The development of spatial light modulators enables the application of active holographic optical elements in electronic speckle pattern interferometry. In our work adaptive comparative measurement is done, where an optically reconstructed image of a recorded or simulated single or double exposure hologram is used for holographic illumination of another object. In this paper, we present experimental results of measuring the difference of two deformations using this technique. The displacement difference can also be obtained numerically, if the wavefront used as a coherent illuminating mask does not belong to an existing object. This type of interferometer can easily adapt to the change of measuring conditions.
Sequentially recorded intensity patterns reflected from a laser illuminated diffuse object can be used to reconstruct the
complex amplitude of the scattered beam. Several iterative phase retrieval algorithms are known in the literature to
obtain the initially unknown phase from these longitudinally displaced intensity patterns. When two sequences are
recorded in two states of the object in similar experimental setups, as is digital holographic interferometry - but omitting
the reference wave-, displacement, deformation, or shape measurement can be done. Although the object-beam-only
setup is not so sensitive to vibrations, several other factors influence the success of the measurements, the position or
angle alignment of the linear stage. The results of initial simulations and measurements of displacement are presented,
and the convergence of the phase retrieval is examined.
As in the classical holography, a major issue in digital holography is the enhancement of the resolution of the digital holograms. It means not only the enhancement of the image quality, but the extension of the upper measuring range too. One natural way can be the application of higher resolution recording devices. Unfortunately the price of a doubled resolution camera is approximately fourfold.
In the presentation a different way of the resolution enhancement is shown.
The resolution enhancement (building the super image) is based on the building of well sampled so called super images from a set of under sampled but dithered input images. Such methods are originated from the drizzle method, and from the Fourier spectrum combination method.
Using these methods not only the resolution of the digital hologram can be increased, but the object distance also can be dramatically shortened.
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.
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 (SLM's) 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.
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 SLM's) 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.
Proc. SPIE. 5144, Optical Measurement Systems for Industrial Inspection III
KEYWORDS: Digital holography, Holograms, 3D image reconstruction, Charge-coupled devices, Spherical lenses, Beam splitters, Digital recording, Monte Carlo methods, Numerical analysis, Computer simulations
Digital holography gives new capabilities to the measurements of size, position, shape or velocity of different objects. In general in-line arrangement is used with a CCD device recording the interference pattern and the reconstruction is performed by numerical methods in computer. Some new ideas were used to improve the process of exposure and calculation techniques. The phase-shift technique is widely used in digital holography. Now it is proposed to use the two-reference-beams arrangement, which can replace the phase-shift methods at different cases. Three holograms -- the interference pattern of the object and the two reference waves and the three intensity distribution (of the beams, separately) are recorded and the phase evaluation can be done numerically. The advantage of FFT-algorithm is also applied reducing the computer running time in reconstruction of the image. We have developed another computational method for the reconstruction using Monte-Carlo simulation.
The record of the interference pattern of the object wave and the reference wave is done by the application of high resolution photoplates in holography. After developing the photoplate the reconstruction of the object wave can be realized illuminating the hologram by the reference beam. In digital holography the photographic process is eliminated for the interference patterns are recorded by a CCD camera and the reconstruction can be done virtually using a computer. In general in-line reference beam is used coding the phases for the low resolution of the available CCD devices. The phase-shift technique (or application of two reference beams) can be applied to recover the phase and the amplitude of the image wve at the plane of the CCD matrix. The FFT algorithm is widely used for the reconstruction. Now it is proposed to apply the Monte-Carlo method simulating the diffracted wave to get the intensity distribution at the image plane. The application of Monte-Carlo simulation has a drawback, that is it can be slower than FFT, but its advantages can be significant, namely the intensity pattern of the diffracted wave can be determined along an inclined plane, just a part of the object can be examined if it is required and additionally there are no difficulties in extending it to phase holograms and 3-D objects.