GB2612916A

GB2612916A - Passive infra red intruder detector

Info

Publication number: GB2612916A
Application number: GB2300889.9A
Authority: GB
Inventors: Pitts Mark; Roberts Simon
Original assignee: Orisec Ltd
Current assignee: Orisec Ltd
Priority date: 2021-03-04
Filing date: 2021-03-04
Publication date: 2023-05-17
Anticipated expiration: 2041-03-04
Also published as: GB2612916B; GB202300889D0

Abstract

A PIR detector comprises a lens having Fresnel facets to focus radiation from outside the detector onto an IR sensor. An anti-masking system includes an IR transmitter and an IR receiver behind the lens. A sequential confirmation PIR detector has first and second PIR detectors. Masking means 32, 34 block IR radiation from non-overlapping coverage areas of the detectors from being focused on the respective sensors (44, 46, Fig. 6). Each lens (12, 14, Fig. 2) has a flange (22, Fig. 2) surrounding a window (20, Fig. 2) covering an aperture in a front cover 18 of a housing. Lens retainer 30, incorporating the masking means, holds the flange to the front cover. The anti-masking system has a light guide 38 to guide radiation from IR transmitter 39 to a point adjacent to the back surface of the lens, or has a second IR transmitter on an outer surface of the detector.

Description

PASSIVE INFRA RED INTRUDER DETECTOR

The present invention relates to a passive infra-red (PIR) intruder detector, particularly to a sequential confirmation PIR detector.

BACKGROUND TO THE INVENTION

Passive infra-red (PIR) detectors for use in intruder alarms are very well known. PIR detectors typically use a pyroelectric sensor, and a lens arrangement to focus infra-red energy onto the pyroelectric sensor. The lens arrangement is typically in the form of plastic window having multiple facets. Each individual facet is a Fresnel lens.

In a PIR detector for use in an intruder alarm system, the pyroelectric sensor has a pair of pyroelectric elements. The two elements of the pair are placed close together and are configured in a differential arrangement. This means that a slow change in infra-red radiation received from all objects in a space, for example because the room is warming up or cooling down, will affect either side of the differential pair equally and the signal will be cancelled out. On the other hand, a localised change in received infra-red radiation, due to a person moving in the room, will cause a strong differential-mode signal. This can be interpreted as an alarm condition.

Intruder alarms can be monitored remotely. Typically the alarm is configured to transmit a signal to an alarm receiving centre (ARC) who can then take follow-on actions, including calling the police. To avoid police time and public resources being wasted by attendance to false alarms, a policy has emerged where the police will attend only where alarms have been properly maintained to agreed standards, and where the signal received at the ARC indicates a "confirmed" alarm condition. One way of generating a confirmed alarm condition is by "sequential confirmation". In sequential confirmation, a confirmed alarm condition is generated by the intruder alarm system, in essence, when two different detectors have indicated the presence of an intruder.

Where one detector detects an intrusion, the alarm will sound and an unconfirmed alarm condition is signalled to the ARC. In this situation the ARC may, for example, call building owners or keyholders, but will not call the police. However if a second detector detects an intrusion the alarm system will signal a confirmed alarm, and in this case the ARC may call the police.

British Standard BS 8243:2010, previously DD 243, sets out in detail the requirements for intruder alarm systems designed to generate these confirmed alarm conditions. One way of meeting the requirement is to provide two independent detectors which use the same technology (for example, passive infra-red), but which do not have overlapping areas of coverage. The detectors are to be independent in the sense that each detector separately reports an alarm condition to the control panel of the alarm system, and the input to one detector does not influence the output of the other detector.

It will therefore be understood that a common FIR detector, having a sensor comprising a differential pair of pyroelectric elements, is one detector for the purposes of generating confirmed alarm conditions. Two independent PIR detectors having non-overlapping coverage areas are needed to meet the requirements of BS 8243:2010.

GB2391936 and GB2421789 disclose devices which comprise of two independent PIR detectors having non-overlapping coverage areas, the two devices being provided in the same housing. Thus there are two separate differential pairs of pyroelectric elements and two separate lens arrangements. Using these devices, an alarm installer can very easily provide PI R coverage of a room which can generate a confirmed alarm condition and hence a police response in the event of an intrusion. These devices are known as "sequential confirmation FIR detectors".

To produce non-overlapping coverage areas, these devices include masks which "blank out" parts of each lens arrangement. The areas covered by one lens arrangement are blanked off in the other lens arrangement, and vice versa. The masks are typically thin opaque flexible plastic pieces which sit just behind the lens arrangement in the assembled detectors.

A sophisticated intruder may try to disable an alarm system in advance of breaking into a building. This is particularly a risk in busy commercial premises to which the public normally have access during opening hours, for example, shops and pubs. A criminal may easily gain access to the premises during opening hours and have the opportunity to tamper with the alarm system. All components of intruder alarm systems are therefore normally provided with tamper detection switches, which will activate if the cover is removed.

