Guenther K.G. Wernicke

Liquid crystal display as spatial light modulator for diffractive optical elements and the reconstruction of digital holograms

Liquid crystal displays, originally designed and fabricated for projection systems, are often used as spatial light modulators in optical correlators or in fringe projection systems. Operating in a linearized phase mode, they can achieve a performance to be applicable as diffractive elements. An adapted driver electronics and adapting measurements of the phase modulation behavior can lead to a dynamic phase modulating system with an almost linear modulation and a maximum phase shift of 2(pi) . The electronic system can directly address the graphics card signal or can picture various video standards. So, computer generated holograms can be addressed at video frame rates. The limiting parameters of the performance are mostly due to physical boundary conditions, such as pixel number and size, response time, transmission etc.. We can assume that the fast growing micro-structuring technology will serve us soon with displays of higher resolutions and efficiency. Beam shaping elements, two-dimensional holograms and the reconstruction of digital holograms will be shown. The latter opens new possibilities for non-destructive testing devices especially in the field of holographic interferometry. We will show results of different systems and derive boundary conditions for applications in holographic reconstruction of coherent masks.

Fault detection and feature analysis in interferometric fringe patterns by the application of wavelet filters in convolution processors

The detection and classification of faults is a major task for optical nondestructive testing in industrial quality control. Interferometric fringes, obtained by real-time optical measurement methods, contain a large amount of image data with information about possible defect features. This mass of data must be reduced for further evaluation. One possible way is the filtering of these images applying the adaptive wavelet transform, which has been proved to be a capable tool in the detection of structures with definite spatial resolution. In this paper we show the extraction and classification of disturbances in interferometric fringe patterns, the application of several wavelet functions with different parameters for the detection of faults, and the combination of wavelet filters for fault classification. Examples for fringe patterns of known and varying fault parameters are processed showing the trend of the extracted features in order to draw conclusions concerning the relation between the feature, the filter parameter, and the fault attributes. Real-time processing was achieved by importing video sequences in a hybrid opto-electronic system with digital image processing and an optical correlation module. The optical correlator system is based on liquid-crystal spatial light modulators, which are addressed with image and filter data. Results of digital simulation and optical realization are compared.

Liquid crystal display as dynamic diffractive element

Liquid crystal panels, originally designed and fabricated for projection systems, are used as spatial light modulator in optical correlators or in fringe projection systems. An adapted driver electronics and measurements of the phase modulation behavior can lead to a dynamic phase modulating system with an almost linear modulation and a maximum phase shift of 2(pi) . The electronics of our Sony-LCD based system can directly address the graphics card signal or can picture various video standards. So, computer generated holograms can be addressed at video frame rates. We demonstrate examples of technical beam splitters, 2D holograms, reconstruction of digital holograms and animation of diffraction patterns. Here we will state to problems of light efficiency, intensity and wavelength dependent modulation. Furthermore, we will show first results of an FLC based system and derive boundary conditions for applications in holographic projection and micro- structuring.

Spatial light modulator system for application as a dynamic diffractive element and in optical image processing

The importance of spatial light modulators (SLMs) in optical systems is increasing. This paper presents our work on an SLM system based on Sony LCD. We show the optimization of the LCD modulation behavior concerning the application as dynamical diffractive element, and the implementation of images and filter functions in optical correlators and pattern projection systems. Measurements of the coupled amplitude and phase modulation and the planarity of the display at different wavelengths lead to correction functions and several optimized operation modes, e.g. gray scale or binary amplitude modulation. The optimization in the amplitude mode leads either to high contrast images of reduced gray levels or to a mostly linearized full gray scale mode. A similar way leads to optimized modes for the phase modulation and to a realization of a total phase shift of 2(pi) . The increase of the processing performance of phase-only filters in optical correlators is presented. Furthermore, diffraction efficiency (DE) measurements of Fourier holograms addressed to the SLM prove the optimization process, compared to the non-optimized and the theoretical DE-values. So, estimations can be made concerning the application of the SLM as dynamical diffractive element.

