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Chapter 5:
Scattering from Particulate Contaminants
All surfaces have some amount of particulate contamination (i.e., dust) that increases their BSDF above the level predicted by their surface roughness. An example of the type of artifact that results from scatter of similar contaminants is shown in this chapter. In this figure, particulates on the camera's optics have scattered sunlight from outside the FOV into its FOV, thus increasing its out-of-field stray light. Scattering from surface roughness results in artifacts similar to those from particulate scatter and also contributes to this artifact. A number of models have been developed to compute the BSDF of contaminated surfaces. As with surface roughness scatter models, the more fidelity the model has, the more input data (and therefore effort to implement) it requires. As the next section demonstrates, the BSDF of a surface due to contamination scatter is a strong function of the particle density function f(D), which is equal to the projected areal density (in units such as 1/mm2) of the particle distribution as a function of particle diameter D, and the difference between the BSDF models presented in this chapter lies in the way they describe f(D). In the most approximate model, f(D) is represented using a simple equation. In the most accurate model, it is represented as a table of values determined by detailed inspection of particles on the surface itself. Details of performing inspection in order to determine the input parameters for either model are also presented in this chapter. Computation of BSDF from f(D) is usually done usingMie scatter theory, and thus this chapter begins with a review of this theory. As with surface roughness scatter theory, a comprehensive review of Mie scatter theory is beyond the scope of this book; however, there are a number of good references.There are also many details related to the control and inspection of particulate contamination that are not covered in this chapter but are covered elsewhere. These references also discuss the effects of molecular contamination, which are discussed briefly here.
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