However, one simple attack is to place an object such as a drink coaster in front of the whole detector. Although obviously visible if the detector is checked, it is unlikely that every detector in a large premises would be sighted on locking up. Therefore "anti-masking" technologies have been designed to detect masking and indicate a fault condition to the alarm system control panel. One way this has been done is by using an active infra-red system. This uses an infra-red source (e.g. an infra-red LED) to transmit infra-red out of the front of the detector, and an infra-red receiver to monitor for infra-red reflected back by objects placed in front of the detector. Note that, although both operate in the infra-red part of the spectrum, the wavelengths used in active IR anti-masking are different from the wavelengths emitted by warm bodies which are detected by passive IS pyroelectric sensors.

These active IR arrangements work reasonably effectively to defeat simple masking attacks, but it has proved difficult to design an active-infra-red anti-masking system which is effective in all cases, due to the wide range of materials which can theoretically be employed to block the passive infra-red radiation which would be emitted by an intruder. Some of these materials are mainly reflective of the active infra-red radiation, resulting in an increase in the signal received by the active infra-red receiver. Other materials mainly absorb the active infra-red radiation, resulting in a decrease in the signal received by the active infra-red receiver, which would otherwise normally be detecting a baseline signal due to reflections from objects in the room. Some materials may cause a very small change in the received signal, and below a certain threshold this small change may be indistinguishable from noise.

It has been found particularly difficult to produce very effective active IR anti-masking systems in sequential confirmation PR detectors. This is because the masks which are placed behind the lenses of sequential confirmation detectors obstruct the active IS transmitter. The mask will be at least partly reflective, and can reflect significant infra-red radiation from the transmitter, directly back to the receiver. This can saturate, or nearly saturate, the receiver, making a change in the signal indicative of a masking attack unlikely to be detected.

With some current P15 detectors, it has been found that the lens can be removed from the front without activating the tamper, and then replaced without causing any visible damage. By removing the lens, access is gained to the pyroelectric sensor, which can simply be covered up for example with a piece of tape or Blu Tack (RTM). This will prevent the detector from detecting anything at all in the room and, since the obstruction is very small and can be placed precisely over the pyroelectric sensor, will not usually activate anti-masking technologies.

It is an object of the invention to reduce these problems.

STATEMENT OF INVENTION

According to the present invention there is provided a sequential confirmation passive infra-red (P1 R) detector for use in an intruder alarm system, the sequential confirmation PI R detector comprising: a first FIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; a second FIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; a housing including at least a front cover, the front cover having an aperture corresponding with each lens arrangement, each lens arrangement having a window portion which covers a respective aperture, and a flange portion which at least partially surrounds the window portion and is disposed against an interior side of the front cover; and at least one lens retainer fixed to the interior side of the front cover to hold the flange portions of the lens arrangements between the front cover and the lens retainer(s), the lens retainer(s) incorporating integrated masking means disposed behind sections of the window portions of each lens arrangement to block infra-red radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first FIR detector and a coverage area of the second PI R detector, the coverage areas being non-overlapping.

By incorporating the masking means into the lens retainer, parts of the lens retainer are disposed not only behind the flange on the periphery of the lens arrangement, but also behind parts of the window portion. The lens retainer is preferably made from a rigid material, and the integrated mask and retainer provides a very secure retention of the lens arrangement to the front cover of the housing, defeating any attempt to remove the lens arrangement from the front of the housing in order to interfere with the infra-red sensor.

The lens retainer is made from a substantially rigid material such as ABS or ASA.

However, the lens retainer can be another plastic or any reasonably rigid material.

Two lens retainers may be provided, one for each lens arrangement. However, it is preferable to provide a single integrated lens retainer which holds both lenses in place by being disposed behind at least the flange portions and parts of the window portions of both lens arrangements. Not only does this improve the security of the fixing, but also reduces part count and improves ease and reliability of assembly. An example prior art sequential confirmation PIR detector includes two lens arrangements, two masks, two lens retainers, and four screws to fix each lens retainer to the housing, making a total of twelve parts. By integrating the masks with the lens retainer and providing a single retainer, which can be fixed to the housing by two screws, this can be reduced to five parts (two lens arrangements, a single integrated mask and retainer, and two screws). Also, the risk of a mistake being made in assembling the device is reduced. In particular, the risk of putting the same mask behind each lens assembly so that the coverage areas overlap is eliminated.

Preferably an arrangement of lugs and recesses is provided to assist with retaining the flange portion of the lens arrangements. Lugs may be provided on the forward-facing surface of the flange portion of the lens arrangement, corresponding with recesses on the inner side of the front cover; and/or lugs may be provided on the backward-facing surface of the flange portion of the lens arrangement, corresponding with recesses or apertures in the lens retainer(s); and/or lugs may be provided on either or both of the inside surface of the cover and the forward-facing surface of the lens retainer(s), corresponding with recesses or apertures in the flange portion of the lens arrangements.

The lens arrangement may be injection-moulded in a three-dimensional shape. This provides more options than common known lenses which are stamped from a flat flexible sheet and then bent into a curved shape, typically into a part-cylindrical shape or similar. In particular, the window portion of an injection-moulded lens arrangement may be curved as part of the surface of a sphere.

In some embodiments, the two lens arrangements may be incorporated into one part formed from a single piece of material. However, in a preferred embodiment the lens arrangements are separate, but identical parts. The lens arrangements may be made from a relatively thin, and to some extent flexible material. However, rigidity may be provided both as a result of the shape of the lens arrangement, and by the lens retainer(s).

