Three error sources, phase shift accuracy, misalignment of the test surface, and wavefront error propagation in the imaging system of an interferometer are analyzed in this paper. The phase shift error addressed here is not caused by poor repeatability [1-4] of the PZT (piezoelectrical transducer) but by transverse phase variations due to its curved Fizeau cavity when the test or Fizeau element is translated. Misalignment of a curved surface under test is due to the fact that fringes are not completely nulled. Tilt and power or defocus can introduce coma and spherical aberration errors which are not insignificant to the interferometer accuracy. For example, a half wave of defocus of a test surface with a NA of .5 can produce a 10th wave of spherical aberration error in the measurement and a half wave tilt of the same surface can create an 8th wave of coma error. The propagation error results when a test wavefront is sheared with a reference wavefront in the imaging system of the interferometer.
© 1992 Optical Society of AmericaPDF Article
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