1. Field of the Invention
The present invention relates generally to the field of optical correlators. More specifically, the present invention discloses an optical correlator using ferroelectric liquid crystal spatial light modulators.
2. Statement of the Problem
Optical correlators were first suggested shortly after the advent of the laser in the early 1960's by A. Vander Lugt et al. at the University of Michigan. In early optical correlators, the input scene was introduced into the correlator by means of a photographic film transparency. The spatial filter was provided by means of a holographic film created by generating a hologram of the filter image's Fourier transform. Special care had to be taken to: (1) generate an acceptable spatial filter because of the generally large dynamic range of the Fourier transform; and (2) align the input scene's spectrum and filter encoded on the hologram. This special care translated into many hours of filter preparation and tedious mechanical alignment. However, once correlation was achieved, the classical Vander Lugt correlator did a good job of recognizing and locating patterns.
One major shortcoming of such simple matched spatial filtering is that the filter is extremely sensitive to differences between the object in the input scene and the object from which the filter is generated. If the difference is more than a few degrees of in-plane rotation or a few percent in scale, the focused points of light in the correlator quickly defocus and the intensity of the signal diminishes. Thus, a robust pattern recognition system for a dynamic application required a large number of filters for each input to cover the different potential orientations and scales of the target. This problem was magnified prior to the mid-1980's by the fact that the process of changing the filter meant replacing the piece of holographic film to within a few wavelengths of light. Thus, until recently, optical correlation was viewed as good physics, but was not practical for most pattern recognition applications in the field.
A key enabling technological development in recent years is the spatial light modulator, or SLM. SLM's can be thought of as programmable transparencies or pieces of film. The use of SLM's, instead of film, in an optical correlator allows the system to rapidly change the input scene and the spatial filter without mechanically moving or replacing pads, thus accommodating the multiple-filter requirement necessary for practical pattern recognition system. SLM's have different programming speeds and resolutions depending on the type of material and the technique used to encode the scene information on the SLM. The two types of SLM's that appear best suited for use in two-dimensional pattern recognition systems are the magneto-optic SLM (MOSLM) and the family of liquid crystal SLM's.
MOSLM devices are commercially available in pixel densities of up to 256.times.256 and have been demonstrated to operate at over 2000 Hz in short bursts, with more practical operating frame rates of 500 Hz for a 128.times.128 device and 100 Hz for a 256.times.256 device. The modulating principle of the MOSLM is Faraday rotation of the polarization vector of the incident light as the light transmits through the MOSLM. The pixels are independently and electronically addressed and are capable of binary amplitude, binary phase, or ternary phase-amplitude (combination of binary amplitude and binary phase) modulation.
Liquid crystal technology has long been used for incoherent imaging in such applications as digital clocks, watches, and television displays. The majority of these devices use a nematic liquid crystal material that provides analog modulation, but is limited in switching speed. The most prominent nematic liquid crystal SLM for coherent imaging is the liquid crystal light valve, or LCLV. Unlike an MOSLM, an LCLV is optically addressed, rather than electrically addressed. This requires the LCLV to be programmed by another light source such as the illumination from a mini-CRT display. An LCLV uses the birefringence property of the crystalline structure and a controlled design thickness to achieve its modulation capability. The device has a maximum resolution of approximately 30 line pairs per millimeter, which equates to pixel densities on the order of 750.times.750. It has a maximum operating speed of approximately 25 to 30 Hz with the ability for modulating approximately 10 to 15 linear gray levels.
A second type of liquid crystal SLM that has recently made significant advances in performance capabilities is the ferroelectric liquid crystal (FLC) used in the present invention. The basic performance differences between FLC SLM's and LCLV SLM's are their addressing capabilities and frame rates. FLC's can be either optically or electrically addressed, whereas LCLV's are only optically addressed. In addition, LCLV's operate at only approximately 30 Hz. Optically addressed FLC's have been operated at over 4500 Hz, and electrically addressed FLC's have been operated at approximately 10,000 Hz. The modulation principle of the FLC SLM is similar to the MOSLM in its operation (i.e., rotation of the modulator's optic axis), but the rotation is due to a reorientation of the liquid crystal molecules under an applied electric field instead of a Faraday effect. The FLC SLM is optically more efficient than the MOSLM since its axis rotation is .+-.22 degrees versus a few degrees for the MOSLM.
A number of optical correlators and SLM's have been patented in the past, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Yu 4,695,973 Sept. 22, 1987 Brooks 4,815,035 Mar. 21, 1989 Javidi 4,832,447 May 23, 1989 Moddel et al. 4,941,735 July 17, 1990 Juday 5,029,220 July 2, 1991 Marsh et al. 5,050,220 Sept. 17, 1991 Johnson et al. 5,073,010 Dec. 17, 1991 Capps 5,086,483 Feb. 4, 1992 Liu et al. 5,150,228 Sept. 22, 1992 Takesue et al. 5,150,229 Sept. 22, 1992 Moddel 5,177,628 Jan. 5, 1993 Moddel et al. 5,178,445 Jan. 12, 1993 Stappaerts et al. 5,221,989 June 22, 1993 ______________________________________
Yu discloses a real-time programmable optical correlator that incorporates a magneto-optic spatial light modulator (MOSLM) and a liquid crystal light valve (LCLV).
Javidi discloses an optical correlator that employs a spatial modulator operating in a binary mode at the Fourier plane. The reference and input images are illuminated by a coherent light at the object plane of a Fourier transform lens system. An image detection device, such as a charge couple device (CCD) is placed at the Fourier plane of this Fourier transform lens system to detect the intensity of images. A thresholding network generates a binary output for each pixel of the Fourier transform interference intensity indicating whether the image intensity for that pixel is greater than the median intensity.
Juday discloses an optical correlator for real-time tracking of the position of the retina during laser eye surgery.
Capps discloses a hybrid optical/electronic processor in the general configuration of a Vander Lugt optical correlator with an input SLM 12, a first Fourier transform lens 16, a target SLM 14, a second Fourier transform lens 18, and an electronic processing array 20. The processing array 20 consists of a two-dimensional array of cells 40, each of which is connected to its nearest neighbors to facilitate peak detection.
Takesue et al. disclose an optical correlator that generates pictorial patterns of a sum of two patterns of pictorial information to be compared and of a difference between the two patterns by a phase conjugate wave form. The system then transforms the pictorial patterns into first Fourier transform images, generates a pictorial pattern of a difference between an intensity distribution of the first Fourier transform images by the phase conjugate wave form, and transforms the pictorial pattern of a difference between an intensity distribution of the first Fourier transform images into second Fourier transform images. The optical correlator detects a cross-correlation peak of the two patterns of pictorial information for comparison at a high signal-to-noise ratio.
Liu et al. disclose another example of an optical correlator using liquid crystal TV's (LCTV1 and LCTV2) to change the input and reference images in real time.
Marsh et al. disclose an optical correlator for fingerprint identification. Two spatial light modulators 28 and 32 are employed to input the unknown fingerprint and a sequence of reference fingerprints for comparison.
The patents to Johnson et al., Moddel, and Moddel et al. disclose several types of optically addressable spatial light modulators incorporating ferroelectric liquid crystals.
Brooks discloses an example of a scrolling spatial light modulator using an array of ferroelectric liquid crystal cells.
Stappaerts et al. disclose an example of a spatial light modulator using non-ferroelectric PLZT ceramic.
3. Solution to the Problem
None of the prior art references uncovered in the search show an optical correlator using electrically addressable FLC spatial light modulators in the present optical configuration. The optical design is compact and facilitates easier system alignment. The system offers real-time pattern recognition at high image rates and at high resolution.