Error sources in deferred heterodyne moire deflectometry: an analytical study

Effects of aperture size and shape of the photodetector and effects of the structure of the grating lines on the performance of deferred electronic heterodyne moire deflectometry are theoretically investigated. Deferred deflectometry is used for measurements of nonsteady phase objects for which it is difficult to complete the analysis of the field in real time. It has been shown that scanning of a moire fringe pattern parallel to an unshifted fringe yields periodical variations in the heterodyne phase and amplitude, which cause severe errors in the measurements. Theory indicates that these variations may be minimized by using a detector with square aperture of size rho/p = 1.0,2.0,3.0 ... or a circular detector with size rho/p =1.25,2.25,3.25... To minimize errors in deflection angle measurements, the fringe inclination due to the phase object should not exceed 15 degrees for a square detector and 13 degrees for a rho/p = 3.25 circular detector. Ronchi gratings with structure factor 0.5 less, similar q less, similar 0.7 are recommended.