JPS60181877A - Optical correlator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to optical correlators, in particular combined transform correlators (JO!
Ilt transform Correlator
) related to

In the combined transform correlator, the light from the first spatial light modulator that produces a phase or gray scale image with coherent light is focused by a first lens or lens system, and is focused on the internuclear lens or lens on the Fourier transform plane. The system causes interference with light from a second spatial light modulator juxtaposed with the first spatial light modulator. A hologram recording device is provided on the Fourier transform surface, which is irradiated with coherent light focused by a second lens or lens system and records the Fourier transform surface of the second lens or lens system. A correlated image is formed on a two-dimensional image receiving opto-electrical converter provided above. The transducer is typically constituted by an array of photodetectors, but at least in principle is a simple unit of the type whose surface is scanned to extract image information, such as is used in some types of video cameras. A single large-area detector may be used instead.

Details of the working principle of the combined transform correlator can be found in D., Casas
ENT's paper [Optical computing technology for radar and sonar], Journal of the Society of Photographic and Equipment Technology, Vol. 118.

1977 ioo page (“optical c Om
Putting Techniques for Rada
r and 3onar 3ignalProcess
ing”, proceedings of the
5ociety of Photo-1strume
ntation Enoineers Vol, 118.
1977, 100).

One application of an optical correlator is to determine whether a particular object, hereinafter referred to as a target object, is present within a field of view under investigation that typically includes other objects and/or clutter noise. If a target object is present within the field of view under investigation, the correlator is normally required to confirm its position within the field of view. These verification and localization functions are performed by correlating the field of view under investigation with a reference field of view that includes a reference object or prototype of the target object.

Prior art optical correlators work well when there is perfect alignment between the target object in the field of view under investigation, including background clutter, and the prototype in the reference field. In a combined transform correlator, the position of the target object is ascertained from the position of the correlation peak on the output surface on which the two-dimensional transducer is provided. However, if there is a deviation from perfect matching between the target object and the prototype due to orientation mismatch or scale variation, a significant drop in the correlation peak occurs. Conventionally, there has been limited information on these deviations obtained by analyzing the shape of correlation peaks.

SUMMARY OF THE INVENTION The present invention is directed to a correlation system that uses a correlation signal output by a servo feedback means to change the representation of a prototype in a reference field of view, thereby optimizing correlation peaks and reducing deviations.

Means for Solving the Problems The present invention provides first and second spatial light modulators that form phase or gray scale images of adjacent first and second coherent lights in the input or target plane of a correlator. , a dynamic hologram recording device provided at a position away from the target surface, and a first lens or lens system interposed therein so that the dynamic hologram recording device is located in a Fourier transform plane of a coherent optical image. a coherent light source for illuminating the dynamic hologram recording device; a photoelectric converter for receiving a two-dimensional image; and the converter is interveningly provided to be located on the Fourier transform plane of the dynamic hologram recording device. a second lens or lens system, and the second spatial light modulator is configured to be electronically controlled by a servo feedback system to maximize the sharpness of the correlated image formed on the transducer. A combined transform correlator is provided.

Embodiments Below, embodiments of the optical correlator according to the present invention will be described.

The basic configuration of the combined transform correlator shown in FIG. a lens or lens system 13; a dynamic hologram recording device 14 used for temporary storage of holograms on the Fourier transform surface; a second] coherent light source 15;
a second lens or lens system 16, a CCD photodiode array 17 on the correlation surface, and an output from the CCD array.
A servo feedback system 18 controls the size and size of the prototype displayed by the spatial light modulator 12.

The first coherent light source 10 is accompanied by a beam expander 19 which spreads the laser and its output beam sufficiently to hit the two spatial light modulators 11 and 12; These devices (
The spatial light modulators 11 and 12) are reflective and not transmissive, so the light from the laser 10 is directed through the beam splitter 20. (If a transmissive device is used, the beam splitter is omitted and the position of the laser is changed accordingly.) The spatial light modulator is a liquid crystal cell matrix that is addressed through an active silicon backing of the liquid crystal layer. An example of this device is UK Patent No. 21183 by the applicant.
No. 47A.

