GB2230125A

GB2230125A - Pattern recognition apparatus

Info

Publication number: GB2230125A
Application number: GB8907777A
Authority: GB
Inventors: S C J Parker; A G Levenston; T G Slack
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1989-04-06
Filing date: 1989-04-06
Publication date: 1990-10-10
Also published as: GB8907777D0

Abstract

Pattern recognition apparatus comprises an input beam 24 modulated in accordance with a viewed scene which passes via Fourier transform lenses 28 and 34 to form a Fourier transform of the viewed scene at a plane P3. A rotating disc 36 presents a series of optical matched filters 38 at the plane P3 and the co-ordinates of any matched features are sensed at the correlation plane by an X-Y detector 42. <IMAGE>

Description

"Pattern Recognition Apparatus" This invention relates to pattern recognition apparatus.

A known form of coherent optical correlation system was developed in 1964 by Vander Lugt. In this system, a matched spatial filter is holographically recorded and used as a template which is subsequently compared with images to determine their similarity.

These techniques may be used to identify a target which is hidden within a cluttered scene.

A major problem which restricts the use of optical correlation is that the target orientation and size acquired by the imaging system is frequently different to the stored template. For example, the image will alter due to changes in target aspect angle and scale.

To overcome this limitation, the applicants have previously proposed a design which uses a computer generator diffractiongrating to split the input image into a number ofdiffr-acti orders. These diffracted orders are then used separately to address a pre-recorded array of matched filters. The main disadvantage with this approach is that, for large arrays, the highest orders are significantly displaced from the optical axis and consequently suffer aberrations. Also, the number of holograms which may be stored in an array is restricted by the "cross-talk" which occurs between closely spaced filters. This earlier system, and alternative proposals, have generally operated on the principle of parallel processing so that the image of the viewed scene is presented to each of the matched filters in parallel.

We have found that it is possible, in the arrangements illustrated herein, to provide an arrangement in which alibraryof matched filters is serially addressed and the output detected by a t.v. camera in real-time. Broadly stated, in one aspect this invention consists of pattern recognition apparatus comprising means for providing a beam of coherent radiation modulated in accordance with a viewed scene, Fourier transform lens means for forming a courier transform of said modulated beam at a focal plane thereof, optical matched filter storage means for successively presenting a series of optical matched filters at said focal plane, and detection means for detecting the diffracted beam passed bysaid optical matched filters.

The invention also extends to any novel combination of features substantially as disclosed herein.

The invention will now be described by way of non-limiting example, reference being made to the accompanying drawings, in which: Figure 1 is a schematic diagram of a pattern recognition apparatus in accordance with the invention; Figure 2 is a schematic diagram of an incoherent coherent radiation converter for use in the apparatus of Figure l, and Figure 3 is an isometric view of the spinning disc unit used in the apparatus of Figure 1.

The optical pattern recognition apparatus illustrated in Figure 1 uses a spinning disc to serially address one hundred matched filters with the Fourier transform of a scene of interest. The apparatus allows the interrogation of a library of filters which account for changes in aspect angle and class of target.

Referring now to Figure 1, a laser source 10, for example an He-Ne gas laser or laser diode, emits a laser beam 12 which is reflected at 14 to pass through a beam expander 16 which has a pin-hole spatial filter to remove spatial non-uniformities in the beam. The beam is then collimated by a collimating lens 18. The collimated beam 20 then passes to beam splitter 22 where it is split into two components 24 and 26. The component 24 (object beam) passes via an input plane P1 to a first Fourier transform lens 28 which forms at the focal plane P2 a Fourier transform of the image supplied at input plane P1. A spatial filter at plane P2 provides a difference - of - Gaussian (DOG) filtering function which serves to remove from the Fourier transform the DC component and noisy high-frequency components.

In practice, this gives rise to an edge-enhanced image.

The filtered beam then passes through a further relay lens 30 thence to the spinning disc unit 32 where the beam passes through a final Fourier transform lens 34, to provide a Courier transform of the beam at the final Fourier transform plane P3. The holographic disc 36 is rotatably mounted in plane P3.

