ASTM E811-09(2015)
(Practice)Standard Practice for Measuring Colorimetric Characteristics of Retroreflectors Under Nighttime Conditions
Standard Practice for Measuring Colorimetric Characteristics of Retroreflectors Under Nighttime Conditions
SIGNIFICANCE AND USE
5.1 This practice describes a procedure for measuring the chromaticity of retroreflectors in a nighttime, that is, retroreflective, geometry of illumination and observation. CIE Standard Source A has been chosen to represent a tungsten automobile headlamp. Although the geometry must be specified by the user of this practice, it will, in general, correspond to the relationship between the vehicle headlamp, the retroreflector, and the vehicle driver's eyes. Thus, the chromaticity coordinates determined by the procedures in this practice describe numerically the nighttime appearance of the retroreflector.5
SCOPE
1.1 This practice describes the instrumental determination of retroreflected chromaticity coordinates of retroreflectors. It includes the techniques used in a photometric range to measure retroreflected (nighttime) chromaticity with either a telecolorimeter or telespectroradiometer.
1.2 This practice covers the general measurement procedures. Additional requirements for specific tests and specifications are described in Section 7.
1.3 The description of the geometry used in the nighttime colorimetry of retroreflectors is described in Practice E808 and the methods for calculation of chromaticity are contained in Practice E308.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E811 −09 (Reapproved 2015)
Standard Practice for
Measuring Colorimetric Characteristics of Retroreflectors
Under Nighttime Conditions
This standard is issued under the fixed designation E811; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 CIE Documents:
CIE Publication No. 15.2 Colorimetry, 2d ed.
1.1 This practice describes the instrumental determination
CIEStandardS 001⁄ISO IS10526,ColorimetricIlluminants
of retroreflected chromaticity coordinates of retroreflectors. It
CIE Standard S 002⁄ISO IS 10527, Colorimetric Observers
includes the techniques used in a photometric range to measure
CIE Technical Report 54.2 Retroreflection: Definition and
retroreflected (nighttime) chromaticity with either a telecolo-
Measurement
rimeter or telespectroradiometer.
1.2 This practice covers the general measurement proce-
3. Terminology
dures. Additional requirements for specific tests and specifica-
3.1 The terms and definitions inTerminology E284 apply to
tions are described in Section 7.
this practice.
1.3 The description of the geometry used in the nighttime
3.2 Definitions:
colorimetry of retroreflectors is described in Practice E808 and
3.2.1 chromaticity coordinates, n—the ratios of each of the
the methods for calculation of chromaticity are contained in
tristimulus values of a psychophysical color to the sum of the
Practice E308.
tristimulus values.
1.4 This standard does not purport to address all of the
3.2.1.1 Discussion—Chromaticity coordinates in the CIE
safety concerns, if any, associated with its use. It is the
1931 system of color specification are designated by x, y, z and
responsibility of the user of this standard to establish appro-
in the CIE 1964 supplementary system by x , y , z .
10 10 10
priate safety and health practices and determine the applica-
3.2.2 CIE 1931 (x, y)-chromaticity diagram—the chroma-
bility of regulatory limitations prior to use.
ticity diagram for the CIE 1931 standard observer, in which the
CIE 1931 chromaticity coordinates are plotted with x as the
2. Referenced Documents
abscissa and y as the ordinate.
2.1 ASTM Standards:
3.2.3 CIE 1931 standard observer, n—ideal colorimetric
E284 Terminology of Appearance
observer with color matching functions x¯(λ), y¯(λ), z¯(λ) corre-
E308 PracticeforComputingtheColorsofObjectsbyUsing
sponding to a field of view subtending a 2° angle on the retina;
the CIE System
B4
commonly called the “2° standard observer.” [CIE]
E691 Practice for Conducting an Interlaboratory Study to
3.2.3.1 Discussion—The color matching functions of the
Determine the Precision of a Test Method
CIE 1931 standard observer are tabulated in Practice E308,
E808 Practice for Describing Retroreflection
CIE Publication No. 15.2, and CIE Standard S 002.
