We have recently developed the gold coating of carefully roughened metal substrates to be used as diffuse reflectors. This particular example, based on Aluminium, has a consistent surface roughness of Ra=6um and would be useful in the near infra red.
The surface roughness needs to be random enough to allow the surface to function as an isotropic diffuse reflector for infra red wavelengths. Additionally the magnitude of the surface roughness needs to be high enough so the reflectance will be perfectly diffuse and have no enhanced reflectance in the specular direction. We are happy to coat customer supplied material and we can provide measurements of the surface texture.