WO1996006865A1

WO1996006865A1 - Infrared intrusion detector with obscuring detecting apparatus

Info

Publication number: WO1996006865A1
Application number: PCT/EP1995/001501
Authority: WO
Inventors: Boris Zhevelev; Nahum Tchernihovsky; Yaacov Kotlicki
Original assignee: Visonic Sicherheitstechnik Gmbh
Priority date: 1994-08-28
Filing date: 1995-04-20
Publication date: 1996-03-07
Also published as: IL110800A0

Abstract

This invention discloses an anti-obscuring intrusion detector for supervising a region comprising a housing having an input radiation window, an intrusion sensor, in said housing, which views the region through the window and provides a first output signal responsive to incident far infrared radiation, a radiation source which emits radiation of a preselected wavelength range partly to a wide field-of-view in the vicinity of the detector and partly into the input window, an obscuring sensor, in said housing, which views the window and provides a second output signal responsive to incident radiation of the preselected wavelength range and obscuring detection circuitry which determines whether the intrusion detector is obscured by determining whether the output of the obscuring sensor is outside a reference window having a lower threshold and an upper threshold.

Description

DESCRIPTION

INFRARED INTRUSION DETECTOR WITH OBSCURING DETECTING APPARATUS

FIELD OF THE INVENTION The present invention relates to intrusion detectors in general and, more particularly, to motion detectors using passive infrared detectors.

BACKGROUND OF THE INVENTION

Passive infrared detectors are widely used in in¬ truder, e.g. burglar alarm systems. Since intruder alarm systems are generally designed for detecting the presence of humans, the infrared detectors of such systems gener¬ ally respond to radiation in the far infrared range, preferably 7 - 14 micrometers, as typically irradiated from an average person. A typical passive infrared detec¬ tor includes a pyroelectric sensor adapted to provide an electric output in response to changes in radiation at the desired wavelength range. The electric output is then amplified by a signal amplifier and received by appropri¬ ate detection circuitry.

To detect movement of a person in a predefined area, typically a room, passive infrared detectors are provided with a discontinuously segmented optical element, e.g. a segmented lens or mirror having at least one optical segment, wherein each segment of the lens or mirror collects radiation from a discrete, narrow, field-of-view such that the fields-of-view of adjacent segments do not overlap. Thus, the pyroelectric sensor receives external radiation through a segmented field-of-view, including a plurality of discrete detection zones separated by a plurality of discrete no-detection zones. The system detects movement of a person from a given zone to an adjacent zone by detecting, for example, a sharp drop or a sharp rise in the electric output of the pyroelectric sensor.

It is appreciated that abrupt changes in ambient temperature may result in abrupt changes in the output of the pyroelectric sensor and, thus, false alarms may occasionally be detected by the intruder alarm system. To avoid this problem, most intruder alarm systems use a dual-element sensor arrangement including two, adjacent, pyroelectric sensor elements. The two elements and the segmented optics are arranged such that the detection zones of the two elements are interlaced and do not overlap. The electric outputs of the two elements have opposite electrical polarities, such that the absolute value of the net signal received by the amplifier is substantially zero as long as radiation from the same source is received by both elements simultaneously and greater than zero only when radiation is detected by one element and not by the other element.

Thus, the use of dual-element sensors improves the reliability and the detection resolution of intrusion detectors. Furthermore, since movement perpendicular to the segmented fields-of-view, i.e. from one detection zone to the next, necessarily includes, in dual-element sensor systems, movement from a detection zone of one element to a detection zone of the other element, any movement detected by a single-element sensor system should be detected by a corresponding dual-element sensor system. However, even dual-element sensor systems occa¬ sionally suffer from false alarms due to the undesired influences of internal systematic noise, radio frequency (RF) or other external noise, or random noise known as "spikes". The results of these influences are generally overcome by increasing detection thresholds or by using pulse-counting techniques known in the art, thereby decreasing the detection sensitivity. Existing intrusion detectors are also exposed to intentional or unintentional obscuring of the far infra¬ red radiation to be received by the pyroelectric sensor. Obscuring of the radiation may result from intentional or unintentional placement of an obstructing object near the detector, blocking at least part of the segmented field-of-view of the pyroelectric sensor, or by inten¬ tional covering of the detector or the segmented optics using obscuring tape or spray or other material which are not transparent to far infrared radiation. When such obscuring occurs during non-detection periods, normally during the day when the alarm system is disarmed to avoid false alarms due to normal movement of people, it is likely to remain unnoticed and, consequently, intrusions during subsequent detection periods when the system is re-armed are not detected. This problem is particularly serious when the obscuring tape or spray or other materi¬ al used for tampering with the detector is substantially transparent to visible light and, thus, invisible to the eye but blocking far infrared radiation. Furthermore, since existing intrusion detectors do not normally re¬ spond to extremely slow movement, obscuring material can be applied to the detector by a person slowly approaching the detector even during detection periods.

To overcome obscuring of the far infrared radiation, some existing systems include a "walk-test" circuit which provides an indication, preferably a visual indication, in response to human movement during non-detection peri¬ ods. However, past experience shows that such tests are not efficient due, inter alia, to difficulty and incon¬ venience in monitoring the test.

U.S. Patent 4,709,153 to Schofield describes a passive infrared intrusion detector with obscuring detection means including an external far infrared gener¬ ator, mounted on the detector, which directs far infrared radiation pulses to the pyroelectric sensor in the detec- tor. When the optical input window of the detector is covered, for example by tape or spray, passage of radia¬ tion from the far infrared generator is blocked and a continuous drop is detected in the electric output of the sensor. This system is inefficient since the use of a far infrared generator, i.e. a heater, consumes a sub¬ stantial amount of energy. Furthermore, since the heater is located on the detector, any obstruction situated beyond the heater is not detectable. For example, cover¬ ing of the entire detector with a non-transmissive box will not be detected by the system suggested in the ' 153 Patent.

