World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 6
T E C H N I C A L
.
M A N U A L
Another type of optical detector is immediately suggested by the beam detector, when
one considers what happens to the light in the beam that is
not
returned to the sensor.
Very little of this light will be ‘absorbed’ (e.g. translated to another energy form). Most
of it will be scattered out of the light beam, and hence not returned to the sensor. The
suggestion is, that if the scattered light itself is sensed, its intensity would be a direct
measure of the scattering medium or smoke. This principle is used very successfully in
many detectors although it is more expensive to realise than an ionisation type detector,
but gives a more reliable and potentially more sensitive detector.
A second type of optical light scattering detector uses a tightly focused laser beam to
produce pulses of light which are scattered from individual particles in an air stream
passing through the point of focus of the laser. This is the ‘particle counter’ detector,
which was originally designed for monitoring the quality of Clean Rooms (microelec-
tronics and pharmaceutical manufacture and research). For smoke detection it is only
viable when used in an aspirating system.
Light Scattering
Detectors
Most fires will generate light of one sort or another, infrared light being the most
prominent. Extremely sensitive infrared cameras are available, which could be used in
an alarm system. Such cameras are not used due to their high cost and the problem of
identifying the position of a fire but they could provide a means of identifying very small
temperature rises. The source of heat must be visible to the detector, either directly or
by reflection. This is not the case though with most other types of detector. A simplified
version that did not produce an image could be acceptable for fire detection although
it would not pinpoint the source of the heat in the same way that a camera would.
A flaming fire will produce a flickering ultraviolet light, and an ultraviolet sensor can be
used to detect it while also discriminating against other sources of ultraviolet, arc
welding for example, by means of the flicker rate. When materials burn they produce
light of different colours depending upon the elements that constitute the material.
These colours are produced in certain proportions and sensors can be made selectively
responsive to these colours by means of colour filters. By sensing the colours of light in
the required proportions an alarm can be triggered. The response time of such a
detector can be extremely rapid.
Light sensing fire detectors tend to have specialised uses, furthermore, although the
Light Sensing
Detectors
