Optical Correlation Recognition Based On LCOS

Due to the characteristics of accurate positioning and high signal-to-noise ratio,Vander Lugt correlator is widely used in optical pattern recognition. Among so many typesof Spatial Light Modulator, Liquid Crystal On Silicon(LCOS) succeeds in serving as amatched filer for the recognition of the letters without distortion owning to its large openrate, uniform amplitude and continuous phase modulation characteristics. That LCOS isapplied in distortions-invariant recognition is scarcely reported. In this paper, a modifiedreal-time Vander Lugt correlator based on the LCOS is presented by means of applyingoptimization algorithms to the design of the filter so that the correlator can be invariant tothe distortions while maintaining good recognition performance.Firstly, the development and progress home and abroad of optical pattern recognitionare presented. A review is given on Vander Lugt correlator, the measures for filter andinfluences of distortions. Then the realization methods of distortions-invariant areresearched. On the basis of synthetic discrimination function, a modified filter is putforward so that the performance of the correlator is more superior. Allowing for the phasemodulation characteristics of LCOS, the phase-only filter is calculated by Euclideandistance, Successive-forcing, Relaxation algorithm respectively. Meanwhile this paper hasmade appropriate modification to the algorithms, which offers theoretical instruction forthe LCOS correlator.The structure and wave-front modulation principle of LCOS are presented in detail.When the polarization direction of the light is consistent with the optical axis of the liquidcrystal molecules, LCOS is in pure phase mode. This paper utilizes the fringe shift methodand surface shape method measuring modulation characteristics. As a result, themodulation range is from0to-0.7349λ, so the encoding algorithm should be adopted for the filter leading to the maximum correlation energy.In the experimental aspect,(1) The auto-correlation and cross-correlation have beenrealized by use of LCOS distinguishing the authenticity of the target.(2) This paper studiesthe distortions-invariant recognition characteristics of the LCOS correlator, which takesadvantage of continuous phase filter making-90°~90°rotation or0.55~1.45scalingdistortions-invariant recognition come true.(3) A new pose estimate method is put forwardthrough establishing a hierarchical structure which consists of different distortion tolerancefilters. As a consequence, simply by31times recognition, the correlator can obtain theaccurate posture of the target.