GB2302598A

GB2302598A - Reflective scatter detector

Info

Publication number: GB2302598A
Application number: GB8821565A
Authority: GB
Inventors: Paul William Walland
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-09-02
Filing date: 1988-09-02
Publication date: 1997-01-22
Anticipated expiration: 2008-09-02
Also published as: SE8902898D0; FR2741449A1; GB8821565D0; GB2302598B

Abstract

A radiation scatter detector mountable on or adjacent to a surface 2 of a small helicopter, comprises an optical system of mirrors and/or lenses arranged to receive light scattered from a region of the surface and to direct such radiation to a photosensor. The system includes a part-toroidal mirror M1 which concentrates the radiation and focuses it onto a detector. The device is particularly suitable for detecting laser radiation from a hostile source but may also be used as a communications link or to gather sunlight.

Description

REFLECTIVE SCATTER DETECTOR This invention relates to an arrangement for detecting scattering of radiation from an object, and relates particularly though not exclusively to the scattering of laser radiation from an aircraft fuselage.

In a copending application UK-A-2001750 there is disclosed a prismatic reflector mounted on a tank for directing incident laser radiation onto a photosensor. However this is not suitable for an aircraft, e.g. helicopter, where the radiation may not be directly incident upon the reflector.

In our copending application 8613297, there is described a laser scatter detector arranged to detect the presence of a laser beam, the scatter detector comprising a sensor which is arranged to respond to reflected light from a vehicle surface and which is mounted a fixed distance from the vehicle surface. Such a detector has been employed with large helicopters but has been found unsuitable for small helicopters since detector units spaced some distance away from the vehicle give undesirable aerodynamic characteristics.

In order to avoid this problem, the present invention provides a radiation scatter detector mounted on or adjacent to a vehicle surface, the detector comprising an optical system of mirrors and/or lenses arranged to received radiation scattered from a region of the surface and to direct such radiation to a photosensor.

In accordance with the invention, it is possible to mount the detector on or adjacent to a vehicle surface, e.g. the surface of the fuselage of a small helicopter, and still detect radiation scattered from the surface.

As preferred, the system of mirrors and/or lenses comprises a first annular mirror arranged at an angle to gather scattered light from a region surrounding the detector and to reflect such gathered light to a second mirror or lens which directs the light to a third mirror or lens of annular form, the cross section of the annulus being arcuate which concentrates and focuses the light onto a central photosensor. The advantage of such an arrangement is that it provides a defined field of view by appropriate selection of dimensions and parameters of the optical system.

As preferred the optical system provides a circular annular field of view surrounding the detector. Alternatively an elliptical field of view may be desired or any other field of view, for example of cruciform shape where the detector is mounted on a winged aircraft.

In the case where only mirrors are provided, it is possible to detect infra-red radiation which might otherwise be absorbed in refractive arrangements.

Although envisaged principally for use as a means of detecting laser radiation from a hostile source incident upon an aircraft or other vehicle, the detector could be used to detect other forms of radiation, e.g. in a communications link or to gather sunlight for solar power.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a schematic diagram of the detector according to the invention mounted on a vehicle surface; Figure 2 is a perspective view of the optical system of the detector of figure 1; Figure 3 comprises three different views of a part-toroidal mirror employed in the system of Figure 2; Figure 4 is a sectional view of the optical system of figure 2 showing the way in which light is reflected onto a photosensor; and, Figure 5 is a sectional view of scale of a preferred example of the embodiment.

Referring now to Figure 1 of the drawings, this shows a detector mounted on the surface 2 of a small helicopter, for example a lynx helicopter, and comprising a detector housing unit 4 of part conical form extending a distance of about 6 inches from the surface of the helicopter and having an optical unit 6 positioned at the top of the housing. Optical unit 6 has a field of view subtending an angle 8 which permits a large area having a width R to be viewed on all sides of the detector so that should radiation, for example laser radiation from a hostile source, impinge on the surface 2 of the helicopter, scattered radiation within the angle W will be detected by the detector. Typically the distance R will be 2 metres.

