NATIONAL BUREAU OF STANDARDS REPORT

4421

Design and Calibration

of a Remote-Indicating, Photoelectric Brightness Meter

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing

of the divisions and sections engaged in technical work. In general, each section is engaged in

specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instru- ments. Magnetic Measurements. Process Technology. Engineering Electronics. Electronic

Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenic Physics.

Engines and Lubrication. Engine Fuels.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid

State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems.

Analogue Systems.

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services.

Radio Propagation Engineering. Frequency Utilization Research. Tropospheric Propagation

Research.

Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0201 -20-2327 November 29, 1955

NBS REPORT

4421

Design and Calibration

of a Remote-Indicating, Photoelectric Brightness Meter

By

C. A„ Douglas and I. Nimeroff

Prepared for

Instrument Division

Weather Bureau

Department of Commerce

Washington 25, D. C„

NBS.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

rfApp

?

ved

t for ?ublic Please by the

d' rector of the National Institute of

2S.D.C. Such pe Stanc|ards and Technology (NIST)

cal ly prepared If On October 9, 2015

' In part, is prohibited

tandards, Washington

eport has been speclfireport for its own use.

REPORT

on the

Design and Calibration

of a Remote-Indicating, Photoelectric

Brightness Meter

1. SCOPE

This report describes the design of a remote-indicating,

photoelectric brightness meter and gives the results of the

calibration and test of the instrument. The instrument was designed for use in making horizon-sky brightness measurements

during the visibility tests being conducted at Newark Airport.

2. APPARATUS

The instrument was designed to use, in so far as possible,

available experimental transmissometer equipment. Figure 1 is a system block diagram of the instrument.

2.1

Optical Design of the Receiver

The optical portion of the instrument is shown in figure 2.

This portion consists of a PJ-14B photocell, an optical filter,

a^fiblcl stop, a lens, and stray-light shield.

2.1.1

Phototube. A type PJ-14B was selected because the spectral

response of this type tube is relatively uniform throughout the

visible portion of the spectrum, has a low dark current, and is

stable.

2.1.2

Filter Design. The purpose of the optical filter is to

provide a phototube-filter combination with a spectral response

which is essentially the CIE standard observer luminosity function,

y.

Thus f%

= 11^ (1)

where T^ is the spectral transmittance of the filter and S^

is

the spectral response of the phototube at wavelength X.

Figure 3 shows the relative spectral response of the phototube and the standard luminosity function.
f

IP

1

p

Am

'

i

frj

i (lr
The spectral transmittance of a desired filter was determined

by solving equation 1 for KT^. Having determined the desired

function of KT^ a suitable combination of glasses was chosen,, The

spectral transmittance of samples of each glass was determined.

From these data the desired thickness of each component was computed. The components were then ground and polished to the

desired thicknesses and the spectral transmittances were measured.

In those instances in which the measured spectral transmittance

did not agree sufficiently well with the desired spectral transmittance. the components were reground to a thickness which gave

the desired spectral response. (It is, of course, necessary to be

certain that the components are never ground thinner than is re- quired.) The component filters are given in table I.

Table 1

Component Filters

Corning Designation Required Thickness

Figure 4 shows the spectral transmittance of the desired and

the designed filter, indicating a reasonably good approximation.

2.2 Electronic Design

The electronic circuitry is basically that of the GMQ-1G

transmissometer as is indicated in figure 1. Hence a detailed ex- planation of the circuits will not be given. Only the significant

differences in the circuitry will be discussed.

Figure 5 is a detailed block diagram and figure 6 an interconnection diagram of the transmissometer system from which the

units were taken. They are directly applicable to the brightness

meter except that the projector shown is not used. Note that in

these and the figures following the numbering of the components is

not the same as that used in the GMQ-10 manual.
}

'

1

1

•l

I

L
-3-

2.2.1 Receiver and Amplifier-Power Supply. Figure 7 is a circuit diagram of the receiver and the amplifier -power supply. The

circuitry of these units differs significantly from that of the

GMQ-10 in the following respects;

V1201. A type PJ-14B phototube is used instead of the type

919 phototube.