The holographic disc 36 has a series of optical matched filters 38 spaced in a concentric array about the spin axis of the disc (see Figure 3). The filters 38 together make up a library of filters each matched to a specific target at a specific aspect angle. In one particular example, five different types of vehicle were represented at every horizontal aspect angle in 5 y 180 increments of 100, thus making a total of 10 = 90 filters. The filters 38 are equispaced around the holographic disc 36 and rre formed holographically in a silver halide emulsion creating the individual filters 38 stepwise.In each step, a transparency representing a particular target at a particular aspect angle is inserted in the input plane P1 and the filtered and Fourier transformed signal at plane P3 forms a hologram when interfered with a reference beam 26. The holographic disc 36 is exposed for the appropriate exposure time, then indexed to a new position so that a filter 38 corresponding to the next slide input at plane Pl may be formed. In this way, the complete series of filters 38 may be exposed holographically. The holographic disc 36 is then removed and developed to compelte formation of the filters. The reference beam 26 is only required for creating filters 38 and is blanked in normal use.

In use, a coherent image of the viewed scene is supplied to the input plane Pi to be filtered and Fourier transformed to form a Fourier transform at plate P3. the holographic disc 36 is spun so that each of the filters 38 thereon in succession is presented to - or addressed by - a Fourier transform of the viewed scene. A detection system comprising a lens 40 and an X-Y detector 42 provided in the correlation plane detects correlation between the viewed scene and the target images represented by the optical matched filters. Thus, if a viewed scene contains a particular target at a particular aspect angle, when the filter 38 corresponding to that target and aspect angle is on the optical axis of the system, an intense correlation signal will be formed at the correlation plane.The X and Y co-ordinates of the correlation peak will represent the X and Y co-ordinates of the target within the viewed scene.

As shown in Figure 1, the output from the X-Y detector 42 may be supplied to a control 44, together with an index signal from the spinning disc assembly 32 so that, when a target is detected, the filter 38 which gives the highest correlation peak may be identified so that the specific type of target and its aspect angle may be determined, together with its position within the viewed scene, Figure 2 illustrates an arrangement which may be used to provide the coherent image of the viewed scene which is required at the input plane P1 during normal operation of the device. In this arrangement, a t.v.

camera 50 views the scene and supplies a picture signal to a cathode ray tube 52 which displays an image of the viewed scene. The image on the cathode ray tube 52 impinges on a spatial light modulator (SLM) 58 butted up against the cathode ray tube 52 and which is read by a coherent beam 54 supplied via a polarising beam splitter 56.

The coherent output beam 60 is modulated in accordance with the viewed scene.

Referring now to Figure 3, the spinning disc unit 32 comprises a housing 62 which supports by means of spaced air bearings 64 a shaft 66 which supports the holographic spinning disc 36. In the particular arrangement illustrated, the bearings ensure that the radial drift of the holographic spinning disc 36 does not exceed 1m- The shaft 66 is driven by means of an electric motor 60 via a belt 70. An indexing system includes a rotary position encoder 72 which provides a readout of the angular position of the spinning disc 36 to the control. The housing 62 also locates the final fourier transform lens 34.

In the above arrangement the matched filters are separately addressed as the disc rotates.

As the filters are recorded on a moving medium, there is less of a limitation on the number of filters which may be stored. In one arrangement, the complete filter library may be interrogated in the time taken for one t.v. frame of the X-Y detector 42 and, provided that the correlation signals are sampied at a rate greater than 2.5 kHz, the correct matched filter may be isolated. Hence, the serially addressed system can be operated in real-time and incurs no time penalities over its parallel addressed counterparts.

In the illustrated arrangement, a DOG filtering function is performed at plane P2. The DOG filter function has a general profile defined by:

Where, d - filter diameter in the xf co-ordinate frame n - ratio of the standard deviations of two Gaussians.

Studies by the applicant show that when the frequency bf maximum transmission d is increased, the correlation peak at the output correlation plane is less sensitive to movement of the matched filter. In a particular example, the effect of high-pass spatial filtering is to increase the tolerance to shift by a factor of 2.5 over the case with no filtering. Another advantage of using a larger diameter filter is that the correlation signal is much narrower (e.g. about 0.15 of the width of the original object function). This property is a consequence of the sharp edge enhancement produced by the spatial filter when passing only high spatial frequencies and is particularly useful when accurately tracking the position of an object.

