iNatonai Bureau oi Manoaras

tibrary, E-01 Admin. Bld£. APR 1 4 1967

NBS MONOGRAPH 75

Colors of Signal Lights

Their Selection, Definition,

lyieasurement. Production, and Use

U.S. DEPARTIVIENT OF COIVIiyiERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring maximum apphcation of the physical and engineering sciences to the advancement of

technology in industry and commerce. Its responsibihties include development and maintenance

of the national standards of measurement, and the provisions of means for making measurements

consistent with those standards; determination of physical constants and properties of materials;

development of methods for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the estabhshment of

standard practices for incorporation in codes and specifications; advisory service to government

agencies on scientific and technical problems; invention and development of devices to serve

special needs of the Government; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simphfied trade practice recommendations;

administration of programs in cooperation with United States business groups and standards

organizations for the development of international standards of practice; and maintenance of a clearinghouse for the collection and dissemination of scientific, technical, and engineering infor- mation. The scope of the Bureau's activities is suggested in the following fisting of its three In- stitutes and their organizational units. Institute for Basic Standards. Apphed Mathematics. Electricity. Metrology. Mechanics.

Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics.

Radio Standards Laboratory:* Radio Standards Physics; Radio Standards Engineering. Office of

Standard Reference Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic

Materials. Reactor Radiations. Cryogenics.* Materials Evaluation Laboratory. Office of

Standard Reference Materials.

Institute for Applied Technology. Building Research. Information Technology. Perform- ance Test Development. Electronic Instrumentation. Textile and Apparel Technology Center. Technical Analysis. Office of Weights and Measures. Office of Engineering Standards. Office

of Invention and Innovation. Office of Technical Resources. Clearinghouse for Federal Scientific and Technical Information.**

*Located at Boulder, Colorado, 80302. **Located at 5285 Port Royal Road, Springfield, Virginia, 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Alexander B. Trowbridge, Acting Secretary

NATIONAL BUREAU OF STANDARDS A. V. Astin, Director

Colors of Signal Lights

Their Selection, Definition, Measurement, Production,

and Use

F. C. Breckenridge

Institute for Basic Standards

National Bureau of Standards Washington, D.C. 20234

National Bureau of Standards Monograph 75

Issued April 3, 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents
Contents

Page Foreword Hi Preface iv

1. Introduction 1 2. Outline of problem 1 2.1. The three essential steps 1 2.2. Basic chromaticity definitions 2

2.3. Master specifications for color standards 2

2.4. Procurement specifications 3

2.5. Regulatory requirements 3

3. Mathematical representation of colors 3

3.1. Three attributes of color 3

3.2. CLE. coordinate system 6

3.3. RUCS coordinate system 7

3.4. Chromaticity diagrams 7

3.5. The z coordinate 7

3.6. Relative distortion of diagrams 7

3.7. Properties of RUCS diagram 12

4. Recognition of signal colors 12

4.1. General principles 12

4.2. Results of research 12

4.3. Results of experience 16

5. Production of signal colors 18

5.1. Source and filter 18

5.2. Effect of colorants 18

5.3. Effect of filter thickness 19

5.4. Effect of color temperature of source 21

5.5. Effect of filter temperature 25

5.6. Relation of transmittance to chromaticity 26

5.7. Effect of filter temperature on transmittance 29

5.8. Plastic fihers 29

5.9. Gaseous discharge and fluorescent lamps 29

6. Control of signal colors 32

6.1. Selection of standards 32

6.2. Types of color standards 32

6.3. Inspection testing 32

6.4. Laws and regulations 32

7. Chromaticity boundaries 33 7.L Selection of boundaries for chromaticity

definitions 33

7.2. Constant hue boundaries 33

7.3. Constant saturation boundaries 34

Page

7.4. Comparison of rational and adopted boundaries 34

7.5. Boundaries for blue and green 34

7.6. Boundaries for red and yellow 35

7.7 Boundaries for white 35

7.8. Mathematical simplicity 36

7.9. Relation of chromaticity boundaries to primary

limit standards 36

7.10. Boundaries for duplicates 36

8. Requirement for chromaticity characteristics similar to standard 37

8.1. Purpose of requirement 37

8.2. Statement of requirement 38

8.3. Effect of requirement 38

8.4. Application to plasticware 39

9. Instrumentation 39

9.1. Chromaticity measurements 39

9.2. Transmittance measurements 40

9.3. Photoelectric measurements 41

9.4. SampUng 42

10. Use of signal hght colors 42

10.1. General principles 42

10.2. Number of colors 42

10.3. The primary signal colors 42

10.4. Systems with yellow and white signals..., 43

10.5. The secondary signal colors 45

10.6. Signals for colorblind observers 45

10.7. Backgrounds 46

10.8. Naming of signal colors 48 Appendix 1. References 48 Appendix 2. Bibliography of general works 49 Appendix 3. List of specifications and standards 50 Appendix 4. Comparative table of boundary equations,

CLE 50 Appendix 5. Comparative table of boundary equations,

RUCS 52 Appendix 6. Comparative diagrams of signal color boundaries 54

Library of Congress Catalog Card No. 64-60008
Foreword

The safety of the traveler, whether by rail, highway, air, or sea, depends on

the reliabihty with which signal Ughts of various colors are seen and their meaning

correctly interpreted. In August 1964, the United States Standard for the Colors

of Signal Lights, developed under the auspices of the U.S. National Committee

on Color for Signal Lights, was issued as National Bureau of Standards Handbook

95. The colors selected, the method of their specification, and the methods of testing signal ware for compHance with the specifications were all based upon

technical information, much of it unpublished.

It is the purpose of this Monograph to make available a complete analysis

of the problem of signaling by lights of various colors, combined with a detailed

reproduction of the basic data used to derive the solution of this problem given

in Handbook 95. An effort has been made to present the material in such a manner as to facihtate comparison between the technically established information

and the requirements.

This Monograph is designed as a guide to the engineer who must implement

the United States Standard, the psychophysicist who is concerned with the meaningful presentation of research in this field, and the administrator who is responsible for the use of signal lights. Those who seek, in years to come, to improve

on the current signaling systems, should find their task simplified by the availability of this Monograph.

A. V. ASTIN, Director.

iii
Preface

The work of many investigators, authors, and colleagues has provided the

material from which this Monograph has been prepared so that it is not possible

to call each by name, but the assistance derived from many discussions with Deane

B. Judd have contributed so much to the analysis of the problem that the writer

desires especially to acknowledge his indebtedness to him and to express his appreciation for his collaboration. Other highly valuable contributions to the

work have come out of correspondence with J. G. Holmes of Great Britain and

from Lorenzo Plaza, of the Institute of Optics, Madrid, Spain, who, before computer codes were available, very kindly carried out several of the more laborious

tasks of transforming diagrams, which had been available only in the CLE.

system, into the RUGS system. Other contributions have been acknowledged

in connection with the figures and tables. Finally, I would like to thank Mrs.

R. L. Meyers for assembling the references, Mr. R. E. Lee, Jr., for assistance

with many of the computations and drawings, and especially Mrs. Ellen B. Way

for her devotion to the task of completing the myriad of details that are essential

to the final correction and completion of any manuscript.

F. G. Breckenridge.

iv
Colors of Signal Lights:

Their Selection, Definition, Measurement, Production, and Use

F. C. Breekenridge

This Monograph is intended to serve as a reference work for all those concerned with the selection, specifications, and use of signal-light colors. It discusses the nature of the problem, and the mathematical representation, recognition, production, control, and use of such colors. The characteristics of different types of chromaticity boundaries and the purpose and effect of the requirement for similarity of chromaticity characteristics are given special consideration. The treatment is varied according

to the intended use. The discussion of the control of colors and the section on the use of colors are nontechnical, whereas the section on the production of signal colors is designed for the colorimetrist who is faced with the problem of selecting limit filters or drafting a specification.

1. Introduction

All forms of organized transportation depend to a very considerable extent upon signal lights of dif- ferent colors for safe operation. Whether one

travels by aircraft or railroad, automobile or ocean

liner, his safety requires the correct interpretation

of signal colors. Radio, radar, and other electronic

aids have supplemented the signal Ughts in some

forms of transportation, but in no form of transportation have the hghts been abandoned. Other

methods of identifying hghts exist, such as flashing

them and arranging them in distinctive shapes and

configuration. These are also used by the transportation services, but they do not eliminate the need

for color identification. On the other hand, the

increasing density of traffic is constantly increasing

the dependence upon signal Ughts. This situation seems to warrant additional attention to the specification and control of their colors. This Monograph, which is an outgrowth of an

informal pubUcation,' has been prepared to furnish

those interested in specifications for the colors of

fight signals with the technical information nec- essary for preparing and appraising such specifications. It is intended to provide a basis for interpreting the recommendations of the International Commission on Illumination (Commission

Internationale de I'Eclairage, or CLE.) which were

set forth in the Proceedings of the Thirteenth Session, held in Ziirich in 1955 [CIE 1955] ^ and later pubhshed as CLE. Publication No. 2 [1959], Colours

of Light Signals. It also explains the background

of the U.S. Standard [NBS 1964]. Most of the

CLE. recommendations have been incorporated

into this Monograph at appropriate places.

2. Outline of Problem

2.1. The Three Essential Steps

As one examines the specifications and recommendations for the colors of fight signals, he finds a confusing assortment of requirements. Some

of the specifications are lengthy with a diversity

of provisions; some are relatively brief. Some de- pend entirely on numerical values, while others

make reference to standard filters. Some of the

provisions are directly related to what the observer

sees. Other provisions refer only to items of equip- ment. All of these recommendations and specifications aim at similar final results, namely, the

insurance that those who use the signal hghts wiU

be shown the most refiably distinctive colors prac

ticable for their particular requirements.

To understand how the various requirements and

provisions contribute to this end, it is necessary

to recognize that the effective control of the colors of signal fights always involves three steps: the

selection of the colors to be used, the selection of

color standards,^ and the appfication of those

standards to control the colors of the signals. AH

three steps may not be immediately evident. The

selection of the colors and the standards may, for example, be done at the same time. In other cases

the controUing agency may define the acceptable

' NBS Letter Circular 484, no longer available.

^ Brackets enclosing a date indicate a reference listed in Appendix 1.

' "standard(s)" with lower case "s" in this monograph refers to a filter in combmation

with a stipulated light source. When a pubhcation is meant it will be called a Stand- ard" with capital "S".

1
colors and leave the selection of the standards

and their application to the manufacturers who

produce the equipment. In general the three steps

are accomplished by basic chromaticity definitions,

master specifications for color standards, and pro- curement specifications, although these may appear

under other titles or may be combined into a single

specification.

If instruments suitable for measuring the signal

colors directly in colorimetric terms were available,

it might be possible to eliminate the need for color

standards selected for specific applications, but

such instruments are not as yet available. The

possible use of such instruments is discussed in sec. 9, Instrumentation.

2.2. Basic Chromaticity Definitions

Basic chromaticity definitions are statements of

chromaticity limits within which a signal is acceptable for use in a system of signal colors. Such a system may be established by a government agency, an association of state or local authorities, or by a

national or international technical body. For

example, two very similar international systems

have been adopted for signal-Hght colors, the recom- mendations of the International Commission on

Illumination [1955, 1959] and the Standard of the

International Civil Aviation Organization [1964].

Most specifications for signal-light colors in use

in the United States contain statements of basic

chromaticity requirements. The most complete

systems are those contained in part I of the "U.S.

Standard for the Colors of Signal Lights," the two

government specifications for aeronautical lights,

and the two specifications of the Association of

American Railroads, Signal Section. These are

all listed in Appendix 3 to this Monograph.

Other specifications used in the United States have paragraphs devoted to basic chromaticity

requirements. Some of these, Hke the two rail- road specifications mentioned above, are valid in Canada as well. These also are listed in Appendix

3. For basic definitions, mathematically expressed

chromaticity hmits apphcable to the emitted signal are used. Such hmits are reproducible at any time

and place if competent personnel and suitable

instruments "are available for their estabhshment.

This makes them especially adaptable for inter- national agreements and agreements between dif- ferent agencies within a single country, since such

agreements are expected to remain in force over

long periods and may be used by many different

laboratories. Basic definitions are also suitable

for governmental regulations because such defini- tions apply to hght signals as shown and allow the

maximum freedom of means of accomplishment.

In principle the basic chromaticity definitions are the foundations upon which detailed specifications are built. For this reason it is desirable that these

definitions be based entirely upon established principles of color recognition. The necessary re- searches, however, have turned out to be so labor- ious and expensive that thus far it has been nec- essary to base these definitions upon experience

interpreted in the light of the research results. These will be discussed in section 4, Recognition

of Signal Colors.

2.3. Master Specifications for Color

Standards

To bridge the gap between the basic definitions i| and the purchase specifications, the U.S. Standard

j for the Colors of Signal Lights includes master

specifications to control physical standards so that equipment purchased on the basis of such standards

will provide signals in accordance with the basic

definitions. These master specifications for stand- ' ards for the colors of light signals are specifica-

^| tions giving chromaticity tolerances and other

requirements for filters to be used in combination

with specified light sources to constitute standards

for the control of equipment, or the hght-filtering'

components of equipment, to insure that such equipment will give a satisfactory color when placed in

service.'' If there are basic chromaticity definitions

which are applicable to a type of signal hghts, then

the requirements for standards must be such as to insure that the colors of those lights will conform j, to those definitions. If there are no appropriate:

j basic definitions, standards are chosen on the basis

| of experience with other types of signal lights.

| Historically, the relationship of the standards i 1 and the basic chromaticity definitions has generally ] been the reverse of the ideal one described in the , ( last section. In most cases the standards were i chosen first by those responsible for the colors of | | the lights and the definitions were later devised to i describe what was giving satisfactory service. i i The master specifications are prepared for the , j use of laboratories, which may be presumed to have ' ] suitable spectrophotometric and colorimetric i

i equipment as well as light sources calibrated for]

p color temperature. They are applicable to flat) f filters, the spectral transmittances of which are t tj readily measured. Such specifications, therefore, 3 f have generally been stated as mathematically ex-j

p pressed chromaticity limits for the filter-source e j combinations which are to serve as physical stand-;; 51 ards; but spectrophotometry of adequate accuracy | jf is difficult and costly, whereas the required accuracy can be obtained with the expenditure of much less time by differential colorimetry. This makes it

' Anotlier approach readily occurs to the spectrophotometrist. Two spectrophoto- metric curves may be drawn with the requirement that the filters used in lights have

spectrophotometric transmissions that remain between these limits for all wavelengths "1' between specified limits. For an example of this method see Otyi'oky [1937]. This J Jji method presents a difficulty arising from the nature of the filters used in practical equipment. Very few signal light filters are in such a form that they can be spec-

trophotometered on American spectrophotometers. To sort acceptable from non- ~

acceptable signal glasses by this method on a practical production scale would require , an elaborate automatic spectrophotometer with an electronic evaluator which does |, not exist in this country. Furthermore, this approach unnecessarily restricts the con- ( 7* tractor since a variety of spectrophotometric curves can produce the same chromaticity I !!' and all may be acceptable.

2
simpler in some instances to select primary ^ standards, and control duplicates by setting limits for the maximum colorimetric differences. No

examples of specifications for limit standards issued

apart from basic chromaticity definitions or pro- curement specifications are known. Part II of the U.S. Standard for Colors of Signal Lights is designed to be a separate master specification for physical standards to control the colors of light

signals, but at present it is available only in the

complete Standard, NBS Handbook 95 [1964].

The most complete provisions for such standards

to be found in specifications previously in use are those in Federal Standard No. 3 [GSA 1951],*^ and

those in the appendix of each of the two specifications for signal light colors issued by the Signal

Section of the Association of American Railroads

[1960. 1962]. These specifications contain chromaticity tolerances and transmittance requirements

for filters to be used as limit standards, but they

leave to the manufacturer the responsibihty of

seeing that the filters are satisfactory in other

respects.

2.4. Procurement Specification

As the name implies, procurement specificatioiis are specifications governing the purchase of equip- ment. Since they are to be used for distinguishing

between satisfactory items, which are to be paid

for, and unsatisfactory items, which constitute a

loss to one of the parties to the contract, they should

be as definite as it is practicable to make them.

Most of those in use in the United States are based

on standards. If such standards are not stipulated

in the specification, whoever is responsible for compliance should provide himself with standards

unless the item and available instruments are such

that it is practicable to measure the chrornaticity

of each item to make certain that it is satisfactory.

This could mean measuring the spectral transmit- tance of each item with a spectrophotometer, computing the corresponding chromaticity coordinates

from the spectral transmittances, and checking

these mathematically for compliance with the

equations of basic chromaticity definitions. A

possible compromise might be the measurement of a few representative items with a spectrophotometer

and the acceptance or rejection of the remaining

items by comparison with the measured items.

In this case the measured items become standards

for the purpose of checking the unmeasured ones.

If suitable colorimeters were available, it might

be feasible to use mathematical hmits for procurement purposes. Such a procedure, however, would

involve the necessity for maintaining accurate

cahbrations, and the use of more elaborate and

expensive equipment than is required when standard filters are used. Moreover, it is not clear that the use of mathematical Hmits would provide any

economies or other advantages to offset the diffi- culties that would be involved in their use for practical inspection procedures.

Examples of procurement specifications are well

known but it should be pointed out that only a part

of such specification as the Mihtary Specification

for Colors of Aeronautical Lights and Lighting

Equipment [MIL SPEC 1963] or one of the railroad

specifications mentioned in sec. 2.3 [AAR-1960,

1962] is to be considered as a procurement specification in the sense in which the term is being used

in this pubhcation. Generally the suppher is responsible only for the production of signal-hght

equipment and not for the manner in which it is used. The manner of use, however, may affect the color of the signal, as when a wrong lamp is installed in signahng equipment or a wrong voltage

is applied to a signal lamp. Procurement specifications, therefore, should apply to the equipment,

and not the light signal, unless the procurement

contract includes the maintenance and operation

of the equipment.

2.5. Regulatory Requirements

The apphcation of legal requirements such as

safety regulations generally involves all of the three

steps of controUing signal-hght colors that have

been discussed above. The initial laws or regulations are generally written in the form of chromaticity definitions applying to the light emitted as

signals, but if the field inspectors check the colors

they must carry out their work by testing the signal

equipment in comparison with physical standards.

This is generally most easily done at the plant of the

equipment manufacturer. Between the law and

the inspector comes the colorimetric laboratory,

which provides the necessary physical standards,

icferees doubtful cases, and tests the signal equipment when unusual problems are involved.

3. Mathematical Representation of Colors

3.1. Three Attributes of Color

Several references have been made to the mathematical expression of chromaticity and as the

discussion proceeds there will be occasion to repre-

'The term "primary" is used here in a general sense to differentiate between two

types of fillers, each of which is composed of more than one class. For a general

discussion of the naming of classes of standards see iMcNish (1958). For a classifica- tion directly applying to signal lights see the "U.S. Standard for Colors of Signal Lights," NBS Handbool< 9.5 [1964). Part I. Definitions 2.12 to 2.19 inclusive. •Sec. .5.3.2. and table IV.

sent chromaticities in diagrams. To understand

the equations and diagrams, it is necessary to reahze

that the perception of color '' is essentially three- dimensional, that is, three numbers are required

to describe a color. The most famihar of these

three attributes is hue. The gradual variation

from red to violet from one side to the other of a

' In this Monograph the word color is used as a general word equally apphcable to the stimulus and the perception.
rainbow is familiar to everyone. The same transition is even more clearly discernible in continuous

spectra. This is the first attribute of color. When

hues of all wavelengths are combined in the necessary proportions, the result is white light. The

rainbow colors, while differing from each other in hue, differ from the more vivid colors which can be

obtained in spectra because there is usually some

white hght shining through the rainbow and the

resulting chromaticity is paler than the rainbow

itself. This variation from pure colors to white

constitutes the second attribute of color and is called saturation. The more the color departs

from white or gray, the greater the saturation. The

third attribute is the quantitative aspect to which

one refers when he observes that one spectrum is brighter than another. While the most common

name for this attribute is brightness, two other

terms are also used, lightness and brilliance. Lightness refers to variations from black to white.

It is perceived by comparing unknown objects or surfaces with each other in an environment that contains familiar elements. It is applicable to

opaque surfaces and transparent volumes but not

to signal lights, which are always seen either as self-luminous areas or as point sources. Brilliance

is applicable only to point sources and is, therefore, used mostly for signal Ughts and stars. Thus far we have described the attributes of

color in psychological terms.* Such terms, however, cannot be numerically evaluated. The

radiant energy by means of which the signal hghts

and surfaces are seen could, in theory at least,

be described by giving its energy distribution with

respect to wavelength in each case, but in practice

this has turned out to be too cumbersome for use. To avoid this difficulty, psychophysical terms and

relationships have been developed. These terms

correlate with the physical terms on the one hand

and with the psychological terms on the other. This correlation is shown in table 1. The correlation of the psychophysical with the physical is ac- complished by deriving the psychophysical values

from the physical quantities through the medium

^ For a more extensive treatment oi this aspect of the subject, the reader is referred

to ASTM Special Technical Publication No. 297, Deane B. Judd [1961J.

J

Table 1. Correlation of terms

The table below shows the relationship of photometric and colorimetric terms used in signal lighting (upper sections) to similar terms used in describing transmitted light (third section) and reflected light (lowest section). Each line shows the correlation between the terms of physics, psychophysics, and psychology.

Application Physics

Psychophysics

(color stimulus)

Quantitive Qualitative

Psychology

(color perception)

Magnitude Quality

Point source,

descriptive

Point source,

quality

Point source, quantity

Point source,

visibility

Spectral

intensity

distribution

Radiant

intensity

Irradiance ^PxlA)dk

Chromaticity =

Dominant wavelength and purity

[VKPxloi)dk* Luminous

intensity

J

{K,PJco)dk

Illuminance

f {K^PJA)dk

Brillance or magnitude

Chromaticness =

Hue and

saturation

1

Self luminous

source.

descriptive

Self luminous,

quality

Self luminous,

quantity

Self luminous,

quantity/area

Spectral flux distribution PJPk„

Radiant flux j

PKdk

Radiance

Chromaticity =

Dominant wavelength and purity V^P^dk*

Chromaticness =

Hue and

saturation

{PJ(o)dklA

Luminous flux

I

K^Pxdk

Luminance

(Photometric

brightness)

(KxPxloo)dkl.4

Brightness

4
Table 1. Correlation of fermi — Continued

Application

Transparent volumes

(Filters), descriptive

Filter and

source,

quality

Filter and

source, quantity

Filter and

source, transmittance

Physics

Psychophysics

(color stimulus)

Quantitive

Spectral

transmittanr-e

Transmitted

radiant flux

Radiant

transmittance

Transmitted luminous flux

Luminous

transmittance

I

K^T^P^dkIjK,P>,dk

Qualitative

Chromaticity =

Dominant wavelength and purity

f V>,TKPxdk*

Psychology

(color perception)

Magnitude Quality

Clear to dense

Chromaticness = Hue and

saturation

Reflecting

surfaces, descriptive Surface and

source,

quality

Surface and

source, quantity

Surface and

source,

reflectance

Surface and

source, quantity per

unit area

Spectral

reflectance (P2/POX

Chromaticity =

Dominant vt^ave- length and purity

f V^RxPKdk*

Chromaticness = Hue and

saturation

Reflected

radiant flux

I

R^Pydk

Radiant

reflectance j R,P,dkIj

P,dk

Radiance

f {R,PJoj)dklA

Reflected luminous flux K,R,P,dk

Luminous

reflectance jK,R,P,dk^jK,P,dk

Luminance

iK>.R,PJaj)dklA

Lightness

(black to white)

Brightness

(invisible to dazzling)

'Symbolic formula, actual mathematical relationship is too complex for inclusion in table.

of the CLE. Standard Observer, which is numerically represented by three functions of wavelength

known as the CLE. tristimulus distribution

functions.