Pulse-Rate Meter. The pulse-rate meter has two ranges

designed for 12000 pulses per minute and 4000 pulses per minute

full scale instead of a single 4000 pulse- per-minute range. This

is accomplished by the addition of C209 and R206 and the modification of S205.

2.2.2 Indicator. Figure 8 is a circuit diagram of the indicator.

The circuitry of this unit differs from that of the GMQ-10 indicator in the following respects;

Counter Stage. A counter stage is included in the indicator.

The purpose of this stage is to provide sufficient power to pulse

a relay or operate a counter when the input pulse rate is too

slow to produce a satisfactory indication on the meter and the

recorder so that the time between individual pulses can be determined.

In this instrument the relay K405 is located in the sensitivitycontrol unit. The plate of tube V311 is connected to B+ through

S304 of the indicator and through the contacts of K404 and the

coil of K405 of the sensitivity control, ^hen the B+ circuit is

complete, tube V311 is triggered once by each signal pulse coming

into the receiver and momentarily energizes keying relay K405.

Because of the energy required to operate the relay, the capacitance of €316 is much larger than that of the corresponding capacitor €306 of the pulse-rate integrator stage V3Q3. Hence the

counter stage cannot respond to pulse rates as high as can the

pulse-integrator stage. It will, however, respond satisfactorily

to pulse rates higher than 2 pulses per second. Since counting of

pulses is not required until the pulse rate falls below 0.5 pulse

per second, this response is satisfactory.

Pulse-rate Integrator Stage. The pulse-rate integrator

stage is designed to have sensitivity ranges of 20,000 (LOW),

400 (MEDIUM), and 800 (HIGH) pulses per minute instead of only

4000 and 800 pulses per minute. This requires that the resistor

chain in the cathode circuit of tube V303 be divided into three

sections instead of two. Switch S303 is a four-pole four-position
I

f

I

*1

I

1
switch. The first three positions, LOW, MEDIUM, and HIGH, place

the indicator on the corresponding sensitivity range. In the

fourth position, AUTO, the sensitivity resistors are connected to

J302 so that the sensitivity may be controlled by a control outside

the indicator. Note that the sensitivity switch does not actuate

the recorder marker pens. Thus when the sensitivity is controlled

manually, the sensitivity range should be noted on the chart of

the recorder.

The capacitance of C306 has been temporarily increased from

2200 ppf to 7200 pjxf by the addition of a 5000 ppf capacitor,

C306A, in parallel with C306 so that full-scale meter readings

can be obtained with pulse rates of the order of 6000, 1200, and

240 pulses per minute.

2.2.3 Sensitivity Control. A sensitivity-control unit which automatically places the indicator on the optimum sensitivity range

is included in the system. In addition, the sensitivity-control

unit contains a keying relay which, when operated from the

counter stage of the indicator, will record individual pulses.

Figure 9 is a circuit diagram of the sensitivity control.

The sensitivity control is actuated by galvanometer relay

K401 whose coil is in series with the indicator meter, M301, and

the recorder. This relay is adjusted so that it closes to L when

the indicator output falls to 0.4 ma (a reading of 8 on M301) and

to H when the indicator output rises to 4.8 ma (a reading of 96

on M301). As the contacts of this relay are rated at only 200 ma

at 6 volts, auxiliary relays K402 and K406 are used to operate

the selector relays K403 and K407. Relays K402 and K4Q6 also

provide a time delay of about 45 seconds so that momentary changes

in the indicator output will not operate the sensitivity-selector

relays.

