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NBS TECHNICAL NOTE 1125

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

PHOTOMETRY AND COLORIMETRY

OF RETROREFLECTION:

State-of-MeasurementAccuracy Report

DO

5753

3 . 1125

980
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ot Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

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physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

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THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

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provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. Some divisions within the center are located at Boulder, CO 80303.
PHOTOMETRY AND COLORIMETRY OF RETROREFLECTION:

State-of-Measurement-Accuracy Report

Kenneth L. Eckerle

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

NATIONAL BTOEAU

OF 8TANDAWJ8

UBBABY

JUL 3 1980

QGIOO

no. \i^-S'

^

0F **

\lchrwm note.

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

. . NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1980
National Bureau of Standards Technical Note 1125

Nat. Bur. Stand. (U.S.), Tech. Note 1125, 44 pages (July 1980) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.25

(Add 25 percent for other than U.S. mailing)
TABLE OF CONTENTS

Page

I. Introduction 1

II. Materials 2

III. Measurement Geometry 3

A. Nighttime Conditions 3

B. Daytime Conditions 4

IV. Quantities to be Measured 4

A. Photometric 4

1. Coefficient of Luminous Intensity (C.I.L.) .... 4

2. Coefficient of Retroreflection 5

B. Colorimetric 5

V. Instrumentation 6

A. C.I.L. or Some Derived Quantity 6

B. Chromaticity Coordinates (x,y) 7

C. The NBS Reference Retroreflectometer 8

VI. Method of Calibration 8

A. C.I.L. or Some Derived Quantity 8

1. Direct Method 8

2. Relative Method 8

3. Substitution Method 9

B. Chromaticity Coordinates (x,y) 9

1

.

Spectroradiometer 9

2. Tel ecolorimeter 10

VII. Important Parameters That Should Always be Included in

Specifications 10

A. Geometrical and Other Factors Affecting Both Photometric

and Colorimetric Values 10

i i i
Page

1. Observation Angle 10

2. Entrance Angle 10

3. Rotation Angle 11

4. Observation Distance 11

5. Size and Shape of the Sample 11

6. Subtended Angles of the Photoreceptor and Source

Apertures 11

7. Reference Center and Axis 12

8. Uniformity 12

9. Source-Polarization and Illuminant 12

10. Wavelength Error 12

B. Additional Items to Specify for C.I.L. and Derived

Quantities 12

1. Specific Quantity to be Measured 12

2. Minimum Acceptable Value 12

C. Additional Items to Specify for Color Determination . . 12

1. Chromaticity Coordinate Limits 12

2. Color Limits for Standard Filters 13

VIII. Test Methods and Specifications 13

IX. Some Prior Research and Intercomparisons 13

A. C.I.L. and Derived Quantities 13

1. Stephenson 13

2. MCCA 14

3. Nimeroff and Hall 14

4. AAMVA 14

B. Chromaticity Coordinates (Nighttime geometry) .... 15

iv
Page

1. Stephenson 15

2. Nimeroff and Hall 15

3. Geometry- Dependent Studies 15

X. Current Issues and Problems 16

XI. Conclusion 17

XII. References 18

XIII. Appendix A: Bibliography 20

LIST OF TABLES

Table

I Partial Collection of Specification and Test-Method

Standards for Retroreflectors 25

II Intercomparison of Prismatic Reflector Measurements

Conducted by American Association of Motor Vehicle

Administrators for the National Bureau of Standards,

C.I.L 26

III Percent Deviation from the Mean for C.I.L. values

listed in Table II 27

LIST OF FIGURES

Figure

1 Retroreflectors 28

2 Schematic of Geometry 29

3 The CIE 1931 Chromaticity Diagram with the spectrum

and Planckian loci and the chromaticities of some illuminants (CIE A, B, C and D65 ) 30

4 Projector 31

5 Sample Carrier 32

6 Receiver Baffle Tube / 33
Figure Page

7 Normalized C.I.L. versus Observation Angle a for

(A) Cube-Corner Retroreflector, (B) Encapsulated

Lens Sheeting, (C) Enclosed Lens Sheeting, and (D)

Pavement Stripe at 86° Entrance Angle 34

8 Relative Signal vs Entrance Angle 3 or 90° -3 for

observation angle of 0.33°. (A) Cube-Corner

Retroreflector, (B) Enclosed Lens Sheeting, and

(C) Pavement Stripe 35

9 Relative Signal vs Rotation Angle e. (A)

Single-Orientation Hexagon Cube (B) TwoOrientation Hexagon Cube 36

10 Drawing Illustrating Arrangement for AAMVA

Intercomparison 37

VI
PHOTOMETRY AND COLORIMETRY OF RETROREFLECTION:

State-of-Measurement- Accuracy Report

by Kenneth L. Eckerle

A survey of the literature on retroreflection has been conducted,

and information from this survey necessary for the understanding of the

phenomenon of retroreflection is summarized. Items included are materials, measurement geometry, quantities to be measured, instrumentation,

and methods of calibration. Recommendations are given to evaluate or to

write specifications by discussing important parameters. A partial list

of test methods and specifications is presented in tabular form. Some

prior research and intercomparisons are summarized including the results

of a previously unpublished intercomparison using prismatic type retroreflectors. The state-of-measurement-accuracy is inferred from the

information presented. A bibliography is included for those who would

like to obtain more details.

Key words: Accuracy; chromaticity; coefficient of luminous intensity;

intercomparisons; retroreflectance; retroreflector; specifications;

test methods.

I. Introduction.

A "state-of-the-art" measurement is usually a measurement that is

the most accurate and precise that has been achieved thus far. An instrument to perform a measurement of this type must either account for

known systematic errors in its design or some means must be provided to

correct for such errors. In addition, random or statistical errors must

be sufficiently small.

To some of the measurement community, however, a "state-of-the-art"

measurement is one that is presently achievable by a typical laboratory.

This weaker usage may sometimes be appropriate, especially if there are good reasons why a "best possible" measurement can not be performed by a typical laboratory, e.g., it is not cost effective. Often, however,

with the aid of a central laboratory such as the National Bureau of

Standards (NBS), the performance of a typical laboratory or, in the

stronger sense, a "state-of-the-art" laboratory can be improved or maintained through the use of a Measurement Assurance Program (MAP)

service and the use of standards such as Standard Reference Materials

(SRM). This can be accomplished by helping the typical laboratory

analyze its problems, and/or by correcting for some problems through the use of standards.

To the person who must make specifications and require experimental

values to fall within given tolerances, it is necessary to know how well

a typical laboratory can make a measurement. Otherwise, equity in trade

cannot be maintained.

