GB2155722A

GB2155722A - Optical apparatus

Info

Publication number: GB2155722A
Application number: GB08504411A
Authority: GB
Inventors: John Ackroyd
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1984-02-24
Filing date: 1985-02-20
Publication date: 1985-09-25
Also published as: GB8504411D0

Abstract

Optical trackers of the kind which are used as position detectors and which incorporate Fourier transform filters 6, 7 matched to an object 1, 2 to be detected within a field-of-view, e.g. a target 2, indicate the position of object within the field-of-view if it is present as the position of a spot of light in the correlation plane 8 of the tracking system. However, positional errors may be introduced due to inaccuracies in the alignment of the filter axis and the reconstruction lens used to provide the output and absolute positions cannot be obtained. The invention described herein uses a Fourier transform filter 6 matched to two objects which may appear together in a given field-of-view e.g. missile and target, the presence of both objects in the field-of-view being indicated as two spots of light in the correlation plane of the tracker, the relative positions of which being equivalent to the relative positions of the two objects within the field-of-view, which is preceded by a device 4 which spatially modulates the incoming incoherent light onto a laser beam. <IMAGE>

Description

SPECIFICATION Optical apparatus This invention relates to optical apparatus of the character disclosed in UK Patent Specification No.

1,177,632. Therein, there is described an optical tracker wherein radiation from a field-of-view is passed to a Fourier transform filter matched to a particular object within the field-of-view and a detector receives the filtered radiation and indicates the co-ordinates of the object within the field-ofview.

The output from the filter appears as a spot of light in what is sometimes known as the correlation plane, the position of the spot within the plane being related to the position of the chosen object within the field-of-view. However, the absolute position of the spot may not reliably indicate the position of the object. For example, a position error may be introduced due to an inaccurate relationship between the axes of the filter and a reconstruction lens used to inverse-transform the filter output.

According to the present invention, there is provided optical apparatus comprising imageforming means for forming an image of a viewed scene, coherent converter means for receiving said image and for supplying a coherent light beam spatially modulated in accordance with said image, Fourier transform means for receiving said beam and forming a Fourier transform image corresponding to said scene image, an optical transform filter arranged to receive said Fourier transform image, inverse-transform means for receiving the filtered Fourier transform image and for forming a corresponding light pattern, and light sensitive detector means for receiving said pattern, said filter comprising a Fourier transform holographic record of two items which may appear within said scene whereby, when said items are present, said light pattern will comprise respective correlation elements at relative positions within the pattern indicative of the relative positions of said items within said scene, and said light sensitive detector means is operable for producing electrical signals indicative of said relative positions of said correlation elements within said pattern.

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 representation of an optical tracking system suitabie for tracking a missile and its target.

The illustrated tracker comprises a focussing lens 1 which focusses an image of a viewed scene containing a target 2 and a missile 3 onto a coherent converter 4. The converter 4 converts the incoherent light image of the viewed scene into a coherent image beam which beam then passes to a Fourier transform lens 5. The transformed beam is then incident on a Fourier transform filter 6 which is optically matched to the target 2 and missile 3. The filtered beam then passes through an inversetransform lens 7 and forms a light pattern at an output or correlation plane 8. The filter 6 comprises a Fourier transform holographic record of two items respectively corresponding to characteristics of the target and missile. For example, the missile item could comprise a point to match what would be seen from a distance of the missile flare or perhaps a beacon carried thereon.The record is made using the same reference beam for the two items so that the resulting filter incorporates which may be called a logical 'OR' function, i.e. the light pattern produced at plane 8 comprises two spots of light, one for the missile and one for the target, if both are present. If only one of either the missile or target is present then only the corresponding one of the light spots will be present. The positions of the spots within plane 8 will depend upon the positions of the target and missile within the viewed scene but, as mentioned earlier, the position of either spot may not accurately correspond to the absolute position of the corresponding object. On the other hand, the relative positions of the two spots will accurately correspond to the relative positions of the objects.

Accordingly, a spot position sensor 9 senses the co-ordinates of the two spots within plane 8 and sends electrical signals indicative of these coordinates, X, and Y1 for the target 2, and X2 and Y2 for the missile 3, to an adaptive controller 10. The adaptive controller 10 calculates the difference in the spot positions and produces command signals, X and Y, which are transmitted to the missile via a suitable transmitter 11 to guide the missile onto the target. The missile 3 may be aligned with the target 2 by moving the missile until the two spots in the output or correlation plane 8 coincide.

The spot position sensor 9 may be a TV system incorporating a threshold detector in order to provide the co-ordinates of the two bright spots of light.

The coherent converter 4 could comprise an electronic real-time spatial light-modulator arranged to receive the incoherent scene image at its input face and a coherent light beam from a laser at its output face, the beam being spatially modulated in accordance with the scene image.

As mentioned, the filter 6 comprises a Fourier transform holographic record of the missile and target. This could be made by directing coherent light from a laser th rough a photographic transparency of the target onto a holographic plate, a portion of the laser light also being passed directly to the plate to act as the reference beam for the hologram. The transparency is then replaced by a mask containing a pin-hole and a further exposure of the holographic plate made. This second exposure records the missile optical characteristics which, from a distance, forms a point source. The pin-hole could be axially aligned with the target image transparency or be offset to one side. If offset, the resulting filter could be used where it is desired to guide the missile so that it maintains a fixed but not coincident positional relationship with the target. Alternatively, this could be done by using different reference beam angles when making the two holographic plate exposures or by electronically introducing suitable offsets into the signals from the sensor 9 or controller 10.

Claims

1. Optical apparatus comprising image-forming means for forming an image of a viewed scene, coherent converter means for receiving said image and for supplying a coherent light beam spatially modulated in accordance with said image, Fourier transform means for receiving said beam and forming a Fourier transform image corresponding to said scene image, an optical transform filter arranged to receive said Fourier transform image, inverse-transform means for receiving the filtered Fourier transform image and for forming a corresponding light pattern, and light sensitive detector means for receiving said pattern, said filter comprising a Fourier transform holographic record of two items which may appear within said scene whereby, when said items are present, said light pattern will comprise respective correlation elements at relative positions within the pattern indicative of the relative positions of said items within said scene, and said light sensitive detector means is operable for producing electrical signals indicative of said relative positions of said correlation elements within said pattern.

2. Optical apparatus according to claim 1 which further comprises control means operable to guide one of said items within said scene in accordance with said electrical signals indicative of said relative positions.

3. Optical apparatus according to claim 1,2 or 3, wherein the coherent converter means comprises an electronic real-time spatial light modulator.

4. Optical apparatus according to claim 1,2 or3, wherein the light sensitive detector means comprises a TV system.

5. Optical apparatus according to claim 4, wherein the TV system incorporates a threshold detector.

6. Optical apparatus substantially as herein before described with reference to the accompanying drawing.

7. A missile tracker system comprising optical apparatus according to any preceding claim wherein said filter is a holographic record of a missile and a target.