GB2193020A

GB2193020A - Multiple correlation system

Info

Publication number: GB2193020A
Application number: GB08614670A
Authority: GB
Inventors: Michael Bernard Brown; Martin Lawrence
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1986-06-17
Filing date: 1986-06-17
Publication date: 1988-01-27
Also published as: GB8614670D0

Abstract

An optical correlation system comprises coherent light source means 2,3 for supplying coherent light at more than one wavelength and a spatial light modulator 7 for modulating said light in accordance with a viewed scene, means 10 performing a Fourier transform of the spatially modulated light and an optically matched holographic filter 11 positioned in the Fourier transform plane which contains a holographic record of an item to be detected within the viewed scene. A spot in the detection plane 15 indicates the presence of the item to be detected in the viewed scene. Use of more than one wavelength enables the item to be detected despite variations in its size and for orientation, the filter having holographic representations of the item for different orientations for different wavelengths, and also being rotatable. <IMAGE>

Description

SPECIFICATION Optical correlation system This invention relates to optical correlation system for example a system for detecting the presence of a predetermined item in viewed scene.

In a previously proposed optical correlation system, coherent light modulated in accordance with a viewed scene is focussed onto a holographic matched filter which has been made photographically recording the interference pattern produced by a plane reference beam and an image beam from a Fourier transform image of the item to be detected, both the reference beam and the image beam being derived from the same coherent light source. Light passed by the filter is imaged to form a correlation pattern indicative of the presence and position of the item within the viewed scene.

The degree of correlation naturally depends upon the degree of correspondence between the size, orientation and shape of the item it appears in the viewed scene and the size, orientation and shape of the item used to make the filter.

It is therefore an object of the invention to provide an optical correlation system in which improved correlation is obtained irrespective of variations in size, orientation and shape of the item in the viewed scene.

According to one aspect of the present invention, there is provided a system for detecting the presence of a predetermined item in a viewed scene, the system comprising: coherent light source means for supplying coherent light at more than one wavelength; means for spatially modulating said light in accordance with said scene; first focussing means for forming a Fourier transform of said spatially modulated light in a transform plane; filter means comprising a holographic record of said item and being positioned in said transform plane to receive said transform; second focussing means for focussing the light leaving said filter means to form an image at an output plane; and detector means for forming signals indicative of said image.

The light source means may produce the different light wavelengths simultaneously i.e.

it may comprise a plurality of monchromatic sources producing light at respective different wavelengths or a single polychromatic source, i.e. one which emits a series of spectral lines.

Alternatively, the light may have only one wavelength at any one time but that wavelength is scanned or varied as required. In this case, the light source could comprise a tunable laser the lasing cavity of which can be altered to enable light of a range of wavelengths to be emitted.

In one embodiment said filter means comprises a spectraliy sensitive multilayer holographic plate on which a set of holographic records of an item is stored, each record being formed using light of a single wavelength corresponding to one of said wavelengths of said coherent light source means.

As an alternative, said holographic plate may contain a set of holographic records corresponding to different items.

In another embodiment, said filter means comprises a holographic plate on which one holographic record is stored, said record being formed with light having a single wavelength within a range of wavelengths emitted by said coherent light source means, whereby a variation in size of said item with respect to said record can be accommodated.

According to a second aspect of the invention, there is provided a coherent optical correlation system for investigating the correlation between a reference image and another image, the system comprising coherent light source means for producing coherent light at a plurality of different wavelengths, modulating means for spatially modulating said light in accordance with said another image, first focussing means for converging the modulated light to form a Fourier transform of said another image, filter means positioned to receive said Fourier transform and comprising a holographic record of said reference image, and second focussing means for forming an image comprising light leaving said filter means.

According to a third aspect of the invention, there is provided a method of investigating the correlation between a reference image and another image, the method comprising producing coherent light at a plurality of different wavelengths, spatially modulating said light in accordance with said another image, converging the modualted light to form a Fourier transform of said another image, filtering said Fourier transform using a filter comprising a holographic record of said reference image, forming an image from light leaving said filter and investigating the components of this image.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a schematic block diagram of an optical correlation system.

Optical correlation systems such as that illustrated in the figure, operate by forming a coherent image of a viewed scene and then transforming that image to form a Fourier transformed image in a Fourier transform plane. The Fourier image is made incident on an optically matched holographic filter which contains a holographic record of an item which is to be detected within the viewed scene. The output from the filter is then inverse-transformed to produce an output in a correlation or output plane, the output being a bright spot of light if correlation is obtained i.e. if the item corresponding to the holographic record stored on the filter is present within the viewed scene.The filter itself is made by recording an interference pattern between a reference beam and an image beam from the item, both the reference beam and the image beam being derived from the same coherent light source.

In the figure, a coherent record of a viewed scene is formed in an input plane 1 using a coherent light source 2. Light from the source 2 passes to a beam expander 3 which consists of a pair of lenses 4, 5, and is then incident on the 'read' surface 6 of a spatial light modulator (SLM) 7. Light 8 from the viewed scene is incident on the 'write' surface 9 of the SLM 7 to produce a modulated 'read' surface 6 corresponding to the viewed scene.

The coherent record formed in the input plane 1 is then transformed by a Fourier transform lens 10 to produce a transformed image in a Fourier transform plane 11. An optically matched holographic filter 12 is positioned in the transform plane 11 to receive the transformed image and contains a holographic record of an item to be detected within the viewed scene. The output from the filter 12 is an indication of any correlation between the viewed scene and the holographic record, and is then inverse-transformed by a further lens 13 producing an output 14 in a correlation or output plane 15.

