Automated photoelasticity has developed as a topic in the last ten to fifteen years during which time major advances have been made, partly as a result of the availability of new technology in computing and image processing. For a review of the subject see Ajovalasit et al  or Patterson  for work prior to 1988. The major techniques of automated photoelasticity are described in the sections below:
Spectral Contents Analysis:
Earlier developments were by Redner  and by Sanford and Igenyar . Essentially, for the point of interest the light intensity is collected over a range of wavelengths to form a spectrum. A theoretical model of the spectral contents of the point in a fringe pattern is fitted to the experimental data using the fringe order as the fitting parameter. The maximum fringe order that can be recognised is approximately equal to the number of wavelengths at which intensity information is collected [5,6]. Hence an RGB camera can be used to obtain fringe orders up to about three . No information about isoclinic angle is available. Recent work has produced significantly faster algorithms which do not need any a priori knowledge of the range of fringe order being measured . The University of Sheffield has implemented the technology in a number of novel instruments.
Fourier analysis requires the collection of a large number of images, typical 90 for isoclinic map determination. The methods used for determining isoclinic and isochromatic maps are different and have been developed by Morimoto et al  and Quan et al  respectively. The grey-field polariscope developed by Lesniak et al  is not readily classified and falls between Fourier processing and phase-stepping. These techniques produce periodic distributions of isoclinic and isochromatic fringe orders. The latter maps usually require unwrapping. See Ramesh  for more details on digital photoelasticity and associated issues.
Generally monochromatic light is used in phase-stepping to produce maps of isoclinic angle and isochromatic fringe order from a theoretical minimum of three images. In practice an over-deterministic system is preferable and a recent review  found that the six step algorithm pioneered by Wang and Patterson  gave the best results. The technique produces periodic maps of isoclinic and isochromatic fringe order, and the latter normally require unwrapping. Various algorithms for demodulating the isoclinic and isochromatics and unwrapping them have been developed. The disadvantage of phase-stepping is that, whilst multiple fringes can be dealt with by phase unwrapping, the fringe order must provided at a pair of points in order to fix the absolute value of the fringe order map. In transmission photoelasticity this has been achieved by using a small probe based on spectral contents analysis3 and by using white light with a colour CCD camera .
Early efforts to automate photoelastic analysis involved collection of monochromatic images followed by some form of fringe thinning, with the operator required to identify all the fringes and interpolation used to obtain values between the locations of fringes. The grey field polariscope falls across the boundaries between Fourier analysis and phase-stepping. Fourier analysis requires large numbers of images and so is often impractical. Spectral analysis can provide the absolute fringe order but no information about isoclinic angle. Thus its use in isolation produces significant drawbacks. The maximum fringe order that can be recognised is approximately equal to the number of wavelengths at which intensity information is collected. Generally monochromatic light is used in phase-stepping to produce maps of isoclinic angle and isochromatic fringe order from a theoretical minimum of three images. The disadvantage of phase-stepping is that, whilst multiple fringes can be dealt with by phase unwrapping, the fringe order must provided at a pair of points in order to fix the absolute value of the fringe order map.