We have measured angle- and polarization-resolved, hemispherical bidirectional reflectance distribution function (BRDF) for two types of graphite used in the fabrication of high temperature fixed point (HTFP) blackbody cavities. Measurements were made at room temperature using 405 nm and 658 nm laser sources, and the samples were illuminated at angles of incidence varying from normal incidence to 70°. All of the samples exhibited non-Lambertian scattering, including enhanced forward scatter at high incident angles, which was dominated by strong forward scattering of s-polarized incident light. The directional-hemispherical reflectance (DHR) for unpolarized incident light, obtained from integrating the BRDF measured at individual points in a hemisphere over the sample, ranged from 0.08 to 0.1, depending upon sample and incident angle. The potential impact of these measurements on emissivity models for graphite blackbody radiators and radiance temperature scale dissemination is discussed.
Pub Type: Journals
blackbody, brdf, emissivity, graphite, optical scattering, radiation thermometry, reflectance