contrast between WIL's and the display background.

The effects of a tungsten illuminant on the WIL chromaticities is

presented in Figure 18. Examination of this figure indicates

that a tungsten illuminant has the effect of reducing the

relative contrasts between all six WIL's -while still keeping

their same relationship relative to that seen under dark

conditions (except for the large color shift seen for WIL 2).

Under tungsten light, WIL- 2 shifted markedly towards the red

whereas WIL's 5 and 6 shifted slightly the other way. What

occurred for all illuminants was that the WIL colors shifted

toward the illuminant chromaticity. In the case of tungsten

light, this shift was towards yellow or a point actually located

not far from WIL 2, as measured under tungsten.

Figure 19 presents the effects of a metal halide illuminant on

the WIL chromaticities while Figure 20 presents the effects of

mercury lamps. The effects of these light sources are discussed

together because their effects are somewhat similar. As was the

case for all illuminants, there were reductions in the color

contrasts between the WIL's compared to the WIL's when measured

under dark conditions. Both illuminants produced a major shift

in WIL 2 and substantial shifts in WIL's 5 and 6. As one would

expect, WIL #2 shifts significantly toward the white of the

illuminant. Under the mercury lamps, WIL #6 shifted closer to

WIL #4, i.e. away from the chromaticity of WIL #5. Perceptually,

the dark red of WIL #6 and the tan-gold of WIL #4 become

virtually indistinguishable. WIL #5 can be distinguished from

them because of greater brightness. Since both illuminants

produce considerable spectral energy in the shorter wavelengths

(UV), high illumination levels (to be determined) will produce

fluorescence and phosphorescence in tri-color phosphors as cited

earlier. The addition of such fluorescence can significantly

affect the chromaticities of the WILs (or any other color) .

65
EFFECT OF A FLUOR. LAMP ON WILS

SOURCE: DARK ASOURCE: FLUOR- DAYLIGHT XUHITE : DARK + WHITE : FLUORFigure 17. WIL chromaticities as a function of a colorclasser fluorescent source (approximately 10 fc

at the CRT display)

.

66
EFFECT OF A TUNGSTEN LAMP ON NILS

SOURCE: DARK A SOURCE: TUNGSTEN XUHITE : DARK + WHITE : TUNGSTEN

Figure 18. WIL chromaticities as a function of an incandescent

source (approximately 3 fc at the CRT display)

67
EFFECT OF MET. HALIDE LAMP ON NILS

SOURCE: DARK ASOURCE: METAL HALIDE XUHITE : DARK + WHITE : METAL HAL I

C

Figure 19. WIL chromaticities as a function of a metal halide

source (approximately 8 fc at the CRT display)

.

68
EFFECT OF A MERCURY LAMP ON NILS

SOURCE: DARK A SOURCE: MERCURY X WHITE : DARK + WHITE : MERCURY

Figure 20. WIL chromat icit ies as a function of a mercury source

(approximately 4 fc)

.

69
The above discussion of the effects of various ambient light

sources on the chromaticities of the WIL's (as measured by the

SR) indicate the critical role that a building's lighting system

can play in the interpretation of weather information by a meteorologist. Although not a part of this investigation, an

equal (or perhaps a more important) parameter involved in viewing

color-coded, self-luminous displays is the role of the whole

lighting geometry involved in the correct placement of the CRT

display consoles. The data used in this study was objectively

obtained by appropriate measuring instruments. To obtain data on

effects noted by human observers, a subjective study, using human

subjects would be required. Using human subjects is often

required because the human visual response system contains

complexities not yet possible to incorporate into objective

measuring instruments. The principal such complexity is the

process of chromatic adaptation, i.e. the ability of the eye to

partially discount the color of the light source (for many

sources) , thereby seeing colors differently from those

objectively present. This is the source of what is called the

color constancy phenomenon. In the final analysis, it is, of

course, not an instrument but a human who must decode the

information provided by a color-coded display.

4.6 SUMMARY

The first NBS report of this FAA contract demonstrated the

validity of a color calibrator for the RRWDS WIL's. Inexpensive,

off-the-shelf parts and standard electronic circuits were used.

The sensitivity of the color calibrator was sufficient to detect

the smallest color change permitted by the systems design, except

at very low levels of the primary RGB colors. Color calibrator

results were validated against calibrated laboratory measuring

instruments and found to yield accurate and reliable results

under laboratory conditions.

