Optical surfaces are the surfaces of the lenses and mirrors that form the image in an optical system. Though these surfaces are typically very smooth, none are perfectly smooth, and their residual roughness will scatter light. An example of the type of artifact that results from surface roughness scattering is included in this chapter. Surface roughness of the camera's optics has scattered sunlight from outside the FOV into its FOV, thus increasing its out-of-field stray light. Scattering from particulate contaminants results in artifacts similar to those from surface roughness and also contributes to this artifact.
There are a number of ways to model surface roughness scatter; in general, the more fidelity the model has, the more measurements (and therefore the more time and money) required to develop it. The relationship between the accuracy of the BSDF model and the accuracy of the predicted stray light performance of the system depends on the location of the optic in the system; optics that are illuminated influence the system performance more than those that are not. In general, the less the scatter from a particular surface contributes to stray light at the focal plane, the less accurately its scatter needs to be modeled. A few models of surface roughness scatter are presented in this chapter, each requiring different inputs. In order to understand these models, it is necessary to review the relationship between the surface roughness profile, the wavelength, and the BSDF. A comprehensive review of this theory is beyond the scope of this book, so an abbreviated version is presented here. After a review of the basic physics, a number of models are presented that make use of this theory to develop a scatter model of the surface. This chapter also discusses scattering from scratches and digs. Because scattering from any real surface is always the sum of scattering from its roughness and scattering from particulate contaminants on it (there will always be some), the models discussed in this chapter may not accurately describe scatter from real surfaces, especially if they are heavily contaminated.
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