Traditional optical methods for locating an edge are based on light intensi
ty variation with respect to a reference triggering level. Since the intens
ity Variation is subject to stray Light, the intensity Variation of the lig
ht source, and the triggering level variation, the exact position of the ed
ge cannot be determined. We describe a method for edge location that uses a
phase variation in a modified differential interferometer. The maximal poi
nt of the slope of the phase variation across an edge is determined exactly
by the relative position between the focused beam spot and the detected ed
ge if the initial intensity ratio of the two single-frequency interference
beams is kept unchanged. Therefore the phase variation can be used to locat
e the edge with high resolution and accuracy. To make practical use of the
phase variation, the second derivative of the phase was used as a monotonic
zero-crossing signal across the edge. The theoretical and the experimental
verification have been conducted in detail. The results of the experiment
show the feasibility of edge location when phase variation is used. The sch
eme is not affected by stray light and the intensity variation of the Light
source. (C) 2000 Optical Society of America OCIS codes: 260.3160, 180.3170
, 260.5430, 120.3180, 120.3940, 120.5060.