Augmenting CASI® BRDF measurement device to measure out-of-plane scatter with CCD pixel array

Abstract

For a given material, the bi-directional reflectance distribution function (BRDF) spatially describes how much light from any given incident direction reflects into each possible scattered direction. One common simplification in both BRDF measurement and modeling is to assume that material reflectance is isotropic throughout the scattered hemisphere with respect to the azimuthal direction. However, in reality, many materials with directionally-dependent surface characteristics, such as milled metals, are likely to exhibit anisotropic BRDFs, particularly noticeable near specular peaks. Scatter measurement devices similar to the modified Complete Angle Scatter Instrument® (CASI®) operated at the Air Force Institute of Technology (AFIT) are capable of direct specular measurements with high spatial resolution, but constrained within the plane of incidence. Anisotropic measurement techniques often sacrifice spatial resolution, particularly near specular peaks. In this work, AFIT's CASI® is augmented by installing a scientfic-grade monochrome charge-coupled device (CCD) camera on the detector arm, whose pixel array captures both in-plane and out-of-plane specular scatter measurements with high spatial resolution. Camera mounting and alignment processes are presented, including required beam attenuation for the visible red helium neon laser source used. The beam signature is measured and characterized, and the camera's effective dynamic range is extended using various exposure times. Beam signature is converted from raw digital counts to BRDF values, providing the baseline for an idealized perfectly specular material. Ultimately, this work is expected to lead towards improvements in measuring and modeling BRDFs for materials exhibiting anisotropic or out-of-plane reflectance properties for a range of radiometric remote sensing applications.