With the contacts of selector relays K403 and K407 as shown,

the indicator is on the Medium sensitivity range. When the indicator current increases to 4.8 ma, the galvanometer relay, K401,

closes the contact at H thereby energizing the H heater of timedelay relay K402. After about 45 seconds the contact at H2 is

closed, energizing the release coil of the Low-Medium selector

relay, K403 (a latching relay), putting its contacts in *he position opposite to that shown. This (1) opens the Medium range

marker-pen contact (see figure 5 as well as figure 9), thus deenergizing the solenoid of the left-hand marker pen in the recorder,

(2) closes the sensitivity contacts shorting resistors R314 and
'
-5-

R315 in the indicator, thus putting the indicator on the Low sen- sitivity range, (3) opens the limit contact so that further

contact at contact H of the galvanometer relay will have no effect,

and (4) places the transfer contacts in the position so that if

contact is made at the L contact of the galvanometer relay it will

energize the L section of relay K402.

Assume the sensitivity control is in the Low position as described above. Then, when the indicator current decreases to

0.4 ma, the galvanometer relay closes the contact at L energizing

the L heater of time-delay relay K402. After about 45 seconds the

contact at is closed, operating the latch coil of selector relay

K402 returning this relay to the position shown and the indicator

to the Medium range.

Again assume that relays K403 and K407 are in the position

shown. When the indicator current decreases to 0.4 ma, the galvanometer relay closes the contact at L thereby energizing the L

heater of the time-delay relay K406. After about 45 seconds the

contact at Lg is closed, energizing the latch coil of the MediumHigh selector relay K407, putting its contacts in the position

opposite to that shown. This (1) closes the High range marker-pea*

contact, thus energizing the solenoid of the right-hand marker

pen in the recorder, (2) opens the sensitivity contact that shorts

R313 in the indicator, thus putting the indicator on the High

sensitivity range, (3) places the limit contact in the position so that further contact of the galvanometer relay at L will energize

keyer delay relay K404, and (4) places the transfer contacts in

the position so that contact of the galvanometer relay at H will

energize the H section of relay K406.

Assume the sensitivity control is in the High position as described above. Then when the indicator current increases to

4.8 ma, the galvanometer relay closes the contact at H energizing

the H heater of time-delay relay K406. After about 45 seconds

the contact at % is closed, operating the release coil of relay

K407 returning this relay to the position shown in figure 9 and

the indicator to the Medium-S’ange position.

If, when the sensitivity control is in the High position, the

indicator current decreases to 0.4 ma, the heater of time-delay

relay K404 is energized. This relay upon closing completes the

plate circuit of V311 in the counter stage of the indicator.

Then the keying relay is momentarily energized by each pulse on the signal line. This momentarily de-energizes the solenoid of

the right-hand marker pen of the recorder producing a pip on the

record chart.

-6-

2.2.4 Recorder. The recorder is a 5-milliampere EsterlineAngus recorder with a synchronous motor-driven chart drive and

spring-driven rewind drive. It has both a left-hand and a righthand marker pen. These pens are energized through the automatic

sensitivity control and indicate the sensitivity range on which

the indicator is operating as follows t

Sensitivity LH Pen RH Pen

Low In In

Medium Out In

High Out Out

In order to obtain time pips on the record chart by means

of the marker pens, a DPDT relay with a 115-volt, 6Q~eycle coil

has been mounted on the back of the recorder. The contacts of

the relay are so connected that when the relay is energized by a timing pulse the circuit through the solenoid of the left-hand

marker pen is opened, and the solenoid of the right-hand marker

pen is energized. Thus, the position of one or both marker pens

will be changed regardless of the position in which the sensitivity

control has placed them.

