ASTM_F_1252_-_10.pdf

Designation:F125210Standard Test Method forMeasuring Optical Reflectivity of Transparent Materials1This standard is issued under the fixed designation F1252;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 test method covers a procedure for measuring thereflectivity of transparent materials,hereafter known as speci-mens.The results are repeatable without specifying a particularbrand name of instrumentation.1.2 This test method applies to substantially flat parts.Errors in measurement can occur if the parts being measuredare not substantially flat.1.3 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.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.Terminology2.1 Definitions:2.1.1 angle of incidence(i)in the plane of the lightsource,specimen,and photometer,the angle of incidence is theangle between the incident light ray and the normal to thesurface(see Fig.1).2.1.2 angle of reflection(r)in the plane of the lightsource,specimen,and photometer,the angle of reflection is theangle between the reflected light ray and the normal to thesurface(see Fig.1).2.1.3 light sourceunless otherwise specified,the NationalInstitute of Standards and Technology(NIST)diffused nonpo-larized Standard Illuminance A or C light source shall be used.The light source size will be such that there will be sufficientoverlap of the front and rear images on the specimen to overfillthe measurement field size of the photometer.This overlap isillustrated in Fig.2.(As angle of incidence and specimenthickness increase,the two images will diverge.)The lightsource used should be specified and reported as part of the testresults.2.1.4 measurement field sizethe angular extent,in degrees,of the measurement aperture of the photometer.2.1.5 photometerany commercial photometer or photopicfiltered radiometer with a suitable measurement field size(1or smaller is recommended).A model with a viewfinder isrecommended.2.1.6 pivot pointthe point in space at which the incidentlight ray and reflected light ray are to intersect(see Fig.1).2.1.7 reflectivitythe reflectivity of a transparent specimenis defined as the ratio of the luminance of the reflected imageof a light source to the luminance of the light source.Thereflectivity will depend upon several factors:the angle at whichthe reflected light is measured,the thickness,surface quality,and type of material of the specimen,whether the specimen iscoated,the spectral distribution of the light source,and thespectral sensitivity of the measurement device.The reflectivity,as defined here,includes the small amount of scattered lightthat contributes to the luminance of the reflected image.3.Summary of Test Method3.1 The luminance of the standard source is determined bymeasuring it directly with the photometer.The luminance ofthe reflection of the source is then measured off the specimenat a specified geometry.The luminance of the reflection isdivided by the luminance of the source to obtain the reflectivityof the specimen.4.Significance and Use4.1 Reflections from aircraft transparencies of instrumentlights and other cockpit objects have been a concern to manypilots.Attempts to reduce these reflections have been ham-pered by the lack of a repeatable measurement method andvariances in reflection measuring instrumentation.The prob-lem with measuring instrumentation is that different brandswill often give significant value differences using the samespecimen surface.4.2 This test method reduces the instrument variations bystandardizing the light source,calculation method,and area ofspecimen surface being measured;a brand of instrumentationis not specified.Since the reflectivity is defined as the ratio of1This test method is under the jurisdiction of ASTM Committee F07 onAerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 onTransparent Enclosures and Materials.Current edition approved Dec.1,2010.Published January 2011.Originallyapproved in 1989.Last previous edition approved in 2008 as F1252 08.DOI:10.1520/F1252-10.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 two luminance measurements and does not depend on anabsolute measurement,dependence upon the accuracy of themeasuring instrument is reduced.4.3 The test method may be used to objectively compare thereflection characteristics of various transparent materials.Furthermore,the test method may be used to evaluate reflec-tions of