  Traditional phase-shifting interferometry technique cannot be used to measure
time-varying phase distributions. But single shot techniques could resolve the
problem. Many efforts have been made on the phase retrieval methods from a
single shot interferogram. In the paper, a simple and effective method is
presented without complex computation. The interference fringe is transferred
to a phase distribution with a look-up-table. And then it is divided into
different regions according to the parity of every pixel. The pixels in the
same region have the same parity, which determines the wrapped phase.
Additionally, the light spot displacement of a local wavefront is obtained to
solve the global sign ambiguity. The theoretical simulation results indicate
that the PV of wavefront error is 0.00054(lambda) and the rms is
0.000125(lambda), which is much better than the results from the Fast Fourier
Transformation method. We also use it in the experimentally measured
interferogram. Our algorithm has the advantages of simplicity, high precision
and effective for both open and closed interferometer fringes, which will be
valuable for real time monitoring the optical elements shape during their
processing.
