Scratch - Dig Optical Surface Specifications

Reference MIL-O-13830  Optical Components for Fire Control Instruments; General Specification Governing The Manufacturing, Assembly, and Inspection of Glass and BSR/OEOSC OP1.002 Optics and Electro-Optical Instruments – Optical Elements and Assemblies – Appearance Imperfections.

Scratch-Dig refers to the surface quality of optical surfaces.  Owing to their relative complexity, many misconceptions exist about meaning of scratch-dig specifications, and how they are applied.    Conceptually, scratch-dig specifications attempt to set a limit on the amount of area surface defects occupy relative to the overall clear aperture of the optical element.  Because assessments are made relative to the size of the part, a scratch that is unacceptable for a small part may be acceptable for a large part.

Much of the confusion about scratch/dig requirements finds its origins in the fact that there are actually two distinct standards – the “visibility method" and the “dimensional method”.    These two standards differ only in the way scratch widths are categorized.    The dimensional method, which is less prevalent, characterizes scratches by width measurements; whereas, the visibility method uses comparisons to a commercially available visual references.

Scratch:  Any marking or tearing of the part surface.

Dig:  A small rough spot on the part surface similar to a pit in appearance. A bubble is considered a dig.  Surface stains are also considered digs.

Visibility Method
Scratch/Dig:  Surface quality is specified by a number such as 60/40.   The 60 defines a scratch width according to a visual standard (it does not mean a scratch can be 60 um wide).   For reference, scratch numbers of 60 typically refer to maximum allowable scratch widths of ~7um to 8um.  With this in mind, the scratch part of the specifications includes the following four requirements:

  1. The combined length of the scratches with the specified scratch number shall not exceed 25% of the smallest dimension of the clear aperture (*).
  2. When scratches with the maximum allowable scratch number are present, then the sum of the products of the respective scratch numbers times the ratio of their length to the smallest dimension of the clear aperture for all scratches cannot exceed 50% of the maximum allowable scratch number.
  3. When scratches with the maximum allowable scratch number are not present, then the sum of the products of the respective scratch numbers times the ratio of their length to the smallest dimension of the clear aperture for all scratches cannot exceed the maximum allowable scratch number.
  4. Surface with scratch letter requirements between 10 and 20 can have no more than 4 separates scratches in any 0.25 inch circular area.

The second number of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter.  The dig part of the specifications includes the following three requirements:

  1. The maximum number of digs must be less than 1 per 20mm diameter circle.
  2. For every 20mm diameter clear aperture, the sum of the diameters of all digs exceeding 2.5um in diameter cannot exceed 2 times the specified dig letter.
  3. Digs with a dig number less than 10 must be at least 1mm apart.

Dimensional Method
Scratch/Dig:  Surface quality is to be specified by letters such as E/D. The first letter relates 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.   A surface quality callout of E/D would permit a scratch width of 60 microns (0.0024") and a dig diameter of 400mm (0.0158"). The table below provides the correspondence between letters and physical sizes.  Note that the scratch and dig accumulation rules for the dimensional and visibility methods are the same, but are repeated below for completeness.  With this in mind, the scratch part of the specifications includes the following four requirements:

  1. The combined length of the scratches with the specified scratch number shall not exceed 25% of the smallest dimension of the clear aperture (*).
  2. When scratches with the maximum allowable scratch width are present, then the sum of the products of the respective scratch widths times the ratio of their length to the smallest dimension of the clear aperture for all scratches cannot exceed 50% of the maximum allowable scratch width.
  3. When scratches with the maximum allowable scratch width are not present, then the sum of the products of the respective scratch widths times the ratio of their length to the smallest dimension of the clear aperture for all scratches cannot exceed the maximum allowable scratch width.
  4. Surfaces with scratch letter requirements between B and C can have no more than 4 separate scratches in any 0.25 inch circular area.

The second letter of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter.  The dig part of the specifications includes the following three requirements:

  • The maximum number of digs must be less than 1 per 20mm diameter circle.
  • For every 20mm diameter clear aperture, the sum of the diameters of all digs exceeding 2.5um in diameter cannot exceed 2 times the specified dig letter.
  • Digs with a dig letter less than B must be at least 1mm apart.

 (*)  – Note that the length descriptor of the clear aperture is not always simple.  MIL-O-13830 defines the length descriptor as the diameter of a circle with the same area as the clear aperture of the part being evaluated.  Often times, however, optical industry prefers to use the smallest dimension of the clear aperture.

 

 

Dig Identification
Dig ID Maximum Dig or Bubble Diameter Dig or Bubble Separation Distance
Letter mm inch mm inch
A 0.05 0.0020 1.0 0.040
B 0.010 0.0039 1.0 0.040
C 0.20 0.0079 20 0.787
D 0.40 0.0158 20 0.787
E 0.60 0.0236 20 0.787
F 0.80 0.0315 20 0.787