3. CALIBRATION

Prior to the calibration of the instrument as a unit, the

ratio of the Low to Medium and Medium to High sensitivities was made to be 5.00 in the following manner. With C306A disconnected,

a pulse signal of line frequency (60 pulses per second) was applied

to the input of the indicator and the voltage applied to V303

adjusted by means of R321 so that a meter reading of 0.90q was

obtained. Then pulse signals of exactly one-fifth and five times

line frequency (12 and 300 pulses per second) were applied with

the indicator on the High and Low ranges respectively and adjustments of R315 and R313 were made so that meter readings of 0.90q

were obtained on these ranges also. The pulse signals were

obtained by capaci tati vely coupling one of the horizontal deflection

plates of an oscilloscope to the indicator input and using the

patterns generated by a line-frequency test signal applied to the

Y-input of the oscilloscope to determine the frequency.

Capacitor C306 was then connected. The value of this capacitor was chosen so that the indicator would produce a full-scale

meter reading with pulse rates in the region of 20 pulses per

second.
I

I

:

i

1

>
The instrument,, using the receiver identified as No. 2, was

then connected as shown in figure 1 with the receiver placed

before a luminance standard of sufficient size to fill the field

of the receiver. The luminance standard was adjusted to have a brightness of 1000 footlamberts. The capacitance of C1201 (in

the receiver) was chosen so that a pulse rate of about 20 pulses

per second was obtained. (This required the use of a 10 pp£

capacitor,, which is about the minimum capacitance with which

satisfactory operation can be obtained.) This pulse signal was

fed into the indicator. The calibration of the indicator was adjusted so that this signal produced a meter reading of I.OQq

with the indicator on the Medium sensitivity, thus calibrating

the brightness meter for the full-scale reading! given in table II.

Table II

Sensitivity

Range

Brightness for Full-Scale

Jmdiiig

footlamberts

Low 5000

Medium 1000

High 200

The calibrator signal, 60 pulses per second, was then applied

to the indicator. A reading of 0.59 was obtained with the indicator on the Low sensitivity. The indicator calibration point is

therefore a reading of 0.59 on the Low range, not 0.90 on the

Medium Range. By keeping the indicator adjusted to this point by

using the CALIBRATION ADJUSTMENT, the effects of possible drifts

in the indicator are eliminated from the calibration of the brightness meter. The calibration of the instrument is, therefore,

dependent only upon the stability of the phototube V12Q1, the

trigger tube ¥1202, the charging capacitor C1201, and the colorcorrecting filter. The calibration data is summarized in

Appendix 1.

4. PERFORMANCE CHECKS

4.1 Linearity

The linearity of the brightness meter was checked by adjusting

the standard to 200 and to 5000 footlamberts and operating the

indicator on High and Low ranges respectively. These checks

showed the instrument to be linear within the accuracy of the

photometry, the deviations being within 1%.
.

.
-8 -

The spectral response of the receiver was given a rather

rigorous test by measuring the luminous transmittance of highly

selective filters. The spectral transmittance of these filters

is shown in figure 10. The results of this test are given in

table III.

Table III

Filter Transmittance, percent

NBS Desig- Color Measured Computed from Difference

nation w/Brightness SpectrophotoMeter metric Data

3151 Red 16 13 +3

3857 Orange 49 51 -2

5462 Yellow 74 76 -2

5787 Green-Yellow 86 89 -3

6380 Green 39 41 -2

6702 Blue -Green 24 24 0

5. DISCUSSION

The brightness meter was found to perform satisfactorily.

The range of selectivity of the filters used in the spectral check

far exceeds the range of colors for which the brightness meter

was designed.

•0

-9

Appendix 1

Calibration Data of Photoelectric Sky-Brightness Receiver No. 2

Component

Phototube (Type PJ-14B)

Trigger Tube (Type WL-759)

Filter

Charging Capacitance (C1201)

Identifying _MarkLS__

2128

120

3A

10 ppf

Calibration; 0.0204 pulses per second per footlambert

Therefore, adjust indicator calibration by means of the CALIBRATION

ADJUSTMENT so that a pulse rate of 60 pulses per second produces a reading of 59 when the indicator is on Low range.

