World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 13
T E C H N I C A L
.
M A N U A L
true signal given by smoke, and the noise signals with sufficient reliability to
prevent an alarm being triggered by the electrical noise. The amount of light
scattered by a given volume of particles is highly dependent upon the wavelength
of the light, with the amount of scatter increasing rapidly with a decrease in
wavelength. The
direction
of scatter is determined by the relationship between the
wavelength of the light and the particle size and shape. This is particularly the
case with large particles. It is however of great importance to note that all shapes
and sizes of particle give a large amount of scatter in the
forward direction
, that is,
scatter that diverts light only a few degrees from its original path. Detectors that
rely primarily on small angle scatter will be sensitive to a wide range of particle
sizes, almost irrespective of the wavelength of light.
One type of aspirating detector, developed in the late 1960’s (although reaching the
market in the ‘70’s) uses a Xenon flash bulb as the light source. This gives a very wide
spectrum of light output extending into very short wavelengths, well below the visible
spectrum. The intensity of scattered light at these wavelengths is high but is only of use
when a light sensor is used which is sensitive to them. The best response of normal
silicon light sensors, as is used in this detector though, only covers the visible spectrum
and the light at longer wavelengths. The advantage promised by the broad spectral
light output is lost.
Other, more recently developed types use semiconductor lasers giving 5 to 100 mW.
of light power. The types used are monochromatic coherent light sources at the red
end of the visible light spectrum. The coherent wave front of a laser beam allows
excellent manipulation of it by lenses. This does not, at first sight, appear to be a
promising light source, but, as pointed out below, can be entirely satisfactory.
Light emitting diodes can give similar light powers to a semiconductor laser and are also
nearly monochromatic. They are very much cheaper than lasers, however they do not
provide a coherent light source and are usually only (able to be) used in point detectors.
An unfortunate characteristic of Xenon flash tubes is a deterioration of light output over
time. Very expensive tubes can be obtained where this effect is reduced to a minimum,
but they are neither commercially viable or used in this application. This deterioration
Light sources