The symbols in table 1 have the following

meanings:

f\ radiant energy per second of wavelength A.,

(X) solid angle,

luminosity function for relative effect of different wavelengths,

Vx chromaticity function, a three component function relating radiant energy to visual quality, Tx transmittance for wavelength X, R\ reflectance for wavelength k. Table 1 is composed of four sections applying

respectively to point-source signal lights, self- luminous sources, transparent volumes and opaque

surfaces. 'Self-luminous sources", for the pur- poses of this correlation, must be considered as including aU lighting units containing optical sys- tems or diffusing surfaces which cause the observer

to see an appreciable bright area. This is the usual case with traffic signal lights and intervehicular

signals. There is no sharp line of division between

point sources and self-luminous sources. "Transparent volumes" are assumed to have nondiffusing

surfaces and against black backgrounds they are seen by their surface reflections, some of which

acquire color by being transmitted through the

volume. Opaque surfaces are seen by light re- flected from them.

The derivations of the psychophysical correlates

from the corresponding physical quantities, by the

mclusion of the luminosity function for quantitative

219-788 0-67—
purposes and all three of the standard observer

functions for qualitative purposes, are shown by the

formulas following the terms. For qualitative

purposes, that is, chromaticity computations, it is not necessary to know the actual energy distribution, but the relative energy distribution of the source must be known.

Although the psychophysical values are numerically derived from physical quantities, their defini- tions were constructed to correlate with the

psychological terms. Thus the hues of the spec- trum are naturally associated with the wavelengths

of the radiations which give rise to them. Chromaticities which can be matched by a mixture of white

light with monochromatic light of pure spectrum

chromaticity are associated with the wavelengths of

the spectral components, and these wavelengths

are called the dominant wavelengths of the mixtures.

AU of the chromaticities now being used in signal

lighting can be matched by such combinations of

spectrum light and white light. Chromaticities

exist, however, which can not be matched by any

combination of monochromatic light and white

light. In such cases it is possible to match white

light by adding monochromatic light of suitable

wavelength to the given sample. In this case the

wavelength of the monochromatic light is called the

complementary wavelength. In either case the

ratio of the monochromatic light to the total light in the matching mixture is called the purity. As in the case of the saturation, the purity increases as the

proportion of spectral light increases.

While the associations pointed out above are the

basis for the correlations given in table 1, it must not be supposed that the hue of the response is deter- mined solely by the qualitative aspects of the hght

from a source, or object, being viewed. In addition,

the apparent size of a source, the illuminance re- ceived from a small source or the luminance of a

larger one, and the characteristics of the environment in any case have important influences on the

hue perceived. On the other hand, even when

these factors remain the same, a change in the

purity, that is, a change in the ratio of monochromatic to white hght in the mixture received from a source, wiU also cause the observer to select a dif- ferent monochromatic hght as being the same in hue as the mixture.

Of the three psychological terms for the intensity

attribute, lightness, brightness, and brilliance, only

the latter two are appUcable to signal hghts. The

choice of these is determined by the concentration

of flux on the retina of the eye. If the Hght from the

target is spread over an appreciable area of the

retina, the signal is evaluated for brightness. If, however, the flux from the target appears to come

from a point, its magnitude is evaluated for brilh- ance. As indicated in table 1, brightness correlates

with luminance, which is sometimes called photometric brightness, while brilliance correlates with

illuminance.

Colors may be defined by any three numbers

which give their hue, saturation, and brightness or briUiance as, for example, the dominant wavelength,

purity, and transmittance for the combination of a

filter and a hght source. These might be considered

as cyhndrical coordinates in which the dominant

wavelength is plotted as the angle, the purity as a

fraction of a fixed radius, and the transmittance as a

distance perpendicular to the plane of the polar

diagram. Experience has shown, however, that

another form of diagram is more useful. This type

of diagram results from the fact that it is possible

to so arrange chromaticities that the chromaticity

represented by any point on a Une joining the

points representing two given chromaticities can be

produced by a certain mixture of Hghts having the

given chromaticities. These diagrams are sometimes called color mixture diagrams because they

are based upon this property. There are an infinite number of coordinate systems, derivable from each

other by projective transformations, all of which give

rise to diagrams having this characteristic. Two

of these wiU be used in this circular, namely, the

CLE. system and the RUCS system.

3.2. CLE. Coordinate System

The CLE. system of chromaticity coordinates was adopted as a device for the numerical descrip- i! tion of chromaticities by the International Commission on Illumination at its meetings in 1931 [CLE.,

1931; Judd, 1933]. Since that date it has been

regularly used as the standard means of specifying |: chromaticities. This coordinate system rests on

the CLE. Standard Observer of 1931 [Judd, 1933], I which is a set of three functions of wavelength each

expressed as a set of eighty-one values correlated

with the 5 m/A intervals from 0.380 to 0.780 jx. One of these functions is the relative luminosity

function for cone vision, that is, a set of values pro- portional to the relative contribution which light of

different wavelengths makes to the brightness of a surface under daylight conditions. The other two

functions which complete the standard observer

express the results of color matching researches. " An infinite number of coordinate systems are consistent with the standard observer, but any one of

these can be derived from any other by a linear

transformation, which in turn requires the assignment of eight coordinates to completely determine ['

it. As defining coordinates, the CLE. selected > values for the x and y coordinates for the spectrum ' colors of wavelengths 0.4358, 0.5461, and 0.7000 \ microns and CLE. standard light source B. The ^ first two spectrum colors are to be found in the « spectrum of mercury; the third can be produced by t an incandescent lamp with a suitable red filter.

' Source B represents mean noon sunlight but it is < defined in terms of a lamp and filter. By choosing

the values in this way the calculation of the luminous

transmittance or reflectance of a sample could be

found incidental to the calculation of the chroma-
ticity coordinates of the sample. The selection of

the above cardinal stimuU also resulted in a set of

primaries {X, Y, Z) which corresponds under usual

I conditions of observation to colors which may be

j considered as unitary red, green, and blue.

\ 3.3. RUCS Coordinate System

'; The Rectangular-Uniform-Chromaticity-Scale Coordinate system was first proposed by Breckenridge

and Schaub in 1937 [1939]. ^ At the time when the

CLE. coordinate system was adopted, little was known about the perceptibility of chromaticity dif- ferences. At that time it seemed reasonable to place the white point centrally with respect to the

spectrum locus. It was believed that this would

avoid the exaggerated extension or compression of any parts of the color field, that is, it would give

approximately uniform chromaticity scales. Later

research, however, showed that the spacing of the

chromaticities in the CLE. system is far from uni- form. The purpose of plotting chromaticities on a diagram is to assist in the visualization of the rela- tionship between different chromaticities. The

distortions of the CLE. diagram tend to defeat the

purpose of such visual representations of chromaticity and a number of uniform chromaticity scale

diagrams have been developed to reduce the

distortions. The Hmitations of the CLE. and RUCS systems

are discussed in sec. 3.6. As compared with other

available uniform chromaticity systems the RUCS

has four advantages for the present use. Its derivation is based on sources of small angular size (2° or

less), a transformation table is available for converting CLE. values into it, it has already been used in two international publications and there is a con- venient relationship between arrangement of chromaticities and our ordinary conceptions of colors.

3.4. Chromaticity Diagrams

[ Each coordinate system gives rise to a characteristic chromaticity diagram in which every chromaticity is represented by a point. Figures 3—1 and

3-2 are CLE. and RUCS diagrams. In each of

these the curved boundary represents colors such

' as those seen in spectra. It is called the spectrum

locus and the numbers along the boundary are the

wavelengths of the corresponding hght. As already

stated in sec. 3.1, to the observer with normal color

vision these spectrum colors differ from each other

in hue. If white light, which generally contains

light of all the spectrum colors, is added to a single

spectrum color, a new chromaticity results which

' does not match any color in the spectrum. Such

chromaticities are represented by points inside the

spectrum locus. The more white added, the fur-

" The coordinates of the RUCS system may be computed directly from tables, derived by a linear transformation, or closely approximated from a transformation

table. The constants for the first two methods are given in the referenced paper. For the tables see Breckenridge (19481 in the list of references. It is also possible to obtain the uniform spacing, but not the other advantages described in sec. 3.7 by

plotting C.I.E. coordinates on a distorted CLE. diagram as proposed by Holmes [1940].

ther from the boundary wiU lie the point corresponding to the resulting chromaticity.

It is a basic property of both of these chromaticity

diagrams that when two samples of hght, which are represented by two points in the diagram, are mixed

they produce a color which is represented by some

point on the straight Une joining the other two points.

From this it foUows that the points on the straight

segment of the boundary connecting the ends of

the spectrum locus represent mixtures of the ex- treme red and extreme violet. The usual purples

He close to this straight boundary at the short wave

end, while near the other end will be found purphshred chromaticities.

The curved hne within the figure is called the

Planckian locus. It represents the chromaticities

of blackbodies at different temperatures. In practical appUcations, it is accepted as representing the

chromaticities of incandescent bodies such as lamp

filaments. The numbers along this curve on the

C.I.E. diagram, and the corresponding numbers in the margin of the RUCS diagram, indicate the color

temperature in degrees Kelvin, sometimes called

degrees absolute, corresponding to the marked

points.

3.5. The z Coordinate

The initial computation of the C.I.E. coordinates

gives three values one of which is an intensity

factor, usually in the case of signal-light filters the

transmittance. Since only two independent values can be plotted in a plane diagram, it is customary to reduce the three values to three proportional numbers totaling 1. This ehminates the intensity

attribute and results in three coordinates which rep- resent a point in the plane through the three points on the axes at x=\, y— 1, and z=\, respectively.

As may be seen from figure 3-3, the C.I.E. diagram

shown in figure 3-1 is the projection on the x,y plane

of the true x,y,2 diagram.

3.6. Relative Distortion of Diagrams

A system of numerical values derivable from

physical measurements has great advantages over names as a means of specifying colors in that the

values can be made independent of the observer.

Even a colorbhnd individual can determine the cor- rect color coordinates if he is provided with suitable

instrumentation. Moreover, a color once measured

can be reproduced at another time or place if its coordinates in any well-designed system are known.

The C.I.E. coordinate system served this purpose

quite well, and since it was the first system that

appeared hkely to be generally utiUzed, it rapidly came into general use. The only purpose served

by plotting the coordinates of chromaticities, however, is to visuaUze the chromaticity relationships.

For this purpose every distortion means a false visuahzation. The distortions of the C.I.E. system

have already been mentioned in sec. 3.3. It

7
remains, however, to consider in this section the

extent of those distortions. The first attempt to estimate the extent of the

distortions of the C.I.E. diagram was made by D. B. Judd in 1934 [1935]. Judd transformed the C.I.E. diagram into a system of triUnear coordinates designed to give uniformity of spacing in accordance - with the best information then available. He then

fiUed the area of real chromaticities on this Uniform

Chromaticity Scale diagram with a group of equal

tangent circles and transformed these back into the I C.I.E. system. The result was the diagram of

eUipses shown in figure 3-4 [Judd 1936].

8
Figure 3-2. RUCS chromaticity diagram.

A transformation ol the C.l.E, diagram designed to space the chromaticities of small sources uniformly in rectangular coordinates.

9
Figure 3-3. The "z" coordinate.

This diagram shows the relationship of the two-dimensional C.I.E. chromaticity diagram to the three-dimensionaj C.I.E. chromaticity coordinates. The coordinates

plot in the sloping RGB plane. The C.I.E. diagram is the vertical plane at the back.

Numerous efforts have now been made to develop

uniformly spaced diagrams, but most of these have

been directed towards a diagram representative of

the spacing for large color samples.'" Two studies

have been made, however, which are appUcable to the signal hghting problem. The results of one of

these researches are shown in figure 3-5 for the

C.I.E. diagram, and corresponding results for the

RUCS system are shown in figure 3-6. Both of

these figures are taken from a paper by W. D. Wright

[1941]. In each of them, the bars represent small

changes in chromaticity that are equally perceptible

at different locations on the diagram."

Wright's study was made with a 2° field, that is, one with an angular diameter four times that of the

fuU moon. Even this angular size is smaller than

the apparent size of the samples preferred for accurate color matching of panels. On the other hand,

it is a larger target than an ordinary 8" traffic fight

at a distance of 20 feet, a httle more than a car length

away. This is about the largest diameter at which

a signal fight ever has to be recognized, whereas in aviation and marine uses such lights frequently

In 1963 the International Commission on Illumination at its 15th Plenary Session adopted the CIE-UCS coordinate system to provide a generally acceptable system of representing surface colors with uniform spacing. This is applicable only to areas subtending at least 1' for the observer. See C.I.E. Proc 1963 or Judd [1963]. " .Although most of the other researches are of little concern for signal light colors, three may be of interest to readers concerned with chromaticity diagrams. MacAdam

[1943] made a study of the representation of small color differences and later [1959] he published the results of a comparison of small field and large field observations which show clearly that the diagrams most desirable for the representation of sur- face colors are not suitable for signal lights. The other study is that by Burnham and NewhaU [1953]. Although this is a study of small fields, the criterion of uniformity used was the spacing of the Munsell diagram which has been carefully developed to approximate uniform spacing for large areas. The results shown are consequently

quite misleading.

'2 Subtended diameter, 5'.

Figure 3-4. The Judd ellipses.

Circles drawn on the Judd Uniform Chromaticity Scale Diagram of 1934 [Judd 1935) presumably represent chromaticities that are equally different from the chromaticities ! at their centers. A group of such circles were transformed to the C.I.E. chromaticity diagram by Judd [1936]. The ratios of the diameters both within an ellipse and be- ; tween ellipses presumably indicate the degree of nonuniformity throughout the C-I.E, diagram.

appear as "point" sources, fights often less than 1 minute in observed diameter.

Another study has been made by G. N. Rautian

and M. K. Guryeva [1957]. In their study paths of

least chromaticity difference were determined between chromaticities in different areas of the dia- gram and the number of distinguishable steps

between these chromaticities was considered to be a measure of the chromaticity difference. The study was carried out with point sources which adds

to its significance as a test for the uniformity of a diagram intended for signal-light use. Figure 3-7

shows the results of the Rautian-Guryeva research on a C.I.E. diagram and figure 3-8 the same chromaticities on an RUCS diagram.

The results of the studies which have been shown

in figures 3-4 to 3-8 can be expressed numerically

by measuring the longest and shortest distances

corresponding to equal perceptual differences and

computing the ratios. These ratios are as follows:

Judd, C.I.E. in terms of U.C.S. 10/1

Judd, RUCS in terms of U.C.S. 1.1/1

Wright, C.I.E. for 2° field 20/1

Wright, RUCS for 2° field 4/1

Rautian and Guryeva, C.I.E. for point sources (5') H/l

Rautian and Guryeva, RUCS for point sources (5') 6/1

These ratios indicate that the C.I.E. diagram is unsatisfactory and that the RUCS diagram, although

10
/

J \

O / •• Z

3

1 N

\

11 /

// y \

/ //l

, / II u \ c)

OB 07 0,8

Figure 3—5. Color steps for the mean offour observers plotted

in the CLE. diagram.

Each dash represents a chromaticiiy difference estimated to be the same as the

others as determined by Wright [1941]. A comparison with figure 3-4 shows genera] agreement both as to the degree and the extent of the nonuniformity of the C.I.E. diagram.

Figure 3-7. Chromaticity differences for signal light colors in C.I.E. The dots indicate the distinguishable color steps for signal lights as determined by Rautian and Guryeva [1957]. Some similarities to the results shown in figure 3-5

are evident.

0.3

0.2

0.1

-0.1

-0.2

• 0.3

- 0.4.

-0.5

0.495

\ 1- . 0.53

'.AO \

-0.4- -0.3

\

\

^ A

0.54-

0.55

. 10.56 7 \

1

1

1 V

\

J 0.58

\ 1

I0.62

0.6J

0.1 0.2

Figure .3—6. Color steps for the mean of four observers plotted

in the RUCS diagram.

Each dash represents a chromaticity difference estimated equivalent to the others,

as determined by Wright [1941]. A comparison of this diagram with figure 3—5 indi- cates that the RUCS is more uniform than the C.I.E. for small sources. It is also evident that the RUCS is not as uniform as would be desirable.

Figure 3-8. Chromaticity differences for signal light colors in RUCS.

The dots indicate the same points shown in figure 3-7 after transformation to the RUCS diagram. The comparison of figures 3-8 and 3-7 with figures 3-6 and 3-5

shows about the same degree of improvement. Both RUCS figures, 3-6 and 3-8,

suggest that no linear transformation could make all the chromaticity differences shown equal. This is in agreement with MacAdam [1942]. 11
leaving much to be desired, is not nearly as non- uniform as the CLE. While the close comformity

of the RUCS to U.C.S. indicated by the ratio 1.1/1

merely reflects the derivation of the RUCS from the

U.C.S. , it is an indication of RUCS uniformity since

the U.C.S. approximates uniformity by independent

criteria. The numerical values given above are not in themselves sufficient for estimating the distortions. It is also necessary to consider the degree to w^hich the

different results agree as to which areas of the

diagrams are overextended and which too much

compressed. All three agree in indicating the

CLE. exaggerates the blue-green differences in the

extreme green region. All three agree in indicating

that the blue corner of the CLE. is the most compressed area, and that red-blue differences are exaggerated in the red corner.

An examination of the two RUCS diagrams indi- cates that the overextension of the green region has

been corrected and the exaggeration of the red-blue

differences at the red vertex has been reduced.

The Wright data makes it appear that the blue

vertex of the RUCS diagram should be further from

the origin whereas the Rautian-Guryeva results indicate this vertex should be nearer the origin.

This difference, however, should be expected to result from the difference between a 2° source and a point source as we shall see in the next section.

4. Recognition

4.1. General Principles

The first, and in some respects, the most important determinant for the Umits of signal colors

is the ability of the eye to recognize them. Unless

there is a high degree of certainty that the signal

will be recognized for the color it is intended to be,

the signal may be a hazard instead of an aid to safety. The problem is a difficult one, however,

because it is a statistical problem with nine parameters. The probability of correct recognition of a

signal-light color depends upon: (1) the number of

colors in the system, (2) the observer's familiarity

with the system of colors, (3) the opportunity to compare colors if such is present, (4) the degree of

concentration which the observer can devote to the

recognition of the color, (5) the normality of the

observer's vision, (6) the state of his visual adaptation, (7) the luminance of the background, (8) the

solid angle subtended by the signal at the observer's

eye, and (9) the illuminance, or the fixed-light equivalent illuminance at his eye.

" See fig. 4-12 and discussion in text Hues slightly on the blue side of the /'-axis are, however, classified as green by many observers and are used satisfactorily as green signals.

3.7. Properties of RUCS Diagram

Although the chromaticity relationships in the

Uniform Chromaticity Scale Diagram and the

Rectangular Uniform Chromaticity Scale Diagrams

are very nearly identical, in developing the RUCS

diagram convenience of use was taken into consideration as well as uniformity of chromaticity

spacing. From this standpoint it seemed desirable

to use rectangular coordinates and to place a suit- '. able white point, technically known as the achromatic point, at the origin of the coordinate system, i If this is done, all radial lines from the origin become \ loci of approximately constant hue terminating in the

corresponding dominant wavelength at the spectrum j

locus. Circles concentric with the origin approximate loci of constant saturation. By properly

orienting the figure, it was found possible to make

the y-axis correspond to red-green differences, while

the ;c-axis corresponds, in a general way, to yeUowblue differences, although more specifically it con- ; nects violet to greenish yellow. As a result of

this, the four quadrants represent green, yelloworange-red, purple, and blue colors respectively, to a

fairly satisfactory approximation.^^

As will be seen in the next section, no one diagram

can express chromaticity differences uniformly for

all conditions of observation, but, so far as present

knowledge makes it possible to judge, the deforma- i

- tion of the RUCS diagram to fit different conditions. : of observation is simpler than the corresponding e

i deformation of the CLE. diagram.

4.2. Results of Research

No one has found it possible to investigate the

problem of signal color recognition with respect to

all the variables listed in sec. 4.1, but H. J. McNicholas [1936], J. G. Holmes [1941 and 1949], N. E. G. HiU

[1947], and R. M. Halsey [1959] have studied impor- ^ t tant segments of it.^^ A correlation of these four

studies has been made and figures 4—1 to 4-12 and

table 2 are from this correlation [Breckenridge,

I960]. The table gives most of the more important

characteristics of these researches, but it should be

added that only the observations of McNicholas i Series II and Halsey Series III were properly recog- j, ^ nition tests, that is, tests of signal colors separated J

I by regions of unused chromaticities. All the other s t tests included in these four researches were naming' i tests to determine within what regions of chromaticity, and with what consistency, observers would

Many other investigations furnish support to the results of the four studies mentioned. Of these, four are of especial interest: Guild [1928], Ornstein, Eymers, and Vermeulen [1934], Halsey and Chapanis [1954], and Bedford and Wyszecki [1958]. In the Halsey and Chapanis study, many chromaticities were visible at the same time. This may be particularly significant for signal lights seen against a background of J other lights. Guild's work was confined to the selection of a range of yellows, but it is i of especial interest for the unusual method used.

of Signal Colors

12
apply the typical signal color names to the members

of weU-distributed sets of chromaticities.

To understand the correlation of the different

tests, it is necessary to take account of some additional observations on the appearance of point sources of light near the threshold of visibility.

When a Ught is just visible it is nearly colorless. Some observers state that red lights are recognizable as such even at threshold but not all observers

have found this to be true. We must conclude that the recognition of red hghts at threshold is too uncertain for signal purposes, and hence at threshold we hav*^ only '^"'^ signal color. As the intensity

of the Ught is increased, or the observer approaches

closer to it, red hghts may be distinguished from

hghts of other colors, but these continue to look

ahke. At still higher levels of illuminance at the

eye of the observer, green lights become recognizable giving us the common three-color system

composed of red, green, and an intermediate color

which at still higher illuminances becomes differ- entiate into yeUow, white, and blue. To get a

sixth color reliably with point sources, it is nec- essary to use dichroic purple hghts which usually

appear as red lights surrounded by blue rays since

the lenses of the eyes cannot focus for both colors

at once. With sources of appreciable size, trained

observers, and favorable conditions of observation,

it is possible to insert additional colors.

Figure 4-1. McNicholas test colors.

