Modified polarization point diffraction interferometer with extended measurable NA for spherical surface testing

The small pinhole of point diffraction interferometer based on pinhole-point diffraction places ultra-high requirement on both the adjustment of testing system and performance of CCD camera. Besides, poor fringe contrast due to the low reflectivity of test spherical surface would limit the measurement precision in the processing of fringe pattern. A modified polarization point diffraction interferometer with extended measurable numerical aperture (NA) is presented for testing the spherical surfaces with low reflectivity. Measurement error factors as well as the corresponding calibration procedure are introduced in detail. Comparing with the results of Zygo interferometer, measurement accuracy with root-mean-square (RMS) value about 0.0026λ and peak-to-valley (PV) 0.0134λ is achieved. The system has good measurement repeatability, and the standard deviation of measurement results RMS better than 0.0010λ is obtained. The proposed interferometer reduces the difficulty in the adjustment of the system and provides a feasible way for testing the surfaces with low reflectivity and high NA.