NBSIR 74-518

Instrumental Colorimetry of

Retroreflective Sign Materials

I. Nimeroff, W. A. Hall

Optical Radiation Section

Heat Division

Institute for Basic Standards

August, 1974

Prepared for Federal Highway Administration

Office of Research

Washington, D. C. 20590

NBSIR 74-518

INSTRUMENTAL COLORIMETRY OF

RETROREFLECTIVE SIGN MATERIALS

I. Nimeroff, W. A. Hall

Optical Radiation Section

Heat Division

Institute for Basic Standards

August, 1974

Prepared for Federal Highway Administration

Office of Research

Washington, D. C. 20590

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

PREFACE

At the request of the Federal Highway Administration of the U S Department of Transportation, the National Bureau of Standards conducted a study to develop an instrumental procedure by which to specify and measure colors of retroreflective materials, as seen under nighttime illuminating and viewing conditions. The work included a study of the colorimetric properties of retroreflective materials used for highway signs and markings as evaluated under

^hr^'^iv^^r'^^''^?''^

illuminating and viewing geometry, particularly those that simulate nighttime use conditions. The opinions, findings, and conclusions m this report are those of the authors and are not necessarily those of the Federal Highway Administration. Certain commercially available equipment, instruments, or materials are identified m this report to specify adequately the experimental procedures used. In no case should such identification be inferred as recommendation or endorsement by the National Bureau of Standards, nor does the identification imply that the materials or equipment are necessarily the best available for the purpose

i
CONTENTS

'

. Page

PREFACE i

I. Introduction <..... 1

1.1 Statement of Work 2

1.2 Partial Support by Federal Aviation Administration 3

2.. Measurement System 4

2.1 Colorimeters and Standards 4

2.2 Daytime Geometry 16

2.3 Nighttime Geometry 16

2.4 Special Geometries 20

3. Colorimetric Measurements 21

3.1 Materials Used 21

3.1.1 Reason for Only One Brown Sample 21

3.1.2 Reasons for Nonperformance of 1.1.7 25

3.1.3 Size of Samples 25

3.2 Spectrophotometrie Measurements for Daytime Color 25

3.3 Telecolorimeter Measurements for Nighttime Color . 26

4. Results Obtained in the Study 27

4.1 Colorimetric Results 27

4.1.1 Daytime Color 27

4.1.2 Nighttime Color 28

4.1.3 Variability of Nighttime Color 38

4.2 Photometric Results 47

V 4.2.1 Daytime Luminance Factors 48

ii
Page

4.2.2 Nighttime Specific Intensity Per Unit

Area 48

4.2.3 Variability of Nighttime Specific Intensity

per Unit Area Measurements 52

4.3 Comparison of Nighttime and Daytime Color 52

5. Recommended Test Procedure for Nighttime Conditions 57

5. 1 Colorimetric and Calibration Equipment 57

5.1.1 Source 57

5.1.2 Telecolorimeter 57

5.1.3 Supports 57

5.1.4 Calibration Standards 58

5.2 Calibration 58

5.2.1 Arrangement of Equipment 58

5.2.2 Adjustments 58

5.2.3 Reference Filters 59

5.2.4 Color-correction Factors 59

5.3 Color Measurement Procedure 59

5.3.1 Arrangement of Equipment 59

5.3.2 Colorimetric Observations and Computations 59

5.4 Photometric Measurement Procedure . 60

5.4.1 Photometer Specifications 60

5.4.2 Specific Intensity Per Unit Area 60

5.4.3 Special Precautions 63

6. Recommended Color Specifications of Retroref lective

Materials 64

iii
Page

6.1 Color Specifications for Retroreflective Delineators

and Lane Markers at Nighttime 65

6.2 Color Specifications for Retroreflective Sheet

Materials at Nighttime 65

6.3 Present Daytime Color Specifications 69

Acknowledgement 76

iv
Fig\ire No,

List of Figures

Page

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figiire 7.

Figure 8.

Figure 9.

Figiire 10.

Figure 11.

Figure 12.

Spectral characteristics of reference filters

(solid lines) compared with those of colored

retroreflectors (dashed lines)

Spectral transmittance of reference filters

suggested to improve spectral match for green emd

blue retroreflectors, shown in Figure 1.

Filters used in this study as reference filters for

white and silver retroreflectorsFilters used in this study as reference filters

for red retroreflectors.

Filters used in this study as reference filters

for yellow and orange retroreflectors.

Filters used in this study as reference filters

for green retroreflectors

10

11

Filters used in this study as reference filters for

blue retroreflectors . 12

Alternate calibration geometries.

Measurement geometry for nighttime conditions.

Schematic nighttime highway sign entrance and

observation geometry.

Geometry of retroreflectors, with lines, angles

and planes defined.

Chromaticity differences between D/0 and 45/0

geometries for red "engineering" grade and "high

intensity" grade retroreflective sheeting

materials. Boundaries on this figure are:

solid line, NJCUTCD for daytime red surface

colors; dashed line, 3M proposed for L-«S-300B red

retroref lectors in daylight.

13

14

17

18

33

V
Figiore No.

Figure 13,

Figxire 14,

Figure 15.

Figure 16,

Figure 17.

Chromaticity changes with change in entrance and

observation angles for a blue retroreflector

.

Boundary on this figure is 3M recommended limit for

nighttime conditions.

Chromaticity changes with change in entrance and

observation angles for a white retroreflector

.

Boundary on this figure is 3M recommended limit

for nighttime conditions.

Chromaticity data for several white retroreflectors in

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average chromaticity

obtained on the three instruments, ©. Chromaticity

obtained for sample 4 on each instrument, +. X.

Average standard deviations in x and y for all

samples in this figure are a = 0.012 and a = 0.012.

X y Boundarxes are recommended limits for nighttime

conditions: solid lines, 3M; dotted lines, NBS.

N;ambers refer to samples in Table III.

Chromaticity data for several red retroreflectors in

the Interlaboratory set obtained on three different

instruments. Geometry', 5/0.33- Average chromaticity

obtained on the three instruments , 0 . Chromaticity

obtained for sample 16 on each instrument, -, +, X.

Average standard deviations in x and y for all samples

in this figure are a = 0.014 and a = 0.015.

X v Boundaries are recommended limits for nighttime

conditions: solid lines, 3M; dotted lines, NBS.

Numbers refer to samples in Table III.

Chromaticity data for several orange retroreflectors in

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average ciiromaticity

obtained on the three instruments, o. Chromaticity obtained for sample 43 on each instrument, - , +, X.

Average standard deviations in x and y for all samples

in this figure are a = 0.010 and a = 0-015. Boundaries are recommended limits for nighttime conditions:

solid lines, 3M; dotted lines, NBS. Numbers refer to

samples in Table III.

Page

36

37

41

42

43

vi
'4 Figure No.

Figure 18.

Page

Chromaticity data for several yellow retroreflectors

in the Interlaboratory set obtained on three different instrximents . Geometry, 5/0.33. Average

chromaticity obtained on the three instruments, o

Chromaticity obtained for sample 12 on each instrument, *, +, X. Average standard deviations in x and y for all samples in this figure are

a = 0.010 and a = 0.011. Boundaries are recommended limits fo? nighttime conditions: solid lines,

3M; dotted lines, NBS. Numbers refer to samples in

Table III. 44

Figure 19. Chromaticity data for several green retroreflectors in

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average chromaticity

obtained on the three instruments, 0 . Chromaticity

obtained for sample 31 on each instrument, • , +, X.

Average standard deviations in x and y for all samples

in this figure are a = .011 and a = 0.026.

Boiindary is recommenced limit for Nighttime conditions

by NBS and 3M. Numbers refer to samples in Table III.

Figure 20. Chromaticity data for several blue retroref lectors

in the Interlaboratory set obtained on three different instruments. Geometry, 5/0.33. Average

chromaticity obtained on the three instruments, o . Chromaticity obtained for sample 21 on each instrument, •, +, X. Average standard deviations in x and y for all samples in this figure are

a = .005 and a = 0.033. Boundaries are recommended limits f^r nighttime conditions: solid

lines, 3M; dotted lines, NBS. Numbers refer to

samples in Table III.

Figure 21. Recommended color boundaries for retroreflective

delineators and lane markers at nighttime.

Figure 22. NBS recommendations for color specifications of

retroreflective sheet materials at nighttime

Figure 23. Recommendations of the 3M Company for color

specifications of retroref lective sheet

materials at nighttime.

Fic-jure 24. Present surface color boundaries of the

NJCUTCD for daytime.

45

46

67

70

73

75

vii
List of Tables

Table No.

Table I.

Table II.

Table III.

Table IV.

Table V.

Table VI.

Table VII.

Table VIII.

Table IX.

Table X.

Table XI.

Table XII.

Table XIII.

Table XIV.

Entrance and Observation Angles

The NBS Set of Materials Used in this Study

The Interlaboratory Set

Chromaticity Coordinates and Reflectance

Factor for Daytime Conditions

Munsell and ISCC NBS Color Under Daytime

Conditions

Colorimetric Data of the NBS Samples for

Several Geometries

Colorimetric Data of tlie Interlaboratory Samples

Summary of Tristimulus-Value Correction Factors

of Trimmed Instruments for Several Colors

Summary of Variability of Color Measurements,

Interlaboratory Set

Comparison of Daylight Luminance Factor, 3#

of Retroreflective Materials, NBS Set

Comparison of Specific Intensity per Unit

Area Obtained by Two Methods . Geometry

5/0.33, NBS Set

Specific Intensity per Unit Area by Intensity

Method. Geometry: 5/0.33

Summary of Photometric-Correction Factor for

C.I.L./A Measurements for Several Colors

Variability of C.I.L./A in Terms of Average

Standard Deviation a, and Coefficient of

Variation V(%)

Page

19

22-23

24

29-30

31-32

34

35

39

40

49

50

51

53

54

viii
Table No.