E809 Practice for Measuring Photometric Characteristics of
3.2.4 CIE standard illuminant A, n—colorimetric
Retroreflectors
illuminant, representing the full radiation at 2855.6 K, defined
by the CIE in terms of a relative spectral power distribution.
B
[CIE]
3.2.4.1 Discussion—Therelativespectralpowerdistribution
ofCIEstandardilluminant AistabulatedinPracticeE308,CIE
This practice is under the jurisdiction of ASTM Committee E12 on Color and
Publication No. 15.2, and CIE Standard S 001.
Appearance and is the direct responsibility of Subcommittee E12.10 on Retrore-
flection.
Current edition approved July 1, 2015. Published July 2015. Originally approved
in1981.Lastpreviouseditionapprovedin2009asE811 – 09.DOI:10.1520/E0811- Available from U.S. National Committee of the CIE (International Commission
09R15. on Illumination), C/o Thomas M. Lemons, TLA-Lighting Consultants, Inc., 7 Pond
For referenced ASTM standards, visit the ASTM website, www.astm.org, or St., Salem, MA 01970, http://www.cie-usnc.org.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Stephenson, H. F., “The Colorimetric Measurement of Retroreflective Materi-
Standards volume information, refer to the standard’s Document Summary page on als. Progress Report on International Exchange Tests,”Proceedings of the CIE, 18th
the ASTM website. Session (London), pp. 595–609, 1975.
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959. United States
E811−09 (2015)
3.2.5 CIE standard source A, n—a gas-filled tungsten- the datum axis, measured counter-clockwise from a viewpoint
filament lamp operated at a correlated color temperature of on the retroreflector axis.
B
2855.6 K. [CIE] 3.2.17.1 Discussion—Range: –180°<ε≤180°. The definition
is applicable when entrance angle and viewing angle are less
3.2.6 entrance angle, β,n—the angle between the illumina-
than 90°. More generally, rotation angle is the angle from the
tion axis and the retroreflector axis.
positive part of second axis to the datum axis, measured
3.2.6.1 Discussion—The entrance angle is usually no larger
counterclockwise from a viewpoint on the retroreflector axis.
than 90°, but for completeness its full range is defined as 0° ≤
3.2.17.2 Discussion—Rotation of the sample about the ret-
β≤180°.IntheCIE(goniometer)systemβisresolvedintotwo
roreflector axis while the source and receiver remain fixed in
components,β andβ .Sincebydefinitionβisalwayspositive,
1 2
space changes the rotation angle (ε) and the orientation angle
the common practice of referring to the small entrance angles
(ω ) equally.
s
that direct specular reflections away from the photoreceptor as
3.2.18 spectroradiometer, n—an instrument for measuring
negative valued is deprecated byASTM. The recommendation
the spectral distribution of radiant energy or power.
is to designate such negative values as belonging to β .
3.2.19 tristimulus colorimeter, n—instrument that measures
3.2.7 goniometer, n—an instrument for measuring or setting
psychophysical color, in terms of tristimulus values, by the use
angles.
of filters to convert the relative spectral power distribution of
3.2.8 illumination axis, n—in retroreflection, a line from the
the illuminator to that of a standard illuminant, and to convert
effective center of the source aperture to the retroreflector
the relative spectral responsivity of the receiver to the respon-
center.
sivities prescribed for a standard observer.
3.2.19.1 Discussion—In some instruments, the filters may
3.2.9 observation angle, n—angle between the axes of the
be combined into one set placed in the receiver; in such cases,
incident beam and the observed (reflected) beam, (in
caution should be observed when measuring fluorescent speci-
retroreflection, α, angle between the illumination axis and the
mens.
observation axis).
3.2.20 viewing angle, v, n—in retroreflection, the angle
3.2.10 observation axis, n—in retroreflection, a line from
between the retroreflector axis and the observation axis.
the effective center of the receiver aperture to the retroreflector
center.
