Abstract
The optical techniques of stress analysis, such as, holography, photoelasticity, and moiré, produce results in the form of whole-field fringe patterns representing physical measurements. The accuracy of results will depend on how precisely fringe orders and fringe locations are determined. A detailed account of an accurate and automatic method of tracing the centerlines of dark and bright fringes using image processing is presented. The variance of gray-level values across fringe width is used as a criterion for determining fringe direction. The determination of fringe direction is done in an operator window of size 5 x 5, 7 x 7, or 9 x 9 pixels at the central pixel of the window. Sequential application of this condition enables the user to trace the fringe centerline. Using the capabilities of image-processing equipment, the cursor, and the specially developed computer program, holographic fringes of different patterns were traced. A few of these cases are presented as illustrative examples. It is also shown that the technique can be successfully applied to fringes that close on themselves so as to form closed loops.
