Ferenc Gyímesi

Half-magnitude extensions of resolution and field of view in digital holography by scanning and magnification

Digital holography replaces the permanent recording material of analog holography with an electronic light sensitive matrix detector, but besides the many unique advantages, this brings serious limitations with it as well. The limited resolution of matrix detectors restricts the field of view, and their limited size restricts the resolution in the reconstructed holographic image. Scanning the larger aerial hologram (the interference light field of the object and reference waves in the hologram plane) with the small matrix detector or using magnification for the coarse matrix detector at the readout of the fine-structured aerial hologram, these are straightforward solutions but have been exploited only partially until now. We have systematically applied both of these approaches and have driven them to their present extremes, over half a magnitude in extensions.

Holographic illumination for comparative measurement

Abstract The displacement, the shape of an object, or the refractive index distribution change in the case of transparent objects can successfully be measured by either holographic or speckle techniques. However, the demand for comparative measurements of two objects, e.g. master and test by interferometry has both conceptual and practical importance. One way of doing that is using holographic illumination for the test object. Applying holographic technique in recording test interferograms provides high resolution of records. On the other hand, the holographically illuminated test object may also be inspected by electronic speckle pattern interferometry more easily but with different quality. The paper presents results of applications of hologram interferometric and electronic speckle correlation techniques in recording process of the difference patterns showing that both techniques can be effective in comparative measurements of displacements.

The accuracy of the phase difference formula in holographic interferometry

Abstract According to the wave optical description of holographic interferometry, the visibility and the phase of interference fringes are strongly influenced by the aperture of the observing optical instrument. An extra term appears in the phase difference formula compared to the usual one calculated by the method of corresponding points of the undeformed and the deformed object surfaces. Neglecting this extra term could cause (especially at large deformations) a significant error in fringe pattern analysis — even as much as half a period of fringes. In this paper the existence of the extra phase term and its dependence on the aperture of the observing system has been proven experimentally. We have demonstrated that the extra phase term is a periodic function of the ratio of the object displacement to the focal depth of the viewing system. Further on, it has been shown that the visibility is periodic too, although with decreasing maxima. It has the same period as the extra phase term.

Light intensity versus small aperture in two-wavelength contouring by analog difference holographic interferometry

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.

Error factors of comparison in difference holographic interferometry

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.

One-wavelength in-plane rotation analysis in electronic speckle pattern interferometry

The analysis of in-plane rigid-body rotations requires phase-shifting methods to determine the direction of rotation in conventional electronic speckle pattern interferometry (ESPI). The phase-shifting procedure makes the real-time measurements impossible. A quasi-real-time method is published recently, where the usual symmetric illumination is combined with a wavelength change before the second exposure. The present paper proposes a device sparing an alternative to this. The symmetric illumination is retained but the wavelength change is replaced by simple illumination direction changes.