The dynamic hologram recording device is degeenerate (dege
nerate> Four-wave mixer 1 For example, a bismuth silicon oxide type mixer may be used. However, with this function, the two waves used for "writing" are the two waves used for "reading",
That is, it is not necessary to have the same frequency as the output wavelength wave from the second coherent light source 15, and the laser does not need to be the same as the first laser 1o.

Dynamic bologram recording devices use different wavelengths to form a thermal pattern by absorption by the medium of light at the writing wavelength, and form the thermal pattern using light at another wavelength that is substantially transmitted through the medium. A thermal effect four-wave mixer can be used that provides substantially unchanged readout.

One class of such thermal effect four-wave mixers is the class of liquid crystal cells disclosed in the applicant's UK Patent Application No. 8403228. The second laser has a beam expander 21 and a beam splitter 22 like the first laser.

The COD photodiode array is provided on the Fourier transform plane of the dynamic hologram recording device formed by the second lens or lens system 16. This arrangement is accompanied by supporting electronic hardware consisting of an A/D converter, an image memory, and a microprocessor or minicomputer located within the servo feedback system 18.

The positions of the correlated spots formed on the array 17 provide information regarding the position of the target object within the field of view under investigation. Typically the size of the array 17 is two spatial light modulators 11 and 12.
smaller than the combined size, so some reduction is required within the system. This is achieved by making the focal length f+ of lens 13 longer than the corresponding focal length [2] of lens 16. In this case, the reduction ratio is given by f2/f+.

The essential function of the correlation calculator is to vary the representation of the prototype in the reference field of view displayed by the spatial light modulator 12 under the control of the servo feedback system 18 to obtain an optimal correlation peak in the array 17.

There are many ways to program the servo feedback system during this optimization.

In the first method, the servo feedback system is configured to first sequentially rotate the prototype representation to find the best orientation and then sequentially vary the magnitude to find the best-fitting dimensions.

Other methods of target object differentiation can also be used if the spatial light modulator has a good gray scale dynamic range. The size of this superimposed composite image is sequentially varied to find the best-fitting dimensions, followed by the best-fitting orientation, and the magnitude of the correlation peak increases rapidly as there are mismatches in dimension and orientation. decreases to Typically, an azimuth mismatch of 0.2° or a dimensional mismatch of 0.5% results in 1 below 3 dB. In order to cover all possible directions in this way, 1800
Different representations of street orientation and 100 different representations of dimensions are required to cover a 50% range of dimensions. Processing at 50 frames per second therefore requires one hour to match this entire combination. This time, however, can be increased by performing some kind of multivariate analysis on the peak shape rather than using only the peak amplitude of the correlation peak [1].
It is shortened to . Using appropriate digital image processing techniques on the output of the COD array, the orientation spacing can be increased from 0.2° to 29.5° and the dimensional spacing from 0.5% to 5%. In this case, all directions can be covered by 72 expressions, and usage can be covered by 10 expressions, and as a result, the time to match and cover all combinations is shortened to just under 15 seconds. F. on the possibilities of multivariate analysis for this type of purpose.

The paper "Hybrid Optical-Digital Imaging in Pattern Recognition Processing Systems" by Merkle.

Journal of the Society of Photographic and Equipment Technology Vol. 422, 1983, 15
Page 2 (“l-1ybrid 0ptical-Dioi
tal I maae 1nthe Processi
ngsystem ror PatternRecog
tion, proceedings of t
lle 3 ocietyOf Photo-1nst
rtlmentatiOn EnfJineerSV
ol, 422 (1983), 152). Obviously, further optimization is possible for specific purposes. For example, if it is known that the prototype has some degree of rotational symmetry, the time required to express the orientation can be reduced.

