Microfacet Based BRDF Solar Cell Model Modification Using Experimental Data

Light curve analysis is often used to understand satellite activity in geosynchronous orbits which are too far away for resolved imagery from ground-based optical systems. Glinting features can provide identifiable information, but large sources of error have been documented. The bidirectional reflectance distribution function (BRDF) describes the spatial distribution of a material's reflectance by relating incident irradiance to scattered radiance, and BRDFs play an integral role in light curve observation, simulation, and interpretation. Microfacet models are a popular class of BRDF, which assume geometric optics and typically trade accuracy for computing speed in both scene generation and computer graphics. Another popular class of BRDF is comprised of physical optics models which account for wave optics effects, but their main drawbacks are complexity and computing power. For materials that exhibit wave optics effects, if a known wave optics solution can be combined with a mi-crofacet model, perhaps computing speed can be retained while improving accuracy of the model. In this work, new models are suggested as inputs into light curve analysis to reduce glinting error by accounting for distinctive solar cell diffraction features. Previous work extended out-of-plane measurements to identify and then model a clear diffraction effect from a solar cell, and this work conducts validation of the model using alternative laser sources and additional solar cell samples. While prior work focused on the specular region, in this work, the theoretical modification is analyzed using high-fidelity, low-density, out-of-plane measurements gained from a modified Complete Angle Scatter Instrument (CASI) capturing BRDF data at scattered angles not just on the specular direction. This data informs a model used to show out-of-plane behavior resulting in otherwise unexpected reflection patterns. The results suggest that for remote sensing scenarios involving materials with diffractive properties, such as solar cells, incorporating a fundamental known solution for wave optics phenomena with a microfacet model foundation can improve the radiometric accuracy as well as retain computing speed.