The quality of the polish of an optical surface can be evaluated by the amount of light it scatters. Some optical materials, notably Zerodur' and CervitTM, exhibit bulk scatter; that is, light can be scattered from the interior of the material as well as from the surface. The presence of bulk scatter makes it difficult to observe the surface scatter, especially when the level of surface scatter is very low as is the case with well-polished optical surfaces. A novel instrument has been built that can distinguish between surface and bulk scatter by differences in their polarization characteristics. The instrument will give zero output signals for bulk scatter while being sensitive to the surface scatter. The instrument can be said to be blind to bulk scatter.
Erik W Anthon,
"An Instrument For Measurement Of Surface Scatter In The Presence Of Bulk Scatter", Proc. SPIE 0680, Surface Characterization and Testing, (23 March 1987); doi: 10.1117/12.939606; https://doi.org/10.1117/12.939606