Infrared sensor suitable for fire fighting applications

An infrared detector particularly well suited for the detection of heat sources, specifically from fires in an outdoor environment. The infrared detector comprises an infrared sensor receiving infrared radiation from a focused refractive optical unit. The infrared radiation from the optical unit is appropriately filtered so as to optimize the reception of infrared radiation by the detector in a frequency band of within about 2.5 to 5 microns. The sensor is configured utilizing a linear matrix of individual infrared sensing elements which may be flexibly applied to modify the field of view of the sensor. The detector contains appropriate power and signal amplification circuitry so as to provide an electrical output signal corresponding to infrared radiation received in the desired frequency band. The entire detector may be housed in a hermetically sealed unit appropriate for outdoor use and mounted on a movable pedestal for inclusion in an overall fire protection system such as, for example, a forest fire detection and warning system.

FIELD OF THE INVENTION 
The present invention relates to an infrared sensor capable of detecting 
heat sources at temperatures of 200.degree. to 300.degree. C. above an 
ambient background temperature, typically those heat sources coming from a 
fire, while rejecting solar radiation reflections and fluctuations in 
ambient background temperature. 
BACKGROUND OF THE INVENTION 
Currently known infrared sensors which are used to detect infrared 
radiation coming from fires operate in the 1 to 2.5 micron wavelength. 
Although such sensors are capable of detecting infrared radiation 
generated by a fire, they are subject to false alarm conditions due to the 
variation of reflected solar radiation reflected off the ground or off 
vegetation in the area of detection of the sensor. If, however, the 
sensitivity of the infrared sensor is extended beyond to the 4 or 5 micron 
wavelength, the ratio between the infrared radiation from the fire and 
infrared radiation coming from fluctuations of the ambient background 
temperature diminishes, making accurate detection of the fire less 
probable. 
It would therefore be greatly advantageous to have an infrared fire 
detector which is optimized for detecting fires against an ambient 
background temperature with reduced succeptability to false alarms due to 
the variation of solar radiation from reflected sources. 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention is related to an infrared fire detector which is 
particularly well suited for the detection of heat sources in the natural 
environment, particularly from fires. It is generally intended for use in 
fire detection systems used to protect forests from forest fires. Other 
applications which are envisioned are those of hangar and air strip 
surveillance at airports as well as the monitoring of urban refuse depots, 
etc. Since the detector is particularly well suited for fire detection 
outdoors, it is envisioned that the sensor would find optimal use as a 
detection component in an integrated forest fire surveillance system. 
The infrared detector of the present invention optimally detects heat 
sources in the infrared frequency band falling within about 2.5 to 5.0 
microns. It is within this band that the infrared radiation due to wood 
fires is at its maximum, and therefore false fire alarms possibly 
triggered by solar reflections or thermal fluctuations of the ambient 
background temperature are minimized. 
The detector is made up of an infrared sensor which receives infrared 
radiation which has been collected and focused by a refractive optical 
collection unit. Between the infrared sensor and the optical collection 
unit is a spectral filter have a pass band which is selected so as to 
optimize infrared detection of the system to a frequency band of between 
about 2.5 to 5 microns. The desired frequency band is obtainable through a 
suitable combination of materials which make up the optical collection 
unit, the spectral filter and the infrared response curve of the infrared 
sensor itself. 
Suitable electronics are provided to provide bias current to the infrared 
sensor, if such sensor is, for example, of a photoconductive variety, and 
an amplifier is provided to amplify the signal coming from the infrared 
sensor to suitable levels for use in fire detection systems. 
The infrared sensor used in the system may be implemented either as a 
photovoltaic or photoconductive sensor comprised of a single sensing 
element or it may be made up of a multiplicity of sensor elements arranged 
in a linear matrix. By arranging individual sensor elements in a linear 
matrix, the overall field of view of the sensor may be varied. For 
example, if each single detector element has a field of view of one 
degree, then to achieve a field of view of 15.degree. to 20.degree. the 
matrix would require 15 to 20 elements. Of course, the focal length of the 
optics would vary accordingly so as to insure correct collection and 
focusing of infrared radiation for the field of view selected. 
The individual sensor elements may be photovoltaic or photoconductive 
sensors chosen from presently available materials such as InSb, InAs, PbSe 
and HgCdTe. By utilizing these materials, and given the amounts of 
radiation expected to strike the detector for the types of radiation be 
detected, the detector can be non-cooled. The material chosen for the 
individual elements of the infrared radiation sensor, due to the 
variations of bandwidth sensitivity among the materials, will require 
appropriate variation of the optics and of the pass band of the filter so 
as to maintain the overall detector sensitivity within the 2.5 to 5 micron 
wavelength band. 
It is therefore an object of the invention to provide an infrared detector 
particularly well suited for the detection of fires in an outdoor 
environment. It is also an object of the invention to provide and infrared 
detector with a band width sensitivity within about 2.5 to 5 microns so as 
to optimize the detection of infrared radiation generated by wood fires 
while minimizing detection of variations due to solar radiation 
reflections or thermal fluctuations in the ambient background temperature 
within the field of view of the detector, thereby minimizing false alarms. 
