Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1994-09-07
1995-11-07
Swann, III, Glenn
Communications: electrical
Condition responsive indicating system
Specific condition
250340, 250395, G08B 1319
Patent
active
054650801
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to an infrared intrusion sensor. In particular, the invention relates to an infrared intrusion sensor which is a long range passive detection system designed for remote unattended surveillance applications. The invention is expected to find applications in airfield perimeter security, high grade fence line security, vital asset protection and other surveillance environments.
The sensor differs from other infrared intrusion sensors in that it has a superior detection range compared to existing devices. Furthermore it provides more extensive information to the operator. For example, the invention has the capability of indicating the direction of movement of a target, number of targets, false alarm probability, near/far field indication, and failure/tamper indication.
In one existing device designed for military use the useable range is 30 metres although the optimum detection range is stated to be 6 metres. This device is admitted to have difficulties with slow-moving targets between 15 metres and 30 metres. In another military device the stated detection ranges are 3 to 20 metres for personnel and 3 to 50 metres for vehicles.
Domestic intrusion sensors have a typical detection range of less than 20 metres. One known civilian security sensor has a detection range of 100 meters but only provides a simple alarm.
These existing intrusion sensors have technical limitations, the major limitation being the relatively short range capabilities of these devices and unacceptably high false alarm rates. Most existing sensors are not capable of indicating the direction of target movement, or if they can indicate the direction of movement it is at the expense of other facilities.
It is an object of this invention to provide an infrared intrusion sensor having enhanced detection range and low false alarm rate compared to existing devices.
It is a further object of this invention to alleviate one or more of the above mentioned problems or at least provide the public with a useful alternative.
SUMMARY OF THE INVENTION
Therefore, according to one form of this invention, there is proposed an infrared intrusion sensor comprising: infrared radiation impinging upon the detector; to the detector array; detector array; output alarm signals; and
The device operates by passively monitoring the thermal radiation emitted in the 8 .mu.m to 13 .mu.m range from a narrow sector in front of the device. When a body having a thermal signature different to that of the background (ie. a person) passes through the monitored region, its thermal (infrared) radiation is detected. Infrared radiation arriving from the scene is optically modulated, then focussed onto a thin film bolometer detector array operated at ambient temperature. The detected signal is amplified and digitised. Digital signal processing is accomplished with an onboard microprocessor, which can be pre-programmed or directly accessed by the operator. The scene background within the sensor field of view is stored over a preset integration period and regularly updated. Targets are detected as differential signals referenced to the background. This technique ensures a low false alarm rate. In particular the sensor will not respond to background variations which are a source of frequent false alarms in other intrusion sensor equipments.
In preference the optics comprise a Cassegrain style objective telescope and infrared transmitting entrance window. The Cassegrain-style telescope is formed by a primary mirror and a smaller secondary mirror mounted on the dither means. The entrance window provides protection against damage to the internal optics of the device. The window is preferably a material such as germanium to permit transmission of the radiation band of interest between 8 .mu.m and 13 .mu.m. Optional materials include zinc sulphide, zinc selenide, silicon and infrared transmitting plastics.
In preference the infrared transmitting window has a hard carbon coating on an outer surface to provide protection against scratching o
REFERENCES:
patent: 3475608 (1969-10-01), Pardes
patent: 3524180 (1970-08-01), Cruse
patent: 3912927 (1975-10-01), Hoffman, II
patent: 5101194 (1992-03-01), Sheffer
patent: 5299971 (1994-04-01), Hart
Liddiard Kevin C.
Rice Brian W.
Watson Rodney J.
Swann, III Glenn
The Commonwealth of Australia
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