µBRDF measurements over a tiny surface area using a goniospectrophotometer

The increasing use of a spatially varying bidirectional reflectance distribution function (BRDF) to describe the appearance of an object raises the important question of how BRDF values change when measured at a smaller scale. For this reason, we present a new goniospectrophotometer with an ability to measure the BRDF at the micron scale (µBRDF). The instrument produces BRDF measurements with a measurement surface diameter of 50 μm using any angular configuration on the hemisphere. This device is designed to aid in the extension of the BRDF metrological scale from centimeter to micrometer size. We support the credibility of our µBRDF measurements using a special made test sample with uniform diffuse white dots on a uniform black background. This is a sample that we can easily model with a few unknown parameters. The ability of our measurements to fit the model indicates the credibility of the measurement technique. True validation of small-scale measurements is a challenge because, currently, no similar measurement exists and so no direct comparison is possible. Comparison with larger existing beams is also problematic. We discuss this issue and propose a future effort on validation of µBRDF measurements between metrology institutes.