To provide some rigidity in the lens arrangement itself, the lens may include a peripheral side wall, which extends substantially perpendicularly from an inside edge of the flange portion, in a forward-facing direction (i.e. outwards from the front of the detector). The window portion of the lens arrangement then fills the space bordered by the outermost edges (edges distant from the flange) of the peripheral side wall.

In other embodiments, the peripheral side wall may be omitted. The lens arrangement in such an embodiment will have a window portion, with the flange portion being a continuation of the window portion, around its periphery. Lens arrangements may be stamped from a flat but flexible material, but are preferably injection moulded in a three-dimensional shape, which allows for a cylindrically-curved surface of the window portion.

The sequential confirmation FIR detector may be provided with an active-infra-red anti-masking system. Preferably, two active-infra-red anti-masking systems are provided, one being associated with each of the first PI R detector and the second PI R detector.

This is important for a high-security system since an attacker successfully masking either one of the two FIR detectors would potentially avoid a confirmed alarm and therefore avoid a police response.

The or each active-infra-red anti-masking system comprises an infra-red transmitter and an infra-red receiver. The infra-red transmitter is arranged to transmit infra-red radiation of a certain wavelength out of the front of the detector, and the purpose of the infra-red receiver is to measure the amount of radiation of that wavelength which is reflected back. The wavelength used for active IR anti-masking is significantly different from infra-red wavelengths emitted by warm bodies.

The infra-red transmitter and infra-red receiver may be disposed at a distance behind each lens arrangement. For example, the transmitter and receiver may be mounted on a PCB, perhaps around 20mm behind each lens arrangement.

Preferably, a light guide or light pipe is provided to channel the infra-red radiation from the infra-red transmitter to a point close to a front surface of the sequential confirmation FIR detector, immediately behind the lens. This avoids the transmitted signal out of the front of the detector being weakened by the presence of the masking means of the lens retainer. It also prevents the infra-red signal being reflected from the back of the masking means, straight back onto the active-infra-red receiver. Therefore, more effective active-infra-red anti-masking can be achieved within a sequential confirmation detector.

Preferably, one or more additional active infra-red transmitters are provided to transmit an active infra-red signal from the front surface of the front cover, not from behind either lens arrangement. By providing a transmitter behind each lens arrangement, and also a transmitter on the front surface which is not behind either lens arrangement, the anti-masking system can detect masking attempts using a very wide range of materials and techniques. In particular, the transmitters positioned behind the lens arrangements are effective for detecting reflective materials placed very close to the lens, but produce a relatively weak "background" signal at the receiver when there is no problematic obstruction. Therefore an absorbent masking material may produce a relatively small change in the signal measured at the receiver, from an infra-red transmitter positioned behind the lens arrangements. On the other hand, a transmitter which is not behind the lens arrangement produces a much stronger "background" signal at the receiver and thus reliably detects many absorbent masking materials, but reflective materials may produce a weaker response especially if the "background" signal is already nearly saturating the receiver. In general, it is found that by measuring changes in received active infra-red signals from two sources, one behind the lens arrangement and one not behind the lens arrangement, a wider variety of different masking attacks can be reliably detected.

The or each anti-masking system may thus include a first active infra-red transmitter behind a respective PIR lens arrangement, a second active infra-red transmitter not behind the PIR lens arrangement, and an active infra-red receiver. In some embodiments of a sequential confirmation detector, the second active infra-red transmitter, which is not behind either lens, may be shared by two anti-masking systems, one anti-masking system protecting each passive IR detector. The or each anti-masking system may be controlled by a processor or simple controller which switches on the first transmitter for a period of time, for example a few microseconds or hundreds of microseconds, and measures the received signal, and then switches off the first transmitter and switches on the second transmitter for another period of time, and measures the received signal. Signals received by the receiver while each transmitter is active can be measured and stored, and as such the device is "self calibrating" in that it learns the "normal background" retum levels for each transmitter. A significant change in those levels may be interpreted as a masking attempt. For some masking materials, the signal associated with one of the transmitters may change more than the signal associated with the other one of the transmitters, and so by using two transmitters in this way the overall effectiveness of the anti-masking system is improved.

The process of switching on one or more of the transmitters for a short period of time and reading the output of the receiver while the transmitter(s) are switched on preferably takes place, at the very least, several times per second. The monitoring of the detector for potential masking attacks is therefore effectively continuous.

According to a second aspect of the invention, there is provided a sequential confirmation passive infra-red (PIR) detector for use in an intruder alarm system, the sequential confirmation PR detector comprising: a first PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; and a second PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor, masking means being provided behind each lens arrangement to block infra-red radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first PIR detector and a coverage area of the second PIR detector, the coverage areas being non-overlapping, and an anti-masking system associated with at least one of the first and second PIR detectors, for detecting an attempt to block infra-red radiation from reaching the infrared sensor of the detector from the coverage area of the detector, the anti-masking system including an infra-red transmitter disposed behind the lens arrangement of the detector, a light guide for guiding radiation from the infra-red transmitter to a point substantially adjacent to a back surface of the lens arrangement, and an infra-red receiver behind the lens arrangement.

The light guide ensures that a large percentage of the radiation emitted by the infrared transmitter is emitted from the front of the detector, rather than dissipating behind the lens in which case perhaps around half would be absorbed or reflected by the masking means behind the lens arrangement. This results in an anti-masking system having performance at least as good as known anti-masking systems in ordinary PIRs not configured for sequential confirmation and therefore including no masking means.

