Round robin comparison of BRDF measurements

The scattering properties of materials such as coated and painted surfaces are important in the design of low observable materials. These properties are also important to enable accurate modeling of targets in a scene of different background materials. The distribution of light scatter from surfaces can be determined by measurements of the Bidirectional Reflectance Distribution Function (BRDF) using devices such as scatterometers. The BRDF should ideally be possible to measure both in and outside the plane-of-incidence in order to characterize both isotropic and anisotropic scatter and with suitably high angular resolving power and signal to noise ratio at the wavelengths of interest. Both narrow-band light sources (e.g. lasers) and broad-band light sources in combination with spectral band pass filters may be used in combination with appropriate detectors. This type of instrumentation may consist of complex mechanically moving parts and optics requiring careful alignment to the sample surface to be measured. To understand the synergies and discrepancies between the outputs of different BRDF instruments measuring the same sample set, we have compared BRDF measurement results between our research laboratories in a round robin comparison of an agreed set of sample surfaces and measurement geometries and wavelengths. In this paper, the results from this study will be presented and discussed.