1
The NBS is undertaking a MAP service for retroreflectance

measurements. As will be seen below, large differences exist among

values obtained by different laboratories, and it is felt that accuracy

in measurements performed on retroreflectors can be improved. The NBS

has built a reference retroreflectometer for measuring the nighttime

photometric properties of retroreflectors, and it is planned in the

future to modify it so that it can also measure colorimetric properties.

When used during the day, retroreflectors are illuminated diffusely

and viewed directionally while at night, the retoreflectors are illuminated from one direction, and viewed directionally (usually at a small

angle with respect to the direction of illumination). The optical

properties of the retroref! ector for these two cases can be quite different. This report will concern itself primarily with the night- type

usage, and will concentrate on measurement accuracies achievable by

"typical" laboratories. We will use the latest definitions and terminology of the International Commission on Illumination (CIE) especially

those for retroreflection that were approved by a subcommittee of the

CIE at the 1979 Kyoto meeting.

Since the primary purpose of this report is to discuss measurement

accuracies presently attained, some topics will only be briefly mentioned. In each such case, references will be provided so that the

reader can pursue that topic further if he desires.

II. Materials.

Retroreflection is defined by the CIE as "Reflection in which

radiation is returned in directions close to the direction from which it

came, this property being maintained over wide variations of the direction of the incident radiation. "[1]*

The CIE accordingly defines a retroreflector as "A surface or device from which, when directionally irradiated, a relatively large

portion of the reflected radiation is retroreflected."[l]

Three common types of retroreflective elements (optical units which

retroreflect by refraction and/or reflection) are commonly used. They are the "cat's eye" element, the spherical element, and the cube-corner

element shown, respectively, in Figures 1(a), 1(b), and 1(c). Further,

the elements can be exposed, enclosed, or encapsulated as shown, for

spherical elements, in Figures 1(d), 1(e), and 1(f), respectively.

Generally, a retroreflector is made up of many elements, and often

is available in sheet form. Plastic is usually used as the medium in

which to imbed and cover the elements; or in the case of cube-corner

*Numbers in square brackets indicate literature references listed in

Section XII.
retrorefl ectors, the whole retroreflectinq device is commonly made of

plastic althouqh some devices are available with the cube-corner made of

qlass. A colored retrorefl ector may be produced by usinq a colored

plastic medium or colored plastic cover. Thus, the color of a retroreflector will depend on the type of plastic used.

The choice between retrorefl ectors constructed of the different

types of elements is made on the basis of color, the qeometric distribution of the reflected liqht, and the behavior with respect to orientation of the retrorefl ector relative to the incident liqht as will be

described more fully below. To obtain details on the construction of a particular retrorefl ector, it is suggested that the manufacturer be

contacted.

III. Measurement Geometry

A. Nighttime Conditions

In Fiq. 2 the relationships between the various optical elements of

a reflectometer for measurinq retrorefl ectance are shown in detail. The

anqle B, between the reference axis C (generally, but not always, the

normal to the surface of the retrorefl ective device) and the illumination axis D, is called the entrance angle. (Known in the past as incidence angle.) In general, the observation plane A and the entrance

plane B are not mutually perpendicular. In this case, the entrance

angle 3 can be resolved into two components, Bi lying in the observation

plane A, and B2 perpendicular to that plane. In Fiq. 2, Bi = and

B 2 = 3. The angle a, between the illumination axis D and the observation axis E, is called the observation angle. (Known in the past as the divergence angle.) The distance between the retrorefl ector reference center and the receptor aperture is called the observation distance. The reference center is a point on or near the device which is

defined to be the center of the retrorefl ector for the purpose of

specifying its performance. Usually a rotation angle e is also specified (not shown in Fig. 2) which indicates the anqle throuqh which a datum mark (a mark which does not lie on the reference axis and which

indicates the orientation of the retrorefl ector) is turned. These definitions are more riqorously and more completely presented in references

[2] and [3].

Care must be taken when comparing requirements from different

specifications, not all of which use the same set of definitions given

here. Not only are the positive and negative sense of the various

angles sometimes different, but frequently a complicated transformation

between related parameters in different specifications is required.

The particular values specified for these different geometrical

parameters will depend on the use intended for the retrorefl ecting

material

.
B. Daytime Conditions

Two commonly specified geometries for daytime conditions are illumination from a 45° angle and 0° viewing (45/0) and diffuse illumination

and 0° viewing (D/0). The relative spectral power distribution of the

light source for measurement or calculation of color parameters is CIE

Illuminant C or Illuminant Dgr.

IV. Quantities to be Measured.

The photometric and colorimetric quantities for nighttime viewing

have in common the geometrical factors defined above. Also, the source

spectral power distribution is usually required to be CIE Illuminant A.

A. Photometric

Two quantities will be defined for which the CIE definitions will

be quoted.

1. Coefficient of Luminous Intensity (C.I.L.)*

"The quotient obtained from dividing the luminous intensity (I) of the retoreflector in the direction of observation

by the illuminance (E^) at the retroreflector on a plane perpendicular to the direction of the incident light.

Symbol R R = —

E

l

Note 1: In the photometry of retroreflectors this coefficient

is expressed in candelas per lux (cd«lx _1 )**-

Note 2: For accurate measurements of R, care must be taken

that the angular extent of the retroreflector at the point of

observation, the source aperture at the retroreflector, and

the aperture of the detection system at the retroreflector are each sufficiently restricted. The restriction needed depends

upon both the distribution of the retroreflected light and the

measurement geometry.

Note 3: For accurate measurements of R the illuminance (Ej_)

must be sufficiently uniform over the useful area of a retroreflector. "[1], [2]

*The abbreviation is of the French equivalent: Coefficient d'Intersite

Lumineuse.

**[m2 -Sr

_1

]
The C.I.L. is sometimes referred to as Specific Intensity,

2. Coefficient of Retroreflection (of a plane reflecting

surface)

"The quotient obtained from dividing the coefficient of

luminous intensity (R) of a plane retroreflecting surface by

its area (A).

R I/E

l Symbol R' R' = * = -^-1

Note 1: The coefficient of retroreflection is expressed in

candelas per lux per square metre (cd'lx -1«m" 2 )*.

Note 2: This quantity is especially useful for describing

materials in sheet form. For such materials the measurements

are customarily made with the direction of illumination, the

direction of observation, and the normal to the surface all in

the same plane. "[1], [2]

Other supplemental quantities for special purposes are defined in

reference [4]. These include

(a) Coefficient of retroreflected luminance (specific luminance)

(b) Luminance factor

(c) Linear Coefficient of Retroreflection

(d) Coefficient of Retroreflected Luminous Flux

B. Colorimetric

The color is usually specified by using the CIE 1931 system [5],

[6], [7] to obtain the chromaticity coordinates x, y, z. Since

x + y + z = 1, x and y are sufficient to specify the color. The CIE

1931 chromaticity diagram is shown in Fig. 3 with the spectrum locus and some illuminants along the Planckian locus.

