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Excerpt from Field Guide to Interferometric Optical Testing
Another common interferometer is the Mach-Zehnder, which is useful for testing optics in transmission in single pass. If the optic under test has large deviations, this less sensitive test could be better for keeping the wavefront slope within the dynamic range of the interferometer.
Transmission tests are useful for measuring index homogeneity or thickness variation. In addition, they can be used to measure the power of a lens with low power. The Mach-Zehnder can also be used for testing surfaces in reflection by replacing one of the fold mirrors with the test part. The sensitivity is less since the beam is incident at 45° instead of normal incidence.
The imaging lens is chosen such that the test part is conjugate to the plane of the interferogram.