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Abstract
Deflectometry has been introduced as a viable alternative to interferometric measurement of specular freeform optical surfaces. Deflectometry measurements show high repeatability; however, due to the errors remaining after the calibration and data analysis, surface measurements have been limited to systematic micron-level biases in the low-spatial frequency limit. Through better modeling, calibration and data processing, the systematic errors remaining after calibration can be minimized, giving deflectometry the potential to be used as a precision metrology tool. We identify and quantify a large systematic phase bias in the deflectometry measurement that is introduced during the analysis of the measurement data. We show that this bias is a significant contributor to the well-known micron-level low spatial-frequency errors in deflectometry that are commonly attributed to calibration errors. To further minimize the systematic errors, a systematic calibration approach is developed and validated with simulation and experiment. The calibration approach is implemented on a deflectometry system and used to measure a specular flat and a spherical concave surface with a few hundreds of nanometers of uncertainty. A flexible method of geometrical modeling and calibration of the shape of deflectometry screens is also proposed. This method is used to model and calibrate the shape of a spherical screen in a simulated deflectometry measurement and used to experimentally measure the out-of-flatness of the surface of a deflectometry LCD screen. This work enables quantitative form metrology with deflectometry systems using complex screen geometries that are limited to qualitative surface defect inspection.