Colour Adaptation in Three Fringe Photoelasticity K.R. Madhu & R.G.R. Prasath & K. Ramesh Received: 27 May 2006 / Accepted: 26 September 2006 / Published online: 23 January 2007 # Society for Experimental Mechanics 2007 Abstract Three fringe photoelasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field by suitably comparing the colour with a calibration specimen. The fringe order evaluation can be erroneous when the materials for the calibration specimen and the application specimen are different. This is because of the colour variation between the two materials. This is conventionally handled by preparing individual calibration tables for each application. A new methodology to tune the calibration table obtained for a single material to accom- modate the tint variation in TFP is proposed for the use of different specimen materials. Discontinuities in fringe order variation are smoothed using the refined TFP (RTFP) procedure. The elegance of the new methodology for solving a multi-material system is bought out by solving the problem of a bi-material Brazilian disc. The results obtained are compared with the phase shifting technique. Keywords Digital photoelasticity . Refined TFP . Colour adaptation Introduction Use of a colour code to identify fringe gradient direction and to assign approximately the total fringe order has become an accepted method in conventional photoelastic- ity. Three fringe photoelasticity (TFP) is an extension of this technique in digital domain. The total fringe order at a point of interest in the actual model is then established by comparing the R, G and B values at the point of interest with that of the calibration table. The colours tend to merge beyond fringe order three, and hence the technique is termed as three-fringe photoelasticity (TFP) [1]. Since, R, G and B values of a colour image are used, it is also known as RGB photoelasticity (RGBP) [2]. Three fringe photo- elasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field. This is very helpful in situations where one attempts to analyse time varying phenomena. TFP also comes in handy for situations where the number of fringes available for data interpretation is small such as in stress frozen slices. Though various noise reduction strategies have been developed for use in conjunction with TFP for improving the results [1–3], there has been little attention to the influence of colour difference between the application specimen and the calibration specimen. Even if the material used is the same, due to heat treatment (stress freezing, annealing etc.) of the specimen, a colour mismatch may occur. This is studied in this paper with specimens made of different materials and a new technique to solve the erroneous identification of fringe order in such cases is proposed. The result thus obtained is further improved by a refined TFP (RTFP) technique. For the sake of complete- ness the methodology of TFP and RTFP are briefly presented. Methodology of TFP In TFP/RGBP one has to compare the RGB values of a point with the calibrated RGB values assigned with known fringe orders so as to determine the fringe order at a given data point. Ideally, RGB values have to be unique for any fringe order. However, in view of experimental difficulties, Experimental Mechanics (2007) 47: 271–276 DOI 10.1007/s11340-006-9012-x K.R. Madhu : R.G.R. Prasath : K. Ramesh (*, SEM member) Indian Institute of Technology Madras, Chennai 600 036, India e-mail: kramesh@iitm.ac.in