Combining MTF and phase retrieval wavefront sensing for complete image quality diagnostics

Abstract

The modulation transfer function (MTF) is an established metric to evaluate the quality of an imaging system. Nevertheless, this method does not provide detailed, diagnostic information about wavefront aberrations. MTF tells us if a system passes or fails required criteria. However, wavefront testing tells us what types of errors are in the system. This paper demonstrates how wavefront measurements can be performed on conventional MTF measurement systems by implementing a phase retrieval method based on the transport of intensity equation (TIE). This new functionality enables wavefront measurements both on-axis and off-axis across a broad wavelength spectrum (from UV to LWIR) and a wide range of aperture sizes. The algorithm used to reconstruct the wavefront from two defocused point spread functions (PSFs) captured by the device is explained. Zernike polynomials are fitted to the resulting initial wavefront to determine the individual aberration components. Initial results for various samples are presented in comparison with theoretical data. Furthermore, the phase retrieval method is compared with conventional interferometry and standard MTF measurement, utilizing calculated MTF values based on the measured wavefronts. The advantages and flexibility of this method are discussed. In addition, its limitations are examined in comparison to other techniques such as interferometry or Shack-Hartmann sensors.