Then full-scale readings will be

Rangje Footlamberts

Low 5000

Medium 1000

High 200

LU

CO

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CLO

SYSTEM

BLOCK

DIAGRAM

OF

SKY

BRIGHTNESS

METER
Vi


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Figure



MODIFIED FOR USE AS A SKY BRIGHTNESS METER

FIGURE 5

NBS TRANSMISSOMETER INTERCONNECTION DIAGRAM

i FIGURE 6

NOTES © OMITTED ON CXFJ-I-L , L-8

(g) OMITTED ON P - 1, P -2, P -3, CXFJ -l-P

© USED ONLY ON P-6. P-7, CXFJ-- 1-

(2) CONNECTIONS ON- CXFo AS FOLLOWS:

CORRESPONDING TERMINALS

© OMITTED ON CXFJ

© OMITTED ON I- 1, 1-2.1 3

© OMITTED ON S-2, S~3. S-8

(g) PROJECTOR NOT USED ON BRIGHTNESS METER

FIGURE 6

NOTES.

CAPACITOR VALUES = MICROMICROFARADS EXCEPT AS NOTED.

RESISTOR VALUES = OHMS K= 1,000 MEG

CPS - CYCLES PER SECOND

SUPPLY VOLTAGE = 95 -125 VAC, 60 CPS

® SCREWDRIVER ADJUSTMENT

NOTE A: CI20I AND C220I ARE NOMINAL VALUES. EXACT VALUES TO BE DETI

BY THE SENSITIVITY OF VI20I AND VI202 OR OF .V220I AND V2202

NOTE B: VALUES OF CI202 AND RI202 SHOWN ARE FOR A MAXIMUM PULSE RA1

20,000 PULSES PER MINUTE.

c .

FIGURE

7
FIGURE 7 NBS


NDTFt;

NOTES'

K403 (LOW -MEDIUM) SHOW

K407(MEDIUM-HIGH) SHOW

K40I CLOSES TO L AT 0.*

K402 , K404, K406 (TIME I 30 TO 60 SECONDS C

SUPPLY VOLTAGE = 95-125

NOTE A. RECORDER CONE

1 1 5 v; 60 CF P403

FIGURE 9
3-RANGE SENSITIVITY CONTROLS S-6,7

FIGURE 9
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NATIONAL BUREAU OF STANDARDS REPORT

10+08

CHRQMATICITIES EXHIBITING MAXIMAL CONTRAST

By

Deane B. Judd

To

U. S* Department of iiie Air Force

Aerial Reconnaissance Laboratory

Wright Air Development Center

Wright^Patterson Air Force Base, Ohio

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing

of the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instru- ments. Magnetic Measurements. Process Technology. Engineering Electronics. Electronic

Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenic Physics.

Engines and Lubrication. Engine Fuels.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid

State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems.

Analogue Systems.

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services.

Radio Propagation Engineering. Frequency Utilization Research. Tropospheric Propagation

Research.

Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

CHROMATICITIES EXHIBITING MAXIMAL CONTRAST

By

Deane B . Judd

Photometry and Colorimetry Section

Optics and Metrology Division

U0 S. Department of the Air Force

Aerial Reconnaissance Laboratory

Wright Air Development Center

Wright-Patterson Air Force Base* Ohio

Contract No* AF 33(6i6) £2-21

Task Noo 6710k

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprinting, or reproduction of this report In any form, either in whole or In part, is prohibited

unless permission Is obtained in wrltingfrom theOffice of theDlrector, National Bureau of Standards, Washington

25, D.C. Such permission is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to reproduce additional copies of that particular report for its own use,

NBS PROJECT

0201-20-232£ November 19£5

NBS REPORT

U4O8

To
[1

!1

[IQ
Preface

This is one of a series of reports prepared under

National Bureau of Standards Project No. 0201“20-2325<>

entitled 11 Color Reconnaissance Studies financed by the

Aerial Reconnaissance Laboratory,, Wright Air Development

Center 5 Wright-Patterson Air Force Base5 Ohioj Air Force

Contract No. AF 33(616) 52-21,, it is coordinated with

Air Force Contract 33(6l6)-262 under Dr. Hugh T. O'Neill.,

O'Neill Associates^, Annapolis,, Maryland. The present

report resulted from one of Dr. O'Neill's work requests.