Table XV. Comparison of Nighttime and Daytime Color of

Retroreflectors

Table XVI NBS Recommended Color Specifications for

Retroreflective Delineators and Lane Markers

at Nighttime

Table XVII NBS Recommended Color and C.I.L./A Specifications of Retroreflective Sheet Materials

at Nighttime

Table XVIII Chromaticity Coordinates of Intersections of

NBS Recommended Boundary Lines for Retroreflective Sheet Materials at Nighttime

Table XIX 3M Recommended Specifications for Retroreflective Sheet Materials at Nighttime

Table XX Present Color and Luminance Factor

Specifications for Retroreflective Sheet

Materials in Daytime

ix
INSTRUMENTAL

COLORIMETRY OF RETROREFLECTIVE MATERIALS

ABSTRACT

Because color-coded applications of highway signs increase,

specifications of colors and color tolerances are required.

In order to assure that the requirements are met within specified

regions, a measurement technique needs to be developed and

described. To accomplish these goals the U. S. Department of

Transportation contracted with the National Bureau of Standards

to conduct the required studies and make the necessary recommendations. Having previously perfomed a study for daytime

conditions, the National Bureau of Standards was competent to

perform a study for nighttime conditions. The colorimetric

properties of 126 samples of retroreflective materials of 7

different colors were measured with 3 telecolorimeters in simulated nighttime conditions. One spectrophotometer was used to

measure color of 38 of the samples in simulated daytime conditions.

The colors measured were: red, orange, brown, yellow, green,

blue and silver (white) . Differences of color measured by means

of different telecolorimeters on the same samples were evaluated.

As a result of these studies procedures for making colorimetric and photometric measurements were developed and are included in this report. On the basis of the color measurements

and their variability tentative recommendations for color

boundaries were prepared and are also included in this report.

X
INSTRUMENTAL

COLORIMETRY OF RETROREFLECTIVE SIGN MATERIALS

1, Introduction

Highway signs and demarcations are frequently marked with

retroreflective materials. Theoretically, perfect retroreflecting

materials reflect all of the unabsorbed light back along a direction

that is opposite to that of the incident light. Consequently, any

measurement of retroreflectively returned light should be measured

along that direction. In general, however, retroreflective sheeting

materials are not perfectly retroreflective, but reflect light in a distributive manner about the retroreflective direction. In the

daytime they are observed by the driver under diffuse illumination and

at night they are illuminated by the automobile's headlights and

viewed by the driver seated behind the source.

For the Interstate Highway System seven colors are used for

these materials: silver, blue, yellow, red, green, brown, and orange.

The measxurement of the color for compliance with the color and color

tolercince of a particular batch of materials is done visually . For

nighttime use that measurement and specification of the reflective

properties of these materials be made at conditions of unidirectional

illumination with varying entrance angle and small observation angle

viewing. The entrance angle usually is less than 10° from the perpendicular to the sign at distances greater than 300 feet.

Federal Specification L-S-300A, dated January 7, 1970, for

reflective sheeting and tape materials used for general purposes, on

the other hand, states in paragraph 4.4.8 Color

"Determine the color of the reflective

material in accordance with ASTM-E-97-SS

(Geometric characteristics must be con- fined to illiomination incident within 10

deg. of, and centered about, a direction

of 45 deg. from the perpendicular to the

test sxirface; viewing is within 15 deg. of,

and centered about, the perpendicular to the

test surface. Conditions of illvimination

and observation must not be interchanged.)

The standards for calibrating the test

apparatus shall be the Munsell Papers

designated in table I. They must be recently

calibrated on a spectrophotometer. The test

instrument shall be one of the following:

1. Gardner Multipurpose Reflectometer

2. Gardner Model AC-2a Color Difference

1
3. Meeco Model V Colormaster.

4. Hunterlab D25 Color Difference Meter."

. At the request of the Office of Traffic Operations through the

Office of Research of the Federal Highway Administration, the Illuminating Engineering Group of the National Bxareau of Standards was asked

to conduct a research study of the color measurement and specification

of retroreflective highway sign sheeting materials under nighttime

conditions. The objective of this study was to develop an acceptable

instrumental procedure to quantitatively measure the nighttime color

by means of CIE tristimulus data of retroreflective sign sheeting

materials representing the seven standard colors now in highway use.

A contract, dated December 20, 1972, Purchase Order No. 3-1-1011

and confirmed in March, 1973, states the various tasks required to

achieve this objective:

i. 1 Statement of Work

1.1.1 Collect a full representative range of samples of retroreflective highway sign sheeting materials that represent

' ^ ^ all current manufacturers and also represent a full variation in color range normally produced for each of the

six (6) standard highway colors - blue, yellow, red, green,

'

: brown, and orange. Also include silver sheeting. Include

all types of reflectorization that are in regular commercial production as well as 3M high intensity sheeting but

specifically exclude other experimental materials.

". 1.1.2 As a reference, characterize the daytime-appearing color

of each material by means of tristimulus data and chromaticity coordinates by instrinnental means using a method

developed in a previous contract <diffuse illumination and

unidirectional viewing) . Also include a reference visual

comparison of each material to the color tolerance charts

for daytime and nighttime appearing color.

1.1.3 Develop an instrumental procedure to measure the nighttime

appearing retro-reflective color of the above materials.

The method and instrument should be based on tristimulus

color filters and CIE chromaticity coordinates and be

capable of being easily obtained and utilized by State highway departments. The color properties of the light source

in the method developed shall be equivalent to the average

automobile headlight. Preliminary optical geometries to be

studied shall include incidence angles of 2*, 25°, and 30°,

and divergence angles of 0.2°, 0.5°,- and 2.0°. From these

the minimum and most appropriate geometries for a pertinent

and standard measurement shall be evolved which are com- patible with real headlight-sign-driver geometries. The

method and instrumentation evolved shall be applicable for

laboratory as well as field use.

2
1.1.4 Instruments to be utilized in item 3 above shall include

as a minimum the Pritchard and Gamma photometers equipped

with suitable color filters for tristimulus color evaluation.

1.1.5 The study leader shall make an on-site review at the 3M

Company's technical center (J. Elstad) to determine the

technical principles and procedures developed by them to

address this particular problem of color measurement.

Other companies such as American Decal and Manufact\iring Co.,

Rohm and Haas, and the Munsell Co. shall be contacted and

visited if necessary for additional input. Emphasis should

be given to developing a suitable method based on the

above-mentioned commercially available photometers or their

commercial equivalents.

1.1.6 After 4 above, develop a proposed specification and limits

for the color range permitted for nighttime-appearing

retro-reflective color of ref lectorized highway signs for

the six (6) standard colors and silver based on tristimulus

measurements and chromaticity coordinates.

1.1.7 As a supplement to the above work but in an exploratory and

less exhaustive manner perform the work outlined in items 1

thru 6 on both raised reflectorized lane markers and reflectorized roadside delineators colored silver, red and

amber. The types of reflectorization shall include corner

ciibe, lens, beads or other. Raised traffic lane markers

investigated shall include but not be limited to products

from Stimsonite and Ray-o-Lite (corner cube) Borg-Warner

(lens) , Catadots (beaded) , 3M (rods) and other types

and manufacturers. Roadside reflectorized hazard markers

shall be representative of all standard varieties from

various sources. The range of incidence and divergence

angles utilized shall approximate nighttime driver viewing

geometries (as seen from 40 to 400 foot distances from a moving vehicle)

.

1.1.8 Prepare and deliver monthly letter reports, a draft final

report, and final report on the data and findings.

1.2 Partial Support by Federal Aviation Administration

Because of a common interest in the results of the study

reported in this document, the Federal Aviation Administration supported this work in part.

3
2. Measurement System

2.1 [ Colorimeters and Standards

A photoelectric tristimulus colorimeter is a device that

consists of a means for sampling three different parts

of the visible spectrum and evaluating these by a system

which correlates with hiiman response to color. The system

used most extensively is the CIE Standard Observer System,

adopted by the International Commission on Illumination

in 1931. It is possible to design optical filters that,

. . together with a photodetector will closely approximate the

"• ^ three functions of the CIE System. To minimize the effect

of the differences between the approximation and the

actual function requires the use of standards with which

.to calibrate the instrxunentation.

2.1.1 Standards can be of at least two different forms. One"

type of standard makes use of a white diffusing plate of

known spectral reflectance and a series of reference

filters that have spectral characteristics similar to

those of the test materials. Figiire 1 shows the spectral

reflectance of a series of 4 reference filters, available

, from the National Bureau of Standards as standard samples:

2101 Red, 2L02 Yellow, 2103 Green, and. 2104 Blue, com-

pared with the spectral reflectances of a series of typical

. retroreflective sheeting materials. The spectral character-

. . . .• istic red and yellow filters are similar to those of the

red and yellow materials, while the spectral characteristics

of the blue and green filters are sufficiently dissimilar

to render their use impractical. To improve the accuracy

of calibration other blue and green reference filters are

. required for this purpose.

