Dual spectrum infrared fire sensor

Disclosed is a fire and explosion detection system wherein long wavelength radiant energy responsive signals are processed in one channel and compared to short wavelength radiant energy responsive signals which are processed in a second channel. At least one of the channels is responsive to a wavelength where at least one of the combustion products of the fire or explosion exhibits a strong absorption band in the atmosphere. When the signals from the two channels are coincident in response to a fire or explosion of a predetermined threshold magnitude, an output fire suppression signal is generated.

TECHNICAL FIELD 
This invention relates generally to fire and explosion detection and 
suppression systems, and, more particularly, to a fast acting long and 
short wavelength responsive multichannel radiation detector. 
BACKGROUND ART 
Fire detection systems which respond to the sudden presence of either a 
fire or an explosion to thereby generate an output control signal are 
generally known. Such systems have a very significant utility, for 
example, in applications with a variety of explosive or fuel transport or 
storage tanks, and these systems normally function to trigger the 
operation of a fire suppression mechanism within a few milliseconds after 
the initiation of a fire or explosion. It is frequently desirable to wire 
these fire detectors into military armored personnel carrier vehicles 
which transport various arms and explosives or into rocket engines for 
triggering the fire suppression system and/or automatic shutdowns. A 
possible fire commonly desired to be suppressed by these types of fire 
detection systems is one which is produced in a fuel tank by a high energy 
round of ammunition fired into the fuel tank from a remote location. 
Another possible fire commonly desired to be suppressed by these types of 
fire detection systems is one produced by a component failure or fuel leak 
causing combustion to take place outside the engine. 
Hitherto, fire detection and suppression systems of the above type employed 
one or more photon responsive short wavelength photodetectors. These 
photodetectors sense the energy from radiation emanating from a fire or 
explosion, such as red/blue or ultraviolet radiation in a particular 
spectral band. These systems use color (red vs. blue) comparisons, energy 
per time comparisons, flicker frequency comparisons or ultraviolet 
wavelengths alone to sense a fire. Signals from these photodetectors are 
properly compared and processed in order to generate a fire control output 
signal. A disadvantage with this type of prior art fire detection system 
is that the system is wholly dependent for its proper operation upon 
distinguishing the photon energy from the fire or explosion to be 
suppressed from the photon energy from the non-fire stimuli, where the 
non-fire stimuli can produce signals that are larger in magnitude than 
those of the fire stimuli. These prior art fire detection systems are 
frequently subject to false operation because the non-fire stimuli vary 
greatly from one location to another and can often deceive the fire 
detection systems into unwanted responses. 
Various circuit techniques have been devised to discriminate against these 
latter sources of extraneous radiation. But these techniques have not been 
totally practical or satisfactory for all conditions of operation and in 
the many environments in which the fire detection system must be capable 
of operating. 
In U.S. Pat. No. 3,931,521, issued Jan. 6, 1976, and assigned to the 
present assignee, there is disclosed a basic dual-channel fire and 
explosion detection system which operates to eliminate the prior art 
problem of false triggering in response to extraneous noise radiation in a 
particular spectral band. Briefly, this operation is accomplished in the 
above patent by the use of a long wavelength-responsive radiation 
detection channel and a short wavelength-responsive radiation detection 
channel. These two channels respond respectively to separate wavelength 
ranges of incident electromagnetic radiation and thereby eliminate the 
above possibility of false triggering, either by extraneous non-fire 
sources or by chopped radiation from a constant energy source, such as the 
sun. 
U.S. Pat. No. 3,825,754, issued July 23, 1974, and assigned to the present 
assignee, provides further novel and useful improvements to U.S. Pat. No. 
3,931,521 by providing means for discriminating between large explosive 
fires on the one hand and high energy flashes/explosions which cause no 
fire on the other. The latter could be, for example, a penetration of a 
High Energy Anti-Tank (HEAT) round of ammunition which does not 
subsequently cause a full scale explosive fire. 
