BRDF measurements of graphite used in high-temperature fixed point blackbody radiators: a multi-angle study at 405 nm and 658 nm

We have measured the polarization-resolved bidirectional reflectance distribution function (BRDF) for two types of graphite used in the fabrication of high-temperature fixed point blackbody cavities in and out of the plane of incidence. 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 degrees. All of the samples exhibited non-Lambertian behaviour, including enhanced forward scatter at high incident angles, especially for s-polarized incident light. The directional-hemispherical reflectance for unpolarized incident light, obtained by integrating the BRDF measured at individual points in a hemisphere over the sample, ranged from 0.083 to 0.101, 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.