ASTM E809-21

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: E809 − 08 (Reapproved 2013) E809 − 21
Standard Practice for
Measuring Photometric Characteristics of Retroreflectors
This standard is issued under the fixed designation E809; 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 general procedures for instrumental measurement of the photometric characteristics of
retroreflective materials and retroreflective devices.
1.2 This practice is a comprehensive guide to the photometry of retroreflectors but does not include geometric terms that are
described in Practice E808.
1.3 This practice describes the parameters that are required when stating photometric measurements in specific tests and
specifications for retroreflectors.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this The values
given in parentheses after SI units are provided for information only and are not considered standard.
1.5 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E284 Terminology of Appearance
E308 Practice for Computing the Colors of Objects by Using the CIE System
E808 Practice for Describing Retroreflection
2.2 CIE Documents:
CIE Publication No. 54.2 Retroreflection—Definition and Measurement
CIE Publication DS 17.2/E:2009 International Lighting Vocabulary
CIE Publication No. 69-1987 Methods of Characterizing Illuminance Meters and Luminance Meters
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 Jan. 1, 2013Jan. 1, 2021. Published January 2013February 2021. Originally approved in 1981. Last previous edition approved in 20082013 as
E809 – 08.E809 – 08 (2013). DOI: 10.1520/E0809-08R13.10.1520/E0809-21.
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 the CIE Webshop at http://www.cie.co.at.U.S. National Committee of the CIE (International Commission on Illumination) (http://www.cie-usnc.org) or
the CIE (cie.co.at) Webshop.
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E809 − 21
3. Terminology
3.1 Terms and definitions in Terminology E284 and E808 are applicable to this practice. In general, the terminology in this practice
agrees with that in CIE Publications DS 17.2/E:2009 and 54.2.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 annular aperture, n—the difference between the angular diameters of the external boundary circle and the internal boundary
circle.
3.2.2 circular aperture, n—the angular diameter of a circular aperture surface.
3.2.3 goniometer, n—an instrument for measuring or setting angles.
3.2.4 photopic receiver, n—a receiver of radiation with a spectral responsivity which conforms to the V (λ) distribution of the CIE
Photopic Standard Observer that is specified in Practice E308.
3.2.5 receiver aperture, n—angular dimensions from the retroreflector center to the entrance aperture or pupil of the receiver.
3.2.6 rectangular aperture, n—the angular height and width of a rectangular aperture surface.
3.2.6.1 Discussion—
The orientation of the sides of the rectangular aperture surface should be supplied together with the angular height and width.
3.2.7 reflected illuminance, E , n—illuminance at the receiver measured on a plane perpendicular to the observation axis.
r
3.2.7.1 Discussion—
This quantity is used in the calculation of the coefficient of luminous intensity,
R : R = (I/E ) = (E d )/E , where d is the distance from the retroreflector to the receptor.
I I ' r '
3.2.8 retroreflectometer aperture angles, n—the maximum angular diameter of the pencil of light (see Fig. 1).
3.2.8.1 Discussion—
In practice the illumination arrives at the retroreflector center within a narrow pencil of light surrounding the illumination axis and
the light reflected to the photoreceptor is contained within another narrow pencil. The distribution of light within such pencils is
the “aperture” function and the maximum angular diameter of the pencil is the “aperture angle.” It is generally assumed that the
aperture functions are rotationally symmetrical and even uniform, but this is often false, especially for illumination.
3.2.9 retroreflector aperture surface, n—the aperture surface of a retroreflector is given by the retroreflector itself, or by a
diaphragm enclosing part of the retroreflector.
3.2.10 retroreflector element aperture, n—angular dimension of the aperture surface of a retroreflective element as seen from the
receiver’s center.
3.2.10.1 Discussion—
The element aperture quantifies an error source in the setting of the observation angle. This is a critical feature for testing large
retroreflective elements or at short distances. When using collimated optics, placing the source and receiver at virtual infinity, the
retroreflector element aperture is virtually zero.
FIG. 1 Illustration of Apertures used in Retroreflection Measurement
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3.2.11 retroreflector (or specimen) aperture, n—angular dimensions from the source point of reference to the aperture surface of
the retroreflector (or specimen).