- As no single filter has the required spectral characteristics, a future investigation may attempt to design

filter combinations that approximate the spectral reflec-

.. tance characteristics of green and blue retroreflectors

.

The spectral curves shown in Figure 2, to approximate the

curves shown in Figure 1 for green and blue retroref lective

sheeting materials are based on nominal spectral transmittance data of combinations of the following Corning

glasses

Green: 4060 (2.0) + 40S4 (4.5)

Blue: 3750 (5.0) + 3962 (2.5) + 5330 (3.4)

Numbers in parentheses are nominal filter thicknesses in

millimeters

.

4
100 100

u

o

<

^00 500 600

100

500 600

\00 500 600 700

700 400 500

WAVELENGTH, nm

600 700

Figure 1. Spectral characteristics of reference filters (solid lines)

compared with those of colored retroref lectors (dashed lines)

5
6
The filters on hand were used as reference filters to calibrate our telecolorimeters . The spectral transmittances

of the filters compared with the spectral reflectances of

the retroreflective materials are shown in Figures 3 to

7 for white, red, yellow and orange, green, and blue,

respectively.

A geometric arrangement for calibrating the instalment

with white diffuser and filter is shown on the lower part

of Figure 8. The angle of 45" is chosen because the . spectral characteristics of the diffusers are generally

known for this geometric condition. The entrance angle

and observation angle for which the spectral reflectance

of the diffuser is known should be used.

The reason for using a diffusing material in the calibration

is to provide an illuminated area at a finite distance on\

which the telecolorimeter can be focussed and to minimize

effects of slight misalignment. Misfocussing, a possible

source of errors, should be avoided, minimized, or its

effect evaluated and correction applied.

There are several materials that can be used as white

diffuse reflectors of known spectral characteristics.

These are: MgO, MgCO^. and BaSO^. The last one, BaSO^,

may be the most convenient to use because it is less

fragile than MgO and whiter than MgCO^* The diffuser

should be of such size that the colorxmeter looks at an illuminated area within the area of the diffuser.

The upper part of Figure 8 shows an alternate calibration

method, employing a diffuse transmitting material of

known spectral characteristics, such as a singly-flashed

opal diffuser, also is available. This method was used

in calibrating our colorimeters with which we made the

color measurements reported in this paper. The measurement

geometry of entrance angle and observation angle is

shown in Figure 9.

Photoelectric tristimulus colorimeters, when properly calibrated can give reasonably stable and accurate results.

The filters are quite good approximations to the CIE

standard colorimetric observer fxinctions and the electronics

are quite stable and reliable.

What constitutes a proper calibration standard for a photoelectric tristimuliis colorimeter? Ideally, when the

test material is highly selective either in its spectral

7
400 SOO 600 700

Waveltnjth, nm Figure 3. Filters used in this study as reference filters for white and

silver retroreflectors

8
Wavtleirjth, nm

re 4. Filters used in this study as reference filters for r retroref lectors

9

11
400 500 600 700

SO

80 -

Blu« R^troreflsctors

60 -

8

£ 50 -

4C0 500 600 7^

WavaJ«ngth, nm Figure 7. Filters used in this study as reference filters for blue

retroreflectors

12
13
UJ

14
or geometric reflectance properties, as retroref lectors

are, then materials of essentially the same spectral and

geometric characteristics as the test material

should be used as standards with which to calibrate

the instrument. Before such standards are used,

however, care should be taken to assure that their

assigned values are reasonably correct for the re- quired geometric conditions. An effort is being

made at NBS to develop such standards.

Once the instrument (telecolorimeter) is set to read the

correct tristimulus values of the white diffuser then the

proper reference filter is inserted in the light path and

the tristimulus values are read. Correction factors are then derived as the ratio of the reading to the known

values for the filter-diffuser combination.

With the components, source, test sample, and receptor in

place, measurements can be made without further adjustments to the telecolorimeter. To obtain the correct

tristimulus values X, Y, Z, the readings X', Y', Z' should be multiplied by the correction factors, F^^, F^,

F , obtained thus:

z X=X' F^= (X'^ + X'j^) F^

y = Y' Fy

Z = Z- F^

From these correct tristimulus values the chromaticity

coordinates can be computed.

X = X/ (X + Y + Z)

y = Y/ (X + Y + Z)

Average correction factors were derived and used to obtain

tristimulus values from the telecolorimeter readings

.

Chromaticity coordinates were computed from these corrected

tristimulus values.

2.1.2 The other type of standard makes use of actual retroreflective materials of which the spectral and geometric

characteristics are similar to the test materials. This

type of standard has the advantage over the use of glass

filters in that the standards would have similar geometrical

retroref lective characteristics as the test samples. This

is important with materials that are as geometrically

selective as retroref lectors . As is shown later in this

report, the color appearance of retroreflective materials

15
depends on the geometries of incidence and viewing.

2 . 2 Daytine Geometry

In daytime a highway sign is illxaminated by skylight and

viewed in a direction that is nearly perpendicular to the

plane of the sign. If at all feasible, the color measurement gecraetry should simulate these geometric use con- ditions, that is, diffuse illumination with daylight

spectre,! dxstribution and approximately perpendicular

- viewing. This geometry has been recommended in Report

FHWA-RI)TM71-I dated November, 1971. (This report is

' available from the National Technical Information Service,

Springfield^ Va. 22151, under Document Accession No.

P3 204-58b.

}

2.3 Nighttime Geometry

At night a highway sign is illuminated by the automobile

; headlight-s operating at about 2856K. A schematic representation of the geometric situation is shown in Figvure

10. The sign is illuminated along an incidence line

different from the reference axis, the line perpendicular

to the sign at its center. The angle between these two

lines is the entrance angle. The light that is retroreflected by the sign is usually viewed at an angle

slightly different from the incidence angle. The angle

between the incidence line and the observation line is

called the observation angle. These angles, entrance and

observation, change dynamically as the observer approaches

the sign. A general description of the geometry of

retroreflectors in the nighttime illuminating and viewing

situation is shown in Figure 11. Consequently, the

geometry under which the photometric and colorimetric

measurements are made should approximate the condition ' under which the sign is seen and read during the approach

by the observer.

A relationship between the entrance angle z^, the observation angle a, the viewing angle e^^ and the position

angle y iti^Y be established by application of geometric

and trignometric principles. This relationship is:

COS a cos + sin a sin cos y = cos

' '

' Several special cases readily can be used to test this

; . relationship.

16
z

o

17
Geometry of Retroreflectors

Figure 11. Geometry of retroreflectors, with lines, angles and planes defined

18
Conditions Results

I. Y = 0°, cos Y = 1 cos a cos e + sin a sin e = cos

J. cos (a - e ) 2 a — e

2

cos

1 =

^1

II. Y = 180", cos Y = -1 cos a cos - sin a sin cos

"l cos (a + e^) cos ^1

a + ^2 =

^1

Ill, Y = 90°, cos Y = 0 cos a cos ~

cos (cos a cos Gj^"

IV. a. = 0° , cos a + 1

sin a = 0 cos e = cos e

^2 = ^1

The usual entrance angles for measurement of specific

intensity or specific luminance are -4°, 15°, and 30°,

and the usual observation angles, angles between the

entrance and observation lines for these photometric

measurements are usually 0.2°, 0.5°, and 2.0°. Recent

discussions among retroreflector manufacturers and

government laboratories have suggested that 0.2° observation be replaced by 0.33° and that 5° entrance angle be

added. These angles are listed in Table I.

Table I. Entrance and Observation Angles

Entrance Angles Observation Angles

- 4° 0.2°

5° 0.33°

15° 0.5°

30° 2.0°

19
2.4 Special Geometries

For some special purposes, such as highway delineator

markers and highway lane stripes, geometries other than

those described above are required. Since the painted

highway lane stripes are placed on a horizontal plane,

for example, the entrance angle would be rather large,

close to 90' from the reference axis. Entrance angles

used for this purpose are specified in Federal Specification TT-P-85D, Paint, Traffic, Reflectorized, for

Airfield Runway Marking (drop-on-type) , and are either

75° or 88°. Observation angle is specified in this

document as 1'' 20' for either entrance angle.

Because raised lane delineators are sloped relative to

the road, the entrance angle is less than that for

painted highway stripe.

20
3. Colorimetric Measurements

3.1 Materials Used

The materials used were those made by three manufacturers.

These consisted of 32 retroreflective sheeting materials

made by two manufacturers and 6 molded cxabe-corner

retroreflectors made by a third manufactiarer . Of the

retroreflector sheeting materials made by one of the manufacturers two were the so-called "diamond grade" com- posed of diamond-shaped sections with very small,

possibly embossed, corner-cube retroreflective elements,

five were the so-called "high intensity" grade and had

hexagon-shaped sections, and two were of glass-bead

impregnated fabric.

The retroreflective materials with diamond- shaped sections

have different retroreflecting characteristics for

different orientations of the major and minor diagonals

of the diamond sections. The hexagonal section dividers

support the outer covering which encapsulates the

retroreflective elements. The remaining 25 retroreflective sheeting materials were of the engineering

grade. In Table II are listed the various retroreflective

materials that were used in this study together with their

trade numbers, nominal color, and other special descriptive remarks. All of these were used for the daytime

color measurements. Some, however, were not used in the

nighttime color measurements. The group of samples listed

in Table II will be called the "NBS set" in this report.

