The invention relates an apparatus and method for forming a diffuse reflector. In one embodiment of the invention, a diffuse reflector is formed by exposing transmissive particles of a pre-determined purity to pressure and forming a material having desired diffuse reflective properties. The transmissive particles may further be thermally treated, such as by sintering, to form material having desired diffuse reflective properties. The treated transmissive particles may then be disposed in a vessel and define a cavity therein to form an integrated cavity diffuse reflector.