BSDF measurements and analyses of specular samples using a table-top goniometer in support of remote sensing instruments

Bidirectional Scattering Distribution Function (BSDF) measurements of selected specular samples were made using the Table-Top Goniometer (TTG) in the Diffuser Calibration Lab (DCL) at NASA GSFC in the support of NASA remote sensing instruments and programs. The same TTG system has also been used in the BRDF measurements for diffuse samples. The tunable laser-based TTG possesses the advantages of small incident beam profile and configuration flexibility and is able to meet various BSDF test requirements on specular samples with flat and curved surfaces. It also has a useful capability in characterizing instrument straylight due to surface roughness and in determining the scattering light distribution function of optical surfaces. The BSDF measurements on specular samples can be performed over 8 orders of linear dynamic range with correction of instrument signatures. In this paper, we present BSDF results on two types of specular samples: a witness flat fold mirror for the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) project and four Multi-Layer Insulation (MLI) samples for the Restore project at NASA GSFC. BRDF measurements in the viewing angle range of ± 90° were acquired at 500 nm, 700 nm, and 2000 nm and at incident angles of 0°, 8°, and 25° for the PACE sample, and at 500 nm, 633 nm, 700 nm, 900 nm, 1000 nm, 1550 nm, and 1800 nm at incident angles of 10° and 25° for the MLI samples. For both types of samples, the ABg model was applied to fit the BSDF data to generate the parameters for optical modeling. The ABg model is able to fit the BSDF data on the polished surface of the flat mirror very well. However, two scattering components were seen in the MLI BSDF fitting results attributed to wrinkle and surface morphology issues. Total Hemispherical Reflectance (THR) and Total Integrated Scatter (TIS) measurements were also made on the samples and were compared to the BSDF results. The details of the BSDF measurement setup and the methodology for realization of the BRDF scale for the specular samples are also described.