Forty-four additional samples of retroreflecting sheeting,

part of an international interlaboratory study, were

supplied by their manufacturer. These were used to make

photometric and colorimetric measurements under nighttime conditions. They were mounted on 1/16" aluminum

plate, 6 inches square. This set of samples consisted of

white, red, orange, yellow, green, and blue "engineering"

and "high intensity" grade retroreflective sheeting. Table

III shows the designated numbers, name, and grades of

these samples of retroreflective sheeting. This group of

samples will be called the "Interlaboratory Set" in this

report

.

3.1.1 Reason for Only One Brown Sample . In response to a request of manufacturers of retroreflective materials of

"engineering" grade or better, only one manufacturer siabmitted a brown retroref lective sheeting sample. Apparently

this is the only brown retroreflective sheeting available

in the "engineering" grade or better.

21
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23
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24
A brown retroreflector is actually dark orange or, in

some instances dark yellow or dark red, when seen in

daylight. But when seen at night as a retroreflector, it

appears as a dim source that may be mistaken for these

other colors.

3.1.2 Reasons for Nonperformance of 1.1.7 . A number of circumstances delayed the effective start of the measurement

part of the study. Because of this, the large number of

samples collected, the large amount of work required,- and

the low priority given in 1.1.7 of the work statement,

measurements were not made on raised reflectorized lane

markers and reflectorized roadside delineators. However, cxibe-corner retroreflective roadside delineators

often have characteristics similar to those of the retroreflective plaques listed in Table II. The procedures

developed and reported in Section 5, Recommended Test

Procedure for Nighttime Conditions are also applicable to

these materials.

3.1.3 Sample Sizes . The NBS set of retroreflective sheeting

materials listed in Table II were supplied in 12- inch

squares. From these, 2- inch squares were cut for the

spectrophotometric measiirements described below and 6-inch

squares were cut for the colorimetric measiarements with

the telecolorimeter

.

The molded plaque cube-corner retroreflectors were measured

with the spectrophotometer and the telecolorimeters in

the sizes supplied. The sizes of these retroreflective

materials are shown in Table II.

Spectrophotometric Measurements for Daytime Color

The spectrophotometric measurements were made in terms of

the spectral radiance factor.

Measurements of spectral radiance factor of the NBS Set

of Materials (listed in Table II) were made by means of a

Cary Model 14 recording spectrophotometer equipped with a

Model 1411 integrating sphere type reflectance attachment,

a linear slidewire (0 to 100%T) , and a Hamamatsu R136

(near S-20 surface) multiplier phototube detector. The

integrating sphere of the Model 1411 reflectance attachment v/as irradiated with non-dispersed flux from a tungsten source focused on the opening at the bottom of

the integrating sphere. The specimen is irradiated

hemispherically and that portion of the irradiating flux

which is reflected in a direction 6° from the normal to

the specimen surface is passed through the monochromator

for analysis. This geometry is nominally diffuse illumination and perpendicular vieving and may be represented
as D/0. Strictly speaking however, it is D/6. The

spectral band-pass was approximately 1 to 2nin. The width

of the slits of the monochromator is automatically adjusted

throughout the spectral scan in order to maintain a constant energy signal as sensed by the electronics

during the reference beam cycle.

The photometric scale of the instrument was set to

record 100% by means of the identical MgO references

located at the sample and reference openings of the integrating sphere. The spectral radiance factor of a reflectance standard (white-glass standard TWl-Bl) was

measured together with the specimens in order to obtain

data with which to adjust the measured values of spectral

radiance factor of the specimens to be relative to

freshly prepared smoked MgO. Each of the specimens was

measured for two orientations; the first was with the

specimen identification on the back of the specimen

positioned so that it could be read when the specimen was

mounted against the sample port of the integrating sphere

and the second measurement with the specimen rotated

clockwise 90° in its own plane. Zero calibration curves

were recorded with the sample beam blocked.

Telecolorimeter Measurements for Nighttime Color

3.3.1 Color Measurements . Color measurements were made with

a large variety of geometries. Using the procediires

described below, in Section 5, Recommended Test Procedures,

the color measurements were made on a Pritchard photometer

and a Gamma photometer equipped with telescope and suitable

tristimulus color filters which convert them into telecolorimeters .

The Gamma Telecolorimeter, Model 2009K, is manufactured by

Gamma Scientific, Incorporated, 3777 Ruffin Road, San

Diego, California 92123. The Pritchard Telecolorimeter

is the Pritchard Photometer, Model Spectra 1980, equipped

with tristimulus filters, and is manufactured by Photo

Research, 3000 N. Hollywood Way, Burbank, California 91505.

3.3.2 Specific Intensity Measurements . Specific intensity

measurements were made with the Pritchard and Gamma photometers, as described below in Section 5, Recommended Test

Procedures. Only one geometric condition was used, that

of 5/0.33.

26
4. Results Obtained in the Study

The colorimetric and photometric resxilts obtained on the materials

used in this study are given in this section.

4. 1 Colorimetric Results

4.1.1 Daytime Color . The daytime color of the samples was

obtained from the spectrophotometrie measxirements described

in Section 3.2. The tristimulus values X, Y and Z and

the chromaticity coordinates x, y, z were obtained by

the CIE standard observer system by means of the following

equations:

X = Js(X)3(X)x(A)dX

Y = Js(X)3(X)y(X)dX

Z = /s(X)3(X)z{X)dX

X = X/(X + Y + Z)

y = Y/(x + Y + Z)

z = Z/ (X + Y + Z) ,

where

S(X) = spectral distribution of the source for

daytime

3(X) = spectral radiance factors of the samples for

diffuse irradiation

/ x(X)/ y(X), z(X) = spectral tristimulus values of

the standard observer.

When the spectral radiance factor 3(X) is used in the

computation of tristimulus value Y, Y is called the

luminance factor 3- For daytime conditions the spectral

distribution of the source was represented in the computations by CIE Source C and by CIE Source D^^. CIE Source

C is intended to represent average daylight with a correlated color temperature of approximately 6774K.

CIE Source represents a phase of daylight with a correlated color temperature of approximately 6504K.

CIE Source D^^ incorporates atmospheric absorption bands

as well as some ultraviolet radiation, but CIE Source C

does not. (The International Commission on Illumination

anticipated that at some future time Source C will be

dropped from the list of standard illuminants .

)

27
The spectral reflectance factors of the samples were

obtained under conditions of diffuse illumination and

unidirectional, 6° view. Table IV lists the results of

chromaticity coordinates x, y and luminance factors 6/

obtained for the NBS set of retroref lective samples.

Also listed for comparison are data, obtained on a photoelectric tristimulus colorim.eter with 45" incidence

angle and 0° viewing angle (45/0) , as submitted by the

manufacturer of the samples. Table V shows the ISCCNBS color names and the Munsell notations of these

samples derived from the colorimetric data obtained vmder

D/0 and 45/0 geometry for CIE Source C. Figure 12 shows

for four red retroreflectors , a typical plot of the

chromaticity coordinates of the samples obtained under

these two geometric conditions. The solid lines are

NJCUTCD specification for red surface colors, and the

dashed lines are boundaries proposed by 3M for daylight

red retroreflector specifications in L-S-300B.

4.1.2 Nighttime Color . If spectral reflectance factor data

were obtained on a spectroradiometer or spectrophotometer

that simulates nighttime geometry, the equations given

in 4.1.1 would apply to the computation of nighttime

color. The spectral distribution of the soxirce in this

application would be that of 2856K.

The colorimetric data x, y obtained with the NBS set of

samples for several geometric conditions are listed in

Table VI. The particular geomet2:y is indicated by the

entrance angle over the observation angle. Thus an

entrance angle of say 5° and an observation angle of say

0.33" is shown as 5/0.33.

Table VII shows the colorimetric data x, y obtained with

the Interlaboratory set of samples for a 0.33'* observa-

. tion angle and entrance angles of -4°, 5°, and 10°.

It has been shown by several investigations that the

nighttime color varies slightly with entrance angle

and with observation angle. We have also found this

effect as is shown on Figure 13 for blue retroreflectors and on Figure 14 for white and silver

retroreflectors. Boundary lines on these figures

are those recommended in Section 6.2.2.

For the interlaboratory study it was suggested that one

entrance angle, 5°, and one observation angle, 0.33°,

designated 5/0.33, be used. But as seen in Figure 14,

if the data are close to the edge of the tolerance area

for one geometry some data may fall outside, for another

28
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i'lrr,^^-^ J.2. Chromaticity differences between D/0 and 45/0

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intensity" grade retroref lective sheeting

materials. Boundaries on this figure are:

solid line, NJCUTCD for daytime red surface

colors; dashed line, 3M proposed for L-S-300B red

retroref lectors in daylight.

