Isochromatics demodulation from a single image using the regularized phase tracking technique

Abstract

In this work we propose a robust fringe demodulation technique applied to the analysis of a single isochromatics fringe pattern produced in photoelasticity. The method used is a regularized phase tracking algorithm with a new sequential scanning technique speci cally adapted for the characteristics of the isochromatic fringe patterns: possible apparition of closed fringes, large dynamic range in its spatial frequency content and low noise. The performance of the method is discussed and experimental results are presented. IntroductionDuring the last few years much eå ort has gone into developing methods for the automatic analysis of photoelastic fringe patterns. For this purpose diå erent techniques, such as phase shifting [1], Fourier transform [2], spectral content analysis [3], tricolour image analysis [4] and load stepping [5], have been applied. One of the objectives for all these techniques is the analysis of photoelastic fringe patterns by using the smallest number of images. In the case of isochromatic fringe patterns, the tricolour, spectral contents and Fourier transform techniques need only one image.The spectral content analysis technique [3] determines the isochromatic fringe order by using a white light source and measuring the output spectrum emerging from a polariscope. This technique is very accurate but the measurement is pointwise, and a spectrometer is needed to perform the spectrum analysis. Related to the spectral content analysis technique, the tricolour isochromatic method [4] is based on the construction of a given red, green and blue (RGB) look-up table which gives the relation between a RGB triplet and the fractional isochromatic fringe order. In this case the experimental set-up is simple: a polariscope and a colour charge-coupled device (CCD) camera. The main problem is that using 8 bit digitization for each RGB channel this technique can discriminate only up to 4-5 orders owing to the lack of contrast in the colour signal as the fringe order increases.A totally diå erent approach is used by the Fourier transform technique for isochromatic fringe analysis [2]; here monochromatic illumination is used and a carrier is introduced by means of a quartz wedge, with further Fourier transform

Digital photoelasticity – A comprehensive review

“…There have been several attempts to evaluate the total fringe order using a single isochromatic image with monochrome, tricolour, or white light as source of illumination. Servin and Quiroga [69] used a single monochrome bright field image to obtain unwrapped isochromatics, by applying a RPT with a three-level quality map. Yoneyama et al [70] developed a tricolour photoelastic system for the evaluation of isoclinic and isochromatic parameters using a single tricolour image; however, they reported that the methodology lacked repeatability.…”