Based on measuring the contrast of moiré fringes produced by digital subtraction, an alternative method for object positioning is presented. We use a model of fringe visibility mismatch due to defocusing and search for a global maximum as the best focusing criterion. Both laser and white light illumination are tested. Experimental results shows an accuracy of 0.5 mm. #1999 Society of Photo-Optical Instrumentation Engineers. S0091-3286(99)01008-9 Subject terms: positioning techniques; digital moiré; focusing. Paper 980150 received Apr. 16, 1998; revised manuscript received Oct. 27, 1998; accepted for publication Feb. 3, 1999.
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