Referring now to Figure 2 of the drawings, there is shown the optical system of the detector of the present invention as comprising a plane mirror M3 which is arranged in an annular configuration at an angle P to the surface on which the detector is mounted. A mirror M2 is provided beneath mirror M3 to receive light reflected from mirror M3. Mirror M2 is plane and annular in shape and is separate from mirror M3 and mounted into a fixed position relative to mirror M3. Mirror M2 is in the form of an annulus and contains a detector D at its centre. A part-toroidal mirror M1 is positioned within mirror M3 and is arranged to receive radiation reflected from mirror M2 to concentrate such radiation and focus it onto detector D.The form of mirror M1 is more clearly shown in Figure 3 which shows the mirror in the form of an annulus, a cross section of the annulus being in the form of a cross-section of a circle. Mirror M1 is integrally formed with mirror M3 by a plastics moulding process. All three mirrors are formed of plastics having silvered reflective surfaces. As an alternative, the mirrors may be formed of another material, for example reflective aluminium.

Figure 4 shows a cross section of the optical arrangement. The system is symmetric about the centre-line C, and is formed in 3 dimensions by a rotation about C. Construction lines are drawn through the system, indicating the propagation of optical or infra-red radiation through the system. Two rays are shown, being the extreme rays, which therefore define the limits of the field of view of the system. The relative positions of the elements are important to ensure that the rays just fall as shown. The rays Al and A2, falling at angles L1 and L2 land from an adjacent plane surface 2 of the aircraft to give a circular field of regard having inner and outer radii determined by the angles and the distance of the optics from the aircraft surface (N.B.The surface need not be plane; any complex shape can be covered, although the protected area will not necessarily be circular). Although separate for clarity, note that ray Al is virtually coincident with the portion of ray A2 reflected from M3 and M2. A portion of Al is reflected from mirror M3. Since the detector is in the focal plane of the surface of M1, then the rays Al and A2 represent the extreme rays of parallel ray bundles of the edges of the detector. The angle of these bundles of mirror M1 can be derived from the principle ray which strikes M1 at its apex.

Given a detector width d and the extreme angles L1 and L2 it is possible to calculate all other dimensions and angles using the formulae given below.

tan N1 = 2f/D (1) tan N2 = 2f/(D+2d) (2) Nl + P = 900 - N2 (3) L2= N2 - 2P (4) L1 = N2 (5) The widths of mirrors M2 and M3 can also be calculated using the equations below. Note that the width of mirror M1 has already been calculated as D. The widths calculated are horizontal dimensions, not the absolute dimensions of the mirror M3, thus the total width of the optical arrangement can be calculated simply by summing the mirror widths Ml and M3.

M2 = f/tan Nl + f/tan N2 (6) M3 = 4f(l+( 2tan P) (7) 4.f.(1+ 2tanP) (1 - 2 tan P) (l-2tanP) Referring now to figure 4 which is a cross-sectional view of a preferred embodiment to scale of the optical system of the detectors, it may be seen that mirror M3 is an annular mirror 37.6 mm in width arranged at an angle of 11.130 to the horizontal. Mirror M2 is spaced from mirror M3 at a distance of 5.7mm and is 15.8 mm wide.

Mirror M1 is in the form of a portion of a toroid with a width of 8.9mm and of circular arcuate cross-section. The central part of the toroid is a solid portion 7mm across. Detector D is also of 7mm width and receives the light reflected from mirror M1. The whole arrangement is mounted on a detector housing about 150 mm) above a surface from which scattered radiation is to be detected.

Various modifications may be made to the described arrangements without departing from the invention as claimed. For example the mirrors M1, M2 and M3 may be formed as the outer surfaces of a solid perspex prism, the outer surfaces being silvered so as to define reflective surfaces. Naturally the dimensions will be altered to take account of the refractive index of the perspex. Since perspex does not pass infrared radiation, such a solid prism could be made from germanium if it is desired to pass infrared radiation.