3.3 Definitions of Terms Specific to This Standard:
3.3.1 telecolorimeter, n—a tristimulus colorimeter equipped
3.2.11 retroreflection, n—reflection in which the reflected
with collection optics for viewing a limited area at a distance
rays are preferentially returned in directions close to the
from the instrument.
opposite of the direction of the incident rays, this property
being maintained over wide variations of the direction of the 3.3.2 telespectroradiometer, n—a spectroradiometer
B
incident rays. [CIE] equipped with collection optics for viewing a limited area at a
distance from the instrument.
3.2.12 retroreflective device, n—deprecated term; use ret-
roreflector.
4. Summary of Practice
3.2.13 retroreflective sheeting, n—a retroreflective material
4.1 Two procedures are described in this practice (see also
preassembled as a thin film ready for use.
E809). Procedure A is based on a calibrated light
Practice
3.2.14 retroreflector, n—a reflecting surface or device from
source,coloredreferencefilters,awhitereferencestandardand
which, when directionally irradiated, the reflected rays are a telecolorimeter equipped with tristimulus filters. In this
preferentially returned in directions close to the opposite of the
procedure, measurements of the incident light on the white
direction of the incident rays, this property being maintained standard at the specimen position are made using the colored
over wide variations of the direction of the incident rays. [CIE,
filters and correction factors developed.Then the retroreflected
B
1982] light is measured under the test geometry and the corrected
relative tristimulus values are computed. In Procedure B,
3.2.15 retroreflector axis, n—a designated line segment
spectralmeasurementsaremadeoftheincidentlightandofthe
from the retroreflector center that is used to describe the
retroreflected light under the test geometry required. From
angular position of the retroreflector.
these spectral measurements, the relative tristimulus values are
3.2.15.1 Discussion—The direction of the retroreflector axis
determined.Inbothprocedures,thechromaticitycoordinates x,
is usually chosen centrally among the intended directions of
y are based on the CIE 1931 Standard Color Observer.
illumination;forexample,thedirectionoftheroadonwhichor
with respect to which the retroreflector is intended to be
5. Significance and Use
positioned. In testing horizontal road markings the retroreflec-
5.1 This practice describes a procedure for measuring the
tor axis is usually the normal to the test surface.
chromaticity of retroreflectors in a nighttime, that is,
3.2.16 retroreflector center, n—a point on or near a retrore-
retroreflective, geometry of illumination and observation. CIE
flector that is designated to be the center of the device for the
Standard Source A has been chosen to represent a tungsten
purpose of specifying its performance.
automobile headlamp. Although the geometry must be speci-
3.2.17 rotation angle, ε,n—the angle in a plane perpendicu- fied by the user of this practice, it will, in general, correspond
lar to the retroreflector axis from the observation halfplane to to the relationship between the vehicle headlamp, the
E811−09 (2015)
retroreflector, and the vehicle driver’s eyes. Thus, the chroma- E308, Fig. 1.The outline in the figure encloses the entire range
ticity coordinates determined by the procedures in this practice of combinations of x and y that correspond to real colors. The
describe numerically the nighttime appearance of the retrore- points at which monochromatic radiation of various wave-
flector. lengths falls are indicated on this boundary, with the more
nearly neutral colors being represented by points toward the
6. Use of the CIE Chromaticity Diagram for the
center of the bounded region.
Specification of Color
6.4 Specifying Color Limits—Acolor point representing the
6.1 Tristimulus Values for a Colored Sample—The spectral
xand ychromaticitycoordinatesofatestsamplecanbelocated
nature of the light coming to the eye from a retroreflector
on the CIE diagram. A specification for a specific retroreflec-
depends upon the spectral distribution of the radiation from the
tive color limit would require that the color point for a sample
source, S(λ), and a quantity proportional to the spectral
of this color fall within specified boundaries of the diagram.
reflectance of the retroreflector, R(λ). For nighttime colorimet-
The area within these boundaries is referred to as a color area,
ric measurements of retroreflectors, S(λ) is Illuminant A. The
and is defined exactly by specifying four sets of chromaticity
spectral tristimulus values, x¯, y¯, and z¯, the illuminant power
coordinates in the specification.