Application of wavelet filters for feature extraction in interferometric fringe patterns

The fast and reliable localization and classification of fault indicating fringe patterns in interferometric images is a major task in holographic non-destructive testing. For the purpose of feature extraction from gray value images, wavelet transformation has proved to be a suitable tool. In contrast to the Fourier transformation the local feature information will be preserved and furthermore the applied transforming wavelet can be adapted--under certain constraints--to the given problem.

Red sensitive dichromated gelatine: investigations on the influence of some parameters on diffraction efficiency

Dichromated gelatin with methylene blue is red sensitive. Tetramethylguanidine intensifies this red sensitivity. Holographic plates covered with such an emulsion were used to investigate the influence of some procedure and development parameters on the structure and diffraction efficiency of holographic gratings. The ratios and concentrations of prehardener, fixer, and sensitizing solution, as well as the pH dependence and storage time, have been investigated. The best results were obtained by a six-step development process. The optimized process needs 3 h from the first step to the final hologram grating. By adapting all investigated parameters, more than 70% diffraction efficiency was achieved. Volume effects in layers of different thickness have been investigated.

Holographic interferometric microscope with conjugate reconstruction for measurement of three-dimensional displacements

A holographic microscope with conjugate reconstruction for the interferometric determination of 3-D displacement vectors is described. Three double-exposed holograms of different illumination directions recorded on one hologram plate are reconstructed conjugately, and phase-stepping techniques are used to evaluate the interferograms. Only by conjugate reconstruction it is possible to obtain a perfectly optimized interferometer for the static evaluation method. Using both phase-step and filtering techniques, the evaluation of interferograms greatly disturbed by speckle noise can be performed successfully.

Diffractive elements for generating microscale laser beam patterns: a Y2K problem

Lasers are widely used in industrial fabrication for engraving, cutting and many other purposes. However, material processing at very small scales is still a matter of concern. Advances in diffractive optics could provide for laser systems that could be used for engraving or cutting of micro-scale patterns at high speeds. In our paper we focus on the design of diffractive elements which can be used for this special application. It is a common desire in material processing to apply discrete as well as continuous beam patterns. Especially, the latter case is difficult to handle as typical micro-scale patterns are characterized by bad band-limitation properties, and as speckles can easily occur in beam patterns. It is shown in this paper that a standard iterative design method usually fails to obtain diffractive elements that generate diffraction patterns with acceptable quality. Insights gained from an analysis of the design problems are used to optimize the iterative design method. We demonstrate applicability and success of our approach by the design of diffractive phase elements that generate a discrete and a continuous Y2K pattern.

Optical processing for the detection of faults in interferometric patterns

The detection and classification of faults is a major task for optical nondestructive testing in industrial quality control. Interferometric fringes, obtained by real-time optical measurement methods, contain a large amount of image data with information about possible defect features. This mass of data must be reduced for further evaluation. One possible way is the filtering of these images applying the adaptive wavelet transform. The wavelet transform has been proved to be a capable tool in the detection of structures with definite spatial resolution. In this paper it is shown the extraction and classification of disturbances in interferometric fringe patterns, the application of several wavelet functions with different parameters for the detection of faults, and the combination of wavelet filters for fault classification. Furthermore the implementation of complex valued wavelet filters and correlation filters is shown. We will present an algorithm to classify interferometric fringe patterns. In order to achieve real-time processing a hybrid opto-electronic system with a digital image processing and an optical correlation module is favored. The calculated wavelet filters are implemented into the optical correlator system that is based on liquid-crystal spatial light modulators. So, all discussed items were verified experimentally in the optical setup.

Analysis of interferometric fringe patterns by optical wavelet transform

For the application of interferometric methods in industrial quality control and particularly in non-destructive material testing an extensive analysis of the obtained image data is needed. In the field of optical image processing wavelet transformation has been proved to be a capable tool in the detection of structures with definite spatial resolution. The selection of the analyzing wavelet function and the variation of parameters lead to a wide range of selective wavelet filters, which are able to perform detection and classification of structures. The presented paper demonstrates numerical simulations and their optical realization in an SLM- based correlator. To locate defect patterns of different classes in interferometric fringe patterns, several wavelet functions, which are optically realizable by a transmission distribution, were utilized. The optical experiments indicate a good accordance with the computer simulations.