The chromaticity coordinates are computed from the tri stimulus

values X, Y, and Z by the equations

X x

=

X + Y + Z

- Y y * X + Y + Z

*[sr

_1

]
Z X + Y + Z The tristimulus functions are defined by the CIE as

X = k

j S (x) P (x) x (x) AX

Y = k

J S (X) p (X) y (X) AX

Z = k

J s (x) p (x) 7 (x) AX

where:

S(x) is the relative spectral power distribution of the source;

p(x) is the spectral reflectance of the surface;

x (x), y(x), and z(x) are the spectral tristimulus values of the

CIE 1931 standard observer;

AX is the wavelength interval for a term of the summation;

and 100 s S(x) y (x) ax,

k

" x so that Y is the luminous reflectance of the surface.

Since only the chromaticity coordinates, x and y, are required,

p(x) can be a reflectance factor - that is it does not have to be on an absolute basis. Since the same factor is included in all three tristimulus values, it will divide from the numerator and denominator in

the equations for x and y above. In this sense, the chromaticity co- ordinates are relative quantities whereas the C.I.L. described above can

be regarded as an absolute quantity. This does not mean, however, that

one will obtain the same chromaticity coordinates for different geometries, since the spectral reflectance is geometry dependent.

V. Instrumentation

A. C.I.L. or Some Derived Quantity

The basic components of the apparatus consist of a photoreceptor, a light-projector source with stable output, and a goniometer sample

holder. The photoreceptor is equipped with a filter so that the spectral responsivity of the photoreceptor-filter combination matches that

of the CIE photopic standard observer - the V(x) [or y(x)] curve. Sometimes a telephotometer is used as the photoreceptor. The light

projector is such that the light incident on the sample spectrally

matches CIE illuminant A (a tungsten lamp operated at a color temperature of 2856K). The goniometer sample holder must have enough degrees

of freedom and range of adjustment for independently setting the
entrance angles 3i and 3 2 as we ^ as the rotation angle e. Since most

measurements are made with samDle-photoreceptor distances of 10 m to

30 m, a dark, enclosed photometric range is required along with suitable

baffles to reduce stray light.

Sometimes a color-correction factor is used to account for deviation

of the system from the ideal CIE specifications. In such cases, correction factors are derived by using the system to measure filters whose

spectral transmittance curves nearly match the shape of the spectral

retroreflectance curve of the sample. (In making these measurements, it

is necessary to find some means of holding the filter in a manner such

that inter-reflections between filter and photoreceptor do not affect

the measurement.) The true colors (including transmittances) of these

filters are normally obtained by calibrating with a spectrophotometer.

B. Chromaticity Coordinates (x,y)

The basic components of the apparatus consist of either a spectroradiometer equipped with collection optics or a telecolorimeter; a light

projector source with nearly constant output; a goniometer sample

holder; and a photometric range. The same qeometrical problems outlined

above for photometric measurements also exist for equipment to measure

the color of retroreflecting devices, along with the additional com- plication of color measurement.

(a) The spectroradiometer will disperse the light and measurements

will be made at specified wavelengths with specified instrumental bandpass. If this instrument is used, it is not necessary to specify the source temperature if the instrument is used as described in Sec. VI.B.l

below. It may be necessary to limit the acceptance cone to narrow

angles or to use collection optics similar to those used in a telecolorimeter.

(b) The telecolorimeter will normally be equipDed with CIE 1931

tristimulus-value filters. These will normally include xre(j and

*blue filters to match the long and short wavelength portions of the x function. Also, y and z filters to match the other components of

the spectral tristimulus functions are supplied. The source is specified to have the relative spectral power distribution of CIE Illuminant

A. If color correction factors are to be used, filters and filter

holders must be supplied as in Section A above.

(c) The other components of the apparatus will have similar re- quirements to those previously discussed under Section V.A. as necessary

for the measurement of C.I.L.

(d) The above equipment is used for nighttime color and other means are normally used for daytime color. (For examples of daytime

measurements, see, for example, references [19] and f20l .

)
C. The NBS Reference Retroreflectometer

A long-ranqe type retroreflectometer for measuring C.I.L. has been

documented and a description has been published [8], This instrument

has been well characterized, and its known sources of error evaluated.

The systematic errors for measured neutral bead sheeting and cubecorner devices have been estimated to be approximately 1% and 2%,

respectively. Figures 4, 5, and 6 show schematically the projector,

sample carrier, and receiver, respectively. This instrument, if proper

corrections are made, is capable of making "state-of-the-art" measurements. There are many laboratories with similar instruments or instruments using telephotometers, but the disagreement between laboratories

is quite large. There exists another class of instrumentation known as portable instruments, which can not be calibrated as described below,

but which require reference to a standard retroreflector or a special

standard mirror to make accurate measurements.

VI. Method of Calibration

A. C.I.L. or Some Derived Quantity

1

Direct Method

First, a lamp with known luminous intensity is placed at the

test-sample position to calibrate the photometer scale. The photometer

will then read directly the luminous intensity of the retroreflector.

A separate illuminance meter, which is calibrated, is then used to

measure the illuminance at the test-sample position. The ratio for a spectrally neutral retroreflector gives the C.I.L. directly. If the

retroreflected light is colored, calibrated filters havinq spectral

transmittance curves similar to the spectral reflectance curve of the

sample may be placed in front of the lamp source to determine the

correction for errors caused by differences between the shape of the

photometer responsivity curve and the CIE V(x) curve.

2. Relative Method

This method is generally more accurate than the direct method since

it is self-calibrating - it does not depend on additional calibrated

equipment and standards. If the same receiver is used to measure Ej_ and I, it follows from the CIE definition for C.I.L. that when the

receiver aperture is filled

R = (S2 /S 1 ).D1 2

where S 2 is the signal obtained with the receiver at the observation

position. Si is the signal obtained with the same receiver at the

sample position, and D x is the observation distance. The signals S 2 and Si should be adjusted for any system non-linearity. If the re- troreflector is colored, a spectrally similar colored filter of known

luminous transmittance may be used to correct for any deviations of

8
the shape of the responsivity curve of the receiver from the shape of

the CIE V(x) curve.

3. Substitution Method

This method employs a standard retroref lector to which direct com- parison of an unknown retroref lector can be made by substitution. If

the system is linear, the value of the unknown can be obtained from the

ratio of the measurements. This method is used for portable instruments

also.

B. Chromaticity Coordinates (x,y)

1. Spectroradiometer

The use of this type of instrument does not require any particular

source temperature and the spectroradiometer output need not be calibrated except for linearity. However, the wavelenqth scale must be

calibrated. The entire retroreflector should fall within the field of

view of the spectroradiometer especially if the sample is nonuniform.

