Hybrid optical and electronic associative memory

What is claimed is:

1. A hybrid optical and electronic associative memory system capable of recalling a stored image when the memory system is provided with a distorted input image which is a partial or distorted version of the stored image, comprising:

a hologram including at least one stored image, said stored image comprising a first stored image of a first object, said first stored image being recorded with a first reference beam, said first stored image being a transform of a first real image of said first object;

image transforming means for providing a transform of the distorted input image to said hologram, so as to generate a distorted reference beam;

means for transforming said distorted reference beam into a correlation image related to said first stored image and distorted input image; and

means for enhancing said correlation image, including:

an electro-optical thresholding means for thresholding said correlation image and eliminating low level spurious light therein while preserving shift-invariance, said electro-optical thresholding means including an electronic threshold processor for thresholding said correlation image, and

optical means for amplifying and conjugating the thresholded correlation image to generate an amplified correlation image signal, wherein said optical means includes a spatial light modulator.

2. The hybrid optical and electronic associative memory system of claim 1, wherein the image transforming means comprises a Fourier transform lens.

3. The hybrid optical and electronic associative memory system of claim 1, wherein the hologram is a Fourier transform hologram.

4. The hybrid optical electronic associative memory system of claim 1, wherein the hologram is a Fresnel hologram.

5. The Hybrid optical electronic associative memory system of claim 1, wherein the hologram is a volume hologram.

6. The hybrid optical and electronic associative memory system of claim 1, wherein said optical amplification means comprises a CCD-LCLV.

7. A hybrid optical and electronic associative memory system capable of recalling a stored image when the memory system is provided with a distorted input image which is a partial or distorted version of the stored image, comprising:

a hologram including at least one stored image, said stored image including a first stored image of a first object, said first stored image being recorded with a first reference beam, said first stored image being a transform of a first real image of said first object;

image transforming means for providing a transform of the distorted input image to said hologram so as to generate a distorted reference beam;

means or transforming said distorted reference beam into a correlation image related to said first stored image and distorted input image; and

means for enhancing said correlation image, including an electro-optical thresholding means, for thresholding said correlation image and eliminating low level spurious light therein, wherein said electro-optical thresholding means includes:

(a) a first vidicon camera for receiving said correlation image and generating a vidicon signal;

(b) an electronic threshold processor for processing said vidicon signal to threshold and enhance the resolution of said vidicon signal and provide a thresholded output signal; and

(c) A CRT having a CRT output screen, the thresholded output signal being provided to said CRT so as to generate an electro-optical image signal at said CRT output screen;

optical amplification means for amplifying the threshold correlation image to generate an amplified correlation image signal, wherein said means for amplifying includes a spatial light modulator positioned in juxtaposed adjacent alignment with said CRT output screen, said spatial light modulator modulating a high intensity readout beam in response to the thresholded output signal, so as to generate the amplified correlation image signal.

8. The hybrid optical and electronic associative memory system of claim 7, wherein said spatial light modulator is a liquid crystal light valve (LCLV).

9. The hybrid optical and electronic associative memory system of claim 8, wherein said system acts as a resonator by further including:

(a) a second vidicon camera for receiving said output image from said hologram and generating a second vidicon signal;

(b) a second threshold pressure for processing said second vidicon signal and providing an object leg threshold signal;

(c) a second CRT, having an output screen, said second CRT for generating a CRT signal responsively to said object leg thresholded signal;

(d) a second LCLV, operatively linked to said second CRT, for modulating a second high intensity readout beam, and

(e) means for directing the modulated second high intensity readout beam to said hologram.

