Zoltan Fuzessy

Difference displacement measurement using digital holography

A new digital holography method for the measurement of difference of displacements is presented, which is based on a pointwise non-linear mathematical operation in the image plane. The method can measure sum of displacements too. Basic principles and experimental verification are described. Some experiments concerning the measuring range of the method were performed. A new approach, named artificial phase-shifting, producing phase-shifted interferograms without use of an actual phase-shifting device in the experiment, is also presented.

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.

Difference displacement measurement by digital holography by use of simulated wave fronts.

A concept called fringe compensation was first presented in phase-shifting electronic speckle-pattern interferometry. We apply a similar principle to digital holographic interferometry; here the phase of a wave front is known and can be manipulated. The basic mathematical formulation of fringe compensation and some experimental results are shown with relatively large, simple rigid-body rotation and circular membrane deformation.

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.

Difference displacement measurement using digital holograms as coherent masks

The rapid development of spatial light modulators in the past few years opened an exciting new area in coherent optical metrology. Commercially available liquid crystal spatial light modulators (LC SLMs) are capable to optically reconstruct digital holograms in quite good quality, so the reconstructed real image of an object can be used as a coherent illuminating mask in optical measurement methods like digital holography (DH) or Electronic Speckle Pattern Interferometry (ESPI). In our work we present experimental results of measuring the difference and sum of two displacements of an object pair (master and test object) using these two techniques. We describe the measurement setups in a DH and an ESPI arrangement, which are capable to project the real image of the master object -- using its previously recorded digital holograms in the SLM device -- onto the test object. If two digital holograms, recorded before and after the deformation of the master object, are used to illuminate the test object in its initial and deformed state, four images can be recorded either in the DH setup or in the ESPI setup. Using these four-four images, the contour fringes of the difference and sum of the master and test object displacements can be calculated. In the case of DH, these images are digital holograms, which are subject to numerical reconstruction, and in the case of ESPI the four images are plain speckled images, which can be used to obtain ESPI fringes (correlograms). As a result, we present several fringe images of our object pair made with these two methods.

Adaptive systems in speckle-pattern interferometry

The concept of adaptivity in television holography is discussed, and various realizations of adaptivity are presented. In one possible variation, functions of the components of the optical arrangement may be changed to adapt them to measurement conditions. An additional peculiarity of the technique is that reference waves are produced by holographically reconstructed virtual images. A method, believed to be new, is introduced for synthesizing the phase front of the master object beam that is produced by a simple holographic optical element and is used as a smooth or a speckled reference beam in the electronic speckle-pattern interferometer. An adaptive interferometer is presented as a measuring device for various measuring tasks. Selected applications are shown, demonstrating different aspects of adaptivity.

Difference holographic interferometry (DHI): single reference beam technique

Abstract Difference holographic interferometry can be used for a direct optical comparison of two nominally identical objects (master and test) and to display the result of the comparison in the form of an interference pattern related to the difference in deformation, shape or refractive index change of the two objects. Reconstructed real images of the master object in its initial and final states are used for holographic illumination of the test object in both its states. The master interferogram can be recorded by two reference beams and the real images can be reconstructed separately. On the other hand, if the master interferogram is recorded by a single reference beam, the wavefronts of the initial and final states of the master object can only be reconstructed simultaneously. The theory of this new technique is discussed and its application for differential measurements of refractive index changes is presented.

Measurement of 3-D displacement: sandwich holography and regulated path length interferometry

An analysis of sandwich and regulated path length interferometries is used to determine 3-D displacement. The combination of the two techniques provides a reliable means for unambiguous determination of displacement. To demonstrate the efficiency of the combination displacement measurement is made of object surface points on a steel beam. The accuracy of sandwich hologram interferometry is replicable for a series of measurements and does not fall below the value needed in industrial measurements.

Analogue and Digital Holography: Absolute and Comparative Measurements and Measuring Range Extension (Analoge und Digitale Holografie: Absolute und vergleichende Messungen sowie Messbereichserweiterungen)

Summary In the paper a brief survey of research at the Department is given that had a twofold aim: to support the project DISCO in indirect and direct ways as well. In this respect the physical state of the art of the analogue basis of DISCO is discussed: principle of the comparative technique named difference holographic interferometry (DHI) is analysed and applications for comparative deformation and shape control in analogue holography are presented. Peculiarities of holographic illumination as well as requirements for repositioning of the master and test objects at comparative measurements are treated. Investigations of the measuring range extension in holographic interferometry are described. Detailed analysis is devoted to the puzzle-read-out extension technique. Finally, application of the principles of DHI in comparative deformation control by digital holography and the developed portable laboratory device for comparative deformation control are presented.

Comparative displacement measurement by digital holographic interferometry

Digital holography is a powerful tool in NDT. Different measuring methods have been developed to perform more flexible measurements and to alleviate the drawbacks of this technique. The rapid development of spatial light modulators in the past few years opened an exciting new area in coherent optical metrology. Commercially available liquid crystal spatial light modulators (SLMs) are capable to optically reconstruct digital holograms in good quality, so the reconstructed real image of an object can be used as a coherent illuminating mask in optical measurement methods like digital holography. Combination of digital holography and TV holography (ESPI) is also possible. In the present work five methods of digital holography are investigated which are able to implement comparative measurement. Both the experimental arrangements and measuring results are presented.