The forwardmost end of the light guide may be touching the back surface of the lens arrangement in some embodiments, or may be very close to, for example less than about 2mm away from, the back surface of the lens arrangement. The requirement is for the light guide to terminate close enough to the back surface to substantially avoid infra-red radiation from the transmitter from being incident on the back surface of the masking means which is provided behind the lens. The actual distances required may therefore be different in different embodiments, depending on the configuration of the non-overlapping coverage areas and therefore the arrangement of the masking means.

The light guide is any conduit capable of channelling the radiation from the infra-red transmitter from one end to the other through internal reflection, minimising losses through absorption or escape from the sides of the light guide. The light guide is typically made of a transparent! translucent plastic. For example, the light guide may be moulded from clear acrylic. Light guides are well known for making indicator status LEDs visible on the front cases of PIR detectors and indeed many other electronic devices. The same light guides may be suitable in this application, the wavelength used by active infra-red anti-masking systems being typically around 950-980nm, just outside the visible part of the spectrum.

Preferable and/or optional features of the second aspect of the invention are set out in claims 17 to 20. Preferable and/or optional features of the second aspect of the invention are also set out in clauses 12 to 15.

According to a third aspect of the invention, there is provided a passive infra-red (PIR) detector for use in an intruder alarm system, the PIR detector comprising: an infra-red sensor for detecting passive infra-red radiation from warm bodies; a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; an anti-masking system for detecting an attempt to block infra-red radiation from reaching the infra-red sensor of the detector, the anti-masking system including a first infra-red transmitter disposed behind the lens arrangement of the detector, a second infra-red transmitter disposed on an outer surface of the detector, and an infra-red receiver behind the lens arrangement.

By providing two infra-red transmitters, one behind the lens arrangement and one on an outer surface of the detector, i.e. not behind the lens arrangement, the anti-masking system is capable of detecting a wide range of masking attacks, including use of both absorbent and reflective materials. A controller! processor may alternately switch on one or the other of the infra-red transmitters and read the amount of radiation received back at the infra-red receiver. A significant change from a baseline associated with either one of the infra-red transmitters may cause a "masked" condition to be detected.

In some embodiments, both of the infra-red transmitters may be switched on at times.

Hence with two infra-red transmitters, there are three possible modes: both switched on, first transmitter only switched on, second transmitter only switched on. All three of those modes may be monitored for significant changes to the amount of radiation received by the receiver.

More generally, in other embodiments, more than two infra-red transmitters may be provided, and may be turned on in any combinations. In particular, for a sequential confirmation PIR detector as described above with two lens arrangements and at least three active infra-red transmitters, one behind each lens arrangement and one not behind any lens arrangement, there are seven different combinations of one, two or three transmitters which may be turned on at a particular time. In addition, the infra-red radiation received by each of two or more receivers may be monitored when determining whether to detect a "masked" condition.

Preferable / optional features of the third aspect of the invention are set out in claims 2 to 5. Preferably! optional features of the third aspect of the invention are also set out in clauses 17 to 20.

Embodiments may include combinations of features of the first, second and third aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 is a perspective view of a sequential confirmation detector according to the invention; Figure 2 is a perspective view from behind of a front cover of the detector of Figure 1, showing lens arrangements in place; Figure 3 is a perspective view from in front of a lens arrangement of the detector of Figure 1; Figure 4 is a perspective view from in front of a lens retainer of the detector of Figure 1; Figure 5 is a perspective view from behind of the front cover and lens retainer, being components of the detector of Figure 1; Figure 6 is a perspective view from in front of the detector of Figure 1, with the front cover not shown but with the lens retainer in place; Figure 7 is a cross section through the parts of the detector shown in Figure 6. DESCRIPTION OF PREFERRED EMBODIMENTS Referring firstly to Figure 1, a sequential confirmation FIR detector is indicated generally at 10. The sequential confirmation detector is essentially two independent PIR detectors in one enclosure. In particular it has two separate pyroelectric-effect passive-infra-red sensors (each sensor having a pair of pyroelectric elements), and each sensor is associated with a separate lens arrangement 12, 14. The lens arrangements 12, 14 in this embodiment are identical to each other. Each lens arrangement 12, 14 comprises a plurality of Fresnel facets. Each facet is designed to focus infra-red radiation from a particular region in the space covered by the detector, onto the respective pyroelectric-effect passive-infra-red sensor.

The housing of the detector 10 is substantially in two parts. A rear casing 16 is designed to be fixed to a wall or other structure, for example with screws. The rear casing 16 houses the electronic components, including the passive-infra-red sensors. A front cover 18 fits in front of the rear casing 16. The front cover retains the lens arrangements 12, 14, as will be seen. The detector 10 may be disassembled relatively easily by removing the front cover 18 from the rear casing 16. This is done when installing the detector in order to fix the rear casing 16, e.g. with screws, to a wall or other structure. Also, in a wired embodiment the casing needs to be opened to allow wires to be connected. Some embodiments may communicate wirelessly with an alarm control panel, in which case the casing will need to be opened from time to time to allow batteries to be replaced.

The casing components 16, 18 may therefore be releasably fixed together with, for example, clips, hinges, screws, or combinations of those, or any other suitable means.