This diagram, reproduced from a paper by McNicholas [1936], shows how McNicho- FlGURE 4-3. McNicholas results plotted on RUCS diagram.This diagram was prepared to facilitate a comparison of the results obtained by

las undertook to arrange his test colors so as to correlate his results. The numbers McNicholas with those obtained in later researches. The numbers on the curves show

identify the filters he used to obtain the test colors. the fraction of answers agreeing with ti.e color shown.

TTi i m I I Li_i 1

1 mil 11 rn—mrm 1 1 in i 1 1 1 1

1—n 1—i i i i ii

/ORANGE-YELLOWS ^ ^ \ SERIES I

// RED \*0RANGE\/ yellow

JJJ

FILTER POSITIONS

Figure 4-2. Results of signal color recognition tests by McNicholas [1936].

Colors are arranged in the order shown on the path drawn in figure 4-1. The ordinates are the percentage of answers indicated for the names shown under the curves, would appear from this figure thai none of McNicholas" colors were completely reliable under the conditions of observation, but these were relatively severe. 13

219-788 0-67—
Figure 4^. Signal color identification with 1 sea mile candle FIGURE 4r-6. The effect of increasing the number of observers. illuminance. ' From report by Holmes [1941]. This figure represents results obtained under the Determination from report by Holmes [1941]. Note that only red signal colors reach same conditions as figure 4-5, but with 50 observers instead of 6 "average" observers,

as high as 90 percent reliability under the conditions of observation. '< was the author's conclusion that reliable results were being obtained with the smaller number of selected observers.

-0.4 -0.3 -0.2 -0.1 0 *0.l x"

Figure 4^7. Identification of signal colors with 100 sea mile Figure 4^5. Signal color identification with 10 sea mile candles candles illuminance. illuminance. These results from Holmes' report show no further improvement in the reliability From report by Holmes [1941]. The increase in certainty of color identification as of red and green signals, but indicate for the first time the distinguishability of blue compared with figure 4^ is quite evident. signals. 14
Figure 4^8. Identification of signal light colors ivith 3400 sea FIGURE 4-10. Distinguishahility of blue, green, and white mile candles of illuminance. * signals.

This figure, the lasi from Holmes' report, shows a further increase in ihe distin- These resuhs taken from a paper by Halsey [1959] were made with a relatively uni- gaishabiliiy of blue signal lights and also in purple signals. form distribution of target chromaticities throughout the test region as indicated by

the small circles. For illuminance levels see text.

Figure 4^. Signal color identification at 2 mile candles

illuminance.

These results were obtained by Hill in 1947. An attempt to distinguish blue from

green signals at this illuminance proved impracticable. Apparently the distinction

of blue from preen fades at a higher iliuminance than the other distinctions which are represented in this figure. (See also figure 4-12 and discussion in sec. 4.2.)

Figure 4^1 1 . Effect of confining target colors to normally

acceptable chromaticities.

This lest, also by Halsey [1959] (not published), discloses the effect of omitting target chromaticities which were considered unreliable so that one might hope that the re- maining chromaticities would be identified correctly 100 percent of the time. It also indicates the effect of giving observers some training. 15
Figure 4-12. Correlation oj Holmes' results. The results shown in hgures 4-4, 4-S, 4-7, and 4-8 have been compressed or expanded along the axis to compensate for the eflfects of low-illuminance tritanopia. It should be noted that in the case of all except the leftmost figure, the white contours are approximately circular around a point near the origin. The leftmost figure has shrunk to a vertical hne, signifying that the yellow-blue discrimination has been lost because of the tritanopic effect of low illuminance.

From the above discussion it becomes clear that

if we were to represent signal colors in diagrams

prepared to correspond to different levels of illu- minance at the observer's eyes, we should make the one for threshold illuminance consist of a single

point. At the next level of illuminance we shall need a short hne with red represented at one end

and non-red at the other. As the illuminance in- creases the line must be made longer and finally

the figure takes on width and develops into something approximating the RUCS diagram.

An attempt has been made to prepare a series

of diagrams on this principle by using the contours

of Holmes' extensive investigation as a guide to the factor for the yellow-blue contraction, or ex- pansion, of the RUCS diagram required for each

level of illuminance. The results of this study are shown in figure 4^12 [Breckenridge, I960]. The

diagrams of this figure are those of figures 4^4, 4^5,

4^7, and 4^8 with the x" coordinate contracted, or expanded by the factor indicated for x" above each

figure. Only that portion of the RUCS diagram nec- essary to include Holmes' test chromaticities has

been included in each case. If these transformations had resulted in diagrams entirely in accord

with the RUCS intent, the inner white contours

would have been circles around the origin in each ' case. '

4.3. Results of Experience

While the contributions of experience to our ^ knowledge of the recognition of signal colors are ' less definite than those of research, their influence \ upon the actual selection of limits for the signal ' colors in use has been greater. It is, of course, ' possible that a definition of a signal color might ' be used for many years without the realization that *

it permits certain chromaticities that could be mistaken under some conditions. Considerable areas ; of chromaticity which are within the definitions are

' not used in service. Chromaticities near the ex- tremes of those represented by the standard limits ' presumably do not occur frequently and it may be ' much more rare to find such a borderline filter in a ' situation in which color recognition is particularly '

difficult. Finally, not every error in color identifi- ! cation causes results that reveal the mistake. ' Nevertheless the long and successful use of the ' same colors for marine and railroad signals is a

| strong indication that chromaticities within the

limits of those used in these services are satis- '

16
Table 2. Summary of procedures for color identification >esearches*

-

1 r Colors Observers Observations Source Research niu mi- target Exposure Number

Colora- Grouping Adaptation by nance size Other of colors Number Aaes Total Names

tion lights X runs nu mber all d

smc ininutcs jsconds Years McNicholas

Series I 0-5/8.2 5 69 (Glass [Street Partly RYOr RPW* Below horizon on dark

C - IT Grouped 38* 20*/60 5800* IKYGBFW or background of woodland „ [Filters [Lights Grouped (R YOGB W * and southern sky Holmes 0.7 1 on/1 on 256 Synthe- None Continuous 6x3 22/42 4608 a YGBP

5 minutes before test sized 13 X 1 3328 do

10 2.2 -

1 1 glass > 6x1 „ 1536 do

2 2 filter

- 50 X 1 18/63 12800 do. 10 2.2 primaries 5x3 22/42 3840 R YG W

10 do. 5x3 3840 RYGN

10 2 2 do. 5X3 3840 RG W

10 2.2 do. Desk

lamp* 4X3 3072 RYGBP

Reading by desk lamp

between observations

100 7.0 do. None 6X3 4608 do. 3400 40.0 do.

" „ 4X3 3072 do.

Hill 1 3 0 6 1 5 73 17 Wratt»n Dim Contmuous 9X20# 20-4O# R(YO)(G + B)W# 10 Minutes before testing

2.6

filters reddish 30420 General .0001 c/ft.^ with 0 6 1 5 73 -f white glow 20^# do. 15' disc Uluminated to .0005 c/ft. for 1.5" before

rlsisey In signal 6 2/280 4 1/7 2 2 50 4 Wratten Red & Contmuousj 100 X 1 17-35 5000 RYGBPW .001 ftL on data pads for

I 1 8/83 filter yellow 100 XI 5000 do. 15 minutes before test

ir „ primaries fixation 100 X 1 17-28 5000 RYGBW and during it lit 1.8/83 do. lights 100 XI 5000 do. do. 2.3/104 * 22 do. do. 3 100 X 1 approx. 2200 RYGBPW Dimly illuminated room

do. do. Groups 100 X 1 as 2200 RYGBW do. lUc do. do. do. 100 X 1 2200 GB W do.

IIIo do. do. do. 100 X 1 do!"^" 2200 GBW do. Notes •Flashed

as long

as de- sired

•Between

observations

*In two

series #Some 30

limes

*3 <20

#17 > 40! •In the two

series

*Note orange

included in one test #orange re- corded as yellow

*(Breckenridge, 1960] Reproduced from report by author to Sixth International Lighthous Conference, Washington, 1960.

factory for their purposes. It does noi follow that

, chromaticities outside of those limits are unsafe.

I While the Umits for the red and green signals used

I in marine and railroad practice were selected before

the researches mentioned in the last section were

carried out, their selection was not casual. The

earlier limits used for railroad signal colors were

not satisfactory and at its second meeting the newly

organized Railway Signal Club appointed a committee in April 1895 [AAR, p. 74, 1953] to investi- gate the question of colors for night signaling. At

first the problem was which colors should mean

j clear, go ahead, and which danger, stop. At that

j time the usual colors for railroad signals were white

j for safety, red for danger, and green for caution.

This was the practice in most countries in accord- ance with an agreement reached at a congress of

railroad men in Birmingham, England in 1841.

:1 The system had been developed in France and it is

i| interesting to note that the originators had reached

' the conclusion in the course of their experimenting

that "the visibihty of a red light was but one-third

j that of a white light of the same intensity; that of a

[ green light one-fifth; and that of a blue hght oneIj seventh" [AAR, p. 73, 1953]. A comparison of

i these values with present information evidences the

progress which has been made by the glass manufacturers in producing signal filters of higher

transmittance.^^

The 1841 standard proved defective because a signal with a broken glass frequently appeared

white, converting a danger or caution signal into a safety signal. The same mistake occurred if an engineer mistook an ordinary white light for a

signal. On this account English practice was

changed to use green for the safety signal with no

caution signal. The first use of the present red- yellow-green system in this country was by the New

York, New Haven, and Hartford Railroad in 1899. Starting about this time interest began to de- velop in the scientific aspects of signal-light rec- ognition. Laboratory work was started at Yale

University and later continued at the Corning Glass

Works by Dr. William Churchill to select the

most effective colors for signal lights [Churchill,

1914]. Between 1914 and 1930 several committees

appointed by the Signal Section of the Association

of American Railroads made field tests with the

cooperation of the Corning Glass Works to deter- mine the most favorable limits for the colors. These tests had the advantage that they were made

by the men who used the lights in service and they were made under conditions approximating some- what the conditions under which such lights are normally used.

This assumes that the "same intensity" refers to the lamps without their filters and that the relative visibilities were measured by the distances at which the lights became visible. 17
The original selection of colors was made at a time when all railroad signal lights used kerosene

wick lamps. The selection of glasses for use with

electric lamp sources was carried out under the

technical leadership of Dr. H. P. Gage at the

Corning Glass Works. Dr. Gage [1928] reviewed

the entire problem systematically. He found that

in addition to meeting the requirements of recognizability, the railroad representatives considered

it desirable that lights of the primary signal colors

(red, yellow, and green) should be about equally

visible. Since the green glass was thought to give

the least conspicuous signal, the limits for green were determined first. Yellow and red glasses

5. Production

5.1. Source and Filter

The second determinant which controls the se- lection of limits for signal colors is the practicability

of producing initially, and maintaining in service,

signals that will be within the proposed limits. This is a problem with many aspects, as indicated

by the subdivisions of this section.

The color of the signal seen by the user depends

upon the chromatic qualities of the source of light

and those of the filtering elements interposed be- tween it and the observer.^'' Some signal colors can be obtained from autochromatic sources that require no filters, but most signaling is done with

units that derive their light from incandescent lamps

which for signal purposes are regarded as white

and require glass, or plastic, filters when other colors are required. In some installations lamps have

been used which contribute a weak coloration that

is strengthened by a filtering element. In the fol- lowing section, we will consider first, and chiefly,

signals produced by filtered incandescent light,

with a brief discussion of gaseous discharge lamps

.in the last subsection.

5.2. Effect of Colorants

If a desired signal color is to be obtained by

filtering light from an incandescent lamp, the first question is whether suitable glass, plastic, or other

material of the necessary spectral transmittance is available. The curves of figure 5-1 show chromaticities that are obtainable with different types of

glass. In many cases slight changes in the composition of the colorant would move the corresponding curve to one side or the other of the path shown.

It is evident from the distribution of these curves

over the green and most of the blue quadrants that a considerable variety of chromaticities is avail-

" For a somewhat parallel discussion of this aspect of the problem from a diflferent point of view, see "Coloured Glasses for Lighthouse Purposes," Holmes [1937]. There

is also some parallel information in Holmes' "Colorimetry in the Class Industry" [1947].

were then selected to obtain signals that were ref garded as comparable in conspicuity as weU as j safely recognizable.

The ranges of acceptable color variation were

identified by filters representing the palest accept-

able ware and the minimum acceptable transmittances. These filters were called limits. The J selection of limits for marine, aviation, and high-j

way signal lights in the United States has been^

based to a considerable degree on the limits se- - lected for railroad signals and such consistency

as exists between these different systems results largely from the influence of Dr. Gage in the selection of all of these limits.

of Signal Colors

able for these colors. In the yellow-red quadrant

any desired degree of redness is available with

high purity but this may not be apparent from the

diagram since for wavelengths greater than

k = 0.585)U, the curve for chromaticities obtainable

from the practically available selenium glasses

virtually coincides with the spectrum locus and is not represented by a separate line. In the purple

quadrant only one type of glass is shown, but the

-.4 -3 -2 -.1 0 X" +.1

Figure 5-1. The effect of colorant selection.

With the exception of the Planckian locus, which is distinguished by the cross bars

indicating the different color temperatures, each of these curves is the locus of chromaticities which result from changing the density of the colorant. In three cases, two

types of glass have been combined in fixed proportions to obtain curves in the regions

not commonly used for signal light purposes, but all the other loci correspond to actual ; filters. 18
chromaticities in this quadrant are not generally

used for signal lighting purposes.

Unfortunately no data for plastic filters comparable to that shown in figure 5-1 for glass filters are available to the author, but red and yellow

plastics with spectral transmittances very similar

to those of the red and yellow glass have been

measured at the National Bureau of Standards as weU as green plastics of suitable chromaticity for signal purposes.

The fundamental difference between filters of

different colors derives from the different degrees

with which light of different wavelengths is trans- mitted. Figures 5-2 and 5-3 have been included

to show typical differences in spectral transmit- tances for filters of different colors.'* The curves have been labeled to assist in coiTelating the curves

of these figures with those of figure 5-1. For each

color, two curves are shown with the same label, the lower curve representing a thicker filter than

the upper curve. The filters represented by the curves of spectral transmittance are not the same

as those of figure 5-1 but they are similar to some

of them and would give chromaticity loci in the

regions indicated.

S.3. Effect of Filter Thicknes

One of the means of controUing the color of a

signal fight is to vary the thickness of filter used, or

, if this is not feasible, to change the concentration of the colorant in the filter. In figure 5-1 it will be

seen that all the curves for the filter chromaticities

' converge to a single point. This point corresponds

to filters of zero thickness which do not change the

color of the incident fight at all and consequently

transmit fight that is the color of the source. Theoretically any filter that is not neutral can be in- creased in thickness or concentration until its transmittance becomes virtually monochromatic.

In the figure this would be represented by prolonging

each curve until it reached the spectrum locus. There are practical reasons why this is not feasible. Even at the outer ends of most of the curves shown

' in figure 5-1 the transmittance is below the miniI mum practicable for signal filters. In production

I there may also be chemical difficulties. There

may be reactions between the colorant and the

body material that will change the spectral trans- mittance characteristics of the colorant; or such a reaction may have a deleterious effect on the physical qualities of the glass. In the practical production of colored glassware

there is always a variation of thickness from piece

' to piece, and from lot to lot there may be variations

of the composition and concentration of the colorant

0 "The speclrophotometric transmittance of the national standard filters are given

If in the U.S. Standard for the Colors of Signal Lights [NBS 1964] and also in Gibson, Haupt, and Keegan [1945] and [1946].

"T—' I '

I

' I' I

I

I—I—I

I

I

'—I

'

I

I

I—f—1—> I

'

I INTERNAL REFLECTANCE

Figure 5—2. Spectral transmittance curves for National Standard

filters. Two filters of each of four colors are included. The upper full line line curves repre- sent pale limits. A lower broken curve in each case represents a denser filter of

similar type.

Figure 5-3. Spectral transmittance curves for three types ofglass available for signal ware.

Lunar-white and carbon-yellow glasses have been used for railroad signal-light ware.

Sextent green glass could be used if a yellow green signal were desirable. Broken

curves are for thicker filters than those represented by the full line curves.

which cause corresponding variations in the chromaticity of the signal. In the case of green and blue ware, the possible variation is considerable. It is, in fact, so considerable that it is difficult to set up

chromaticity limits for some ware that will keep

within the limits regarded as safe for the recognition

of the signal and at the same time allow enough

variation to permit reasonably priced production.

In the case of red and yellow selenium ware, the

variation of chromaticity with thickness is much less and is mainly parallel to the spectrum locus. Similar, but larger, changes are produced by heat treat- ing selenium glass. Since these sharp cutoff sele- nium red and yellow filters, excepting some too pale

for signal purposes, produce colors close to the

spectrum locus, there is a negligible change of

19
x" (or z) not only with changes of thickness but with

changes in any of the other parameters we shall consider. For this reason the changes of chromaticity of the red and yellow selenium glasses may

be represented diagrammatically by using y" or y

as an index of redness.

Variations in the chromaticity of plastic ware are

less than those for glass but similar in character

because the methods of production permit more

rigid controls of thickness and composition. There

is, on the other hand, greater risk of change of chromaticity through fading with age in the case of

plastics. Some plastic colorants, however, are re- sistant to fading.

Curves showing the variation of chromaticity as represented by one of the coordinates as a function

of the filter thickness are useful in the duplication

of standard filters. Figure 5-4 shows this variation

for three similar green glasses. To prepare the curve it is necessary to know the spectral transmit- ' tance of the filter, but after the curve is drawn all other chromaticity measurements may be made

with a differential colorimeter (see sec. 9.1). An

ordinary spectral transmittance curve does not give

values that enable one to compute chromaticity

coordinates with the accuracy suggested by the curves of figure 5-4, and for this reason the first grinding of the blank should be to a thickness that

^°20 .21 .22 ,23 ,24 ,25 ,26 .27 .28 .29 ,30 X

Figure 5-4. Variation of chromaticity with thickness.

The center curve shows the variation in the x coordinate for the chromaticity of a green filter transmitting Hght from a source 2854 as the thickness of the filter is varied thru a range of approximately 1 to 2. This type of curve is useful in estimating

the change of thickness necessary to produce a required change in chromaticity when

filters are being ground to come within a given tolerance. The central curve is for a national standard filter and the upper and lower curves are for other filters of similar types of glass. It is to be noted that the tangents for all three curves are nearly the same for any given value of x. In adjusting chromaticities by grinding, it is the rate of change that is important. This enables us to use a standard curve even though the

particular piece of glass to be adjusted may have a slightly displaced curve.

•100 .200 300 .400 -500 .600 X

Figure 5-5. Variation {CLE.) of chromaticity in service for green glass filters. This is the type of diagram which must be prepared to show that the signal color will remain within the basic chromaticity definitions in service. The curves in this figure are based on U.S. Standard Filter 7.134. Those labeled with color temperatures show the variation with thickness using light sources of the labeled color temperatures. Curves labeled with thicknesses show the variation with light-source color temperature for filters of the labeled thicknesses. The straight lines show the boundaries of the basic chromaticity defini- tions for signal green, intermediate green, and restricted green. To be acceptable, the chromaticities for the proposed colorant must remain inside the applicable basic chromaticity

definitions for all points within the area bounded by the minimum and maximum light source color temperatures to be expected in service and by the curves for two thicknesses

representative of the minimum and maximum thickness to be expected in the filter ware.

20
Figure 5-6. Variation (RUCS) of chromaticity in service for green fitters. The full line curves here represent the same variations as in figure 5-5. The dotted line curves represent the chromaticities to be expected on the basis of a green filter manufactured by a different company.

allows for a possible error of at least the estimated

uncertainty. The differential measurement pro- vides a more accurate value of the chromaticity, and

if this were plotted on a diagram corresponding to figure 5-4 it would probably not fall on the curve. However, we now know how much change in ;c or y we desire, and we know the slope of the curve for the chromaticity as a function of thickness with

sufficient accuracy so that we can compute the re- duction in thickness required to bring the filter within the tolerances.

5.4. Effect of Color Temperature of Source

It has been stated in sec. 5.1 that not only the

characteristics of the filtering element but also the

color of the source affects the color of the signal.

Most signal lights today operate with incandescent

lamps, but signal lights using kerosene wick flames,

acetylene flames, and mantle burners may still be

in use, and each of these has its characteristic spectral energy distribution. The incandescent lamps

also differ in spectral energy distribution because of

differences in the operating temperatures of the fila- ments. It is customary to specify the energy distri- bution of an incandescent lamp by giving its color

temperature. This is not the physical temperature

of the filament but the temperature of the blackbody

or complete radiator which would most nearly match

the filament in chromaticity.

The differences in filament temperature result

in part from differences in lamp design and in part

from variations in operating voltage and the latter may be either incidental or intentional. Lamps for

21

219-788 0-67—
*.02

Figure 5-7. Variations (RUCS) of chromaticity in service for blue filters.

This figure is analogous to figure 5-6. In the case of blue signal filter ware, the variations caused by change of light source color temperature and those caused by change in the thickness of the filter tend in nearly the same direction, with the resuh that the entire range of colors actually used is compressed within a relatively narrow belt across the region defined by the basic chromaticity definitions. For this reason it is not practicable to show as many hnes as in the case of green.

different applications are designed for different

periods of life and this means different filament

temperatures. Lamps now in use for signal lighting vary from about 2300 °K to about 3100 °K in color temperature at designed voltage. This varia- tion should be compensated by a proper selection

of colorant density in the filtering element, and in selecting the limit standards it is necessary to take

account of the color temperature of the lamps to be

used with the ware to be controlled.

It is difficult to estimate the variations in supply

voltage for signal light installations in general but one survey of traffic signal fights showed that the

lamps in the installations studied were operating at a minimum of 100 volts and a maximum of 130

volts. This could cause color temperatures vary- ing through an interval of 200 °K. The largest color

temperature variations are those caused by varia- tions in lamp current which are made intentionally

in order to adapt the intensity of the signal to prevaifing conditions. Approach and runway fights

designed for use at airports under conditions vary- ing from daytime fog to clear night darkness are

designed to have their lamp filaments vary from

3100 °K at maximum intensity to 1500 °K at minimum intensity.

While it is not feasible in most signal-light applications to make provision for varying the intensity

of the lights to compensate for the difference be- tween fog and clear air, it would be feasible in some

cases to make allowance for the variation between

night and day requirements. With the increasing

intensities required for traffic signals on high-speed

roads, such compensation may soon be imperative

for traffic signals. The increasing intensities used

for vehicular signals make them glaring at night

and it has been proposed to cut these intensities when headlights are used. Tests made in connection with the development of approach and runway

lighting for airports have shown that approximately

1000 times the intensity is needed in the daytime

to obtain the conspicuity equivalent to that considered necessary at night with the same atmospheric

transmissivity.

Figures 5-5 to 5-9 show the effect of light-source

color temperature on the chromaticity of the signal

22
tor glasses of different colorant "densities." For

green glasses, figures 5-5 and 5-6 also show the

loci of constant colorant "densities." These curves and those for constant color temperature follow

quite different paths so that it is feasible to show

both sets on the same diagram as functions of :r" and y" . Similar curves are shown for blue glasses

in figure 5-7, but these may be found somewhat confusing because in this case the two types of loci

lie much closer to each other, the change of

chromaticity with source color temperature and with

colorant density being similar. This similarity of

variation is shown even more clearly in figure 5-8

which shows the corresponding variations of chromaticity for lunar white and blue white filters. In

figure 5-9, however, we have plotted the change of

the y" coordinate as a function of the color temperature of the source since the chromaticity of red and

yellow filters can be represented to a good approximation by a single parameter.