10. An associative memory system capable of associating a first body of data with a second incomplete body of data, comprising:

(a) data storage means for storing a first transformed set of data corresponding to said first body of data, and a reference set of data corresponding to a transform of a plurality of delta functions;

(b) data input means for providing said data storage means with a second transformed set of data corresponding to the second incomplete body of data so as to generate a composite product of the transform of said first body of data, the second transformed set of data and the reference set of data;

(c) means for enhancing the composite product including a first electronic data processing means for thresholding the composite product and generating a thresholded signal, and

a data amplifier operating in association with said electronic data processing means,

for receiving a readout beam and modulating said readout beam with the thresholded signal such that a modulated signal is generated;

(d) means for redirecting the modulated signal to said data storage means, and

(e) associative processing means, operatively linked to said data storage means, for retrieving said first body of data, by identifying an association between said first body of data and said second incomplete body of data, when said data storage means is provided with the modulated signal.

11. The system of claim 10 further comprising:

a data resonator which provides adjustable thresholding of said incomplete body of data, said resonator comprising:

a second electronic data processing means for thresholding, disposed on the side of said data storage means opposite from said first electronic data processing means, so that the modulated signal iteratively loops back and forth between the first and second electronic data processing means.

12. A hybrid optical and electronic associative memory system for recalling a first stored image when the memory system is provided with a distorted incomplete input image, comprising:

(a) a hologram having the first stored image of an object recorded therein as a scattered field pattern transform;

(b) means for providing the distorted incomplete input image to said hologram so as to generate a distorted reference beam;

(c) electro-optical means for resolving the reference beam so as to generate an enhanced reference beam, including:

a correlation lens;

image processor having an adjustable control for setting the threshold of the reference beam, and generating a thresholded reference beam;

means for modulating a readout beam responsively to the thresholded reference beam, so as to generate the enhanced reference beam, and

(d) an object leg of the system disposed on the side of said hologram opposite to said electro-optical means, to form a resonator with said electro-optical means, said object leg including:

a second means for modulating a second readout beam, and

a second thresholding means, the enhanced reference beam, from said electro-optical means iteratively looping through said hologram between said electro-optical means and said object leg, and reconstruction an output image corresponding to said first stored image.

13. The hybrid optical and electronic associative memory system of claim 12, wherein said first and second means for modulating are directly electronically controlled to modulate, respectively, said first and second readout beams.

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FIELD OF THE INVENTION

This Invention relates to Associative Memory Systems and in particular to Associative Memory Systems using electronic and optical components.

BACKGROUND OF THE INVENTION

The speed and computational accuracy of modern digital computers are well-known. However, all digital computers solve problems in a sequential fashion through the use of numerical computation. While the processing unit contained in a simple pocket calculator can easily out-perform the human brain in number crunching tasks, digital computers are able to accomplish this sophisticated numerical analysis only on a step-by-step basis. Digital computers exhibit their best abilities when presented with a serially programmable algorithm.

Digital computers are not capable of sophisticated parallel processing, such as that required when a human undertakes the task of pattern recognition. Problems such as comparing the fingerprint found at the scene of a crime with a data base full of fingerprints is the sort of practical and necessary problem that arises and yet is not easily solved by a digital computer. To the extent that digital computers have been programmed to match the fingerprint found at the scene of the crime with an existing fingerprint in the files, lengthy serial searches of memory are required to digitally achieve accurate pattern recognition.

A matrix algebra based on an associative memory model was described by J. J. Hopfield in his paper "Neural Networks and Physical Systems with Emergent Collective Computational Abilities," proceedings of the National Academy of Science U.S.A., 1982, Vol. 79, pp. 2554-2558. The Hopfield model utilizes feedback and nonlinear thresholding to force the output pattern to be the stored pattern which most closely matches an input pattern presented to the associative memory system. A digital emulation of this model requires large storage and computational effort for the manipulation of an association matrix used in the model. For example, in order to store two-dimensional image patterns consisting of N.times.N pixels, the model requires a matrix with N.sup.4 entries be used.