3.2.11.1 Discussion—
As the source and receiver are generally close to each other, distinction is not made between aperture angles seen from the source
and receiver. When using collimated optics where the source and receiver are at virtual infinity, the retroreflector aperture is
virtually naught. The retroreflector aperture describes the maximum variation of the entrance angle of the aperture surface of the
retroreflector.
3.2.12 source aperture, n—angular dimensions from the retroreflector center to the exit aperture stop or pupil of the light source.
4. Summary of Practice
4.1 The fundamental procedure described in this practice involves measurements of retroreflection based on the ratio of the
retroreflected illuminance at the observation position to the incident illuminance measured perpendicular to the illumination axis
at the retroreflector. From these measurements, along with the geometry of test, various photometric quantities applicable to
retroreflectors can be determined.
4.2 Also described are methods of comparative testing where unknown specimens are measured relative to an agreed-upon
standard retroreflector (a substitution test method).
5. Significance and Use
5.1 This practice describes procedures used to measure photometric quantities that relate to the visual perception of retroreflected
light. The most significant usage is in the relation to the nighttime vehicle headlamp, retroreflector, and driver’s eye geometry. For
this reason the CIE Standard Source A is used to represent a tungsten vehicle headlamp and the receptor has the photopic, V (λ),
spectral responsivity corresponding to the light adapted human eye. Although the geometry must be specified by the user, it will,
in general, correspond to the relation between the vehicle headlamp, the retroreflector, and the vehicle driver’s eye position.
6. Uses and Applications
6.1 Coeffıcient of Retroreflection—This quantity is used to specify the performance of retroreflective sheeting. It considers the
retroreflector as an apparent point source whose retroreflected luminous intensity is dependent on the area of the retroreflective
surface involved. It is a useful engineering quantity for determining the photometric performance of such retroreflective surfaces
as highway delineators or warning devices. The coefficient of retroreflection may also be used to determine the minimum area of
retroreflective sheeting necessary for a desired level of photometric performance.
6.2 Coeffıcient of Luminous Intensity—This term is used to specify the performance of retroreflective devices. It considers the
retroreflected luminous intensity as a function of the perpendicular illuminance incident on the device. It is recommended for use
in describing performance of RPMs, taillight reflex reflectors and roadway delineators.
6.3 Coeffıcient of Line Retroreflection (of a Reflecting Stripe)—This term may be used to describe the retroreflective performance
of long narrow strips of retroreflective materials, when the actual width is not as important as is the reflectivity per unit length.
6.4 Reflectance Factor (of a Plane Reflecting Surface)—This is a useful term for comparing surfaces specifically designed for
retroreflection to surfaces which are generally considered to be diffuse reflectors. Since almost all natural surfaces tend to
retroreflect slightly, materials such as BaSO can have a reflectance factor much higher than one (as much as four) at small
observation angles. Such diffuse reflectance standards should be used for calibration only at large observation angles, for example,
45°.
6.5 Coeffıcient of Retroreflected Luminance (also called Specific Luminance)—This term considers the retroreflector as a surface
source whose projected area is visible as an area at the observation position. The coefficient of retroreflected luminance relates to
the way the effective retroreflective surface is focused on the retina of the human eye and to the visual effect thereby produced.
It is recommended for describing the performance of highway signs and striping or large vehicular markings which are commonly
viewed as discernible surface areas.
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6.6 Coeffıcient of Luminous Flux per Unit Solid Angle, R —This measurement is used to evaluate retroreflectors on the basis of
Φ
flux ratios. It is numerically very nearly equal to the coefficient of retroreflected luminance at small entrance angles. It is
recommended for use in the design of retroreflectors but not for specification purposes.
7. Requirements When Measuring Retroreflectors
7.1 When describing photometric measurements of retroreflectors, items in paragraphs 7.1.1 – 7.1.11 must be included. Refer to
Fig. 2 for a diagram of measurement geometry terminology.
7.1.1 Retroreflective photometric quantity, such as: coefficient of luminous intensity (R ), coefficient of retroreflected luminance
I
(R ) (also called specific luminance), coefficient of retroreflection (R ), coefficient of line retroreflection (R ), reflectance factor
L A M
(R ), or coefficient of luminous flux per unit solid angle (R ).