To simplify the discussion the term "density"

has been given a generalized meaning above. In the case of blue glass the change of "density" can

be made either by changing the concentration of the

colorant or by changing the thickness. In the case

of green glass the change in concentration is liable

to have undesirable chemical effects and the change

of "density" must be considered as referring to changes of thickness primarily. With red and

yellow glass of the selenium, sharp cutoff type a change of thickness has little effect upon the chromaticity. For these glasses the chromaticity is largely determined by the wavelength at which the

cutoff occurs. This is to a considerable extent de- termined by the heat treatment given the glass. In the case of the blue and green glasses we have

23
Present Practice

RESTRICTED

REO

^

Figure 5-9. Variation {RUCS) of chromaticity in service for

red and yellow filters.

In this figure the value of /' has been used as an index of redness and yellowness. Since all these chromaticities lie so close to the spectrum locus that the distance between them and the spectrum locus is not significant, it is feasible to represent the chromaticities by a value for y alone. Limits shown by vertical arrows are for specifications in use prior to the adoption of the U.S. Standard. The chromaticity limits of the Standard are shown by horizontal lines as labeled.

-.50

Figure 5-10. Effect of physical temperature on spectral transmittance.

Spectral transmittances of six red and yellow selenium glass filters as measured

by Leberknight and Stone [1955] compared with the spectral transmittance of one of them at four different physical temperatures. The filters are identified by giving their approximate transmittances at the temperature nearest room temperature.

SAME FILTER T= 0.325 AT 270*0

T= 0.421 AT ISQ-'C

T= 0.509 AT 90" C

T= 0.567 AT 23.9=CT= TRANSMITTANCE

FOR 2654=

SOURCE

ROOM TEMP

T = 0.567 AT 23,9'C T= 0457 AT 23.9°C

0.348 AT 23.9-C 0253 AT 25.S°C

0.188 AT 26.6° C

0.126 AT ZS.S-C

420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 WAVELENGTH, ( m /i )

24
Figure 5-11. Effect of physical temperature on the chromaticity for selenium filters.

(a) These curves show how ihe chromalicity of the Iransmitted hghl, as measured by the y coordinate, varies with the physical temperature. Six of the curves are for the same

filters shown in figure 5-10 and are identified by their transmittances at room temperature as shown in that figure; two are from measurements made by K. S. Gibson [1916]: and one is from the researches of Holland and Turner [1941].

(bl These curves are based on the same red and yellow filters shown in the upper curves (a), but y (RUCS) is used in place of y (CLE.) as the index of redness.

used thickness as a parameter of colorant "density"

and in the case of the red and yellow glasses we

have selected filters of different cutoff as the equivalent of different "densities" in the other cases. From figures 5-5, 5-7 and 5-9, it is evident that there must be a correlation between the varia-

|- tions of colorant "density" required for a practical

manufacturing tolerance, the changes of source

color temperature required by the conditions of

service, and the variation of chromaticity allowed

by the basic chromaticity definitions. Especially

in the green and the yellow it is necessary to consider the manufacturing and service limitations in adopting the definitions and in all cases it is necessary to consider them in adopting limits for the control of production.

5.5. Effect of Filter Temperature

Another source of variation which must te con- sidered is the change in the chromaticity of the sig- nal resulting from changes in the temperature of the

filtering element. This effect is most marked in the case of selenium glassware. To understand

the nature of the change in transmittance with the

temperature of the glass, it is well to compare the curves of spectral transmittance for different glasses

at different temperatures shown in figure 5-10.

These curves, which are due to Leberknight and

Stone [1955], show six glasses at room temperature

and one of these glasses at three additional temperatures. There is a considerable similarity between

the changes from glass to glass and those from

25
-4 -.3 -.2 -.1 X" 0

IM 1 1

1 1 I 1 11

1 11 1

1

1 '

I 1

1 1 - RECTANGULAR UNIFORM CHROMATICITY

SCALE DIAGRAM " '

1

1 -4S0

; r

'- 1

/

w 0 —

; \V\

:

1 cl -

: \ \^ 30 - X\

"n'

\ \ \

^ \

^

* \

I

1

Figure 5-12. Contours of maximum transmittance.

The curves are contours of maximum possible transmittance for assumed source

as computed by D. L. MacAdam [1935]. Source is at 2850 TC. These curves were computed by assuming 100 percent transmittance throughout one or two sections of the spectrum and zero transmittance for all other wavelengths. The original diagram

in C.I.E. coordinates has been transformed to RUCS coordinates by Plaza.

temperature to temperature. In both cases the curves differ from others of the same set in the

wavelength region in which the cutoff occurs, and

the effect of increasing the temperature of the fiher

is much the same as that of substituting a somewhat

redder fiker except that heating reduces the trans- mittance as well as moves the cutoff. Figures 5-lla and 5-llb show the change in redness as measured by the y (C.I.E.) and y"

(RUCS) coordinates for the chromaticities of the

six filters of figure 5-10 as a function of physical

temperature. The light source is at 2854 °K. These figures include, in addition to the filters measured by Leberknight and Stone, two measured by

K. S. Gibson [1916] and one measured by Holland

and Turner [1941]. The filters are all selenium red

glasses, which indicates that they are all sharp

cutoff filters. Although some of the measurements

used for these curves were published as early as 1916 and some as late as 1955, although some were made in this country and some in Great Britain,

and although the glass of three different glassmakers was measured, the results are in substantial agree- ment. Glass of other colors changes slightly with

temperature but not sufficiently to constitute a problem.

-0.4 -0.3 -O.Z -0.1 0.0 +0.1

4M^, ®

SOO

®

0

(lo/^

vAsJo

J 5'0

I 550 —

450/

440/

®

Vmu, I

120

030 560

.40

iFsTo"

510

!80\

\

r

!850'J ^

-0,4 -0.3 -O.Z -0.1 0-0

Figure 5-13. Transmittance obtainable with blue and green

glasses. The smooth curves which fan out from the point 2850 °K are the loci resulting from

changing the thickness with the same type of filter. The irregular lines are contours

of equal transmittance for the filters included. The diagram gives some indication

of the maximum transmittance it is reasonable to expect for different chromaticities, but service glassware will have somewhat lower transmittances for the same chromaticities because of losses incidental to the designs. Types of glasses are shown in the

following table by designation in use at the time the glasses were received at the Bureau.

Types of Glasses Corning Glass Works Designations

1 G 401 CZ 7 Combination

2/3 thickness G 403 ED

2 G 408 N 1/3 thickness 552 PY

3 G 408 F 8 Combination

1/3 thickness G 403 ED

4 G 406 AY 2/3 thickness 552 PY

5 G 40 D 9 552 PY

6 G 403 ED 10 557

5.6. Relation of Transmittance to Chromaticity

AU those variations which affect the chromaticity

of a signal light also affect the transmittance of the

filtering element and so increase or decrease the

intensity of the signal.'^' There are, moreover,

limits to the transmittance which it is possible to obtain for any given degree of saturation in a se- lected hue. This is evident from the fact that the

selective coloration of the light transmitted by the

filtering element is obtained by absorbing the undesirable wavelengths. These limits have been computed by D. L. MacAdam [1935].2i One of his figFor similar information on glass made by a different manufacturer, see Holmes,

[1937] and [1946].

2° It has now been found, as was long suspected, that the luminous intensity of a colored signal light computed from the transmittance of the filter is not an accurate measure of the relative brilliance of the light as seen at long range. (See Middleton

[1957] and Dressier [1953].)

2* Later MacAdam [1950] extended his investigation of this problem to the case of the maximum attainable luminous efficiencies with any possible sources. He found

these to be monochromatic sources or pairs of such sources in every case.

26
t3

-4

a

o /

• -NATIONAL STANDARD FILTERS

• -AAR STANDARD FILTERS o - LEBERKNIGHT AND STONE FILTERS AT ROOM TEMPERATURE

o /

L-i\>^ rii_i cno

o /

•o

V

•/

0 .1 .2 .3 4 .5 .6 7 .8 .9 1.0 TRANSMITTANCE

Figure 5-14. Maximum and practical transmittance for red and yellow filters. The continuous line represents the theoretical limit of transmittance (T) as a function of redness as indicated by y. Allowance has been made for surface losses of 0.08. The

points marked by the several symbols represent actual filters of selenium glass.

ures, transformed into the RUCS system by Plaza,^^

is shown in figure 5-12. In practice the available

transmittance is always somewhat lower than the

theoretical maximum. Figure 5-13 shows the

transmittances obtainable in different parts of the

green and blue areas through the use of different

types of glass in different thicknesses. These

curves were computed for some of the same filters selected for figure 5-1. Figure 5-14 shows the relation of the transmit- tance to the y" coordinate, representing redness,

for typical selenium glass filters transmitting light

from sources at 2854 °K color temperature. The

closer y" approaches —0.5, the redder the chromaticity represented. The theoretical limit for the

maximum transmittance possible for each value of

y" with a light source at this color temperature is

" Lorenzo Plaza, of the Institute of Optics of Madrid, assisting as a guest associate

at the National Bureau of Standards. ®It is customary to consider that the transmittance determined spectrophotometrically can be used to determine range and apparent brightness (brilliance) of a signal

light, notwithstanding the experience of signal-light engineers that red lights often ap- peared brighter than this assumption warranted. Middleton and Gottfried [1957] have made tests confirming the qualitative observations. A corresponding effect with large sources has been found by several investigators. For a review see Dressier

[1953], Another practical consideration is the differential transmittance of smoke and

haze. See Breckenridge [1931].

also shown on this diagram. The values for the

typical filters are shown by open and solid circles. The reader can readily draw a curve to rep- resent the maximum transmittances obtainable in practice for any desired redness. In using this curve as a basis for estimating the characteristics

of glassware, a further allowance must be made

for the differences between polished filters and

practical filter ware, that is, for losses due to scat- tered light and additional internal reflections. This

allowance may be more closely estimated when the

transmittances of similar glassware of a somewhat

different color are known, in which case it is only

necessary to multiply the transmittance for the

known ware by the ratio of the transmittances given

by the curve for the desired and the known chromaticities. The ratios obtainable from this curve can be very useful in revising transmittance re- quirements to allow for changes in chromaticity

requirements.

Figure 5-15 shows the variation of transmittance

with the color temperature of the hght source for typical filters of the more commonly used signal

colors.

27
Figure 5-15. Variation of transmittance with color temperature of light source.

The figure shows the relationship between the transmittance (T) and the color temperature of the Hght source for five of the standard filters included in the U.S. Standard N.B.S.

[1964]. It should be noted that while the blue, green, and lunar-white filters increase in transmittance with the increase in color temperature of the source, the red and yellow filters decrease.

1.0 p—I

1

1

1

1 1

1

I

I

I

1

I

I Z

Figure 5-16. Effect of physical temperature on transmittance of selenium filters. The curves show the decrease in transmittance as the physical temperature of the

filter increases. Three different researches are included. They are distinguished by

the names of the investigators: Gibson [1916]; Holland and Turner [1941]; and Leberknight and Stone [1955]. The numbering of the curves is the same as in figures 5-10 and 5-11. The two sets of measurements made in the United States appear consistent, notwithstanding that Gibson made his measurements on Corning glass filters and

Leberknight and Stone made theirs on Kopp glass filters. The results by Holland and

Turner indicate a more rapid change of transmittance with temperature than was found

by either of the other researches.

0.0 0 1

I 1 I I

I I I 1 1 1 1 1 1

-200 -100 0 +100 +200 -t-300 + 400

TEMPERATURE °C 28
Figure 5-17. Chromaticities of green plastic filters {CLE.).

These curves correspond to those shown in figure 5-5. The more nearly horizontal curves flabeled with color temperatures) show the variations of chromaticity with change of thickness for the filter used with a light source of the color temperature indicated for each curve. The more nearly vertical curves Oabeled with the thickness of the filter) show

variations of chromaticity for changes of light source color temperature. The straight lines are chromaticity boundaries from the U.S. Standard. N.B.S. [1964].

5.7. Effect of Filter Temperature on

Transmittance

The changes in the chromaticity of selenium

filters with the temperature of the glass are accompanied by changes in their transmittances as shown

in figure 5-16. The glasses for which values are given in these curves are the same filters for which

the chromaticity characteristics are given in fig- ures 5-lla and 5-llb. Some other types of filters show changes in transmittance with change ot

filter temperature but such changes are important

only if the change of temperature amounts to several

hundred degrees and the requirements are exacting.

5.8. Plastic Filters

Filters of any color required for signal lighting can be made of plastic material and spectral trans- mittance data are available for a few red, yellow,

and green plastic filters. Unless care is taken to choose color-fast dyes, however, blue and green

filters are liable to fade in service. Figures 5-17

and 5-18 show the chromaticities for hght sources

of color temperatures ranging from 1500 to 3250 °K

for a plastic filter which has been found to have a resistance to fading that was adjudged satisfactory

for use in a beacon cover. This same figure also shows the variation in chromaticity with thickness

for this filter for all these light sources. Red and yeUow plastic filters are available with

sharp cutoffs that give as high purity of color as is available in glass. Yellow filters are also available

which have chromaticities very similar to those of

carbon-yellow glass although the spectral transmit- tance characteristics are somewhat different. Some of these red and yeUow filters are fluorescent

which interferes with making reliable spectrophotometric measurements for them on some of the com- mon types of spectrophotometers.

5.9. Gaseous Discharge and Fluorescent

Lamps

Gaseous discharge lamps offer the possibihty of

obtaining signal-light colors without the use of

filters although such lamps may also be used with

filters to improve their chromaticity. Such lamps

29
-.04 -.02 0 X)2 .04 .08 .22

.20

.08

.06

.04

10 mm

9 mm

8 mm

-7 mm —

^6 mm

V^Smm

\

V\4mn

m

3250°k\ 285^ 2mm

2365°K'

1904 °K \

\

\lmm

I500°K

Figure 5-18. Chromaticities of green plastic filters {RUCS).

This figure is a transformation intn RUCS coordinates of the curves shown in figure 5-17.

are sometimes assumed to have characteristic

colors that are dependable. This is not an entirely safe assumption. The chromaticity of neon lamps

varies with the pressure as indicated in figures

5-19a and 5-19b. When the lamps are new the

pressures used in them are not normally low enough

to give chromaticities paler than the limit allowed

in the U.S. Standard [NBS 1964] and CLE. Recommendations [195>5]. In use, however, the neon

lamps decrease in pressure and this may cause the

chromaticity to become paler than the specified

limit. In the case of discharge lamps which contain

mercury, there may be a large change of chromaticity as the ambient temperature varies. At

lower temperatures the vapor pressure of the mercury is reduced with consequent decrease in the

intensity of the mercury spectrum while the spectral distribution of the light from any rare gases in the lamp is little affected by the change in temperature.

Similar difficulties have been experienced with

sodium lamps which contain neon. At low temperatures the sodium is not volatilized and the neon

gas which is used to start the discharge contributes

most of the light. At such temperatures, the light

is nearly red enough to meet the definition of avia- tion red. When the lamp is fuUy heated the color

is slightly redder than the sodium D lines. One

lamp measured at the National Bureau of Standards was found to have a chromaticity approximating

;«; = 0.578, y = 0.422 U" = + 0.075, y" =-0.160)

which would make it an acceptable yellow under any

of the signal-color specifications in common use in the United States. Fluorescent lamps are seldom used as signal

lamps because of their low luminance. Figure

5-20, however, shows the chromaticities for a number of common types of fluorescent lamps. To

distinguish those lamps which lie within the boundaries of the U.S. Standard [NBS 1964] the boundaries for the signal colors are also shown.

30
.336

.334

Y

.33 2

.330

.328

.32 6

.324

(a) RU :s

CLE

(b)

-.310

-320

y"

-.330

V-.340

6 7" PRESSURE

8 0 12 min.

Figure 5-19. Chromaticity of neon lamps.

The curve shows the variation in the chromaticity in a group of neon lamps which had been filled with different pressures by the manufacturer. There was no way to determine how accurately the pressures had been measured. Inaccuracies in adjusting the pressures to the nominal values indicated may account for the irregularities in the values plotted. The pressure is expressed in mm of Hg. (a) Redness indicated by V' iRL'CS). )b) Redness indicated by y (C.I.E.).

Figure 5-20. Chromaticities offluorescent lamps (RUCS).

The circles show chromaticities obtainable with the use of commercially available fluorescent lamps, plotted in RUCS coordinates. Additional chromaticities are obtain- able with mercury, neon, and the sign-lighting type of lamps. Those shown in the

figure along the Planckian locus are known as daylight, cool-white, white, and warmwhite. Those with more saturated colors are blue, deep-blue (it is nearer the spectrum

locus), green, gold, pink, and red.

•0.3

y"

*0.2

-0.2

-0.3

-0.5

RUCS DIAGRAM

EQUAL ENERGY AT X'-O.y'.Q

C IE ILLUMINANT "A" AT 2854'

I COLOR

I TEMPERATURE- - 20 000"!

7 - 10 000"

I- 6 500" 570 4 800°' - 3 600°

I- 2 854° 580 ^ I- 2 360°

- 1 900= -590

31
6. Control of Signal Colors

6.1. Selection of Standards 6.3. Inspection Testing

In addition to making sure that specifications and

regulations controlling signal-light colors describe

colors which are recognizable and which may be

produced in service with practicable means, it is also essential to be certain that the requirements be

such that they may be applied most effectively in the various situations for which they are intended.

Usually the control of signal-light colors involves

the use of chromaticity standards which are rep- resented by filters in combination with stipulated

light sources.

6.2. Types of Color Standards

Most of the signal-light applications have so much

in common that it is possible to have a single set of

filters which, with a light source of 2854 °K, can be

used as national standards for most purposes. A

set of 18 such national standards is now in process

of adoption. Fourteen suitable glasses are avail- able among the thirty filters which the Signal Section of the Association of American Railroads

adopted between 1931 and 1935.24 The chromaticity

characteristics for the other four have been ap- proximately defined, but glasses have not yet been

selected. All of the national standards have been

checked in relation to the basic chromaticity defi- nitions and found satisfactory.

The national standards have been selected to provide for the colors that are at present required

by more than one organization. There are a few

other applications such as those for which the

railroads use lunar white, and perhaps the walk

signals used at pedestrian crossings, which did

not appear to be sufficiently general to warrant

the establishment of national standards. Provision is made in the U.S. Standard for the Colors

of Signal Lights [1964] for organization standards

to meet these needs. Organization standards must

be checked against the basic chromaticity defini- tions to make sure that the signals will remain

within these definitions under all the conditions of

use.Neither the national standard filters nor the

filters of organization standards are available for inspection purposes. Duplicates are required for such purposes and the U.S. Standard provides for such duplicates by establishing tolerances for them.

Even the duplicates are likely to be considered too valuable for routine operations, and manufacturers

and laboratories will generally make their own dupli- cates for use as working standards. No standard

tolerances have been provided for these since they

are solely the responsibility of the manufacturers.

See Association of American Railroads [1953]. ^ For the status of national standards see McNish 1958.

The final step required to assure that the signal

colors conform to the selected standards is the

inspection testing of the equipment. This inspection may be made on completely assembled equipment, but more often it is made on the filtering

element, for example, on a piece of glassware or

a plastic cover. In either case the problem is to compare the light from the unit, or the light trans- mitted by its filter, with light from a source of known

color transmitted through a duplicate standard

and to decide from that comparison that the light

from the unit is, or is not, within the specified

chromaticity Hmits.

Several researches have been carried out to develop a photoelectric colorimeter for this purpose.

The objective was to build an instrument which

would give a direct reading of the chromaticity

coordinates for the light intercepted by the meter. Few, if any, colorimetric laboratories have such

equipment today. At this time, we know of no

manufacturer who has an instrument of this type

that is suitable for use on routine inspections. It would appear practicable to design photoelectric

equipment that could be adjusted to distinguish between acceptable and unacceptable ware made

with a predetermined colorant, but such equipment

is not yet available.

An instrument has recently become available

which measures small color differences photoelectrically and which is proving useful for some

inspection purposes. Unless such instrumentation

is available, visual comparisons must still be relied

upon.

Since visual comparisons must be used for carry- ing out inspection tests, it is important to take into

consideration the types of comparison which can

be made by the usual inspector. These comparisons are limited to discriminations of hue, saturation, and brightness. In view of this, it is essential

to draft specifications so that only direct compari

sons of hue and saturation are required in testing

equipment for conformity with the chromaticity

requirements. Specifications worded in this manner are most easily correlated with basic

chromaticity definitions if the boundaries of these

definitions are lines of constant hue and saturation.

The practical problem of inspection is greatly

simplified if there is a requirement in the specification that the chromaticity characteristics be

similar to those of a standard filter. This type of

requirement is discussed in sec. 8.

6.4. Laws and Regulations

A somewhat distinct problem in the control of

signal-fight colors arises in connection with laws and

regulations. These are usually written in terms of

32
color designations that are defined in technical

Standards. To be suitable for such legal citation

the Standard must define the signal as observed

by the user, in order to be impartial, and this definition must be stated with sufficient precision

to avoid any significant uncertainty of interpretation.

As has been mentioned in sec. 2.5, the application

of laws and regulations to the performance of signallight units involves the same procedures as the in- spection of ware on procurement. Whether the

enforcement is by an inspector on the staff of

the regulatory authorities, or by an inspector in the

employ of the manufacturer, his basis for ac- ceptance or rejection will be similar to that in a

procurement transaction, that is, it will be a comparison of hght from a test piece with hght of known

chromaticity. For this reason the same principles

apply to the choice of boundaries for chromaticity

definitions underlying laws and regulations as in setting up procurement specifications. In this case, however, there is more likelihood that no standard

having chromaticity characteristics "^^ similar with

respect to the test item will be available, and the

visual comparison may have to be made with a greater chromaticity difference than is commonly

encountered in the inspection of purchased filter ware. This makes it even more necessary that the type of judgment required be one of hue or

saturation.

7. Chromaticity Boundaries

7.1. Selection of Boundaries for Chromaticity

Definitions

I Chromaticity boundaries are involved in two

I ways in the control of signal-light colors, in the

basic chromaticity definitions and in the tolerances

for the duplication of standard filters. The problems are essentially different in these two applications. The basic definitions will be considered

first. As in the case of signal color specifications as a whole, it is essential that the chromaticity definitions

in particular be so designed that they not only de- scribe chromaticities that can be dependably recog- nized and can be practicably produced, but also that the boundaries differentiate between acceptable

and unacceptable chromaticities in ways that afford the most prospect of positive differentiation in practice. These definitions are used mainly for three purposes, namely: (1) the control of color

standards: (2) the inspection of filter ware; and

(3) the application of regulations.

For the selection of standards, any positively

defined Unes in a coordinate system that can be

computed from spectral transmittances will suffice because standard filters are regularly tested, either

by spectrophotometry or differential colorimetry,

both of which methods of measurement provide the

chromaticity coordinates of the filter source

combination.

