# ASTM E811-09 (Reapproved 2015)

Designation: E811 − 09 (Reapproved 2015)Standard Practice forMeasuring Colorimetric Characteristics of RetroreflectorsUnder Nighttime Conditions1This standard is issued under the fixed designation E811; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes the instrumental determinationof retroreflected chromaticity coordinates of retroreflectors. Itincludes the techniques used in a photometric range to measureretroreflected (nighttime) chromaticity with either a telecolo-rimeter or telespectroradiometer.1.2 This practice covers the general measurement proce-dures. Additional requirements for specific tests and specifica-tions are described in Section 7.1.3 The description of the geometry used in the nighttimecolorimetry of retroreflectors is described in Practice E808 andthe methods for calculation of chromaticity are contained inPractice E308.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E284 Terminology of AppearanceE308 Practice for Computing the Colors of Objects by Usingthe CIE SystemE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE808 Practice for Describing RetroreflectionE809 Practice for Measuring Photometric Characteristics ofRetroreflectors2.2 CIE Documents:3CIE Publication No. 15.2 Colorimetry, 2d ed.CIE Standard S 001 ⁄ISO IS 10526, Colorimetric IlluminantsCIE Standard S 002 ⁄ISO IS 10527, Colorimetric ObserversCIE Technical Report 54.2 Retroreflection: Definition andMeasurement3. Terminology3.1 The terms and definitions in Terminology E284 apply tothis practice.3.2 Definitions:3.2.1 chromaticity coordinates, n—the ratios of each of thetristimulus values of a psychophysical color to the sum of thetristimulus values.3.2.1.1 Discussion—Chromaticity coordinates in the CIE1931 system of color specification are designated by x, y, z andin the CIE 1964 supplementary system by x10, y10, z10.3.2.2 CIE 1931 (x, y)-chromaticity diagram—the chroma-ticity diagram for the CIE 1931 standard observer, in which theCIE 1931 chromaticity coordinates are plotted with x as theabscissa and y as the ordinate.3.2.3 CIE 1931 standard observer, n—ideal colorimetricobserver with color matching functions x¯(λ), y¯(λ), z¯(λ) corre-sponding to a field of view subtending a 2° angle on the retina;commonly called the “2° standard observer.” [CIE]B43.2.3.1 Discussion—The color matching functions of theCIE 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—colorimetricilluminant, representing the full radiation at 2855.6 K, definedby the CIE in terms of a relative spectral power distribution.[CIE]B3.2.4.1 Discussion—The relative spectral power distributionof CIE standard illuminant A is tabulated in Practice E308, CIEPublication No. 15.2, and CIE Standard S 001.1This practice is under the jurisdiction of ASTM Committee E12 on Color andAppearance and is the direct responsibility of Subcommittee E12.10 on Retrore-flection.Current edition approved July 1, 2015. Published July 2015. Originally approvedin 1981. Last previous edition approved in 2009 as E811 – 09. DOI: 10.1520/E0811-09R15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3Available from U.S. National Committee of the CIE (International Commissionon Illumination), C/o Thomas M. Lemons, TLA-Lighting Consultants, Inc., 7 PondSt., Salem, MA 01970, http://www.cie-usnc.org.4Stephenson, H. F., “The Colorimetric Measurement of Retroreflective Materi-als. Progress Report on International Exchange Tests,”Proceedings of the CIE, 18thSession (London), pp. 595–609, 1975.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.5 CIE standard source A, n—a gas-filled tungsten-filament lamp operated at a correlated color temperature of2855.6 K. [CIE]B3.2.6 entrance angle, β,n—the angle between the illumina-tion axis and the retroreflector axis.3.2.6.1 Discussion—The entrance angle is usually no largerthan 90°, but for completeness its full range is defined as 0° ≤β≤180°. In the CIE (goniometer) system β is resolved into twocomponents, β1and β2. Since by definition β is always positive,the common practice of referring to the small entrance anglesthat direct specular reflections away from the photoreceptor asnegative valued is deprecated by ASTM. The recommendationis to designate such negative values as belonging to β1.3.2.7 goniometer, n—an instrument for measuring or settingangles.3.2.8 illumination axis, n—in retroreflection, a line from theeffective center of the source aperture to the retroreflectorcenter.3.2.9 observation angle, n—angle between the axes of theincident beam and the observed (reflected) beam, (inretroreflection, α, angle between the illumination