Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-07-02
2001-04-03
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250SDIG001
Reexamination Certificate
active
06211522
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of passive infra-red intrusion sensors, and especially lens arrays for use in such intrusion sensors.
BACKGROUND OF THE INVENTION
Several types of passive infra-red sensors have been described in the prior art for detecting intruders in high security areas. Such sensors detect the changes in the infra-red radiation falling on an infra-red detector caused by movement of the infra-red emitting intruder in the field of view of the sensor. The area under intrusion surveillance is focused onto the infra-red sensitive detector by means of an array of lenses to produce a number of discrete zones. As the intruder crosses from zone to zone, the changes in the detector output above the ambient level from the surroundings are amplified by suitable circuitry, and an alarm signal generated.
In order to maintain uniform nomenclature in describing this invention, the term “sensor” is used as far as possible to describe the complete intrusion detection unit, and the term “detector” to describe the element within it which converts the infra red radiation to an electrical signal. Since these terms are used interchangeably in the prior art and the industry in general, this delineation is not always possible, especially in quotations, and the context should always be checked when there is any doubt as to the meaning.
The effectiveness of a passive infra-red intrusion sensor system as a function of the cost of manufacture, depends to a great extent on the effectiveness of the optical system in covering the whole of the area to be protected. Various schemes for providing as full cover as possible are described in the prior art. The major problem addressed is that of providing high detection sensitivity both for intruders moving at a distance from the sensor, and for those moving close to the sensor.
In addition, a related problem is that of discrimination between signals caused by real human intruders and those caused by pets such as dogs, cats, etc., moving close to floor level around the protected areas. This problem, known as pet immunity, is especially serious, since a proliferation of false alarms resulting from pets may cause the owners to switch off an otherwise effective alarm system, and in a situation where the presence of pets may be desirable, as they may add an additional element of protection.
In U.S. Pat. No. 4,058,726, assigned to Cerberus AG of Switzerland, the inventors describe a focusing system composed of a vertical array of cylindrical lenses with vertical axes. When correctly located in front of the detector, the array divides the surveillance area up into a number of vertical strip-shaped sectorial zones fanning out from the sensor. There are a number of limitations and disadvantages to a simple array of this type. Firstly, having only one sort of lens, the sensor is unable to effectively deal both with the far and the close fields of view. In particular, since cylindrical lenses magnify in one direction only, objects distant from the sensor do not fill the lens aperture well, and the far field sensitivity is therefore low. Only a convex focusing lens, usually, for this geometry in Fresnel form, is able to give good sensitivity at the far field, and practically all such systems currently produced use convex focusing lenses for the far field.
In addition, two further disadvantages of this system have been enumerated in U.S. Pat. No. 4,740,701, which too was assigned to Cerberus A. G. The authors state that such a simple cylindrical array is unable to provide good 90° vertical angular coverage when a “protective curtain” has to be provided in front of an opening such as a door or window. Furthermore, as it moves closer to the sensor, an object becomes poorly focused, both because of the non-optimal focusing distance, which has to be optimized for some median distance, and because of increased image aberrations as the object falls off-axis at angles below the sensor. Because of these two effects, to a large extent, almost all infra-red intruder detection systems suffer from reduced sensitivity in the lower fields of view which cover middle and near distances, and this is a problem which they have to solve in order to maintain good wide range coverage. At the same time, this property does have the positive effect of providing an inherent element of pet immunity for all such systems.
In the above mentioned U.S. Pat. No. 4,740,701, the inventors therefore describe a sensor which attempts to overcome these problems by using substantially cylindrical Fresnel lenses each having a longitudinal axis, a focal point and a focal length, the lenses being curved along an axis perpendicular to the longitudinal axis, to form a sector of a circle with radius equal to the focal length, and with the infra-red detector located at this focal center point. This prior art provides a well focused, narrow zone of surveillance area, which extends over a vertical angle of up to 90°. It also allows coverage of more than one direction, by using a separate Fresnel lens for each direction rather than an array of lenses. However, like the previously described Cerberus focusing system, the use of only one sort of lens prevents the sensor from being able to deal effectively both with the far and the close fields of view. Furthermore, the narrowness of the surveillance zones is in itself a serious drawback for covering a larger area, and it is thus limited to situations which require only one or two protective curtains.
In U.S. Pat. No. 4,604,524, Y. and M. Kotlicki describe a sensor which overcomes some of these drawbacks, especially in the far field areas, by means of an array composed of multi-faceted convex focusing lenses of differing configurations, each configuration corresponding to differing protection zones. The arrays are spatially arranged to define several protection zones at various angles to the horizontal. The lens array is curved around a vertical axis, such that the sensor is able to cover an azimuthal angle of up to 140°. The vertical angle covered is from 25° above the horizontal, to 10° below the horizontal. However, as is apparent from the ray coverage scheme shown in FIG. 6B of this patent, this system, too, is primarily effective for wide area coverage, and is limited in its ability to detect intruders in areas closer to the sensor. As the intruder approaches the sensor, the emitted radiation coming from the lower zone becomes more and more off-axis with respect to the normal to the sensor, and the sensitivity of the system to such radiation falls off considerably.
In U.S. Pat. No. 4,734,585 assigned to Racal Guardall, one of the leaders in the field of intruder detection, the inventor I. Owers points out that a further cause of reduced sensitivity close to the sensor, and therefore in the lower zone since the sensor is mounted high on a wall, is that the signals generated by objects moving in these areas are of high frequency, to which the detector is less sensitive. His invention attempts to fulfill the requirement of increasing sensitivity close to and at angles below the sensor by means of a composite lens array which includes some slotted sections. This array has an upper section composed of Fresnel lenses (which from their described function must be convex lenses), providing multi-zone long range coverage at the horizontal level, and two lower sections of Fresnel (convex) lenses, each curved at an increasing angle below the horizontal, to provide medium and short range coverage. The latter two sections include vertical slots between the lens segments, through which the infra-red radiation passes without any focusing action. These slots define zones with large fields of view, determined solely by the dimensions of the slots and their location with respect to the infra-red detector.
This slot technology has a number of disadvantages. The inventor himself states that because of the lack of any focusing effect, the sensor has a large field of view, which the image of an intruder may not fill. The detection sensitivity i
Kotlicki Yaacov
Moldavsky Mark
Tchernichovski Nachum
Zhevelev Boris
Darby & Darby
Hannaher Constantine
Israel Andrew
Visonic Ltd.
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