U.S. Patent 4,752,768 to Steers describes an intru¬ sion detector employing an anti-obscuring device. In addition to a far infrared emitter similar to the genera¬ tor of Patent '153, the anti-obscuring device of Patent ' 768 includes a source of near infrared radiation within the detector, a mirror located at the far end of the detection zone and a near infrared sensor within the detector. Near infrared radiation emitted from the source is reflected by the mirror at the far end of the detec¬ tion zone and the reflected radiation is detected by the near infrared sensor. When an object is placed anywhere along the optical path from the detector to the mirror and back, a drop or a rise in the output of the near infrared sensor is detected.

The main draw-back of the arrangement of Patent '768 is that only obstructions along the narrow optical path between the detector and the mirror are detected, while other obstructions are not detected. Another draw-back is that, in practice, the system would require a retrore- flecting far-end mirror since a simple mirror would need constant adjustment. Furthermore, this system does not distinguish between obstructions in the vicinity of the detector, which are generally related to intentional obscuring of the detector, and remote obstructions which are generally not related to such intentional obscuring.

U.S. Patent 4,242,669 to Crick describes another passive infrared intrusion detector with an anti-obscur¬ ing device. This anti-obscuring device includes a near infrared source and a near infrared sensor which are both mounted within the detector, whereby the near infrared radiation is transmitted and received through the input window of the detector. Placement of an obscuring object near the detector is detected by detecting a rise in the electric output of the near infrared sensor due to reflection from the obscuring object. However, obscuring is not detected if the obscuring object is highly trans¬ parent or, alternatively, highly absorptive, to near infrared radiation.

U.S. Patent 4,982,094 also describes an infrared detector with anti-obscuring means.

It should be appreciated that the performance of existing anti-obscuring devices, including the devices described above, is substantially affected by changes in ambient conditions, such as temperature and humidity, and other factors, such as accumulation of dust on the detec¬ tor. To avoid false detection of radiation obscuring, high detection thresholds are used by the signal proces¬ sors of existing anti-obscuring devices, resulting in rather poor obscuring detection resolution.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a passive infrared intrusion detector with improved obscuring detection means. According to one aspect of the present invention, infrared radiation emitted by a near infrared source of the intrusion detector is directed, partly, to illuminate a region in the vicinity of the intrusion detector and, partly, to provide feedback illumination to a near infrared sensor within the intru¬ sion detector. The near infrared radiation is received through an input window which is also used for collecting the far infrared radiation. The input window may include segmented optics. Alternatively, the input window may be a simple transparent window, in which case segmented optics, such as a segmented lens or a segmented mirror, are within the detector.

The electric output of the near infrared sensor in response to the near infrared feedback illumination provides the intrusion detector with a reference, also referred to herein as the reference signal, indicative of non-obscuring conditions. When an obscuring object is brought to the vicinity of the intrusion detector, near infrared radiation reflected by the object enters the detector through the input window and onto the near infrared sensor and, thus, the output produced by the near infrared sensor is increased. When obscuring materi¬ al covers at least a portion of the input window, the output produced by the near infrared sensor is reduced. Thus, obscuring of the present intrusion detector is detected either when the output of the near infrared sensor exceeds a first, predetermined, threshold or when the output of the near infrared sensor drops under a second, predetermined, threshold lower than the first threshold.

It should be appreciated that the difference between the first and second thresholds, hereinafter referred to as the detection signal window, determines the obscuring detection sensitivity of the system.

According to another aspect of the present inven¬ tion, the intrusion detector includes circuitry for dynamically adjusting the first and second thresholds described above, according to a predetermined adjustment schedule, while preferably maintaining the detection signal window substantially constant. Adjustment of the first and second threshold is preferably performed peri¬ odically, in accordance with periodic measurements of the near infrared sensor output. This allows correction of the first and second thresholds in accordance with var¬ ious changes in the system or in ambient conditions which affect the reference signal, resulting in more accurate positioning of the detection signal window. The accurate positioning of the detection signal window allows the use of a narrower detection window which, in turn, provides improved detection sensitivity.

Although the use of near infrared radiation for obscuring detection is preferred, it should be appreciat¬ ed that other spectral ranges, for example ultraviolet radiation, may be equally suitable for the purposes of the present invention.

In accordance with a preferred embodiment of the invention there is thus provided an anti-obscuring intru¬ sion detector for supervising a region including: a housing having an input radiation window; an intrusion sensor, in said housing, which views the region through the input window and provides a first output signal responsive to incident far infrared radia¬ tion; a radiation source which emits radiation of a prese¬ lected wavelength range partly to a wide field-of-view in the vicinity of the detector and partly into the input window; an obscuring sensor, in said housing, which views through the input window and provides a second output signal responsive to incident radiation of the preselect¬ ed wavelength range; and obscuring detection circuitry which determines whether the intrusion detector is obscured by determining whether the output of the obscuring sensor is outside a reference window having a lower threshold and an upper threshold.

In a preferred embodiment of the invention, the detector further includes segmented optics which provide the intrusion sensor with a predefined, segmented, field- of-view of the region supervised by the detector. In one preferred embodiment, the segmented optics includes a segmented lens. The segmented lens is preferably associ¬ ated with the input window. Alternatively, the segmented optics include a segmented mirror.