axis and theobservation axis).3.2.10 observation axis, n—in retroreflection, a line fromthe effective center of the receiver aperture to the retroreflectorcenter.3.2.11 retroreflection, n—reflection in which the reflectedrays are preferentially returned in directions close to theopposite of the direction of the incident rays, this propertybeing maintained over wide variations of the direction of theincident rays. [CIE]B3.2.12 retroreflective device, n—deprecated term; use ret-roreflector.3.2.13 retroreflective sheeting, n—a retroreflective materialpreassembled as a thin film ready for use.3.2.14 retroreflector, n—a reflecting surface or device fromwhich, when directionally irradiated, the reflected rays arepreferentially returned in directions close to the opposite of thedirection of the incident rays, this property being maintainedover wide variations of the direction of the incident rays. [CIE,1982]B3.2.15 retroreflector axis, n—a designated line segmentfrom the retroreflector center that is used to describe theangular position of the retroreflector.3.2.15.1 Discussion—The direction of the retroreflector axisis usually chosen centrally among the intended directions ofillumination; for example, the direction of the road on which orwith respect to which the retroreflector is intended to bepositioned. In testing horizontal road markings the retroreflec-tor axis is usually the normal to the test surface.3.2.16 retroreflector center, n—a point on or near a retrore-flector that is designated to be the center of the device for thepurpose of specifying its performance.3.2.17 rotation angle, ε,n—the angle in a plane perpendicu-lar to the retroreflector axis from the observation halfplane tothe datum axis, measured counter-clockwise from a viewpointon the retroreflector axis.3.2.17.1 Discussion—Range: –180°ε≤180°. The definitionis applicable when entrance angle and viewing angle are lessthan 90°. More generally, rotation angle is the angle from thepositive part of second axis to the datum axis, measuredcounterclockwise from a viewpoint on the retroreflector axis.3.2.17.2 Discussion—Rotation of the sample about the ret-roreflector axis while the source and receiver remain fixed inspace changes the rotation angle (ε) and the orientation angle(ωs) equally.3.2.18 spectroradiometer, n—an instrument for measuringthe spectral distribution of radiant energy or power.3.2.19 tristimulus colorimeter, n—instrument that measurespsychophysical color, in terms of tristimulus values, by the useof filters to convert the relative spectral power distribution ofthe illuminator to that of a standard illuminant, and to convertthe relative spectral responsivity of the receiver to the respon-sivities prescribed for a standard observer.3.2.19.1 Discussion—In some instruments, the filters maybe combined into one set placed in the receiver; in such cases,caution should be observed when measuring fluorescent speci-mens.3.2.20 viewing angle, v, n—in retroreflection, the anglebetween the retroreflector axis and the observation axis.3.3 Definitions of Terms Specific to This Standard:3.3.1 telecolorimeter, n—a tristimulus colorimeter equippedwith collection optics for viewing a limited area at a distancefrom the instrument.3.3.2 telespectroradiometer, n—a spectroradiometerequipped with collection optics for viewing a limited area at adistance from the instrument.4. Summary of Practice4.1 Two procedures are described in this practice (see alsoPractice E809). Procedure A is based on a calibrated lightsource, colored reference filters, a white reference standard anda telecolorimeter equipped with tristimulus filters. In thisprocedure, measurements of the incident light on the whitestandard at the specimen position are made using the coloredfilters and correction factors developed. Then the retroreflectedlight is measured under the test geometry and the correctedrelative tristimulus values are computed. In Procedure B,spectral measurements are made of the incident light and of theretroreflected light under the test geometry required. Fromthese spectral measurements, the relative tristimulus values aredetermined. In both procedures, the chromaticity coordinates x,y are based on the CIE 1931 Standard Color Observer.5. Significance and Use5.1 This practice describes a procedure for measuring thechromaticity of retroreflectors in a nighttime, that is,retroreflective, geometry of illumination and observation. CIEStandard Source A has been chosen to represent a tungstenautomobile headlamp. Although the geometry must be speci-fied by the user of this practice, it will, in