The effect of speckle on BRDF measurements

摘要

Many satellite instruments operating in the reflective solar wavelength region between 400 nm and 2500 nm require accurate and precise determination of the Bidirectional Reflectance Distribution Function (BRDF) of on-board diffusers used in their pre-flight and on-orbit calibrations. In this paper we study the characteristics and effects of speckles in the measurement of the BRDF of Spectralon diffusers. Two types of light sources were used in this study: a monochromator based broadband source and a coherent laser source at 632.8 nm. The Spectralon diffuser was measured over a range of incident and scatter angles. The speckle effect is known to be a significant issue in the measurement of Spectralon BRDF using laser sources. In this study, three different speckle pattern minimization techniques are examined. These include moving the Spectralon sample, expanding the incident spot size, and depolarizing the incident laser light. The results were compared with measurements using the incoherent monochromator-based source and the degree to which speckle was reduced is described. Speckle effects are found to be easily minimized using these simple techniques. The experimental data were obtained using the out-of-plane scatterometer located in NASA's Goddard Space Flight Center's Diffuse Calibration Facility (DCaF). The typical measurement uncertainty of reported BRDF measurements is 0.7% (k=1).