In accordance with a preferred embodiment of the invention, the radiation source includes a disperser which disperses the radiation to the vicinity of the detector and into the input window.

In a preferred embodiment of the invention, the preselected wavelength range includes a near infrared wavelength range. Preferably, the near infrared wave¬ length range is centered at a wavelength of approximately 0.9 micrometers. Alternatively, the preselected wave¬ length range includes an ultraviolet wavelength range.

In one preferred embodiment of the invention, the radiation source is in the housing. In another preferred embodiment of the invention the radiation source is at least partly external to the housing.

In one preferred embodiment of the invention, the detector further includes a reflector, on said housing, which reflects some of the radiation emitted by the radiation source into the input window.

Further, in accordance with a preferred embodiment of the invention, the obscuring detection circuitry includes a window adjustment circuit which adjusts the upper and lower thresholds of the reference window in accordance with measurements of the second output signal. The window adjustment circuit is preferably activated periodically in accordance with a preselected time sched¬ ule.

In a preferred embodiment, the radiation source includes at least one light emitting diode (LED) . BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the following detailed description of preferred embodi¬ ments of the present invention, taken in conjunction with the following drawings in which:

Fig. 1 is a simplified pictorial illustration of an anti-obscuring intrusion detector constructed and opera¬ tive in accordance with one preferred embodiment of the present invention;

Fig. 2 is simplified, partially cutaway, schematic illustration of the interior of the intrusion detector of Fig. 1;

Fig. 3 is a cross-sectional, schematic, illustration of the intrusion detector of Fig. 1 showing paths of near infrared radiation generated by the intrusion detector;

Fig. ^'4A is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 1, showing paths of near infrared radiation reflected by an obstruction in the vicinity of the intrusion detector;

Fig. 4B is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 1, showing paths of radiation reflected or absorbed by an obstruction on the intrusion detector;

Fig. 5 is a simplified pictorial illustration of an anti-obscuring intrusion detector constructed and opera¬ tive in accordance with another preferred embodiment of the present invention;

Fig. 6 is a cross-sectional, schematic, illustration of the intrusion detector of Fig. 5 showing paths of near infrared radiation generated by the intruder detector;

Fig. 7A is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 5, showing paths of near infrared radiation reflected by an obstruction in the vicinity of the intrusion detector;

Fig. 7B is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 5, showing paths of near infrared radiation reflected or absorbed by an obstruction on the intrusion detector;

Fig. 8 is a simplified pictorial illustration of an anti-obscuring intrusion detector constructed and opera¬ tive in accordance with yet another preferred embodiment of the present invention;

Fig. 9 is simplified, partially cutaway, schematic illustration of the interior of the intrusion detector of Fig. 8;

Fig. 10 is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 8 showing paths of near infrared radiation generated by the intruder detec¬ tor;

Fig. 11A is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 8, showing paths of near infrared radiation reflected by an obstruction in the vicinity of the intrusion detector;

Fig. 11B is a cross-sectional, schematic, illustra¬ tion of the intrusion detector of Fig. 1, showing paths of near infrared radiation reflected or absorbed by an obstruction on the intrusion detector;

Fig. 12 is a cross-sectional, schematic, illustra¬ tion of an anti-obscuring intrusion detector constructed and operative in accordance with a further preferred embodiment of the present invention, showing paths of near infrared radiation generated by the intruder detec¬ tor;

Fig. 13 is a schematic illustration of circuitry for the intrusion detectors of Figs. 1 - 12 according to one preferred embodiment of the present invention;

Fig. 14 is a schematic illustration of circuitry for the intrusion detectors of Figs. 1 - 12 according to another preferred embodiment of the present invention;

Fig. 15 is a schematic illustration of circuitry for the intrusion detectors of Figs. 1 - 12 according to yet another preferred embodiment of the present invention which incorporates digital processing; and

Fig. 16 is a schematic flow chart which outlines a preferred algorithm for the digital processing incorpo¬ rated by the circuitry of Fig. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to Fig. 1 which schematically illustrates an anti-obscuring intrusion detector 8 con¬ structed and operative in accordance with one preferred embodiment of the present invention. Intrusion detector 8 preferably includes a housing 10 having a cover portion 12. Cover portion 12 preferably accommodates a segmented Fresnel lens 14 which provides a segmented field-of-view to sensors (not shown in Fig. 1) inside housing 10, as known in the art, and a near infrared output window 16. As described in detail below, lens 14 is preferably transmissive to both far and near infrared radiation. A reflector 18, which may be slightly angled as shown in Fig. 3, is preferably mounted on cover portion 12 near output window 16. Alternatively, as known in the art, segmented lens 14 may be a separate unit situated under¬ neath cover portion 12 of housing 10.

Reference is now made to Fig. 2 which schematically illustrates internal elements of intrusion detector 8 which are enclosed in housing 10. As shown in Fig. 2, intrusion detector 8 includes a circuit board 20 adapted for accommodating intrusion detection circuitry and anti- obscuring circuitry as described below with reference to Figs. 13, 14 and 15. At least one light emitting diode 22 (LED) is preferably mounted on circuit board 20 so as to be located underneath window 16 of cover portion 12 when board 20 is mounted in housing 10. According to the present invention LED 22 emits near infrared radiation, preferably at a wavelength of approximately 0.9 microme¬ ters, which is transmitted via output window 16 and illuminates a wide field-of-view of the space surrounding detector 8. The near infrared radiation is preferably generated in short pulses, thereby to minimize the energy consumption of detector 8 and to distinguish the near infrared radiation over other ambient radiation. In a preferred embodiment of the invention, detector 8 is mounted on a wall or a ceiling of an enclosed area to be monitored by intrusion detector 8, and the near infrared radiation emitted by LED 22 covers a wide field-of-view in the vicinity of detector 8. This allows coverage of substantially any attempt to intentionally obscure detec¬ tor 8.