.55 .60

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2

3

4

5

6

7

. 8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

VII. Colorimetric Data of the Interlaboratory Samples

-4/0.33 5/0.33 10/0. 33

X y X y X y

0.419 0.426 0.421 0.423 0.419 0.425

.413 .415 .412 .418 .412 .418

.390 .404 .392 .402 .394 .406

.456 .449 .458 .447 .463 .444

.417 . .436 .418 .438 .422 .439

.444 .432 .463 .450 .466 .450

.530 .469 .533 .465 C ^5 o .533 .466

.538 " .461 .538 .460 COO .538 .461

.535 .458 .536 .453 .535 /CI .457

.512 .514 .474 C T C .515 .474

.540 .454 .540 .454 .544 .450

.566 .430 .564 .431 .566 .430

.658 .341 .656 .342 .657 .342

.631 .363 .632 .362 .633 .362

.654 .345 .654 .344 .654 .345

.627 .366 .628 .366 .629 .366

.643 .352 .644 .352 .646 .349

.655 .341 .657 .341 .656 .341

.643 .351 .642 .354 .646 .348

.677 .323 .673 .326 .672 .326

.100 .310 .099 .310 .099 .307

.104 .244 .106 .242 .110 .242

.107 .268 .108 .267 .108 .269

.112 .351 .112 .350 .111 .350

.086 .257 .087 .256 .086 .256

.086 .202 .087 .203 .085 .204

.083 .252 .082 .252 .081 .252

.172 .537 .172 .538 .171 .536

.190 .545 .190 .544 .190 .540

.217 .593 .216 .592 .214 .595

.196 .577 .196 .579 .194 .580

.186 .559 .185 .562 .185 .561

.173 .590 .171 .592 .172 .589

.167 .611 .166 .611 .167 .610

.181 .643 .180 .642 .181 .646

.618 .377 .618 .378 .616 .379

.628 .370 .627 .370 . 629 .369

.535 .401 .534 .400 .537 .400

.515 .390 .512 .392 .516 .391

.570 .390 .572 .388 .572 .389

.559 .373 .555 .374 .561 .374

.620 .374 .622 .372 .620 .374

.582 .385 .582 .386 .583 .385

.592 .382 .596 .379 .596 .380

35
0.350

0.300

15/0.2^-4/0.2

30/0.2

0.250

45/0.2

-4/0.5

15/0.5

30/0-5

45/0.5

0.200

0.100 0.150 0.200

Figure 13. Chromaticity changes with change in entrance and

observation angles for a blue retroreflector.

Boundairy on this figure is 3M recommended limit for

nighttime conditions.

36
Figure 14. Chromaticity changes with change in entrance and

observation angles for a white retroreflector

Boundary on this figure is 3M recommended limit

for nighttime conditions.

37
geometry. Hence, we may conclude that color measiared

under one geometry may not be adequate.

The tristimulus-value correction factors of the telecolorimeters used in this study are listed in Table VIII.

For the Gamma telecolorimeter these factors listed

represent the correction factor for electrical trimming

of the responses. For the Pritchard telecolorimeter the

factors listed represent the correction factors for mathematically trimmed responses. Mathematical trimming was

necessary on the Pritchard instrument because it had

been designed as a telephotometer and tristimulus filters

were added.

1.3 Variability of Nighttime Color Measurements . To determine

how well color measurements can be replicated, the variability of measurements for one geometry 5/0.33 was

studied. Two sets of measurements were made on one instrument at the National Bvireau of Standards, a set of measurements was made on each of two different instruments at the

National Bureau of Standards and a set of measurements

were made on each of three instruments, two at the National

Bureau of Standards and one in another laboratory. The

variabilities of the measurements for these combinations,

represented by the average standard deviations a and the

average coefficient of variation, V = (a/Mean) for several

samples of each nominal color, are listed in Table IX.

The following figures show the data obtained on the two

NBS telecolorimeters and at another laboratory on one telecolorimeter for the retroreflective materials on the Interlaboratory set of samples for one geometry. Figure 15

shows the colorimeter results (chromaticity coordinates)

for the silver and white retroreflectors. The circles

represent the average chromaticity measured on the three

instrximents. Also shown on this figure are the difference

between the three measurements and their average for one

sample. The differences among the measurements for the

other samples of this color in the Interlaboratory Set is

similar to the one shown. The standard deviations in x

and y are also shown in this figxire's caption.

The results obtained for the other colors are shown in

Figure 15 for red. Figure 17 for orange. Figure 18 for

yellow, Figure 19 for green, Figure 20 for blue, retroreflectors. Boundary lines on Figure 15 through 20 are

those recommended in Section 6.2.

38
Table VIII. Summary of Tristimulus-Value Correction Factors*

of Trimmed Instruments for Several Colors

Gamma Instrument Pritchard Instrument

F ^2

0., 893 0..948 / 0. 265 0.. 902 0..917 0 . 166

Yellow/Orange 0-,996 0..972 1. 055 0..980 0..891 .004

j Green 1.,002 1.,051 0. 882 0..817 0..940 0..934

Blue 0,,707 0.,944 0. 921 0,.594 0..691 0..941

White 1.,000 0.,998 0. 953 0.,957 0..974 0.,989

*Not all of the correction factors for these instruments met the

requirements discussed in section 5.2.4 below. Although correction factors for red samples / and F^ for blue samples are

relatively unimportant, better approximations to the CIE functions

can be achieved by the manufacturer. '

39
Table IX. Summary of Variability of Color Measurements, Interlab. Set

Color

3 Inst.,

0X

2 Lab,

ay

2 Inst.,

aX

1 Lab.

ay

1 Inst.,

aX

1 Lab.

c

y

White 0.012 0.012 0.005 0.009 0.0012 0.0014

Yellow .010 .011 .014 .015 .0015 .0020

Red .014 .015 .016 .016 .0010 .0009

Blue .006 .033 .004 .020 .0007 ,0009

Green .011 .026 .010 .018 .0005 .0010

Orange .010 .015 .010 .011 .0012 .0010

Mn 0.010 0.019 0.010 0.015 0.0010 0.0012

40
.500

Figure 15. Chroraaticity data for several vvhite retroref lectors in.

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average chromaticity

obtained on the three instruments, © . Chromaticity

obtained for sample 4 on each instrument, • , +. X.

Average standard deviations in x and y for all

saunples in this figure are o = 0.012 and o = 0.0J2.

Boundaries are recommended Ixraits for nighttime

conditions: solid lines, 3M; dotted lines, N3S.

Numbers refer to samples in Table III.

41
Figure 16. Chromaticity data for several red retroref lectors in

the Interlaboratorv' set obtained on three different

instruments. Geometry, 5/0.33. Average chroniaticity

obtained on the three instri-unents , o. Chromaticity

obtained for sample 16 on each instriiment, +, X.

Average standard deviations in x and y for all samples

. ' in this figure are o = 0.014 and o - 0.015.

X V

Boundaries are recom-"\ended limJ-ts foi nighttime

conditions: solid lines, 3M; dotted linos, NBS.

Nuirvbers refer to samples in Table III.

42
FLgvcce 17. Chromaticity data for several orange retroref lectors in

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average chromaticity

obtained on the three instruments, o. Chromaticity obtained for sample 43 on each instrument, •

, -S-, X.

Average standard deviations in x and y for all samples

in this figure are a = 0.010 and c = 0.015. Boundaries are recommended limits for nighttime conditions:

solid lines / 3M; dotted lines, KBS. Numbers refer to

samples in Table III.

43
in the Interleiboratory set obtained on three different instruments. Geometry, 5/0.33. Avercige

chroraaticity obtained on the three instruirients , q.

Chrornaticity obtained for sample 12 on each instrument, , -h, X. Average standard deviations in x and y for all samples in this figure are

0 = 0.010 arid o O.OJl. Boundaries are recomX y mended lim.its for nighttiir.e conditions: sclid li.nes,

3M; dotted lines, NBS. NunolDers refer to saip.plt. r. in

Table III.

44
Figure 19. Chrorr.aticity data for several green retroref lectors in

the Interlaboratory set obtained on three different

instruments. Geometry, 5/0.33. Average chromaticity

obtained on the three instruments, © . Chromaticity

obtained for sample 31 on each instrument, +, x. Average standard, deviations in x and y for all samples

in this figure are a = .011 and a ^ = 0.026.

Boundary is recommenced limit for nighttime conditions

by NBS and 3M. Numbers refer to samples in Table III.

45
Figure 20. Ctiromaticity data for several blue retroref lectors

in the Interlaboratory set obtained on tliree different instruments. Geoinetry, 5/0.33. Avera^je

chromaticity obtained on the three instruiricnLs , o . Chromaticity obtained for s:ijnple 21 on each instrument, •/ X. Average standard deviations in x and y for all samples in this figure are

Q = .006 and o = 0.033. Boundaries are recomX V

: - mended limits fOr nighttime conditions: solid

lines, 3M; dotted linos, N3S. Numbers refer to

sajrnples in Table III.

46
The variability among the colorimetric results obtained

in this study is consistent with that obtained in earlier

studies. It is our hope that as we improve our selection

of reference calibration filters and our calibration

techniques that this variability will be reduced.

4. 2 Photometric Results

Under typical daytime conditions the luminance of a retroreflective sign, or legend, or other object is not a significant factor in determining visibility. Instead

the contrast between an object or legend, and its background determines visibility . Since under most daylight

conditions the illuminance on the object and on its

background are equal and independent of distance, or

nearly so, the contrast is a function only of the luminance

factor of reflecting materials, such as retroreflectors

and paints, is required for the evaluation of their

daytime performance.

On the other hand, under nighttime conditions the

luminance of a sign or object, or the intensity of a delineator, produced by the lights of an automobile varies

inversely as the square of the distance. The liaminance

may be so low that the color of the object can not be

recognized, or the object might not be seen. The

luminance of the background is usually much lower than

the luminance of a retroreflective sign or legend and

has no significant effect on visibility.