Mirror M2 need not be plane but could have some magnification. Mirror M2 could be replaced by a lens arrangement as could mirror M1. Toroidal mirror M1 could be other than a circular section, for example hyperbolic or parabolic. However a circular section is easier for manufacture.

Mirror M1 may be replaced by a frequency sensitive element such as a diffraction grating, to select the frequency of radiation focussed on the detector.

In a modification the photo sensor detector element may be segmented such that the position on the adjacent plane at which radiation falls can be deduced by analysis of the position of radiation on the detector.

This invention is primarily intended as a detector for use as a scattered radiation detector on military or other vulnerable platforms as part of a laser detection and warning system. It could also be used in any application where a laser pulse or pulses is incident upon an area, but cannot be precisely aligned upon a small detector, such as in a communication or data-link system. This may e.g. arise when data is to be transferred from an aircraft overflying a detection area by the use of coded laser transmission. In a highly turbid atmosphere this detector could be used to detect laser radiation passing close to, but not incident upon, the detector system, which is scattered from the particles suspended in the atmosphere or other medium. Since the present invention is a reflective optical system it could equally well operate within an aqueous or other liquid medium without degradation.

Claims

CLAIMS:

1. A radiation scatter detector mounted on or adjacent to a surface, the detector comprising an optical system of mirrors and/or lenses arranged to receive light scattered from a region of the surface and to direct such radiation to a photosensor.

2. A detector as claimed in claim 1, wherein said system includes a first annular mirror arranged at an angle to the surface in order to define a field of view of an annular region of the surface.

3. A detector as claimed in claim 2 wherein said angle is approximately 110, and the width of the mirror is approximately 100mm.

4. A detector as claimed in claim 2 or 3 including a plane mirror for redirecting the light reflected from said first annular mirror.

5. A dector as claimed in any preceding claim including a third annulas mirror, the annulus having an arcuate section for concentrating and focussing scattered light onto a photosensor.

6. A detector as claimed in claim 5 wherein the third mirror has a circular arcuate section.

7. A detector as claimed in claim 5 or 6 as appendant to claim 2 wherein said first annular mirror and said third mirror are formed as an integral plastics pressing.

8. A radiation detector, substantially as described with reference to the accompanying drawings.

8. A dector as claimed in any preceding claim, wherein the dimensions of the optical system conform to the equations set forth as equations (1) to (7) above.

9. A radiation detector including a mirror system for defining a field of view of the detector and an annular mirror, the annulus having an arcuate section for concentrating and focussing radiation within the field of view onto a photosensor.

10. A detector as claimed in claim 9 including any of the features of claims 1 to 8.

11. A radiation detector substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radiation scatter detector mountable on or adjacent to a surface, the detector comprising an optical system of mirrors and/or lenses defining a field of view of the detector for receiving radiation scattered from the surface; and an annular mirror, the annulus having an arcuate section for concentrating and focusing radiation reflected from the optical system onto a photosensor.

2. A detector, as claimed in claim 1, wherein said system includes a first annular mirror arranged at an angle to the surface in order to define the field of view of an annular region of the surface.

3. A detector, as claimed in claim 2, wherein the angle of the first annular mirror is approximately 110 and the width of the mirror is approximately lOOmm.

4. A detector, as claimed in claim 2 or 3, including a plane mirror for redirecting the light reflected from the first annular mirror.

5. A detector, as claimed in any preceding claim, wherein the arcuate annular mirror is of part toroidal shape having a circular or parabolic or hyperbolic section.

6. A detector, as claimed in any preceding claim as appendant to claim 2, wherein the first annular mirror and arcuate annular mirror are formed as an integral plastics pressing.

7. A detector, as claimed in any preceding claim, wherein the dimensions of the optical system conform to the equations set forth as equations (1) to (7) above.