A natural implementation of an associative memory model would be one which uses optical technology. Optical associative memory systems store information as patterns; so that, upon the introduction of a stored pattern to the system, the system is able to recall the stored pattern and perform a match. These Optical systems achieve massive parallel processing. The ability of an optical associative memory to perform such a function has wide application in the fields of pattern recognition and image understanding. Used in conjunction with a laser beam, specially treated photosensitive film or plates act as holograms. A hologram is a frozen "picture" of an object wherein the image of the object is recorded on the film plate as an interference pattern between a reference beam of plane waves (which is directed only at the photographic film) and an object wavefront (which is created by reflection from the object, where the object wavefront is made by the same coherent source that produced the reference beam). Holograms are characterized as having extremely good spatial coherence. The light used to produce the hologram, normally a laser beam, exhibits a high degree of temporal coherence. In order to view the recorded holographic image, one redirects coherent light along the same path as the reference beam which originally recorded the hologram. A viewer views the hologram along the same line of sight that connected the object and the hologram during its recording. Directing a new reference beam on the hologram causes an image to appear which, in a lensless environment, gives rise to a three-dimensional image. The lifelike dimensionality of a lensless image produced in a hologram is due to the fact that, unlike a photograph, a hologram stores not only amplitude changes but also records phase changes as interference fringes resulting from the interaction between spatially coherent object and reference beams.

Holograms are characterized by very precise and lifelike imaging. In addition, a hologram, when viewed from different angles, produces different views of the recorded image. The hologram is programmable for use in storing a plurality of images, by varying the angle of the reference beam used to record the image. The information stored within a hologram is recorded throughout the holographic medium; even a portion of the hologram retains the complete record. It therefore can be seen that holograms are quite useful in parallel processing systems. Furthermore, holograms are inherently useful for optical pattern recognition mechanisms.

Among the types of holograms known in the art are the volume, Fresnel, and Fraunhofer holograms. The volume holograms have a thickness and can be used to record either amplitude or phase modulated images without the generation of both primary and conjugate waves that is inherent with thin holograms. Fraunhofer holograms are characterized as holograms that record distant objects. Larger and closer positioned objects produce Fresnel holograms.

The Fourier transform hologram uses a lens and is adaptable for memory storage purposes. As is well known in the numerical analysis arts, the Fourier transform is a mathematical tool wherein any function may be broken up into a sum of sinusoidal superimposed patterns. This manner of dividing a function into its Fourier components is known as defining the Fourier transform of a function. In Fourier transform holography, one captures an object's wave front holographically, after it has undergone a Fourier transformation. To do this, one places a photographic holographic plate at the back focal plane of the lens. A flat object, such as a transparency, is placed at the same distance in front of the lens as the photographic plate is behind it. The object's wave front, when it reaches the plate, has been Fourier-transformed by the lens. The holographic pattern produced as an image is quite unlike the original object. If the object is illuminated only by coherent light, such as a laser beam, and if a reference beam is provided at an angle to the plate, the Fourier transform can be recorded as a hologram.

Pattern recognition has used Fourier transform holograms in another fashion to perform the operation of convolution. The best way to understand convolution is to look at an example. If one were to convolve a first transparency having three dots with a second transparency having one triangle, using a holographic Fourier transform, one obtains three triangles, one at each position of the dots. A related operation mathematically similar to convolution is correlation. The result of correlating two identical objects is a sharp peak at a position corresponding to the shift value which superimposes the two objects. The peak is greatly reduced if the two objects are not identical, making correlation useful in pattern recognition.

To correlate two transparencies (also referred to as objects) one simply positions a first object one focal length in front of a lens and a Fourier transform hologram of a second object one focal length back of this lens. A second lens is positioned in back of the Fourier transform hologram of the second object. The correlation of the first and second objects appears one focal length behind the second lens.

An example of optical pattern recognition using correlation would be where in a printed page of text, one could recognize a particular word or letter at some position on a page. Wherever the particular word appears in the text, a bright spot of light highlights the word in the correlated image. Wherever the word occurs on the page, there will be a corresponding bright spot of light in the correlated image called a correlation peak. Thus, the nature of holograms and lenses combined in an optical system using a coherent light source allows the operation of pattern recognition to occur. Such a device has been characterized as an optical neural computer. The term "neural" is derived from the fact that the parallel processing of a hologram to provide an associative memory is similar to that of a human brain's neural system in that the stored information is not localized.

