GB2504249A

GB2504249A - Apparatus for detecting varying intensity coherent optical radiation

Info

Publication number: GB2504249A
Application number: GB8919930.1A
Authority: GB
Inventors: Hillary Gil Sillitto
Original assignee: Selex Galileo Ltd
Current assignee: Leonardo UK Ltd
Priority date: 1989-09-04
Filing date: 1989-09-04
Publication date: 2014-01-29
Anticipated expiration: 2009-09-04
Also published as: GB2504249B; GB8919930D0

Abstract

A cylindrical objective lens 10 views a field of view within which a source of radiation may be located. The objective lens 10 produces a line image on a mask 11 located in the focal plane of the objective lens. The mask carries a number of parallel tracks extending perpendicular to the line image, each track comprising a sequence of opaque and translucent regions. Optical means comprising further cylindrical lenses 13, 14, 15 are provided to image the illuminated portions of the tracks on the mask 11 on to a linear array of detectors 12. Circuit means are provided which respond to the electrical outputs of the detectors to provide an indication of the direction of the source relative to a datum direction. The mask tracks may form a Gray code and further tracks may be provided to guard against illumination by a source of large angular subtense or incomplete illumination of the mask providing a false indication. A second similar apparatus may be modified to provide an indication of wavelength (figure 4, not shown).

Description

APPARATUS FOR DETECTING VARYING-

INTENSITY COHERENT OPTICAL RADIATION

The use of sources of coherent opticaL radiation, i.e. lasers, for range finding and target designation in military theatres makes it necessary for potential targets to be able to identify their illumination for such purposes. Laser warning receivers are in themselves well known but are of limited use in determining the direction of the threat posed by the source of radiation. Military vehicles such as tanks may be provided with a number of detectors each covering an angular field of 300 or more, but these are generally unable to do more than indicate that the source lies within the field of view of one of these detectors. In a situation where speedy action may he necessary against the source a far more accurate indication of its position is necessary.

Of course, the requirement for such accurate determination may be satisfied by known technology, but only by resorting to high cost and complexity in equipment and signal processing ability.

It is an object of the invention to provide a relatively simple apparatus for detecting coherent radiation and determining the direction of the source to a high degree of accuracy.

According to the present invention there is provided apparatus for detecting varying-intensity coherent optical radiation from a source located within a field of view, which includes a linear array of detectors responsive to optical radiation of a wavelength to be detected, a cylindrical objective lens arranged with its longitudinal axis parallel to the orientation of the array, a mask comprising a number of parallel tracks extending in a direction substantially perpendicular to a line image of the source formed by the objective lens on the mask, each track comprising a sequence of opaque and translucent regions, optical means arranged to form an image of each track of the mask on to a separate detector of the array and circuit means responsive to the electrical output signals from the optical detectors to provide an indication of the direction of the source of coherent radiation relative to a datum direction.

The term "varying-intensity optical radiation" is used to indicate the intensity of radiation as received by the detector array and thus applies equally to a pulse source and to a continuous source the radiation from which is moved across the objective lens.

The invention will now be described with reference to the accompanying drawings, in which:-Figure 1 is a schematic diagram of the optical elements of the apparatus according to a first embodiment; Figure 2 is a representation of one form of mask used in the embodiment of Figure 1; Figure 3 is a schematic block diagram of electronic circuitry suitable for use with the optical elements of Figure 1; and Figure 4 is a schematic diagram showing optical elements suitable for determining the wavelength of detected optical radiation.

Referring now to Figure 1, the optical system of the apparatus has a cylindrical objective lens 10 which gathers optical radiation from a field of view of, say, 10 degrees.

This objective lens forms a line image on a mask 11 which located in the focal plane of the objective lens. The line image extends in a direction parallel to the longitudinal axis of the objective lens 10. The mask 11 carries a pattern of coded tracks, each track comprising alternate opaque and translucent areas extending in a direction perpendicular to the line image formed on the mask. The tracks are shown schematically in Figure 1 and will be described in more detail below.

A linear array of optical radiation detectors, shown generally at 12, is provided. This array contains at least as many detectors as there are tracks on the mask 11. Setween the mask and the detector array is an optical system comprising, in the embodiment illustrated, three cylindrical lenses 13, 14 and 15. Lens 13, the relay lens, forms an image of the illuminated part of each track of the mask 11 on to the appropriate one of the array of detectors. The other pair of lenses 14 and 15 form a demagnified image of the objective on the detector array.

