Stray Light Implications of Scratch/Dig Specifications

Conference · Mon Jul 22 00:00:00 EDT 1991

OSTI ID:5183421

Lewis, Isabella T. ^[1]; Ledebuhr, Arno G. ^[1]; Bernt, Marvin L. ^[2]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
2. Toomay, Mathis and Associates, Inc., Bozeman, MT (United States)

The bidirectional transmittance distribution function (BTDF) oftwo sets ofscratch/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 MEL-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 ofthe 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 MEL-STD inspection area criteria, did not catastrophically increase the 0.05 B₀ (B₀ is the BTDF at 0.57°) 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 os a well-polished optic (typically 4A 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.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 5183421
• Report Number(s): UCRL-JC-106887; CONF-9107115--47; ON: DE91018461
• Country of Publication: United States
• Language: English

Related Subjects

656003* -- Condensed Matter Physics-- Interactions between Beams & Condensed Matter-- (1987-)
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
DEFECTS
DISTRIBUTION FUNCTIONS
FUNCTIONS
LIGHT SCATTERING
MATHEMATICAL MODELS
OPTICAL PROPERTIES
OPTICAL SYSTEMS
OPTICS
PHYSICAL PROPERTIES
SCATTERING
SURFACE PROPERTIES