Optical correlation by filtering in the Fourier plane of an optical processor has recently received much attention, due to the advent of rapidly reconfigurable Fourier plane filters - spatial light modulators (SLMs). Many algorithms have been developed to generate filters, mostly binary in nature in line with the capabilities of current SLMs. This thesis reviews optical correlation techniques and SLM technology, before turning attention to the consequences of the pixellated structure of the Fourier plane filters generally necessary to enable arbitrary filter patterns to be written to an SLM. Conventionally the pixels are identical and regularly spaced on a rectangular grid. This is shown to lead to replication in the output plane of the correlator, and aliasing if the input exceeds dimensions related to the pixel spacing in the Fourier plane flter. If the input is also provided by an SLM, this requires the number of pixels across the filter SLM (its space-bandwidth product) to be greater than or equal to that of the input, even though the target itself may occupy only a small area of the total input scene, in order to prevent aliasing and misleading correlation results. If the replication and aliasing could be prevented, the space bandwidth product of the filter need only be matched to that of the target to be detected, rather than the entire input scene. The replicas arise through convolution with distinct spectral orders in the SLM's Fourier spectrum. A means of eliminating the spectral orders other than the zero order through randomisation of the pixel positions is presented and analysed, and implemented on transmissive matrix addressed SLMs custom designed and built for the work of this thesis.
|Heddle, Steven B.
|University of Edinburgh
|Electronic Thesis or Dissertation
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