ADDITIVE-SUBTRACTIVE PHASE-MODULATED SPECKLE INTERFEROMETRY - FRINGE VISIBILITY UNDER PARTIAL DECORRELATION

Additive-subtractive phase modulated speckle interferometry (ASPM-SI) is a technique that minimizes the susceptibility of speckle methods to environmental noise while providing fringes of good visibility. The m ethod requires the acquisition of two consecutive video frames of addi tive-speckle images of the same two deformed states of an object at a rapid enough rate such that ambient noise is not a problem. The additi ve-speckle images as expected are of very poor visibility due to the p resence of the self-interference term. An interframe phase-modulation is introduced and the two additive-speckle images are digitally subtra cted to improve the fringe visibility by removing the self-interferenc e term. The ASPM-SI method works with in-plane and out-of-plane deform ation sensitive ESPI as well as with displacement-gradient sensitive s peckle-shearing interferometry. It is shown that the ASPM-SI scheme ha s higher visibility than conventional additive-SI and performs consist ently better than subtractive-SI schemes in the presence of partial in terframe speckle decorrelating optical noise. Furthermore, it is shown that the fringe visibility of the out-of-plane displacement sensitive interferometer which uses a protected reference beam separate from th e object beam can be made to be essentially unity even at complete int erframe decorrelation. Copyright (C) 1996 Elsevier Science Ltd.