ASTM F1252-21

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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: F1252 − 16 F1252 − 21
Standard Test Method for
Measuring Optical Reflectivity of Transparent Materials
This standard is issued under the fixed designation F1252; 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 test method covers a procedure for measuring the reflectivity of transparent materials, hereafter known as specimens. The
results are repeatable without specifying a particular brand name of instrumentation.
1.2 This test method applies to substantially flat parts. Errors in measurement can occur if the parts being measured are not
substantially flat.
1.3 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 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. Terminology
2.1 Definitions:
2.1.1 angle of incidence (Θ ), n—in the plane of the light source, specimen, and photometer, the angle of incidence is the angle
i
between the incident light ray and the normal to the surface (see Fig. 1).
2.1.2 angle of reflection (Θ ), n—in the plane of the light source, specimen, and photometer, the angle of reflection is the angle
r
between the reflected light ray and the normal to the surface (see Fig. 1).
2.1.3 light source, n—unless otherwise specified, the National Institute of Standards and Technology (NIST) diffused nonpolarized
Standard Illuminance A or C light source shall be used. The light source size shall be such that there shall be sufficient overlap
of the front and rear images on the specimen to overfill the measurement field size of the photometer. This measurement field size,
and front and back reflected image overlap, are illustrated in Fig. 2. (As angle of incidence and specimen thickness increase, the
two images will diverge.) The light source used shall be specified and reported as part of the test results.
2.1.4 measurement field size, n—the angular extent, in degrees or arc minutes, of the measurement aperture of the photometer.
This test method is under the jurisdiction of ASTM Committee F07 on Aerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 on Transparent
Enclosures and Materials.
Current edition approved April 1, 2016May 1, 2021. Published April 2016May 2021. Originally approved in 1989. Last previous edition approved in 20102016 as
F1252 – 10.F1252 – 16. DOI: 10.1520/F1252-16.10.1520/F1252-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1252 − 21
FIG. 1 Apparatus Set-Up
FIG. 2 Photometer Measurement Field Size (Aperture) Compared to Specimen Front and Back Surface Reflections
2.1.5 photometer, n—any commercial photometer or photopic filtered radiometer with a suitable measurement field size (1° or
smaller is recommended). A model with a viewfinder is recommended.
2.1.6 pivot point, n—the point in space at which the incident light ray and reflected light ray are to intersect (see Fig. 1).
2.1.7 reflectivity, adj—the reflectivity of a transparent specimen is defined as the ratio of the luminance of the reflected image of
a light source to the luminance of the light source. The reflectivity will depend upon several factors: the angle at which the reflected
light is measured, the thickness, surface quality, and type of material of the specimen, whether the specimen is coated, the spectral
distribution of the light source, and the spectral sensitivity of the measurement device. The reflectivity, as defined here, includes
the small amount of scattered light that contributes to the luminance of the reflected image.
3. Summary of Test Method
3.1 The luminance of the standard source is determined by measuring it directly with the photometer. The luminance of the
reflection of the source, from both the front and back surfaces of the specimen, is then measured off the specimen at a specified
geometry. The luminance of the reflection is divided by the luminance of the source to obtain the reflectivity of the specimen.
4. Significance and Use
4.1 Reflections from aircraft transparencies of instrument lights and other cockpit objects have been a concern to many pilots.
Attempts to reduce these reflections have been hampered by the lack of a repeatable measurement method and variances in
reflection measuring instrumentation.
4.2 This test method reduces the instrument variations by standardizing the light source, calculation method, and area of specimen
surface being measured; a brand of instrumentation is not specified. Since the reflectivity is defined as the ratio of two luminance
measurements and does not depend on an absolute measurement, dependence upon the accuracy of the calibration of the measuring
instrument is reduced.
F1252 − 21
4.3 The test method may be used to objectively compare the reflection characteristics of various transparent materials.
Furthermore, the test method may be used to evaluate reflections of a specified spectral distribution light source (for example, a
monochromatic light-emitting diode) by using that source in place of the standard light source.
4.4 Provisions are made to check for polarization effects of the sample and to record the reflectivity of a standard specimen. These
provisions are offered as an option to the tester; it is up to the user or the requiring agency to determine the significance and use
of these data.
4.5 Since the reflections are measured photopically, the results are representative of what the pilot would visually perceive.
5. Apparatus and Setup
5.1 The apparatus shall be set up as shown in Fig. 1.
5.2 The angle of incidence Θ shall be determined by the user or requiring agency. Since Θ = Θ , the total angle of reflection
i i r
Θ = 2Θ = 2Θ . Θ and Θ shall be accurate to within 60.5°, hence Θ shall be accurate to within 61°.
i r i r
5.3 The distance from the light source to the specimen and from the specimen to the photometer is not critical. However, it is
desirable to position the light source relatively far from the sample (for example, 50 cm or more) to minimize the effects of
scattered light from the specimen contaminating the reflectivity measurement. The light source to specimen distance must be such
that the reflected image viewed through the photometer is sufficiently large to overfill the photometer measurement field (see 2.1.3
and Fig. 2). The distance from the specimen to the photometer must be short enough to ensure the reflected images overfill the
measurement aperture but long enough to ensure the photometer can focus on the image.
5.4 The testing shall be done in a room with controlled lighting such that the photometer reading with the reference light off is
less than 0.1 % of the reflection reading measured with the reference light on. This will ensure ambient room light contamination
of the results is less than 0.1 %.
5.5 A flat black surface (such as black velvet) may be positioned behind (but not touching) the specimen during measurement to
reduce possible ambient light contamination effects.
5.6 The photometer measurement aperture size (for example, 1°), the reference light source emitting surface size (for example 5
cm circular), the distance from the reference light source to the specimen, the distance from the photometer to the specimen and
the
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