F Φ
7.1.1.1 In specifications, a minimum acceptable quantitative value is usually established.
−1 −2
7.1.2 Units in which each quantity is to be measured (for example cd·lx ·m ).
7.1.3 Observation angle.
7.1.4 Components of the entrance angle, (β and β ).
1 2
7.1.4.1 When both β and β are near zero, care must be taken to prevent specular reflection from entering the photoreceptor.
1 2
−1
7.1.4.2 Entrance angle β equals cos (cosβ cosβ ).
1 2
7.1.5 Rotation angle and the datum mark position shall be specified if random rotational orientation of the test specimen is not
suitable.
7.1.6 Test distance or minimum test distance.
7.1.7 Test specimen size and shape.
7.1.8 Photoreceptor angular aperture.
7.1.9 Source angular aperture.
7.1.10 Retroreflector center.
FIG. 2 View of Test Geometer for Measuring Retroreflection
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7.1.11 Retroreflector axis. The retroreflector axis is usually perpendicular to the surface of retroreflective sheeting. In such
complex devices as automobile or bicycle reflectors, the retroreflector axis and retroreflector center may be defined with respect
to the illumination direction.
8. Apparatus
8.1 General—The apparatus shall consist of a photoreceptor, a light projector source, a specimen goniometer, an observer
goniometer, (sometimes known as the observation angle positioner), and a photometric range.
8.1.1 Aperture angles are a very important consideration when measuring retroreflectors as Fig. 1 illustrates. See Table 1 for
recommendations for maximum angular aperture of optical elements. See 9.1 on selection of angular apertures.
8.2 Photoreceptor—The photoreceptor shall be equipped as follows:
8.2.1 Photopic Filter—The photoreceptor shall be equipped with a light filter such that the spectral responsivity of the receptor
should match the V(λ) response of the CIE Standard photopic observer with an f ' tolerance no greater than 3 %. Spectral correction
filters to the V(λ) function may be used provided that they are determined on material which has been previously measured by
spectroradiometric means and closely corresponds in their spectral coefficient of retroreflection to the specimen under test. See
Annex A1 for uncertainty tests and compensation.
8.2.2 Photoreceptor Stability and Linearity—The stability and linearity of the photometric scale reading must be within 1 % over
the range of values to be measured (see Annex A2). The responsivity and range of the photoreceptor should be sufficient such that
readings of the projector light source and the retroreflector under test will have a resolution of at least 1 part in 50.
8.2.3 Photoreceptor Angular Aperture—The photoreceptor must be equipped with a means to limit the angular collection of
retroreflective luminous flux. This may be accomplished with an objective lens and field aperture or with light baffling. The field
of view shall be limited such that the effect of stray light is negligible. The field of view should be limited to the smallest aperture
that includes the entire test specimen or the illuminated area when testing horizontal coating materials. When an objective lens is
used, it shall be capable of focusing at the test distance. Angular apertures for the photoreceptor are specified in degrees subtended
at the specimen. The responsivity across the aperture shall be uniform.
8.3 Light Projector Source—The light source shall be a projector type capable of uniformly illuminating the specimen with
appropriate reflector and lenses to provide illumination on the test sample with a spectral power distribution conforming to the
1931 CIE Standard Illuminant Source A (a tungsten filament lamp operated at a correlated color temperature of 2856°K2856 K 6
20K,20 K, see Practice E308). The normal illuminance on the sample shall be uniform within 5 % of the average normal
illuminance over the area of the retroreflector at the test distance. The light projector shall be equipped with an adjustable iris
diaphragm or a selection of fixed apertures. The intensity of light shall be regulated and shall not vary more than 1 % for the
duration of the test.
8.3.1 The current of the projection lamp must be adjusted to provide a correlated color temperature of 2856°K.2856 K. An
adjustment procedure is described in Annex A3. Such adjustment often requires lowering the power from the nominal value since
many projector lamps are designed to operate at correlated color temperatures greater than 2856°K.2856 K.
