A multi-period fringe-projection interferometry with back-propagation method is described to measure surface profiles
of a glass plate. Phase distribution of the multi-period fringe formed by interfering the two calumniated laser beams is
utilized to determine the position of the two surfaces of the glass plate. The multiple optical fields of the different fringe
periods on the two surfaces, which are obtained from the sinusoidal phase-modulated interference signals, are back-propagated
to a position where all of the phases of the multiple optical fields become zero. At the same time, the
amplitude of the sum of the multiple back-propagated fields becomes maximum. The distances of the back-propagation
provide the positions of the two surfaces of the glass plate. In the experiment a glass plates of 2mm-thickness is
measured with a precision of 2.3&mgr;m.