Apparatus and method for remotely detecting the presence of chemical warfare nerve agents in an air-released thermal cloud

An apparatus and method for remotely detecting the presence of chemical ware nerve agents in a suspect thermal cloud. Infrared radiation emitted by a suspect thermal cloud is analyzed by means of a forward looking infrared thermal imager equipped with a spectral filter wheel having four passband filters; three of the spectral filter elements are spectrally optimized to respond to infrared radiation emissions characteristic of chemical warfare nerve agents. The variations in image contrasts of the thermograms generated by selectively filtering the infrared radiation emitted by the suspect thermal cloud are compared to determine the presence or absence of chemical warfare nerve agents in the suspect thermal cloud.

BACKGROUND OF THE INVENTION 
This invention relates generally to infrared radiation detection apparatus, 
and more particularly, to an apparatus and method for remotely detecting 
the presence of chemical warfare nerve agents in an air-released thermal 
cloud by analyzing its infrared radiation emission characteristics. 
Certain potential enemies of the U.S. possess the capability to direct 
air-releases of a wide spectrum of lethal and incapacitating chemical 
warfare nerve agents against the U.S. Fleet. Air-releases of chemical 
warfare nerve agents, forming thermal clouds, may be effected by dispersal 
from aircraft or air-burst projectiles. These chemical warfare nerve 
agents are all organo-phosphorus compounds with strong, narrow infrared 
absorption bands near 9.8 and 10.75 microns. An interrogation of the 
infrared characteristics of an air-released thermal cloud must readily 
permit the detection of chemical warfare nerve agent constituents so that 
the Fleet may undertake effective countermeasures. The detection of 
chemical warfare nerve agents must be equally effective in a diurnal or 
nocturnal environment. The detection of chemical warfare nerve agent 
thermal clouds must be possible against the varied infrared backgrounds 
and naturally occuring infrared-emitting objects of the naval environment. 
The interrogation process must be sensitive enough to discriminate between 
chemical warfare nerve agent thermal clouds and interferent thermal 
clouds, i.e., those dispersants which mimic the visual characteristics of 
a chemical warfare nerve agent thermal cloud such as air-releases of JP-4 
aviation fuel and the explosion products from the air-burst of 
high-explosive projectiles. 
The prior art discloses gas analyzers which use infrared radiation to 
ascertain the chemical composition of a given gas sample; representative 
examples being U.S. Pat. Nos. 4,297,579 to T. Spaeth, 4,035,643 to J. 
Barrett, and 4,013,260 to McClatchie et al. These analyzers operate on the 
principle that a given gaseous element or compound will absorb infrared 
radiation at specific absorption bands characteristic of that element or 
compound. Typically, these devices use an active radiation source to 
generate infrared radiation which is then filtered so as to pass only 
infrared radiation in a specific absorption band, this absorption band 
being characteristic of one of the gaseous elements or compounds presumed 
to be a constituent of the gas sample. The filtered infrared radiation is 
then passed through a cell containing the gas sample and impinges upon an 
infrared radiation detection means. The detected infrared radiation is 
converted into an electrical output signal by means of electronic 
processing circuitry, the strength of the output signal being inversely 
proportional to the degree of absorption of the filtered infrared 
radiation passed through the gas sample. Variations in strength of the 
output signals at selected absorption bands may then be compared with 
reference signals generated by passing the same selected infrared 
radiation bands through the cell without the gas sample to determine the 
gaseous elements or compounds present in the gas sample. These gas 
analyzers are active, i.e., to operate they require an infrared radiation 
source as a component of the apparatus, and the gas sample to be analyzed 
must be contained within a cell in the apparatus. In contradistinction, a 
chemical warfare nerve agent detection apparatus, to be efficacious in a 
naval environment during a conflict scenario, must be a passive device, 
i.e., function based upon the infrared radiation emissivity 
characteristics of the chemical warfare nerve agents rather than their 
absorption characteristics, and must be capable of detecting chemical 
warfare nerve agents at remote distances. 
U.S Pat. No. 2,930,893 to Carpenter et al discloses an apparatus and method 
for the remote detection of atmospheric contaminants, particularly highly 
toxic chemical warfare nerve agents. An infrared radiation source-receiver 
is encircled by a series of remotely positioned reflectors such that coded 
infrared radiation is transmitted through a detection area and then 
reflected back through the detection area to the receiver. An echelette 
grating in the receiver passes infrared radiation at selected infrared 
wavelengths; a detection band of mean wavelength of 9.8 microns at which 
the contaminant of interest exhibits strong absorption and reference bands 
at mean wavelengths of 9.25 and 10.4 microns to reduce the likelihood of 
any other contaminant in the detection area triggering a false alarm. The 
received infrared radiation is processed electronically so that a 
comparator circuit will generate an alarm whenever the toxic agent of 
interest is present in the detection area. This invention requires an 
active source of infrared radiation for the detection of chemical warfare 
nerve agents which detracts from its applicability in a mobile naval 
environment. Operationally this invention was designed for the detection 
of chemical warfare nerve agents over a fixed detection area inasmuch as 
the infrared radiation source-receiver must be encircled by fixed 
reflectors; this would reduce the invention's efficacy in a mobile naval 
environment. In addition, the invention's detection area is limited to the 
line-of-sight between the infrared radiation source-receiver and the fixed 
reflectors. 