A problem with any optical correlator is that the system has a low tolerance for imperfections in the spatial light modulator. Such imperfections can affect the light wavefront transmitted or reflected by such a device. This introduces distortion which increases the systematic error/noise, which causes the holographic correction element 24 to
and 2b, it is corrected by using it as shown in FIG. Here, FIGS. 28 and 2b show a part of the correlator shown in FIG. 1 which has been modified to include the correlator pit correction element. The present configuration uses spatial light modulators 11 and 12 as a phase image generating device, i.e., to transfer spatial field information to an interrogating beam from laser 10.
(ng beam) It is suitable for use as a device that applies light on a phase-wise basis without changing the amplitude of the light.

FIG. 2a shows the configuration used to create the required pattern of correction elements 24. Laser 10 (not shown)
After passing through a beam splitter 20, the parallel light beams are incident perpendicularly onto the spatial light modulators 11 and 12 (for this purpose, the laser 10 and associated beam expander 19 are temporarily shown in FIG. 1). (in the same figure, it is necessary to move from the position occupied by lens 13). The light reflected by the spatial light modulators 11 and 12 and reflected by the beam splitter 20 is incident perpendicularly onto the correction element 24 . At this position, first beam splitter 2
0, and then reflected by the plane mirror 25, and is configured to interfere with the light that passes through the beam splitter 20. The plane mirror 25 is tilted by a small angle "a" so that the two beams interfere. The interference pattern thus generated is recorded on the correction element 24 with no data applied to either of the spatial modulators 11 and 12.

This configuration is changed as shown in FIG. 2b when the interference pattern recorded on the correction element is fixed as a phase object. The mirror 25 is removed and the correction element is returned to its original position.
All other correlator components are constructed as shown in FIG. The only exceptions are laser 10 and beam expander 1.
9, which are positioned so that the expanded beam is incident on the correction element at an angle [a,1. As a result, the light incident on the element is made into a diffracted beam, and after being inverted by Q=J by the beam splitter 20, it is incident perpendicularly on the spatial light modulator.

Only the diffracted beam is subjected to wavefront correction, and therefore it is preferable to reduce the amplitude 11J of the undiffracted light. This requires that the reflectance of the mirror is matched to that of the spatial light modulator. Note that the diffraction efficiency of exposed silver-based photographic emulsions at this point is surpassed by other types of emulsions, such as gelatin dichromate, later. Therefore, it is desirable to use one of these non-silver emulsions unless there is a particular problem in terms of sensitivity depending on the wavelength used.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the correlator, and Figures 28 and 2b are the correlators shown in Figure 1. A modification including a correction element to suppress the error will be shown. 10.15...Coherent light source, 11,12...
Spatial light modulator, 13... first lens or lens system, 14... dynamic hologram recording device, 16... second
lens or lens system, 17... CCD small 1-diode array, 18... Servo feedback system, 19...
Beam expander, 20...Beam splitter, 24...
- Correction element, 25...plane mirror.

Claims (2)

[Claims] (1) first and second spatial light modulators that are adjacent to each other and that form phase or gray scale images of the first and second coherent lights in the input or target plane of the correlator; a first lens or lens system interveningly provided so that the dynamic hologram recording device is located in a Fourier transform plane of a coherent optical image, and the dynamic hologram a coherent light source that illuminates a recording device; a photoelectric converter for receiving a two-dimensional image; and a second lens or lens system that is interposed so that the converter is located in a Fourier transform plane of the dynamic hologram recording device. a second spatial light modulator, the second spatial light modulator being electronically controlled by a servo feedback system to maximize the sharpness of the correlated image formed on the transducer; correlator. (2) The optical correlator according to claim 1 or 2, further comprising a holographic correction element in the optical path to correct unnecessary wavefront distortion caused by the spatial light modulator. 2. An optical correlator according to claim 1, wherein the servo feedback system includes electronic processing means responsive not only to the correlation amplitude peak but also to the shape of the correlation peak.