2 May 2007 Time-delayed source and interferometric measurement of domes and windows
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Measurement of the transmitted wavefront of domes and windows is a long-standing problem. One may use a large return sphere and measure the interference cavity without the dome present and again with the dome present. The difference between the two measurements is a double-pass measurement of the transmitted wavefront of the dome. Even so, the long coherence length of the source results in many extraneous fringe patterns. Windows may be tested by using a collimated source and return flat. A time-delayed source (TDS) having a short-coherence length is used to obtain a single interference pattern due only to interference of light reflected by the two surfaces of a dome or window. Standard phase shifting algorithms may be used with the TDS to measure the optical thickness of a dome or window without errors due to multiple reflections. Since most of the interferometer is common-path, environmental sensitivity is reduced and alignment is straightforward compared to typical interferometers. Finally, since there is no reference surface, stitching of sub-aperture measurements is simplified.
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William P. Kuhn, Matthew Dubin, "Time-delayed source and interferometric measurement of domes and windows", Proc. SPIE 6545, Window and Dome Technologies and Materials X, 65450O (2 May 2007); doi: 10.1117/12.718810; https://doi.org/10.1117/12.718810

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