S(λ), and the reflectance quantity R(λ) are used together to
6.5 Daytime versus Nighttime Color Limits—Different color
calculate three numbers, the tristimulus values X, Y, and Z as
limits are required to specify daytime and nighttime color.
follows:
Nighttime and daytime color limits are different for two major
reasons: the quality of the illuminating light and the geometry
X 5 k S ~λ! R~λ! x¯~λ!dλ
*
A
or direction of the illuminating light. Daytime color is viewed
under a source of daylight quality, and nighttime color is
viewed under Source A (a CIE source corresponding to an
Y 5 k S ~λ! R~λ! y¯~λ!dλ
*
A
incandescent lamp, such as an automobile headlamp). Illumi-
nation in the daytime is from skylight, and diffusely reflected
Z 5 k S λ R λ z¯ λ dλ
* ~ ! ~ ! ~ ! light is observed; illumination in the nighttime comes from a
A
point very near the observer, and retroreflected light is ob-
where: served.
S (λ) = spectral power distribution of Illuminant
A
7. Requirements to be Stated in Specifications
A,
R(λ) = spectral reflectance factor of the sample,
7.1 When stating colorimetric retroreflective requirements,
and
the following requirements shall be given in the specification
x¯(λ), y¯(λ), z¯(λ) = color matching functions of the CIE stan-
for the material:
dard observer.
7.1.1 Limits of the color area on the 1931 CIE chromaticity
diagram (usually four pairs of chromaticity coordinates (x and
100/k 5 S y¯ λ dλ
* ~ !
A
y) are required to define an area on the diagram).
7.1.2 Chromaticity coordinate limits and spectral transmit-
Integration of each curve across the visible region (380 to
tance limits of the standard filter when Procedure A is used.
740 nm) give the numerical value for the corresponding
(These may be specified by giving the filter glass type and
tristimulus value X, Y, or Z.
thickness or the manufacturer’s part number of the filter.)
7.1.3 Observation angle (α).
6.2 Chromaticity Coordinates—The chromaticity coordi-
nates x, y, and z are computed from the tristimulus values X, Y, 7.1.4 Entrance angle (β) and when required the components
and Z as follows: of the entrance angle β , and β . (When specifying entrance
1 2
angles near 0°, care must be taken to prevent “white” specular
x 5 X/ X1Y1Z
~ !
reflection from entering the receptor. Therefore, instead of
y 5 Y/~X1Y1Z!
specifying 0°, the entrance angle is usually specified so that
z 5 Z/~X1Y1Z!
specular light is reflected away from the receptor.)
The normalization constant k in the equations for X, Y, and
7.1.5 Rotation angle (ε) and the location of the datum mark,
Z cancels out in calculating x, y, and z.Thus, x, y, and z express
if random orientation of the test specimen is not suitable.
the color of the reflected light without regard to its intensity.
7.1.6 Observation distance (d).
Because the sum of x, y, and z is always equal to one, only two
7.1.7 Test specimen dimensions and shape.
of these qua
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E811 − 09 E811 − 09 (Reapproved 2015)
Standard Practice for
Measuring Colorimetric Characteristics of Retroreflectors
Under Nighttime Conditions
This standard is issued under the fixed designation E811; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice describes the instrumental determination of retroreflected chromaticity coordinates of retroreflectors. It
includes the techniques used in a photometric range to measure retroreflected (nighttime) chromaticity with either a telecolorimeter
or telespectroradiometer.
1.2 This practice covers the general measurement procedures. Additional requirements for specific tests and specifications are
described in Section 7.
