Toward SI Traceability of a Monte Carlo Radiative Transfer Model in the Visible Range

A 3-D Monte Carlo (MC) ray-tracing radiative transfer model is tested for its ability to simulate the bidirectional reflectance factors (BRFs) of a grooved artificial target given SI-traceable measurements of the optical and topographic properties of the target’s surface. The optical properties of a grooved target and an identical flat target were measured with the goniospectrophotometer at the National Metrology Institute of U.K. (NPL) and are traceable to the NPL scales of radiance factor. The topographic measurements were performed with the coordinate measuring machine at the National Metrology Institute of Finland (MIKES), and are traceable to the realization of the meter. The BRFs of the flat target were used to parameterize analytical scattering functions for rough surfaces. Similarly, the topographic measurement results were used to construct a structural model of the grooved target. Each element within this structural model then had its optical properties defined by the parameterized scattering function before the 3-D MC model simulated the BRFs of the grooved target under well-defined illumination and viewing conditions. The measured and modeled BRFs agreed for 72% of the measured geometries in the plane of incidence within the measurement and modeling uncertainties. The relative root-mean-squared (RMSE) error was 0.19. In the plane orthogonal to the plane of incidence, the measured and modeled BRFs agreed for 45% of the measured geometries, and the relative RMSE between measured and modeled values was 0.65.