World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 14
T E C H N I C A L
.
M A N U A L
provides a very difficult problem to overcome where a detector is intended to behave
as an absolute measuring instrument. Due to the deterioration in light output, the
signal will deteriorate to a commensurate degree, requiring the detector to be
frequently re calibrated. The high electrical noise generated by a Xenon tube will offset
the large signals it generates, and the promise of high sensitivity is not fully realisable.
The light from a Xenon flash tube is impossible to focus into a tight beam and it is not
possible to detect much light scattered at small angles from a wide angled beam.
However, complex (dust accumulating) iris systems in the light sensors viewing path
can allow a reasonable signal to be obtained from light scattered at larger angles. The
Xenon flash tube has a high energy requirement for its drive, which is a disadvantage
when intrinsically safe systems are required, or when a system runs from its stand-by
battery source. As a consequence, although at first sight the Xenon lamp looks ideal as
a light source, it has problems that effectively rule it out as a technically viable one with
the advent of semiconductor lasers. Although future developments could conceivably
change this situation, it is difficult to envisage that the effort required would be practical
or cost effective when the alternative of a laser exists.
The semiconductor laser is at present the subject of many semiconductor manufactur-
ers development programmes because of its application in printers, compact disc
players photo- copiers and fibre optic systems. These are expanding markets and auger
well for future improvements in cost and performance. Using a laser as the light source
in a smoke detection chamber has immediate advantages and disadvantages when
compared with a Xenon flash tube. The main disadvantage is that it gives light at a single
wavelength and this is at the long wavelength end of the visible spectrum, where the
amount of scatter signal is low. However, the single wavelength, coherent light source
that it provides, can be focused to a very tight beam allowing light scattered at very
small angles to be sensed without complex iris arrangements. This may be seen to more
than offset the disadvantage of the low intensity of scatter at these wavelengths, since
the intensity of scatter at small angles is consistently comparatively high, regardless of
particle size. Practical signals can be obtained, as with the Xenon flash tube, for particles
down to large molecule (gas) size. The semiconductor laser gives a consistent intensity
of light output with time and, when it is correctly driven, has an extraordinary length
of life. These advantages score heavily in a competition against the Xenon flash tube.