If a retroreflective material under nighttime conditions

has an area so small that it appears as a point source,

for example a roadside delineator, its intensity, in

candelas, is its pertinent photometric quantity. Therefore, its reflecting properties are characterized by the

intensity produced by incident unit illuminance in candelas

per footcandle, called specific intensity (abbreviated

S.I.), or in international units candelas per lux, called

coefficient of luminous intensity (abbreviated C.I.L.).

If a sign of retroreflective material is so large that

it is seen as an object, rather than as a point, then

the pertinent photometric quality is its luminance or candelas per unit area. Therefore, its reflecting

properties are characterized by the luminance produced by

incident unit illuminance in specific intensity per

square foot, called specific intensity per unit area

(abbreviated S.I.A.), or in international iinits co- efficient of luminous intensity per unit area (abbreviated

C.I.L./A or C.I. A. )

47
4.2.1 Daytime Luminance Factors . A measure of the daytime

appearance of the color of retroreflectors other than its

chromaticity is the luminance factor. Since the measured

values of the luminance factor of retroreflective

materials is very dependent upon the geometry of

measurement, it is important that this geometry simulate

the geometry of the use condition. Average luminance

factors obtained for several colors are shown in Table X

for two geometries, diffuse, 0* (D/0) and 45°, 0" (45/0).

Also shown in this table for comparison are luminance

factors, with D/0 and 0/45 geometries for similar materials

, . obtained by A. Fisher, University of New South Wales,

^ Australia, for the Standards Association of Australia

and presented at the International Conference on Photometry

and Colorimetry, Verna, Bulgaria, June, 1973. Geometries

0/45, used by Fisher, and 45/0, used by NBS, are ex- pected to yield similar luminance factors.

4.2.2 Nighttime Specific Intensity per Unit Area . A measure of

the nighttime appearance of a retroreflector is the

specific intensity per unit area, S.I.A. - S.I. /A. This

quantity may be considered either as the Ixaminance per

unit illuminance L'cos ^^/^^ the coefficient of

Ituninous intensity per unit area, C.I.L./A or C.I. A.

These two approaches lead to two methods of measurement.

These may be called the luminance method (for large areas) or the intensity method (for small areas) , respectively.

The essential difference between these two methods is

that in the Ixaminance method this field stop is filled

while in the intensity method the field is not. 2"^^^® units in ejther method can be either cd/(fc ft ) or

cd/(lx • m )

Both of these methods were used in this study. It was

found that the intensity method of measurement is in

reasonably good agreement with the luminance method. The

values for the NBS set are shown on Table XI for one instrument and geometry 5/0.33.

Table XII shows the C.I.L./A values obtained on the

Interlaboratory Set for 5/0.33 geometry, using the intensity method. These values represent averages of measurements made with two instnaments at NBS. The variability

of these data are discussed in the next section.

48
Table X. Comparison of Daylight Luminance Factor, 3f

of Retroreflective Materials, NBS Set

Luininance Factor for D/0 and 45/0 Geometry

Retroreflector NBS P I SHE R

Colors No. D/0 45/0 No. D/0 0/45

Red 4 0.08 0.06 7 0.12 0.09

Brown 1 .065 .060

Orange 3 .23 .21

Yellow 6 .40 .36 6 .40 .34

Green 2 .09 .06 6 .09 .05

Blue 3 .07 .05 4 .03 .02

White 7 .53 .44 7 .51 .44

49
Table XI. Comparison of Specific Intensity per Unit Area

Obtained by Two Methods. Geometry: 5/0.33,

NBS Set

C . I . L . / A * Manufacturer s Luminance Intensity

Color Name Method Method

Ruby Red 41.2 39.3

Red 27.2 26.4

Red - - 18.2 17.6

Red 22.7 20.9

Brown 2.7 2.8

Orange 69.9 65.8

' Orange 38.0 36.9

Orange 26.8 25.5

Orange 21.1 21.7

Yellow 142.9 138.3

Yellow 56.5 57.2

Yellow 56.0 55.2

Yellow 33.1 32.9

Yellow 68.7 64.3

Gold ^

• - 70.4 69.4

Lemon Yellow 50.8 50.6

Green 40.6 44.1

Green 13.1 13.1

Green 16.6 17.8

Grey Blue 9.8 11.1

Blue 4.7 5.4

Blue 17.6 16.9

White 94.3 93.9

White 77.3 73 1

Silver 273.1

Imperial White 63.7 63.7

Silver 101.1 99.8

Silver 50.6 52.6

Silver 97.9 96.7

Black 18.4 18.6

* See Paragraph 4.2

50
Table XII. Specific Intensity per Unit

Area by Intensity Method.

Geometry: 5/0.33

Sample

No. and Color C.I.L./A*

1 White 221.0

2 W 175.0

3 W 61.8

4 w 107.9

5 w 99.0

6 w 94.5

7# Yellow 126.6

8 Y 137.2

9 Y 57.6

10 Y 45.2

11 Y 53.1

12 Y 25.1

13 Red 42.2

14 R 21.1

15 R 19.0

16 R 23.0

17 R 18.8

18 R 26.5

19 R 12.0

20 R 12.9

21 Blue 20.9

22 B 6.3

23 B 9.7

24 B 22.9

25 B 6.0

26 B 3.8

27 B 4.8

28 Green 35.0

29 G 26.0

30 G 8.9

31 G 9.3

32 G 11.1

33 G 34.8

34 G 11.4

35 G 15.0

36 Orange • 56.4

37 0 52.6

38 0 36.

39 0 30 0

40 0 33.4

41 0 25.7

42 0 53.8

43 0 25.7

44 0 21.2

* See Paragraph 4.2.2

51
If the white and silver retroreflectors were spectrally ' nonselective there would be no need for correction

factors in their photometry. But, as can be seen from

Figure 3, these retroref lectors are not spectrally nonselective and require correction factors, as do retroreflectors of other colors. Photometric correction

• factors for colored materials used for the data obtained

on the Gamma and Pritchard Telecolorimeters are listed in

Table XIII.

4.2.3 Variability of Nighttime Specific Intensity per Unit Area

To determine how well measurements of specific intensity

per unit area are replicated, the variability for one

geometry was studied. Two sets of measurements were

made on one instrument at NBS, a set of measurements was

made on two different instrximents at NBS, and a set of

measurements was made on each of three instrxunents, two

at NBS and one at another laboratory* These variabilities,

for these sets of data, represented by the average standard

deviation a for several samples of each nominal color,

are listed in Table XIV.

Comparison of Nighttime and Daytime Colors

It would be convenient if the daytime and nighttime

chromaticity requirements of the color of a retroreflective material could be specified by using the same

chromaticity boundaries. This, of course, can not apply

to these materials. In daytime use they are illuminated

diffusely with daylight (source D^^ spectral distribution.

In nighttime they are illuminated nearly perpendicularly

with light of tungsten filament lamps of color temperature

2856K (source A)

Table XV shows a comparison of the chromaticity coordinates

of nighttime and daytime color of retroreflective materials

in the NBS set. From these data it can be seen that in

daytime the color of the red, orange and yellow retroreflectors appears weaker than in nighttime while the color

of white, green, and blue retroreflectors appears bluer

than in nighttime.

52
Table XIII. Summary of Photometric-Correction Factor for

C.I.L./A* Measurements for Several Colors

Color Gamma Instrument Pritchard Instrument

Red 0.892 0.917

Yellow Orange .944 .891

Green . yyy o/. r» . 94U

Blue .882 .691

WhJLte .983 .974

* See paragraph 4.2

53
Table XIV. Variability of C.I.L./A* in Terms of Average

Standard Deviation a, and Coefficient of

- - Variation V(%)

Retrore fleeter

Color

3 Instruments

2 Laboratories

2 Instruments

1 Laboratory

1 Instrument

2 Readings

u O o

White 3.4 2.7 1,1 2.1 0.6 0.5

Yellow 1.8 2.4 1.6 2.1 .5 0.7

Red 1,3 6.4 0.6 2.8 .1 0 5

Blue 1.0 11.4 1.2 13.7 .1 1.1

Green 1.4 7.4 1.5 7.9 .1 0.5

Orange 1.4 3.7 0.8 2.1 .5 1.3

All Colors 1.7 5.7 1.7 5.1 0.3 0.8

* See paragraph 4.2.2

54
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5. Recommended Test Procedure for Nighttime Conditions

On the basis of this study the following procedure is recommended for instriimental color measurement of retroref lective materials

under nighttime illuminating conditions.

5. 1 Colorimeter and Calibration Equipment

The colorimetric equipment shall consist of a source

with its power supply, a telecolorimeter with its

power supply, and meter, supports or holders for the

sovirce, colorimeter, and test samples. Calibration

equipment shall consist of calibrated filters and a calibrated reflecting surface, such as MgO, BaSO^ or MgCO^- The equipment shall be placed in a room that is

at least 100 feet long and can be darkened as needed.

5.1.1 Souroe. The source shall be a projector with a tungsten

filament lamp and a lens having maximum diameter of two

inches. The lamp shall be operated at a voltage to

yield a color temperature of the projected light of

2856K. To maintain the lamp voltage the source power

supply shall be equipped with voltage regulating equipment that controls voltage to within 0.1 volt.