Heretofore, one such optical associative memory has been proposed by Abu-Mostafa and Psaltis in Scientific American, vol. 256, no. 3 in an article entitled "Optical Neural Computers," at page 88 (March, 1987). In that article an optical thresholding device and a pinhole array were used as part of a double hologram associative memory system.

The applicant has previously disclosed (as a coinventor) in a pending patent application an associative memory system entitled "ASSOCIATIVE HOLOGRAPHIC MEMORY APPARATUS EMPLOYING PHASE CONJUGATE MIRRORS", Ser. No. 06/786,884, filed Oct. 11, 1985. Also, the applicant is a coinventor in a now pending application "ASSOCIATIVE HOLOGRAPHIC MEMORY APPARATUS EMPLOYING PHASE CONJUGATE MIRRORS IN A TWO-WAVE MIXING CONTRA-DIRECTIONAL COHERENT IMAGE AMPLIFIER", Ser. No. 06/821,237, filed Jan. 22, 1986. (The disclosures contained in both applications are hereby incorporated by reference.) Hughes Aircraft company, the assignee of this application, is also the assignee of these two pending applications. These systems also employ primarily all-optical elements.

As indicated above, optical elements, such as the hologram, make excellent associative memory storage devices. When a distorted input image is presented to a system which includes at least one hologram (containing a clear representation of that image), the system processes light through its components in such a manner as to correlate and match the distorted input image with one of the images stored on the hologram. The sharper the correlation peaks, the better the match. All optical systems are excellent parallel processors but generally may not be shift-invariant and furthermore, they may exhibit optical and gain losses in the system as the image is processed. In order to achieve a good match, an optical associative memory must have good thresholding and gain so that the correlation peak which reconstructs the reference beam (when the image is to be reconstructed) is sharp and bright. Losses of light intensity in the system are inevitable as the light is processed through an optical system as disclosed in the above-incorporated applications or as that disclosed in the Abu-Mostafa article, supra. Additionally, reconstruction and phase conjugation of the reference beam in the all-optical systems described in the pending patent application Ser. Nos. 06/786,884 and 06/821,237, is achieved inherently by use of phase conjugate mirrors, (PCMs) using for example BaTiO.sub.3 material. In such systems, thresholding is determined by physical processes in the PCMs and is not easily alterable nor readily adjustable. Also, such optical systems heretofore have required at least a second for the PCM to respond. BaTiO.sub.3 --based optical techniques are relatively slow, in a computer sense. Phase conjugate mirrors of an all-optical component system may be used to fully reconstruct and return an image to its point of origin to achieve pattern recognition. Nonlinearities in the phase conjugate mirrors are used to select those stored objects which exceed a threshold, based on the overlap of computed integrals of the object input with the stored objects. Although, experimentally, store-and-recall of two objects with shift invariance, was achieved, the gains achieved by phase conjugate mirrors were not enough to overcome hologram losses. Additionally, the nonlinearities of the phase conjugate mirrors were difficult to control.

Therefore, an all-optical system using PCMs has certain advantages over an electronic computer in performing massive parallel operations, such as pattern recognition; however, such a system is relatively slow and the thresholding is not easily controlled.

It is therefore an object of this invention to provide a system which makes use of the pattern recognition properties of a hologram but in such a manner that optical losses are kept to a minimum, thresholding achieved, and sharp correlation of images at the hologram accomplished, with shift invariance and at video frame rates.

U.S. Pat. Nos. 4,546,248 and 4,556,986, both issued to Glenn D. Craig and assigned to the United States (NASA), disclose electro-optical systems used to process images with incoherent light sources. The systems represent attempts to vary spatially the optical gain of signals without thresholding or enhancement of optical images. Such references show the state of electro-optical art, but do not in themselves advance the achievement of the objects of this invention to provide a fast reaction shift invariant associative memory system.