In the case of a uniform retroreflector a spot which does not include

the edges of the retroref lector may be viewed. The readings S^x) of

the retroreflected light as measured with the spectroradiometer in the

observation position are taken at desiqnated wavelengths spaced at

regular intervals throughout the visible spectrum. The readings S 2 (x) are taken at the same wavelengths with the spectroradiometer in the

sample position, responding to incident light from the entire source.

An alternative way to get the values S 2 (x) is to place a white (spectrally neutral) diffusing plaque in the beam and view the plaque with

the spectroradiometer at a distance small enouqh to get a signal of

suitable magnitude and at an angle larqe enouqh to avoid any retroreflectance by the white diffusing plaque. The tristimulus values are

X = C z (S 1 (x)/S 2 (x)) S(x) x(x) AX

A

Y = C

j (S 1 (x)/S 2 (x)) S(x) y(x) ax

Z = C

j (S 1 (x)/S 2 (x)) S(x) z(X) ax

where S(>) is the relative spectral power distribution for Illuminant A

and where C is a constant which need not be determined since the chromaticity coordinates x and y are:

x = „ . v, . -, and v =

X + Y + Z

u,,u X + Y + Z and C divides from both the numerators and denominators. If the data are

not taken at regular intervals, the equations for X, Y, and Z must be

modified.
2. Telecolorimeter

The field-stop aperture must be filled with light when the telecolorimeter is focused on the retroreflecting surface. Note that this

is only to measure chromaticity, not the C.I.L.. The tristimulus

values are read using the filters supplied. Color correction factors

may be applied to the measured tristimulus values through the use of

spectrally similar filters of known tristimulus values. One method of

doing this is described in Federal Test Method 373. The chromacity

coordinates are then calculated.

VII. Important Parameters That Should Always Be Included in Specifications

The accuracy with which these parameters are known will affect the

accuracy of a measurement. However, the results very often also depend

on the retroreflecting sample, and a tolerance given as adequate for one sample may be inadequate for a different sample. A general mathematical discussion of many types of errors is given in reference [9],

A. Geometrical and Other Factors affecting both Photometric and

Colorimetric Values

1

.

Observation Angle

The observation angle a must be known. Since the entire range of

observation angles over which the retoreflector reflects significant

amounts of the incident light is quite small, the accuracy with which

this angle is set is important. This is especially true for cubecorner type retroref lectors and high quality bead sheeting. See" Fig. 7 for some typical curves showing how rapidly C.I.L. (R) drops off with

increasing observation angles. These data are from reference [9] and

were obtained by Norbert Johnson. Setting the observation angle off by

only a few thousandths of a degree can still cause a non-negligible

error. The values used are not only critical for measurements of

C.I.L. , but are also important for chromaticity coordinates. Rennilson's

[10] measurements show that the nighttime color of retroreflectors

varies with the geometry.

2. Entrance Angle

Both components 3i and 32 of the entrance angle 3 must be specified. The setting accuracy required for the entrance angle normally , becomes more critical for large values of 3. The required setting

accuracy for the entrance angle is approximately 100 times less stringent than that for the observation angle. See Fig. 8 for some typical curves showing how the retroreflected light drops off with increasing

entrance angle. These data are from reference [9] and were obtained by

Moerman, 1976, and Morren, 1978.

10
3. Rotation Angle

The rotation angle e must be specified. Normally, this angle is

not so critical for bead-type sheeting as for prismatic-type cubecorner retroreflectors. An example of variations due to changes in e for two types of cube-corner retroreflectors is shown in Fig. 9. The

data are from reference [9], and were obtained by Uding.

The elements of the retroreflectors are cube corners. Each cube- corner has six edges which form an appearance of a hexagon array. All

of the cube-corners are oriented in one direction for the one-orientation

device. The two-orientation device is divided in the center with the

cube- corners on the one half oriented in one direction and the cube- corners in the other half rotated 180° with respect to the cube-corners

in the first half.

4. Observation Distance

The observation distance between the sample and the photoreceptor

must be specified. The magnitude must be sufficiently large so that

the observation angles specified can be realized. (That is, the linear

separation between source and receiver corresponding to the specified

angles must exceed the smallest physically achievable separation.) The

observation distance must also be small enough so that signal-to-noise

levels of suitable magnitude are attainable. Distances of the order 10

to 30 m are normally specified.

5. Size and Shape of the Sample

The size and shape of the sample are normally not very critical.

The size must be sufficiently large to get suitable signal-to-noise

levels, and it must be small enough to subtend an angle at the photoreceptor which will not affect the results of the measurements. The

influence on accuracy of measurement due to angular apertures has been

reported by J. J. B. Moerman [11]. Stephenson [12] states that the

angle subtended by the retroreflector is relatively unimportant for

subtended angles up to 80 minutes of arc for photometric measurements.

6. Subtended Angles of the Photoreceptor and Source Apertures

The angle subtended at the sample by the photoreceptor aperture

and the angle subtended at the sample by the source aperture must be

sufficiently small. On the other hand, it must be large enough to

obtain practical signal levels. The errors caused by these two angles

is additive and Moerman [11] gives guidance on their proper selection.

Also, Johnson [13] and Johnson and Stephenson [14] have investigated

the selection of maximum apertures for different types of materials.

Generally, the subtenses are both of the order of 6 minutes of arc. If

the aperture sizes are fixed, however, these subtenses will vary with

observation distance.

11
7. Reference Center and Reference Axis

The reference center and the reference axis must be specified.

The reference axis usually is perpendicular to the surface of the

retroreflecting device. However, the device can be designed so that,

for example, the reference center and reference axis are defined relative to the viewing direction.

8. Uniformity

Non-uniformity of the illuminance on the sample, non-uniformity of

the sensitivity of the photoreceptor across its field of view, and non- uniformity of the sample can all cause errors [9], [13]. Although

these effects are usually not large, limits can be set on the allowable

variations, which can be checked periodically.

9. Source-Polarization and Illuminant

The source must produce unpolarized light and the photoreceptor

must be polarization insensitive since the retroreflector may change

the state of polarization of the incident light; in addition, the

source must usually have the relative spectral power distribution of

CIE Illuminant A.

10. Wavelength Error

The maximum allowable wavelength error must be specified if spectral

measurements are made; or if photometric or tristimulus filters are

used, the type of color correction, if any is to be used, should be

specified.

B. Additional Items to Specify for C.I.L. and Derived Quantities.

1. Specific Quantity to be Measured

The specific quantity to be measured must be specified along with

the system of units to be used in the measurement.

2. Minimum Acceptable Value.

The minimum value acceptable must be specified.