A tamper switch is included which will send a tamper detection signal to an alarm control panel in the event that the casing is opened. Therefore, an attacker who does not know the correct codes / procedure to disable tamper protection on the alarm control panel will not be able to access the inside of the detector 10 without being detected, even though actually opening the casing is a simple matter with ordinary tools.

Figure 2 shows the front cover 18, when detached from the rear casing (16). In Figure 2, it is the back (i.e. inside) surface of the front cover 18 which is visible. The front cover has a first aperture which is covered by the first lens arrangement 12, and a second aperture which is covered by the second lens arrangement 14. Each lens arrangement includes a window portion 20a, 20b, and a flange portion 22a, 22b surrounding the window portion. Each lens arrangement 12, 14 is positioned behind the front cover 18 so that the window portion 20a, 20b fills a respective aperture of the front cover 18, and so that the flange portion 22a, 22b is in contact with an inside surface of the front cover, surrounding the window portion 20a, 20b. Assembly of the lens arrangements 12, 14 into the detector 10 is therefore very straightforward, and furthermore the lens arrangements 12, 14 can be replaced if necessary (their front surface is exposed and hence they are vulnerable to damage, especially in a "high risk" environment where there may be public access and some level of vandalism or risk of accidental damage). However, it is impossible to remove the lens arrangements 12, 14 from the front of the detector 10 without disassembling the detector and therefore activating the tamper, and without causing visible damage.

Figure 3 shows one of the lens arrangements 12, 14. The two lens arrangements 12, 14 are in fact identical parts, and so it is not possible to make an error in assembly / maintenance where the wrong lens arrangement is put in the wrong aperture in the front casing 18. The lens arrangement has a window portion 20. In this embodiment this is in the form of a curved surface. The curvature of the window portion is spherical. The window portion comprises a plurality of Fresnel facets. Individual facets are not shown in the drawing, but may be moulded into the lens arrangement 12, 14 in a manner familiar to the skilled person. The flange 22 substantially surrounds the window portion.

The lens is manufactured by injection moulding and, although the material may be slightly flexible / resilient, since it is fairly thin, the lens does not have to be deformed during assembly.

In this embodiment there is a side wall 24 which extends substantially perpendicularly of the flange portion 22, and approximately perpendicularly also to the window portion 20 (the window portion being curved and thus the side wall is more or less perpendicular to different parts of the window portion). The perpendicular side wall 24 extends from an inside edge of the flange 22, to an outside edge of the window portion 20. In this way, the window portion is set in front of the flange, for example by about 3-5mm. When assembled into the front cover 18, the front of the window portion 20 of the lens arrangement may form together with the front of the front cover 18, a substantially continuous front surface of the device 10.

Three lugs 26 are provided, extending from the front surface of the flange portion 22.

One lug 26 is provided to either side of the window portion 20 (only one of which is visible in Figure 3) and one lug 26 is provided above the window portion 20. The lugs 26 fit into recesses on the inner surface of the front cover 18 of the detector 10. They locate the lens arrangement 12, 14 in place, assisting in assembly by correctly locating the lens arrangements 12, 14 before the lens retainer is fitted, and also ensure that it is impossible to remove the lens arrangement from the front of the detector 10 when assembled, without activating the tamper.

Apertures 28 are also provided around the flange portion. Pegs may be provided in corresponding positions on the inside surface of the front cover 18, and/or in a front surface of the lens retainer.

Figure 4 shows the lens retainer 30. The lens retainer is fitted behind the lens arrangements 12, 14. Hence the flange portion 22 of each lens arrangement 12, 14 is sandwiched between an inside (rear) surface of the front cover 18 and a front surface of the lens retainer 30. In this embodiment, a single lens retainer is provided for retaining both of lens arrangements 12, 14. This is preferred because it reduces part count and reduces the possibility of assembly error. The lens retainer 30 is made from a rigid material, for example ABS or ASA plastic.

The lens retainer includes two masking portions 32, 34. Each masking portion is made from the same rigid material as the rest of the retainer 30. The masking portion substantially conforms to the shape of the window portion (20) of each lens arrangement (12, 14). When assembled, masking portion 32 sits immediately behind lens 12 and masking portion 34 sits immediately behind lens 14. Preferably, the front surface of each masking portion 32, 34 is in contact with the rear surface of its corresponding lens arrangement 12, 14. As is clear from Figure 4, each masking portion has a pattern of cut-outs. The Fresnel facets of the lens arrangements which correspond to the cut-outs will allow infra-red radiation through, and focus it onto the corresponding pyroelectric-effect infra-red sensor. However, no infra-red radiation will reach the infra-red sensor through the Fresnel facets which are blocked by the masking means. Since each Fresnel facet focuses radiation from a particular region of the space covered by the detector, the pattern of the cut-outs in the masking portions 32, 34 define a coverage area for the first PIR detector, and a coverage area for the second PI R detector. The coverage areas are arranged to be non-overlapping, in accordance with the requirements for a sequential confirmation detector.

With the flange portion (22) of each lens sandwiched between the inside surface of the front cover 18 and the lens retainer 30, and the rigid masking portions 32, 34 of the lens retainer also being placed directly behind the window portion (20) of each lens, the lens is very firmly secured in place when the detector is assembled, and there is no possibility of removing the lens, without causing visible damage, and without activating the tamper by disassembling the detector 10.

