The previous chapter outlined non-interferometric means for testing optical surfaces and systems. However, much of optical testing relies on the interference of light to provide sensitive wavefront measures. This chapter examines basic interferometry setups and describes their function. As with the previous chapter, there are two main cases for the testing procedure. The first case, again, involves a single optical surface and a testing procedure that compares the wavefront reflected from that surface to an ideal wavefront. The wavefront at the test optical surface is typically analyzed to avoid changes in shape due to propagation, and errors in the fabrication of the surface are encoded in the wavefront shape. This technique provides feedback as to the quality and accuracy of the surface. The second case involves testing the transmitted wavefront through an optical system. The performance of the system compared to the original design is assessed to determine if the system meets some quality specification. The system may be evaluated at multiple field angles, and the wavefront is analyzed in the exit pupil. Again, the term pupil will be used when describing the location at which the wavefront is being measured, with the understanding that this refers to the surface in the surface testing and the exit pupil in the system testing.
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