Stray-light implications of scratch/dig specifications

Isabella T. Lewis; Arno G. Ledebuhr; Marvin L. Bernt

doi:10.1117/12.50493

1 December 1991 Stray-light implications of scratch/dig specifications

Isabella T. Lewis, Arno G. Ledebuhr, Marvin L. Bernt

Proceedings Volume 1530, Optical Scatter: Applications, Measurement, and Theory; (1991) https://doi.org/10.1117/12.50493
Event: San Diego, '91, 1991, San Diego, CA, United States

Abstract

The bidirectional transmittance distribution function (BTDF) of two sets of scratch/dig standard sets were measured. These sets were representative of the inspection standards used in the optical industry to characterize polished surface defects. Measurements were taken with a small (1 mm diameter) illumination beam to maximize signal. The increase in average BTDF that results from a single scratch or dig over the MIL-STD 20 mm diameter surface was then calculated to determine what overall impact a defect will have on system stray light above base surface scattering due to surface micro-roughness. A BTDF measurement was taken with the illumination beam centered on the defect, then with it centered on a smooth section of the reference sample to find the increase in scattering caused by the defect. Results show that dig scattering, when normalized to account for the single dig per 20 mm MIL-STD inspection area criteria, did not catastrophically increase the 0.05 B0 (B0 is the BTDF at 0.57 degree(s)) at 633 nm characteristic of a high quality optical surface. As intuitively expected, dig scattering was angularly symmetric. Scratches, however, scattered highly directionally. Normalized BTDF is substantially increased from a smooth surface's typical 0.05 B0 perpendicular to the scratch axis, but is unaffected in other angles. On average, the scratches may not have increased net surface scattering. Scattering from the defects on the surfaces below the 40-20 scratch/dig level was found to not cause a catastrophic increase in scattering over the level as a well-polished optic (typically 4 angstroms rms roughness). Since comparisons with scratch/dig samples only serve to provide a measure of the localized defects, and fail to be useful in determining the low-level scattering from the surface microroughness, one should not assume that a '40-20' surface is necessarily a low-scattering optic.

Citation Download Citation

Isabella T. Lewis, Arno G. Ledebuhr, and Marvin L. Bernt "Stray-light implications of scratch/dig specifications", Proc. SPIE 1530, Optical Scatter: Applications, Measurement, and Theory, (1 December 1991); https://doi.org/10.1117/12.50493

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