8.3.2 The size and shape of the projector exit aperture and the angle this aperture subtends at the test specimen must be specified.
The radiance across the aperture shall be uniform.
8.4 Specimen Goniometer (Test Specimen Holder)—This goniometer shall be capable of movements in three axes and sufficiently
large to support the test specimen in the prescribed geometric arrangement. The motions of the axis shall be in accordance with
A
TABLE 1 Optical Element Angular Apertures
Standard apertures 0.05° 0.−1° 0.167° 0.333°
Angular aperture of an individual 0.01° 0.02° 0.04° 0.08°
retroreflective element, ° max max max max
A
Optical element angular aperture maximum requirements apply to all non-
collimating instruments.
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Practice E808. For most materials, the tolerance of setting the angles β and β should be less than 0.1°. The rotation angle ε
1 2
tolerance should be less than 60.2°. The setting tolerance refers to the goniometer mechanism alone. The goniometer must be set
in accordance with 11.1.4.
8.5 Observer Goniometer—This goniometer is used to accurately set the separation of the projector (light source) and
photoreceptor. This setting determines the observation angle. This is sometimes referred to as an observation angle positioner
(OAP). The positioning tolerance of the photoreceptor with respect to the light source should be held to 1 % of the angular aperture
of the photoreceptor. For example, at 10m, a standard aperture of 0.1° would be equal to 60.001° or 0.17 mm separation.
8.6 Photometric Range—The photometric range provides the dark work area for testing retroreflectors. To minimize the effect of
stray light, the background behind the test specimen shall be flat black. Light baffles shall be located, as necessary, between the
projector and the test specimen. Goniometer parts, exposed range walls, ceiling, and floor not baffled and exposed to the light beam
shall be painted flat black.
9. Selection of Photometric Range Parameters
9.1 Selection of Angular Apertures:
9.1.1 Standard Circular Apertures—The following uniform circular apertures are considered standard.
9.1.1.1 0.05° (3 arc min) for both light source and photoreceptor.
9.1.1.2 0.1° (6 arc min) for both light source and photoreceptor.
9.1.1.3 0.167° (10 arc min) for both light source and photoreceptor.
9.1.1.4 0.333° (20 arc min) for both light source and photoreceptor.
9.1.1.5 For all standard circular apertures, the tolerances are 68 %.
9.1.2 Discussion—With standard circular aperture, the defined observation angle is based on the center to center separation of the
apertures.
9.1.3 Commonly used standard circular apertures are:
9.1.3.1 0.05° (3 arc min) for observation angles of 0.1°.
9.1.3.2 0.1° (6 arc min) for observation angles from 0.2° to 0.5°.
9.1.3.3 0.167° (10 arc min) for 0.33° spectral measurements.
9.1.3.4 0.333° (20 arc min) for 1.0° observation angles and larger.
9.1.4 In theory, retroreflection is defined with apertures that are infinitely small. Measurements using the standard angular
apertures in 9.1.1 will not always be equal to measurements using much smaller apertures. The standard apertures give sufficient
sensitivity for practical measurement and ensure reproducibility between laboratories providing the same standard aperture pairs
are used.
9.2 Selection of Observation Distance—The observation distance and illumination distance must be specified in testing
retroreflectors. They are limited by angular aperture requirements, the requirement to test a minimum sample area, for example
0.01 m in the case of retroreflective sheeting or the desire to test an entire retroreflector at once. The observation distance and
the illumination distance should not differ by more than 20 mm (for a 15 meter illumination distance) so as to not introduce errors
in the observation angle over the test specimen. The tolerance on the setting of the observation and illumination distances should
be 60.05 %.
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10. Test Specimen
10.1 The test specimen shall consist of one entire retroreflector. A large retroreflector may be tested by summing the values
obtained from segments of the device.
10.2 When testing retroreflective sheeting, it is recommended that the test area be between 0.01 and 0.1 m . This may be
accomplished, for example, by selecting a single square test specimen 0.2 m on each side or by averaging the measurements over
several representative pieces totaling between 0.01 and 0.1 m in area.
