Introduction

In general, phase demodulation methods make use of an optical phase modulation for which a additional phase term ζ(x,y,t) is added to the useful phase . The fringe pattern expression can thus be rewritten as:

This phase modulation can be purely temporal; in this case, it will produce a uniform phase shift in the plane of the fringe pattern:

The modulation can also be purely spatial:

In this latter case, the modulation introduces a phase variation in the image plane which is called "spatial carrier".

The parameters u0, v0 , ƒ0 are the carrier frequencies of the modulation. To describe the demodulation process, it is judicious to focus on the parameters of the carrier frequencies, since the terms a(x,y,z,t) and bn(x,y,z,t) are generally independent from the modulation terms. A generic process of demodulation must then determine the phase   independently from a(x,y,z,t) and bn(x,y,z,t). Combining various techniques of phase extraction with one of the two types of modulation leads to a large variety of methods, each of them having their own algorithm, applicability, and performance.

Most of these methods assume a sinusoidal profile, which implies that the decomposition contains only one harmonic, but most of them can be applied to other profiles with minor modifications.