G. S. Pati

In-plane displacement measurement using a photorefractive speckle correlator

Abstract The present work describes the use of a photorefractive correlator in the measurement of in-plane displacement. A correlation technique has been proposed and demonstrated for precise displacement measurement by recording objective speckles. Speckle interference patterns are generated in situ using a converging beam illumination and two-beam coupling recording geometry in a photorefractive barium titanate crystal. Intermediate digital interfacing through an electrically addressed spatial light modulator enables one to introduce digital processing techniques (viz., thresholding) for improving the fringe contrast resulting in a better quality of correlation output required for measurement accuracy and reliability. This also increases the range of displacement measurement. In-plane object displacement being proportional to the separation between the correlation peaks, a measure of it gives the magnitude of object displacement. Both simulation and experimental studies have been performed on double as well as multiple exposure speckle interference patterns. The effect of threshold binarization on speckle correlation has also been studied. The method promises high degree of accuracy and increased range for displacement measurement in real-time.

Illumination sensitivity of joint transform correlators using differential processing: computer simulation and experimental studies

Differential and binary differential joint-transform correlators are found to be associated with high tolerance to illumination variation of the reference and target. This makes them suitable for illumination-independent pattern recognition applications. Computer simulation results show that a tolerance limit as high as an amplitude ratio of 0.25 between the target and the reference image can be attained. As a refinement to the above mentioned techniques, we suggest the use of isotropic difference operators. We also present some experimental results for the differential type JTCs.

Three-dimensional displacement measurement using chirp modulation in a photorefractive correlator

A speckle correlation technique based on chirp modulation of the speckle interference pattern is proposed and demonstrated for the measurement of 3-D rigid body motion. The chirp modulation is a con- sequence of the out-of-plane displacement component associated with object motion. The Fresnel zone plate action introduced by chirp modu- lation causes the correlation peaks to focus at different planes along the longitudinal direction. The separation between the correlation peaks along the transverse direction gives a measure of in-plane displacement of object motion. Both simulation and experimental studies are per- formed to establish the proposed technique.

An improved photorefractive joint-transform correlator utilizing beam-fanning in a high-gain barium titanate crystal

A photorefractive nonlinear joint-transform correlator is demonstrated by utilizing the nonlinear intensity-dependent behavior of beam-fanning in a photorefractive barium titanate crystal. The results obtained reveal a narrowing of the correlation profile, and improved peak-to-side lobe ratio resulting in a better discrimination ability of the correlator. The compressional beam-fanning nonlinearity is modelled for a simulation study in order to give a comparison with the experimental observation.

Multichannel image addition and subtraction using joint-transform correlator architecture

A simple technique for multichannel image addition and subtraction has been proposed using a basic joint-transform correlator architecture. Computer simulation and experimental results are provided. The geometry can also be modified to measure small relative displacement between objects, and to implement Hartley transform.

Photorefractive joint transform correlator using incoherent erasure in two- and four-wave mixing geometries in a BaTiO3 crystal

The outcomes of some experimental and simulation studies performed on an incoherent-erasure photorefractive joint transform correlator (PR JTC) in the two-wave- and four-wave-mixing geometries are demonstrated. Computer simulation results show the effectiveness with which nonlinear transfer characteristics can be achieved in the incoherent-erasure configuration of the JTC. These can be exploited to attain different filter operations, e.g., matched, phase-only, and inverse filters by varying beam ratios. Experimental limitations with PR barium titanate crystal entail the incoherent erasure (IE) JTC operation with all filter actions and encourage to this goal further investigations with fast PR materials.

Photorefractive joint-transform correlator using incoherent-erasure in two-wave-mixing geometry

We propose and investigate a photorefractive joint-transform correlator that uses an incoherent erasure beam in a two-wave-mixing arrangement. Computer simulation and experimental results are presented showing its performance in terms of discrimination ability and noise robustness. Simulation studies show that this JTC can exploit a nonlinear transfer characteristic inherent to the energy transfer process.

Noise immunity and pattern discrimination using wavelet feature-based generalized fringe-adjusted joint transform correlator

A wavelet feature-based generalized FAJTC has been proposed to improve the performance of fringe-adjusted type JTCs in the presence of noise in the input scene. Computer simulation results are produced for test images containing single as well as multiple targets and synthetic textured input scene noise. The results illustrate the effectiveness of wavelet processing in reducing the detrimental effect of noise and enhancing pattern discrimination ability of conventional generalized FAJTCs.

Polychromatic light-based photorefractive correlator for 3D motion measurement

We have proposed the use of polychromatic light based photorefractive correlator for 3D motion measurement. The double exposures of speckle patterns can be recorded in a BaTiO3 crystal, thus providing a set of interference fringes due to the polychromaticity of the light source and the 3D displacement. Due to the fringes corresponding to the two illuminating wavelengths used in our experiment, a set of correlation output is generated which helps in a better averaging of the results of measurement. This technique further contributes to the enhancement of accuracy and range of displacement measurements.

Scheme for implementation of an all-optical non-zero-order joint transform correlator using contrast reversal technique

A technique for realizing an all-optical non-zero order JTC has been proposed and investigated using computer simulation studies. A simple contrast reversal operation forms the basis of the proposed scheme. Effect of quantization and device nonlinearity has been studied to draw closer resemblance to a practical non-zero order image recognition system.