In two-wavelength contouring by difference holographic interferometry, the test object is illuminated holographically by real images of the master object belonging to the two different wavelengths. In this process, however, the illuminating holograms have to reconstructed together. The illuminations belonging to the other wavelengths are unwanted, but fortunately they are reconstructed with some direction shift. Thus they can be filtered out by a proper aperture in the Fourier plane of a lens. Because the shift of the corresponding spot in the Fourier plane is relatively small, the similarly small filtering aperture leads to a significant reduction of the master object wave intensities in the recording steps. This in practice may result in a low-quality master hologram and consequent poor holographic illumination as well. To overcome this, the paper suggests two techniques to increase the light intensity of the filtered master object waves. First, a shape change of the aperture is proposed, and its extension to an aperture system. Second, a special optimization of the optical arrangement geometry is suggested to maximize the applicable aperture size in the Fourier plane. The two techniques provide an order of magnitude of intensity increase, even up to more than half of the nonfiltered value.
The development efforts in two-wavelength contouring methods of holographic interferometry are mainly along one line: The lateral shift between two images that are recorded at two different wavelengths but are both reconstructed at one of them is to be eliminated as automatically as possible. The present paper takes an opposite approach. A manual elimination process is intentionally maintained, and it is supported by precise visual monitoring. The advantage of this approach is threefold. First, the final accuracy of the elimination is visually displayed and controlled with interferometric precision. Second, exaggerated tilt compensation can result in the subtraction of an extra compensation fringe system from the real one to be measured. In this way, object positioning inaccuracies can be compensated and in addition the upper limit of the measuring range can be extended. In our experiments, a threefold measuring range extension was demonstrated. Third, the intensities of the two reconstructed images can be adjusted to each other if needed. Tilt compensation and intensity adjustment are especially advantageous features at the realization of the comparative two-wavelength contouring by difference holographic interferometry.
The paper summarizes main researches done at the Department of Physics for DISCO project - Distant Shape Control. The main contribution to the project is the comparative technique - difference holographic interferometry (DHI) - elaborated earlier and developed continuously at the Department. Applications of digital holography are presented which include direct and comparative displacement measurement with both digital and analogue reconstructions. A special attention is taken to the increasing practical upper measuring limit of both analogue and digital holographic interferometry that is well below its theoretical value and is determined by evaluation system used and by peculiarities of the actual interference pattern. Because of the space and time limitations the main ranges of the work are presented here; for further details the reader is kindly referred to papers of F. Gyimesi at al., ("Two wavelength contouring in difference holographic interferometry and DISCO") and J . Kornis at al. ("Comparative displacement measurement by digital holographic interferometry") in this volume.
The first two-wavelength contouring method of difference holographic interferometry (DiffHI) has been developed - as a more sensitive and more user-friendly alternative to the already existing two-refractive-index contouring method of DiffHI. It is built on "compensation-friendly" two-wavelength contouring method and on its tilt compensation version, developed specially for this purpose. Experimental verification is presented for the numerically correct functioning of the two-wavelength contouring method of DiffHI - although it is limited temporarily to mirror objects, only, because of intensity problems of the available laser. However, we hope to get the same results on diffuse objects, too - just in the near future. Two ideas seem to be applicable in the digital holographic version of the two-wavelength contouring method of DiffHI and subsequently in the Distant Shape Control project (DISCO), too. The first one is the application of the extra adjustment-monitoring mirror surface to control alignment accuracies in holographic illuminations and in remote duplication of the optical arrangement. The second one is the tilt compensation process which can correct the previous inaccuracies. The first one, on the other hand, can be used for the control of the accuracy of the calculated reference direction change, as well - at contouring with non-holographic illumination, too.
Difference holographic interferometry (DHI) is a holographic technique for direct optical comparative measurement of deformation, shape or refractive index distribution changes of two objects (master and test). At this procedure the test object is illuminated by reconstructed real images of the master object recorded previously. The holographic illumination is the spirit of the technique providing the unique possibility for direct optical comparison of the two objects. The differences in phases of
the wave fronts belonging to the different states of the two objects lead to an interferogram that displays the difference of the measured quantities directly. The accurate comparison requires that the phases of the master and test object interference patterns should be compared at their corresponding points. There are two typical sources of error: (1) false reversing the master reference beams and (2)imperfect positioning of the test object. In the present paper a short summary of DHI is given, the above mentioned error sources are analyzed, techniques for minimizing their effects are suggested and the allowed shift of interference phase is estimated.
Holographic interferometry has the advantage of high sensitivity but at practical loads, the fringe systems get soon too dense to be observed conveniently. One special way of overcoming this problem is provided by the comparative methods. In the present paper, comparisons of deformations up to the millimeter region will be reported in difference holographic interferometry and an extension possibility to electronic speckle pattern interferometry will be demonstrated, too. The most direct approach would be to magnify the image to the required great extent and then build up the complete fringe system from the observed tiny parts. A practical method will be suggested here which alleviates this cumbersome procedure. In addition, if the very dense fringes are already washed away by the speckles -- some integration along the fringes proves to be of real help.
Conventional holographic interferometric and speckle pattern interferometric techniques are tailored basically to compare the two states of the very same object, which may have optically rough surface or optically distorting transparent walls around. The direct comparison of the behavior of two different--but macroscopically quite similar--objects does not fit naturally in the process. At present, this type of extension can go three different ways. In holographic interferometry, one can mix the interferograms of the two objects at some sublevel of interferogram making and the moire effect of the individual fringe systems can be produced. In speckle pattern interferometry, the reference surface can be replaced by the other object itself and it serves as a live reference: changing according to the test object. Finally, although really first if time of birth is regarded, holographically recorded and reconstructed images of a master object can be used for illumination of test objects--in an otherwise conventional holographic or speckle interferometric arrangement. The present paper deals with this latter technique. Some new developments are reported in difference holographic interferometry and partly as a consequence of this--a really successful realization is introduced in holographically illuminated difference ESPI.
Difference holographic interferometry (DHI) can be used for direct optical comparison of two nominally identical objects (master and test) and gives the results of the comparison in the form of an interference pattern displaying the difference in deformation, shape, or refractive index distribution change of the two objects. The concept of the DHI is discussed and the main techniques and typical applications are presented. Finally, theory is briefly overviewed.
The usual holographic interferometric techniques provide the possibility for deformation, shape, and refractive index change measurements of objects with diffuse reflection properties but do not allow their comparison in a direct interferometric way. To compare two different but similar objects (e.g., master and test objects, or the original object and the same one after some alterations or wear) two interferograms are made and evaluated numerically. Difference holographic interferometry (DHI) makes the direct interferometric way possible, and, in addition, DHI does work even if the individual interferograms to be compared would be invisible, dense, or badly localized. The present paper summarizes the idea, theory, and experimental evidences of DHI and points out its useful application possibilities in prosthesis development too.