Conventional Joint Transform Correlator Research Articles

Images encryption system based on a fractional joint transform correlator and nonlinear filtering

A new optical security system for image encryption based on a fractional joint transform correlator and nonlinear filtering is proposed. The position of the lens in the proposed optical encryption setup can be chosen, so that an additional key is introduced in the security system. The distributions at the input and output planes of the encryption system are related by a fractional Fourier transform (FrFT) at a given fractional order that is defined, among other parameters, by the focal length and the position of the lens in the setup; this fractional order acts as an additional key of the security system. The optical intensity of the complex distribution in the fractional Fourier domain (output plane) is captured by a CCD camera. The nonlinearity introduced in the last step of the encryption process, maintains the encrypted function as a real-valued function. The security system proposed in this work is an generalization of the encryption system based on a conventional joint transform correlator (JTC), from the Fourier domain to the fractional Fourier domain. Additional advantages of the proposed system are: new degrees of freedom for the optical setup, alleviated alignment requirement, and the introduction of an additional key (the fractional order of the FrFT) that improves security. Numerical simulations verify the validity of this new optical security system.

Cryptanalysis of an image encryption scheme based on joint transform correlator with amplitude- and phase- truncation approach

In this paper, a novel optical cryptosystem is proposed that uses amplitude- and phase-truncation approach in joint transform correlator (JTC) architecture. Due to amplitude- and phase- truncation, the encryption scheme becomes asymmetric and cryptanalysis of the scheme reveals that it is resistant to hybrid attack. This hybrid attack is the combination of specific attack on asymmetric cryptosystem and chosen-plaintext attack on JTC. We also carry out authentication verification using this correlation geometry. The scheme can be implemented digitally or optically employing a conventional JTC. Computer simulation results validate the proposed method.

Noisy face recognition using compression-based joint wavelet-transform correlator

A new method for noisy face recognition by incorporating wavelet filter into compression-based joint transform correlator (JTC) is proposed. The simulation results show that the proposed method has advantages over the conventional compression-based JTC in that regardless of the contrast and the noise level of the target, the wavelet filter can optimize the recognition performance to be higher than the classical JTC, provided compressed references have high contrast.

Color Pattern Recognition with Recombined Single Input Channel Joint Transform Correlator

Joint transform correlator (JTC) is a well known tool for color pattern recognition for a color image. Color images have red, green and blue components, thus in conventional JTC, three input channels of these color components are necessary for color pattern recognition. This paper proposes a new technique of color pattern recognition by decomposing the color image into three color components and recombining those components into a single gray image in the input plane. This new technique needs single input channel and single output CCD camera, thus a simple JTC can be used. We present various kinds of simulated results to show that our newly proposed technique can accurately recognize and discriminate color differences.

Wavelet filter for improving detection performance of compression-based joint transform correlator

A new method for improving fingerprint detections by a compression-based joint transform correlator (JTC) via wavelet filter is proposed. The simulation results show that the proposed method has advantages over the conventional compression-based JTC in that detection performance can be maximized to be higher than a classical JTC by using a smaller file size for the compressed-reference images.

Use of a joint transform correlator architecture for study of speckle displacements

The optical speckle–displacement correlation technique (OSDCT) is proposed for study of in-plane speckle displacements. The joint transform correlator architecture is used to evaluate the displacements of speckle subimages of strained surfaces. The procedure of the correlation response producing as a result of cross-correlation of strainless and strained surfaces in a conventional joint transform correlator (JTC) is considered. The convenient techniques for a joint power spectrum transformation are selected. The systematic and random errors of a mean speckle pattern displacement of a rigid body motion calculated by using the OSDCT and the digital speckle–displacement measurement technique are compared. The robustness of some JTC versions (a conventional JTC, a JTC with median thresholding, a JTC with subset median thresholding, and a fringe adjusted filter JTC) to input noise of speckle patterns and output noise of a correlation response is studied.

Comparison of detection efficiency for nonzero-order and conventional joint transform correlator

The detection efficiencies for the nonzero-order joint transform correlator (N0JTC) and the conventional joint transform correlator (CJTC) are evaluated. We show that the N0JTC outperforms the CJTC under both the noiseless and noisy conditions, due to the high diffraction efficiency (DE) of the N0JTC. We also show that false alarms created by intertarget modulation can indeed be avoided using the N0JTC.

Joint wavelet transform correlator with power spectrum subtraction for improved performance

Power spectrum subtraction is introduced into the joint wavelet transform correlator to improve its performance. Because the joint wavelet transform correlator is not a matched recognition, its performance is quite poor when the conventional joint-transform correlator is employed. By subtracting the power spectra of the target image and the reference image, the power spectrum subtraction technique may greatly improve the joint wavelet transform correlator. In comparison with the conventional joint wavelet transform correlator, the new correlator has narrower dc output, bigger space-bandwidth product, stronger wavelet transform output, and greater background-noise resistance. Both theoretical analysis and computer simulation verify its efficiency.

