GB2205156A

GB2205156A - Optical detector

Info

Publication number: GB2205156A
Application number: GB08811153A
Authority: GB
Inventors: Graham Wild
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-05-16
Filing date: 1988-05-11
Publication date: 1988-11-30
Also published as: GB8711609D0; GB8811153D0

Abstract

An optical detector for detecting light radiation particularly infra red, has a sensor (6) and an optical focusing device which acts to focus the radiation on to the sensor (6). The focusing device is formed by a sheet (7) provided with a Fresnel pattern. The sheet is flat or has a shallow curvature so that the distance between the sensor (6) and the sheet is less than the radius of curvature of the sheet (7). The Fresnel pattern has segments of different focal length and radiation is focused onto the sensor (6) from edges of the sheet (7) as well as from the centre. The detector may be used in an intruder detector. The sheet (7) may be a Fresnel lens or a Fresnel mirror; a combination of a lens and a mirror may also be employed. <IMAGE>

Description

OPTICAL DETECTOR This invention relates to an Optical Detector for detecting light radiation. As used herein the term light compasses both the visible and near-visible spectrum particularly including infra-red.

So called passive infra-red detectors are used in intruder alarm systems. The Detector may be mounted in a corner of a room and comprises an infrared sensor behind a lens. Infra-red radiation reflected off the body of an intruder within the radiation collection field of the lens is focused on the sensor and can trigger an alarm. It is desirable for the lens to give efficient focusing with a wide collection field and using a relatively inexpensive structure. With this in mind it is known to use a Fresnel lens structure comprising a moulded flexible plastics sheet having raised curved ribs defining lens segments. One possibility is to mount the Fresnel sheet in part-cylindrical conformation with the infra red sensor at the focus of the curved surface. This ensures good focusing and a wide collection angle.

However, the lens structure can be bulky and unsightly. Another possibility is to mount the Fresnel sheet in a flat conformation over a centrally located sensor This arrangement involves an acceptable physical structure but there is the problem of attaining an effective collection angle.

An object of the present invention is to provide a light-ccllecting structre for an Optical Detector which gives good focusing and which has a wide collection angle and yet which is of relatively inexpensive structure and is not unduly bulky or unsightly.

According to the invention therefore there is provided an Optical Detector for detecting light radiation comprising a sensor responsive to such light radiation and an optical focusing device for forcusing light from a field on to said sensor characterised in that the focusing device comprises a curved sheet provided with a Fresnel pattern, the sensor being located on the concave side of the sheet in the central region thereof at a distance from the sheet less than its radius of curvature, and the Fresnel pattern having segments of different focal length and being adapted to focus radiation on to the sensor from edge regions as well as the central region of the sheet.

With this arrangement good focusing and a wide collection field can be achieved with a relatively compact and inexpensive structure.

The focusing device may be a lens in which case the sensor is arranged behind the curved sheet which is made from a material which is transparent or semi-transparent to the radiation to be detected.

Alternatively, the focusing device may be a mirror in which case the sensor is arranged in front of the curved sheet which is silvered or otherwise made reflective. It is also possible to use a lens and mirror in combination with the sensor therebetween to give enhanced long range sensitivity. In this case each of the lens and mirror may constitute the focusing device of the invention, or only one may constitute such device, the other therefore being a conventional lens or mirror.

The curved sheet of the focusing device of the invention may comprise a moulded plastics sheet having an appropriate pattern of curved ribs of the shape of lens segments of different focal length. The sheet may be moulded to adopt the desired curvature. Alternatively the sheet may be flexible and may be constrained to adopt the desired curvature by mounting within a frame structure.

The sensor may be an infra red sensor and the Detector may be used in an intruder alarm system.

The radius of curvature of the sheet is preferably relatively large so that the curvature is relatively shallow. It is even possible, and may be preferred, to use an infinite radius of curvature whereby the sheet is wholly flat.

The invention will now be described further by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of one form of a Detector according to the invention, Figure 2 is a plan view of a lens structure of the Detector, and Figure 3 is a diagrammatic sectional view showing the mounting of the lens structure in the Detector.

The Detector is part of an intruder alarm system and comprises a plastics box 1 containing a printed circuit board 2 supporting various electronic components. The box 1 has a rear mounting structure (not shown) by means of which it can be attached to a wall, e.g. in a room or corridor. On the front wall there is a window 3 containing a lens structure 4 and below this a hole 5 through which projects an infra red emitter [e.g. light emitting diode] mounted on the printed circuit board 2. An infra red sensor 6 [e.g. a photo transistor] is mounted on the P.C.B.

within the box behind the lens structure 4.

The lens structure 4 comprises a rectangular moulded flexible plastics sheet 7 having a raised pattern of ribs on one side defining a Fresnel lens system. The sheet 7 is translucent to light in the visible spectrum but transparent to infra red. The sheet 7 is clamped at its edge within the box between a curved supporting framework 8 and a clamping frame 9, whereby the main central part of the sheet is free of any obstruction and the sheet 7 is constrained to adopt a part cylindrical curvature of say IlOmm radius.

The sensor 6 is located say 30mm directly behind the centre of the longitudinal dimension of the sheet 7.

The sensor 6 is also approximately centrally located with regard to the transverse dimension of the sheet although provision is made for adjusting the position of the sensor in this direction as explained below.

The Fresnel pattern is made up of curved ribs 10 of triangular cross-section as is customary. The pattern is divided into upper and lower longitudinal regions 11, 12 and each such region is divided into strips 13. As shown in Fig. 2 the radii of curvature of the ribs 10 increase towards the longitudinal end 14 of the sheet (in accordance with a single cosine offset) and the separation between adjacent ribs 10 decreaqes towards the 'ongitadinal edges 15 of the sheet. With this pattern it is ensured that radiation can be collected from a relatively wide angle and effectively focused on the sensor 6. By adjusting the position of the sensor vertically it is possible to adjust the actual shape of the collection field to suit requirements (e.g. depending on whether it is a long narrow corridor or a short wide room which is to be monitered).

It will be noted that the shape of the lens is conducive to a compact, attractive structure.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.

Thus, for example, if desired the sheet 7 may be flat rather than curved. In this case the framework 8 and frame 9 are shaped to hold the sheet in a flat conformation.

Claims

1. An optical detector for detecting light radiation comprising a sensor responsive to such light radiation and an optical focusing device for focusing light from a field on to said sensor, characterised in that the focusing device comprises a curved sheet provided with a Fresnel pattern, the sensor being located on the concave side of the sheet in the central region thereof at a distance from the sheet less than its radius of curvature, and the Fresnel pattern having segments of different focal length and being adapted to focus radiation on to the sensor from edge regions as well as the central region of the sheet.

2. An optical detector according to claim 1, wherein the focusing device is a lens and the sensor is arranged behind the curved sheet which is made from a material which is transparent or semi-transparent to the radiation to be detected.

3. An optical detector according to claim 1 or 2, wherein the focusing device comprises a moulded plastics sheet having a patern of curved ribs of the shape of lens segments of different focal length.

4. An optical detector according to any one of claims 1 to 3 wherein the sheet has an infinite radius of curvature and is therefore flat.

5. An optical detector according to any one of claims 1 to 4, wherein the sheet is flexible and is mounted in a frame which holds the sheet in a desired conformation.

6. An optical detector according to any one of claims 1 to 5, wherein the sensor is an infra red sensor.

7. An optica] detector substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.