1.3 The description of the geometry used in the nighttime colorimetry of retroreflectors is described in Practice E808 and the
methods for calculation of chromaticity are contained in Practice E308.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E284 Terminology of Appearance
E308 Practice for Computing the Colors of Objects by Using the CIE System
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E808 Practice for Describing Retroreflection
E809 Practice for Measuring Photometric Characteristics of Retroreflectors
2.2 CIE Documents:
CIE Publication No. 15.2 Colorimetry, 2d ed.
CIE Standard S 001 ⁄ISO IS 10526, Colorimetric Illuminants
CIE Standard S 002 ⁄ISO IS 10527, Colorimetric Observers
CIE Technical Report 54.2 Retroreflection: Definition and Measurement
3. Terminology
3.1 The terms and definitions in Terminology E284 apply to this practice.
3.2 Definitions:
3.2.1 chromaticity coordinates, n—the ratios of each of the tristimulus values of a psychophysical color to the sum of the
tristimulus values.
This practice is under the jurisdiction of ASTM Committee E12 on Color and Appearance and is the direct responsibility of Subcommittee E12.10 on Retroreflection.
Current edition approved Dec. 1, 2009July 1, 2015. Published January 2010July 2015. Originally approved in 1981. Last previous edition approved in 20012009 as
E811 – 95 (2001).E811 – 09. DOI: 10.1520/E0811-09.10.1520/E0811-09R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from U.S. National Committee of the CIE (International Commission on Illumination), C/o Thomas M. Lemons, TLA-Lighting Consultants, Inc., 7 Pond St.,
Salem, MA 01970, http://www.cie-usnc.org.
3.2.1.1 Discussion—
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E811 − 09 (2015)
Chromaticity coordinates in the CIE 1931 system of color specification are designated by x, y, z and in the CIE 1964 supplementary
system by x , y , z .
10 10 10
3.2.2 CIE 1931 (x, y)-chromaticity diagram—the chromaticity diagram for the CIE 1931 standard observer, in which the CIE
1931 chromaticity coordinates are plotted with x as the abscissa and y as the ordinate.
3.2.3 CIE 1931 standard observer, n—ideal colorimetric observer with color matching functions x¯(λ), y¯(λ), z¯(λ) corresponding
B4
to a field of view subtending a 2° angle on the retina; commonly called the “2° standard observer.” [CIE]
Stephenson, H. F., “The Colorimetric Measurement of Retroreflective Materials. Progress Report on International Exchange Tests,”Proceedings of the CIE, 18th Session
(London), pp. 595–609, 1975.
3.2.3.1 Discussion—
The color matching functions of the CIE 1931 standard observer are tabulated in Practice E308, CIE Publication No. 15.2, and
CIE Standard S 002.
3.2.4 CIE standard illuminant A, n—colorimetric illuminant, representing the full radiation at 2855.6 K, defined by the CIE in
B
terms of a relative spectral power distribution. [CIE]
3.2.4.1 Discussion—
The relative spectral power distribution of CIE standard illuminant A is tabulated in Practice E308, CIE Publication No. 15.2, and
CIE Standard S 001.
3.2.5 CIE standard source A, n—a gas-filled tungsten-filament lamp operated at a correlated color temperature of 2855.6 K.
B
[CIE]
3.2.5 CIE standard source A, n—a gas-filled tungsten-filament lamp operated at a correlated color temperature of 2855.6 K.
B
[CIE]
3.2.6 entrance angle, β, n—the angle between the illumination axis and the retroreflector axis.
3.2.6.1 Discussion—
The entrance angle is usually no larger than 90°, but for completeness its full range is defined as 0° ≤ β ≤ 180°. In the CIE
(goniometer) system β is resolved into two components, β and β . Since by definition β is always positive, the common practice
1 2
of referring to the small entrance angles that direct specular reflections away from the photoreceptor as negative valued is
deprecated by ASTM. The recommendation is to designate such negative values as belonging to β .
3.2.7 goniometer, n—an instrument for measuring or setting angles.
3.2.8 illumination axis, n—in retroreflection, a line from the effective center of the source aperture to the retroreflector center.