The inspection of ware and the enforcement of

regulations, on the other hand, usually involve

visual comparisons and the only such chromaticity

judgments that can be directly made are those of

, hue and saturation. It follows that reliable judg-

\ ments with reference to whether a difference indi- cates a chromaticity inside, or outside, of the

chromaticity definitions can only be made if the

definition boundary is a line of constant hue or constant saturation.

7.2. Constant Hue Boundaries

It was stated in sec. 3.4 that the CLE. and RUCS

diagrams are color-mixture diagrams with pure

spectrum colors situated along the curved outside

boundary and white located somewhere in the center. The selection of the white point, which is technically known as the heterogeneous stimulus,

should properly be determined by the conditions of

observation. In developing the RUCS diagram, the

equal-energy point, that is, the point representing a mixture of all visible wavelengths in equal parts, was

selected as the white point because it is centrally

located among the points usually advocated as desirable white points and also because it is easily

defined. For signal-lighting purposes it might be

preferable to use a point corresponding more closely

to the color temperature of the lights which are most

commonly seen at night, but the difference between

the equal energy point and the point representing

2854 °K, for example, produces no difficulties that are serious so far as present requirements are concerned.

It was pointed out in sec. 3.4 that in any mixture

diagram all the points lying between two points on

a straight line through them represent chromaticities that may be produced by adding light of the

chromaticities represented by the two selected

points. It foUows, therefore, that all the points on a line from the origin in the RUCS diagram

through any point on the spectrum locus represent

chromaticities which may be produced by adding

white light to the spectrum color represented by

the point on the spectrum locus. Such a line is a line of constant dominant wavelength and one would expect it to be a line of constant hue. This

is approximately true.

^ See NBS Hanhbook 95. p. 2, sec. 2.11 [1964] and chapter 8 of this Monograph.

33
Newhall, Nickerson, and Judd [1943] have deter- mined lines of constant hue from the spacing of

elements in the Munsell uniform color space and

D. L. MacAdam [1950, 51] has made a direct deter- mination. While both of these determinations

show some curvature, the departure from straight

lines is not greater than the uncertainty of applying

determinations made with surface colors to the case of signal lights. In view of this and the lack

of standard definitions for constant-hue lines,

there would seem to be little advantage in complicating a specification to allow for the difference

between the straight and the curved lines.

7.3. Constant Saturation Boundaries

On a uniform chromaticity diagram which

faithfully represented chromaticity differences for given conditions of observation, the loci of constant

saturation would be circles having the white point, or heterogeneous stimulus, as center. If circular arcs on the RUCS diagram were adopted as saturation boundaries for the basic chromaticity defini- tions, the corresponding boundaries on the CLE.

system would be ellipses. Since the CLE. system

is the only one recognized as standard at the present

time, and ellipses are too difficult to apply as criteria

in practical problems, it is necessary to use the

chords of elliptical arcs to express boundaries that approximate boundaries of constant saturation on

the CLE. diagram. Transformed to the RUCS,

these lines become chords of circular arcs concentric with the origin.

-0.4 -0.3 -0.2 -0.1 0 *0.i x"

•0.1

0

-0.2

-0.3

-0.4

-0.5

1 1 1 1 11 M 1 1 1 r 1 1

1 I r 1 ••rrrTTTTV'- 1 1 1

1

1 1 1

1 j 00 , „\ 52 0 ;

r GREEN

-530 : r 1 i-540 : PI

I 4

- 42

460 ^p***

30 y

0 v7

BLUE K

4

r-5

COLOR I TEMPERATURE.- - ZOOOO^K - - 10000° - - 6 500° I

- 3 600° ; ^ 2 854° - 80 : - 2 560° -

:

60 \ \ c'

-5

r

- 1 900° ; 90 '. - 1 500° I

L

1-600 j

- 1 000° RU

EQUAL

C.l£ 1L

CS DIAG

ENERGY AT LUMINflNT "A"

RAM

K".0, Y".0

6T 2854° K

c

i-e

i-6

10 :

20 :

-

. , . . 1 . , 1 .

a\

1

i-630 -

i- 1 : i-640

i- 1 t , 1 , , , ,-

Figure 7-1. Comparison of rational boundaries with those of

the U.S. Standard.

Figure 7—1 shows a comparison of boundaries for signal light colors as defined in the CLE. Recommendations [1955] and the U.S. Standard, N.B.S. [1964] with a system of

radial and chord boundaries on the RUCS diagram. The construction of the diagram

is explained in the text. It is intended to show that there is a general approximation between the spacing of chromaticities in the RUCS diagram and that to i?'' expected

if the limits of the CLE. Recommendations and U.S. Standard are accepted as repre- senting optima] signal colors. The absence of signal colors in the lower left hand

quadrant is explained by the lack of filters giving such chromaticities.

7.4. Comparison of Rational and Adopted

Boundaries

Figure 7-1 shows a comparison of the boundaries

of signal-light colors as defined in the CLE. Recommendations [1955] and the U.S. Standard [NBS

1964] with a system of radial and chord boundaries on the RUCS diagram. The radii and chords of

this figure may be considered as lines approximating

constant hue and constant saturation, as described

in the discussions of sees. 7.2 and 7.3. In addition,

four of these radii have been evenly spaced, since

it is plausible to assume that optimum boundaries

for signal colors can be constructed from such lines

if the RUCS diagram is uniform in its chromaticity

spacing. To construct this diagram, line {A, A ') was drawn from the red terminus of the spectrum

locus at x" = + 0.075, y" = — 0.500 through the origin

and prolonged to intersect the spectrum locus in the upper left quadrant. This is a significant

line on the RUCS diagram because it is both a

line of constant dominant wavelength and close to a locus of confusion for the protanopic type of color- blindness. The angle between (AA ) and the — X"-a.xis has been bisected to obtain {BB'), and

this line has been reflected about AA' to obtain

(CC). Designating the origin as 0, this makes

AX"0B= ABOA = Z. AOC. The chords were drawn

through the points in which an arc centered at the origin intersected the hue boundaries {—X"0,

BO, AO, and CO). The radius of this arc was made

equal to the mean distance from the origin to the

four inner corners of the CLE. blue and green

regions.

7.5. Boundaries for Blue and Green

A comparison of these rationally constructed

boundaries (sec. 7.4 and figure 7-1) with those

which have been adopted on the basis of research

and experience shows that in the blue and green

regions there is a considerable degree of corre- spondence between the rational and the recom- mended boundaries. The coincidence of the blue

boundary recommended for green with the line through the red terminus may be accidental. The

blue boundary of green was, however, intentionally

displaced towards the blue to make it possible

to use green signals that can be differentiated

from red and yellow signals by red-green colorblind

observers. The similarity of the rational divisions

to the chromaticity boundaries of the CLE. Recom34
mendations and the U.S. standard in the blue and

green regions favors the use of a uniform-chromaticity-scale diagram, such as the RUCS diagram, for studying the chromaticities of such signals.

7.6. Boundaries for Red and Yellow

!

' Returning to figure 7-1, it is seen that the boundaries for yellow are displaced clockwise and the

boundaries for red counterclockwise from the

sectors opposite the blue and green. This places

both the red and yellow in the sector opposite the

gap between blue and green. The displacement of

I the yellow is reasonable in view of the fact that the

spectrum locus in the region opposite the blue,

which should have been selected on the basis of

equal arcs, is too close to the white point to be

acceptable as a color distinct from white. The

sector opposite the green, on the other hand, is characterized by much lower transmittances and

saturations than are obtainable in the long wavelength region.

From the fact that both recommended red and

recommended yellow lie in the same sector, it appears that the discrimination is not made on the

basis of hue angle alone, and that the linear distance

of separation on the RUCS diagram rather than the

angle may be the basis for the recognition of these

colors. Since the acceptable regions here lie so

close to the spectrum locus, we may take distance

along the hne a:" = 4-0.075 as a possible basis for assigning boundaries to red and yellow. In figure

7-1 the rational boundaries in the yellow-red quadrant have been drawn by dividing the spectrum locus

in this quadrant into nine equal parts. Two-ninths

have been allotted to yellow and two-ninths to each

of the unused sectors on both sides of yellow, leaving

three-ninths for red. This extra allotment to red

may be, to some extent, justified by the fact that the

longest wavelength portion of the red allotment is not serviceable for some red signals because it cannot be produced wth a transmittance sufficient

for use where long-range visibihty is important. As

in the blue and green regions, the similarity of the

rational divisions of the RUCS diagram to the chroi maticity boundaries of the CLE. 1955 RecommendaI tions is sufficient to indicate that the RUCS diagram

^ is an advantageous one for studying the chromaticities of light signals.

The unused sector along the spectrum locus from

the yellow hmit of green to the Z"-axis is approximately equal to the unused sector between the

1 ^'''-axis and the green limit of yellow. This makes

the total gap between green and yellow twice that

j between yellow and red. As we have just observed,

( however, this long gap is required because the spec- trum locus in this region is so close to the Planckian

locus that both can not be used to represent distinct

colors, and the chromaticities of the Planckian locus

are those of the lights regularly used as white signals.

Having considered the hue limits of red and yellow on the basis of their extent along the spectrum

locus, it remains to complete the boundaries of these

colors. To extend the paler hue limits towards the

white point is to include chromaticities that are paler than some chromaticities these hmits exclude

at the spectrum locus. It seems desirable, therefore, that the boundaries from these points should

foUow loci of constant saturation. A possible way

of completing each of these areas in a manner some- what analogous in form to that of the green and blue

areas would be to follow arcs having their centers

at the origin until these arcs intersect radii from the

origin through the second hue hmits. This would

cause the yellow area to include the chromaticities

of lamps incandescing at low voltage. From the

standpoint of producibihty there is no need to in- clude these chromaticities in the yellow region.

For any system which includes a white signal, it is advantageous to hmit the yellow to a narrow band

along the spectrum locus. In the case of red hghts, an area defined by an arc of minimum saturation

through the pale hue hmit and a radius through the

reddest chromaticity visible would include chromaticities which, according to Hill's results shown in figure 4-9, are much less dependable than those

closer to the spectrum locus. On the other hand,

it is quite practical to close the red and yellow areas with boundaries approximately parallel to the spec- trum locus which is the direction of the natural

chromaticity variations of the glasses used. The

CLE. green boundary of yellow and yellow boundary of red do extend somewhat inward on hues

directed towards the white area but since this is not necessary to secure adequate transmittances, the U.S. Standard uses hues approximating constant

saturation for these boundaries.

7.7. Boundaries for White

White lights are commonly lights without filters, so that the chromaticity definitions for white lights

usually follow the Planckian, or blackbody, locus. This suggests a definition for white in the form

of a maximum permissible departure from the

blackbody locus. It appears feasible to accomplish

this in the CLE. system by a tolerance in y alone. When such a tolerance is transformed into RUCS

coordinates, however, it is evident that the boundaries are far from uniform tolerances. The U.S. Standard [NBS 1964] has tolerances expressed as functions of x and y, and it is a region of fairly

uniform width in RUCS which does not actually

overlap the yellow region.

There is another type of white light which re- quires pale blue filters if the source is an incandescent lamp. These are called "lunar white"

or "blue white" lights. Since they require filters,

it is frequently desired to compute their chromaticities from spectrophotometric data. Curved

boundaries are unsatisfactory in this case because

of the undue amount of computing which may be

required to find out whether the chromaticity is

35
inside or outside the chromaticity definition. In

this region, also, the Planckian locus is much less curved than in the region of the unfiltered white

lights. For these reasons, it is both desirable and

feasible in this region to use a narrow quadrilateral as a boundary.

The U.S. Standard for the Colors of Signal

Lights [NBS 1964] uses straight boundaries for lunar white and blue white since these require

filters and curved boundaries based upon the blackbody locus for beacon white and variable white.

The International Commission on Illumination

[1955] and the International Civil Aviation Organization [1964] solve the problem by using complex

straight-sided figures allowing rather large departures from blackbody chromaticities in some regions

in order not to limit the departure too severely in other regions. The problems encountered in dis- tinguishing yellow from white will be discussed in sec. 10, "Use of Signal Light Colors."

7.8. Mathematical Simplicity

In the past, specification writers have shown a preference for limits that were easily expressed in the CLE. system, as for example, x = constant. This is quite reasonable as long as no reason for prefering other limits is known, but the consideration of mathematical simplicity seems entitled to

little weight in comparison with the importance of

having the limits follow lines representing sequences

of colors which an inspector can recognize; that is, the chromaticities on one side of the boundary

should differ from those on the other in some com- mon, recognizable respect. In another sense mathematical simplicity is a sound principle. The direction and location of a boundary having been determined in accordance

with the general principles discussed above, the

equation used to define the boundary should not be expressed with more significant figures than are warranted by the precision with which the facts are known. On the other hand the use of equations

that are not reduced to mathematically simplest

terms may be warranted if by their use it is possible

to make it clear that the equation passes through a

particular point of interest.

7.9. Relation of Chromaticity Boundaries to Primary Limit Standards

In approving primary standards, the laboratory

concerned must consider the use to be made of

these standards. If they are to be used to test duplicates by direct colorimetric comparison

with a visual instrument, the primary standards

should be far enough within the boundaries as to insure that the duplicates in turn can be used by

direct comparison to keep the ware within those

boundaries. Color standards may, of course, be located even further inside the basic chromaticity

definitions if it is deemed advisable to restrict the

chromaticity of the signals controlled by these

standards to something less than the entire range

permitted by the definition.

7.10. Boundaries for Duplicates

Boundaries for tolerances are quite a different * problem from those for basic chromaticity defini- tions. Although duphcate filters are frequently

tested visually, this testing is done with colorimeters designed to give precise values for the color

differences between the filters being compared.

For this reason, it is not important that the boundaries of these tolerances foUow lines of constant hue

and saturation.

It would be logical and quite simple to assign

tolerances for duplicate filters by requiring that the

chromaticity coordinates for the duplicates, ex- pressed in the RUCS system, faU within a fixed

radius of the point representing the chromaticity of

the primary standard. It is not at present practicable to do this, however. The calibration of the

existing colorimeters used for this work is in terms

of CLE. coordinates and their immediate recali- bration in RUCS would be a considerable task. More than this, the duplicates must sometimes be

ground a number of times to bring them within the

tolerances, and the increase in the complexity of

the calculations involved in checking glasses for compliance with circular tolerance areas would not be offset by any advantage to be gained from ex- cluding the corners of rectangular tolerance areas. A. J. Werner" has developed a rational method of

constructing tolerances for duplicates. When the

glass for duplicates is ground to meet a tolerance,

the chromaticities of all the ground filters will be

found to lie on a curve which is approximately

parallel to that generated by the chromaticity of the t primary calculated for various thicknesses (see

fig. 5-4). Werner proposed to make the slope of ; two sides of the tolerance parallelogram parallel to t the tangent to the locus for the primary filter at the i poirrr'representing the primary itself. The spread

between the curves for glasses from different melts, n all of which represent good practice in the reproduc- . tion of the original melt, determines the separation j of the long sides of the tolerance. If the tolerance area lies close to a boundary for a basic chromaticity , definition, its other two sides may be made parallel

to that boundary. Making these sides parallel to ( the boundary may facihtate the inclusion of the , necessary area within the tolerance without en- , croaching too close to the boundary. i

Unpublished memorandum prepared in connection with the revision of Specifica- ( tion 59-39 (now 59-61), Signal Section, Association of American Railroads. For j further information consult Mr. A. J. Werner, Corning Glass Works, Corning, New 1 York. : 36
Figure 7-2. Illustration offilter tolerance.

The diagram shows the relationship of tolerance boundaries to the chromaticity of

the primary filter and the locus of chromaticities for filters made from the same type

of material but differing in thickness, LL'

Such a tolerance area is illustrated in figure 7-2. In this figure LL ' represents the locus of a primary

standard filter. The straight lines represent a toler- ance area constructed in accordance with Werner's

proposal as used for specifications of the Association

of American Railroads and subsequently adopted for

tolerances in the U.S. National Standard fNBS

1964].

Another construction, preferred by the author,

is to make the slope of the ends approximately

perpendicular to the sides when this is feasible. This construction gives the most compact area

practicable. The distance between the ends of the

parallelogram is determined by the accuracy within

which the optimum thickness may be approximated,

which, in turn, is determined by the difficulties of

computing and grinding. The size of the tolerance

required varies from primary to primary according

to the closeness with which melts can be duphcated

and the precision with which thickness may be

adjusted.

Since chromaticity differences are more easily

measured to the accuracy required in duplicating

tolerances than chromaticity coordinates are, it is preferable that the tolerances be expressed in terms

of the diff'erences between the chromaticity of the

primary and that of the duphcate rather than as equations in the coordinates themselves. This,

however, need not prevent the use of tolerances in accordance with the principles outhned above.

8. Requirement for Chromaticity Characteristics Similar to Standard

8.1. Purpose of Requirement

It is evident from the purpose of standard filters that their selection is primarily controlled by the

basic chromaticity definitions, but since it has been

pointed out in sec. 5 that both the chromaticity and

the intensity of a light depend not only on the

colorant in its filter but also on the filter's thickness,

the color temperature of a light source and, in some

cases, the physical temperature of the filter, the

variation of these factors must also be taken into account. In the case of green filters especially, and

to a lesser extent in the case of blue filters, the

chromaticity variations caused by changes in the source are at substantial angles on the diagram with

those caused by changes in thickness, or in colorant

concentration, with the result that the total variation

caused by both covers an area on a chromaticity

diagram. This was illustrated in figures 5-5 to 5-8.

If a change is made in the colorant, the area of

utilized chromaticities will be changed, and even

though the original standards had been carefully

checked to make certain that ware of the same color- ant would stay within the basic chromaticity defini- tions for all the anticipated conditions of use, the ware with the changed colorant might produce sig- nals outside of these definitions. A requirement

to prevent the acceptance of improperly colored ware is an essential part of a dependable specification.

A change of colorant may also make a satisfac- tory inspection impossible. So long as the chromaticity differences in ware are such as can result from

changes in the thickness of the ware, the variations

in chromaticity will be such that they may be repre- sented on a chromaticity diagram by a smooth curve such as the broken curves in figures 8-1 and 8-2.

It is a simple task for an inspector with normal eyesight to compare the chromaticities produced by

the members of a group of filters which vary in this manner with a pair of standard chromaticities

located on the same curve and determine that the

tested chromaticities are within, or outside of, the

acceptable segment of chromaticities. If the color- ant used in the inspected ware is identical with that used for the standard filters, or if it is a satisfactory

equivalent, the curve of chromaticity variation

will pass through the points which represent the

chromaticities of the color standard. If the colorant

used in the inspected ware gives rise to a curve of

chromaticity variation slightly displaced but

parallel to the curve for the color standard, the

inspector is still able to make satisfactory judgment

and this material should be considered a satisfac- tory equivalent for that used in the standard filter.

If, on the other hand, the curve of chromaticity

variation is considerably displaced from that of the

standard filter, for some thicknesses of the ware, it may be impossible for the inspector to arrive at any

satisfactory judgment. A requirement to assure

that the material submitted can be inspected with

the standard is therefore an essential part of a dependable specification. In addition to these two reasons for restricting

the chromaticities more strictly than the basic

chromaticity definitions alone would restrict them,

a third reason arises from the fact that these defini37
-.51-

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.52 CLE. CHROMATICITY DIAGRAM : EQUAL ENERGY AT i J

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Figure 8-1. Similar chromaticity characteristics (CLE.).

The figure shows the limits allowed in the U.S. Standard, N.B.S. [1964] for similar chromaticity characteristics in the case of national green standard No. 7.134 and

national blue standard No. 8.047. The green standard is shown for hght sources of two color temperatures, 1900 "K and 2854 °K, but in the case of the blue only the

limits for 2854 *K are shown because the curves would overlap each other so much as

to produce a confused pattern.

Figure 8-2. Similar chromaticity characteristics (RUCS).

This diagram shows the same data as figure 8-1, transformed into the RUCS system.

It is evident from this figure that the blue is less closely controlled than the green

notwithstanding that in the CLE. diagram the blue and green tolerances appear about

equal.

tions which are intended only to restrict the colors

used to those that may be relied upon under usual

circumstances, necessarily accept a considerable

range of chromaticities. A purchaser who has

been accustomed to receive material of one type

might well feel he has been provided with an inadequate specification if, upon receiving filters of a noticeably different type, he finds he has no grounds

for rejecting the new material. If a producer is able to improve his product, or finds that efficient production requires a change in his colorant, he

should inform his customers and arrange for the

change of product upon the basis of new standard

filters. The requirement that ware have similar chromaticity characteristics to those of the standard filter cited in the specification was developed for the

three reasons outlined above. It is designed to limit the chromaticity characteristics of the product

furnished without introducing unnecessary restric- tions on the physical characteristics, or chemical

constitution, that might add to the cost by restricting

competition or that might retard improvements

which did not change the color characteristics.

8.2. Statement of Requirement

In the U.S. Standard for the Colors of Signal

Lights [NBS 1964] the similarity requirement is

stated as follows:

Similar Chromaticity Characteristics A light-transmitting material has chromaticity characteristics

similar to a standard material for a given light source over a

stated chromaticity range if the chromaticity coordinates, within

that range, of the light from the given source transmitted by any

thickness of the standard material can be duplicated within

stated tolerances by the chromaticity coordinates of the light from the same source transmitted by some thickness of the

subject material. As used in this Standard and in specifications based upon it, the phrase "similar chromaticity characteristics"

will be understood to include the range of illuminants to be used

with the light-transmitting material in service, and the chromaticity range extends from the chromaticity of the applicable pale

limit to that of the second hue limit, or to the chromaticity of such thickness of the subject material as has the minimum

acceptable transmittance, whichever gives the lesser range.

In this Standard the range of illuminants is that specified in Part II, Table II—1; the minimum transmittance is that stated

in Table II—2; and the tolerance is that given in Table II-3.

8.3. EiFect of Requirement

Figures 8—1 and 8—2 illustrate this requirement

by showing the area within which the chromaticity

must remain to qualify green signal ware as con- forming to the definition for similar chromaticity

characteristics with respect to the pale limits indi- cated. The figures show the application of the

definition with respect to a green pale limit for illuminants at 1900 °K and 2854 °K and for a blue

pale limit for the illuminant at 2854 °K. As stated

38
in the definition, the test is apphed only for the

range of illuminants and chromaticities which may

be involved in the use of the ware to be dehvered

in fulfilling the order or orders which are being

inspected.

