Sven Krueger

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.

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.

Detection of faults in interferometric fringe patterns by optical wavelet filtering

In this paper the dynamic processing of interferometric fringe patterns obtained by real-time optical measurement methods like holographic interferometry is shown. A hologram of the tested component is superimposed with the hologram of the stressed component. The achieved fringe patterns vary according to the degree of stress applied. To evaluate these varying fringe patterns in real time, dynamic filtering is required. A hybrid opto-electronic sytem with a digital image processing and optical correlation module based on liquid-crystal spatial light modulators gives us the possibility to use dynamic filters and input images. In order to process interferometric fringes the adaptive wavelet transformation is applied.

Compact dynamic phase-modulating system for laser beam splitting and beam shaping

A 2D dynamic phase-modulating system (DPS) addressed by a digital signal at video frame rate can realize several optical transmission functions in the field of refractive as well as diffractive optics. Critical parameters are the space-bandwidth product, the phase shift and the light efficiency of the system. We demonstrate result of our current research activities on a compact system based on the SVGA spatial light modulator and a 635 nm laser diode. Points of discussion are limitations of pixilated diffractive optical systems, which cause boundaries for the application in the field of beam shaping and beam splitting. Furthermore, light efficiency and compactness of the systems architecture are main tasks. Examples are shown, which deal with the current state of beam shaping possibilities and which show the potential in the field of rapid-prototyping for optical transmission functions.

Investigation of spatial light modulators for their application in the reconstruction of coherent masks

The application of two dimensional addressable high resolution displays as spatial light modulators for the reconstruction of coherent masks is investigated. Based on their amplitude and/or phase modulating properties they are implementable as dynamic diffractive element and thus they are qualified for the efficient reconstruction of digital holograms. Parameters are deduced for a geometrically exact reconstruction of an object-wave field for its use as coherent mask in an interferometric measuring system. Results for different spatial light modulators are presented.