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Holographic interferornetry is a widely applied method for nondestructive, contactless testing of diffusely reflecting surfaces. For the investigation of very small objects it is necessary to use a magnifying imaging sy'stem for hologram recording. There are two principal setups of holographic microscopes': At first, the light wave front coming from the object surface is directly stored in a hologram, and the reconstructed image is observed through a usual microscope. At second, it is possible to store a magnified real image at or near by the intermediate image plane of a microscope and to observe the reconstructed image through an eyepiece. The first method is used, when a large angular aperture is required, but it suffers from a rather limited resolution. Though there are disturbing effects due to coherent noise (speckle effect), the second method provides resolution limits in the order of magnitude of conventional microscopes 2, and was chosen for our investigations. Two problems have to be taken into consider: At first, using the double-exposure technique for comparison and evaluation of two different object states, imaging errors of the magnifying system could be important. Following a proposal of Smith and Williams , these aberrations may be suppressed by conjugated reconstruction of the holographic interferogram. For this, the hologram is repositioned exactly and illuminated by the conjugated reference wave. In this way, deformation errors of the wave fronts due to the optical setup are cancelled, and the inuge nearly unaffected by lens aberrations can be observed at the original position of the object. At second, due to the coherence of the recording light, a holographic image is always superimposed by a subjective speckle pattern depending on the size and localion of the system aperture. Consequently, magnifying the image must result in magnifying the speckles. There have been some few publications concerning speckle reduction in holographic transmission microscopy, but the methods described do not apply to microscopic interferometry. So there must be found a cornpromise between speckle size and image magnification. To suppress speckle production by the reconstructing beam the hologram should be recorded as an image-plane type that can be reconstructed by incoherent light. A further important problem is the location and visibility of the interference fringe pattern. It is shown that the visibility of the fringe system is not influenced by conjugated reconstruction, but depends as well as its location on the size and the position of the aperture of the imaging system. Following these considerations, a holographic interference microscope was built up to record holographic double-exposure interferograms of surfaces of completely different objects: solder joints of surface mounted devices and germinating seeds.
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