Bidirectional Transmittance Distribution Function Of Several Infrared Materials At 3.39 Microns

Abstract

Bidirectional transmittance distribution function (BTDF) measurements were made on several infrared transmitting materials, including zinc selenide (ZnSe), fused quartz (Si02), mono and polycrystalline silicon (Si), calcium fluoride (CaF2) and spinel (MgA1204). The measurement samples were one inch in diameter, and two or three different sample thicknesses were measured for each material. BTDF measurements were made with a helium neon laser operating at 3.39 microns, at a 10° angle of incidence. Significant variations in BTDF were found among samples that were expected to have the same BTDF levels. A correlation between surface characteristics (such as surface roughness, figure and irregularity) and BTDF was found, explaining many of the BTDF variations between samples of the same thickness and material. The effects of bulk scatter on BTDF were examined through BTDF measurements on samples of different thicknesses. The measurement system is described, and data in the form of BTDF vs scatter angle and BTDF vs sample thickness are presented and compared for all five materials.