A large variation in bidirectional reflectance distribution function (BRDF) for certain GaAs surfaces is proposed as the basis for a variable BRDF reference material. Such a material would provide a range of reference values as opposed to the discrete values of diffuse reference materials. The effect is reasonably stable and uniform, and is most likely the result of a weak grating effect caused by surface striations.
Jeff L. Brown, Jeff L. Brown,
"Variable BRDF reference material", Proc. SPIE 2541, Optical Scattering in the Optics, Semiconductor, and Computer Disk Industries, (1 September 1995); doi: 10.1117/12.218332; https://doi.org/10.1117/12.218332