3.2.9 observation angle, n—angle between the axes of the incident beam and the observed (reflected) beam, (in retroreflection,
α, angle between the illumination axis and the observation axis).
3.2.10 observation axis, n—in retroreflection, a line from the effective center of the receiver aperture to the retroreflector center.
3.2.11 retroreflection, n—reflection in which the reflected rays are preferentially returned in directions close to the opposite of
B
the direction of the incident rays, this property being maintained over wide variations of the direction of the incident rays. [CIE]
3.2.12 retroreflective device, n—deprecated term; use retroreflector.
3.2.13 retroreflective sheeting, n—a retroreflective material preassembled as a thin film ready for use.
3.2.14 retroreflector, n—a reflecting surface or device from which, when directionally irradiated, the reflected rays are
preferentially returned in directions close to the opposite of the direction of the incident rays, this property being maintained over
B
wide variations of the direction of the incident rays. [CIE, 1982]
3.2.15 retroreflector axis, n—a designated line segment from the retroreflector center that is used to describe the angular position
of the retroreflector.
3.2.15.1 Discussion—
The direction of the retroreflector axis is usually chosen centrally among the intended directions of illumination; for example, the
direction of the road on which or with respect to which the retroreflector is intended to be positioned. In testing horizontal road
markings the retroreflector axis is usually the normal to the test surface.
E811 − 09 (2015)
3.2.16 retroreflector center, n—a point on or near a retroreflector that is designated to be the center of the device for the purpose
of specifying its performance.
3.2.17 rotation angle, ε, n—the angle in a plane perpendicular to the retroreflector axis from the observation halfplane to the
datum axis, measured counter-clockwise from a viewpoint on the retroreflector axis.
3.2.17.1 Discussion—
Range: –180°<ε≤180°. The definition is applicable when entrance angle and viewing angle are less than 90°. More generally,
rotation angle is the angle from the positive part of second axis to the datum axis, measured counterclockwise from a viewpoint
on the retroreflector axis.
3.2.17.2 Discussion—
Rotation of the sample about the retroreflector axis while the source and receiver remain fixed in space changes the rotation angle
(ε) and the orientation angle (ω ) equally.
s
3.2.18 spectroradiometer, n—an instrument for measuring the spectral distribution of radiant energy or power.
3.2.19 tristimulus colorimeter, n—instrument that measures psychophysical color, in terms of tristimulus values, by the use of
filters to convert the relative spectral power distribution of the illuminator to that of a standard illuminant, and to convert the
relative spectral responsivity of the receiver to the responsivities prescribed for a standard observer.
3.2.19.1 Discussion—
In some instruments, the filters may be combined into one set placed in the receiver; in such cases, caution should be observed
when measuring fluorescent specimens.
3.2.20 viewing angle, v, n—in retroreflection, the angle between the retroreflector axis and the observation axis.
3.3 Definitions of Terms Specific to This Standard:
3.3.1 telecolorimeter, n—a tristimulus colorimeter equipped with collection optics for viewing a limited area at a distance from
the instrument.
3.3.2 telespectroradiometer, n—a spectroradiometer equipped with collection optics for viewing a limited area at a distance
from the instrument.
4. Summary of Practice
4.1 Two procedures are described in this practice (see also Practice E809). Procedure A is based on a calibrated light source,
colored reference filters, a white reference standard and a telecolorimeter equipped with tristimulus filters. In this procedure,
measurements of the incident light on the white standard at the specimen position are made using the colored filters and correction
factors developed. Then the retroreflected light is measured under the test geometry and the corrected relative tristimulus values
are computed. In Procedure B, spectral measurements are made of the incident light and of the retroreflected light under the test
geometry required. From these spectral measurements, the relative tristimulus values are determined. In both procedures, the
chromaticity coordinates x,y are based on the CIE 1931 Standard Color Observer.