A far infrared sensor 24, such as a pyroelectric sensor, which produces an electric output in response to far infrared, preferably in the wavelength range of between approximately 7 micrometers and approximately 14 micrometers, is mounted on circuit board 20 at a prede¬ termined position underneath segmented lens 14. Sensor 24 is positioned with respect to lens 14 so as to receive far infrared radiation from a preselected, segmented, field-of-view of the region supervised by detector 8, as known in the art. A near infrared sensor 26 which pro¬ duces an electric output in response to near infrared radiation, preferably at the same wavelength as emitted by LED 22, is mounted on circuit board 20 preferably close to sensor 24. To prevent direct illumination of sensor 26 by LED 22, an optical barrier 28 mounted on circuit board 20 between LED 22 and sensor 26 preferably absorbs all near infrared radiation from LED 22 except for radiation entering housing 10 via lens 14.

In an alternative embodiment of the invention, not shown in the drawings, LED 22 is not mounted in housing 10 of detector 8 but, rather, LED 22 is installed in a separate unit located anywhere in the vicinity of detec¬ tor 8 or attached thereto. In this embodiment, housing 10 may serve as an optical barrier between LED 22 and sensor 26 and, thus, optical barrier 28 is not required.

Reference is now made to Fig. 3 which schematically illustrates a cross-section of intrusion detector 8. As shown in Fig. 3, near infrared radiation emitted by LED 22 exits detector 8 through output window 16 and illumi¬ nates a wide-field-of-view in the vicinity of detector 8. Part of the near infrared radiation, hereinafter referred to as the feedback radiation, is reflected by reflector 18 and reenters housing 10 through segmented lens 14. Additionally or alternatively, reflections from objects in the vicinity of detector 8, for example walls, can be utilized to provide the feedback radiation. Inside hous¬ ing 10, part of the feedback radiation is received by near infrared sensor 26 which produces an electric out¬ put, hereinafter referred to as the reference signal, in response to the received near infrared radiation. Far infrared radiation from bodies in the region supervised by detector 8 is received, via lens 14, by far infrared sensor 24 which produces an electric output in response thereto.

Reference is now made to Fig. 4A which schematically illustrates the effect of an obstructing object 30, located in the vicinity of intrusion detector 8, on the infrared radiation paths. Object 30 may be any type of object used for obscuring detector 8, for example a hand of a person approaching detector 8 to obscure the detec¬ tor. As shown in Fig. 4A, part of the near infrared radiation emitted by LED 22 is reflected by object 30 and received, via lens 14, by near infrared sensor 26. In this case, the electric output produced by sensor 26 is higher than the reference signal produced in the no- obstruction situation of Fig. 3. The actual magnitude of the electric output of sensor 26 depends on the proximity of object 30 to detector 8 and on the reflectivity, size, shape, position, etc., of object 30.

Reference is now made to Fig. 4B which schematically illustrates an obstructing object 32, which may also be an obscuring tape or a layer of obscuring material such as spray paint, juxtaposed with or attached to lens 14 of intrusion detector 8. As shown in Fig. 4B, infrared radiation emitted by LED 22 and reflected by reflector 18 is reflected off object 32 or absorbed thereby and, hence, does not enter housing 10 via lens 14. If object 32 does not completely cover lens 14 or if object 32 is partially transmissive to the near infrared radiation, some of the infrared radiation may be received by sensor 26. In either case, the electric output produced by sensor 26 is lower than the reference signal produced in the no-obstruction situation of Fig. 3. The actual magni¬ tude of the electric output of sensor 26 depends on the amount of near infrared radiation admitted through or around object 32.

Reference is now made to Figs. 5, 6, 7A and 7B which schematically illustrate an intrusion detector 58 con¬ structed and operative in accordance with another, pre¬ ferred, embodiment of the present invention. Intrusion detector 58 includes a housing 60, similar to housing 10 of detector 8 (Fig. 1) and having a cover portion 62. Cover portion 62 accommodates a segmented Fresnel lens 64, similar to lens 14 of Fig. 1, which provides a seg¬ mented field-of-view as described above with reference to lens 14, and a near infrared transmissive disperser 66. As described above with reference to lens 14, lens 64 is preferably transmissive to both far and near infrared radiation. The internal elements of intrusion detector 58, described below with reference to Figs. 6, 7A and 7B, are arranged substantially as in Fig. 2 with reference to corresponding internal elements of detector 8. As simi¬ larly for this embodiment, the intrusion detector 58 includes a housing 60 adapted for accommodating intrusion detector circuitry and anti-obscuring circuitry as de¬ scribed below with reference to Figs. 13, 14 and 15.

Reference is now made to Fig. 6 which schematically illustrates a cross-section of intrusion detector 58. As shown in Fig. 6, near infrared radiation emitted by at least one light emitting diode (LED) 72 exits detector 58 through transmissive disperser 66 and illuminates a wide field-of-view in the vicinity of detector 58. Part of the dispersed near infrared radiation, also referred to as feedback radiation is collected by segmented lens 64 and reenters housing 60. Inside housing 60, part of the feedback radiation is received by a near infrared sensor 76 which produces an electric output, also referred to herein as the reference signal, in response to the re¬ ceived near infrared radiation, as in the embodiment of Fig. 3. Far infrared radiation from bodies in the region supervised by detector 58 is received, via lens 64, by a far infrared sensor 74 which produces an electric output in response thereto.