SUMMARY OF THE INVENTION 
In accordance with the invention, still further novel and useful 
improvements to the foregoing U.S. Pat. Nos. 3,931,521 and 3,825,754 are 
provided by setting at least one of the channels at a wavelength where at 
least one of the products of combustion exhibits a strong atmospheric 
absorption band. As a consequence, the fire will exhibit a stronger 
contrast ratio, or greater signal-to-noise ratio (S/N), against various 
backgrounds. Thus, smaller fires can be detected while maintaining the 
same false alarm rejection of the previous techniques. 
The electrical detection system of the invention is responsive to a fire or 
explosion and generates an output signal. The system includes: 
(a) long wavelength channel means responsive to radiant energy in a 
predetermined spectral band greater than about 4 .mu.m of electromagnetic 
radiation and received from a fire or explosion for generating a first 
logic signal; 
(b) short wavelength channel means responsive to radiant energy in a 
predetermined spectral band less than about 3.5 .mu.m of electromagnetic 
radiation and received from said fire or explosion for generating a second 
logic signal; and 
(c) output gate means coupled to receive both said first and second logic 
signals and responsive thereto to generate said output control signal 
which may be further processed to control the suppression of said fire or 
explosion, with the proviso that at least one of said channel means is 
responsive to an atmospheric absorption wavelength associated with at 
least one combustion product of said fire or explosion. 
Depending on the fire and the background conditions present, a S/N 
improvement of at least 2:1 is obtained over that provided by U.S. Pat. 
Nos. 3,825,754 and 3,931,521.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the FIGURE, the multi-channel fire detector, designated 
generally 10, includes a short wavelength responsive channel 12 (channel 
1) and a long wavelength radiation responsive channel 14 (channel 2) 
coupled respectively to receive radiant energy 16 from a nearby or remote 
fire or explosion 18. The system is typically designed so that it is 
highly responsive to high energy fuel-type explosions out to distances on 
the order of 5 m. The radiant energy 16 of interest in channel 12 is that 
radiation in the near infrared region of the electromagnetic frequency 
spectrum, less than about 3.5 .mu.m, whereas the radiant energy from 
source 18 of interest in channel 14 lies in the far infrared region of the 
electromagnetic frequency spectrum, greater than about 4 .mu.m. 
The short wavelength channel 12 includes a suitable conventional optical 
filter 20 for passing radiation wavelengths only in the spectral band of 
interest, which preferably is on the order 0.7 to 3.5 .mu.m. The radiation 
thus passed impinges on a detector 22, such as a silicon and/or germanium 
photodetector, which generates an output detection signal at the input of 
an amplifier 24. The amplifier 24 has its output connected as shown to one 
input 26 of an AND threshold gate 28. 
The long wavelength channel 14 includes a conventional optical filter 30 
for passing radiation wavelengths in the range of about 4 to 30 .mu.m, and 
the energy thus passed impinges on a detector 32. This detector may 
advantageously be a thermal detector such as thermistor, thermopile, or 
other detector sensitive to these wavelengths for generating an output 
signal which is coupled to an input of a frequency compensating amplifier 
stage 34. Alternatively, in the lower portion of this wavelength spectrum, 
photon detectors may also be used. 
The output from amplifier stage 34 is connected to a second input 36 of the 
AND threshold gate 28, and this latter gate is operative in response to 
input signals on lines 26 and 36 to generate an output pulse on line 38, 
as will be further described. The output pulse is further processed in 
driver electronics (not shown) for driving and triggering a suitable fire 
suppression mechanism. 
At least one of the two channels 12 and 14 is responsive at a wavelength 
where at least one of the products of combustion exhibits strong 
absorption bands in the atmosphere. For example, in a hydrocarbon fuel 
fire, one product of combustion is CO.sub.2 and exhibits absorption bands 
approximately centered at 1.4, 1.9, 2.7, 4.4, and 15 .mu.m. Another 
product of combustion is H.sub.2 O and exhibits absorption bands 
approximately centered at 1.4, 1.9, 2.7, 6.5 and 17 .mu.m. 