SUMMARY OF THE INVENTION

A hybrid optical and electronic associative memory system capable of recalling a complete and undistorted stored image when the memory system is provided with an input image which is distorted (or is a part of the complete stored image or both) is disclosed. Such an input image is hereinbelow referred to as a "distorted image." The associative memory system of this invention includes a holographic means for recording and reconstructing a first object transform of a first object. Image transforming apparatus, such as a lens, provides a second transform of a second object to the hologram. The hologram forms the product of the transforms of the first object, second object, and the first reference beam used in recording the hologram. This composite product, known as a distorted second reference beam, is transformed by a correlation lens into a correlation function which is the transform of the product. The correlation function is enhanced by an electro-optic thresholding and light amplification system which also retroreflects the enhanced correlation back to the hologram as the enhanced second reference beam. The enhanced second reference beam then reads out the first object transform stored in the hologram. An image then appears in the output plane as a reconstructed first object after passing through the transform lens.

The electro-optical thresholding and light amplification subsystem of this invention includes a vidicon camera for receiving the reference beam from a correlation lens. This first vidicon camera has a standard response time of 30 frames per second; it can therefore be used to compare a holographic memory image with an input object of short duration. The vidicon camera provides an electronic output signal to an optical processor which (in the preferred embodiment) is an electronic image processor.

The processor refines and thresholds the signal to enhance the correlation peaks found at the correlation plane of the vidicon, thereby improving the quality of the reference beam and the resolution of the image. This first electronic image processor provides a processor output signal to a viewing or CRT screen. The CRT generates an electro-optical image signal at its output screen. The output screen of the CRT is juxtaposed against and aligned with a spatial light modulator, which in one presently preferred embodiment comprises an adjustable liquid crystal light valve (LCLV). The liquid crystal light valve (LCLV) is electronically controllable and modulates a high-intensity readout beam. A polarizing beam splitter amplitude-modulates the high-intensity readout beam according to the phase modulation control of the LC light valve. In this manner, the CRT, LCLV and polarizing beam splitter act as control elements of an optical "amplifier" The CRT, LCLV, and polarizing beam splitter amplify and enhance the reference beam and retroreflect the enhanced reference beam to the hologram. In an alternative embodiment, an imaging lens and a diffuser are positioned between the polarizing beam splitter and the hologram to provide better distribution of the light on the hologram.

It will be noted that electronics are introduced into the system only at the point where the parallel processing has already occurred or an enhanced image is needed to make the parallel processing mechanism of the hologram work more effectively. No attempt is made to use the electronics in place of an optical component where an optical component would better perform the function which is required. Electronic components are deliberately introduced to provide adjustable thresholding of the light beam initially presented to the correlation plane of the first vidicon camera, and to improve system performance by eliminating the PCMs.

A second leg of the system may be provided where-in a second vidicon camera receives a focused image from the hologram. This second vidicon camera provides an electronic signal to a second optical processor, which in the preferred embodiment is an electronic image processor. The second optical processor provides an object leg output signal to a second CRT. An image of the first object is displayed on the second CRT screen which may be threshold limited by the second image processor. The second CRT is operatively linked to a second LCLV and this second LCLV provides a modulated control of a second readout beam using a polarizing beam splitter. When the beam splitter is illuminated by the bright readout beam, an enhanced, and amplified object beam resonates within the memory system, providing a truer match on the holographic plate.

It can be seen that the present inventive system uses the flexibility of electronic control to perform functions which are more advantageously performed using electronics than using optical elements.

The processing of data, for pattern recognition purposes, accomplished by the associative memory of the present inventive electro-optical system may be generalized as an associative memory system where a first body of data is transformed and is recorded within a memory storage means, such as a hologram, using transforms (plane waves) of a reference set of data (delta functions) and the memory system is presented with a second incomplete body of data that is the distorted input image. For example, the nature of the transformation may be a Fourier transform. A second transform set of data corresponding to the second body of data is provided to the hologram. The hologram, in conjunction with a correlation device, generates the correlations of the first and second bodies of data. Such a generalized model also includes a means for enhancing and improving this correlation function to eliminate losses in the system and to achieve threshold. This improved correlation function is transformed as an enhanced reference beam which is then presented to the data storage area where the data stored in the system is kept. This results in the reconstruction of the first body of data.