5. Significance and Use
5.1 This practice describes a procedure for measuring the chromaticity of retroreflectors in a nighttime, that is, retroreflective,
geometry of illumination and observation. CIE Standard Source A has been chosen to represent a tungsten automobile headlamp.
Although the geometry must be specified by the user of this practice, it will, in general, correspond to the relationship between
the vehicle headlamp, the retroreflector, and the vehicle driver’s eyes. Thus, the chromaticity coordinates determined by the
procedures in this practice describe numerically the nighttime appearance of the retroreflector.
6. Use of the CIE Chromaticity Diagram for the Specification of Color
6.1 Tristimulus Values for a Colored Sample—The spectral nature of the light coming to the eye from a retroreflector depends
upon the spectral distribution of the radiation from the source, S(λ), and a quantity proportional to the spectral reflectance of the
retroreflector, R(λ). For nighttime colorimetric measurements of retroreflectors, S(λ) is Illuminant A. The spectral tristimulus
values, x¯, y¯, and z¯, the illuminant power S(λ), and the reflectance quantity R(λ) are used together to calculate three numbers, the
tristimulus values X,Y, and Z as follows:
Rennilson, J. J., “Chromaticity Measurements of Retroreflective Material Under Nighttime Geometry,” Applied Optics, Vol 45, April 15, 1980.
E811 − 09 (2015)
X 5 k S λ R λ x¯ λ dλ
* ~ ! ~ ! ~ !
A
Y 5 k * S ~λ! R~λ!y¯ ~λ!dλ
A
Z 5 k S ~λ! R~λ!z¯~λ!dλ
*
A
where:
S (λ) = spectral power distribution of Illuminant A,
A
R(λ) = spectral reflectance factor of the sample, and
x¯(λ), y¯(λ), = color matching functions of the CIE standard observer.
z¯(λ)
100/k 5 S y¯ λ dλ
* ~ !
A
Integration of each curve across the visible region (380 to 740 nm) give the numerical value for the corresponding tristimulus
value X, Y, or Z.
6.2 Chromaticity Coordinates—The chromaticity coordinates x,y, and z are computed from the tristimulus values X,Y, and Z as
follows:
x 5 X/~X1Y1Z!
y 5 Y/~X1Y1Z!
z 5 Z/ X1Y1Z
~ !
The normalization constant k in the equations for X,Y, and Z cancels out in calculating x,y, and z. Thus, x,y, and z express the
color of the reflected light without regard to its intensity. Because the sum of x,y, and z is always equal to one, only two of these
quantities are needed to describe the chromaticity of a light. The chromaticity coordinates x and y are chosen for this purpose.
6.3 CIE 1931 (x, y) Chromaticity Diagram—The chromaticity coordinates x and y can be plotted as shown in Practice E308,
Fig. 1. The outline in the figure encloses the entire range of combinations of x and y that correspond to real colors. The points at
which monochromatic radiation of various wavelengths falls are indicated on this boundary, with the more nearly neutral colors
being represented by points toward the center of the bounded region.
6.4 Specifying Color Limits—A color point representing the x and y chromaticity coordinates of a test sample can be located
on the CIE diagram. A specification for a specific retroreflective color limit would require that the color point for a sample of this
color fall within specified boundaries of the diagram. The area within these boundaries is referred to as a color area, and is defined
exactly by specifying four sets of chromaticity coordinates in the specification.
6.5 Daytime versus Nighttime Color Limits—Different color limits are required to specify daytime and nighttime color.
Nighttime and daytime color limits are different for two major reasons: the quality of the illuminating light and the geometry or
direction of the illuminating light. Daytime color is viewed under a source of daylight quality, and nighttime color is viewed under
Source A (a CIE source corresponding to an incandescent lamp, such as an automobile headlamp). Illumination in the daytime is
from skylight, and diffusely reflected light is observed; illumination in the nighttime comes from a point very near the observer,
and retroreflected light is observed.
7. Requirements to be Stated in Specifications
7.1 When stating colorimetric retroreflective requirements, the following requirements shall be given in the specification for the
mat
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