Surface flatness is quoted as peak-to-valley deviation in fractions of a test wavelength, normally 633 nm, so lambda over ten means within about 63 nm across the clear aperture. It is the usual spec for laser-grade mirrors and windows, with lambda over four for commercial parts and lambda over twenty for interferometry. People also use it loosely to mean any decent optic, as opposed to cheap parts.
Those are lambda over four windows, fine for the camera but not in the interferometer arm, get lambda over ten at least.
Also heard as
- lambda/10
- tenth-wave
Related terms
Scratch-dig
The surface quality rating for optics, given as two numbers such as 20-10 for the allowed scratches and pits.
AR coating
Antireflection coating: thin dielectric layers that cut surface reflections from about 4 percent to a fraction of a percent.
Beam expander
A pair of lenses arranged as a telescope that enlarges a collimated beam's diameter and cuts its divergence.