GB2251938A

GB2251938A - Passive infrared intruder detector

Info

Publication number: GB2251938A
Application number: GB9200223A
Authority: GB
Inventors: George Heftman
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1991-01-15
Filing date: 1992-01-07
Publication date: 1992-07-22
Anticipated expiration: 2012-01-07
Also published as: GB9100791D0; HK23195A; GB9200223D0; GB2251938B

Abstract

A passive infra-red intruder detector has two detector elements 15 and 16 mounted within a semicylindrical array 12 of lenses 13. A plane mirror 25 and 26 is mounted in front of each detector element so that radiation passing through lenses 13' or 13'' at the periphery of the array is reflected onto the detector element 15 or 16 on that side of the array. The mirrors 25 and 26 are inclined at 45 degrees to the normal N with each mirror aligned edge on to the opposite detector 16 and 15 and such that if radiation from a source is prevented from reaching one detector by its mirror, it can pass to the other detector. <IMAGE>

Description

DETECTOR ASSEMBLIES AND APPARATUS This invention relates to detector assemblies and apparatus including such assemblies.

The invention is more particularly concerned with passive infra-red detector assemblies which are used to detect the presence of an intruder or other person by the infra-red radiation emitted by that person.

In order to ensure that as wide an area as possible is covered, the detector assembly preferably has a wide viewing angle. Alternatively, several assemblies can be used but this adds to cost and mounting difficulties. The viewing angle is usually made as large as possible by incorporating in the assembly an array of lenses located in front of the detector, the lenses being oriented such that they focus onto the detector IR radiation incident on the assembly from different directions. Using such a lens array, it is usually possible to achieve a viewing angle of about 90 degrees.

It is an object of the present invention to provide an improved detector assembly.

According to one aspect of the present invention there is provided a detector assembly including first and second optical radiation detector elements and an array of lenses located in front of the detector elements, a reflector being located intermediate the first detector element and the lens array, and the reflector being oriented and located such that it reflects radiation from one periphery of the lens array onto the first detector element and such that radiation from a source obscured by the reflector from reaching the first detector element passes to the second detector element whereas radiation from a source obscured by the reflector from reaching the second detector element passes to the first detector element.

The assembly preferably includes a second reflector, the second reflector being oriented and located such that it reflects radiation from an opposite periphery of the lens array onto the second detector.element and such that radiation from a source obscured by the second reflector from reaching the second detector element passes to the first detector element whereas radiation from a source obscured by the second reflector from reaching the first detector element passes to the second detector element.

The first reflector, the first detector element and said one periphery of the lens array are preferably located on one side of the assembly, the second reflector, the second detector element and the opposite periphery of the lens array being located on an opposite side of the assembly. The first reflector is preferably aligned edge on with the second detector element, the second reflector being aligned edge on with the first detector element.

The or both reflectors are preferably inclined at an angle of about 45 degrees to a normal to the detector elements.

The or each reflector may be plane. The lenses of the array are preferably of elongate shape, the lens array being of semicylindrical shape with a concave surface presented towards the detector elements. The detector elements are preferably responsive to infra-red radiation, the or each reflector being reflective of infra-red radiation and the lens array focussing infra-red radiation on the detector elements. The outputs of the detector elements may be connected in series in antiphase.

Passive infra-red intrusion detector apparatus including a detector assembly in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a perspective view of the apparatus; Figure 2 is a cut-away plan view of the apparatus; and Figure 3 is an elevation view of a part of the apparatus along the line N.

The apparatus includes a wall-mounted casing 1, in which is mounted the detector assembly 10 and a processing unit 11. A light 2 and audible alarm 3 form a part of the apparatus external of the casing 1. The detector assembly 10 detects the presence of an intruder from his body infra-red emissions and triggers energization of the light 2 and alarm 3. Alternative apparatus need not include both the alarm and/or light but might instead provide an output to energize a camera, solenoid lock, a separate intruder alarm system or the like.