C. Additional Items to Specify for Color Determination

1. Chromaticity Coordinate Limits

The allowable color limit area on the CIE 1931 chromaticity

diagram is usually a quadrilateral, one side of which is a portion of

the spectrum locus (see Fig. 3). (For white, the limit area may have

more than four sides and does not include any part of the spectrum

12
locus.) The limit area can be specified either by giving the chromatid" ty coordinates (x,y) of all the corners of the limiting polygon, or

by giving the equations of all the sides of the polygon other than the

spectrum locus.

2. Color Limits for Standard Filters.

If standard filters are used to correct colorimeter readings, the

color limits allowable should be specified.

VIII. Test Methods and Specifications

Table I contains a partial listing of some test methods and specifications. Some of these documents contain both methods and specifications, and some are broader in scope than others. It is beyond the

scope of this paper to analyze each specification, but each one follows

to some extent the prescription outlined in the preceeding sections.

Lozano [15] compares five specifications for retroreflective materials

used on road signs. It should be mentioned that many individual states

have specifications which have not been listed, and many foreign

countries other than those listed have specifications.

Due to recent developments, some of these specifications may

change their terminology to conform to the new CIE definitions.

IX. Some Prior Research and Intercomparisons

In addition to a list of references, a bibliography is included at

the end of this report in Appendix A. It is not meant to be complete

since time does not permit, and the omission of any particular paper is

not intentional. It is hoped, however, that the papers cited in this

report, combined with the papers in the bibliography, will provide a starting point for anyone who wishes to do more research on the subject

of retroreflecting materials and devices.

A. C.I.L. and Derived Quantities

1 . Stephenson

H. F. Stephenson [16] conducted an international interchange test

in 1974 and-i975 in which 44 samples of three types of retroreflective

sheeting (encapsulated lens, enclosed lens, and white samples screened

with colored process paste) were investigated by nine laboratories.

The chromaticity coordinates (x and y) were measured (results are presented in section B below) and also the SCIL -- coefficient of

retroreflection or specific coefficient of luminous intensity

(cd«lx ^m"2

) -- was measured. Results are tabulated in his Table 8

for each laboratory along with the mean and standard deviation for each

sample. The coefficients of variation* measured for white samples

*Standard deviation divided by the mean.

13
were about 6%, varying for individual samples from approximately 5% to

7%. Similar results were obtained for the yellow, red, and orange

colors with slightly larger values for two of the red samples. The

coefficients of variation for blue and green samples typically ranged

from about 5% to 16%. The spread between laboratories was regarded to

be of an unacceptable level (approximately 3 times the coefficient of

variation), and more research needs to be done.

2. MCCA

The Manufacturers Council on Color and Appearance conducted five

intercomparison tests on retroreflectance in collaboration with the

NBS[17]. Four different colors of sheeting were randomized and distributed to the participating laboratories for measurement with two

geometries for a/3 (0.27-4° and 2°/30°). The sign for 3 follows the

CIE convention as described fully in Reference [3]. Although the co- efficient of variation varies from approximately 5% to 10% for the

0.2°/-4° geometry, the results for the other geometry had coefficients

of variation as large as 20%. This does not include data from some laboratories that had to be excluded. If these laboratories were

taken into account, the range in values was occasionally 100% to 150%.

3. Nimeroff and Hall

I. Nimeroff and W. Hall [18] also did some measurements on SCIL.

They compared measurements obtained on six colors of sheeting with 3

instruments by two laboratories. The coefficients of variation obtained

indicate that results could vary by as much as 2% to 14%, and this

variation was primarily of instrumental cause.

4. AAMVA

The American Association of Motor Vehicle Administrators under the

direction of Mr. Armond Cardanelli conducted an intercomparison of

measurements performed on prismatic retroreflectors and presented the

data to the NBS. Eleven laboratories from six countries including the

United States participated. The C.I.L. of two white, one yellow, and

three red prismatic reflectors were intercompared. The instructions

given were as follows:

"The test conditions for measuring these reflectors for the purpose

of this laboratory intercomparison are illustrated on the accompanying

diagram. The plane of the black backing of the reflector is to be perpendicular to the direction of incidence (0° entrance angle). Since

the front surface of these reflectors is curved, a portion of the

radiation reflected specularly by the front surface will be included in

the measurements. However, since this specularly reflected flux is

spread out over a rather large solid angle it will not contribute a large portion of the measured flux and no attempt should be made to

exclude it. If the apertures and distances indicated in the diagram

cannot be used, the closest available approximation should be used and

14
the departures should be described in the report giving the measured

values of the coefficient of luminous intensity (C.I.L.). Specifically,

if a different observation distance than the one shown is used, the

aperture sizes, source-to-reflector distance, and their associated

tolerances should be changed proportionally. The CIL of each reflector

under the given conditions should be reported in candela per incident

lux or, alternatively, in candlepower per incident foot-candle. Please

indicate which units were used when reporting the CIL data."

The arrangement for the measurements is shown in Fig. 10. Table

II contains the reported results along with the mean, standard deviation,

and coefficient of variation. It is noted that the coefficients of

variation ranged from about 8% to 17%. In Table III, the percent deviation from the mean is tabulated. Use of colored retrorefl ectors

changes the percent deviation from the mean of some laboratories by

large amounts.

B. Chromaticity coordinates (Nighttime geometry)

1

.

Stephenson

Stephenson's 1974 and 1975 interchange cited in Section A above

[16] also contains data on chromaticities . It is not possible to

reproduce his discussion here, but in most instances the coefficients

of variation ranged from about 1% to 3% and for the blue sample the

coordinates varied as much as 11%. For the interested reader, he

presents tables containing the data for each laboratory, the standard

deviation and the range. Also, CIE chromaticity diagrams illustrating

the data are presented. He reported that the spectral data usually had

a smaller standard deviation than the tristimulus data. Venable,

Stephenson, and Terstiege [9] suggest some of the tristimulus telecolorimeters used in the interchange may not have achieved as great an accuracy in filter matching as is possible using current practices.

2. Nimeroff and Hall

I. Nimeroff and W. Hall [18] did a study for color measurements

similar to that they did for SCIL. They compared the values for x and

y from data obtained with 3 instruments and two laboratories. The

coefficients of variation usually vary approximately from 1% to 5% with

the valuesTor the blue sample varying as much as 6% to 10%. Again,

their results indicate that the variations were primarily due to instrumental differences.

3. Geometry-Dependent Studies

In addition to the differences between laboratories such as those

shown by Stephenson, there are geometry effects on chromacity. Nimeroff

published a report [19] which is primarily concerned with daytime color

with 45/0 and D/0 geometries. He shows that the D/0 geometry correlates

better with visual observations. Nimeroff published another paper with

15
Hall [18] which concerns nighttime geometry primarily. On the basis of

his measurements, tentative color boundaries for retroreflector chromaticities were prepared and reported. Lozano [20] published a paper

which shows that no present recommendation can be used for both daytime

and nighttime conditions. He makes proposals for color tolerance

areas. In addition, Rennilson [10] shows that nighttime color also

depends on geometry, and presents data which will be useful for establishing color tolerances.