Figure 2 illustrates a simple form of the mask 11 of Figure 1. The mask is a sheet of material such as nickel which carries a series of parallel coded tracks. The outermost tracks 20 shown in the drawing are guard tracks, the function of which will be described later. Tracks 21 to 24 define a binary Gray code, the property of which is that adjacent codes obtained by reading all four tracks vary by only one bit. The code tracks are made up of alternate opaque and translucent or transparent elements.

The mask also carries a further three tracks 25 to 27 the purpose of which will be described later.

The operation of the apparatus with the Gray-code mask tracks is as follows:-A point source of variable-intensity coherent radiation located within the field of view of the objective lens 10 and directed at the objective lens will result in a line image 28 being formed on the mask 11 located in the focal plane of the objective lens 10. The position of the line on the mask will depend upon the direction of the source from the objective relative to a datum of the apparatus. The line 28 shown in Figure 2 results from a source located slightly off-centre, and the line may occupy any position on the mask dependent upon the position of the source of radiation. The Gray code produced by the line illumination of the mask as shown in Figure 2 is 0100. Thus of a maximum of four possible points of radiation which could pass through the gray code tracks of the mask, only one is present in this instance. This is focussed by relay lens 13 on to the appropriate one of four detectors in the array 12 to produce an electrical output from that detector. It will be seen therefore that the combination of output signals from the four detectors of the array will define the direction of the soUrce of radiation within the limits of resolution of the mask. The resolution clearly depends upon the number of Gray-code tracks on the mask and a practical form of the mask would probably have seven or eight tracks.

It is necessary to be able to distinguish a point source of radiation directed at the objective lens from a source of large angular subtense or perhaps a source outside the field of view which achieves a direct strike on the objective.

Clearly in both of these latter cases any output from the apparatt2s will be spurious, and the tracks 25 to 27 are provided to cater for these possibilities. These three tracks provide a three-element repeating code such that a line image such as that shown at 28 cannot illuminate all three tracks simultaneously.

Three further detectors are added to the array to detect the radiation from these three tracks. If all three detectors provide an output then the source of radiation must be one which causes the objective lens 10 to illuminate a large area of the mask IL and therefore cannot be a point source. Circuitry is provided which responds to this situation to indicate that any directional indication produced by the outputs of the Gray-code detectors is spurious.

The two guard tracks 20 are provided to ensure that outputs resulting from a point source having an elevation angle such that only part of the mask is illuminated are also ignored as erroneous. Only if both guard tracks are illuminated is the source considered to be within the field of view of the objective. Two additional detectors in the linear array provide the signals caused by radiation passing through the guard tracks and the absence of either of both of these indicates that the position readout is spurious.

As has already been stated, the nature of the threat to be detected means that the source is unlikely to be continuous and unmoving, that is there is unlikely to be a constant output from any detector in the detector array. In practice varying atmospheric conditions, including the presence of smoke or atmospheric scintillation, will tend to cause even radiation from a constant source to fluctuate. However general background radiation and truely constant sources may be eliminated by providing a.c. coupled circuits to handle the detector outputs, so that only time-varying signals will be used in the subsequent signal processing. Although the range of magnitude of the illumination received by any detector may vary considerably it is also preferable to define a minimum useful signal level or threshold.

The circuitry necessary to process the signals from the detector array is well known, and Figure 3 shows only a schematic block diagram setting out the necessary functions. -The diagram shows a separate detector corresponding to each of the tracks on the mask 11 of Figure 1. For example the two detectors 30 relate to the guard tracks 20, detectors 31 to 34 relate to the Gray-code tracks 21 to 24, whilst detectors 35 to 37 relate to tracks 25 to 27. The outputs from each of the detectors 31 to 34 pass through threshold amplifiers, shown collectively at 38 to a converter or decoder 39. This changes any combination of signals from the four detectors 31 to 34 to a single output which indicates the direction of the source in azimuth relative to the datum direction. The decoder 39 may be a simple R011 acting as a look-up table or may be an active decoding circuit.

The outputs of detectors 30 and 35 to 37 are used in son manner to indicate when the outputs of detectors 31 to 34 are spurious. Clearly this may be done in several ways. For example, as shown in Figure 3 the output from the decoder 39 may be blocked by the output of a gate 310 which blocks the output of the decoder 39 if both guard detector outputs are not present. Similarly the outputs of detectors 35 to 37 may be applied to a circuit 311 which provides an output only if one or two of the outputs, but not all three, are present and this may be used to enable a further gate 312. Clearly other enabling or blocking arrangements may be used. The same arrangement may be used to control the application of signals to the decoder 39 rather than to the output signals from the decoder.

Alternatively the outputs from detectors 30 and 35 to 37 may be used to apply a simple "error" indication to accompany the decoder output.