11. Calibration
11.1 The following components required in this practice must be calibrated prior to use.
11.1.1 Projector Source—The source must be calibrated to a correlated color temperature of 2856°K2856 K 6 20K20 K and
closely duplicate the spectral power distribution of CIE Standard Illuminant Source A. A method of calibration is described in
Annex A3 based on tristimulus colorimetry. Spectroradiometric methods of calibration are also suitable.
11.1.2 Photoreceptor Spectral Responsivity—The photoreceptor spectral responsivity must be verified in terms of the spectral
power distributions measured in this practice. A procedure for verification of spectral responsivity is described in Annex A1. Errors
in the photopic fit of the receptor are direct systematic errors in the test result. Determination of the error f ' should be followed
from CIE Publication 69. The f ' should be no greater than 3 %.
11.1.3 Photoreceptor Linearity—The procedures in this practice require the measurement of both incident and reflected light levels
which may be several orders of magnitude different in value. To ensure accuracy, the photoreceptor and readout system must be
linear or appropriate corrections for nonlinearity must be applied. Annex A2 describes a method for verification of photoreceptor
linearity.
11.1.4 Goniometer Calibration—The goniometer shall be calibrated at the 0° entrance angle position. All measurements shall be
made relative to this point and shall be checked each time the goniometer or light projector is moved. If measurements are to be
made at extreme angles of 75° to near 90°, it is recommended that the goniometer be calibrated in the same 75° to 90° range of
entrance angle for greatest accuracy.
11.1.4.1 Calibration of the goniometer at the 0° entrance angle position may be accomplished by several means. One example is
by substituting an approximately 200 mm (8 in.) square high quality plane mirror in place of the sample. A 200 mm cross, centered
on the surface of the mirror can be made with photographic black tape. A 400 mm square piece of white construction paper, with
a small (5 mm) hole in the center, can be centered over the light projector exit aperture. By observing the white paper, the
goniometer can be adjusted so that the shadow of the cross is reflected directly on the exit aperture of the projector. This position
of the goniometer is the 0° entrance angle.
12. Test Procedure
12.1 The geometry used to determine the photometric performance of retroreflectors shall be in accordance with Practice E808.
There are several methods that can be used in determining this performance. These are the ratio method, the substitution method,
the direct luminous intensity method, and the direct luminance method.
12.2 The Ratio Method—In this method, use the same instrument with the same apertures and field of acceptance to measure the
reflected illuminance (E ) and the normal illuminance (E ). Therefore, the photoreceptor need not be calibrated, and the
r '
uncalibrated meter readings of E and E are referred to as m and m , respectively. Do not use different instruments to measure
r ' 1 2
E and E .
r '
12.3 Procedure A—Ratio Method.
12.3.1 General—Select the smallest available field aperture large enough to include both the entire retroreflector as seen from the
photoreceptor, and the source as viewed from the retroreflector, for measurement of M and m . Measure the normal illuminance
1 2
at the face of the sample by substituting the photoreceptor for the sample. Place the photoreceptor entrance aperture where the test
specimen is mounted and record m . (Alternatively the light source may be substituted for the test specimen at the test distance
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and the incident normal illuminance can then be measured without moving the photoreceptor.) Then, return the photoreceptor and
the test specimen to their original positions, and record m in the same units as m .
1 2
12.3.2 Measure the amount of stray light by replacing the test specimen with a black surface of the same shape and area at angles
such that the gloss does not affect the reading. A high gloss black surface is preferred. In some cases a flat black with reflectance
less than 4 % could be used. Subtract the stray light readings, m from the reading m . The value m ' in the following equations
0 1 1
is the value of m less the stray light reading m .
1 b
12.3.3 Unless the photoreceptor has a repeatability of 60.3 % between power-on cycles, it is recommended that the photoreceptor
remain energized between measurement of m and m '.
2 1
12.3.4 If the photoreceptor is deficient in its correction to the CIE photopic standard observer, a color correction factor must be
applied (see Annex A1). This correction factor K is applied by means of a filter having a spectral transmittance proportional to the
spectral retroreflectance of the test specimen.
12.3.4.1 Warning—If close spectral matches in permanent filters are not available, it is recommended that the correction factor
not be used. If the correction fact
...