Optimization of a shape measurement system based on spatial light modulators

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Abstract
We describe the results of an experimental study to optimize the measurement precision of a whole-field shape measurement system based on projected fringes produced by a spatial light modulator (SLM). Investigated issues include (1) the relative performance of different phase-shifting algorithms (4 frame, 7 frame, and 15 frame); (2) the relative performance of different temporal phase unwrapping algorithms (in particular, the reversed exponential and Fourier transform ranging methods); and (3) the effect of projector defocus. The last point is shown to be a crucial limiting factor in the performance of such systems. Focusing the projector within the measurement volume produces data with high systematic noise content due to the pixel structure of the SLM, whereas focusing the projector a long way beyond the measurement volume introduces significant random noise due to the reduced fringe modulation. At the optimum focal position, a measurement precision of better than 1 part in 20,000 of the field of view is achieved.

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