It is a further object of this invention to provide a non-cooled infrared 
detector which can be packaged in a hermetically sealed housing for 
flexible deployment in an outdoor environment. 
Other objects and features of the present invention will become apparent 
from the following detailed description considered in conjunction with the 
accompanying drawings. It is to be understood, however, that the drawings 
are designed solely for the purposes of illustration and not as a 
definition of the limits of the invention, for which reference should be 
made to the appended claims.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
With initial reference to FIG. 1, the infrared detector 20 is shown in side 
view. The individual components are housed in a hermetically sealed 
container 5 to which may be attached a suitably sized mounting bracket or 
pedestal 6. 
In operation, as can be seen in greater detail in FIG. 2, infrared 
radiation from a heat source, typically a fire, strikes the detector 20 
and is collected and focused by an optical collection unit 3, typically 
comprised of silicon crystal optics. The optical unit 3 focuses the 
radiation received from the heat source and passes the focused infrared 
radiation through filter 2, after which the filtered infrared radiation 
reaches infrared sensor 1. Filtered infrared radiation striking sensor 1 
causes the generation of an electrical signal from sensor 1 to be fed to 
amplifier 4, wherein the signal is amplified and made available at an 
output 8. 
The detector 20 is configured so as to optimize the detection of infrared 
radiation falling within a frequency range of about 2.5 to 5 microns in 
wavelength. It is within this frequency range that infrared radiation as a 
result a wood fire is maximally detected while infrared radiation usually 
resulting from reflected solar radiation or thermal fluctuations in the 
ambient background temperature are minimally detected. This increases the 
sensitivity of the detector for the particular detection mode desired 
while minimizing the possibility of false alarms. 
The achievement of the desired infrared pass band results from the matching 
of the type of sensor material used for infrared sensor 1 and the pass 
band characteristics of filter 2, in combination with the optics 3. 
Optical collection unit 3 is comprised of reflective silicon crystal 
optics having a diameter on the order of 50 mm and a high relative 
aperture. The infrared sensor itself may be comprised of commonly 
available photovoltaic or photoconductive elements. Suitable materials 
currently available are InSb, InAs, PbSe and HgCdTe. Given the sensitivity 
requirements of the system and taking into account the amount of radiation 
anticipated to strike the detector, the sensor 1 may be non-cooled. 
Through the utilization of silicon crystal refractive optics in optics 
unit 3, filter 2 must be capable of filtering out wavelengths less than 
2.5 microns. The cut-off at wavelengths greater than 4 or 5 microns may be 
obtained by utilizing a bandwidth limited sensor, such as one comprised of 
InAS, or adjusting the filter passband appropriately to filter out 
infrared radiation above these wavelengths. Such a situation may be 
encountered if the infrared sensor was comprised of PbSe, for example. In 
any case, the combined characteristics of optics 3, filter 2, sensor 1 
must result in a detection sensitivity such that infrared radiation in the 
wavelength band of about 2.5 to 5 microns is maximized while wavelengths 
outside that band are minimized. 
As required, a power unit 7 provides power for signal amplification unit 4 
and, in the case where a photoconductive sensor is utilized, provides bias 
current to infrared sensor 1. 
The field of view of the sensor is adjustable to meet design requirements 
based upon the implementation of sensor 1. Sensor 1 is comprised of 
individual infrared sensor elements 10, as seen in FIG. 3. Each sensor 
element 10 has a particular field of view characteristic. Sensor elements 
10 are configured in a linear matrix to achieve the required field of view 
by adjusting the number of sensor elements 10 utilized in the matrix of 
sensor 1. A typical field of view for the infrared sensor 1 when utilized 
in a forest fire detection system, for example, is for sensor 1 to have a 
field of view of approximately 15.degree. to 20.degree.. In such a system 
the sensor elements 10 would possess individual fields of view of 
1.degree. each, and therefore a linear matrix of approximately 15 to 20 
elements is required to achieve the desired 15.degree. to 20.degree. 
overall field of view of sensor 1. Given the field of view of the matrix 
of sensor elements 10 within infrared sensor 1, the optics unit 3 must 
have a focal length which conforms to the desired field of view angle to 
provide focusing within the field of view desired. 
As shown in FIG. 1, all of the elements may be housed in a hermetically 
sealed housing 5 and mounted as appropriate via a movable pedestal 6 for 
flexibility of application. When so configured, with a pass band 
sensitivity in the range of approximately 2.5 to 5 microns, in a field of 
view on the order of 15.degree. to 20.degree. the sensor can be utilized 
to detect heat sources at temperatures of 200.degree. to 300.degree. C. 
above an ambient background temperature. In standard weather conditions a 
detector so configured is able to detect a 6 meter fire at a 10 kilometer 
range. The detector therefore possess significant advantages when used as 
part of an overall fire detection system deployed outdoors, such as in a 
forest. 
Thus, while there have been shown and described and pointed out fundamental 
novel features of the invention as applied to preferred embodiments 
thereof, it will be understood that various omissions and substitutions 
and changes in the form and details of the disclosed invention may be made 
by those skilled in the art without departing from the spirit of the 
invention. It is the intention, however, therefore, to be limited only as 
indicated by the scope of the claims appended hereto.