X. Current Issues and Problems

A few of the areas of current interest will be mentioned. Efforts

are underway to standardize nomenclature and definitions for test

methods and specifications. These efforts will be aided by the work

done by the CIE mentioned earlier [2], [3], Research is being done to

evaluate the effect of the various geometrical and photometric parameters on the measurements. Supplementing this research will be programs

such as the NBS (U.S.) MAP service under development to assist the

experimenter in evaluating his instrument.

Although not a primary concern of this paper, some issues for daytime measurements are under discussion. One controversy involves the

relative merits of D/0 versus 45/0 geometries for daytime color. The

geometry D/0 includes the wings of the retroreflectance curve but on the

other hand the geometry D/0 might be closer to the situation for actual

usage. Other possibilities such as D/45 geometry have been proposed.

Fluorescence of the retroreflector will be especially important for daytime color and for portable instruments but it will probably not pose a problem for nighttime viewing since the fluorescence is weaker and

diffuse. It has been suggested that because of fluorescence, Illuminant

D65 rather than Illuminant C be used for daytime color measurements.

Another area under discussion is the relative merits of tristimulus

colorimetry versus spectral colorimetry for both daytime and nighttime

geometries. At least one manufacturer of instruments thinks they should

be equivalent if proper care is used in the design and use of the

instruments

.

Specifications for chromaticities of retroref! ectors involve color

boundaries which must account somehow for the fact that the color of a retroreflector varies with geometry under both daytime and nighttime

geometries.

The accuracy of portable instruments is a topic of concern and

methods of measurements in the field have not been adequately specified.

Design of portable instruments is also a topic of controversy. As an example, it has been suggested that for short range instruments, it

might be adequate to integrate over larger solid angles than usual to

obtain an average for a given observation angle and still give adequate

information for some purposes.

16
XI. Conclusion

The state of the measurement system for retroreflectors is not at

present of the quality necessary for national and international equity

in trade. However, the outlook suggests that the uncertainties in

measurements of retroref! ective optical properties will become smaller

in the future. Much interest is being shown both nationally and internationally in the state of the measurement system as it relates to

optical properties of retroreflectors. The CIE and many standardizing

organizations are preparing or revising recommendations, methods, and

specifications. In the U.S., the NBS is conducting research to support

a Measurement Assurance Program service and standards for C.I.L..

Although the NBS reference retroreflectometer does not at present

measure chromaticity, it is hoped that this capability will be developed

in the future. The factors which affect the quality of measurements are becoming better understood. It is therefore probable that the stateof-the-measurement system will improve.

**********

ACKNOWLEDGEMENTS

This report partially fulfills the needs of, and was supported by,

two programs. This first is the NBS-OSHA Project on Criteria for Signs

in Workplaces, a joint effort of the Occupational Safety and Health

Administration (OSHA) and the Center for Building Technology of the NBS

The second is the Office of Measurement Services for the Measurement

Assurance Program (MAP).

17
XII. References

(1) International Lighting Vocabulary, 3rd Edition, Publication CIE

No. 17 (E.l.l) 1970, available from U.S. National Committee,

J. Tech, Secretary, National Bureau of Standards (Code 534),

Washington, D.C. 20234

(2) CIE TC 2.3 Subcommittee Report on Retroreflection, August, 1979.

(Unpublished)

(3) Venable, William H., Jr., and Johnson, Norbert L. "Unified

Coordinate System for Retroreflectance Measurements", Applied

Optics 19, 1234 (1980)

(4) Federal Test Method Standard 370. March 1, 1977. "Instrumental

Photometric Measurements of Retroreflective Materials and

Retroreflective Devices".

(5) Nimeroff, I., NBS (U.S.) Monograph 104. January 1968 "Color

imetry".

(6) Billmeyer, Fred W. , Jr. and Saltzman, Max, Principles of Color

Technology (Interscience Publishers, New York, 1966.)

(7) Wyszecki, Gunter, and Stiles, W.S., Color Science (John Wiley

and Sons, Inc., New York, 1967.)

(8) Eckerle, K. L. , Hsia, J. J., Weidner, V. R. , and Venable,

W. H., Jr., "NBS Reference Retroreflectometer" , Applied Optics

19, 1253 (1980).

(9) Venable, William H., Jr., Stephenson, H. F., and Terstiege,

Heinz. "Factors Affecting the Metrology of Retroreflecting

Materials". Paper presented at the 1979 CIE Kyoto meeting.

A version of this paper appears in Applied Optics 1_9, 1242

(1980).

(10) Rennilson, J. J., "Chromaticity Measurements of Retroreflective

Material Under Nighttime Geometry". Paper presented at the

1979 CIE Kyoto meeting. A version of this paper appears in

Applied Optics 1_9, 1260 (1980)

(11) Moerman, J. J. B., Lighting Research and Technology, 9_, 85

(1977). "Accuracy of photometry of retroref lectors and

retroreflective materials".

(12) Stephenson, H. F. , Proceedings of the SPIE Vol. 146 (1978),

"The Photometry of Retroref lectors".

(13) Johnson, N. L. , Proceedings of the SPIE Vol. 196 (1979),

"Accuracy in the Photometry of Retroreflectors".

18
References (Con't)

(14) Johnson, N. L. , and Stephenson, H. F., "Influence of

Aperture Size on the Photometry of Retroreflectors" . Applied

Optics 1_9, 1247 (1980)

(15) Lozano, R. D. , Lighting Research and Technology, 8_, 107 (1976).

"Measurement standards for retroreflective materials used on road signs".

(16) Stephenson, H. F. , Proceedings of the CIE, 18th Session

(London 1975) pp. 595-609. "The Colorimetric Measurement of

Retro-Reflecting Materials. Progress Report on International

Interchange Tests"

.

(17) Horlick, J. and Stevenson, J., NBS-MCCA Collaborative Reference

Program - Color and Appearance - Retroreflectance. Reports 1 through 5 (1977 and 1978)

(18) Nimeroff, I., and Hall, W. A., NBS Interagency Report - NBSIR 74-518, August, 1974. "Instrumental Colorimetry of

Retroreflective Sign Materials".

(19) Nimeroff, I., NBS Report No. FHWA-RD-71 -1 , November, 1971.

"Instrumental Color Measurement of Retroreflective Highway

Sign Materials".

(20) Lozano, R. D. , Die Far be 24, 167 (1975). "The Problem of

Retroreflective Materials".

