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The 1970s and 80s generated considerable concern over scatter in optical systems. Although it was often recognized in advance that low-scatter optics were required for a given application, the specifications were usually either nonexistent (a best-effort requirement) or inappropriate. The easiest, most available, and cheapest scatter measurement was the TIS measurement. Until the mid-1980s, most of the specifications written to handle scatter concerns were either TIS (usually given without angle or frequency limits) or rms roughness found from profile data. Surface roughness was often specified to control scatter, even though it was recognized that it would be difficult, futile, and sometimes impossible to attempt to relate the roughness parameter(s) to actual component scatter. But at least the direction was right (no sign error), as smoother surfaces do generally mean less scatter. Then in the late 1980s, serious work was begun on BSDF standards through an ASTM committee funded in part by the U.S. Air Force. The result is a document (ASTM Standard #E1392-90) that not only details minimum measurement requirements, but also gives a data format system. This enables the easy transfer of data between labs and increases the ease with which specifications can be checked—and rechecked. This chapter addresses the problem of selecting the scatter measurement(s) that will make appropriate specifications for a given component, system, or process. The preceding chapters have presented the definitions and techniques for quantifying, measuring, and analyzing optical scatter. The issue is now approached from the other direction. That is, given that the metrology exists, what specifications should be called out to assure component quality or to provide control of a critical process? This is a key issue. Specifications that are too loose, or too tight, will waste, rather than save, money and time. Scatter specifications are pretty much the bottom line for this book. You need them in order to qualify parts and/or systems. You even need scatter specs to build a scatterometer. And to be appropriate, specifications need to address the issue at hand; they have to be application specific.
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