On the influence of schlieren in large field-of-view applications of multiwavelength holography

Abstract

High-precision applications of multiwavelength holography typically requires stable laboratory-like environments, which is hard to achieve in industrial applications. The influence of schlieren is crucial, especially in large fieldof-view applications, where long working distances can result in optical paths up to one meter. Schlieren is a well-known effect in interferometry and can be seen in the reconstructed object wavefronts. Small perturbations in temperature change the refractive index of air resulting in local variations of the optical path. Proper encapsulation or vacuum techniques are typically employed to compensate for this. In this work, we investigate the impact of schlieren on multiwavelength holography and propose a compensation method. The sensor used is a Mach-Zehnder-based interferometer with a field-of-view of 17.9 mm × 13.4 mm and a camera with 9344 px × 7000 px. A mirror was positioned at a distance of 1 m in front of the sensor. We performed holographic and interferometric measurements with and without an encapsulating pipe around the beam to investigate the influence of schlieren. The deviations of the phase shifts of the holographic data were laterally resolved using a modified version of the algorithm proposed by Cai et al. The fringe patterns of the interferometric data captured with different exposure times and frame rates were analyzed using a sinusoidal fit and discrete Fourier transformations (DFT) to show lateral frequency deviations. Both methods show that encapsulation leads to improved measurements. A potential compensation method is proposed.