As already pointed out, the advantages of this requirement are so considerable from the standpoint of both inspection and service as to warrant

the drafting of purchase specifications for signal ware to require ware having chromaticity characteristics similar to those of the pale-limit and

transmittance-standard filters adopted to control

the purchases. If a purchasing agency finds that such a restriction limits the competition unnecessarily, then it is possible to adopt two or more

alternative sets of standard filters. Experience to date suggests that this will rarely, if ever, be necessary. The additional standard filters, if required,

should be obtained from prospective bidders who

are not satisfied with the existing standards. The

requirement for similar chromaticity characteristics

places no burden on manufacturers who have pro- duced acceptable material so long as such manufacturers do not change the chromaticity characteristics

of their ware. When they do desire to make such a change, and when a new bidder offers ware, an

analysis of the chromaticity characteristics of the new colorant is necessary to allow the effects 'of the change to be evaluated. Moreover, since the

manufacturers who have established the characteristics of their ware have presumably already fur- nished standard filters to customers who regularly

purchase from them, or to inspection agencies which

inspect the ware for them, it is equitable to require

new bidders to furnish at least a pair of filters for each color. Preferably a bidder should submit

duplicate filters, one of which would be returned

for his guidance and the other held by the purchaser

for inspection purposes. That no more difficulty

has been experienced in the past in checking the

chromaticities of glassware must be attributed to the fact that manufacturers have adhered quite

closely to material that was similar in its chromaticity characteristics to existing pale-hmit standards

or to samples submitted to customers.

8.4. Application to Plasticware

Plasticware should be required to meet the same

specifications for similarity of chromaticity characteristics as glassware although the requirements

may be based upon different standards. Thickness

in this case is Hkely to offer little difficulty because

it usually varies much less than in the case of glass- ware. Changing the color temperature of the source can cause the chromaticity of a sample to depart from that of the standard by more than the

allowable amount, but since the variation in thick- ness is likely to be less than with glass, variations

from changes in source color temperature should

be easier to keep within the basic chromaticity

definition. This is because the chromaticity limits can be more favorably located within the area allowed by the chromaticity definitions. Information on the variability of dyes from mix to mix is unfortunately not available. It would be of considerable interest.

9. Instrumentation

9.1. Chromaticity Measurements

The measurement of signal-light chromaticities

is carried out, for the most part, on the same principles as other color measurements. When spectrophotometric measurements are to be made, the

measurements are made with photoelectric spec- trophotometers and the results are evaluated with

the use of accepted mathematical functions. Such

measurements are regarded as free from subjective

estimates. On the other hand, nearly all direct

colorimetry of signal lights is carried out with visual

instruments which depend upon visual estimations.

In general, the photoelectric colorimeters which

have been developed for the measurement of sur- face colors are not satisfactory for use in measuring

the colors of the light transmitted by signal ware. The lack of suitable photoelectric instrumentation is partly due to the difficulties inherent in the problem

of making such an instrument and partly the result of the small number of laboratories which would be

justified in purchasing such instruments. A new

approach to the problem of objective instrumentation is discussed in sec. 9.3, Photoelectric Measurements.

Whenever new standards are to be adopted, the

spectral transmittances of the filters must be measured. This is essential because it is necessary to compute the chromaticities to be obtained with the

type of colorant represented in different densities

(thicknesses or concentrations) and also with sources of different color temperatures. The pro- cedures of spectrophotometry have been described

in NBS Circular 484 [Gibson, 1949] to which refer- ence may be made for details of procedure and the

probable accuracy to be expected.^^'

^The fact that chromaticity differences are more easily and more accurately measured than the chromaticity itself suggests the desirability of standardizing a set of precision standards judiciously spaced over the regions of interest and making all other chromaticity measurements by chromaticity difference. Even if it is important to know the spectral characteristics of a filter the results of the spectrophotometry should be checked against the chromaticity as determined by difference with respect to the precision standards. W. D. Wright [1959] has advocated this procedure for surface

color measurements and it seems even more advantageous in the case of filters. ^'To assist in keeping the errors of spectrophotometry to a minimum, the National Bureau of Standards has made sets of standard filters designed for this purpose

available. See Keegan, Schleter, and Judd [1%2].

39
In sec. 6.2 it was stated that the U.S. Standard

makes provision for dupHcates of the national and

organization standard filters by requiring that they

conform to definite chromaticity tolerances. Such

duplicates are polished squares and may be measured with a spectrophotometer. More frequently,

however, they are tested by visual methods with

colorimeters which measure the chromaticity dif- ferences between the duplicates and the originals.

This method is generally preferable when there are

satisfactory reasons for assuming that the dupli- cates are similar in chromaticity characteristics to the standard filters they represent. The chromaticity-difference measurements are more accurate than the ordinary spectrophotometric measurements

and take less time. The instruments and proce- dures used for these chromaticity-difference meas- urements are described in NBS Circular 478 [Judd,

1950]. In principle the procedures for inspecting ware

are those of colorimetry, and from the colorimetric

standpoint they are relatively simple. They are, however, unusual in having to be made on ware that has optical properties which make it desirable that the chromaticity measurements be made at some

distance from the test specimen in order that the

measurements represent an average chromaticity

such as is seen in service. For this reason the in- strumentation is hkely to resemble that used for the

visual photometry of signal units rather than the

usual colorimeters. The instrument may, in fact,

be an ordinary Lummer-Brodhun photometer with one side illuminated with light of a known chromaticity.

The variations of chromaticity which have to be

checked in an inspection of signal ware will usually

constitute a one-dimensional system, that is, they

aU lie on a smooth curve. The problem is to dis- criminate between ware which transmits light

falling within an acceptable interval from ware which transmits light of a chromaticity outside of

that interval. If the similarity of chromaticity

requirement has been met, all differences of chromaticity are of a clearly recognizable nature al- though small differences may require more careful

observations than large ones. If such a requirement is applicable and the inspector experiences

difficulty in determining whether units are inside or outside of the hue limits, he should refer samples

to a colorimetric laboratory equipped to determine

whether or not the similarity requirement has been

met. If there is no similarity requirement, the

inspector should likewise feel free to seek the assist- ance of a well-equipped colorimetric laboratory

since the discriminations required may be such that they can not be made by comparison with one or two

hue limits.

9.2. Transmittance Measurements

Most specifications for signal colors contain

requirements with reference to the transmittance

or transmittance ratio of the ware since this is

\ closely associated with the chromaticity in that both

result from the nature of the colorant and the

amount of it in the path of the light. The trans- mittance of signal ware is usually determined as the

ratio of two intensity measurements, one with test ware in the optical path and one with a standard

filter in the path, if the test ware has no optical

effect on the beam, that is, if there is no designed

convergence or divergence of the hght, nor any

j appreciable scattering at the surface, the simple

interchange of the test ware and standard filter suffices to give the ratio of the transmittances from

which we may compute true transmittance values.

But most signal ware does not meet these conditions

and it is necessary to insert a piece of colorless ware

of the same optical characteristics into the path with

the standard filter when the light transmitted by the

filter is being measured. This results in a nonsymmetrical comparison with two extra glass-air sur- faces in the light path when the standard is being

measured. The symmetry may be restored by

inserting a colorless (neutral) filter into the path

when the test ware is being measured. Numerically, the same result is obtained by introducing the

equivalent of the colorless filter mathematically.

The formulae for transmittance and transmittance

ratio, with and without the colorless flat filter, may

be derived as follows: Let us assume that a seasoned

lamp (figure 9-1), standardized for the color temperature for which the measurements are to be

made, is mounted at a fixed distance from an illumi- nometer (/) which gives readings in proportion to the illuminance at the instrument. Between the

lamp and illuminometer, two lenses are placed successively, the first a standard lens (Ls) which is colorless but of the same design as the lenses to be

tested and the second a test lens {Lt). The lenses

must be properly focused with respect to the lamp

and the illuminometer must be far enough from the

lens so that the inverse-square law holds [Waldran,

1951, 1952]. When the colorless standard lens is in place, a standard filter {Fg) of the color of the test lens is placed between it and the illuminometer.

When the test lens is in place a colorless filter (Fc)

may be substituted for the standard filter, or may be

omitted as indicated by the instructions.

^•^The transmittance values of all filters are ultimately derived from the spectrophotometric measurements on the filter or some other filter of similar spectral transmittance.

In signal lighting most transmittance values can be traced back to the work of Gibson, Haupt and Keegan [1945] and [1946].

^' See p. 41 . eq (4) for the definition and discussion following.

40
Let the pertinent quantities be represented as follows

E = illuminance at illuminometer from standard

lamp, without lens or filter. Z,s= optical effect of standard lens, including

transmittance.

Zr= optical effect of test lens, including trans- mittance.

Ts = transmittance of standard filter. Tc = transmittance of colorless filter, assumed to be 0.92.

/?s = illuminometer reading for standard lens with

standard filter in place.

/?? = illuminometer reading for test lens with

colorless filter in place,

f = illuminometer reading for test lens without

colorless filter in place.

K = calibration constant of illuminometer which

is assumed to give a hnear response and

to approximate the spectral response of

the C.I.E. luminosity function.

From which:

R,K=ELsTs (1) RtK=ELtTc (2)

R'tK = ELt (3)

I (2)/{l) gives L,/L, = {RtIRs) (TJTc) = transmittance ratio of test lens. (4)

or {L,ILs)Tc=-(R,IRs)Ts = "transmittance" of test lens. (5)

(3)/(l) gives LtlLs=(R\IRs)Ts

= transmittance ratio of test lens. (6)

f or {L,IL,)Tc = {R;IRMTs X 0.92) — "transmittance" of test lens. (7)

Quotation marks are used around the word

"transmittance" in eqs (5) and (7) because the ratio

I obtained in this measurement does not, in general,

conform to the usually accepted definition of "trans-

, mittance", namely: "The ratio of the luminous flux

I transmitted by the body to that which it received"

j [CIE 1957]. The purpose of the measurement in

; this case is to obtain an index of the loss of light due

I to the colorant in the specimen. There may be

much fight wasted by a lens or diffusing cover which

does not in principle affect the determined ratio, it being the purpose of the standard lens to cancel this out of the measurement. In actual practice the

measured ratio may be influenced in either direcFlGURE 9-1. Arrangement of parts for transmittance measurements.

La = Standard lens (colorless) L, =Test lens (colored) Fg = Standard filter (colored) = Colorless filter / = Receptor of measuring instrument. See text for procedure.

tion by this wasted light according to the relative amount of fight lost by the test and standard pieces.

The factor Tc, or 0.92, allows only for the surface

reflection which may be small as compared with the

other wasted fight. It is for this reason that the

term "transmittance ratio" was introduced as a term

referring to the relative efficiency of a lens. It matters little to the user of a light signal whether the

light was lost in a poor colorant, or a poorly molded

surface, or an optical design that is less efficient than that of the standard lens. Another procedure may be used which makes it unnecessary to measure either the "transmittance"

or transmittance ratio. The specification writer

may multiply the candlepower distribution expected

for a clear lens by the desired minimum transmit- tance ratio and specify a performance curve in the

colored light. This requires a candlepower dis- tribution curve to be made on enough pieces to insure that all those accepted meet the specification.

9.3. Photoelectric Measurements

There has been, over a period of years, an in- creasing interest in the use of photoelectric in- struments for the measurement of the chromaticity

and transmittance of signal ware. Efforts to produce a suitable instrument seem thus far to have been directed to the development of an

instrument that will read chromaticity coordinates

and the transmittance of the test specimen directly.

This is a difficult objective. A much more simple

approach would be to develop instruments that could be substituted for the human eye in the op- erations which are performed by an inspector.

The simplest approach might be a combination

of photoelectric cell, filter, and meter, which

41
v/ould give readings that are a simple, singlevalue function of the position of a test chromaticity

with respect to that of a standard chromaticity

when both lie on the path of the normal variation

of chromaticity with thickness for the specimen

being measured. A change of photocell, filter, or instrument scale in changing from one color to another would not be a prohibitive complication.

The difficulty in applying this simple approach

lies in the variation in optical efficiency from piece

to piece. Suppose, for example, some small areas

of the test piece are blanked out so that they

contribute nothing to the illuminance on the photocell but do not change the chromaticity at all. This sort of effect has to be distinguished from

those changes that result from changes in color- ant density which do affect the chromaticity. This can probably be done by means of a preliminary

filter which transmits only a narrow band of wavelengths in a region that is relatively well transmitted

by the colorant. This should enable one to in- terpret the readings taken with the chromaticity

filter and determine whether or not the chromaticity

is paler than is allowed by the specifications.

If there is a second hue limit it might be necessary

to have a second chromaticity filter to check

the test piece for this possible deficiency.

To inspect ware for compliance with transmit- tance requirements, a photocell corrected to the

spectral luminous efficiency of the CLE. standard

observer TCIE 1924] can be used and the measurements interpreted in accordance with the appro- i; priate formula of sec. 9.2.

9.4. Sampling ; Measurements are of no value unless the things * measured represent something significant. This I requires a comment on sampling. The variability

of color in most glass filterware is so great that <

it is not practicable to apply to its inspection the ' ordinary principles for sampling material. Instead i of random samples, samples deliberately chosen I to contain the palest and densest specimens in I the lot are required to assure a satisfactory product, i This selection can be based upon a visual inter- t comparison of the items submitted. An instru- r. ment test is then made on the few specimens so t chosen. If all of these specimens are satisfac- I tory, that is good evidence that the whole lot is i within the limits. If some of these are not satis- i; factory, that does not show that the lot is all un- > acceptable but rather that the lot has not had an s adequate inspection by the contractor. When the i inspector is satisfied that the contractor has re- i inspected the lot and replaced those pieces found * outside of the limits, there need be no bar to another i inspection. i In the case of plastic filters and perhaps some

types of glass filters, it is possible to keep the pro- duction well inside of the limits. In such cases

it is possible to use such statistical samphng

procedures as are approved for other materials that are normally well within safe limits.

10. Use of Signal Light Colors

10.1. General Principles

In considering the recognition of signal light

colors in sec. 4, it was pointed out that such rec- ognitions depend upon the conditions under

which the light is seen, and nine elements which

affect the certainty of recognition were listed. In designing or redesigning a system of light

signals, it is important to take these elements into consideration and determine how they will affect the certainty of recognition under the conditions

of service. Unfortunately, there are almost no research results available for correlating these

conditions with service applications. For this reason it is possible to give only a general discussion of this problem. The following simple principles indicate ways in which the most common

sources of signal-color confusion can be reduced

to a minimum.

10.2. Number of Colors

The first condition affecting the recognition of

signal-light colors is the number of colors in the

signal system. The number of colors should be

kept to the minimum that will serve the needs

of the signahng since every color that is added

increases the risk of mistaken identification.

^

10.3 The Primary Signal Colors i

7 The most elementary signal-light system is a one color system. Its use in the form of a bea- ^ con fire on a tower serving as a landfall light 3 can be traced to archeological times and there is i no reason to doubt that signal fires were used by t primitive tribes long before the towers were built. : From the standpoint of color, it might be thought \ that such a system would be completely safe since jj it does not lose its identity at the limit of range, i or even when it is used by a colorblind person. ) There may be, however, the possibility that some 3 stray light may be mistaken for the signal.

If more colors are required, the next step is a

two-color system. If it is required that a single - signal color be distinguishable from the general ag- , gregate of ordinary lights, then such a system must i be regarded as a two-color system since the ordi-

^ nary lights are in effect being given a negative i significance. It might be thought that for maxi- |_ mum dependability the colors of a two-color •

42
signal-light system should be as far apart on the

mixture diagram as is practicable. According

to the RUCS diagram these would be the longest

wavelength red and the most saturated bluish green

available. This theoretical solution, however, has

to be modified in practice since these highly

saturated colors are available only with extremely

low luminance.

With a fixed amount of hght available from the

lamp, the distance at which the signal color may

be recognized is at first increased by the greater

illuminance that may be obtained by transmitting more hght even though the resultant signal is less saturated.^- After an optimum mixture has been

reached, further additions of less saturated light

reduce the recognition distance as the saturation

becomes too low for long range recognition. In

practice a two-color system is hkely to consist of a red signal and a yellow-white signal, that is, a signal

of whatever color the hght source may be. Such a system should have dependable color recognition

almost to the threshold of the red, for just above

threshold all hghts except red ones appear white

or bluish white, but red lights retain some reddish- ness almost, if not quite, to their threshold. If a two-color system is to be used by red-green confusers, a bluish white hght should be substituted

for the yellow-white hght.

The next step is a three-color system in which

red, green, and yellow-white or yellow, lights are all used. This is the most common of signal systems.

It is used for the long-range lights of aviation, for the navigation lights of ships and for aids for marine

navigation with nominally white hghts. With

yellow hghts it is used for highway traffic signals,

and for the primary signaling system of most rail- roads. It is doubtful if any signal system should

contain colors other than these three unless the additional colors are to be used solely under conditions that are favorable to color recognition. These

will be discussed in connection with the four-,

five-, and six-color signal systems.

It has already been pointed out in connection

with the two-color system that there is an optimum

color density for a color filter to be used on a hght

intended for recognition at maximum range, and

this, of course, is equally apphcable to a three- color system.

J. G. Holmes included a determination of the

optimum color density for two green filters in the

report of his investigation of "The Recognition of

Coloured Light Signals" [1941]. For signal green

glass he found that 70 percent correct recognition

**The thresholds of visibility and color recognition have been investigated by Hill [1947] More recently, Middleton and Wyszecki have determined peripheral thresholds

for red, green and white Ughts [1951]. These values are from table 20 of the Holmes report. Holmes notes a risk of confusion with blue if the Chance Bros, signal green glass is used with a light source at 2850 "K and recommends against the practice. The signal green glasses used in this country with sources at 2850 *K are designed for this use and have given no difficulty, although it should be noted that the blue lights in this case are secondary signals for short range identification.

was obtained at maximum range with the following

transmittances:

Light source at 1904 °K, optimum transmittance 0.125 Light source at 2365 °K, optimum transmittance 0.16 Light source at 2854 °K, optimum transmittance 0.12 For 80 and 90 percent correct recognition, the trans- mittances of the optimum densities for light of

color temperature 1904 °K were the same within

the accuracy of the determination, the greater

rehabihty corresponding to a decreased range. Since both increasing and decreasing the density

of the filter tend to reduce its effectiveness as a

signal, the transmittances given above presumably should apply to the middle of the range of

transmittances to be furnished under the specification. The values given above may not be

strictly applicable to a three-color signal-light

system since they are based upon tests in which

the observers were allowed six color names, but

the location of signal green on the chromaticity

diagram is such as to indicate much more probability that unsaturated signals would be called

white than that they would be called blue. It is also probable that the green glass made by Messrs. Chance Brothers and Co. Ltd., of England is somewhat different from the corresponding green

glass made in the United States, but a comparison

of the above values for signal green glass with

those Holmes found for emerald green glass indi- cates that the presumably smaller difference

between glasses of the^ same color made in the

two countries is hkely to have less effect than

that resulting from the changes of light source

color temperature shown above (see footnote ^^). The practice in the United States is to use

green glass of higher transmittance than the

optimum found by Holmes. This may be justi- fied on the ground that the additional range or conspicuity is of more value than the greater

certainty. A light must be noticed before it can

be identified, and in the short interval between

discovery and identification the range is usually

reduced, which increases the certainty of identification. On the other hand green signals are also easUy confused with white at excessive

luminances.

10.4. Systems with Yellow and White Signals

For aviation, both yellow and white signals are used for beacons and for runway-lighting

systems. In accordance with the principles

set forth in the last section these are questionable

practices. So far as they are permissible, it is because of the circumstances of their use, for in both cases the two colors are associated in the same

apphcation in such a way that they are subject

to intercomparisons which enable the observer

to distinguish them. Seadrome beacons flash yellow and white alternately at intervals of 2 to 5 seconds, and this permits comparison of a present

43
flash with the memory image of its predecessor.

In the case of some runway-hght systems, yellow

Ughts have been used to differentiate the last section of a runway from the rest. In this case

the intercomparison is of two groups of Ughts

in the same field of view. Both cases therefore

involve only the distinguishing of chromaticity

differences rather than the recognition of two

chromaticities independently. A much smaller

chromaticity difference is sufficient for such

distinctions than is required to assure the cor- rect recognition of two chromaticities independently.

The justification of the use of both yellow and

white lights on runways has to be qualified. In

clear weather when the whole length of the runway

lighting system is visible at once, the contrast be- tween the two segments is easily detected and a pilot can keep the transition point in view until he is on

the ground and nearly abreast of the point where

the color changes. In fog, however, the interval

during which both types of lights are visible may

be limited to 2 or 3 seconds and under the circumstances this is not long enough to insure that the

distinction will be observed. This method of marking the last segment of a runway has never been

universally used in this country and it has been

used very little, if at all, in any other country.

The requirements for use of yellow both in sea- drome beacons and on runways are stiU in effect but seadrome beacons are becoming less common

and there is a growing recognition that the runway

use is not dependable. These uses, therefore,

do not constitute safe precedents for any similar

usage in any other application.

The importance of exercising caution in the use

of both yellow and white as primary signal colors

in the same system was appreciated by the Inter- national Civil Aviation Organization at the time

when it approved its Standards and Recommended

Practices for "Aeronautical Ground Light and Sur- face Marking Colours" [ICAO 1964]. This is shown

by the following quotation from Annex 14 to the

Convention on International Civil Aviation (p. 52). These paragraphs are quoted as being the best

summary available of the precautions which should

be exercised if circumstances seem to make it unavoidable to use both yellow and white lights as primary signal colors.

2.2 — Discrimination Between Yellow and White

Lights

2.2.1 Recommendation. — If yellow and white are to be dis- criminated from each other, they should be displayed in close proximity of time or space as, for example, by being flashed successively from the same beacon.

Note. — The limits of yellow and white have been based on

the assumption that they will be used in situations in which the

characteristics (colour temperature) of the light source will be

substantially constant.

2.2.2 Recommendation. —The colours variable-yellow and

variable-white are intended to be used only for lights that are to be varied in intensity, e.g., to avoid dazzling. If these colours

are to be discriminated from each other, the lights should be

so designed and operated that:

a) the chromaticity of the yellow lights will be represented

by coordinates such that y is not greater than x — 0.160 at any

time when the chromaticity of the white lights is represented

by coordinates of which x is greater than 0.470; and

b) the disposition of the lights will be such that the yellow

lights are displayed simultaneously and in close proximity to the white lights.

Highway traffic signal systems do not include

white lights, but the assumption that the yellow

traffic lights are recognizably diff"erent from ordi- nary lights commonly seen along streets and

highways involves the same difficulties as using

yeUow and white in the same system. The warning light of the traffic signal system is generally

recognizable for reasons other than color. In some installations it does not appear except in association with the green light and in this use any color could serve the purpose. In other in- stallations where this is not true, the warning

fight is seen following a green one or preceding

a red one, and this change of colors, if seen, is an indication that both signals are traffic fights. In stiE other installations the yellow fight is flashed

and this differentiates it from street lights and miscellaneous lights which may be within the view of

the driver. At short range it is possible for drivers

to recognize that the warning light is coming from

a traffic light fixture. AU of these considerations

contribute to making the use of yellow traffic signals possible notwithstanding that many other

yellowish fights are frequently visible to drivers. The realization that the circumstances associated with the use of yellow as a traffic signal

have been irnportant to the satisfactory experi- ence with this color makes it clear that it is not

safe to conclude that such yeUow lights may be

used as independent signals. In particular, it is not safe to depend upon the exposure of a fixed

yeUow light as a warning signal to automobile

drivers unless the situation has been examined to make certain that drivers cannot mistake such a

light for a stray light. This becomes an increas- ' ingly important consideration with the growing

use of traffic lights on open highways where the

higher speeds of traffic, and the longer distances

from which lights must be observed, make it more difficult for drivers to distinguish between

traffic lights and stray lights than is the case in i city driving.

