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Comparison Of Visibility Of Standard Scratches

The increasing requirement for stringent standards relating to surface imperfections on certain optical components has highlighted the need for objective methods of measuring such defects. In recent years, several Standards Committees and many individual companies have developed their own series of standard defects which are compared visually with a defect observed on a test piece. This paper describes a comparator technique for measuring the visibility of a test defect relative to any chosen standard and discusses the relationship between the uncertainty of this measurement and the various design parameters such as lens aperture, focal setting, magnification and source coherence. The measured visibility of a selection of standard scratches as recommended in BS, DIN and MIL standards are recorded, together with an in-house comparator set provided by Kodak Ltd. The method enables any defect to be quantified in terms of a line-equivalent width and therefore provides means for the intercomparison of different standards.