An electronic data processor for resolving (by thresholding and amplifying) the correlation function is used to direct a composite product set of data back to the transformed data storage means. An associative processing occurs wherein the transformed data storage combines and correlates these first stored and transformed set of data with the second transformed input data and the reference data to form the composite product set of data, whereby a first stored body of data is associated with a second incomplete body of data. In applicant's presently preferred embodiment, the parallel processing and pattern recognition are accomplished optically, while thresholding and gain are accomplished electronically. However, the operation of the system may lend itself to use in a fully digital setting, where proper algorithms are programmed to drive the computer.

In any event, the hybrid optical and electronic associative memory system of this invention, which is capable of recalling a first stored image when the memory system is provided with a distorted input image, includes the hologram having an optical medium which records a first stored image of an object as a scattered field pattern transform which is frozen into the hologram. A light source produces an illuminated but possibly distorted input image for comparison with the stored image on the hologram. The stored image is reconstructed when the distorted image is presented to the hologram and the distorted image is correlated with both the first stored image and then combined (through convolution) with an enhanced reference (such as a delta function) which enables the first stored image of the object to be reconstructed through the hologram for viewing at the output plane.

The first electro-optical means includes a correlation means and a light amplification and thresholding means. The light amplification and thresholding subsystem are electronic and result in enhancing the reference beam so that the output of the inventive system is the stored image which best matches the input image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified first embodiment of the hybrid optical and electronic associative memory of this invention.

FIG. 2 shows the configuration of FIG. 1 in greater detail.

FIG. 3 shows a hybrid electronic and optical resonator associative memory system in accordance with a preferred embodiment of this invention.

FIGS. 4(a) and 4(b) show the appearance of the input and correlation planes during simultaneous recording four different objects.

FIG. 5(a) shows the input image of the object A which is used to match the object A of the recorded image of 4(a).

FIG. 5(b) shows the positioning of the correlation spot on the correlation plane of the vidicon of FIG. 1 of this invention corresponding to the input image of FIG. 5(a).

FIG. 6(a) shows a masked input of an image of object B. FIG. 6(b) shows the positioning of the corresponding correlation spot.

FIG. 7(a) shows an image input selection of the object C.

FIG. 7(b) shows the corresponding positioning of the correlation spot on the correlation plane to the image C of FIG. 7(a).

FIG. 8(a) shows a masked selected object D of the four objects recorded in FIG. 4(a).

FIG. 8(b) shows the relative positioning of the correlation spot corresponding to the object D in the correlation plane of the first vidicon camera of FIG. 1.

FIG. 9 shows an alternative configuration of the embodiment of the associative memory system shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIGS. 1 and 2, the preferred embodiments of the invention of this application are generally shown and may operate in a first recording mode and a second reconstruction mode. This description details the associative memory system operating in the reconstruction mode. A hologram 18 has stored within it at least two coherent wave amplitudes generated by at least two different objects. The hologram 18 carries within its light-sensitive medium both phase and amplitude information with regard to the objects stored therein. The particular hologram 18 which is described in detail in the following description is a Fourier transform hologram. However a Fresnel or a volume hologram can also be used. Utilizing a Fresnel or a volume hologram will, however, result in the loss of shift invariance, which is an important advantage of the Fourier transform hologram. By "shift invariance", it is meant that an object will be recognized and reconstructed regardless of its position in the input plane.

When the hologram 18 is irradiated by a complex wavefront which is a distorted version of the stored image, the hologram 18 may be used in conjunction with the other components of the system, to match the incomplete input image with the stored image on the hologram 18.