Reference is now made to Fig. 7A which schematically illustrates the effect of an obstructing object 80 in the vicinity of intrusion detector 58. As shown in Fig. 7A, part of the near infrared radiation emitted by LED 72 which is not directly collected by lens 64 is reflected off object 80 and received, via lens 64, by near infrared sensor 76. In this case, the electric output produced by sensor 76 is higher than the reference signal produced in the no-obstruction situation of Fig. 6. The actual magni¬ tude of the electric output of sensor 76 depends on the proximity of object 80 to detector 58 and on the reflec¬ tivity, size, shape, position, etc., of object 80.

Reference is now made to Fig. 7B which schematically illustrates the effect of an obstructing object 82, which may also be an obscuring tape or a layer of obscuring material such as spray paint, juxtaposed with or attached to lens 64 of intrusion detector 58. As shown in Fig. 7B, infrared radiation emitted by LED 72 and dispersed by disperser 66 towards lens 64 is reflected off object 82 or absorbed thereby and, hence, does not enter housing 60 via lens 64. If object 82 does not completely cover lens 64 or if object 82 is partially transmissive to the near infrared radiation, some of the infrared radiation may be received by sensor 76. In either case, the electric output produced by sensor 76 is generally lower than the reference signal produced in the no-obstruction situation of Fig. 6. The actual magnitude of the electric output of sensor 76 depends on the amount of near infrared radia¬ tion admitted through or around object 82.

Reference is now made to Figs. 8, 9, 10 11A and 11B which schematically illustrates an anti-obscuring intru¬ sion detector 108 constructed and operative in accordance with yet another, preferred, embodiment of the present invention. Referring to Fig. 8, intrusion detector 108 preferably includes a housing 110 having a cover portion 112. Cover portion 112 preferably accommodates a segment¬ ed Fresnel lens 114 which provides a segmented field-of- view as described above with reference to lenses 14 and 64, and a LED cap 116 which is transmissive to near infrared radiation. As described above with reference to lenses 14 and 64, lens 114 is preferably transmissive to both far and near infrared radiation.

Reference is now made to Fig. 9 which schematically illustrates internal elements of intrusion detector 108 which are enclosed in housing 110. As shown in Fig. 9, intrusion detector 108 includes a circuit board 120 adapted for accommodating intrusion detection circuitry and anti-obscuring circuitry as described below with reference to Figs. 13, 14 and 15. At least one light emitting diode (LED) 122 is preferably mounted on circuit board 120 so as to be located underneath LED cap 116 of cover portion 112 when board 120 is mounted in housing 110. At least one additional LED 125 is mounted in dis¬ persive cap 116 and is directed substantially toward segmented lens 114. According to the present invention LED 122 emits near infrared radiation, preferably at a wavelength of approximately 0.9 micrometers, which is transmitted via output window 116 and illuminates a wide field-of-view of the space surrounding detector 108. LED 125 emits near infrared radiation, preferably at the same wavelength as that of LED 122, which is preferably dis- persed by a portion of dispersing cap 116 and proceeds in the general direction of lens 114.

A far infrared sensor 124, preferably a pyroelectric sensor, which produces an electric output in response to radiation in a far infrared wavelength range, preferably between approximately 7 micrometers and approximately 14 micrometers, is mounted on circuit board 120 at a prede¬ termined position underneath segmented lens 114. Sensor

124 is positioned with respect to lens 114 so as to receive far infrared radiation from a preselected, seg¬ mented, field-of-view of the region supervised by detec¬ tor 108. A near infrared sensor 126 which produces an electric output in response to radiation in a near infra¬ red wavelength range, preferably centered at approximate¬ ly 0.9 micrometers, is preferably mounted on circuit board 120 preferably close to sensor 124. To prevent direct illumination of sensor 126 by LEDs 122 and 125, detector 108 is further provided with an optical barrier 128, mounted on circuit board 120 between LEDs 122 and

125 and sensors 124 and 126, which preferably absorbs all near infrared radiation from LEDs 122 and 125 except for radiation entering housing 110 via lens 114.

Reference is now made to Fig. 10 which schematically illustrates a cross-section of intrusion detector 108. As shown in Fig. 10, the near infrared radiation emitted by LED 122 exits detector 108 through LED cap 116 and illu¬ minates a wide field-of-view in the vicinity of detector 108. Some of the near infrared radiation emitted by LED 125, also referred to as the feedback radiation, is dispersed by LED cap 16 and renters housing 110 through segmented lens 114. Inside housing 110, part of the feedback radiation is received by near infrared sensor

126 which produces an electric output, also referred to as the reference signal as described above, in response to the received near infrared radiation. Far infrared radiation from bodies in the region supervised by detec- tor 108 is received, via lens 114, by far infrared sensor 124 which produces an electric output in response there¬ to.

Reference is now made to Fig. 11A which schematical¬ ly illustrates the effect of an obstructing object 130 located in the vicinity of intrusion detector 108. As shown in Fig. 11A, part of the near infrared radiation emitted by LED 122 and dispersed by LED cap 116 is re¬ flected off object 130 and received, via lens 114, by near infrared sensor 126. In this case, the electric output produced by sensor 126 is generally higher than the reference signal produced in the no-obstruction situation of Fig. 10. The actual magnitude of the elec¬ tric output of sensor 126 depends on the proximity of object 130 to lens 114 and on the reflectivity, size, shape, position, etc., of object 130.

