An algorithm for accurately extracting the local fringe direction is presented. The algorithm estimates, in the neighborhood of n × n points, the direction of the gradient that points normal to the local fringe direction. The performance of four different derivative kernels is also 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 moiré deflectogram of a spherical and a progressive addition ophthalmic lens, resulting in a map of the refractive power of these lenses. The results are compared with the data obtained with a commercial focimeter. This technique is useful for analyzing the fringe patterns where the fringe direction is variable and must be obtained locally.
© 1998 Optical Society of America
Original Manuscript: December 12, 1997
Revised Manuscript: March 19, 1998
Published: September 1, 1998
Héctor Canabal, J. Antonio Quiroga, and Eusebio Bernabeu, "Automatic processing in moiré deflectometry by local fringe direction calculation," Appl. Opt. 37, 5894-5901 (1998)