H. J. Keegan

Project Leader

G HROMATICITIES EXHIBITING

MAXIMAL CONTRAST

By

Deane B. Judd

I. REQUEST

;

1. Which two hues will give the greatest contrast

visible to the eye?

2. Which will give the least contrast?

3. Is there any table,, chart, or graph,, published or unpublished* which would give this information in terms of the CIE

chromaticity diagram?

ho Could we have a copy of such a diagram showing areas

of color contrast?

II. Introduction

Color contrast may be defined as the perceptual size of the

difference between two colorso As shown in NBS Report 3773* Determina

tion of Color of Maximum Contrast, [l

j

*

color contrast may be broken

down into a lightness component and two chromatic components, and the

relative importance of the lightness component, depends upon the observing conditions, particularly upon the angular subtense of the

elements to which the colors are perceived to belong* For fields of

small angular subtense (0*01°) the lightness component determines the

color contrast, and, indeed, no chromatic component is perceptible.

For fields of large angular subtense (10°), the chromatic component

of the color contrast assumes an importance at least as great as that of the lightness component. The question, ,rWhich two hues will

give the greatest contrast visible to the eye?*1 , is interpreted as having reference only to the chromatic components of contrast. It

might be rephrased? "What two colors of identical luminance axe

perceived as maximally different?*1 , or "What two chromatieities are perceived as maximally different?"

As in NBS Report 3773, the basic information is to be found in

formulas already derived and verified within about a factor of 2 for

various specific observing conditions j see formulas 28, 29, 30, 31*

32, 33* 3h;> and 35* given by Judd (1952) [2[J. The determination of

the two maximally different chromaticities is especially convenient

^figures in brackets indicate the index references at the end of

this report.
:• - •/' .
from formula 30, which expresses the differenced E, between two colors

of equal luminance ass

s kgA S (l)

where dS is proportional to the distance between the two chromaticity

points plotted on a uniform-tshrcm^tieity-soal© diagram,, such as that

by Judd (1935) Oils MacAdam (1937) Uf?5 or Breckenridge and Sehaub

(1939) f5J.

III. Maximally different chromatic ities for 2® fields

The method of finding maximally different chromatic! ties suggested

by equation 1 is by inspection of the uniform-chromaticity-scale (UCS)

diagram valid for the particular field size in quest! on . The Judd”UCS

diagram (see Figc lks reference 2) applies to field sizes of about 2

a

. The area inclosed by the spectrum locus and the straight line joining

its extremes is the locus of points representing ehromaticities producible by all additive mixtures of spectrum lights, that is, all

possible chroma tic i ties o From the shape of this locus it is evident

that one of the maximally different ehromaticities must be that of

the long-wave (red) extreme of the spectrum. The second ehromaticiiy

to be paired with this is not sharply definedo What is evident from

inspection of this locus is that .any spectrum ehromaticiiy from about

510 (bluish green) to U65 n|x (blue) is separated from extreme spectrum

red by about the maximum amount. The Breckenridge-Schaub-RUCS diagram

(Figure 1 of this report, or Fig. 775 reference 2) gives the same indication, as does, indeed, the MacAdam-UCS diagram (reference U).