In the illustrated arrangements, a continuous wave He-Ne laser is used. In certain applications, however, a pulsed laser may be used. In this case, the pulsed laser may be triggered from a signal supplied by the position encoder 72 to ensure that the laser is pulsed each time a filter 38 is aligned. with the optical axis of the apparatus. Alternatively, a set of fiducial marks may be recorded on the disc at the locations of the matched filters. A tracking unit, for examPle one developed from compact disc technology, may be used to follow the surface of the disc using a laser diode. The pulsed laser could be triggered from these fiducial marks.

Theability of thematched filter to discriminate against shapes, other than that of the stored image, is shown to vary for head-on and side-on tanks. The applicants sties confirm that the high-pass spatial filter selects out objects which contain a large proportion of high spatial frequencies. Hence, the smaller head-on targets, which contain significant high frequencies in the horizontal direction, are more easily distinguished than side-on targets. Consequently when storing the matched filters it is advantageous to implement a range of elliptical band-pass spatial filters. This would permit the tailoring of the horizontal pass-band to the size of the target. Care needs to be taken however, to ensure that the exposures of the filters are kept uniform so that the diffraction efficiency of each hologram is kept constant.Also, the auto- correlation signals from each filter need to be optically or electronically normalised. The various spatial filters could be addressed by an electronically updateable transmission SLM, placed in the plane P2 of the system; at present, conventional glass filters are submerged in a liquid gate fitted with optical flats to remove any phase discrepancies between different filters.

Whilst in the above example, the matched filters represent the target at equal l0o increments of aspect angle, it-may be advantageous to make the increments nonlinear because the sensitivity of the correlation will vary non-linearly with the aspect angle of the particular target.

In the above example, a single band of filters is arranged on the holographic spinning disc 36. To increase the capability of the system, several bands of optical filters may be located on the disc with suitable means for supplying a Fourier transform of the input image to each band and a detector at the output side of each band.

The concepts disclosed herein may be applied to a variety of both civil and military pattern recognition tasks; for example: quality control, machine vision and surveillance.

Claims

1. Pattern recognition apparatus comprising means for providing a beam of coherent radiation modulated in accordance with a viewed scene, Fourier transform lens means for forming a F'.ourier transform of said modulated beam at a focal plane thereof, optical matched filter storage means for successively presenting a series of optical matched filters at said focal plane, and detection means for detecting thediffracted beam passed by said optical matched filters.

2. Pattern recognition apparatus according to Claim 1, wherein said plurality of filters makes up a library of filters representing at least one target in a range of different aspect angles.

3. Pattern recognition apparatus according to Claim 1 or 2, wherein said optical matched filter storage means includes a rotatable disc means carrying a plurality of optical matched filters spaced angularly around the disc means.

4. Pattern recognition apparatus according to Claim 3, wherein said disc means includes a plurality of filters arranged in concentric bands extending around said disc.

5. Pattern recognition apparatus according to any preceding claim, wherein said detection means includes relay lens means for forming an output correlation plane of the radiation beam passed by said optical matched filters.

6. Pattern recognition apparatus according to Claim 5, wherein said output correlation signal is incident on a detector located at the correlation plane of said -relay lens means, for detecting correlation of said viewed scene and said optical matched filters.

7. Pattern recognition apparatus according to Claim 2, or any claim dependent thereon, which includes index means associated with each of said optical matched filters for identifying the particular target aspect angle represented thereby, and identification means responsive to said index means and said detection means for identifying the aspect of a target in a viewed scene.

8. Pattern recognition apparatus according to Claim 7, wherein said library of filters contains filters relating to more than one target, and said identification means is operable to identify the particular target in a viewed scene.

9. Pattern recognition apparatus according to any preceding claim, wherein a spatial filter is provided at said focal plane of said first mentioned courier transform lens means for applying a difference of Gaussian (DOG) filtering function to the Courier transform of said modulated laser beam.

10. Pattern recognition apparatus according to Claim 9, wherein the DOG filtering function may be modified dependent on the particular optical matched filter presented to said focal plane.

11. Pattern recognition apparatus according to any preceding claim, wherein said beam of coherent radiation is generated by a pulsed laser means, and means are associated with said optical matched filter storage means for successively triggering said pulsed laser means substantially in synchronism with the successive presentation of said optical matched filters.

12. Pattern recognition apparatus according to any preceding claims, wherein said detection means includes a detector operating at a preset frame rate and said optical matched filter storage means is operable to present the whole series of said optical matched filters at substantially the same rate.

13. Pattern recognition apparatus substantially as hereinbefore described with reference to, and as illustrated in ansT of'the accompaninc drawings.