Scratch-Dig

Scratch-Dig is a specification used to quantify the surface quality of an optical component (such as a lens, mirror, or filter). It defines the allowable level of surface defects—specifically scratches and pits (digs)—on the polished surface of the glass. It is primarily a cosmetic standard but becomes a performance standard in high-power laser systems or high-contrast imaging.

The Notation The specification is expressed as two numbers separated by a hyphen, following the US Military Standard MIL-PRF-13830B.

  • Format: [Scratch Number] - [Dig Number]
  • Example: 60-40 (read as "sixty-forty")

The Components

The Scratch (The First Number)

The "Scratch" number refers to the visibility or brightness of a scratch, not necessarily its physical width.

  • How it works: A scratch on the optic is compared visually against a calibrated standard (a master set of scratches). If the scratch on the optic is no brighter than the standard #60 scratch, it passes the "60" rating.
  • Common Misconception: The number (e.g., 60) does not directly equal the width in microns. It is a weight classification based on visual appearance.
  • Standard Ratings: 80, 60, 40, 20, 10.

The Dig (The Second Number)

The "Dig" number refers to the physical size of pits, bubbles, or pinholes in the glass. Unlike scratches, this is a direct measurement of diameter.
  • The Math: The Dig number represents the diameter of the defect in units of 1/100th of a millimeter (10 microns).
  • Calculation: Diameter = Dig Number x 0.01 mm
  • Example: A Dig 50 specification allows a defect with a diameter of 0.5 mm (50 × 0.01).

Common Industry Grades

Different applications require different levels of surface quality. Tighter tolerances (lower numbers) increase manufacturing costs significantly.

Grade Specification Typical Application
Commercial Quality 80-50 Standard consumer projection, photography, low-cost sensors. Defects may be visible to the naked eye.
Precision Quality 60-40 Research optics, microscopy, standard low-power lasers. The most common standard for off-the-shelf optics.
High Precision 40-20 High-performance imaging, telecommunications, and low-light detection systems.
Laser Quality 10-5 High-power laser cavities and UV applications. Necessary to prevent laser-induced damage (LIDT) and minimize scatter.

Inspection Method

Under the MIL-PRF-13830B standard, inspection is performed visually by a trained technician.

  • Setup: The optic is viewed against a matte black background.
  • Lighting: Specific side-lighting is used to make surface defects "light up" or scatter light, making them easier to see.
  • Comparison: The technician compares the optic's defects to a certified "calibrated scratch and dig pad."

Why It Matters

  • Scattering: Surface defects scatter light, which reduces contrast (Modulation Transfer Function) and system throughput.
  • Laser Damage: In high-power laser systems, scratches act as "energy sinks." Laser energy concentrates in the defect, causing the optic to shatter or burn (Laser Induced Damage Threshold).
  • Cleanability: Deep scratches or digs can trap contaminants (dust, oil) that are difficult to clean and may degrade coatings over time.
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