Verifire AT Fizeau Interferometer (Zygo Corperation)
The Zygo VeriFire AT Fizeau interferometer (shown to left) measures the deviation of height of a surface from a reference surface. It accomplishes this using phase shifting interferometry. The figure error of optical surfaces up to 150 mm in diameter at normal incidence, or larger optics at grazing incidence angle ( < 1500 mm), can be measured with this instrument.
A picture of a grazing incidence measurement on a Far-IR beamline planar mirror (M3), in so-called "double pass" mode is shown below. A collimated HeNe laser beam (l=632.8 nm, 150 mm diameter) is output from the Fizeau Interferometer through a reference transmission flat at the left, reflected off the mirror under test (right) and then reflected back with a "return flat" (top right) to the detector in the interferometer. "Double pass" refers to the fact that the collimated beam is reflected twice off the test mirror.
Below is shown the 3D height profile of a transmission flat, measured in "single pass" normal incidence mode. The transmission flat under test is 150 mm in diameter.

The height profile shown (i.e. difference between the transmission flat under test and the "reference" test flat) is displayed in terms of wavelength, where l=632.8 nm (i.e. HeNe laser). Maximum peak to valley height over the test transmission flat is ~l/23.26, or 0.043l wavelength. Our best quality optical cavity, consisting of a "matched" high quality reference transmission flat and "return" flat, is approximately l/50 in quality (combined, as an optical cavity), so flatness measurements to ~l/50 are certainly possible with this instrument.

The surface/wavefront profile (curve in green) shows a slice (user selectable) through the test transmission flat (taken just off somewhat from the center line), and clearly shows that a slight concavity (i.e. raised edges and sunken center) exists in this test transmission flat.

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