PRINCIPLES OF OPERATION
6
2.3.3
IMMUNITY TO SOLAR RADIATION
Modulated radiation from direct or reflected sunlight as well as modulated radiation from strong
sources of artificial lighting can produce an unwanted response from infrared flame detectors. To
counter this possibility, the FV300 detector looks for the flame in a very narrow waveband where
most of the sun radiation is absorbed by CO
2
gases in the atmosphere. Secondary re-radiation
effects from sun heated optical components is minimised by an additional long wave IR filter on
the guard channel and a special sun-block coating on the array lens.
To further eliminate the risk of unwanted alarms from modulated sun radiation or radiation from
sources of artificial lighting, the FV300 incorporates a third sensor looking specifically for
radiation in the visible and short wave infrared band. The output signal from this sensor is mixed
with that of the long wavelength infrared filter to operate as a spectral guard for modulated sunlight
or any other strong artificial lights.
2.4
DETECTION RANGE
The FV300 FlameVision detectors can detect a fully developed 0.1 m
2
n-heptane or petrol
(gasoline) fire at 50m. The collimating optics and the ability of the detector to separately process
signals from individual sensing elements of the array enables the detector to correct signal losses
due to off-axis incident angles. This results in a ‘flatter’ response throughout the detector field of
view. Performance details are given in the System Design Information section.
2.5
MULTIPLE FIRES DETECTION
The FV300 sensor array, together with its associated digital signal processing, can identify more
than one fire event occurring at the same time within the detector field of view. At any one time,
up to four alarms from the strongest flames can be reported and signalled. Fires that are
sufficiently close or in the same line of sight will generate merged activity clusters on the sensor
array and can only be identified as a single fire event.
2.6
DETECTION OF FLAME IN THE PRESENCE OF BLACKBODY
The ability of the detector to identify and process multiple activity clusters within its field of view
allows radiation signals from fire sources and blackbody sources to be analysed separately for both
their spectral and time domain signature. Thus, in the presence of modulated blackbody sources,
the detector will not generally be desensitised when responding to a fire event. In some
applications, large blackbody sources may overshadow areas of the field of view where detection
is required. In these cases, care should be taken in the number and positioning of detectors that are
needed for achieving the degree of protection required.
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