For hydrocarbon fires, the short wavelength channel is preferably set to 
one of the following wavelengths (spectral passband in parentheses): 
1.4 .mu.m (1.3 to 1.5 .mu.m) 
1.9 .mu.m (1.8 to 2.0 .mu.m) 
2.7 .mu.m (2.4 to 3.0 .mu.m). 
The long wavelength channel is preferably set to one of the following 
wavelengths: 
4.4 .mu.m (4.2 to 4.7 .mu.m) 
6.5 .mu.m (5.5 to 7.5 .mu.m). 
15 .mu.m (14 to 16 .mu.m) 
17 .mu.m (16 to 30 .mu.m). 
Most preferably, both channels are set to one of the foregoing wavelengths 
suitable for that channel. 
Although the fire sensor apparatus of the invention covers devices whose 
windows of operation are synonymous with known atmospheric absorption 
bands, little problem is expected with the atmosphere itself attenuating 
the radiation from a flame. This is because the normal application of 
these fire sensors is for very close-in distances, a matter of a few 
meters. For atmospheric attenuation to be significant, one needs to 
consider a path length of greater than 100 meters in most cases, and few 
measurements of atmospheric attenuation are made over a path length of 
less than a kilometer. 
The products of combustion in the case of a hydrocarbon fire are mainly 
CO.sub.2, H.sub.2 O and carbon particles. All of these constituents are 
present in the atmosphere also, but in greatly reduced concentrations. 
All materials including gas molecules emit radiation in accordance with 
Planck's radiation law. This law specifies that given the object's 
temperature and emissivity, the radiation at any desired wavelength can be 
determined by plugging values into the Planck's law equation. 
For radiation directed at an object, the radiation is either transmitted 
through the object, reflected by the object, or absorbed by the object. 
Absorption here would be essentially the same as emissivity. For the 
absorption bands in the atmosphere, then, radiation passing through the 
atmosphere is absorbed due to the high emissivity of the H.sub.2 O, 
CO.sub.2, etc., molecules present. 
However, since these H.sub.2 O, CO.sub.2, etc, molecules are at a 
relatively low temperature (ambient temperature at low altitudes), the 
amount of radiation emitted by these molecules is insignificant for 
wavelengths below 10 .mu.m, even though the emissivity is high at the 
various absorption bands of interest. For a fire, however, where the 
molecules are at a temperature in excess of 1000.degree. C., the high 
emissivity (in the atmospheric absorption bands) plus the high temperature 
combine to yield a significant signal advantage. 
By setting at least one of the channels at a particular wavelength where at 
least one of the products of combustion exhibits strong absorption bands 
in the atmosphere, the fire will exhibit a stronger contrast (greater S/N) 
against various backgrounds. Consequently, smaller fires can be detected 
while maintaining the same false alarm rejection of the previous 
techniques. A S/N improvement of at least 2:1 over the previous 
techniques, depending on the fire and background conditions present, is 
achievable by the apparatus of the invention. 
The system shown in the FIGURE is thus operative to compare the radiant 
energy in two different spectral bands and generate an output signal on 
line 38 only during the presence of both long and short wavelength energy 
from source 18 at levels above a chosen threshold level or levels. This 
threshold level may, of course, be controlled internally in either the 
electronics of the amplifiers 24 and 34 or the internal electronics of the 
AND threshold gate 28. Thus, the system in the FIGURE will discriminate 
against radiant energy from short wavelength only sources or from long 
wavelength only sources and against any other radiant energy sources which 
generate radiation below a given pre-established energy threshold. The 
system disclosed herein was specifically conceived to respond to fires or 
explosions where there is always the presence of a combination of long and 
short wavelength radiation above given thresholds. 