5.1.2 Telecolorimeter . The telecolorimeter shall be equipped

with tristimulus filters and electrical trimming of the

output of the photodetector in combination with each of

the filters so that tristimulus values have proper

relative values. The colorimeter shall have means of

focussing the test sample image on the field stop. The

field stop shall be of such a size that the field of view

is 6 minutes. The telecolorimeter shall have its own

power supply, amplifier and output meter. To achieve

high accuracy the correction in chromaticity coordinates

of test samples resulting from multiplication of the

tristimulus value readings by the color-correction

factors shall be within ± 0.015 in x and in y, the

linearity of the scales shall be within ± 0.010 over the

range of 100 to 0.01 and the stability shall be within

± 0-002 in any range.

5.1.3 Supports . There shall be suitable tripods or other

supports for the source, telecolorimeter, and samples and

means for rotation of the samples as required so that the

geometric arrangement required for calibration and measurement can be made. These arrangements are shown in Figures

8 and 9, respectively.

57
5.1.4 Calibration Standards. Suitable filters, those with

relative spectral transmittance curves similar in shape

to those of the spectral reflectance of the test samples

shall be used. By similar shapes is meant that the ratio

of the spectral reflectance of the sample to the spectral transmittance of the filter is approximately

constant. If no such filters are available, then the

NBS reference filters (item 4.10 of the NBS Std. Ref.

Materials catalog) may be used. Standard reflecting

materials, 4-inch square plaque of MgO, MgCO^, of BaSo^

may be used as a white standard for calibration.

5.2.1 Arrangement of Equipment . To provide a spectrally nonselective surface, place a white standard plaque in the

, :,: Sample position. Arrange the source, operating at 2856K,

the white standard and the telecolorimeter as shown in

Figure 10. At a viewing distance of 100 feet and with a

6 minute field stop in place, focus the telecolorimeter

upon the center of the white standard plaque. (At 100

feet a 6' field stop limits the field of view of the

photometer to approximately a 2-inch circle, thereby

permitting use of 4-inch square plaques.)

5.2.2 Adjustments of Meter . Follow the directions in the manu-^

facturer's instruction manual for the telecolorimeter,

and adjust the zero settings and sensitivity so that the

instrument is calibrated to read luminance. Remove

the photopic correction filter before the photoreceptor

of the telecolorimeter and replace it with the

Y-tristimulus filter. Adjust the distance between the

source and the white plaque to obtain a reading of approximately 100 on the meter dial. Next, with each tristimulus filter, positioned in turn before the photoreceptor,

adjust the meter instrument so that the meter indicates

the correct values for the white plaque. For MgO these

are

X ' = 104.45 Y' = 100.00

5.2 Calibration

r 5.38 Z 35.55

where

where.

X

r is the reading with filter for the long-wave

portion of the X-function.

'b is the reading with filter for the short-wave

portion of the X- function.

58
Y' is the reading with the filter for the spectral

luminance efficiency function, and

Z' is the reading with the filter for the Z- function.

5.2.3 Reference Filters . With the telecolorimeter focussed

on the illuminated white plaque, insert each of the

standard filters in turn into the auxiliary filter slot as needed, read, and record the tristimulus values,

5.2.4 Color-Correction Factors . From the data obtained with

the standard filters and the white standard, calculate

the correction factors, F^, F and F^ for the tristimulus

values of each of the standard filters by means of the

following equations.

F^ = X/(X'^ + X'^)

F^ = Y/Y'

F^ = Z/Z',

where X, Y, and Z are the correct tristimulus values of

the standard filters.

The magnitude of the correction factors for the tristimulus

values are indications of the closeness of the fit to the

CIE functions. To ascertain that the instrxoment '

filters fit, limits should be set on the size of the

correction factors. We suggest that the correction

factors lie between 0.8 and 1.2, except that for red

retroreflectors the F^ factor may be no less than 0.2.

(The instruments used in this investigation did not meet

these requirements .

)

5.3 Color Measurement Procedure

5.3.1 Arrangement of Equipment . Reposition the source telecolorimeter, and sample holder to achieve the arrangement of components shown in Figure 9. Make no changes

in the adjustments of the telecolorimeter . To minimize

errors that may result from variations of source color

temperature from 2856K, use the same light source in the

color measurement as was used in the calibration.

5.3.2 Colorimetric Observations and Computations . Read the

tristimulus values of the samples, X r ^i^d Z' , to

three significant figures. Record tSese as simple

readings. Correct the tristimulus values of the sample

colors by the use of the following equations

59
X

Y

X

z Z' F,z'

where F , F^ and F are the color-correction factors

obtained in the calibration above for the reference

filter with spectral characteristics similar to those

of the test samples. Compute the CIE chromaticity

coordinates x, y of the test sample color by the use of

the following equations:

X = X/(X + Y + Z)

y = Y/(X + Y + Z)

5.4.1 Photometer Specification . Because a large variety of

photometric instruments may be used/ including a telephotometer, it is essential that the instrument actually

used be described. The entrance and observation angles

must be specified and test geometry defined either by

angular apertures of source, receptor, and sample or by

their dimensions together with the test distance. Further,

specify the position angle if different from 0°, and

specify the rotation angle if sample characteristics are orientation dependant.

The photometric equipment shall consist of the same

components as described in 5.1 for the colorimetric

equipment. The receiver shall be either color corrected

or color-correction factors for the Y tristimulus value be

known and applied. The linearity of photometric scale

shall be within ± 0.010 over a range of values from 100

to 0.01. The instrumental stability shall be within

±0.002 in any range.

5.4.2 Specific Intensity per Unit Area . Set the specified

distance from the test sample surface to the source and

to the photometer. (See 5.4.3). Measure the illuminance

of the face of the reflector by substituting a receptor

for the retroreflector , with its entrance window at the

place where the face of the retroref lector will be mounted.

If a telephotometer is used, choose the smallest field

stop which is large enough to include the entire image

of the source. Record reading as . Put the retroreflector in place. Orient it so that the desired

entrance and orientation angles are obtained. (See

Figure 9) . Place the receptor at a lateral distance, d,

from the source so that

5.4 Photometric Measurement Procedure

60
tan d/D = a

where a is the desired observation ajigle. See Figure 9.' If a telephotometer is used choose the smallest field

stop which is large enough to include the entire image

of the retroreflector inside the field stop.

Determine -its luminous intensity by measuring the

illuminance at the receptor in the same units as R^^ (reading R^) - The coefficient of the luminous intensity

(C.I.L.)/ or in the United States usage the specific

intensity (S.I.), may then be computed from the relation

2

C.I.L. or S.I. = R2^^'^

where D' is the distance between the retroreflector and

the receptor. When colored samples are measured it is

usually desirable to correct for the deficiencies in the

y filter of the photometer by means of a filter having a spectral transmittance approximating that of the retroreflector. Obtain calibration factor K by the formula

K = Rj^T/Rj,,

where is illiaminance of the face of the retroreflector determined as described above,

Rp is the reading of the receptor at the

same position as for R^ but with a color

filter placed immediately in front of

the receptor, and

T is known (total) luminous transmittance

of the filter.

The corrected specific intensity of the retroreflector is

then

2 2

C.I.L. or S.I. = K R2^°'^

/-^i ^ -^2^°'^ ^ ^

Note that this method is applicable only when the same receptor is used to measure both the normal illuminance

at the retroreflector , E and the normal illuminance

produced by the retrorefSector at the receptor, E' . If different receptors are used for these measurements,

then the color-correction factor of the receptor used to

measure E' must be determined by a separate set of

measureraencs . Note that if the same photometer is used

for the determination of R, (and R^) and R^ , calibration

of the photometer is not required if the instrument is

sufficiently linear. If separate photometers are used.

61
or if the instrument is not sufficiently linear, each

(range used) shall be calibrated by using a standard

lamp.

Because of the wide variety of photometric instruments,

a complete description of the calibration procedure for

all types of instruments would be too lengthy for this

document. Accordingly, it is recommended that users

follow the usual procedure given by the manufacturer

\inless required to do otherwise in special applications.

Since, by definition, specific luminance (S.L.) is the

specific intensity (S.I.) per unit projected area,

S.L. = S.I. /A cos or

S.L. = KR^D^/R^ A cos

where

A is the effective retroreflector surface area and

is the entrance angle

Specific intensity per unit area (C.I.A^lor S.I. A.) is

given by —

2

C.I.A. = C.I.L./A = K R2^°'^ /^l^

where the meter is the unit of length, or

S.I. A. = S.I. /A = KR^{D')^/R^A,

where the foot is the unit of length.

A luminance meter is often used as the receptor in the

evaluation of retroreflecting sheet instead of an illuminance meter. When a luminance meter is used care

must be taken to insure that the image of the retroreflector completely fills the field stop of the

luminance meter. The photometric procedures are essentially unchanged except that the color-correction factor

of the luminance meter k must be obtained separately and

the following relations apply:

S.L. = kR /R cos e LI ^ and

S.I. A. = kR,/R
where

is the reading of the luminance meter in

candelas per unit area.

