A cosmetic surface quality spec written as scratch number then dig number. The scratch number is judged visually against reference scratches, while the dig number is the allowed pit diameter in hundredths of a millimeter. 60-40 is commercial grade, 20-10 is typical for laser optics and 10-5 for high-power or intracavity optics. Lower numbers cost more and reduce scatter and damage sites.
Intracavity optics need to be 10-5 scratch-dig, the 40-20 parts we got for the beam delivery won't survive in there.
Also heard as
- S-D
- scratch and dig
Related terms
Lambda over ten
Shorthand for an optic flat to one tenth of a wavelength, used to mean good-quality laser optics.
Damage threshold
The maximum fluence or power density an optic can take before it is permanently damaged.
AR coating
Antireflection coating: thin dielectric layers that cut surface reflections from about 4 percent to a fraction of a percent.