general, correspondto the relationship between the vehicle headlamp, theE811 − 09 (2015)2retroreflector, and the vehicle driver’s eyes. Thus, the chroma-ticity coordinates determined by the procedures in this practicedescribe numerically the nighttime appearance of the retrore-flector.56. Use of the CIE Chromaticity Diagram for theSpecification of Color6.1 Tristimulus Values for a Colored Sample—The spectralnature of the light coming to the eye from a retroreflectordepends upon the spectral distribution of the radiation from thesource, S(λ), and a quantity proportional to the spectralreflectance of the retroreflector, R(λ). For nighttime colorimet-ric measurements of retroreflectors, S(λ) is Illuminant A. Thespectral tristimulus values, x¯, y¯, and z¯, the illuminant powerS(λ), and the reflectance quantity R(λ) are used together tocalculate three numbers, the tristimulus values X, Y, and Z asfollows:X 5 k *380740SA~λ! R~λ! x¯~λ!dλY 5 k *380740SA~λ! R~λ! y¯~λ!dλZ 5 k *380740SA~λ! R~λ! z¯~λ!dλwhere:SA(λ) = spectral power distribution of IlluminantA,R(λ) = spectral reflectance factor of the sample,andx¯(λ), y¯(λ), z¯(λ) = color matching functions of the CIE stan-dard observer.100/k 5 *380740SAy¯~λ!dλIntegration of each curve across the visible region (380 to740 nm) give the numerical value for the correspondingtristimulus value X, Y, or Z.6.2 Chromaticity Coordinates—The chromaticity coordi-nates 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, andZ cancels out in calculating x, y, and z. Thus, x, y, and z expressthe color of the reflected light without regard to its intensity.Because the sum of x, y, and z is always equal to one, only twoof these quantities are needed to describe the chromaticity of alight. The chromaticity coordinates x and y are chosen for thispurpose.6.3 CIE 1931 (x, y) Chromaticity Diagram—The chroma-ticity coordinates x and y can be plotted as shown in PracticeE308, Fig. 1. The outline in the figure encloses the entire rangeof combinations of x and y that correspond to real colors. Thepoints at which monochromatic radiation of various wave-lengths falls are indicated on this boundary, with the morenearly neutral colors being represented by points toward thecenter of the bounded region.6.4 Specifying Color Limits—A color point representing thex and y chromaticity coordinates of a test sample can be locatedon the CIE diagram. A specification for a specific retroreflec-tive color limit would require that the color point for a sampleof 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 chromaticitycoordinates in the specification.6.5 Daytime versus Nighttime Color Limits—Different colorlimits are required to specify daytime and nighttime color.Nighttime and daytime color limits are different for two majorreasons: the quality of the illuminating light and the geometryor direction of the illuminating light. Daytime color is viewedunder a source of daylight quality, and nighttime color isviewed under Source A (a CIE source corresponding to anincandescent lamp, such as an automobile headlamp). Illumi-nation in the daytime is from skylight, and diffusely reflectedlight is observed; illumination in the nighttime comes from apoint very near the observer, and retroreflected light is ob-served.7. Requirements to be Stated in Specifications7.1 When stating colorimetric retroreflective requirements,the following requirements shall be given in the specificationfor the material:7.1.1 Limits of the color area on the 1931 CIE chromaticitydiagram (usually four pairs of chromaticity coordinates (x andy) are required to define an area on the diagram).7.1.2 Chromaticity coordinate limits and spectral transmit-tance limits of the standard filter when Procedure A is used.(These may be specified by giving the filter glass type andthickness or the manufacturer’s part number of the filter.)7.1.3 Observation angle (α).7.1.4 Entrance angle (β) and when required the componentsof the entrance angle β1, and β2. (When specifying entranceangles near 0°, care must be taken to prevent “white” specularreflection from entering the receptor. Therefore, instead ofspecifying 0°, the entrance angle is usually specified so thatspecular light is reflected away from the receptor.)7.1.5 Rotation angle (ε) and the location of the datum mark,if random orientation of the test specimen is not suitable.7.1.6 Observation distance (d).7.1.7 Test specimen dimensions and shape.7.1.8 Receptor angular aperture, usually either 6 min or 10min of arc.7.1.9 Source angular aperture, usually either 6 min or 10min of arc.7.1.10 Reference center of the retroreflector.7.1.11 Refere