Temporal demodulation of fringe patterns with sensitivity change

There are many phase measuring experimental setups in which the rate of temporal phase variation cannot be easily determined. In the case of phase stepping techniques, asynchronous phase measuring techniques were developed to solve this problem. However, there are situations for which the standard asynchronous techniques are not appropriated, like experiments with a sensitivity variation in the phase. In this work, we present an asynchronous demodulation technique for which the only requirement is the monotonicity of the phase in time. The proposed method is based in the computation of the quadrature sign (QS) of the fringe pattern and afterwards the demodulation is performed by a simple arccos calculation, that thanks to the QS extends its range from half fringe to a modulo 2π calculation. The presented demodulation method is asynchronous, direct, fast and can be applied to a general n-dimensional case. We have applied the proposed method to a load stepping experimental fringe pattern obtaining good results.