The distorted image is provided to the associative memory system by the object input plane 12. Light from the object input plane 12 is directed at the beam splitter 14. The beam splitter 14 redirects light from the object input plane 12 to the Fourier transform lens 16. It will be noted that the object input plane 12 is one focal length distance in front of the Fourier transform lens 16. The hologram 18 is one focal length in back of the Fourier transform lens 16. A composite product beam which is a distorted reference beam generated by the object wavefront incident on the hologram is provided through the correlation lens 32. The correlation lens 32 is in back of the hologram 18.

The distorted reference beam is provided to the beam splitter 20. This distorted reference light beam is then simultaneously provided to the correlation plane of the vidicon 22 and the liquid crystal light valve 28, each of which is one focal length in back of the correlation lens 32. The image received by the vidicon 22 and the light crystal light valve 28 may be characterized as the correlation of the stored image of the object, stored within the hologram 18, and the distorted input image from the object input plane 12.

In simplified mathematical terms, let

a=an object "a";

A=a Fourier transform of the object "a" stored within the hologram 18;

b=a reference "b", the Fourier transform of which, B, is also stored in the hologram

The quantities A and B are in general complex. Then, the amplitude transmittance of the hologram 18 is proportional to the magnitude squared of the sum of A and B or .vertline.A+B.sup.2.

If now a distorted input image a' is provided by way of the Fourier transform lens 16 to the hologram 18, a transformed input image A' is presented to the hologram 18. A Fourier transform hologram 18, when arranged in a system as shown in FIG. 1, gives rise to the correlation of the distorted image a' with the quantity a which is in turn convolved with b.

It is well known that the convolution of two functions in the spatial domain equals the product of the Fourier transforms of each function in the spatial frequency domain. If the symbol "*" is used herein to indicate convolution, then, b * (a' *.circle. a) means b convolved with the quantity consisting of the correlation (*.circle. ) of a' with a.

In the spatial frequency domain, the amplitude transmitted by the developed hologram is proportional to the expression A' .vertline.A+B.vertline..sup.2. If one were to expand this product, one would derive the following expression:

Rearranging these terms, one would obtain the following expression:

where A is the complex conjugate of A and B is the complex conjugate of B.

The first two terms of this expression, are respectively, the zero order term and the -1 order term, neither of which are of direct concern to this invention; however, the last term, A' (AB) is one which is important to this invention. B is a tilted plane wave, and this plane wave is derived from a reference b, which is a shifted delta function. Therefore, in the spatial domain, the quantity b*(a' *.circle. a) represents a shifted version of the correlation of the distorted input image a' with the stored a. FIG. 1 shows that the above quantity is present simultaneously at the vidicon (correlation plane) 22 and the liquid crystal light valve (LCLV) 28.

The above mathematical results are in keeping with what is experimentally observed in optical systems. If a first object is placed one focal length in front of a Fourier transform lens and a Fourier transform hologram of a second object is placed in back of the same Fourier transform lens by one focal length; then, a second lens is positioned one focal length behind the Fourier transform hologram of the second object, a screen which is one additional focal length behind the second lens will produce a correlated and convolved image of the first object and the second object which is stored in the Fourier transform hologram. If the stored image transform B is the Fourier transform of a delta function b, (i.e. a tilted plane wave), then the remaining terms of interest A' (A) in the frequency domain correspond to the spatial domain correlation of input image a' with the stored image a. These relationships of association arise intrinsically when a Fourier transform hologram 18 is used within an associative memory as shown in FIG. 1. The correlated images, as stated hereinbefore, are provided by the beam splitter 20 to the correlation plane of the vidicon 22 and the liquid crystal light valve 28.

One of the significant aspects of the invention and system disclosed in this application is the manner in which the correlated image provided to the vidicon 22 and the liquid crystal light valve 28 is enhanced by the iteration loop 21.

