Gardner, Mark Charles; (2003) Phase object pattern recognition by optical correlation using a liquid crystal display for spatial phase modulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Optical correlation is demonstrated as a means of automatic inspection of 3D objects with depth features in the order of a wavelength of light. The distorted wave-front from the object is directly subjected to pattern recognition. Thus, use is made of the full complex light field from the object. Normally in optical correlation the phase part, and hence any useful information associated with it, is discarded through electronic capture and redisplay of the object. A commercial Liquid Crystal Display (LCD) is utilised as an optical Fourier plane filter medium, affording the high spatial resolution of 'purpose made' spatial light modulators (SLMs), but at lower cost and with simple drive signal requirements. While LCDs are designed as intensity modulators, it is known that in the spatial frequency domain, phase manipulation is more effective than intensity manipulation. The LCD's polariser and analyzer orientation is engineered to provide up to 229° of phase modulation (despite an LC layer thickness of less than 4.5μm) with insignificant coupled intensity modulation. A model developed in the UCL/Sira research group to show complex modulation as a function of polariser / analyzer configuration is experimentally evaluated. An improvement to the model showing the importance of including effective LC cell capacitance is also detailed. The LCD system's response to high spatial frequency images is investigated, and an algorithm, devised to process images prior to display, is described. While proving useful for visual display purposes, it was not found suitable for correlation filter display. A complete correlator is designed and constructed to accommodate small holographic optical elements at the input. The correlator is shown to discriminate successfully between real (not displayed) transparent objects, with 3D relief patterning for, it is believed, the first time. Experiments show good agreement with computer simulations despite the poor display of high spatial frequencies in the filter plane. While identically patterned input objects of varying depth yield sluggish discrimination, simulations were run which suggested adequate depth discrimination would be achievable using full complex Fourier plane filtering. Finally an automatic inspection system is proposed as a potential application.

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