Reference is now made to Fig. 11B which schematical¬ ly illustrates an obstructing object 132, which may also be an obscuring tape or a layer of obscuring material such as spray paint, juxtaposed with or attached to lens 114 of intrusion detector 108. As shown in Fig. 11B, near infrared radiation emitted through LED cap 116 towards lens 114 is reflected off object 132 or absorbed thereby and, hence, does not enter housing 110 via lens 114. If object 132 does not completely cover lens 114 or if object 132 is partially transmissive to the near infrared radiation, some of the near infrared radiation may be received by sensor 126. In either case, the electric output produced by sensor 126 is generally lower than the reference signal produced in the no-obstruction situation of Fig. 10. The actual magnitude of the electric output of sensor 126 depends on the amount of near infrared radiation admitted through or around object 132.

Reference is now made to Fig. 12 which schematically illustrates a portion of a cross-section of an anti- obscuring intrusion detector 158 constructed and opera- tive in accordance with a further, preferred, embodiment of the present invention. Intrusion detector 158 includes a housing 162 having an input window 164, an output window 166 and a reflector 168. Input window 164 is preferably transmissive to both far and near infrared radiation. Mounted in housing 162, there is a circuit board 170 adapted for accommodating intrusion detection circuitry and anti-obscuring circuitry as described below with reference to Figs. 13 , 14 and 15. At least one light emitting diode (LED) 172 is preferably mounted on circuit board 170 so as to be located directly behind window 166 of housing 162.

A concave mirror 165, preferably a segmented concave mirror, is mounted in housing 162 behind input window 164. Mirror 165 is adapted to reflect radiation entering housing 162 via window 164 so as to provide a segmented field-of-view to sensors 174 and 176 on circuit board 170, as described below. Alternatively, window 164 may include a segmented lens and, in such case, mirror 164 may be a non-segmented mirror.

According to the present invention LED 172 emits near infrared radiation, preferably at a wavelength of approximately 0.9 micrometers, which is transmitted via output window 166 and illuminates a wide field-of-view of the space surrounding detector 158. In a preferred embodiment of the invention, detector 158 is mounted on a wall or a ceiling of an enclosed area to be monitored by intrusion detector 158, and the near infrared radiation emitted by LED 172 covers a wide field-of-view in the vicinity of detector 158. This allows monitoring of substantially any attempt to reach detector 158 while, at the same time, only activities in the vicinity of detec¬ tor 158 are monitored.

Far infrared sensor 174, which produces an electric output in response to radiation in a far infrared wave¬ length range, preferably between approximately 7 microme- ters and approximately 14 micrometers, is mounted on circuit board 170 at a predetermined position with re¬ spect to mirror 165. Sensor 174 is positioned with re¬ spect to mirror 165 so as to receive far infrared radia¬ tion from a preselected, segmented, field-of-view of the region supervised by detector 158. A near infrared sensor 176, which produce an electric output in response to radiation in a near infrared wavelength range, preferably centered at approximately 0.9 micrometers, is mounted on circuit board 170 preferably close to sensor 174. Direct illumination of sensor 176 by LED 172 is avoided by mounting LED 172 and sensor 176 on opposite sides of circuit board 170, which preferably absorbs all near infrared radiation from LED 172 except for radiation entering housing 162 via input window 164. Alternatively, sensor 176 may be positioned at other suitable locations in housing 162, for example on mirror 165. In a preferred embodiment of the invention additional optical barriers, other than circuit board 170, are provided to more thor¬ oughly absorb undesired radiation within the boundaries of housing 162.

The near infrared radiation emitted by LED 172 exits detector 158 through output window 166 and illuminates a wide field-of-view in the vicinity of detector 158. Part of the near infrared radiation, also referred to as the feedback radiation, is reflected by dispersing reflector 168, reenters housing 162 through input window 164 and is, again, reflected by mirror 165. Part of the feedback radiation is received by near infrared sensor 176 which produces an electric output, also referred to herein as the reference signal as described above, in response to the received near infrared radiation. Far infrared radia¬ tion from bodies in the region supervised by detector 158 is received, via window 164 and mirror 165, by far infra¬ red sensor 174 which produces an electric output in response thereto. The response of detector 158 to placement of objects in the vicinity thereof is substantially as described above with reference to detectors 8, 58 and 108, i.e. an increase in the electric output of sensor 176. Similarly, the response of detector 158 to placement of objects on window 164, obscuring at least part of the feedback radiation from reflector 168, is substantially as de¬ scribed above with reference to detectors 8, 58 and 108, i.e. a decrease in the electric output of sensor 176.

It should be appreciated that variations of segment- ed-mirror detector 158, some of which may incorporate relevant features of the segmented-lens detectors de¬ scribed above with reference to Figs. 1 - 11B, are also within the scope of the present invention.

Reference is now made to Fig. 13 which schematical¬ ly illustrates preferred intrusion detection and anti- obscuring circuitry which may be used for any of intru¬ sion detectors 8, 58, 108 and 158, in accordance with one preferred embodiment of the present invention. The cir¬ cuitry of Fig. 13 includes a far infrared signal amplifi¬ er 216 which amplifies the output of a far infrared sensor 224, such as pyroelectric sensors 24, 74, 124 or 174 of the preferred embodiments described above. The output of amplifier 216 is received by a far-infrared signal window comparator 218. The output of window compa¬ rator 218, which is responsive to variations in the output of amplifier 216, is connected to one of the inputs of a main controller 214. When an intruder crosses the segmented field-of-view of the intrusion detector, the output of amplifier 216 changes abruptly and, conse¬ quently, window comparator 218 generates an intrusion detection signal to controller 214. An intrusion alarm circuit of controller 214, activated in response to the intrusion detection signal, then provides an intrusion alarm signal which operates a loudly sounded alarm or some other indication, near the detector or at a remote monitoring station.

