Infrared Detector

Infrared detectors operate in a similar way to the microwave devices in that a generator radiates a beam which is picked up by the receiver in the infrared detector. If the beam is broken by an intruder the alarm is triggered. The infrared rays are usually produced by a gallium arsenide crystal and hence they are sometimes called gallium arsenide rays, but they can be generated by a wide variety of means.

Any source of heat radiation will usually produce infrared radiation because both lie just below visible light, in the same part of the electromagnetic spectrum. Thus lamp bulbs, heaters, and even a pocket torch generate both to a greater or lesser extent. This opens the possibility of an intruder defeating and infrared detector by directing a portable infrared source at the receiver as he moves across and breaks the infrared detector's original beam.

This type of interference is prevented by pulsing the beam of the infrared detector. The frequency of the pulses varies, but 200 pulses a second is common. The infrared detector's receiver only responds to a pulsed beam of the same frequency; if the pulses are replaced by a steady beam, it signals an alarm.

The infrared detector's projector and receiver are housed in identical cases; hence it is not possible to identify which is which. This adds difficulty to anyone trying to defeat the system, and the problem is compounded by the use of additional dummy housings which are available for the purpose.

The only way to evade the infrared detector's ray is by avoiding it, such as by crawling underneath it. To do this the intruder must be aware of the presence and path of the infrared detector but even so evasion becomes impossible if the beam is laced across a vulnerable point in zigzag fashion. This can be done by means of mirrors; unlike the microwave beam which is too diffuse, the infrared detector can be reflected at several points by small reflectors just like rays of visible light.

The rays from the infrared detector will also penetrate clear glass just like light and so protection can be extended through partitions and windows. Here again is a reduction in range for each glass pane although the reduction is less than for mirror reflections. For 2 mm window glass the range is reduced to 84 percent for a single sheet; for a second sheet the range becomes 70 percent, a third reduces it to 60 percent, and a fourth drops it to 50 percent.