Coupling for capturing an displaying hologram systems for real-time digital holographic interferometry

Abstract

Conventional (analog) holographic interferometry (HI) has been used as a powerful technique in optical metrology since sixties of XX century. However, its practical applications have been constrained because of the cumbersome procedures required for holographic material development. Digital holography has brought significant simplifications due to digital capture of holograms and their further numerical reconstruction and manipulation of reconstructed phases and amplitudes. These features are the fundamentals of double exposure digital holographic interferometry which nowadays is used in such applications as industrial inspection, medical imaging, microscopy and metrology. However another very popular HI technique, namely real time holographic interferometry has not been demonstrated in its digital version. In this paper we propose the experimental-numerical method which allows for real-time DHI implementation. In the first stage a set of digital phase shifted holograms of an object in an initial condition is captured and the phase of an object wavefront in the hologram plane is calculated. This phase is used to address a spatial light modulator, which generates the initial object wavefront. This wavefront (after proper SLM calibration) propagates toward an object and interfere with an actual object wavefront giving real-time interference fringes. The procedure works correctly in the case when CCD camera and SLM LCOS pixel sizes are the same. Usually it is not the case. Therefore we had proposed two different methods which allow the overcome of this mismatch pixel problem. The first one compensates for lateral magnification and the second one is based on re-sampling of a captured phase. The methods are compared through numerical simulations and with experimental data. Finally, the implications of setting up the experiment with the object reference phase compensated by the two approaches are analyzed and the changes in an object are monitored in real time by DHI.