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Description of an integrated scatter instrument for measuring scatter losses of 'superpolished' optical surfaces

An instrument has been developed for measuring integrated scattering (TIS) of low-scatter optical surfaces. In the case of TIS measurement using an Ulbricht sphere, Rayleigh scattering on air was found to be the limiting factor in improving the resolution limit of the instrument. Measures for the suppression of Rayleigh scattering are described. The achieved resolution limit for the integrated scattering in the reflection hemisphere is 0.1 ppm (1 ppm=10-6) of the incident light intensity. This improved resolution offers the capability of measuring surface roughness induced scatter losses of high quality multilayer-coated optics and even uncoated transparent 'superpolished' fused quartz substrates. The result is given of a developed theoretical model describing the straylight emission of the two boundaries of a transparent substrate. Scatter measurement results from uncoated 'superpolished' substrates are presented, showing scatter losses as small as 0.6 ppm corresponding to 0.1 nm rms surface roughness.