We present a new computational method of white light scanning
interferometry for 3-D surface mapping. This method accomplishes
the task of detecting the true peak of the interference fringe in two steps.
The first step is global search locating the envelope peak by fitting
sampled intensity data directly to a symmetric quadratic polynomial. The
second step is fine-tuning to precisely determine the fringe peak by compensating
for the phase shift on reflection using the absolute fringe order
identified by the envelope peak obtained in the first step. This two-step
method offers an efficient means of computation to provide a good measuring
accuracy with high noise immunity owing to its inherent reliance
on least squares principles.