Photo elastic analysis using a full-field spectral analyser

A new method for analysis of full-field isochromatic fringe patterns in birefringent specimens is proposed. It is based on the point-by point method of spectral contents analysis. The method uses a CCD camera as a 256 x 256 pixel diode array and narrow band filters for the isolation of eight wavelengths on the range of 400 to 750 nm. The system uses a polariscope set for plane and circularly polarized light. This results in full-field images of intensity normalised to zero load conditions for both polariscope setups. The normalised intensity values from the circular polariscope are used to represent the spectrum on light for each pixel and comparison with the theoretical spectrum using a minimisation procedure produces the absolute retardation and consequently the fringe order N. In this manner each of the 65,536 pixels in the 256 x 256 image is analysed to produce a full field map of calibrated isochromatic. Input of the retardation and the normalised intensity spectra from the plan polariscope yields the isoclinic parameter. The viability of the technique was shown through the analysis of a frozen stressed specimen and results showed good agreement with manual analysis.