Another use for yellow light signals on highways j is in turn signals on vehicles. In this use yeUow

and white have been used somewhat interchangeably on the front end of vehicles without difficulty.

This would seem to be primarily because the location of the signals on the vehicles has indicated

their purpose and the location has been recognizable from the outfine of the car in the day time

and from its relation to headlights or tail-lights at night. The Society of Automotive Engineers has now adopted a standard which requires all turn

signals to be yellow signals and this should in time

| simpfify the situation constructively.

44
10.5. The Secondary Signal Colors

In some signaling systems one set of colors is needed for operations at long range and additional colors are needed for purposes which permit

observing the lights at shorter range and with more dehberation. For these secondary applications it must be possible to design the fixtures so

that by the time a decision must be made the

hghts will either be observed as small surfaces,

rather than as point sources, or they will be seen

at luminances which are neither very high nor

very low. Under these conditions it is possible

to use as many as three additional colors. The

colors usually adopted for such purposes are lunar white (which could equally well be called

lunar), blue, and purple. The general policy in such systems should be to define the primaries

for optimum recognition as if the secondaries were

not used because the primaries perform the more

important functions and do it under the more severe

conditions. The blue and lunar secondaries are then fitted in to give the best differentiation possible

with the green and yellow. The transmittances

obtainable for the lunar glasses are about the same as those for the green glasses, but the trans- mittances of the blue glasses are about one tenth

of those feasible in the green. Blue signal ware

should be required to be free from an excessive

transmission of red light.

Purple signals have been used in the United

States to differentiate certain fixed signals from

blue hand lanterns used in railroad yards, but the

present trend is away from their use for this pur- pose. Purple signals fall into a class by themselves in that they are recognized by their dichroic

character. The red and blue components of the

light transmitted by purple glasses are separated

by the chromatic aberration of the eye and are seen separately, usually as a reddish signal sur- rounded by a blue halo. For this reason the

specifications for purple signal ware must contain

a requirement to insure tTiat tne proportion ot red light transmitted is not too low. Otherwise

the signal is liable to be mistaken for a blue sig- nal. On the other hand, too high a proportion of

j red in the transmitted light may cause a risk

j that the signal wiU be mistaken for a red signal.

If the system uses both blue and purple signals,

the hmitations for the transmittance of red by

I the purple ware have to be held within narrow lim-

! its and the complementary requirement that the

j blue ware not transmit too much red hght becomes

essential.

j Requirements relating to the proportion of red

I light transmitted by a blue or purple filter are repj resented as minimum or maximum limits for TrjTy,.

' Tit,, the transmittance for "white" light, is the

; total transmittance for light of the specified color

temperature, and Tr is the transmittance of red

light which has been defined for this purpose as hght of wavelengths longer than 0.650/u,. Tr and Tw

are not difficult to compute from spectrophotometric measurements in cases where spectrophotometric measurements are available, but the

inspection of signal ware has to be carried out with- out spectrophotometry. As a practical equivalent,

the blue or purple light may be measured through

a deep red, sharp-cutoff filter, but it is important

that the use of this procedure be clearly defined in the specification in order to avoid possible misunderstanding and litigations based upon the dif- ference between the actual cutoff of the filter and

the abrupt cut implied by the reference to a wavelength.

Table 3 shows the defining red filters, color temperatures for testing, and the limits prescribed for blue ware in the specifications and standards

named.

Table i — Maximum transmittance of red light allowed for

blue ware

Test Color Maximum

Specification filter temperature TrlT^

Federal Standard No. 3 3113A 2854 °K 0.015 Colors Aeronautical

Lighting

Military Specification, 3055A 2854 °K 0.015 MIL-C-25050

Colors Aeronautical Lights and Lighting Equipment

(p. 9) A.A.R. Specification 59-61 A.A.R.* 1900 °K 0.029 Glasses for Kerosene Hand 211 Lanterns (p. 4, 8) A.A.R. Specification 69-59 A.A.R. 2854 °K 0.006

Signal Glasses (p. 11) 86

•Expressed as light of wavelength greater than 0.650/x but tested with filter.

10.6. Signals for Colorblind Observers

Signal hght systems are generally planned for persons having normal color vision. As pointed

out in section 3.1, for such persons the world of

color is three dimensional. In looking at surface

colors, luminances are relative and are governed

by the reflectances of the surfaces. Such differ- ences are recognized as true color differences and

in normal surroundings a baby blue ribbon is easily

distinguished from one of navy blue by any person

with normal color vision even though the hue and

saturation may be the same. In signal hghting

the transmittance of the signal ware, which is the

analog of reflectance, merely affects the intensity,

and while this is technically part of the color, it

is not available for distinguishing signals since the

apparent intensity is dependent upon the distance

to the signal and the state of the intervening at- mosphere. In the interpretation of signal hghts, even the normal observer, or trichromat, is hmited

to chromaticity differences which constitute a two

dimensional space.

45
In contrast to the normal trichromats, the completely colorblind monochromats appraise what they

see with reference to a single primary. These

persons have no chromatic world and no signal

light colors can be of any use to them for they see

only differences in brightness or brilliance. Inter- mediate between these groups are the partially

colorblind dichromats who live in one dimensional

chromaticity worlds.^* Since very few persons are totally colorblind,

whereas a substantial fraction are dichromats, it seems reasonable to consider the possibility of de- signing a system of signal colors that can be used

by at least a part of the dichromats. A dichromat,

however, may lack any one of the three normal proc- esses and hence there are three kinds of dichromats, known as protanopes, deuteranopes, and

tritanopes. The differences between these three

groups can be appreciated from figures 10—1, 10—2,

and 10—3. In each of these figures there is a family

of lines radiating from a focus outside the spectrum

locus, that is, outside the region of visible colors. These are chromaticity confusion lines and each

set corresponds to the vision of one of the three

types of dichromats. All the chromaticities along

any one line seem the same to a colorblind ob- server of the type to which the locus applies. D. B. Judd has designed a three-color specification for use in control panels for electronic equipment which should be satisfactory for protanopes

and deuteranopes as well as for normal observers.

This system consists of aviation red, aviation blue,

and aviation green with its yellow boundary moved

to the line

y = 0.667(1-%).

These areas have been indicated in figures 10-1,

10—2, and 10-3 by the boundaries of the aviation

colors, the allowable portion of the green area being

distinguished by shading. It appears from figures

10-1 and 10-2 that the system should be useful to protanopes and deuteranopes but figure 10—3 indi- cates that tritanopes will easily confuse the blues

and greens. The Judd proposals have been tested

by Sloan and Habel [1955] in a research with 22

deuteranopes and 18 protanopes using signals subtending 1° at the observer's eye. Normal observers,

all the deuteranopes, and 13 protf nopes recognized

the colors correctly. Five protanopes experienced

difficulty with some of the test colors, making 16

errors out of 120 identifications. The results confirm the principle but indicate that further study is desirable before depending upon such a system in any situation involving appreciable risks. The

test must also be considered inconclusive for the more common applications of signal light colors

in which the subtended angle is a few minutes more

or less.

For discussions of the response functions of protanopes and deuteranopes see Judd [1944] and [1949].

FlGVnE 10-1. Confusion loci for protanopes. m''

All points lying on any one of the radii through P will appear to a person having partial

' color blindness of the type caUed protanopic as having the Same chromaticness. ' This diagram is a transformation into RUCS coordinates of the diagram given in the

original paper by Pitt [1935]. ' The boundaries for signal red. green and blue and intermediate green (shaded) as dehned in the U.S. Standard have been added showing that protanopes may be expected

to have difficulty distinguishing between signal green and red. '

The possibility of a system designed to be recognizable by all three types of dichromats deserves ^ consideration. Three small areas have been cross- ^ ^ hatched on figure 10-3 to make a modification of * the Judd system for this purpose. This proposal j has not been tested because the most important ]' application for such a system would seem to be in ( ^ connection with the control of traffic. For this use, ^ however, the proposal has a serious handicap in ^ the very low transmittance (1 or 2 percent) which ^ ^ must be accepted in order to obtain sufficiently

saturated green and blue signals. '

10.7. Backgrounds ' ^

*

I- The background of a signal light can have a con- \ f siderable effect on its conspicuity and itS' recogni- 1" tion. The conspicuity of a signal is greatly reduced i is if it must be seen among a number of other lights

from which the observer receives an equal or 1 lij greater illuminance. A yeUow traffic light, for J lij example, hanging in the center of a number of high- ' « way lights installed to illuminate a crossing may be I

li{ difficult to find even though it is known to be there. ' ot A driver who has noticed a green light must trans- ' lii fer his attention to the roadway surface to check 1 lli his alinement and make sure his path is still clear,

J

in and he may see only a cluster of yellowish lights ' tli when he looks back to check the traffic signal. If ' 4

46
Figure 10—2. Confusion loci for deuteranopes.

All points lying on any one of the broken vertical lines will appear to have the same

chromaticity to a person of deuteranopic vision. The spacing of the lines near the speclmm locus is based upon the results obtained by Pitt [1935]. The lines are drawn

vertical because different investigators disagree as to the location of the conversion

point in the case of deuteranopes. The conversion point found by some researches would lie at a fairly large negative value, whereas another research would place the conversion point at a positive value. In view of this uncertainty, it is felt that the

vertical lines represent the facts as far as known adequately. The other lines have

the same significance as in figure 10—1.

the driver is traveling at a good speed, he cannot

devote much attention to playing hide and seek with

hghts, and so he reverts to his relation to the roadvk^ay surface. When he has reassured himself

that all is well here, he looks again at the lights to find a red one among them. Unless he has reduced

his speed as a response to a realization that he is dealing with a situation that he may not completely

understand, he must now make an emergency stop,

if he can, before he reaches the crossing.

Green lights given a high intensity to insure their

visibility by dayhght look quite pale to the darkadapted eye at night. This may cause a green

traffic hght to become lost among highway hghts

with mercury lamps, which are frequently quite

greenish as seen from a few hundred yards away.

The obscurption of red traffic lights by neon signs

is a common source of concern. The loss of conspicuity because of competing

lights is not as serious a problem with city traffic lights as it would otherwise be because drivers be- come accustomed to the exact location of traffic lights with reference to the driving lane in their own city and city driving speeds allow more attention to be given to hghts. A study, however, of

the whole problem of conspicuity is needed. The

mere increasing of the intensities may increase

the problem of recognition and do little to solve

the problem of conspicuity.

Figure 10-3. Confusion loci for tritanopes.

In this case all of the points on any one of the converging lines will have the same

chromaticity for a tritanope. The conversion point represents a mean of the locations found by Thomson and Wright (1953], Judd [1944], and one computed from the MuUer

theory [Judd, 1949). The other lines have the same significance as in figure 10-1. Closely related to the problem of conspicuity

is the problem of visibility. At night the presence

of many bright hghts wiU destroy the dark adaptation and make it impossible for observers to see

distant signals if they are faint. This is a situation

which has to be avoided in marine and aeronautical

navigation, and it has received some attention in these fields, but it has not been thought a serious

problem in land transportation where signals are

closer to the observer. The increasing use of highway lights and the higher intensities used for headlamps and brakehghts is making it a problem on the

highway.

Both visibihty and conspicuity may be reduced

by too high a background brightness. This is particularly true of traffic hghts hung over the center

of an intersection which by day are seen against

the sky. This problem has long been recognized

in railroad signahng, and high intensity railroad

signals used in the daytime' are regularly provided

with artificial backgrounds by surrounding the

light with a black shield. A promising field for the improvement of the

color recognition of traffic lights is the possibility

of improving their backgrounds. While it is probably impracticable to use as large background

shields as are used for some of the railroad signals,

it should be possible to redesign the traffic light fixtures so as to present the largest practicable

black cross section surrounding the signals. An

effort might be made to have special lighting units

designed for use in locations near traffic signals

47
in order to eliminate the competition between

street and highway Ughts and traffic hghts. The

location of traffic signals should be considered with

reference to competing lights and means sought

to eliminate lights from the background if they are

interfering with the conspicuity of a needed trafffic light. A first approach might be to seek the cooperation of landowners in order to educate the public with reference to the problem. If this proves

inadequate legal means should be considered.

These are only the obvious ways in which the

situation might be improved. It is quite probable

that a thorough study of the problem would develop

other and possibly more important steps that could

be taken to improve the recognition of traffic signals.

The same principles apply to other types of signal

lights when such lights are used in close proximity

to stray lights.

10.8. Naming of Signal Colors

The naming of signal colors might not be ex- pected to have much importance but one problem

has arisen that merits some attention. It is com- mon practice to think of an ordinary incandescent

lamp seen through colorless signal ware as white.

This has been the accepted practice in connection

with lighthouses and aviation beacons and it is probable that people in general seldom ascribe any

color to the lights used for illumination. If such a

color, at moderate brightness, is shown an observer

From an engineering point of view the projection of hazardous photons ' seem to be essentially different from the projection of hazardous pebbles.

uld not

whose eyes are adapted to ordinary daylight, he

will generally classify it as yellow. In the case of

airport approach and runway lights the situation is aggravated by the necessity of dimming the lights

to a color temperature close to 1500 °K which is redder than candlelight and almost at the red limit of some signal yeUow lights although not as satu- rated. At full intensity these lamps may operate

at more than twice this color temperature. It is necessary to have a single name for this entire

range of colors. The International Civil Aviation

Organization selected the term "aviation variable

white." The Recommendations of the International Commission on Illumination for colors of

signal lights refer to these lights as "white/yeUow

lights." The U.S. Standard for the Colors of Signal

Lights calls it "variable-source white," and this may be a satisfactory name provided all concerned

can be made to realize the scope of the term. The

limitations which should be placed on the use of

both yellow and white in the same system have

already been discussed in sec. 10.4. A minor difficulty encountered in the effort to bring the several specifications for signal-light

colors used in the United States into a more consistent relationship has arisen from the practice

of naming signal colors with the names of the serv- ices in which they are used. It may seem quite

inappropriate to railroad signal engineerings, for example, to use an aviation green. For this reason

the names used in the U.S. Standard [NBS 1964]

have been designed to avoid reference to any

applications.

Appendix 1. References

Association of American Railroads, Signal Section, American

Railway Signaling Principles and Practices, Chapter I, History and Development of Railway Signaling, 73-74 (1953). Association of American Railroads, Signal Section, Part 136, Specification 69-59, Signal Glasses (Exclusive of Kerosene

Lantern Globes) (1960). Association of American Railroads, Signal Manual, Part 66, Specification 59-61, Glasses for Kerosene Hand Lanterns

(Exclusive of Electric Lantern Glassware) (1962). Bedford, R. E., and Wyszecki, G. W., Wavelength Discrimination for Point Sources, J. Opt. Soc. Am. 48^^129 (1958). Breckenridge. F. C., Transmission of Light through Fog, Commission Internationale d'Eclairage, Proceedings, 8th Session,

Cambridge & Edinborough (1931); also Trans. LE.S. (New

York) 27, 215, (1932). Breckenridge, F. C. and Schaub, W. R., Rectangular-UniformChromaticity-Scale Coordinates, J. Opt. Soc. Am. 27, 226

(Abstract) (1937); J. Opt. Soc. Am. 29, 370 (1939); Internationale Beleuchtungskommission (CLE.) Proc. 2, 81 (1939). Breckenridge, F. C, Tables for Transforming Chromaticity Coordinates From the I.C.I. System to the R-U-C-S System, NBS

Letter Circular LC897 (1948). Breckenridge, F. C, A Correlation of Signal Color Recognition

Tests, Paper No. 5-1-22, Sixth International Technical Conference on Lighthouses and Other Aids to Navigation, U.S. Coast Guard (1960). Burnham, R. W., Newhall, S. M., Color Perception in Small

Test Fields, J. Opt. Soc. Am., 43, 899 (1953). Churchill, William, Signal Glass, Proceedings New York Rail- road Club (1914). Commission Internationale d'Eclairage, Proceedings, 6th Session, Geneva (July 1924).

Commission Internationale d'Eclairage, Proceedings, 8th Session, Cambridge (1931). Commission Internationale d'Eclairage, Proceedings, 13th Session, Zurich (1955). Commission Internationale d'Eclairage, CLE. Publication 1.1, International Lighting Vocabulary, p. 42, No. 20-120 (1957). Commission Internationale d'Eclairage, Colours of Light Signals, Publication No. 2 (W-1.3.3) (1959). Commission Internationale d'Eclairage, Proceedings, 15th

Session, Resolution adopting CIE-UCS Coordinate System,

pages 33 (French), 35 (English), 37 (German), (1963). Dressier, A., The Non-Additivity of Heterochromatic Bright- nesses, Trans, LE.S. (London) 18, 141 (1953). Gage, H. P., Practical Considerations in the Selection of Standards for Signal Glass in the United States, CLE. Proceedings,

7th Session, Saranac Inn, N.Y., p. 834 (1928). General Services Administration, Federal Standard No. 3, Colors, Aeronautical Lighting (21 March 1951). Gibson, K. S., The Effect of Temperature Upon the Coefficient

of Absorption of Certain Glasses of Known Composition,

Phys. Rev. N.S., 7, 194 (1916). Gibson, K. S., Haupt, Geraldine W., Keegan, H. J., Specification

of Railroad Signal Colors and Glasses, NBS J. Res. 36, 1 (1946); RP1688: also, J. Opt. Soc. Am. 35, 772 (1945). Gibson, K. S., Spectrophotometry, NBS Circular 484 (1949). Guild, J., Selection of Yellow Glass for Railroad Signals, CLE.

Proceedings, 7th Session, Saranac Inn, N.Y., p. 862 (1928). Halsey, R. M., Chapanis, A., Chromaticity-Confusion Contours

in a Complex Viewing Situation, J. Opt. Soc. Am. 44, 442

(1954). Halsey, R. M., Identification of Signal Lights, I. Blue, Green, White, and Purple, J. Opt. Soc. Am. 49, 45 (1959); U. EUmi48
nation of the Purple Category, 49, 167 (1959): III. Effects of Limiting Stimuli (not yet publisheH. but tables were made avail- able to author).

. Hill. N.E.G.. The Recognition of Coloured Light Signals Which

' are Near the Limit of Visibihty. Proc. Phys. Soc, London 59,

• 560(1947).

1 HUl. N. E. G.. Measurement of Chromatic and Achromatic

Thresholds of Coloured Point Sources Against a White Background. Proc. Phys. Soc. 59, 574 (1947). Holland and Turner, The Variation with Temperature of the

'j Transmission of Glasses, J. Soc. Glass Technol. 25, 164 (1941).

1 Holmes. J. G.. Coloured Glasses for Lighthouse Purposes, Proc.

, Int. Lighthouse Conf., 99 (German), 77 (EngUsh) (1937).

, Holmes. J. G.. Use of Uniform Chromaticity Scales. Proc. Phys.

j Soc. 52, 359. (1940). Holmes. J. G.. The Recognition of Coloured Light Signals, Trans.

' 111. Eng. Soc. (London) 6, 71 (1941). Holmes, J. G., Colorimetry in the Glass Industry, Proc. Phys. Soc.

I 59,592(1947).

Holmes, J. G., Colour Recognition of Very Small Light Sources,

I Documenta Ophthalmologica 'S-Gravenhage 3, 240 (1949).

I International Civil Aviation Organization, International StandI ards and Recommended Practices. Aerodromes, Annex 14 to the Convention on Civil Aviation, Fourth Edition (1964).

I International Commission on Illumination (see Commission

International d'Eclairage (CIE)). Judd, D. B.. The 1931 Standard Observer and Coordinate Sys- tem for Colorimetry. J. Opt. Soc. Am. 23, 359 (1933). Judd. D. B.. A Maxwell Triangle Yielding Uniform Chromaticity

Scales. J. Opt. Soc. Am. 25, 24 (1935): J. Res. NBS 14, 41

( 1935) RP 756.

,| Judd. D. B.. Estimation of Chromaticity Differences and Nearest

I Color Temperature on the Standard 1931 ICI Colorimetric Coordinate System. J. Opt. Soc. Am. 26, 421 (1936); J. Res. NBS 17,771 (1936)RP944.

Judd, D. B.. Standard Response Functions for Protanopic and

Deuteranopic Vision, J. Res. NBS 33 , 407 (1944) RP1618:

' J. Opt. Soc. Am. 35,199(1945).

I Judd, D. B., Response Functions for Types of Vision According

I to the MuUer Theory, J. Res. NBS 42, 7 (1949) RP1946.

I ludd, D. B., Colorimetry, NBS Circular 478 (1950).

' Judd, D. B.. A Five-Attribute System of Describing Visual Ap1 pearance. Am. Soc. for Testing Materials, Special Technical

I Publication 297(1961). Judd, D. B., International Commission on Illumination (report of adoption of new coordinate system). Editor's Page, August, J.

ij Opt. Soc. Am. 53,1012(1963).

'I Keegan. H. J.. Schleter, J. C, and Judd, D. B.. Glass Filters for Checking Performance of Spectrophotometer-lntegrator Sys- tems of Color Measurement, J. Res. NBS 66A, 203 (1962). Keegan, H. J. Schleter, J. C. and Belknap, Marion, Recalibration of the NBS Glass Standards of Spectral Transmittance,

I J. Res. NBS 67A, 577 (1963). Leberknight, C. E.. and Stone, G. E.. Color and Transmission

Properties of Sharp Cut-Off Red Glass Filters at Elevated Temperatures, Kopp Glass Inc. (1955). MacAdam, D. L., Maximum Visual Efficiency of Colored Materials, J. Opt. Soc. Am. 25, 361 (1935).

' MacAdam, D. L., Visual Sensitivities to Color Differences in DayUght, J. Opt. Soc. Am. 32, 247 (1942).

MacAdam, D. L., The Graphical Representation of Small Color

Differences, J. Opt. Soc. Am. 33, 675 (1943). MacAdam, D. L., Maximum Attainable Luminous Efficiency of Various Chromaticities, J. Opt. Soc. Am. 40, 120 (1950). MacAdam, D. L., Loci of Constant Hue and Brightness Deter- mined with Various Surrounding Colors, J. Opt. Soc. Am. 40,

589(1950). MacAdam. D. L.. Influence of Visual Adaptation on Loci of Constant Hue and Saturation, J. Opt. Soc. Am. 41, 615 (1951). MacAdam, D. L.. Small-field Chromaticity Discrimination, J. Opt. Soc. Am. 49, 1143 (1959). McNicholas. H. J.. Selection of Colors for Signal Lights, J. Res. NBS. 17,955(1936) RP956. McNish, A. G., Classification and Nomenclature for Standards

of Measurement, IRE Transactions for Instrumentation 1-7 371 (Dec. 1958). Middleton, W. E. K. and Gottfried, H. S. T.. Apparent Intensities of Colored Signal Lights, lU. Eng. 52, 192 (1957). Middleton, W. E. K. and Wyszecki, G. W., Visual Thresholds in the Retinal Periphery for Red, Green, and White Signal Lights,

J. Opt. Soc. Am. 51,54(1961).

Military Specification MIL-C-25050 (ASG), Colors, Aeronautical Lights and Lighting Equipment, General Requirements

for (Dec. 2, 1963). National Bureau of Standards, U.S. Standard for the Colors of Signal Lights, Handbook 95 (1964). Newhall, S. M., Nickerson, D., and Judd, D. B., Final Report of the O.S.A. Subcommittee on the Spacing of the Munsell

Colors, J. Opt. Soc. Am. 33, 385 (1943). Ornstein, Eymers, and Vermeulen, Farbenerkennungspriifungen

mit Rucksicht auf die Brauchbarkeit von Signalglasern, Proc. Kon. Acad. Wet 37, 385 (1934). Otyfoky and Fanderlfk, New Czechoslovakian Signal Glasses,

Jour. Soc. Glass Tech. 21, 95 (1937).