Special Precautions . For cube-corner reflectors in

vehicular and highway use, the suggested measurement

geometry is reflector-receptor distance 30 meters (100

feet), receptor diameter 1.25 cms (0.5 inch) and source

diameter 5 cms (2 inches) . Such equipment is also

suitable for a material such as beaded sheet that has a relatively flat curve of specific intensity versus

observation angles, but for cost reasons the laboratory

testing only beaded sheet may prefer a reflectorreceptor distance of 15 meters (50 feet) , receptor size

of 2.5 cms (1 inch) and a source size of 2.5 cms (1 inch)

This geometry is adequately precise for beaded sheet but

not for the cube-corner reflector. Much longer distances

are needed for the very precise cube-corner reflectors.

Care should be exercised to minimize the effects of stray

light. The background of the retroreflector should be

flat black. The field of view of the receptor should be

made as small as feasible. Baffles should be used where

appropriate to limit the spread of the source. By the

use of a black surface of the same shape and area as the

test sample a measiorement of the amount of stray light

can be made. The stray-light reading so obtained should

be subtracted from the reading R^, obtained with the

retroreflector in place.

When measTirements are being made of the output of colored

retroreflectors it is essential that the source have the

specified color temperature, or spectral distribution.

Because the intensity, as well as the spectral distribution, of a light source varies with the applied voltage,

care should be taken to assure that the voltage be stable.

To stabilize the intensity of the light source to within

1% the voltage should be stabilized to within 0.3%.

In reporting data for retroreflectors, conditions of

measurement — observation angle, entrance angle, viewing

angle, and rotation angle — should always be reported.

Accordingly, the geometry of the measurement equipment

should be specified either by reference to a standard

(such as SAE J 594 or LS-300) or by itemizing source size,

receptor size and retroref lector-receptor distance.

63
6. Recortmiended Color Specifications of Retroreflective Materials

The Interim Report on Recommendations for Surface Colors for

Visual Signalling by the Subcommittee on Surface Colors of the CIE

Committee TC 1.6, Fundamentals of Light Signals and Signs contains the

following comment on the nighttime appearance of retroreflective (sheet)

materials

" Night-time appearance . This docxoment makes no specific recommendations for the measurement of the colours of retro-reflective

materials with the special geometries that are typical of the illuminating and viewing conditions at night-time. The reasons for the omission

of definite advice are related to the current technical difficulties of

making measurements on retro-reflective materials at small observation

angles, and the present lack of reproducibility of results between

different colorimetric laboratories. Consequently, the night-time

appearance of the colours of retro-reflective materials should be

assessed by visual inspection with the illuminants likely to be used, and

the illuminating and viewing geometries likely to occur."

Elsewhere in the subcommittee's report the following comment is

made about all material surfaces with the color brown;

" Brown . This docxament makes no specific recommendation with regard to any color commonly called brown."

Because brown cannot be distinguished from yellow or orange, if Ixaminances can not be compared, its color will not be specified.

In this report, tentative recommendations will, nevertheless be

made for specification of the colors of retroreflective materials, ex- cluding the color brown, for nighttime conditions, based on the results

obtained in this study. These specifications are based only upon

available chromaticity characteristics and not upon visual aspects of

the problem.

Because colors under nighttime conditions differ from the colors vmder

daytime conditions, different sets of specifications are required.

Present daytime color specifications are also included for comparison.

The color specification requirements for retroreflective materials

such as delineators and line markers which are viewed as point sources

are different from those for retroreflective sheet materials viewed as surfaces. These two types will be treated separately. The color

boundaries are described by equations. When, however, it is desired to

plot the chromaticity regions on a chromaticity diagram, it is often

easier and quicker to do this by joining by straight lines points

corresponding to the intersections of the boundaries than by drawing the

boundaries from their specifying equations. The coordinates of these

points are therefore given.

64
6. 1 Color Specifications for Retroreflective Delineators and Lane

Markers at Nighttime

For retroreflectors of small size which appear as point, or near-point, sources color boundaries recommended by the International Commission on Illumination

(CIE) should be used. Because these markers are used

by drivers with color deficient vision as well as normal, restricted boundaries of the CIE signal light

colors have been selected for this pvirpose. The

equations for the boundaries selected and intersection

points are listed in Table XVI. The boundaries are

plotted on Figure 21.

For red color specifications, the restricted boundaries

for persons who are red-green confusers were recommended.

The use of the color orange in delineators and lane

markers is not recommended because orange sources

of small angular subtense will be confused with red and

yellow.

For yellow, the restricted white boundary recommended to

obtain greater discrimination from white was chosen.

For green, the restricted yellow boundary recommended

for persons who are red-green confusers and the restricted

blue boundary to obtain greater discrimination from

blue was chosen.

For white, the coded white boundary recommended to obtain

greater discrimination from yellow was chosen.

6.2 Color Recommendations for Retroreflective Sheet Materials at

Nighttime

6.2.1 NBS Recommendations for Nighttime Color Specifications

Color specifications of retroreflective sheet under

night conditions are tentatively recommended. The

boundaries for each color have been chosen to include

as large an area as practicable and to be consistent

with the data obtained in this study. The option of

supporting any CIE recommendations if and when they

are formulated and receive international acceptance

at some time however, is reserved.

The specifications for the coefficients of luminous

intensity, C.I.L./A, and color recommended for retroreflective sheeting materials in nighttime conditions

under 5/0.33 geometry are listed in Table XVII. The

65
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66
Figure 21. Reconmiended color boundaries for retroref lective

delineators and lane markers at nighttime.

6?
8O

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68
boundary lines for these colors are shown in Figure 22.

(The boundary lines are also shovm on Figures 15 to 20.)

In Table XVIII are listed the x, y chroiuaticity coordinates to aid in plotting the boundary lines

6.2.2 RecoiTCT.endations by 3M Company for Nighttime Color " Specifications . The 3M Corupany has also irade recomendations for nighttime color specifications of retroreflectiva sheeting materials. These regions specified

exe considerably broader than the NBS recomme-ndations

For comparison the 3M recomruendations are given in

Table XIX, together with the x# y chroraaticity coordinates

of the intersection points of the boundary lines. The

boundaries are also shown on Figure 23. The follovjing

will be noted from this figure:

1. Orange over- laps red on one side and yellow on

the other2. IThite is too close to yellow.

3. Blue extends into the region of the diagram

usually described as green.

6.3 Present Daytirae Color Specifications

To complete the specification description and to serve

as a convenient comparison; present daytime color and

luminance factor specifications are included. These

are the standard highway sign color and lurairiance

factor specifications for daytime, listed in Ftr.JA Data

Sheet TO-21, September, 1971, and the NJCUTCD boundaries

for surface colors, which are given in Ta±)le XX, The

boundaries have been plotted on Figure 24,

69
70
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AcIiTiOwl edgemants.

The authors take this opportunity to thank Messrs. C„ A, Douglas,

E. L. Walters and V. R. Weidner of the National Bureau of Standards for

their assistance and the representatives of several manufacturers of

retroref lective materials for collaborating with and supplying the

sample iraterials. We v/ish also to thank the Departraent of Transportation,

Federal Highway Administration, for requesting and supporting the investigations reported in this document, and the Federal Aviation Administration for their support after the work vras begun.

76
U.S. C£PT. OP COMM.

BIBLIOGRAPHIC DATA

SHEET

1. FUHLICATION OR REPORT NO.

74 518

2. Gov't Accession

No.

i|7T1TLE and SUBTITLE

INSTRUMENTAL COLORIMETRY OF RETROREFLECTIVE SIGN MATERIALS

. AUTHORCi)

I I. Ninsroff and W. A. Hall

3. Recipient's Accession No.

5. Publication Date

6. Performing Organization Code

8. Performing Organ. Report No.

PERFORMLNG ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Wotk Unit No.

11. Contract/Grant No.

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

I Federal Highway Adiainistration

Office of Research

I Washington, D. C. 20590

13. Type of Report & Period

Covered

Final, Dec. 1972 to

Aug . 30; IW-— 14. sponsoring Agency Code

.SUPPLEMENTARY NOTES

. ABSTRACT (A 200-word or less factual summary of most significant information-^ If document includes a significant

bibliography or literature survey, mention it here.)

Because color-coded applications of highway signs increase, specifications of colors

and color tolerances are required. In order to assure that the requirements are

met within specified regions, a measurement technique needs to be developed and

described. To accomplish these goals the U. S. Department of Transportation

contracted with the National Bureau of Standards to conduct the required studies and

make the necessary recommendations. Having previously performed a study for daytime conditions, the National Bureau of Standards was competent to perform a study

for nighttime conditions. The colorimetric properties of 126 samples of retroreflective materials of 7 different colors were measured with 3 telecolorimeters in

simulated nighttime conditions. One spectrophotometer was used to measure color of 38

of the samples in simulated daytime conditions. The colors measured were: red,

orange, brown, yellow, green, blue and silver (white) . Differences of color measured

by means of different telecolorimeters on the same samples were evaluated.

As a result of these studies procedures for making colorimetric and photometric

measurem.ents were developed and are included in this report. On the basis of the

color measurements and their variability tentative recommendations for color

boundaries were prepared and are also included in this report.

l'. KEY UORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

<*ame; separated by semicolons)

Colorimetry; highv;ay signs; photoelectric tristimus colorimeters; photometry of retroreflective materials; retroreflective sign materials

^.AVAILABILITY Unlimited r

19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1

' For Official Distribution. Do Not Release to NT IS UNCLASSIFIED

76

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ft'ashinKton, D.C. 20402. SD Cat. No. CP.

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22. Price

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USCOMM.OC 29j42-K'74

r