Keywords: digital holographic interferometry, Fresnel transform algorithm, phase unwrapping algorithms.

Digital holographic interferometry is a useful technique for measuring the deformation or displacement of the surface of an object by recording at least two speckle patterns, one before and one after the object is deformed. Holographic interferometry utilizes interference between the speckled image of an object illuminated by a laser and a reference beam derived from the same laser. Any change in the shape of the object results in local changes in the intensity distribution in the holographic fringe pattern. This optical technique allows detecting deformation with sensitivity smaller than the wavelength of light. The holograms are generated directly on a charge coupled device (CCD) target and stored electronically. The speckle patterns can be taken quickly analyzed using holographic interferometry and Fourier transform method.

Two digital holograms are captured at different states of the object, on separate frames of the CCD camera. The first is made with the undeformed object in its original state and the second is made with a stress applied to the object. The holograms are generated directly on a charge coupled device (CCD) target and stored electronically. This leads to noisy recordings which makes the post-processing difficult. Intensity and phase maps of each hologram are calculated through digital reconstruction of the recorded holograms using the Fresnel transform algorithm.

The subtraction of these two phase maps results in a wrapped phase map. To obtain the continuous phase distribution, noise filtering and unwrapping algorithms are used to solve the discontinuities in the noisy phase map. From this unwrapped phase map, the displacement distribution of the surface during deformation can be calculated.

The paper is organized as follows. In Section 2, the principles of digital holographic interferometry are presented. In Section 3, we describe the numerical method used to calculate the displacement from phase map, the phase unwrapping algorithm and the image processing programs used to reduce speckle noise in the phase map. In Section 4, various numerical tests are presented, and concluding remarks are discussed in Section 5.

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