13 November 2014 The art of specifying surface quality
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Abstract
If you open a catalog to buy an optic its surface quality will likely be given by a scratch/dig specification. This is useful information and low scratch/dig numbers usually mean a lower rms roughness and lower scatter, but you cannot find the rms roughness or make estimates of scatter levels from scratch/dig. If you measure a surface with a profilometer you can get bandwidth limited values of the rms roughness and the surface power spectral density function, but this information is often limited to spatial frequencies below about 0.1/μm and this is not high enough to even estimate visible scatter beyond about 5 degrees from specular. Scatter can be measured in BRDF or BTDF units, but instrumentation and measurements are expensive and change with incident angle, wavelength and polarization and cannot always be directly related to roughness statistics. This paper reviews what we know about the relationship between these various parameters and offers suggestions on how to approach application specific issues by defining and specifying surface quality.
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John C. Stover, Sen Han, "The art of specifying surface quality", Proc. SPIE 9276, Optical Metrology and Inspection for Industrial Applications III, 92760M (13 November 2014); doi: 10.1117/12.2071006; https://doi.org/10.1117/12.2071006
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