A scratch is defined as any marking or tearing of a polished optical
surface. In principle, scratch numbers refer to the width of the reference
scratch in ten thousandths of a millimeter. For example, an 80 scratch is
equivalent to an 8-µm standard scratch. However, this equivalence is
determined strictly by visual comparison, and the appearance of a scratch
can depend upon the component material and the presence of any coatings.
Therefore, a scratch on the test optic that appears equivalent to the 80
standard scratch is not necessarily 8 mm wide.
If maximum visibility scratches are present (e.g., several 60 scratches on a 60-40
lens), their combined lengths cannot exceed half of the part diameter. Even with
some maximum visibility scratches present, MIL-0-13830A still allows many
combinations of smaller scratch sizes and lengths on the polished surface.
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