A wavelength-tuned Fizeau interferometer is applied to the problem of flatness testing of transparent plates. When the plate is positioned at a specific distance from the reference surface and an integermath 13-frame phase-shifting algorithm is applied, the system directly filters out unwanted interference arising from backsurface reflections. The resulting front-surface profile exhibits less than 2 nm of residual error attributable to spurious reflections from within the plate.
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