Joint Transform Power Spectrum Research Articles

Photochromic diarylethene-film-based compact all-optical nonlinear joint transform correlator

We construct an all-optical nonlinear joint transform correlator utilizing the rewritable photochromic diarylethene film, which serves as a joint transform power spectrum recorder. The computer simulations indicate that, with the nonlinear response characteristic of the diarylethene film, the SNR and peak sharpness of the correlator can be improved. A compact correlator prototype with a 5-cm focal length is constructed and the experiment results verify that the physical length can be reduced remarkably owing to the high resolution of the diarylethene film.

Color pattern recognition using image encoding joint transform correlator

AbstractA color pattern recognition scheme with encoding technique based on the nonzero order joint transform correlator is proposed. Each color target image is organized as a new grayscale encoded image. A Lagrange multipliers approach is also used to design an optimized synthetic reference function. A joint transform power spectrum subtraction method is applied to remove the zero order terms in addition. Simulation results verify the performance of the proposed technique. This correlator can reduce the channel number in the input and still maintain good ability in color image recognition. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1665–1669, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22554

Constrained optimization on the nonzero-order joint transform correlator constructed with the Mach–Zehnder configuration

Although the implementation of a nonzero-order joint transform correlator by the Mach–Zehnder technique directly removes the zero-order component from the joint transform power spectrum in one step, the large sidelobe energy reduces the recognition ability. In this paper, a new method to improve the performance is achieved by using a constrained optimization technique to synthesize a reference function in the input spatial domain. The purpose is to reduce correlation sidelobes and to sharpen the correlation peak. Numerical results are presented to show the feasibility of the proposed technique.

Novel real-time joint-transform correlation by use of acousto-optic heterodyning.

To replace the film recording aspect of performing optical correlation, conventional real-time joint-transform correlation (JTC) optical systems make use of a spatial light modulator (SLM) located in the Fourier plane to record the joint-transform power spectrum (JPS) to achieve real-time processing. The use of an SLM in the Fourier plane, however, is a major drawback in these systems because SLMs are limited in resolution, phase uniformity, and contrast ratio, which are, therefore, not desirable for robust applications. We propose a hybrid (optical/electronic) processing technique to achieve real-time joint-transform correlation. The technique employs acousto-optic heterodyning scanning. The proposed real-time JTC system does not require an SLM at the Fourier plane as in other real-time JTC systems. This departure from the conventional scheme is extremely important as the proposed approach does not depend on SLM issues. We develop the theory of the technique and substantiate it with experimental results.

Phase-shifting joint transform correlation with phase-iterative algorithm: effect of the dynamic range limit

With phase-shifting joint transform correlation (PSJTC) one uses multiple phase shifts to recover the phase difference between Fourier transforms of the input and the reference. In PSJTC systems the resulting phase-only function is used instead of the joint transform power spectrum (JTPS). Provided it can be recorded linearly, the JTPS reduces to a modulated sinusoidal fringe, especially when the target image matches the reference image. In practice, the JTPS has a wide dynamic range, and a CCD camera has a nonlinear response. Correspondingly, the recorded JTPS turns out to be different from a perfect sinusoidal fringe. Here we study the dynamic range effects of realizing PSJTC with the phase-iterative algorithm.

Robustness of joint transform correlator versus VanderLugt correlator

The robustness of the joint transform correlator (JTC) versus the VanderLugt correlator (VLC) are evaluated based on the transversal and the longitudinal misalignment. We show that the VLC is very sensitive to the transversal misalignment, while the JTC is not. Although the VLC performs better in terms of longitudinal alignment, the tolerable misalignment in recording the joint transform power spectrum (JTPS) is in the order of a millimeter, which can be easily adjusted in practice. In short, we conclude that the JTC is far more robust than the VLC.

Binary joint transform correlator based on differential processing of the joint transform power spectrum

A novel binary joint transform correlator with differential processing in the joint transform power spectrum is proposed. The new model of the binary joint transform correlator can eliminate the dc component and sharpen the correlation intensity peaks, hence increasing its discrimination ability but with a lower complexity of computation. Preliminary results are shown by computer simulations.

Implementation of a non-zero-order joint-transform correlator by use of phase-shifting techniques

The implementation of non-zero-order joint-transform correlators (JTC's) is presented. The zero-order spectra (i.e., the autocorrelation power spectra) are removed from the joint-transform power spectrum by use of phase-shifting techniques by which the output diffraction and input spatial domain can more efficiently be utilized. Applications of the phase-shifting techniques to both conventional JTC's and phase-transformed input JTC's (PJTC's) are discussed. Compared with the conventional JTC, the PJTC has the advantages of higher light efficiency, a better signal-to-clutter ratio, and the simplicity to realize phase shifting. We anticipate that the proposed non-zero-order JTC's should have a significant impact on the future development of more efficient JTC's.

Polychromatic target identification with a color liquid-crystal-TV-based joint-transform correlator

A real-time polychromatic target-detection technique that uses a computer-addressable color liquid-crystal- TV-based joint-transform correlator (JTC) is presented. One of the major features of the polychromatic JTC is that the spectra content of the object can be exploited. Since the liquid-crystal-TV panel is computer addressable, the JTC system can be made adaptive, by which the joint-transform power spectrum can be manipulated. Experimental demonstration of the proposed polychromatic JTC is provided, in which we show that the spatial and the spectral content of targets can be detected.

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.

Phase modulation in a liquid‐crystal TV based joint transform correlator

AbstractA phase‐encoded object has been modeled as the input to a joint transform correlator. The joint transform power spectrum is modeled as a binary phase object to calculate the correlation output. The results have been experimentally verified using a liquid‐crystal television operating in the phase modulation mode.

Effects of thresholding in joint-transform correlation

The joint-transform power spectrum of two identical objects can be represented as a one-dimensional sinusoidal grating modulated by a Fourier transform, and the correlation peaks can be regarded as the first-order diffraction of the grating. The peak intensity and the width are then determined by the aperture and the modulation of the grating. Based on this analysis, it is shown that dc blocking, hard clipping, or binarization of the power spectrum results in higher correlation peak intensity and a narrower peak width. Direct-current blocking is also found to be preferable if the input pattern to the correlator is corrupted by noise.

Using the hybrid modulating properties of liquid crystal television

The phase modulating capabilities of a commercially available liquid crystal television (LCTV) have been investigated and applied to the joint transform optical correlator architecture. Operating the LCTV in a phase modulating mode requires a much smaller coherent light source while still producing a good joint transform power spectrum and good correlation signals.

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.

New binarization techniques for joint transform correlation

In this paper the application of joint transform correlator (JTC) techniques for use in automatic target recognition using actual sensor data is addressed. The problem of interest is the detection and classification of objects in forward-looking infrared (FLIR) images. A JTC architecture using a single magneto-optic spatial light modulator and a chargecoupled device camera is tested. Unique techniques for binarizing the JTC input and the subsequent Fourier transform fringe pattern, commonly called the joint transform power spectrum (JTPS), are presented. Computer simulations and experimental results are provided.

Mfsk signal detection using acousto‐optic joint transform correlator

AbstractA multichannel acousto‐optic joint transform correlator is presented. On‐plane acousto‐optic (AO) cells and a square‐law converter are employed. The AO cells are driven by frequency multiplexed temporal signals to produce spatially modulated signals. The joint transform power spectrum is generated with a square‐law converter. By inverse Fourier transformation with a coherent readout, the cross correlation of input signals can be obtained. Since the architecture can process wideband signals, it may be applied to detect multiple frequency shift keying (MFSK) signals.