This report covers field validation of the color calibrator as

well as the development of a RGB color look-up table (i.e. the

tolerance ranges for the calibrator output voltages presented in

Table 4) for use by field maintenance personnel. Due to the

small number of field sites visited and large inter-site

differences in the WIL chromaticities, the color calibrator

look-up table needs further refinement through sampling of a much

larger number of field sites. Despite the small number of field

systems evaluated, the field validation results showed that the

color calibrator can be a valuable tool in maintaining color

consistency of the RRWDS WIL's. The color calibrator is both

more accurate and consistent in measuring the WILs than the usual

method of simply "eye-balling" them. In addition, the objectivity of the measurements taken by the color calibrator should have

greater acceptance by the courts than "eyeballing" in

establishing whether WIL colors were adjusted according to

specifications

.

70
Field experience using the color calibrator suggested some

engineering modifications to the color calibrator's detector head

to improve ease of manufacturing and reduce the cost of the final

product. These modifications would allow the manufacturer to

preset the dark current and sensitivity of each RGB photoreceptor

and make the head a throw-away item. The discardable detector

head would avoid the need for field technicians to make

adjustments to the color calibrator except to exchange the

quick-disconnect detector head.

The field tests also showed the significant effect of the total

lighting geometry on the ability to discriminate between the

WIL's. Field data indicated that any light source providing an

illuminance greater than 1-5 fc on the RRWDS screen, whether

located overhead or to a side, will reduce color discriminability

and brightness contrasts. Consequently, the last part of this

report presents objective data on the effect of room

illumination, as reflected from the CRT display, on WIL

chromaticity shifts as well as the loss of color and brightness

contrast. These laboratory results indicate that the geometry,

spectral composition and luminance of the ambient lighting should

be considered in the placement of field CRT consoles.

Also, a review of the literature on color-coding, where color is

used as the source of information, clearly indicates that the

presence of irrelevant color or too many colors produces an

interference effect that seriously degrades performance.

(Obviously, as the number of colors in a color-coded display is

increased, the amount of information that can be presented in the

display is increased.) Thus, the problem with using too many

colors is the potential for information overloading. The use of

too many colors may increase the time to locate and make correct

color discrimination as well as increase the amount of time

validating the color against a color-code legend located on or

near the display. The result may be that less time is available

for extracting the desired information from the display.

Although the literature does not provide clear cut answers for

color coding, it would appear that more colors can be used in

static or slow-changing displays than in dynamic faster-changing

displays, especially if the latter are not made of neat,

geometrical, and easily recognizable forms or patterns. Based on

the research literature, the author suggests that the optimum

number of colors for a weather display should be between six and

ten colors to obtain good human performance in extracting weather

information without overloading the observer.

Finally NBS suggests that the color calibrator be upgraded to a relatively inexpensive "true" CRT colorimeter. This colorimeter

would still use the color calibrator detector head and add a small microprocessor with built-in hardware/software programs.

These programs would use matrix algebra to calculate the

International CIE (x,y) coordinates from the RGB outputs of the

detector head. In order to keep the colorimeter inexpensive, the

matrix algebra equations require that the chromaticit ies of a

71
CRT's RGB phosphors be known in advance. A much more expensive

version—a true general colorimeter--would allow the instrument

to measure the chromaticities of the RGB, store this information

in memory, and then use the stored values in the matrix algebra

equations. NBS recommends upgrading the color calibrator to a "true" CRT colorimeter because the output would be the WIL

chromaticities in CIE coordinates which are the internationally

recognized values for color measurement and specification.

Hence, the colorimeter outputs would have an additional value in

court test.

72
5.0 REFERENCES

1. Banks, W. W.; Gertman, D. I.; and Petersen, R. J.: "Human

Engineering Design Considerations for CRT-Generated

Displays," Report No. EGG-SSDC-5610 . Prepared for the U.S.

Nuclear Regulatory Commission under U.S. Department of

Energy Contract No. DE-AC07-761D01570 , October 1981.

2. Cakir, A; Hart, D. J.; and Steward, T. F. M.: "Visual

Display Terminals," John Wiley & Sons, New York, New York,

USA, 1980.

3. Christ, R. E.: "Review and Analysis of Color Coding

Research for Visual Displays," Human Factors, 1975; 17, pp.

542-570.

4. Egeth, H. and Pachela, R.: "Multidimensional Stimulus

Identification," Perception and Psychophysics, 1969, 5, pp.

341-346.

5. Eriksen, C. W.: "Location of Objects in a Visual Display as

a Function of the Number of Dimensions on Which the Objects

Differ," Journal of Experimental Psychology, 1952, 44, pp.

56-60.