Intensity compensation fiber for joint transform correlation peak enhancement

A spatial filtering technique is proposed to sharpen the correlation peak for a joint transform correlator (JTC) by using the inverse reference power spectrum. Ways of handling the pole problems are discussed under various noise conditions. The minimum mean-square-error method is used to locate the optimum bias value and to estimate threshold level as applied to eradicate the poles. Applications to multitarget recognition and spectral fringe binarization are also studied. Computer-simulated results show that the compensated JTC performs better than the conventional JTC.

Removal of intra-class associations in joint transform power spectrum

A technique to remove the spectral contents of the intra-class association (ICA) with a hybrid joint transform correlator (JTC) is presented. We have shown that the removal of ICA content from the joint transform power spectrum (JTPS) can significantly improve the performance of correlations, the space bandwidth product and efficiency for a joint transform correlator particularly for multiple target detection environment. Simulated results using the ICA removal algorithm are provided, in which we have shown that the removal of the ICA contents performs better as compared with the conventional JTC.

Theoretical expression for the correlation signal of nonlinear joint-transform correlators

The nonlinear joint-transform correlator is obtained by placing a nonlinear spatial light modulator at the Fourier plane of the conventional joint-transform correlator. Here the error-function limiter is used to model the linear and the nonlinear effect of the spatial light modulator. The theoretical expression for the correlation signal of the nonlinear joint-transform correlator is thus derived. It is shown that every part of the interference intensity can be used to construct the correlation signal of the nonlinear joint-transform correlator. In addition, the correlation signal of the nonlinear joint-transform correlator is the convolution of the conventional correlation signal and a nonlinear signal. Since not all of the light energy is used to construct a given order of correlation signal, the light use should not be 100%. The nonlinear joint-transform correlator becomes bipolar when the nonlinearity is a hard limiter. The theoretical expression for the correlation signal of the bipolar joint-transform correlator is also derived. The hard-limiting effect converts the correlation signal at the Fourier plane from an amplitude-modulated signal into a pulse-width modulated signal. An equation for deciding the optimal threshold value to maximize the autocorrelation peak of the bipolar joint-transform correlator is also obtained. This optimal threshold value is shown to be invariant to the separation between the input and the reference signal.

Multiple-object binary joint transform correlation using multiple-level threshold crossing

We investigate the performance of the binary joint transform correlator (JTC) in the presence of multiple objects for two types of thresholding that are used to binarize the joint power spectrum. The first method uses the median of the joint power spectrum of the reference image and the input scene as the threshold value. The second method is a two-dimensional thresholding technique used to maximize the light intensity of the correlation peak, and it eliminates the even-order harmonic terms. The correlation performance of the binary JTC is determined for both thresholding methods. The binary JTC output is determined analytically in terms of multiple input targets. The separation requirements of the binary JTC and the conventional JTC in the presence of multiple targets are computed. Computer simulation and experiments are presented for a limited number of multiple-target images to determine the correlation peak-to-sidelobe ratio and the correlation width for both thresholding techniques. In the experiments, a hybrid optical processor with an optically addressed spatial light modulator is used to implement the binary JTC. The results indicate that, using both thresholding methods, the binary JTC produces a large peak-to-sidelobe ratio and a narrow peak for the multiple-target images used in the tests. The two-dimensional threshold function produces better correlation performance compared with the median thresholding.

Binary nonlinear joint transform correlation with median and subset median thresholding

We investigate the performance of the binary joint transform correlator (JTC) for three types of thresholding used to binarize the joint power spectrum. The first thresholding method uses the median of the joint power spectrum of the reference image, which is independent of the effect of input scene noise. The second thresholding method uses the median of the joint power spectrum of the input scene and the reference image. The third method is the subset median thresholding. The subset median is computed for each segment of the joint power spectrum and is used to threshold the same segment. The threshold computed by the second or third method is dependent on the effect of the input scene noise. The correlation performance of the binary JTC is determined for each thresholding method and is compared with the linear (conventional) JTC. Computer simulation is used to determine the correlation peak intensity, peak-to-sidelobe ratio, and correlation width for various thresholding techniques. The results indicate that the thresholding of the joint power spectrum using the methods that take into account the input scene noise produces a reasonably good correlation performance.

Performance of the binary nonlinear joint transform correlators in the presence of the Fourier plane quantization

The quantization effects of the joint power spectrum on the performance of both binary joint transform correlator (JTC) and the conventional linear JTC are investigated. Computer simulation is used to determine the correlation peak intensity, peak to sidelobe ratio, and the correlation width for various quantization levels. We show that in binary JTC, a capture effect for the number of the quantization levels exists. No correlation signal is produced when the joint power spectrum is quantized below a certain number of quantization levels. For the quantization levels of the joint power spectrum above this capture level, the correlation performance is comparable to that of the unquantized JTC. For conventional linear JTC, the effects of the quantization is to deteriorate the correlation performance steadily.

High-efficiency joint-transform correlator

A technique that increases the efficiency of a conventional joint-transform correlator is proposed. By modifying the joint-transform power spectrum of the input objects by using a spatial sampling method, the readout light as well as the physical area of the square-law detector can be fully utilized. As a result, the output correlation intensity can be substantially increased. Cases of using coherent and partially coherent readout light are discussed, and experimental results are presented.