Colin M. Cartwright

Image feature extraction with various wavelet functions in a photorefractive joint transform correlator

The wavelet transform has found a lot of uses in the field of optics. We present an experimental realization of employing variant wavelet filters into the object space of a photorefractive joint transform correlator to realize image feature extraction. The Haar’s wavelet, Roberts gradient and Mexican-hat wavelet are employed in the experiment. Because of its good optical properties, the photorefractive crystal Bi12SiO20 is used as the dynamic holographic medium in the Fourier plane. Both scene and reference have been detour-phase encoded in a liquid crystal television in the input plane. Computer simulations, experimental results and analysis are presented.

Optical implementation of a photorefractive joint transform correlator with wavelet filters

We have investigated a real-time photorefractive joint transform correlator (JTC) with a bank of wavelet filters. In the experimental set-up, a photorefractive crystal BSO has been used as the dynamic holographic medium and a bank of discrete wavelet functions, derived from a mother Mexican-hat wavelet have been selected to achieve band-pass operations in the JTC Fourier domain. With the proper dilation factors, shaper correlation peaks, higher discrimination ability and higher noise robustness have been obtained. Experimental results and analysis are presented.

Edge enhancement by use of moving gratings in a bismuth silicon oxide crystal and its application to optical correlation

The technique of moving gratings in a photorefractive crystal is applied to the edge enhancement of objects and edge-enhanced optical correlation. The nonlinear dependence of the optimum fringe velocity on the fringe modulation and the variation of the enhancement of the diffraction efficiency with fringe modulation at a fixed fringe velocity appropriate to high fringe modulations are experimentally investigated. It is shown that the diffraction at high fringe modulations, which corresponds to the high-spatial-frequency components of the Fourier spectrum, is enhanced by a factor of approximately 3.7, whereas the diffraction at low fringe modulations is suppressed by a factor of 0.6. The proposed technique has the advantages of real-time enhancement, arbitrary selection of the spatial frequency to be enhanced, and improved stability of the output. Experimental results of the edge enhancement of objects and edge-enhanced correlation are presented.

Real-time edge-enhanced optical correlation with a cerium-doped potassium sodium strontium barium niobate photorefractive crystal

The nonlinear diffraction property of a volume grating written by two-wave mixing in a cerium-doped potassium sodium strontium barium niobate crystal is applied in a coherent image processing system to obtain real-time image edge enhancement as well as edge-enhanced optical correlation. The theoretical analysis of the correlator is given, and the experimental results of optical correlation are presented, which are compared with the computer-simulated results.

Four-wave mixing and edge-enhanced optical correlation in a Ce:KNSBN crystal

In the case of degenerate four-wave mixing in a cerium-doped potassium sodium strontium barium niobate (Ce:KNSBN) crystal, it is found that the transmission grating is dominant when the incident beams are extraordinarily polarized, the crystal response is more rapid, and the conjugate beam is more intense. Furthermore, the variation of the conjugate beam intensity forms a loop as the fringe modulation of the transmission grating varies. Based on this observation, we have implemented edge enhancement of an image and edge-enhanced optical correlation via four-wave mixing in a Ce:KNSBN crystal without the requirement of reversal of the signal-pump-beam intensity ratio.

Implementation of a photorefractive binary joint transform correlator

The use of photorefractive materials such as Bi12SiO20 as dynamic holographic media is becoming an interesting alternative to that of the current liquid-crystal-based modulators in real-time optical image processing. We present an experimental realization of optical correlation for pattern recognition by means of a photorefractive joint transform correlator. The correlator operates with a liquid-crystal television as the input and a photorefractive crystal at the recording plane. We consider two possible ways of registering the Fourier plane information: conventional detection of the joint power spectrum, and utilization of only phase information at the Fourier plane by suitable preprocessing of the scene and the reference at the object plane. We compare the latter case with the performance of a binary joint transform correlator. Analysis, simulations, and experimental results are presented.

Effects of optical bias on moving gratings in bismuth silicon oxide at large fringe modulation

Optical bias has been applied in the formation of moving gratings in bismuth silicon oxide at large fringe modulations. It is shown that optical bias is an effective method of overcoming the problems associated with the sudden drop in the optimum fringe velocity when the fringe modulation is close to unity. It is experimentally found that within a certain range of optical bias the absolute diffraction efficiency can be higher than that without optical bias, which is not the case when a stationary grating is used.

Optimization of the projection screen in a display system for minimal access surgery

Background: The operative image for minimal access surgery currently is displayed on a monitor located outside the sterile field. It is ergonomically advantageous to locate the image adjacent to the surgeon’s hands by projection onto a sterile screen, but there has been no research into the optimal screen material. Methods: Several screen materials were compared for image resolution, brightness, variation of brightness with viewing angle, and image artifact. Results: Glossy materials perform poorly, whereas finely grained surfaces improve image clarity. Excessive roughness and incomplete opacity limit the image resolution. Conventional screen fabrics are unsuitable in this application. Ambient lighting and projector brightness affect image contrast, but a correct choice of material can address this. Practical issues such as moisture absorbency and ease of sterilization are considered. Conclusions: Potential screen materials were rejected because of excessive glare, poor resolution, and image artifact. Finely textured surfaces (e.g., polystyrene sheeting) provide an acceptable screen.

Hybrid diffractive-refractive ultra-wide-angle eyepieces

Summary Two kinds of hybrid diffractive-refractive ultra-wide-angle (70°) eyepieces are designed which are with considerably reduced apparent size and weight and with comparable or improved aberrations as compared with the conventional refractive countertype. The designing strategy including the consideration and the steps of the optimization, the most important specification of the diffractive efficiency, and the manufacture issues is presented. The structures and the performance merits of the designed systems are given in a comparison of the traditional Scidmore eyepiece. The designed eyepieces have potential applications in various kinds of modern head-mounted displays.

Rotationally invariant pattern recognition in a photorefractive joint transform correlator using circular harmonic filters and the wavelet transform

A photorefractive joint transform correlator using circular harmonic filters and the wavelet transform for high discriminant and rotationally invariant pattern recognition is reported. In the experimental implementation, a photorefractive crystal, BSO, is used as the dynamic holographic medium and a bank of wavelet functions, derived from a mother Mexican-hat wavelet have been selected to achieve a band-pass operation in the Fourier domain. The zero-order and second-order circular harmonic components are used in the investigation. It is shown that, with the proper dilation factors, there is a trade-off between peak correlation intensity and signal-to-noise ratio (SNR) with the wavelet transform filters. With the second-order circular harmonic, an improvement of 63% in the SNR is achieved with only a 4% loss in the peak correlation intensity.