The image presented to the vidicon camera 22 by the correlation lens 32 and the beam splitter 20 has partially distorted spurious light associated with this correlated image due to the imperfect input of real time image from the object input plane 12 (a'). In order to remove distortions and provide a threshold value of amplitude intensity of the optical signal to the system, the distorted correlation image captured by the vidicon 22 is electronically provided to the electronic threshold unit 24. In the preferred embodiment, the electronic threshold device 24 may be an image processor which has rapid frame-grabbing capabilities. One such image processing device is the Series 100.TM. computer adaptable circuit component board for IBM PC/ATs made by Imaging Technology Incorporated of Woburn, Mass. 01801. Thirty times a second, a new frame sweep of the screen of the vidicon 22 is made by the electronic threshold 24 and the information provided to the vidicon camera 22 is stored and translated digitally with the electronic threshold 24. As used in this application the term "threshold" refers to the electronic threshold 24 which adjusts its preprogrammed memory in such a manner so as to use the light intensity at the center of the correlation image presented to the vidicon 22 as a reference standard for determining what portion of the vidicon signal is eliminated and what other portion is further processed and sent to the CRT (cathode ray tube) screen 26. In other words, if the image on the vidicon 22 is partially distorted, such as that of a circle having flares or wings, these flares or wings at the outer portions of the correlation are clipped, so that the electronic threshold 24 provides to the CRT screen 26 a smooth, round correlation image.

This correlation image is then presented as writing light to a spatial light modulator, which is a liquid crystal light valve 28 in the preferred embodiment. The liquid crystal light valve is an optical-to-optical image transducer tat is capable of accepting a low-intensity input light image and converting it, in real time, to an output image with light from another source. The device is designed so that the input and output light beams are completely separated and noninteracting. The liquid crystal light valve (LCLV) 28 operates to modulate the phase of the readout beam 36 according to the control provided by the writing light of the CRT screen 26. The liquid crystal light valve provides optical birefringence wherein the birefringent phase of the readout beam 36 is spatially modulated by the light from the CRT. The polarizing beam splitter 30 directs the high intensity coherent readout beam 36 to the LCLV 28 and converts the birefringent phase modulation into amplitude modulation.

The polarizing beam splitter provides half its signal back to the hologram 18 along the bidirectional diverging path to the hologram 18 shown in FIG. 1 Thus, the original correlated image [a'*.circle. a] provided to the vidicon 22 is processed by the e threshold device 24 and provided to the CRT screen 26. This threshold-enhanced signal is boosted by operation of the liquid crystal light valve 28 (LCLV) which provides a higher gain strong signal to the hologram 18. This signal processing assures sharper and clearer correlation of the image from the object input plane 12 with a stored image a of the hologram 18, for viewing at the output plane 34.

With particular reference to FIG. 2, there is presented a detailed preferred system showing a liquid crystal light valve (LCLV) based associative memory of this invention. The input object 12 provides a partially distorted image by way of the beam splitter 14 to the Fourier transform lens 16 and onto the Fourier transform hologram 18. A vidicon 35 receives the output image much as the output image was viewed at the output plane 34 of FIG. 1. The correlation lens 32 is placed in back of the hologram 18 and provides a correlated image to both the vidicon 22 and the liquid crystal light valve 28 simultaneously.

The correlation lens 32 provides a correlation image to the vidicon 22 by means of the beam splitter 20 and the mirror 21. The liquid crystal light valve 28 receives its image from the correlation lens 32 through the beam splitter 20, mirror 40, mirror 38, and onto the liquid crystal light valve 28. The vidicon 22 perceives a partially distorted correlation plane image which is processed by the electronic threshold 24 which, as in FIG. 1, may be an image processor in the preferred embodiment. This electronic threshold 24 passes light from the correlation plane of the vidicon in the form of an electronic signal and displays an image of the light received on the screen of the CRT 26. Only that light which falls on the vidicon 22 of an intensity as great or higher than the center of the image falling on the vidicon 22, is provided to the CRT screen 26. The liquid crystal light valve (LCLV) 28 phase-modulates the readou