AUTOMATIC PROCESSING IN MOIRE DEFLECTOMETRY BY LOCAL FRINGE DIRECTIONCALCULATION

An algorithm for accurately extracting the local fringe direction is p resented. The algorithm estimates, in the neighborhood of n x n points , the direction of the gradient that points normal to the local Fringe direction. The performance of four different derivative kernels is al so compared. Since this method is sensitive to noise and variations in background and amplitude, a preprocessing step is used to limit these error sources. The method has been applied to the moire deflectogram of a spherical and a progressive addition ophthalmic lens, resulting i n a map of the refractive power of these lenses. The results are compa red with the data obtained with a commercial focimeter. This technique is useful for analyzing the fringe patterns where the fringe directio n is variable and must be obtained locally. (C) 1998 Optical Society o f America.