Figure 5 shows the lens retainer 30 in place, fixed to the front cover 18 with two screws 36. Also shown in Figure 5 are two tube-shaped formations 38, 40, which in this embodiment are formed integrally with the lens retainer. The tube-shaped formations each house a light guide or a light pipe, that is a material designed to transmit light (specifically infra-red light) from one end the other with minimal loss by means of repeated internal reflections. A suitable light guide may be made for example from a suitable clear plastic. Such light guides are known for example to transfer the light from status LEDs to the surface of electronic devices. In this case, each light guide is used to transfer the light from an infra-red transmitter, which forms part of the anti-masking system of the detector 10. The light is transmitted from an LED which may be disposed on the surface of a PCB, set back for example a few centimetres from a respective lens arrangement 12, 14. Each light guide transmits the light from its source to a point just behind a respective lens arrangement 12, 14. Preferably, the end of each light guide touches the back surface of the lens arrangement 12, 14.

Figure 6 shows a view from in front of the detector 10, with the lens arrangements 12, 14 and front cover 18 omitted. Note that the detector is never actually in this form in any intermediate stage of assembly, since the lens retainer 30 is fixed to the front cover 18 before the front cover 18 is fitted over the back casing 16.

A printed circuit board (PCB) 42 is mounted to the back casing 16. The PCB in turn mounts all the electronic components required for the detector 10 to operate. These will not be described in detail since they will be familiar to the skilled person and moreover depend on the type of detector (wired, wireless) and features which are included. In this embodiment, connector blocks for connecting power and signalling wires and DIP switches for configuration (for example to set the device address in an addressable system) are included. There is an on-board processor, for example a microcontroller.

Mounted on the PCB 42 are two pyroelectric-effect sensors 44, 46. Infra-red radiation entering the detector 10 through the lens arrangements 12, 14 is focused onto the pyroelectric-effect sensors 44, 46. In particular, radiation entering through lens 12 is focused on sensor 44 and radiation entering through lens 14 is focused on sensor 46.

In Figure 6 the front ends of light guide tubes 38, 40 are shown. Infra-red transmitters (not visible in Figure 6) are mounted on the PCB 42, and the light guide tubes 38, 40 transmit the signal from the infra-red transmitters to a point just behind the lens arrangement Further light guide tubes 48 are provided to transmit light from LEDs mounted on the PCB 42. A set of three light guide tubes 48 is provided above the first lens arrangement (12), and a further set of three light guide tubes 48 is disposed between the first and second lens arrangements (12, 14). The light guide tubes 48 can be used to transmit light from status LEDs mounted on the PCB 42, through corresponding apertures in the front cover (18). Status LEDs may be used, for example, to perform a "walk test" so that the detector can be correctly positioned to cover a room. One or more of the light guide tubes 48 may be used to transmit infra-red radiation from a further infra-red transmitter.

The detector may therefore be protected by an anti-masking system which includes an infra-red transmitter behind light guide 38, and also behind the first lens arrangement 12, an infra-red transmitter behind light guide 40, and also behind the second lens arrangement, and an infra-red transmitter behind one of the further light guides 48 and hence not behind either of the lens arrangements. In some embodiments, there may be even more infra-red transmitters. A controller / processor may switch on the infra-red transmitters in turn, and/or in combinations. For example, the transmitter behind light guide 38 may be switched on for a few microseconds, and then the transmitter behind light guide 48 may be switched on as well for another few microseconds, so that measurements can be made with two transmitters switched on, and then the transmitter behind light guide 38 may be switched off, so that only the transmitter not behind the lens is turned on for a period of time.

In some embodiments there may be a period of time during which all transmitters are switched off, in between each "on" period. This is preferable since the LED transmitters may be over-driven at high current during the "on" cycles. An "off" period therefore allows heat to dissipate and ensures that the transmitters will not fail prematurely.

An infra-red receiver 50 is provided on the PCB 42 behind the first lens 12, and another infra-red receiver 52 is provided on the PCB 42 behind the second lens 14. In each "mode" (i.e. for each sensor switched on individually, or for each combination of sensors), the signal received by the receivers is measured and stored. Thus the detector can learn a "baseline" signal level which is expected by each of the receivers when each transmitter, or each combination of transmitters, is active. Any significant change may indicate a masking attempt and a signal can be generated accordingly.

Figure 7 is a cross-section through the parts of the device shown in Figure 6, to more clearly show the arrangement of light guides. Indicated in Figure 7 is an infra-red transmitter 39 mounted on the PCB 42 behind light guide 38, and LEDs 49 mounted on the PCB 42 behind light guides 48.

