Rupert Charles David Young

High-speed hybrid optical/digital correlator system

Abstract. A hybrid optical/digital configuration to implement a high- speed correlator is described. The input data is digitally Fourier trans-formed at video rates utilizing specialized digital signal processing chip sets and the computed spectral phase data passed to an electrically addressed phase modulating spatial light modulator (SLM). Successivetemplates, which are stored as angle multiplexed volume holograms ina photorefractive crystal, are reconstructed and optically correlated with the current input scene during an interval in which the SLM display is held stable. In this way, shift invariance, one of the major advantages of the correlation technique, is preserved. Of critical importance to thespeed of operation are the energy losses of the coherent wavefront through the system and the rate at which correlation plane data can beconverted to an electronic signal; both of these issues are consideredto estimate that at least a 3-kHz search rate is achievable from a 1000template memory with system components currently available.

Assessment of a Wiener filter synthetic discriminant function for optical correlation

Abstract The Wiener filter (WF), which has been used extensively for image restoration and signal processing, is employed for robust optical pattern recognition and classification. The WF is formulated to incorporate the in-class image (to be detected) and the out-of-class noise image (to be rejected) into a single step filter construction. It is compared with the classical matched filter (CMF) and phase-only filter (POF), demonstrating a superior discriminatory capability. The WF is incorporated into a synthetic discriminant function (SDF); correlation results show that it is tolerant to image distortion. With a 30 ° out of plane rotation between training set images, the WF SDF achieves a 100% success rate in discriminating one class of images from another. The CMF SDF and POF SDF fail to achieve 100% discrimination even at rotation increments of 15 °.

Design and simulation of a synthetic discriminant function filter for implementation in an updateable photorefractive correlator

The design of a highly selective synthetic discriminant function (SDF) filter is described for implementation in an up-dateable correlator based on a non-degenerate four wave mixing interaction in the photorefractive crystal Bismuth Silicon Oxide. The coefficients of the SDF cross-correlation matrix are determined for each member of the training set by modeling the high-pass filtering effect generated by a Fourier transform hologram written to the BSO with a relatively weak reference beam. Simulation show that the SDF so produced results in a correlation peak localization and discrimination ability comparable to that of the MACE filter. The filter is realizable as an all positive real-valued space domain image capable of direct input to the correlator via an SLM. Simulation results are presented for the orientation- independent, shift-invariant recognition of a test industrial component. The filters discrimination ability against another test component, not included in the filter training set, is also examined.

Tuneable edge enhancement filters for optical correlation

Abstract Difference of Gaussian (DOG) filters, a form of wavelet filter, are an extremely useful preprocessing tool for the enhancement of image edge data. The DOG filter is highly tuneable by control of the standard deviations of its constituent Gaussian distributions. These can be used in the frequency plane of optical correlators and implemented in the form of a static non-updateable filter. This article reports on a simple optical technique using photorefractive materials whereby a filter, updateable in real time and very similar to the DOG filter, is implemented by tuning the intensity of the hologram reference wave to give enhanced modulation in a selectable frequency band. This is called a tuneable photorefractive (TPR) filter. The results of the DOG and TPR filters are compared with those of the phase-only filter and classical matched spatial filter with respect to the criteria of signal-to-noise ratio, Horner efficiency and discrimination capability.

Wiener filter: synthetic discriminant function for target identification

The Wiener filter, which has been used extensively for image restoration and signal processing, is employed for robust optical pattern recognition and classification. The Wiener filter is formulated to incorporate the in-class image and the out-of-class noise image into a single step filter construction. It is compared with the classical matched filter (CMF) and phase-only filter (POF), demonstrating a superior discrimination capability. The Wiener filter is incorporated into a synthetic discriminant function (SDF); correlation results show that it is tolerant to image distortion. With a 30 degree out-of-plane rotation between training set images, the Wiener filter-SDF achieves a 100% success rate in discriminating one-class of images from another. The CMF-SDF and POF-SDF fail to achieve 100% discrimination even at rotation increments of 15 degrees.

Performance of tuneable photo-refractive filters

The use of the widely available photorefractive material, Bismuth Silicon Oxide (BSO), as a tunable holographic filter’ is an extremely effective optical technique with which the performance of optical correlators’ can be improved. This approach was physically implemented as part of a matched filter by Young and Chatwin,; it exploits selective erasure of spatial frequencies at the Fourier plane in the photorefractive material BSO. Experimental shows that the tuneable photo-refractive (TPR) filter not only gives sharp correlation peaks and good discrimination ability but is updateable and tuneable in real time via a variable attenuator, see Fig. 1. The spatial frequency weighting of the TPR correlation operation can be tuned by varying the amplitude of the plane wave reference beam U, via the attenuator shown in Fig. 1. The ratio between U, at zero spatial frequency, and U3 is defined as