An overdetermined phase-stepping strategy for the capture of high-quality photoelastic data

An overdetermined extension to the phase-stepping approach to digital photoelasticity is described in order to obtain high-quality photoelastic data for use in the examination of contact problems. A framework is adapted from earlier work by Ambirajan and Look and is applied to the generalization of existing phase-stepping strategies and the synthesis of new overdetermined variants on the method. The optimality of these strategies is demonstrated computationally, and the influence of errors in quarter-wave plate orientation and assumed isoclinic angle is explored. Sample experimental results are presented, which demonstrate the excellent performance of the system when used with narrow-bandwidth filters to approximate the use of monochromatic light. When the internal filters integral with the colour camera were used, the results were also of high quality up to a relative retardation of around 4.5 measured on the green channel, but measurements of higher fringe orders were progressively degraded owing to the broad bandwidths of the camera's internal filters.