19
XIII. APPENDIX A: BIBLIOGRAPHY

AUSTIN 1973

R. L. Austin, "Nighttime Search and Rescue", Presented at the 23rd

Congress of the Permanent International Association of Navigation

Congresses, July 10, 1973.

BILLMEYER 1966

F. W. Billmeyer, Jr. and Max Saltzman, Principles of Color Technology , Interscience Publishers - John Wiley and Sons (1 9661

BREDAHL 1980

T. D. Bredahl , "Factors for Consideration in the Establishment of

Retroreflectivity Specifications for Horizontal Coatings", Report

to ASTM Committee DOT .26.23 dated January 29, 1980

BURG 1978

Albert Burg, and Jinx Beers, "Reflectorization for Nighttime Conspicuity of Bicycles and Motorcycles", J. of Safety Research, 1_0 (2), 69 (1978)

CHANDLER 1958

K. N. Chandler and J. A. Reid, "Reflex Reflectors", Road Research

Technical Paper No. 42, Dept. of Scientific and Industrial Research

London (1958)

CHANDLER 1960

K. N. Chandler, "On the Effects of Small Errors in the Angles of

Corner-Cube Reflectors", J. Opt. Soc. Am. 50 (3) 203 (1960)

CIE 1970

"International Lighting Vocabulary", 3rd Edition, Publication CIE

No. 17 (E.l.l) (1970)

CIE 1978

"Surface Colours for Visual Signalling", CIE Publication No. 39

(TC-1.6) (1978)

CIE 1979

TC 2.3 Subcommittee Report on Retroreflection, August (1979)

COLORADO DIVISION OF HIGHWAYS 1970

Colorado Division of Highways, Planning and Research Division,

"Reflective Traffic Bead Study", Final Report, May (1970) (PB 200

872)

DERDERIAN 1977

George Derderian and Denis R. Breglia, "Retroreflectance Measurement System", Department of the Navy, Navy Case No. 59,414 April

(1977) (AD D003920)

20
APPENDIX A (Cont'd)

ECKERLE 1980

K. L. Eckerle, J. J. Hsia, V. R. Weidner and W. H. Venable, Jr.,

"NBS Reference Retroreflectometer" Applied Optics 1_9, 1253 (1980)

GRIESER 1972

D. R. Grieser, Michael M. Epstein and Robert W. King, "Development

of a New Low-Profile Highway Stripping for Wet-Night Visibility.

Phase 1: Feasibility", Battel le Columbus Laboratories, Federal

Highway Administration Report No. FHWA-RD-72-13, February (1972)

(PB 209423)

GRIESER 1973

Daniel R. Grieser, Michael M. Epstein and Joseph Preston, "Development of a new Low-Profile Highway Stripping for Wet-Night Visibility

PHase 2: Road Tests", Battel le Columbus Laboratories, Federal

Highway Administration Report No. FHWA-RD-73-78, September (1973)

(PB-224 934)

HECK 1976

Edwin A. Heck, "Test Procedure Revision for Retroreflectance Measurements", U. S. Army Mobility Equipment Research and Development

Command, Fort Belvoir, VA, Final Report 2187, August (1976) (AD

A034988)

HORLICK 1977 & 1978

J. Horlick and J. Stevenson, NBS-MCCA Collaborative Reference

Program - Color and Appearance - Retroreflectance, Reports 1 through 5 (1977 and 1978)

HURUKAWA 1977

K. Hurukawa, M. Ishikawa and Y. Nayatani "A Photometric System for

Retroreflective Materials for Traffic Signs, J. Res. Institute for

Electronic Tech., _4J_ (4), 35-42, Japan (1977)

INTERNATIONAL 1971

International Road Federation, "1971 World Survey of Current Research and Development on Roads and Road Transport", The Federal

Highway Administration December (1971) (PB206728)

JOHNSON 1979

N. L. Johnson, "Accuracy in the Photometry of Retroreflectors"

,

Proceedings of the SPIE Vol. 196 (1979)

JOHNSON 1980

N. L. Johnson and H. F. Stephenson, "The Influence of Aperture Size

on the Photometry of Retroreflectors", Applied Optics 19

1247, (1980) —?

21
APPENDIX A (Cont'd)

JUDD 1963

Deane B. Judd and Gunter Wyszecki, Color in Business , Science , and

Industry John Wiley & Sons, Inc. Second Edition (1963)

KINCHEN 1975

R. W. Kinchen and E. J. Leblanc, "Evaluation of Retroreflective

Durability of Raised Pavement Markers" Final Report - Research

Report No. 91, Louisiana Dept. of Highways (PB 249 481) (August

1975)

LOZANO 1975

Robert Daniel Lozano, "The Problem of Retroeflective Materials,

Some Practical Recommendations to the Measurement of their Colours,

Die Farbe 24, 167 (1975)

LOZANO 1976a

R. D. Lozano, "Measurement Standards for Retroreflective Materials

Used on Road Signs", Lighting Research and Technology, j3, 107

(1976)

LOZANO 1976b

Roberto Daniel Lozano, "II Parte: Comparacion Entre Distintas

Especificaciones Y Propuesta De Un Metodo de Control Visual Simple

Y Economico", Luminotecnia 1_0, 13 Argentina (1976)

LOZANO

Roberto Daniel Lozano, "Study on the Visibility, Color and Measuring Requirements of Road Signs for Safe Driving", Instituto

Nacional de Tecnologia Industrial, Buenos Aires, Argentina

(Unpublished)

MOERMAN 1977

J. J. B. Moerman, "Accuracy of Photometry of Retrorefl ectors and

Retroreflective Materials", Lighting Research & Technology, 9_ (2),

85-92 (1977)

NIMEROFF 1968

I. Nimeroff, "Colorimetry" , NBS (U.S.) Monograph 104, January

(1968)

NIMEROFF 1971

I. Nimeroff, "Instrumental Color Measurement of Retroreflective

Highway Sign Materials", NBS, Federal Highway Administration

Report No. FHWA-RD-71 -1 , November (1971) (PB 204 586)

NIMEROFF 1975

I. Nimeroff and W. A. Hall, "Instrumental Colorimetry of Retroreflective Sign Materials", NBS, Federal Highway Administration Final

Report No. FHWA-RD-75-4, January (1975)

22
APPENDIX A (Cont'd)

REID 1956

J. A. Reid and K. N. Chandler, "A Photometer for the Measurement of

Reflex Reflector Performance", Dept. of Scientific and Industrial

Research Research Note Mo. RN/2674/JAR.KNC. (England) January

(1956)

RENNILSON 1979, 1980

J. J. Rennilson, "Chromaticity Measurements of Retroreflective

Material Under Nighttime Geometry". Paper presented at the 1979

CIE Kyoto meeting. A version of the paper is published in Applied

Optics 19, 1260 (1980)

SMITH 1976

G. M. Smith and M. H. Janson, "Reflectorized Flagman Vests",

Research Laboratory Section, Testing and Research Division,

Research Report No. R-1021, Michigan Dept. of State Highways and

Transportation (1976)

SNYDER 1975

J. J. Snyder, Paraxial Ray Analysis of a Cat's-Eye Retroreflector"

,

Applied Optics, 14 (8) 1825 (1975)

SOLOMON 1974

Stephen S. Solomon, "Visibility of Fire Fighters", J. Am. Optometric Assoc, 45 (2), 137 (1974)

STEPHENSON 1975

H. F. Stephenson, "The Colorimetric Measurement of Retro-Reflecting

Materials. Progress Report on International Interchange Tests",

Proceedings of the CIE, 18th Session (London 1975) pp. 595-609.