The detector assembly 10 includes a lens array 12 of semicylindrical shape having many parallel converging lenses 13 of elongate shape extending parallel to the axis of the array. The lenses 13 may be of convex section or may be fresnel lenses. Preferably, the array 12 is a plastics moulding. The array 12 is mounted on the front face 14 of the casing 1 with its axis vertical and presents its concave surface internally of the casing.

The detector assembly 10 also includes a detector with a pair of infra-red responsive detector element such as pyroelectric sensors 15 and 16 mounted in a housing 17 located approximately at the centre of the lens array 12 and at the focal point of the lenses 13. The detector elements 15 and 16 are preferably formed on a common substrate and are located adjacent one another, side-by-side, on a plane parallel to the front face 14 of the casing 1.

In front of both detector elements 15 and 16 there is mounted a narrow, vertical mirror 25 and 26 respectively, of rectangular shape. The mirrors 25 and 26 are both of an opaque plastics material inclined at an angle e of approximately 45 degrees to a normal N to the detector elements 15 and 16, and face outwardly in opposite directions so that they reflect onto respective ones of the detector elements infra-red radiation entering the peripheral lenses 13' and 13" at opposite ends of the array 12. The radiation focussed by these lenses 13' and 1311 is travelling substantially parallel to the front face 14 of the casing 1 from opposite directions, as shown by the broken lines R' and R".The detector element 15, the mirror 25 and the peripheral lenses 13' at one end of the array 12 are located on one side of the normal N of the assembly; the other detector element 16, the mirror 26 and the peripheral lenses 13" at the opposite end of the array are located on the opposite side of the normal N of the assembly.

The normal field of view of the detector elements 15 and 16, without the mirrors 25 and 26, would be about 90 degrees in azimuth. The inclusion of the mirrors increases this to about 180 degrees. The mirrors 25 and 26 are located in front of the detector elements 15 and 16 in such a configuration that each lens 13 in the array 12 has a largely unobscured path onto at least one of the detector elements 15 or 16, either directly or via reflection from one of the mirrors 25 or 26. The part of the field-of-view in front of their respective detector element 15 and 16 that is obscured by the mirrors 25 and 26 is shown by the two hatched regions 35 and 36 in Figure 2. The location of the mirrors and detector elements is such that the part 35 of the field-of-view of detector element 15 which is obscured by the mirror 25 will be visible to the other detector element 16 between the two mirrors 25 and 26. Similarly, that part 36 of the field-of-view of the detector element 16 obscured by the mirror 26 will be seen by the detector element 15. It is, however, possible for there to be some obscuring of both elements in a part of the field-of-view providing that some radiation reaches one of the detectors and that the level of this radiation gives an acceptable signal-to-noise ratio.

Preferably, the mirror in front of one detector element is angled so that it is approximately edge on to the other detector element.

In this way, any infra-red radiation emitted within an azimuth field-of-view of 180 degrees will be focussed by the lens array 12 onto one or both of the detector elements 15 and 16. The location of the mirrors ensures that there is a minimum obstruction to the field-of-view, with maximum sensitivity and an absence of blind spots. Movement of the source in front of the apparatus will, however, cause an image of the source to move across at least one of the detector elements.

Continued movement will cause another image of the source, formed by an adjacent lens 13 to track across one of the detector elements 15 or 16. The output of each detector element 15 and 16 produced by a moving source, will take the form of a signal that increases, as the area of detector illuminated increases, and that then decreases as the area decreases. The output of the detector will be positive or negative depending on which of the elements 15 or 16 receives the radiation. The detector elements 15 and 16 are connected together in antiphase, in series between earth and an amplifier 18. Alternatively, the detector elements could be connected in parallel. As seen by the amplifier 18, the output of the detector assembly 10 appears to be an alternating voltage signal at a frequency dependent upon the rate of change of angle of the source at the apparatus.The magnitude of the signal is dependent on the temperature, proximity and size of the source. By connecting the detector elements 15 and 16 together in antiphase the effects of bulk temperature shift are cancelled.

