Scattering Characteristics Of Optical Materials

In the visible and ultraviolet regions of the spectrum the dominant source of scattered light from optical components is surface scattering by microirregularities. They are typically only a few nanometers in height but cover the entire optically polished surface. Since microirregularity scattering is inversely proportional to the square of the wavelength, the dominant source of scattering in the infrared is often macroscopic defects such as scratches or digs. Pitting of the surface by sand or rain erosion also contributes to defect scattering. Dust particles and other particulates are a third important source of scattered light in the near infrared. When specifying the surface quality of optical components to be used in low scatter applications, at least the rms microroughness of the surface and the scratch/dig specification should be given. (An improved scratch standard would be most helpful here.) Better yet is a functional test of the scattering performance of the component. The most direct functional test is total integrated scatter. A prototype instrument utilizing a Coblentz sphere and a HeNe laser operating at 3.39 p.m, where the dominant source of scattering is scratches and other macro-scopic defects; at 1.15 p.m, where particulates such as dust may be important; and at 0.6328 pm, where microirregularity scattering dominates, has been constructed to perform this functional test. A simple theory allows us to extrapolate the results at these wavelengths to obtain a reasonably complete picture of the total integrated scattering behavior of the component at any wavelength. Additional data are required to understand the angular dependence of the scattered light since the angular dependence of scattered light is related to the slopes as well as the heights of the surface defects. Such data can be measured experimentally and analyzed to give a more complete picture of the surface than is obtainable from total integrated scattering measurements alone.