The detector described provides for sequential confirmation, with two independent PIR detectors with non-overlapping coverage areas in a single package. Hence a confirmed alarm can be generated when an intruder is present in the zone. Activeinfra-red anti-masking using multiple infra-red transmitters, both behind the lens arrangements and not behind the lens arrangements, and using light guides to transmit the signal from the infra-red transmitter to a point just behind the lens, provides for a more effective anti-masking arrangement, able to detect a wide range of masking attempts.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Clauses Various aspects of the invention will now be enumerated in the following clauses: 1. A sequential confirmation passive infra-red (PIR) detector for use in an intruder alarm system, the sequential confirmation PIR detector comprising: a first PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; a second PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; a housing including at least a front cover, the front cover having an aperture corresponding with each lens arrangement, each lens arrangement having a window portion which covers a respective aperture, and a flange portion which at least partially surrounds the window portion and is disposed against an interior side of the front cover; and at least one lens retainer fixed to the interior side of the front cover to hold the flange portions of the lens arrangements between the front cover and the lens retainer(s), the lens retainer(s) incorporating integrated masking means disposed behind section of the window portions of each lens arrangement to block infra-red radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first PIR detector and a coverage area of the second PIR detector, the coverage areas being non-overlapping.

2. A sequential confirmation PIR detector as recited in clause 1, in which the lens retainer(s) are made from a substantially rigid material.

3. A sequential confirmation PIR detector as recited in clause 1 or clause 2, in which a single integrated lens retainer is provided for holding both the first lens arrangement and the second lens arrangement in place.

4. A sequential confirmation PIR detector as claimed in any of the preceding clauses, in which lugs are provided on the flange portion of each lens arrangement, and corresponding recesses or apertures are provided on the front cover and/or the lens retainer(s).

5. A sequential confirmation PIR detector as claimed in any of the preceding clauses, in which each lens includes a peripheral side wall extending substantially perpendicularly from an inside edge of the flange portion, in a forward-facing direction, the window portion of the lens arrangement filling the space bordered by the outermost edges of the peripheral side wall.

6. A sequential confirmation PIR detector as recited in any of the preceding clauses, provided with an active-infra-red anti-masking system including at least one active-infra-red transmitter and at least one active-infra-red receiver.

7. A sequential confirmation PIR detector as recited in clause 6, in which at least one active-infra-red transmitter is mounted behind at least one of the lens arrangements, and in which a light guide tube is provided to channel the infra-red radiation from the infra-red transmitter to a point immediately behind the respective lens.

8. A sequential confirmation PIR detector as recited in clause 7, in which one or more additional active-infra-red transmitters are provided to transmit an active infra-red signal from the front surface of the front cover, not from behind any lens arrangement.

9. A sequential confirmation PIR detector as recited in clause 8, in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different active- infra-red transmitter being activated, or a different combination of active infra-red transmitters being activated, and the controller operating the active-infrared transmitters in one of the modes for a period of time, and then another of the modes for a period of time.

10. An intruder alarm system incorporating a sequential confirmation PIR detector according to any of the preceding clauses.

11. A sequential confirmation passive infra-red (PIR) detector for use in an intruder alarm system, the sequential confirmation PIR detector comprising: a first PR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; and a second PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor, masking means being provided behind each lens arrangement to block infra-red radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first PR detector and a coverage area of the second PR detector, the coverage areas being non-overlapping, and an anti-masking system associated with at least one of the first and second PIR detectors, for detecting an attempt to block infra-red radiation from reaching the infra-red sensor of the detector from the coverage area of the detector, the anti-masking system including an infra-red transmitter disposed behind the lens arrangement of the detector, a light guide for guiding radiation from the infra-red transmitter to a point substantially adjacent to a back surface of the lens arrangement, and an infra-red receiver behind the lens arrangement.

12. A sequential confirmation PIR detector as recited in clause 11, in which the forwardmost end of the light guide touches the back surface of the lens arrangement 13.A sequential confirmation PIR detector as recited in clause 11 or clause 12, in which one or more additional active-infra-red transmitters are provided to transmit an active infra-red signal from the front surface of the front cover, not from behind any lens arrangement.

14. A sequential confirmation PIR detector as recited in clause 13. in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different active- infra-red transmitter being activated, or a different combination of active infrared transmitters being activated, and the controller operating the active-infra-red transmitters in one of the modes for a period of time, and then another of the modes for a period of time.

15. An intruder alarm system incorporating a sequential confirmation PIR detector as recited in any of clauses 11 to 14.

16. A passive infra-red (PIR) detector for use in an intruder alarm system, the PIR detector comprising: an infra-red sensor for detecting passive infra-red radiation from warm bodies; a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; an anti-masking system for detecting an attempt to block infra-red radiation from reaching the infra-red sensor of the detector, the anti-masking system including a first infra-red transmitter disposed behind the lens arrangement of the detector, a second infra-red transmitter disposed on an outer surface of the detector, and an infra-red receiver behind the lens arrangement.

17. A PIR detector as recited in clause 16, in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different active-infra-red transmitter being activated, or a different combination of active infra-red transmitters being activated, and the controller operating the active-infra-red transmitters in one of the modes for a period of time, and then another of the modes for a period of time.

18. A PIR detector as recited in clause 16 or clause 17, which is a sequential confirmation PIR detector comprising two independent PIR detectors in a single housing, each independent PIR detector including a lens arrangement.

19. A FIR detector as recited in clause 18, in which an active-infra-red transmitter is disposed behind each one of the lens arrangements.