The output of the amplifier 18 is supplied to the processor 19 which sets the limits of sensitivity of the apparatus. According to the application to which the apparatus is put, it may be set, for example, so that it does not respond to low level signals of the kind that might be produced by movement of a cat within the field-of-view. The apparatus may also be set to ignore a very slow moving source such as a warm patch of sunlight which moves across a wall as the sun moves.

When a person or other source of the appropriate characteristics is detected, the processor 19 provides an output to the light 2 and alarm 3. In some applications, the apparatus is used to provide a courtesy light, in which case the alarm 3 would be omitted. The apparatus could be used to trigger a separate intruder alarm system which might have various other intruder detectors of different forms.

The wide field-of-view possible with apparatus of the present invention gives greater freedom of mounting of the apparatus. For example, it may be inconvenient to have to mount the apparatus on a wall that directly faces the zone to be protected because there may not be convenient access to electrical power. Alternatively, there may not be a wall facing in the direction of the zone to be protected.

In some cases, there may be several zones, or a large zone, to be protected which would previously have required the use of several apparatus pointing in different directions. This involves additional cost and inconvenience in mounting and may be unsightly. The present apparatus, because of its wide field of view can usually cover the entire zone to be protected.

It will be appreciated that the apparatus could be modified in various ways. For example, in some applications where it is not necessary to have a 180 degrees field-of-view, it may only be necessary to use one mirror. Such an arrangement could give cover over about 135 degrees. Alternatively, the apparatus could be arranged to give cover over a field-of-view greater than 180 degrees such as by, for example, using more than two mirrors. The mirrors are preferably plane but could, for example, be curved to produce focussing, or be segmented with sections inclined at an angle to one another.

Different forms of lens array could also be used.

Claims

1. A detector assembly including first and second optical radiation detector elements and an array of lenses located in front of the detector elements, wherein a reflector is located intermediate the first detector element and the lens array, and wherein the reflector is oriented and located such that it reflects radiation from one periphery of the lens array onto the first detector element and such that radiation from a source obscured by the reflector from reaching the first detector element passes to the second detector element whereas radiation from a source obscured by the reflector from reaching the second detector element passes to the first detector element.

2. A detector assembly according to Claim 2 including a second reflector, and wherein the second reflector is oriented and located such that it reflects radiation from an opposite periphery of the lens array onto the second detector element and such that radiation from a source obscured by the second reflector from reaching the second detector element passes to the first detector element whereas radiation from a source obscured by the second reflector from reaching the first detector element passes to the second detector element.

3. A detector assembly according to Claim 2, wherein the first reflector, the first detector element and said one periphery of the lens array are located on one side of the assembly, and wherein the second reflector, the second detector element and the opposite periphery of the lens array are located on an opposite side of the assembly.

4. A detector assembly according to Claim 2 or 3, wherein the first reflector is aligned edge on with the second detector element, and wherein the second reflector is aligned edge on with the first detector element.

5. A detector assembly according to any one of the preceding claims, wherein the or both reflectors are inclined at an angle of about 45 degrees to a normal to the detector elements.

6. A detector assembly according to any one of the preceding claims, wherein the or each reflector is plane.

7. A detector assembly according to any one of the preceding claims, wherein the lenses of the array are of elongate shape.

8. A detector assembly according to Claim 7, wherein the lens array is of semicylindrical shape with a concave surface presented towards the detector elements.

9. A detector assembly according to any one of the preceding claims, wherein the detector elements are responsive to infra-red radiation, wherein the or each reflector is reflective of infra-red radiation, and wherein the lens array focusses infra-red radiation on the detector elements.

10. A detector assembly according to any one of the preceding claims, wherein the outputs of the detector elements are connected in series in antiphase.

11. A detector assembly substantially as hereinbefore described with reference to the accompanying drawing.

12. Detector apparatus including a detector assembly according to any one of the preceding claims and alarm means, and wherein the output of the detector means is used to trigger operation of the alarm means.

13. Detector apparatus including a detector assembly according to any one of Claims 1 to 11 and a light, and wherein the output of the detector means is used to trigger operation of the light.

14. Detector apparatus substantially as hereinbefore described with reference to the accompanying drawing.

15. Any novel feaure or combination of features as hereinbefore described.