Nondestructive testing of specularly reflective objects using reflection three-dimensional computer vision technique

Abstract

Michael Y. Y. HungCity University of Hong KongDepartment of Manufacturing Engineeringand Engineering ManagementTat Chee Avenue, KowloonHong KongE-mail: bcmhung@cityu.edu.hkH. M. ShangNanyang Technological UniversitySchool of Mechanical andProduction Engineering50 Nanyang AvenueSingapore 639798Abstract. We review an optical method referred to as 3-D computervision technique for nondestructive inspection of three-dimensional ob-jects whose surfaces are specularly reflective. In the setup, a computer-generated cosinusoidal fringe pattern in the form of linear, parallel fringelines of equal spacing is displayed on a TV monitor. The monitor isplaced in front of the test object, whose specularly reflective surfacebehaves as a mirror. A virtual image (or mirror image) of the fringe linesis thus formed. For a planar surface, the fringe pattern of the image isundistorted. The fringe lines, however, are distorted according to theslope distribution if the surface is not flat. By digitizing the distorted fringelines, employing a phase-shift technique, the fringe phase distribution isdetermined, hence enabling subsequent determination of the surfaceslope distribution. When applied to nondestructive flaw detection, twoseparate recordings of the virtual image of the fringe lines are made, onebefore and another after an incremental load is applied on the test ob-ject. The difference of the two phase-fringe distributions, or the phasechange, represents the change in surface slope of the object due to thedeformation. As a subsurface flaw also affects surface deformation, bothsurface and subsurface flaws are thus revealed from anomalies in thesurface slope change. The method is simple, robust, and applicable inindustrial environments.