Thorn Security Limited

Paul J Basham

Bernard Etienne Henri Laluvein

Mark E Pleydell

Steven P Maynard

There is described a detector which is suitable for use as a fire detector. The detector has two channels which allow for discrimination between energy received from a fire and a &ldquo;false fire&rdquo;. False fires are sometimes detected as a result of radiation from a so called &ldquo;cold&rdquo;, black body radiation source which flickers at a frequency of between 1 and 20 Hz. In the past, these gave rise to false alarms being triggered. The invention overcomes the problem by having a notch filter which when used in combination with another filter, ensures that detected radiation at, or around, 4.3 &mgr;m is transmitted to a sensor. A processor then compares the received value with a value computed by interpolating between signals received from two other channels. If a threshold value is exceeded an alarm is triggered. The invention thus overcomes disadvantages with prior art systems as signals from cold black body sources are rejected as false.

Detector

Apparatus for detecting the presence of a fire includes a pair of detectors for radiation characteristic of a fire, e.g. at 4.3 microns, and a further detector for radiation characteristic of a black body, e.g. at 3.8 microns. Filters select the detector output components at flame flicker frequencies. The filtered outputs of the detector pair are cross-correlated to produce a relatively noise-free signal. This signal is divided by a factor obtained by cross-correlating the further detector filtered output with the average of the detector pair filtered outputs. The result of the division is applied to a threshold-responsive circuit. Thus the relatively noise-free signal is effectively subjected to a threshold which is proportional to the degree of similarity between the variations with time of the output signal of the further detector and the output signals of the detector pair.

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High Wycombe, United Kingdom

Paul J Basham