Phase unwrapping for large depth-of-field 3D laser holographic interferometry measurement of laterally discontinuous surfaces

Abstract

A phase unwrapping method is developed to mathematically increase the depth-of-field for the 3D optical measurement of objects with laterally discontinuous surfaces, which contain disconnected high aspect ratio regions. This method is applied for laser holographic interferometry precision measurements. The phase wrap identification at boundary pixels, masking and recovery, dynamic segmentation and phase adjustment are developed to overcome the divergence problem in phase unwrapping of laterally discontinuous surfaces. An automotive automatic transmission valve body is applied as an example to demonstrate the developed method. Experimental results demonstrate that the proposed methods can efficiently unwrap the phase to increase the depth-of-field for laterally discontinuous surfaces. Effects of segment size and width of overlapped regions on the computational efficiency are investigated.

Invited Review Article: Review of post-process optical form metrology for industrial-grade metal additive manufactured parts

“…The ambiguity which exists in most fringe projection phase-shifting techniques due to the wrapped phase output being an inherent part of the measurement process is an issue that does not plague laser triangulation. 107 It is nevertheless possible to mitigate the presence of discontinuities through use of various unwrapping techniques, 55,57,108 which of course carry a post-processing time penalty. As far as the overall measurement latency is concerned, however, in general structured light provides a faster measurement compared to laser triangulation even with all the post-processing required, as it can capture a whole view at once rather than raster scan a single laser beam over the measured object point-by-point (or row-by-row when using a laser line scanner).…”

Shape verification using dual-wavelength holographic interferometry

“…Common to these methods is the assumption that the unwrapped phase is a continuous function or that it has small discontinuities. There are also methods handling discontinuities in the unwrapped phases, 28 but even then the discontinuity jumps cannot exceed an upper limit.…”

Phase estimation in digital holographic interferometry using cubic-phase-function based method

“…To estimate a 2k and a 3k , M k ( y) is evaluated using Equation (8) for two positions of y say y 1 and y 2 yielding M k ( y 1 ) and M k ( y 2 ). In other words, we have…”

“…Usually, the interference phase can be estimated in DHI using the arctan function [2]. But the phase thus estimated is generally noisy and wrapped and hence requires filtering [3][4][5] and unwrapping [6][7][8] operations.…”