Development of Simultaneous Dual-Resolution Digital Holography System

Abstract

This research paper is focused on the development of a digital holography system for simultaneous dual-resolution measurements. Digital holography has been widely used for deformation measurements and non-destructive testing (NDT) due to its advantages of high sensitivity, high accuracy, and whole-field, non-touch measurements. A traditional test only has one spatial resolution, which can cause a big deformation to be indistinguishable or minor defects to be ignored. Both large and small fields of view should be observed to reach a multi-spatial resolution measurement. Usually, multiple separate tests are used to observe the different sized fields of view, resulting in higher costs and longer required testing times. Furthermore, these tests may not be repeatable in some cases. This paper presents research on a novel digital holography system that achieves dual spatial resolution measurements simultaneously by testing different-sized fields of view with a single camera. The novel system has two optical channels with two optical layouts of holography to measure deformation. By changing the combined focus length, the two holographic setups have different fields of view, i.e., one has a large and the other has a small field of view. To realize a simultaneous test, the polarization technique is used to avoid cross-interference between the two optical layouts. Finally, spatial carrier fringes with different orientations are introduced into the two holographic setups by appropriately adjusting the reference beam of each setup. The different oriented spatial carrier fringes enable the spectrums of the two interferograms to be separated after a FT (Fourier transform) and the phase distributions of the two interferograms can be extracted and separated by windowing the spectrum to perform an IFT (inverse Fourier transform). The phase distributions can then be used to analyze and calculate the deformations. The experiment using this system is described in this paper and the practicability of this method is verified by the obtained experimental results.