The circuitry of Fig. 13 further includes a LED driver 212 which drives at least one LED 220 such as LEDs 22, 72, 122 or 172 of the preferred embodiments described above. As known in the art, LED 220 is preferably driven in a pulsed mode. The operation of driver 212 is prefera¬ bly controlled by appropriate circuitry in main control¬ ler 214, however, the driver may alternatively be con¬ trolled by a separate pulse generating unit. A near infrared sensor 222, such as sensors 26, 76, 126 or 176 of the preferred embodiments described above, senses the near infrared pulses generated by LED 220 and provides a corresponding output which is, then, amplified by a near infrared signal amplifier 200. Since the near infrared radiation is preferably pulsed, the output of amplifier 200 is also pulsed. Thus, the output of amplifier 200 is preferably connected to the input of a pulse-to-level (P/L) converter 201 which provides a continuous-level output signal, Vs, corresponding to the pulsed output signal of amplifier 200.

Output signal Vs of converter 201 is connected to the input of a differentiator 208 and to one of the inputs of a near infrared window comparator 206. Output signal Vs is also connected, via a switch 202, to a threshold memory 204 which generates top and bottom thresholds, Vt and Vb, respectively, to be used by window comparator 206 as described below. The output of differ¬ entiator 208 is connected to an input of controller 214, via a comparator 210 which compares the output of differ¬ entiator 208 to a preselected threshold input. The output of window comparator 206 is connected to another input of controller 214. Activation of switch 202 is controlled through control signals generated by controller 214 as described below.

When an obstructing object is brought to the vicini¬ ty of the intrusion detector, as described above, the radiation reflected by the object causes an increase in the output, Vs, of P/L 201. When the object is suffi¬ ciently close to the detector such that Vs exceeds upper threshold Vt of window comparator 206, i.e. Vs>Vt, an obscuring detection signal is generated to controller 214. The obscuring detection signal is also generated when an obstructing object, typically an obscuring tape or spray, is placed on the input window or the segmented lens of the intrusion detector, depending on which embod¬ iment of the invention is used, such that Vs drops under lower threshold Vb of comparator 206, i.e. Vs<Vb. In either case, the obscuring detection signal may activate an obscuring alarm circuit of controller 214 to provide a preselected indication of the obscuring event, on the detector or at a remote monitoring station.

It is appreciated, however, that very slow changes in output signal Vs are generally not caused by obscuring attempts but, rather, they are caused by other factors such as changes in ambient conditions. Therefore, such slow changes should be distinguished from more abrupt changes which are typical of obscuring attempts. The provision of differentiator 208 enables such a distinc¬ tion. The output of differentiator 208, which corresponds to the time derivative of output signal Vs, i.e. dVs/dt, is compared at comparator 210 with a predetermined threshold, Rt, which preferably corresponds to a time- derivative lower than the time-derivative resulting from typical obscuring activity. Thus, in a preferred embodi¬ ment of the invention, the obscuring alarm circuit of controller 214 is activated only when (dVs/dt)>Rt, in addition to the criteria described above.

In accordance with a preferred embodiment of the present invention, the window defined by window compara¬ tor 206 is not fixed but, rather, it is dynamically adjusted as will now be described. At an initial time, preferably when the detector is installed in a new loca- tion, switch 202 is activated and an initial reference reading of output signal Vs, hereinafter Vr, is stored in memory 204. Thresholds Vt and Vb of comparator 206 are preferably adjusted according to reference signal Vr, such that Vt is higher than Vr by a first, preselected, difference and Vb is lower than Vr by a second, prese¬ lected, difference. The first and second differences may be fixed, for simplicity, or they may be dependent on the absolute value of Vr. The first and second differences may or may not be equal.

Further, in accordance with a preferred embodiment of the invention, a timer circuit in controller 214 activates switch 202 periodically, in accordance with a preselected time schedule. During each such activation of switch 202, a new reference signal Vr is stored in memory 204 and new thresholds Vt and Vb are set for window comparator 206. In a preferred embodiment of the inven¬ tion, updating of the thresholds is performed very fre¬ quently, preferably every few seconds, for example every ten seconds. This allows the system to adjust to new environmental or other conditions, for example changes in temperature and/or humidity or accumulation of dust, which may affect Vr but are not related to obscuring activity. It is appreciated, that the use of dynamically adjustable thresholds, also referred to herein as the floating window, allows the use of a narrower window and, thus, improves the obscuring detection sensitivity of the detector.

Reference is now made to Fig. 14 which schematical¬ ly illustrates intrusion detection and anti-obscuring circuitry which may be used for any of intrusion detec¬ tors 8, 58, 108 and 158, in an alternative, preferred, embodiment of the present invention. The circuitry of Fig. 14 is generally the same as that of Fig. 13, except for the circuitry for adjusting top and bottom thresh- olds, Vt and Vb, of window comparator 206. In the embodi¬ ment of Fig. 14, a controller 314 adjusts thresholds Vt and Vb through a threshold generator 303, which is pref¬ erably activated for threshold calibration during initial installation of the detector and for subsequently updat¬ ing the thresholds according the time schedule described above.