If the item corresponding to the holographic record is present in the viewed scene and corresponds in shape, size and orientation to the holographic record stored in filter 12, the output 14 takes the form of a bright spot of light in the output plane 15. This spot of light is then detected by a detector (not shown) and which may be a colour television camera for example. The position of the spot of light in the output plane 1 5 is an indication of the position of the item within the viewed scene.

If, however, the item is present in the viewed scene but is of a different size and orientation, correlation may still be obtained but it may not be determined as easily, i.e. the spot of light obtained in the output plane 1 5 may be below the threshold level at which the detector distinguishes it from the background. In order to accommodate changes in size and orientation a different filter has to be used for each possible orientation and size of the item.

Variations in orientation of an item to be detected within the viewed scene can be accommodated by using a multiple wavelength coherent light source and a multiple exposure holographic filter, each exposure of the filter corresponding to a different orientation of the item. In such a case, each exposure would be made using light of a different wavelength. For example, if red, green and blue wavelengths are used, the red wavelength may be used to make a record of the front of item, the green, a record of the side elevation and the blue, a record of the rear of the item. If correlation is obtained, i.e. the item is present within the viewed scene, the spot of light will be coloured red, green or blue respectively according to which of the three records is being detected within the viewed scene.

Similarly, a multiple exposure filter can be used to detect different items which may be present within a viewed scene. Variations in size of the item in the viewed scene, when compared with the holographic record used, may be accommodated using a single optically matched filter and a multiple wavelength coherent light source. The filter would be made using light of a wavelength within the range of the multiple wavelength source i.e. if the multiple wavelength source emits light in the red, green and blue regions of the spectrum, the filter could be made using green coherent light. This filter would be able to reconstruct the holographic record of the item using both the red and blue light as well as the green.

The corresponding sizes of the record would be reduced and enlarged respectively, and when the coherent record of the viewed scene is formed in the input plane 1 using the multiple wavelength source, correlation would be obtained if there is a variation in size of the item but having the same orientation, the colour of the spot of light in the output plane 15 being an indication of the size of the item detected. Due to a multiple wavelength source being used to form the record in the input plane 1, a series of Fourier transforms are obtained in the transform plane 11, each transform corresponding to a single wavelength of the source. The scale of the transforms being proportional to the radiation used and inversely proportional to the size of the record in the plane 1.

Additionally, accommodation of variations in size of an item could be achieved together with accommodation of variations in orientation by forming records of the item in different orientations using green light, say, each orientation being recorded using a different angle of the reference beam used. In this case, by rotating the filter, a range of different orientations and size can be accommodated in a single optically matched filter.

The multiple wavelength source may be a single coherent source emitting a number of spectral lines simultaneously. As an alternative, the multiple wavelength source may be a tunable laser source in which the wavelength range can be scanned by mechanical or electronic means. Naturally, the multiple wavelength source may be implemented by individual coherent sources each emitting a particular wavelength of light.

The coherent record of the viewed scene formed in the input plane 1 may be produced in some other way instead of using SLM, for example, a transparency may be used.

Claims

1. A system for detecting the presence of a predetermined item in a viewed scene, the system comprising: coherent light source means for supplying coherent light at more than one wavelength; means for spatially modulating said light in accordance with said scene; first focussing means for forming a Fourier transform of said spatially modulated light in a transform plane; filter means comprising a holographic record of said item and being positioned in said transformed plane to receive said transform; second focussing means for focussing the light leaving said filter means to form an image at an output plane; and detector means for forming signals indicative of said image.

2. A system according to claim 1 wherein the light source comprises a plurality of monochromatic sources for producing light at different wavelengths.

3. A system according to claim 1 wherein the light source comprises at least one polychromatic source.

4. A system according to claim 1 wherein the light source is adapted to emit light the wavelength of which varies with time.

5. A system according to claim 4 wherein the light source comprises a tunable laser.

6. A system according to any preceding claim wherein the filter means comprises a spectrally sensitive multilayer holographic plate on which a set of holographic records is stored each record being formed using light of a single wavelength corresponding to one of the wavelengths of the coherent light source means.

7. A system according to claim 6 wherein the holographic plate carries a set of holographic records of a single item.

8. A system according to claim 6 wherein the holographic plate carries a set of holographic records corresponding to a plurality of items.

9. A system according to any one of claims 1 to 5 wherein the filter means comprises a holographic plate on which one holographic record is stored which record is formed with light having a single wavelength within a range of wavelengths emitted by the coherent light source means whereby a variation in size of said item with respect to said record can be accommodated.

10. A coherent optical correlation system for investigating the correlation between a reference image and another image, the system comprising coherent light source means for producing coherent light at a plurality of different wavelengths, modulating means for spatially modulating said in accordance with said another image, first focussing means for converging the modulated light to form a Fourier transform of said another image, filter means positioned to receive said Fourier transform and comprising a holographic record of said reference image, and second focussing means for forming an image comprising light leaving said filter means.

11. A system substantially as herein described with reference to, and as illustrated in, the accompanying drawing.

12. A method of investigating the correlation between a reference image and another image, the method comprising producing coherent light at a plurality of different wavelengths, spatially modulating said light in accordance with said another image, converging the modulated light to form a Fourier transform of said another image, filtering said Fourier transform using a filter comprising a holographic record of said reference image, forming an image from light leaving said filter and investigating the components of this image.

13. A method substantially as herein described with reference to, and as illustrated in, the accompanying drawing.