Optical wavefront metrology
Optical wavefront or figure metrology is concerned with measuring the departure of a spherical wavefront or optical surface from a purely spherical (or flat as a special case of spherical) form. Form resolution can be as good as 2.5 nm, but environmental effects often limit repeatability at any point on the surface to several times this. Lateral resolution is generally no more than 100 cycles per aperture diameter and often substantially less. The maximum departure from pure spherical form that can be measured accurately—that is, to within a few times the resolution of the instrument—is about 10 wavelengths of visible light or perhaps 5 μm.
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