Use of radiation in the atmospheric absorption regions of combustion 
products by a least one of the channels optimizes the overall S/N of the 
system; the signal being the fire and the noise being the sun and other 
non-fire radiation sources. Typical hydrocarbon fires radiate the greatest 
amount of IR energy in the 2 to 6 .mu.m band; but the sun also emits a 
great deal of energy in the same band, and as a result, the sun is capable 
of falsely triggering the detection system. Such false triggering could 
occur, for example (and in the absence of channel 12), if some object were 
to pass between the sun and the detector 32 to thereby produce a time 
varying signal at detector 32 and acceptable by the bandwidth of channel 
14. The same false triggering could be produced, for example, by electric 
heaters, heating lamps, quartz lamps, hot exhaust manifolds, or other 
steady state sources of radiation in the bandwidth of channel 14 and 
capable of producing a time varying signal at detector 32 when momentarily 
shielded by a moving object. This possibility of false triggering has been 
eliminated herein by the use of channel 12 whose 0.7 to 3.5 .mu.m 
bandwidth response is below that of most steady state radiation sources 
capable of generating an output signal in channel 14 and by setting at 
least one of the channels to respond to an atmospheric absorption band of 
at least one combustion product. 
Tests were made using a radiometer having seven parallel channels, each 
covering a different spectral band. Each channel was adjusted to respond 
equally to a pan of burning fuel (hydrocarbon fire) used as a reference. 
Two of the channels were identical to the fire sensor described in U.S. 
Pat. No. 3,931,521. The other five channels operated in atmospheric 
absorption bands centered at 1.4 .mu.m, 1.9 .mu.m, 2.7 .mu.m, 4.4 .mu.m, 
and 6.5 .mu.m. 
The intent of the testing was to determine the signal to noise ratio 
improvement of the 1.4, 1.9, and 2.7 .mu.m channels over the 0.9 .mu.m 
channel, and the 4.4 and 6.5 .mu.m channel over the 7 to 30 .mu.m channel. 
Here, "signal" is defined as the response to the standard fire and is 
adjusted to be equal for all channels. "Noise" is defined to be the worst 
case response under outdoor conditions to non-fire stimuli. 
In some channels, this worst case "noise" occurred with the radiometer 
looking directly at the sun, while other channels responded more for other 
configurations. The minimum improvement of the 1.4 .mu.m, 1.9 .mu.m and 
2.7 .mu.m channels over the 0.9 .mu.m channel was a factor of 20, 28 and 
134, respectively. The minimum improvement of the 4.4 .mu.m and 6.5 .mu.m 
channels over the 7 to 30 .mu.m channel was a factor of 8.0 and 2.1, 
respectively. 
It is to be understood that the present invention is not limited to the 
particular type of detectors used. For example, germanium, silicon, lead 
sulfide, thermopile detectors or thermistor bolometers can be used in the 
short wavelength channel 14, whereas lead selenide, thermopile, 
mercury-cadmium-telluride, zinc-doped germanium, copper-doped germanium 
detectors or thermistor bolometers can be used in the long wavelength 
channel 14. Other detectors may also be used. 
The present invention is not limited to the particular type of fire being 
suppressed. For example, while the invention has been disclosed in terms 
of suppressing hydrocarbon fires, others, such as titanium, magnesium and 
electrical fires which generate combustion products of known atmospheric 
absorption wavelength, may also be suppressed employing the detection 
apparatus of the invention. 
It should also be understood that the present invention is not limited in 
its use to any particular type of output fire suppression means. One 
suitable technique for suppressing fires and explosions in enclosed spaces 
and which is most compatible for use with the detection system described 
above utilizes a plurality of pressurized Freon gas bottles, each of which 
are electromechanically driven by a count down register (not shown) at the 
output of the above-described system. Each successive output pulse 
generated by the system can be utilized to drive the count down register 
(which is of conventional design), so as to activate a separate bottle 
each time there is a fire or explosion. In this manner, the system can be 
used to fully guard against a condition where the system operates to 
extinguish an initial fire, and then is not equipped for further response 
to a delayed or a secondary fire, or even to a second primary fire which 
occurs later at the same location. As a practical matter, the pressurized 
bottles of Freon are presently commercially available and contain the 
necessary gas exit orifices, so that the Freon gas exits these orifices 
under a very high pressure and completely empties in about 10 milliseconds 
or less. 
Other suitable techniques involve the release of various dry chemicals, 
powders, or foam for suppressing fires in open or well-ventilated spaces.