6. Letter report of May 8, 1981 from Robert A. Glass to Mr.

James Norton, F.A.A:

7. Grandjean, E. and Vigilioni, E.(Eds.): Ergonomic Aspects of

Visual Display Terminals; Taylor and Francis, Ltd., London,

1980.

8. Gummerman, K.: "Successive Processing of Color and Form

From Brief Visual Displays," Perceptual and Motor Skills,

1975, 40, pp. 31-41.

9. Heldenbrand, J. L. and Porter, L. G.: "Color Calibrator for

RRWDS & Radar Remote Weather Display System," NBSIR 84-2924,

September 1984.

10. Howett, G. L.; Glass, R. A. and Tibbott, R. L.: "Proposed

Chromaticity Specifications for Color Weather Radar," NBS on

February 14, 1980.

11. Kopala, C. J.: "The Use of Color-Coded Symbols in a Highly

Dense Situation Display," in Proceedings of the Human

Factors Society 23rd Annual Meeting. Santa Monica, CA:

Human Factors Society, 1979, pp. 397-401.

12. Luder, C. B. and Barber, P. J.: "Redundant Color Coding on

Airborne CRT displays," Human Factors, 1984, 26, pp. 19-32.

73
13. Luria, S. M. and Strauss, M. S.: "Eye Movements During

Search for Coded and Uncoded Targets," Perception and

Psychophysics, 1975, 17, pp. 303-308.

14. Noble, M. and Sanders, A. F.: "Searching for Traffic

Signals While Engaged in Compensatory Tracking," Human

Factors, 1980, pp. 89-102.

15. Smith, S. L. and Thomas, D. W.: "Color Versus Shape Coding

in Information Displays," Journal of Applied Psychology,

1964, 43, pp. 137-146.

16. Teichner, W. H.? Christ, R. E. and Corso, G. M.: "Color

Research for Visual Displays," (Report No. AD-A043 609/7ST)

,

Las Cruces, NM: New Mexico State University, June 1977.

17. Wagner, D. W.: "Experiments with Color Coding on

Television," (Report No. NWC TP 5952), China Lake, CA: U.S.

Naval Weapons Center, 1977.

18. Weidner, V. R. and Hsia, J. J. "Reflection Properties of

Pressed Polytetraf luoroethylene Powder." Journal of the

Optical Society of America . Vol. 7, #7, July 1981.

19. Williams, L. G.: "The Effect of Target Specification

on Objects Fixated During Visual Search," Perception

and Psychophysics, 1966, 1, 315-318.

74
NBS-114A (rev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

86-3403

2. Performing Organ. Report No. 3. Publication Date

JULY 1986

4. TITLE AND SUBTITLE

Final Evaluation of a Color Calibrator for a Radar Remote Weather Display System

5. AUTHOR(S)

Louis G. Porter

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

FAA

U.S. Department of Transportation

Washington, D.C. 20591

10.

SUPPLEMENTARY NOTES

I | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report is the final report on the development and field testing of an inexpensive

color calibrator for the standardization of the Weather Intensity Level (WIL) colors

used in the FAA' s Radar Remote Weather Display System or RRWDS. The report covers the

field validation of the color calibrator and, as an end product, the construction of a tentative look-up table that identifies whether the six WIL colors are within

acceptable limits. The look-up table is tentative due to the small (5) number of

field sites visited. The development of a color lookup table finalizes the validation

of the color calibrator as a useful tool for calibrating the WIL colors used in RRWDS.

Moreover, the color calibrator can be used to calibrate the colors of other tricolor

display systems, provided an appropriate lookup table is developed.

In addition, this report includes a general review of significant literature on

color-coding, since RRWDS color codes weather information. The report presents firstof-its-kind objective data on the effects of ambient room lighting on colors used in a self-luminous display. The effects were chromaticity shifts due to the presence of

veiling reflections or glare from the face of the CRT console. This report may,

accordingly, have significant impact on the design of building environments where

color-coded CRT's are to be used.

12.

KEYt WORDS (S/'x to twelve entries; alphabetical order; capitalize only proper names: and separate k,ey words by semicolon s) building lighting environments; cathode ray tube (CRT) ; chromaticity snifts; coloi

calibrator; color coding; room illuminant reflections; self-luminous displays;

specular glare from CRT's; veiling luminances; video display terminals (VDT)

13. AVAILABILITY

[XJ Uni imited

| | For Official Distribution. Do Not Release to NTIS

"H Order From Superintendent of Documents. U.S. Government Printing Office, Washington. D.C

20402.

[]£] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

85

15. Price

$9.95

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