GB2221984A

GB2221984A - Optical detector

Info

Publication number: GB2221984A
Application number: GB8811152A
Authority: GB
Inventors: Graham Wild
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-05-11
Filing date: 1988-05-11
Publication date: 1990-02-21
Also published as: GB8811152D0

Abstract

A wall mounted optical detector for use in an intruder alarm system has a sensor mounted in a tray behind a one-piece moulded front cover (15). The cover (15) has a Fresnel pattern moulded into its surface to focus radiation onto the sensor. <IMAGE>

Description

OPTICAL DETECTOR This invention relates to an optical detector for detecting light radiation. As used herein the term light encompasses 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 infra-red 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 partcylindrical conformation with the infra-red sensor at the focus of the curved surface. This ensures good focusing and a wide collection angle and the curved Fresnel sheet can form a robust rigid structure.

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 and to use a Fresnel pattern which gives increased focal lengths (using a double cosine offset) towards the edges of the sheet so that light radiation incident on the edge region of the sheet can be focused on the sensor. However, with this arrangement, there is the problem of ensuring adequate support for the flat flexible Fresnel sheet which is freely exposed and therefore liable to be displaced or damaged e.g. when handled during installation.

An object of the present invention is to provide a light-collecting structure for an optical detector which gives good focusing and has a wide collection angle and yet which comprises a relatively inexpensive robust structure which 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 focusing light from a field on to said sensor characterised in that the focusing device comprises a one-piece moulded plastics self-supporting cover structure mounted over the sensor, a part of the surface of said cover structure being such as to permit transmission of said light radiation therethrough and having a Fresnel pattern moulded therein.

With this arrangement, good focusing and a wide collection angle can be attained by appropriate design of the Fresnel pattern and an inexpensive structure which is robust and unobtrusive can be achieved due to the integral, one-piece incorporation of the Fresnel pattern into the self-supporting cover structure.

In a preferred embodiment the detector comprises a support structure which supports the sensor and the cover structure is interengaged with the support structure to define therewith a housing which encloses the sensor. The support structure may be provided with a fixing bracket or the like for use in mounting the support structure on a wall surface. Further, the support structure may support electronic components connected to the sensor for example on a printed circuit board.

The cover may comprise a central portion including or consisting of the said part containing the Fresnel pattern, and a peripheral portion comprising inwardly projecting walls defining a rim around the central portion.

The Fresnel pattern may be moulded on the inner surface of the cover structure.

The part containing the Fresnel pattern may be flat or slightly curved, preferably flat. Where a curved surface is used the radius of curvature is preferably greater than the spacing of the sensor from the sheet.

The Fresnel pattern is preferably adapted to focus radiation on to the sensor from edge regions as well as the central region of the sheet.

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

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 front perspective view of one form of an optical detector according to the invention; Figure 2 is a sectional view of the detector; and Figure 3 is a view of the inner surface of a cover of the detector.

The detector is part of an intruder alarm system and comprises a one-piece rigid square plastics tray 1 which contains a printed circuit board 2 supporting various electronic components. The tray 1 has a square base 3 and upstanding side walls 4 around three sides, the other side 5 being largely open. On the outer surface of the base 3 there is detachably mounted a bracket structure 6 for attachment to a wall e.g. in a room or corridor.

Within the tray 1 and projecting therefrom there is mounted, on top of the printed circuit board 2, a shielding structure 7 comprising a one-piece rigid plastics moulding having side walls 8 and a central inwardly directed funnel 9 tapering from a wide square opening 10 to a narrow square opening 11 located close to the pcb 2.

Over the shielding structure 7 there is a top structure 12 which is detachably engageable with the tray 1 so as to define therewith an enclosure containing the pcb 2 and the shielding structure 7. The top structure 12 comprises a one-piece rigid square plastics frame 13 having at one side a projecting part 14 which acts to close the open side 5 of the tray 1. The frame 13 is arranged to snap-fit into engagement with the tray 1.

On the frame 13 there is a one-piece square plastics cover 15.

The plastics material of the cover 15 is thin compared with the material of the frame 13, the tray 1 and the shielding structure 7 whereby the material of the cover 15 is of a relatively flexible nature. However, the cover 15 is shaped so that overall it'is relatively rigid and is self-supporting. Thus, the cover 15 has a flat central square part 16, four flat side parts 17 which are inclined away from the side edges of the central part 16, and four interconnected side walls 18 which extend at right angles to the central part 16, the resulting structure being of the form of a tray.

The side walls 18 fit tightly within peripheral grooves 19 or slots formed in the frame 13 and the arrangement is such that the periphery of the square central part 16 rests on the periphery of the square wider end 10 of the funnel 9.

The components on the pcb 2 include an infra-red sensor 20 (e.g. a phototransistor) mounted in alignment with the narrow square opening 11 of the funnel 9. The pcb 2 is movable to permit adjustment of the alignment of the sensor 20. The frame 13, the tray 1 and the shielding structure 7 are formed from relatively thick white pigmented plastics which is largely opaque to infra-red radiation to which the sensor 20 is sensitive. The cover 12 is translucent to light in the visible spectrum but transparent to infra-red.

On the inner surface of the square central part 16 of the cover 12 there is formed a square Fresnel lens pattern 21. The pattern comprises a raised arrangement of ribs moulded in the material of the cover 12 so as to be formed integrally in one piece with the cover 12.

The pattern comprises curved ribs of triangular cross-section which act to focus infra-red on to the sensor 20. The pattern varies across the surface so as to collect radiation from a relatively wide angle and focus this on the sensor 20, the actual shape of the collection field being determined by the nature of the pattern which is selected to suit requirements (e.g. depending on whether it is a long narrow corridor or a short wide room which is to be monitored).

The infra-red radiation may be directed into the room or other space to be monitored from a source mounted within the optical detector or elsewhere as desired.

The optical detector is capable of effective operation yet is in the form of a compact attractive structure which is robust and relatively inexpensive.

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

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 one-piece moulded plastics self-supporting cover structure mounted over the sensor, a part of the surface of said cover structure being such as to permit transmission of said light radiation therethrough and having a Fresnel pattern moulded therein.

2. An optical detector according to claim 1 wherein said detector comprises a support structure which supports said sensor and the cover structure is interengaged with the support structure to define therewith a housing which encloses the sensor.

3. An optical detector according to claim 2 wherein said support structure is provided with a fixing bracket for use in mounting the structure on a wall surface.

4. An optical detector according to claim 2 or 3 wherein said support structure supports electronic components connected to the sensor.

5. An optical detector according to claim 4 wherein the electronic components are supported on a printed circuit board.

6. An optical detector according to any one of claims 1 to 5 wherein a central portion thereof includes or consists of the said part containing the Fresnel pattern, and a peripheral portion comprises inwardly projecting walls defining a rim around the central portion.

7. An optical detector according to any one of claims 1 to 5 wherein the Fresnel pattern is moulded on the inner surface of said cover structure.

8. An optical detector according to any one of claims 1 to 7 wherein said part containing the Fresnel pattern is flat.

9. An optical detector according to any one of claims 1 to 7 wherein said part containing the Fresnel pattern is slightly curved.

10. An optical detector according to claim 9 wherein said curved surface has a radius of curvature greater than the spacing of the sensor from the sheet.

11. An optical detector according to any one of claims 1 to 10 wherein the Fresnel pattern is adapted to focus radiation on to the sensor from edge regions as well as the central region of the sheet.

12. An optical detector according to any one of claims 1 to 11 wherein said sensor is an infra-red sensor.

13. An optical detector substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

14. An intruder alarm system incorporating a detector according to any preceding claim.