Pitt, F. H. G., Characteristics of Dichromatic Vision, With An

Appendix on Anomalous Trichromatic Vision, Report No. 14, Reports of the Committee Upon the Physiology of Vision, Medical Research Council, London (1935). Rautian, G. N. and Guryea, M. K.. Color Distinction of Point Sources, Transaction of the Academy of Sciences of U.S.S.R. 112, No. 6 (1957). Sloan, Louise L., and Habel, Adelaide, Recognition of Red and Green Point Sources by Color-Deficient Observers, J. Opt.

Soc. Am. 45, 599 (1955). Sloan, Louise L., and Habel, Adelaide, Color Signal Systems for the Red-Green Color Blind — An Experimental Test of the ThreeColor Signal System Proposed by Judd, J. Opt. Soc. Am. 45,

592 (1955): USAF Av. Med. Proj. Report. Rep. No. 55, 20 (1955). Thomson. L. C. and Wright, W. D., The Convergence of the Tritanopic Confusion Loci and the Derivation of the Fundamental Response Functions, J. Opt. Soc. Am. 43, 890 (1953). U.S. National Committee on Colors of Signal Lights (F. C. Breckenridge, Editor), United States Standard for the Colors

of Signal Lights, Handbook 95, National Bureau of Standards

(1964). Waldram, J. M.. The Photometry of Projected Light, Trans. lES, London 16, 187 (1951): 111. Eng. 47, 397 (1952). Wright, W. D.. Sensitivity of the Eye to Small Colour Differences, Proc. Phys. Soc. London 53, 93 (1941). Wright, W. D., Color Standards in Commerce and Industry,

J. Opt. Soc. Am. 49, 384 (1959).

Appendix 2. Bibliography of General Works

Handbook of Colorimetry, Arthur C. Hardy, The Technology

Press, Mass. Inst, of Tech., Cambridge, Mass. (1936). The Science of Color, Committee on Colorimetry, Optical Society of America, Thomas Y. Crowell Company, N.Y. (1953). Color in Business, Science and Industry, Judd, Deane B.. and

Wyszecki, Gunter, John Wiley & Sons, Inc., New York (1963). Photometry, Walsh, J. W. T., Constable & Company Ltd., 10 Orange .Street, London, WC2 (1953). Vision Through the Atmosphere, Middleton, W. E. K., University

of Toronto Press, Toronto (1952).

Commission Internationale de L'Eclairage, Proceedings 1924, 1928, 1931, 1935, 1939, 1948, 1951, 1955, 1959. 1963. Address: Central Bureau CLE., Assistant Secretary, J. J. Chappat, 57 Rue Cuvier, Paris 5, France.

I. E. S. Lighting Handbook, 1959, Address: Illuminating Engineering Society, 345 East 47th Street, New York, N.Y.

Fifty Years of Signal Lighting, F. C. Breckenridge, 111, Eng. 53,

311(1958).

49
Appendix 3. List of Specifications and Standards

Commission Internationale de I'Eclairage, Colors of Light Sig- nals, Publication CLE. No. 2 (W-L3.3) (1959). Address:

Central Bureau of CLE., Assistant Secretary, M. J. J. Chappat, 57 Rue Cuvier, Paris 5, France, or Secretary, U.S. National Committee, L. E. Barbrow, National Bureau of Standards, Washington, D.C

International Civil Aviation Organization, International Standards and Recommended Practices, Aerodromes, Annex 14,

Int. Civ. Av. Org., Fourth Edition (1964). Address: 1080

University Street, Montreal 3 (Quebec), Canada.

U.S. Standard for the Colors of Signal Lights, National Bureau

of Standards Handbook 95 (1964). Address: Superintendent

of Documents, Government Printing Office, Washington, D.C,

20402. Federal Standard No. 3, Colors, Aeronautical Lighting (21 March,

1951). Address: Superintendent of Documents, U.S. Govern- ment Printing Office, Washington, D.C, 20402. MiUtary Specification MIL-C-25050 (ASG), Colors, Aeronautical Lights and Lighting Equipment, General Requirements for (2 December, 1963). Address: Superintendent of; Documents, U.S. Government Printing Office, Washington,

D.C, 20402. Association of American Railroads, Signal Section, Part 136, Specification 69-59, Signal Glasses (Exclusive of Kerosene

Lantern Globes) (1960). Address: Secretary AAR, Signal

Section, 30 Veasey Street, New York, N.Y.

Association of American Railroads, Signal Manual Part 66, || Specification 59-61, Glasses for Kerosene Hand Lanterns

(Exclusive of Electric Lantern Glassware) (1962). Address:

Secretary AAR, Signal Section 30 Veasey St., New York, N.Y. American Standards Association, Adjustable Face Traffic Control Signal Head Standards, ASA DlO.1-1958, UDC-656.057

(1958). Address: American Standard Association, 70 East

45th Street, New York, N.Y.

Society of Automotive Engineers, Inc., SAE Standards, TR-34,

Lighting Equipment and Photometric Tests. Address: Soci- ety of Automotive Engineers, Inc., 485 Lexington Ave., New

York 17, N.Y.

Other Countries

Austraha

Standard in preparation.

Germany DIN 6163 Blatt 1: Farben und Farbgrenzen fiir Signallichter, Allgemeines

DIN 6163 sheet 1: Colors and color limits for signal lights; General. DIN 6163 Blatt 2: Farben und Farbgrenzen fiir Signallichter, Ortsfeste Signalhchter an See-und Binnenschiffahrtstrassen DIN 6163 sheet 2: Colors and color limits for signal lights; Fixed signal lights at maritime and inland waterways. DIN 6163 Blatt 3: Farben und Farbgrenzen fiir Signallichter, Signallichter an Strassenfahrzeugen und Strassenbahn DIN 6163 sheet 3: Colors and color limits for signal lights: Signal lights on road vehicles and tramways. DIN 6163 Blatt 4: Farben und Farbgrenzen fiir Signallichter, Signallichter der Eisenbahn DIN 6163 sheet 4: Colors and color hmits for signal lights; Signal lights of the railways. DIN 6163 Blatt 5: Farben und Farbgrenzen fiir Signallichter, Ortsfeste Signallichter im Strassen-und Strassenbahnverkehr DIN 6163 sheet 5: Colors and color limits for signal hghts: Fixed signal lights in road and tramway traffic.

DIN 6163 Blatt 6: Farben und Farbgrenzen fiir Signallichter, Signalhchter an Wasserfahrzeugen DIN 6163 sheet 6: Colors and color limits for signal hghts;

Signal lights on vessels. DIN 6163 Blatt 7: Farben und Farbgrenzen fiir Signallichter,

. Luftfahrtfeuer und Signalhchter zur Luftverkehrsregelung DIN 6163 sheet 7: Colors and color hmits for signal lights; Air

navigation lights and signal Hghts for air traffic regulation. DIN 6163 Blatt 8: Farben und Farbgrenzen fiir Signallichter, Signallichter an Luftfahrzeugen DIN 6163 sheet 8: Colors and color limits for signal hghts;

Signal lights on aircraft. DIN 6171: Aufsichtfarben fiir Verkehrszeichen, Farben und

Farbgrenzen DIN 6171: Control colors for traffic signals; Colors and color

limits. Deutschen Normenausechusses, Berhn W 15. United Kingdom

British Standards Institution, British Standard 1376:1953, Colours of Light Signals (1953). Address: British Standards

Institution, British Standards House, 2 Park Street, London

Wl, England.

Appendixes 4 and 5

The tables of 4 and 5 show the equations used as boundaries the coordinates of the CLE. system, which is the system used

for the signal-Ught colors in the U.S. Standard [N.B.S. 1964], in all the specifications, but since the RUCS equivalents of the together with the corresponding equations used in the Standards CLE. recommendations are more significant for color differen- of the International Civil Aviation Organization [1964] and the tiation, App. 5 has been added to give these also. For the titles Recommendations of the International Commission on Illumi- of the Standards and the addresses of the issuing authorities, nation [1955]. All of the equations of App. 4 are expressed in see App. 3.

Appendix 4. Corresponding: Boundary Equations

CIE Coordinates

Colors and Boundaries' International Commission on

Illumination^

International Civil Aviation

Organization'' U.S. Standard for the Colors of Signal Lights'*

Signal Red

Purple boundary

Yellow boundary

j>: = 0.980 -y

y = 0.335 y = 0.980 -a: y = 0.335

A: + y= 0.992 x-y = 0.330

Intermediate Signal Red

Purple boundary

Yellow boundary No equivalent No equivalent

50

a: + y = 0.992 jc-y = 0.380
Appendix 4. Corresponding Boundary Equations

CIE Coordinates — Continued

Colors and Boundaries ' International Commission on

Illumination - International Civil Aviation Organization ' U.3. Standard tor the Colors of Signal Lights *

Restricted Signal Red

Purple boundary

Yellow boundary

} — U.VOD — X 7 = 0.300 No equivalent

„ 1 „, A nO'7 X + y — u.yy / x-y = 0.405

Signal Yellow Red boundau-y

White boundary Green boundary

y = 0.382 y=U. /yU — U.OO/5C y=;c-0.120

y = 0.382

A -70A C\ (^(^1 y — u. /yu — u.oo /X x = 0.550

y = 0.383

„ I „ A 0*70 _1_ A OAA^ X + y — U.o (/ + U.zUUx x = 0.550

Intermediate Signal Yellow Red boundary

White boundary Green boundary

No equivalent No equivalent y = 0.383

^ 1 „, A 0*70 _1_ A OAA^ X + y — u.o/z + U.^UUr x-y=0.124

Restricted Signal Yellow Red boundary

White boundary Green boundary

!No equivalent y = 0.400 y — U. /yu — \j.oolx x = 0.560

y= 0.400 X +y — U.o /Z + U.ZUUX x-y = 0.124

Signal Green

Yellow boundary

White boundary

Blue boundary

x = 0.360 -O.OSOy

jc = 0.650y y = 0.390-0.171x

x = 0.333 x = 0.650y

y=0.390-0.171a:

x = 0.360 -0.080y

X — u.ooUy

y=0.390-0.171x

Intermediate Signal Green

Yellow boundary

White boundary

Blue boundary

No equivalent No equivalent y = 0.730 (1-x)

X — u . oDUy

y = 0.500 -0.500X

Restricted Signal Green

Yellow boundary

hite boundary

Blue boundary No equivalent y = 0.850 -a:

a: = 0.650y — 0.030 y = 0.390-0.171x

y = 0.730 (1-x)

X = 0.425y y = 0.500 -0.500x

Signal Blue Green boundary

White boundary

Purple boundary

y = 0.065 + 0.805;c X = 0.400 -y

;c = 0.133 + 0.600y

y = 0.805a: + 0.065 y = 0.480 -X

x = 0.600y+ 0.133

y = 0.065 + 0.805x y = 0.400 -X

x = 0.100 + 0.700y

Signal White Variable source white is nearest equivalent Variable source white is nearest equivalent

Signal white includes all chromaticities which qualify

Qc (in^i r»T trill riw intr iniir

Variable Source White

Green boundary

Purple boundary

fy=0.150 + 0.640x y — U.44U

fy = 0.382 |y = 0.050 + 0.7.50a:

y = 0.640 -0.400x y= 0.050 + 0.750% y = 0.390

} y-yo = 0.003

}.yo-y = 0.003

Yellow boundary

Blue boundary

r:c = 0.255 + 0.750y

|y=0.790-n.667;(

i: = 0.310

}x = 0.560 x = 0.310

X = 0.555 + 0.750(y- 0.405) x = 0.325

Beacon White Green boundary

Purple boundary

Yellow boundary

Blue boundary

fy=0. 150 + 0. 640a: h =0.440

[y= 0.382 |v = 0.050 + 0.750a: x = 0.500

a: = 0.310

y= 0.640 + 0.400X y=0.150 + 0.640x y= 0.050 + 0.750X y = 0.390 x = 0.500 x = 0.310

} y-yo = 0.003

} yo-y= 0.003 X = 0.490 + 0.500(y - 0.410) x = 0.325

Lunar White

Green boundary

Purple boundary

Yellow boundary

Blue boundary

y = 0.150 + 0.640* y = 0.050 + 0.750a:

a: = 0.440

a: = 0.310

No equivalent

y = 0.195 + 0.500x y = 0.170 + 0.500x X = 0.445 + 0.333(y- 0.405) x = 0.325

51
Appendix 4. Corresponding Boundary Equations

CIE Coordinates — Continued

Colors and Boundaries ' Internalional Commission on

Illumination - International Civil Aviation Organization ^ U.S. Standard for the Colors of Signal Lights

Blue White Green boundary

For X < 0.330 roT X > 0.330 Purple boundary

For X < 0.330 For X > 0.330 Yellow boundary

Blue boundary

No equivalent No equivalent

y = 0.030 + a: and

y — u. ivo \).o\)\JX

y = 0.005 + x or y = 0.170 + 0.500;t X = 0.445 + 0.333(y- 0.405) X = 0.265 - 0.250(y- 0.280)

1 Boundary names are from the U.S. Standard. In the case of the I.C.A.O. the correspondence is readily evident with these possible exceptions; "variable yellow" is classified as signal yellow because it is the most inclusive; restricted green is an I.C.A.O. recommended practice; the use of the CLE. white boundary for blue is recommended in I.C.A.O. The correspondence in the case of the CLE. is clear except for white lights. In this case the correspondence has been determined from the uses indicated in the two documents.

2 Recommendations, Publication No. 2. "International Standards and Recommended Practices. Aerodromes. Annex 14.

' NBS Handbook 95.

Appendix 5. Corresponding Boundary Equations RUCS Coordinates

Colors and Boundaries' International Commission on

Illumination^

International Civil Aviation Organization^

U.S. Standard for the Colors of Signal Lights'*

Signal Red

Purple boundary

Yellow boundary

Aa:" = x"- 0.075

x" = + 0.070 / = - 0.317 -4Ajc"

Z^" = jc"- 0.075 y = + 0.070 / = - 0.317 -4A;c"A;t" = a:"- 0.075

«" =+ 0.0725 /=-0.317 + 0.16Ax"

Intermediate Signal Red

Purple boundary

Yellow boundary No equivalent No equivalent

x" = + 0.0725 / = - 0.375 + 0.4A:«;"

Restricted Signal Red

Purple boundary

Yellow boundary

x" = + 0.071 / =— 0.400 — 4Ay No equivalent

y' = + 0.0741 y =-0.407 + 0.5Aa;"

Signal Yellow Red boundary

VV IIIIC UOUHUal y Green boundary

y' = - 0.225 -4A;i;"

x" = + 0.063 -0.050/

y' = -0.134-0.50Ax"

/ = - 0.225 -4A:t:"

y' = + 0.063 -0.050/

/ = - 0.120 + 3Aa:"

/ =- 0.225 -4Ay'

x" = + 0.068 -0.026/

/ = - 0.120 + 3Aa:"

Intermediate Signal Yellow Red boundary

White boundary Green boundary

No equivalent No equivalent

/ =- 0.225 -4A:c"

a;" = +0.068 -0.026/

/=-0.136 + 0.5Aa;"

Restricted Signal Yellow Red boundary

White boundary Green boundary

No equivalent

/ = -0.194-4A;("

y' = + 0.063 -0.050/

/ = - 0.134 + 3Aa:" / = -0.194-4Ajc"

x" = + 0.068 -026/

/ =-0.136 + 0.5Ax"

Signal Green

Yellow boundary

White boundary

Blue boundary

a;" = + 0.715/

/ = + 0.085 -0.600a:"

x" = - 0.150/

x" = + 0.875/

/ = + 0.085 -0.600y'

*" = - 0.150/

x" = + 0.715/

/ = + 0.085-0.600x"

V' = - 0.150/

Intermediate Signal Green

Yellow boundary

White boundary

Blue boundary

No equivalent No equivalent

x" = + 0.045 -0.0075/

/ = + 0.085 -0.600x" a;"=- 0.020/

Restricted Signal Green

Yellow boundary

White boundary

Blue boundary

No equivalent = + 0.047 + 0.050/

/ = + 0.104- 0.650;c"

a:" = -0.150/

a:" = + 0.045 -0.0075/

/ = + 0.142 -0.450a:"

a:" = - 0.020/

52
Appendix 5. Corresponding Boundary Equations

RUCS Coordinates — Continued

1 rtlr»rc a n ri Rrii inrianf^ ^ International Conimission on

Illumination - International Civil Aviation Organization ^ *J.O. OiailUdlU lOl Lllc v>OiOJ 3 Ol Signal Lights *

Signal Blue Green boundaryWhite boundary

Purple boundary

x" =- 0.950/

a:" = -0.125 + 0.350/

/=-0.205x"

x" =- 0.950/

x" = - 0.075 + 0.250/

/ = -0.205y'

y' = - 0.950/

y = -0.125 + 0.350/

/=-0.370y'

Signal White Variable source white is nearest equivalent Variable source white is nearest equivalent

Signal White includes all chromaticities which qualify as one of the following four

Variable Source White Green boundary

Purple boundary

Yellow boundary

Blue boundary

\x" =+ 0 . 020 — 0 . 700/

lx" =+ 0.042 -0.247/

[/ = + 0.021 -0.240/

U" = - 0.026 -0.900/

fy'=- 0.161+ 0.086a:"

[x" =+ 0.063 -0.050/

y' =- 0.025 + 0.984/

X = + 0.055 — 0.100/

a:" = + 0.020 -0.700/

y' =- 0.026 -0.900/

a:" = + 0.024 -0.240/

1 y' = + 0.115 + 0.300/

a:" = -0.025 + 1.000/

Curved boundaries, no simple

transformation / = -0.1515 y = - 0.000 + 0.900(/ - 0.009)

Beacon White

Green boundary

Purple boundary

Yellow boundary

Blue boundary

ry' =+ 0.020-0.700/

1^' = + 0.042 -0.247/

rx" = + 0.021 -0.240/

lx" =- 0.026 -0.900/

x" = + 0.100 + 0.420/

y' =- 0.025 + 0.984/

a:" =+ 0.055 -0.100/

y' = + 0.020- 0.700/

x" =- 0.026 -0.900/

y' = + 0.024 -0.240/

x" = + 0.100 + 0.420/

y' = - 0.025 + 1.000/

^ Curved boundaries, no simple

transformation / = - 0.094 + 0.400(a:" - 0.056) y = - 0.000 + 0.900(/ - 0.009)

Lunar White

Green boundary

Purple boundary

Yellow boundary

Blue boundary

a:" =+ 0.020 -0.700/

y' = - 0.026 -0.900/

y' = + 0.074 + 0.536/

y' = - 0.025 + 0.984/

No equivalent

y' = + 0.016 -0.571/

a:" = -0.002 -0.586/

/ = - 0.060 + 0.700(a:" - 0.045)

x" = + 0.000 + 0.900(/ - 0.009)

Blue White

Green boundary

r or < 0.330 For X > 0.330 Purple boundary

For X < 0.330 For jr > 0.330 Yellow boundary

Blue boundary

No equivalent No equivalent

/ = + 0.021 -0.844x"

y' = + 0.016 -0.571/

/ = + 0.004 -0.794x"

a:" = -0.002 -0.586/

/ = - 0.060 + 0.700(y' - 0.045)

x" = - 0.037 + 0.700(/ - 0.040)

1.2. 3.4g£e corresponding notes to Appendix 4.

I

53
Appendix 6. Comparative Diagrams of Signal Color Boundaries

Figures A-1, A-IA, and A-IB. and figures A-2, A-2A and A-2B show a comparison of the chromaticity boundaries of the three standards for signal hght colors listed in App. 3 of the Appendix: the U.S. Standard, the CLE. Recommendations,

and the I.C.A.O. Standards and Recommended Practice. To

avoid confusion, some boundary lines in the yellow-white region and one in the red region have been omitted from figure A-2. The RUCS boundaries for restricted red in the CLE. Recommendations may be fairly well located by noting the location of these boundaries on the CLE. diagram in comparison with other boundaries that are shown on both the CLE. and RUCS dia- grams. Space limitations have also dictated the use of different types of lines to distinguish the boundaries of the three standards. When the same equation has been used as a boundary in more than one standard, a combination of the different types of

lines has been used to represent the common boundary. The

significances of the different types of lines have been shown in the figures. Labels have been used to distinguish boundaries

of the same standard which are specified for different uses and

again it has been found necessary to conserve space by abbreviat- ing the terms. The following code has been used:

U.S. Standard: SIG =Sienal Color (broadest category) INT = Intermediate Color RES = Restricted Color (highest certainty category)

BW = Blue white (special category)

LW = Lunar white (distinguishable from yellow) CLE. Recommendations:

RES = Restricted (red only) LC.A.O. Standard and Recommended Practices: VAR = Variable (color of signal varies with source as intensity is changed for different conditions) CON = Constant (signals not in previous category)

REC = Recommended practice.

In the case of the CLE. white boundaries, the selection of the boundary depends upon the number of colors in the system and

this seemed to make it impracticable to label the CLE. boundaries. Reference should be had to the Standard itself for the

interpretation of the boundary lines.
.45 .50 .55 .60 .65 \,580

585 / VAR 8 SIG

SOURCE ^"^^ \^CON \»-INTa RES

Ni \590

RES a CON CON— VflR \595

^X-INTaSIG

VAR \,600

A.C.A.O. ^b

o

a> bo

A-IB

585 ,^SIG

REC —

:::gEacoN"'t

SOURCE^

-^>^INTaRES \ \590

RES

in

i_ 1-

b

o

\595

INT a SIG

A-IA

50

,45

.40

60535

.50

45

40

.35

.45 .50 .55 .60

Figures A-IA and A-IB.

.65

'0.3

y"

*0,2

-0,1

-0,2

-0,3

-0,4

-0,5

U,S, STO

• CI.£.REC

I.C.A,0 STO

C.I,E, ai,C,AO,

- US. a CLE.

U.S., a I.C.A.O- ALL THREE

R.U.C.S, DIAGRAM

EQUAL ENERGY AT x"-0, Y". c Clf. ILLUMINANT 'A* AT 2854*1

- 520

^530

r I :-540

r I ^550

-

I COLOR

I TEMPERATURE. -560

[- 20 000-1

- - 10000°

I*- 6 500° -570 4 600°'

. — 3 600'

2 854° -580

- 2 365°

-590

1 500"

--RES

INT a SIG 600

^6 30

I 640

- I i- 660

\7

Figure A-2.

+ 0.025 -1-0.050 +0.075 +.025 +0.050 +0.075 0050

^RES a CON 0.200

INT a SIG VAR

0,600 -0.250

Figures A-2A and A-2B.

55

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