For each update, threshold generator 303 generates new top and bottom thresholds, Vt and Vb, based on the updated value, Vr, of voltage Vs. The relationship between thresholds Vt and Vb and reference signal Vr is preferably as described above with reference to Fig. 13. Since controller 314 does not receive a direct feedback of voltage Vs, updated reference Vr is determined indi¬ rectly, as described below, using voltage generator 303 and window comparator 206. According to one embodiment of the invention, generator 303 gradually changes the level of thresholds Vb and Vt, i.e. gradually increases or decreases both thresholds, until the output of window comparator 206 indicates that voltage Vs has reached either of thresholds Vb or Vt. Then thresholds Vt and Vb are increased or decreased, depending on which of thresh¬ olds Vb and Vt was reached, by a preselected fixed volt¬ age so as to maintain the desired relationship between thresholds Vb and Vt and reference voltage Vr.

The preferred embodiments of Fig. 13 and 14 de¬ scribe implementation of the present invention using dedicated hardware. It should be appreciated that the invention can be also implemented using an electronic computer or microprocessor programmed with appropriate software. Such an implementation of the present invention is illustrated schematically in Figs. 15 and 16. The circuitry of Fig. 15 includes a processor 340 programmed to perform part of the algorithm outlined in Fig. 16. According to this preferred embodiment of the present invention, the analog output of amplifier 200 is convert¬ ed by an analog to digital (A/D) converter 330 to a corresponding digital output which is received by proces¬ sor 340. Processor 340 also receives the output of far- infrared signal window comparator 218.

Referring to Fig. 16, the pulsed output generated (block 354) in response to the near infrared pulses (block 352) is amplified (block 356) and, then, converted to a corresponding level signal (block 358), as in the embodiments of Figs. 13 and 14. The digital output of A/D converter 330 (Fig. 15) is stored in a memory associated with processor 340 and thresholds Vt and Vb are deter¬ mined as described above (block 360) .

Referring to blocks 366, 368 and 370, when the following conditions are met:

(a) time derivative dVs/dt exceeds threshold rate Rt (block 366); and

(b) signal Vs is not within the window defined by thresholds Vt and Vb (block 368), then the obscuring alarm is activated (block 370) as described in detail above.

As indicated at block 362, when the time, Tc, lapsed from the last update of thresholds Vt and Vb reaches a predetermined update time, To, preferably on the order of 10 seconds, time derivative dVs/dt is com¬ pared with threshold Rt (block 364) . If dVs/dtζRt, then an updated value of Vs is memorized as described above with reference to block 360. If dVs/dt>Rt, then Vs is compared with thresholds Vb and Vt (block 368) and, if Vs is not within the window, the obscuring alarm is activat¬ ed as described above. Although a preferred detection algorithm includes rate comparison as indicated at block 364 of Fig. 16, such rate comparison is merely optional as described above with reference to the embodiments Figs. 13 and 14. In a preferred embodiment of the invention, the obscuring alarm can be deactivated by the user as indi¬ cated at block 372. When the alarm is deactivated, the above mentioned procedure is repeated after an updated value of Vs is memorized as indicated at block 360.

Although the present invention was described above as using near infrared radiation for detection of obscur¬ ing, other spectral ranges, for example ultraviolet radiation, may be equally suitable for the purposes of the present invention and are, thus, also within the scope of the present invention.

It should be appreciated that the present invention is not limited to what has been thus far described with reference to preferred embodiments of the invention. Rather, the scope of the present invention is limited only by the following claims:

Claims

C L A I M S.

1. An anti-obscuring intrusion detector for supervising a region comprising: a housing having an input radiation window; an intrusion sensor, in said housing, which views the region through the window and provides a first output signal responsive to incident far infrared radiation; a radiation source which emits radiation of a prese¬ lected wavelength range partly to a wide field-of-view in the vicinity of the detector and partly into the input window; an obscuring sensor, in said housing, which views the window and provides a second output signal respon¬ sive to incident radiation of the preselected wavelength range; and obscuring detection circuitry which determines whether the intrusion detector is obscured by determining whether the output of the obscuring sensor is outside a reference window having a lower threshold and an upper threshold.

2. A detector according to claim 1 and further compris¬ ing segmented optics which provide the intrusion sensor with a predefined, segmented, field-of-view of the region supervised by the intrusion detector.

3. A detector according to claim 2 wherein said seg¬ mented optics comprises a segmented lens.

4. A detector according to claim 2 wherein said seg¬ mented optics comprises a segmented mirror.

5. A detector according to claim 3 wherein the segment¬ ed lens is associated with the input window.

6. A detector according to any of the preceding claims wherein the preselected wavelength range comprises a near infrared wavelength range.

7. A detector according to claim 6 wherein the near infrared wavelength range is centered at a wavelength of approximately 0.9 micrometers.

8. A detector according to any of claims 1 - 5 wherein the preselected wavelength range comprises an ultraviolet wavelength range.

9. A detector according to any of the preceding claims wherein the radiation source is in the housing.

10. A detector according to any of claims 1 - 8 wherein the radiation source is at least partly external to the housing.

11. A detector according to claim 10 wherein the radia¬ tion source is external to the housing and in the vicini¬ ty thereof.

12. A detector according to any of the preceding claims wherein the radiation source comprises a disperser which disperses the radiation to the vicinity of the detector and into the input window.

13. A detector according to any of the preceding claims and further comprising a reflector, on said housing, which reflects some of the radiation emitted by the radiation source onto the input window.

14. A detector according to any of the preceding claims wherein the obscuring detection circuitry comprises a window adjustment circuit which adjusts the upper and lower thresholds of the reference window in accordance with measurements of the second output signal.

15. A detector according to claim 13 wherein the window adjustment circuit is activated periodically in accord¬ ance with a preselected schedule.

16. A detector according to any of the preceding claims wherein the radiation source comprises at least one light emitting diode (LED) .