20. An intruder alarm system incorporating a PIR detector as recited in any of clauses 16 to 19.

Claims

CLAIMS1. A passive infra-red (PIR) detector for use in an intruder alarm system, the PIR detector comprising: an infra-red sensor for detecting passive infra-red radiation from warm bodies; a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; an anti-masking system for detecting an attempt to block infra-red radiation from reaching the infra-red sensor of the detector, the anti-masking system including a first infra-red transmitter disposed behind the lens arrangement of the detector, a second infra-red transmitter disposed on an outer surface of the detector, and an infra-red receiver behind the lens arrangement.
2. A PIR detector as claimed in claim 1, in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different active-infra-red transmitter being activated, or a different combination of active infra-red transmitters being activated, and the controller operating the active-infra-red transmitters in one of the modes for a period of time, and then another of the modes for a period of time.
3. A PR detector as claimed in claim 1 or claim 2, which is a sequential confirmation PR detector comprising two independent PIR detectors in a single housing, each independent PR detector including a lens arrangement.
4. A PIR detector as claimed in claim 3, in which an active-infra-red transmitter is disposed behind each one of the lens arrangements.
5. An intruder alarm system incorporating a PR detector as claimed in any of claims 1 to 4.
6. A sequential confirmation passive infra-red (PIR) detector for use in an intruder alarm system, the sequential confirmation PIR detector comprising: a first PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; a second PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor; a housing including at least a front cover, the front cover having an aperture corresponding with each lens arrangement, each lens arrangement having a window portion which covers a respective aperture, and a flange portion which at least partially surrounds the window portion and is disposed against an interior side of the front cover; and at least one lens retainer fixed to the interior side of the front cover to hold the flange portions of the lens arrangements between the front cover and the lens retainer(s), the lens retainer(s) incorporating integrated masking means disposed behind section of the window portions of each lens arrangement to block infra-red radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first PIR detector and a coverage area of the second PIR detector, the coverage areas being non-overlapping.
7. A sequential confirmation PI R detector as claimed in claim 6, in which the lens retainer(s) are made from a substantially rigid material.
8. A sequential confirmation PIR detector as claimed in claim 6 or claim 7, in which a single integrated lens retainer is provided for holding both the first lens arrangement and the second lens arrangement in place.
9. A sequential confirmation PIR detector as claimed in any of claims 6 to 8, in which lugs are provided on the flange portion of each lens arrangement, and corresponding recesses or apertures are provided on the front cover and/or the lens retainer(s).
10. A sequential confirmation PIR detector as claimed in any claims 6 to 9, in which each lens includes a peripheral side wall extending substantially perpendicularly from an inside edge of the flange portion, in a forward-facing direction, the window portion of the lens arrangement filling the space bordered by the outermost edges of the peripheral side wall.
11.A sequential confirmation PIR detector as claimed in any of claims 6 to 10, provided with an active-infra-red anti-masking system including at least one active-infra-red transmitter and at least one active-infra-red receiver.
12. A sequential confirmation PI R detector as claimed in claim 11, in which at least one active-infra-red transmitter is mounted behind at least one of the lens arrangements, and in which a light guide tube is provided to channel the infrared radiation from the infra-red transmitter to a point immediately behind the respective lens.
13. A sequential confirmation PIR detector as claimed in claim 12, in which one or more additional active-infra-red transmitters are provided to transmit an active infra-red signal from the front surface of the front cover, not from behind any lens arrangement.
14. A sequential confirmation PIR detector as claimed in claim 13, in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different activeinfra-red transmitter being activated, or a different combination of active infrared transmitters being activated, and the controller operating the active-infra-red transmitters in one of the modes for a period of time, and then another of the modes for a period of time.
15. An intruder alarm system incorporating a sequential confirmation PIR detector according to any of claims 6 to 14.
16. A sequential confirmation passive infra-red (PIR) detector for use in an intruder alarm system, the sequential confirmation PIR detector comprising: a first PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infrared sensor; and a second PIR detector including an infra-red sensor and a lens arrangement, the lens arrangement having a plurality of Fresnel facets, each Fresnel facet being configured to focus radiation from a region outside the detector onto the infra-red sensor, masking means being provided behind each lens arrangement to block infrared radiation from being focused on a respective infra-red sensor from selected regions, defining a coverage area of the first PI R detector and a coverage area of the second PIR detector, the coverage areas being non-overlapping, and an anti-masking system associated with at least one of the first and second PIR detectors, for detecting an attempt to block infra-red radiation from reaching the infra-red sensor of the detector from the coverage area of the detector, the anti-masking system including an infra-red transmitter disposed behind the lens arrangement of the detector, a light guide for guiding radiation from the infra-red transmitter to a point substantially adjacent to a back surface of the lens arrangement, and an infra-red receiver behind the lens arrangement.
17. A sequential confirmation PIR detector as claimed in claim 16, in which the forwardmost end of the light guide touches the back surface of the lens arrangement.
18. A sequential confirmation PIR detector as claimed in claim 16 or claim 17, in which one or more additional active-infra-red transmitters are provided to transmit an active infra-red signal from the front surface of the front cover, not from behind any lens arrangement.
19. A sequential confirmation PIR detector as claimed in claim 18, in which a controller is provided and configured to operate the active-infra-red transmitters in at least two different modes, a different mode comprising a different active- infra-red transmitter being activated, or a different combination of active infra-red transmitters being activated, and the controller operating the active-infrared transmitters in one of the modes for a period of time, and then another of the modes for a period of time.
20. An intruder alarm system incorporating a sequential confirmation PIR detector as claimed in any of claims 16 to 19.