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The presence of stray light is a continuing problem in the design and performance of optical systems. In well-designed systems most of it comes from the optical components themselves. By proper baffling it can be reduced in sensitive areas by orders of magnitude, but ultimately it is necessary to improve the components themselves. The dominant source of component scattering is from the optical surfaces. Bulk scattering can also occur in windows or lenses but it is typically one to two orders of magnitude lower than surface scat-tering. In the visible and ultraviolet regions of the spectrum scattering from surface microirregularities only a few nanometers in height dominates. At longer wavelengths micro-irregularity scattering drops exponentially and defect scattering from surface blemishes, scratches, dust, etc. becomes important. It is often nearly independent of wavelength, and forward scattering is typically somewhat larger than backscattering, as would be predicted from dipole scattering theory. Pitting of the surface by sand or rain erosion increases the importance of defect scattering. More complicated than the case of individual incoherent scattering centers is that of correlated scattering from a repetitive surface. Diamond-point turned optical surfaces are an example. Resonances can occur for such surfaces, particularly when they are overcoated with dielectric films. In the best cases, however, scattered light levels from diamond-turned optical surfaces are remarkably low, and the absence of polishing defects or scratches makes this technique very attractive for producing infrared optics. Scattered light resonances can also occur in the ultraviolet. If aluminum or silver coatings are used, optical excitation of surface plasmons may occur. If they decay radiatively, the result is scattered light. Dielectric overcoatings can enhance scattering in this case; increases of as much as an order of magnitude have been observed. These effects are reduced by using surfaces having very low rms microirregularities. We conclude that when ordering optical components for low scatter applications, one should include a specification of the rms height of microirregularities (often termed the rms roughness of the surface) in addition to or even instead of the traditional scratch/dig specification. The rms roughness specification is particularly important for optics to be used in the visible and ultraviolet regions of the spectrum. In the infrared region defect scattering is of primary importance, and it is here that the scratch/dig specification is most useful.
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