STEPHENSON 1978

H. F. Stephenson, "The Photometry of Retroreflectors" , Proceedings

of the SPIE Vol. 146 (1978)

STOUDT 1978

M. D. Stoudt and K. Vedam "Retroreflection from Spherical Glass

Beads in Highway Pavement Markings. 1: Specular Reflection",

Applied Optics, ]]_ (12) 1855 (1978).

UDING 1978

Kenneth tiding and Fred De Long, "Reflectors", Bicycling, April 1978

VEDAM 197 5 K. Vedam and L. Shuler, "Use of Hemispherical Beads as Pavement

Marking Retroreflectors", Pennsylvania State University, Federal

Highway Administration Final Report No. FHWA-RD-75-67 , April (1975)

(PB-244 653)

23
APPENDIX A (Cont'd)

VEDAM 1978

K. Vedam and M. D. Stoudt, "Retroreflection from Spherical Glass

Beads in Highway Pavement Markings. 2: Diffuse Reflection (A First

Approximation Calculation", Applied Optics, ]7_ (12) 1859 (1978)

VENABLE 1979, 1980

Will imam H. Venable, Jr., H. F. Stephenson and Heinz Terstiege,

"Factors Affecting the Metrology of Retroreflecting Materials".

Paper presented at the 1979 CIE Kyoto meeting. A version of this

paper is published in Applied Optics 19, 1242 (1980).

VENABLE 1980

William H. Venable, Jr. and Norbert L. Johnson, "Unified Coordinate System for Retroreflectance Measurements" Applied Optics

19, 1236 (1980).

WEBB 1973

William E. Webb, "A Road Stripe Reflectometer for Use During Daylight Hours", Univ. of Alabama, Bureau of Engineering Research

Report No. 152-39, September (1973) (PB-226 032)

WYSZECKI 1967

Gunter Wyszecki and W. S. Stiles, Color Science , John Wiley and

Sons, Inc., New York (1967)

24
Promulgated

YES NO Speci fi cations

Test

Methods

X X X

X X X

X X X

TABLE I PARTIAL COLLECTION OF SPECIFICATION AND TEST-METHOD STANDARDS

FOR RETROREFLECTORS

Description

1. Consumer Products Safety Commission Requirements

for Bicycles. Federal Register Vol. 41, No. 19. Part IV, Jan. 18, 1976.

2. SAE J594e Reflex Reflectors

3. SAE J774A Emergency Reflex Reflectors

4. Federal Specification LS-300C - Sheeting and

Tape, Reflective: Non exposed lens X XX 5. Federal Test Method Standard 370 - Instrumental

Photometric Measurements of Retroreflective

Materials and Retroreflective Devices X X

6. Federal Test Method 373 (Colorimetry of

Retroreflectors) X X

7. British Standard 5064 Optical performance of

reflective agents for use in high visibility

garments and accessories X XX 8. ASAE S276.2 Slow Moving Vehicle Emblem X XX 9. E/ECE/TRANS/S05 Regulation 27 (Warning Triangle X XX 10. ISO Safety Colours and Safety Signs ? X 11. CIE Recommended Practices for Photometry and

Colorimetry of Retroreflectors X X 12. ASTM Subcommittee E-12.03 Practice for

Describing Retroreflection X X

13. ASTM Subcommittee E-12.03 Photometry of

Retroreflectors X X

14. ASTM Subcommittee E-12.03 Coefficient of

Retroreflection of Retroreflective Sheeting X X 15. ASTM Subcommittee E-12.03 Colorimetry of

Retroreflectors Under Nighttime Illuminating

Conditions X X

16. DIN 67520 - Retroreflecting materials for safety

and traffic; photometric assessment, measurement

and classification X X

17. DIN 6171 Surfact colours for traffic signs;

CoJ-ours and colour boundaries for illumination

by daylight X X 18. Federal Register Vol. 39, No. 155, Title 49,

Chapter V, Pat 571 Federal Motor Vehicle Safety

Standards X X 19. DOT FP-1974: Std. spec for Construction of roads

and Bridges on Federal Highway projects. X X

25
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(A) (B) (C)

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Figure 1: Retroreflectors. (a) Cat's Eye Element (b) Spherical or Lens Element (c) Cube-Corner Element (d) Exposed Lens Array

(e) Enclosed Lens Array (f) Encapsulated Array

28
Figure 2: Schemata
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SPECTRUM

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Figure 3. The CIE 1931 Chromatici ty Diagram with the spectrum and

Planckian loci and the chromatici ties of some illuminants

(CIE A, B, C and D65 ).

30
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31
Fiqure 5: Sample Carrier. A - Transverse Motion. B - Vertical Motion C - Incident

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32
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34
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Stripe

35
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37
NBS-114A (REV. 9-76)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 1125

4. TITLE AND SUBTITLE

PHOTOMETRY AND COLORIMETRY OF RETROREFLECTION

State-of-Measurement- Accuracy Report

5. Publication Date July 1980

7. AUTHOR(S)

Kenneth L. Eckerle

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/WorK Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

Same as No. 9

13. Type of Report & Period Covered Final

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or learn tactual summary of moat significant information. It document includes a significant bibliography or

literature survey, mention it here.)

A survey of the literature on retroreflection has been conducted and information from this survey necessary for the understanding of the phenomenon of

retroreflection is summarized. Items included are materials, measurement geometry, quantities to be measured, instrumentation, and methods of calibration.

Recommendations are given to evaluate or to write specifications by discussing

important parameters. A partial list of test methods and specifications is

presented in tabular form. Some prior research and intercomparisons are sum- marized including the results of a previously unpublished intercomparison using

prismatic type retroreflectors. The state-of-measurement-accuracy is inferred

from the information presented. A bibliography is included for those who would

like to obtain more details.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Accuracy; chromaticity; coefficient of luminous intensity; intercomparisons;

retroreflectance; retrorelfector; specifications; test methods

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[X^Order From Sud. of Doc, U.S. Government Printing Office, Washington, DC

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