IV. Maximally different ehromaticities for 0.1° fields

If one of the two ehromaticities whose contrast must be maximized

subtends an angle of the order of 0.1® the spacing indicated for 2®

fields does not apply. The perceptibility of chromaticity differences

from the gray point in the purplish blue to greenish yellow direction

becomes vanishingly small. This phenomenon is known as small-field

tritanopia ( Judd* 19h9 DO* Wright, 19h9 , Middle ton and Holmes,*

19U9 C8J). Under these conditions all ehromaticities represented by

ary straight line passing through the tritanopie eo-punctal point

(x = 0.165, y - 0.000 or thereabouts), x^hich is near the short-wave

(violet) extreme of the spectrum locus, become indistinguishable

from each other. A measure of the chromaticity contrast between

two colors viewed with an angular subtense of about 0.1® is the distance between the projections of the corresponding two chromaticity

points from the tritanopie co-punctal point to the straight line

representing the long-wave branch of the spectrum locus. By this

measure the maximum chromaticity contrast would be found between
'

.

e

.
-3-

spectrum red and spectrum blue at about h65 mp,, but nearly as great

contrast is indicated between extreme spectrum red and any spectrum

chromaticity of wavelength between k%0 and 510 mp,. The indication

derived from the 2° condition thus holds fairly well for smaller

field sizes as well.

V. Answers to the specific questions

1. Maximally contrasting pairs of chroma tic i ties are produced

by extreme spectrum red paired with the spectrum bluish green to

spectrum blue.

2. Minimally contrasting chromaticities are those yielding a perfect match, or zero contrast. These pairs may be produced by

colors of any hue.

3. The UCS diagrams show approximately the perceptibility of

the difference between any two chromaticities by the distance between

the points representing them when they are presented in fields of

about 2° subtense.

1|. Fig. 1 is the RUGS diagram developed by Breekenridge and

Schaub [si It is a projection of the CIE chromaticity diagram such

that for viewing in fields of about 2° subtense the distance between

the two points is approximately proportional to the contrast between

the two chromaticities represented. From this diagram there may be

read off immediately a measure (linear distance on diagram) of the

chromaticity contrast between any two spectrum colors. Since it is

also a mixture diagram, the chromaticities of any mixture of two

colors fall on the straight line connecting the points representing

them. This diagram thus yields indirectly a measure of the contrast

between any two colors viewed in fields of about 2® subtense. It

may also be extended to yield estimates of chromaticity contrast

between colors viewed in fields of about 0.1® subtense by distance

between projections of the two chromaticity points from the tritanopia

co-punctal point (x51 ~ -0.391, y

,r - O.Olli) onto the straight line

x" = 0.075, as explained above.

VLc References

-lfrD. Bo Judd, Determination of color of maximum contrast,

NBS Report No. 3773 to WADC, November 12, 195U.

D. Bo Judd, Color in Business, Science, and Industry,

pp 0 256-265, New York, John Wiley & Sons, 1952

o

[

3]

OQ

Do Bo Judd, A Maxwell triangle yielding uniform chromaticity

scales, J. Research Natl. Bur. Standards, lh, ill (1935)

RP7 56.

”

Do L. MacAdam, Projective transformations of I.C.I. color

specifications, J. Opt. Soc. Amer. 2?, 29k (1937).

F. Co Breckenridge and W. R. Schaub, Rectangular uniformchroma ticity-scale coordinates, J. Optical Soc. Am. 29,

370 (1939).

Do Bo Judd, Response functions for types of vision according

to the MttUer theory, J. Research NBS, Ii2, 1 (19h9) RP19U6.

W. Do Wright, Les courbes de melange des couleurs enregistrees

sur la fov6a sous de tres petits champs visuels Rev. optdque,

28, 17U (19U9).

Wo E. K. Middleton and M. C. Holmes, The apparent colors of

surfaces of small subtense - A preliminary report, J. Optical

Soc. Am, 39, 582 (I9ii9).
1

I

l

c 1
Figure 1.

The spectrum locus and Planckian (color

temperature) locus plotted in rectangularTM

uniform-chromaticiiy-scale (RUGS) coordinates

by F. Co Breckenridge and W. R. Schaub ££][•

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