Pixel MTF response effect on non-null interferometry.

The pixel modulation transfer function response degrades the contrast of non-null interferometric surface figure measurements. We experimentally quantify this effect for spatial frequencies ranging from 0 to 363 lp/mm (≈3.33 times the Nyquist limit). Our results show a low SNR spatial frequency band that behaves like a low-pass filter for sub-Nyquist interferometry and a stop-band filter for multiple-wavelength phase-shifting interferometry. We also introduce a multiple-mode, multiple-wavelength interferometry approach to measure optical surfaces with slope departure angles mapping to spatial frequencies in this low SNR band. The extended measurement range of this approach is achieved without using a sparse-array detector.