It will be clear from the above description that the apparatus described gives an indication of the direction of a radiation source in one dimension only. If, as described, the mask tracks are horizontal with the longitudinal axes of the cylindrical lenses aligned accordingly, then the direction of the source relative to a datum direction is determined only in azimuth. Clearly in order to determine the actual position of the source it is necessary to measure also the angular direction of the source in elevation. This is done by providing duplicate apparatus differing only in that the tracks of the mask are vertical and the axes of the cylindrical lenses are aligned appropriately.

A further property of the source which may be of use, in addition to its angular direction in two dimensions, is the wavelength of the coherent radiation emitted *by the source.

This may help to determine the nature of the threat posed by the source.

Figure 4 is a schematic diagram which shows a plan view of the optical elements of apparatus for determining the wavelength, located alongside apparatus for determining the direction of the source as already described with reference to Figure 1. That former apparatus is given the same reference numbers as in Figure 2. It will be seen from Figure 4 that the main difference is the insertion of a dispersing prism 46 and folding mirror 47 in addition to the optical elements 40 to 45 of the existing optical system. The prism 46, which may conveniently be a Pellin-Broca prism, and the folding mirror 47, are located in front of the cylindrical objective lens 4D. The prism produces, for a coherent monochromatic source, an optical output the position of which across the mask 41 depends upon the wavelength of the radiation. Thus circuitry similar to that of Figure 3 may be used to convert the output signals of the detector array 42 into an indication of the wavelength of the radiation emitted by the source.

It will be seen that in Figure 4 the relay lenses 13 and 43 are located between the pairs of field lenses 14, 15 and 44, 45. This is simply one of several alternative arrangements which may be used.

It will be noted that the mask is positioned in the focal plane of the cylindrical objective lens. The intensity of radiation striking the mask may be very high arid the mask must therefore be made from a material, such as nickel, which will withstand this. The detector array is located in the pupil of the optical system by means of the field and relay lenses and hence is not subject to such intense radiation.

Claims

Claims: 1. Apparatus for detecting varying-intensity coherent optical radiation from a source located within a field of view, which includes a linear array of detectors responsive to optical radiation of a wavelength to be detected, a cylindrical objective lens arranged with its longitudinal axis parallel to the orientation of the array, a mask comprising a number of parallel tracks extending in a direction substantially perpendicular to a line image of the source formed by the objective lens on the mask, each tracks comprising a sequence opaque and translucent regions, optical means arranged to form an image of each track of the mask on to a separate detector of the array, and circuit means responsive to the electrical output signals from the optical detectors to provide an indication of the direction of the source of coherent radiation relative to a datum direction.
2. Apparatus as claimed in Claim I in which the mask comprises a number of binary-coded tracks arranged such that the outputs of the detectors associated with those tracks provide an unique indication of the direction of the source.
3. Apparatus as claimed in Claim 1 in which the number of tracks together define a Gray code.
4. Apparatus as claimed in any one of CLaims 1 to 3 in which the mask includes further tracks each having an associated detector in the detector array and arranged to provide an error indication when the source is of large angular subtense.
5. Apparatus as claimed in any one of Claims 1 to 4 in which the mask includes at least two guard tracks each having an associated detector in the detector array and arranged to provide an error indication when the source is located outside the field of view of the objective lens such that the whole area of the mask carrying tracks is not illuminated.
6. Apparatus as claimed in any one of the preceding claims in which the optical means is arranged such that the detector array is located in the pupil of the optical arrangement.
7. Apparatus as claimed in any one of Claims 1 to 6 in which the circuit means is ac coupled to eliminate response to steady-state sources of radiation.
8. Apparatus as claimed in Claim 4 in which the circuit means includes error-detecting means responsive to the outputs of the detectors associated with said further tracks to provide said error indication.
9. Apparatus as claimed in Claim 5 in which the circuit means includes error detecting means responsive to the outputs of the detectors associated with said guard tracks to provide said error indication.
10. Apparatus as claimed in either of Claims S or 9 in which the error detecting means responds to said error indications by blocking the output of the circuit means.
11. Apparatus for detecting varying-intensity coherent optical radiation substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.
12. A detection system which includes first and second apparatus each as claimed in any one of the preceding claims, the direction of the tracks on the mask of a first apparatus being substantially perpendicular to the tracks on the mask of the second apparatus.
13. A detection system which includes at least one apparatus as claimed in any one of Claims 1 to 11 and a second such apparatus modified in that it includes an optical prism located between the source and the objective lens so as to produce on the mask of the second apparatus a line image the position of which is dependent upon the wavelength of the optical radiation emitted by the said source.
14. A detection system substantially as herein described with reference to Figure 4 of the accompanying drawings.