This parameter measures the allowable defects in a coating or on the surface of an optical element, and is specified as a numerical value. For example, a specification may read “Scratch/Dig = 80/50", explained as follows:
Scratch numbers are the apparent widths of hairline scratches allowed in units of 0.001mm. A scratch number of 80 is really 0.08 mm wide. Additionally, the combined length of maximum-size scratches on the surface of the lens in question cannot exceed 1/4 the diameter of the usable lens area.
Digs represent the apparent diameters of allowable defects such as bubbles, pinholes and inclusions on the surface of lens or coating. Digs are specified in units of 0.01mm, so a dig value of 50 is actually .5mm diameter allowable inclusion. The allowable number of maximum size digs within the useful area of the lens is one, and the sum of the diameters of all digs cannot exceed twice the diameter of the minimum size dig number specified.
1. All class 5 and 10 digs must be separated by at least .040 inches.
2. The sum of the dig diameters must be less than 2 times the maximum dig diameter allowed within any .80 inch diameter.