Scratch / Dig Specifications

Purpose:

This document defines glass surface quality on optical components per MIL-O-13830.

Definitions:

  • SCRATCH: Any marking or tearing of the glass surface.
  • SLEEK SCRATCH: A hairline scratch.
  • CRUSH or RUB SCRATCH: A surface scratch or a series of small scratches.
  • DIG: A small rough spot on the glass surface similar to pits in appearance. A bubble is considered a dig.

Procedure:

Surface quality is to be specified by a number such as 60/40. The first digits relate to the maximum width allowance of a scratch as measured in microns. The next digits indicate to maximum diameter allowance for a dig in hundredths of a millimeter. Thus, as can be seen from the table below, a surface quality callout of 60/40 would permit a scratch width of .0024 inches and a dig diameter of .0158 inches. The size of a defect is to be measured through the use of an optical comparator.

Scratch or Dig Number Maximum Scratch Width Maximum Dig or Bubble Diametre Dig or Bubble Seperation Distance
mm Inch mm inch mm inch
120 0.12 0.0047 1.20 0.0473 20 0.787
80 0.08 0.0031 0.80 0.0315 20 0.787
60 0.06 0.0024 0.60 0.0236 20 0.787
50 0.05 0.0020 0.50 0.0196 20 0.787
40 0.04 0.0016 0.40 0.0158 20 0.787
30 0.03 0.0012 0.30 0.0118 20 0.787
20 0.02 0.0008 0.20 0.0079 20 0.787
15 0.015 0.0006 0.15 0.0059 20 0.787
10 0.010 0.0004 0.10 0.0039 1 0.040
05 0.005 0.0002 0.05 0.0020 1 0.040
03 0.003 0.00012 0.03 0.0012 1 0.040