U.S. Pat. No. 3,848,129 to Figier et al is representative of passive 
radiation detection apparatus which is capable of discriminating between 
infrared radiation with different spectral characteristics. An optical 
device collects the incident infrared radiation which is then sequentially 
filtered by first and second bandpass spectral filters to pass infrared 
radiation of relatively long and short wavelengths. The filtered infrared 
radiation impinges upon a single detector to produce first and second 
sampling signals. The first and second sampling signals are compared to 
produce a comparison signal. A target signal is generated whenever the 
amplitude of the comparison signal is equal to or greater than a 
predetermined value. This type of infrared radiation detection apparatus 
is used to detect point sources of infrared radiation by processing 
infrared radiation emissions in two narrow spectral bands; the spectral 
bandpass for potential targets is in the 2.8 to 3.2 micron range while the 
spectral bandpass for point source background radiation is posited to be 
in the 2.0 to 2.5 micron range. Therefore this device functions on an 
either/or basis; intercepted infrared radiation emissions indicate either 
a true target or a false target. Chemical warfare nerve agents emit 
infrared radiation in two narrow spectral bands centered at 9.8 and 10.75 
microns. In the naval environment, however, most naturally occuring 
objects have infrared radiation emission characteristics that change 
slowly with wavelength, i.e., their emissivity over the 8.0 to 14.0 micron 
atmospheric window is well approximated by that of a gray body; at any 
given time these objects, as well as interferents, could be emitting 
infrared radiation in the 9.8 and 10.75 spectral bands. Thus an either/or 
device would be incapable of discriminating between chemical warfare nerve 
agents and naturally occuring objects and/or interferents when their 
spectral characteristics are similar. Finally, an either/or device 
generates a target signal only when the intercepted infrared radiation 
generates a comparison signal that exceeds an internal reference; this 
presupposes a priori knowledge of the signal strength of a target. By 
comparison, the strength of the signal generated by chemical warfare nerve 
agents is dependant upon the apparent temperature difference between the 
chemical warfare nerve agent thermal cloud and its background, the agent 
concentration in the cloud, and the optical path through the agent cloud; 
these parameters vary with the physical, chemical and dispersal 
characteristics of chemical warfare nerve agents and the prevailing 
atmospheric conditions, and therefore, cannnot be postulated beforehand. 
Infrared imaging devices, such as Texas Instruments Model AN/AAS-28, a 
forward looking infrared (FLIR) thermal-imagery sensor, convert an 
invisible infrared image into a two-dimensional video image, i.e., a 
thermogram. Infrared imaging devices are designed to operate within broad 
infrared wavelength regions of atmospheric transparency, the atmospheric 
windows. These atmospheric windows exist at approximately 8 to 14 microns, 
3 to 5 microns, 2 to 2.5 microns, 1.5 to 1.9 microns and wavelengths 
shorter than 1.4 micron; infrared radiation emitted at these wavebands by 
a remote target undergoes minimum attenuation due to atmospheric 
absorption, scattering and particles prior to reception by a FLIR. A FLIR 
collects and collimates infrared radiation emitted by a remote target and 
passes it to infrared detectors which convert the infrared radiation into 
electrical signals which are amplified and processed by electronic 
circuitry for real-time viewing on a cathode-ray tube (CRT) or storage on 
hard copy. Infrared imaging devices spectrally integrate infrared 
emissions over a broad waveband to form a thermogram which is a composite 
of all infrared radiation emitted by a remote target. These basic imaging 
devices do not have the sensitivity required to spectrally discriminate 
between chemical warfare nerve agent thermal clouds and interferents 
and/or naturally occuring objects in the naval environment. 
SUMMARY OF THE INVENTION 
The present invention surmounts the disadvantages and limitations of the 
prior art by means of a forward looking infrared thermal imager (FLIR) 
modified with a spectral filter wheel (SFW) The FLIR collects infrared 
radiation emitted by a thermal cloud suspected of containing chemical 
warfare nerve agents. The incident infrared radiation is horizontally and 
vertically scanned, selectively filtered by the SFW and focused on a 
vertical array of infrared radiation detectors. Irradiance modulations of 
the detected infrared radiation are converted by the detectors to voltage 
fluctuations; these voltage fluctuations are amplified to drive a vertical 
array of light-emitting diodes (LEDs). The LEDs are scanned to focus a 
visible image onto a vidicon. The vidicon image is then converted into a 
thermogram. 
The SFW has four infrared radiation bandpass filters, each of which is 
sequentially positioned to filter the infrared radiation emitted by the 
suspect thermal cloud, so that four separate thermograms are generated. 
Filter 1 passes approximately all incident infrared radiation in the 8 to 
14 micron atmospheric window. Filter 2 is spectrally optimized to minimize 
the FLIR response to chemical warfare nerve agent infrared radiation 
emissions with respect to the background. Filters 3 and 4 are spectrally 
optimized to maximize the response of the FLIR to the two major infrared 
radiation emission bands characteristic of chemical warfare nerve agents. 
A comparison is then made among the the four thermograms as to variations 
in image contrast of the suspect thermal cloud with respect to its 
background to determine if the composition of the suspect thermal cloud 
includes chemical warfare nerve agents. 
It is therefore a primary object of this invention to provide a modified 
infrared imaging apparatus and method for remotely detecting air-releases 
of chemical warfare nerve agents. 
Another object of this invention is to provide a modified infrared imaging 
apparatus and method for passively detecting the infrared radiation 
emissions of chemical warfare nerve agents. 
Yet another object of this invention is to provide an infrared imaging 
apparatus having a spectral filter wheel spectrally optimized to respond 
to infrared radiation wavebands characteristic of chemical warfare nerve 
agents. 
A further object of this invention is to provide a spectral filter wheel 
capable of infrared radiation spectral discrimination between chemical 
warfare nerve agents and interferents and/or naturally occuring objects in 
the naval environment. 
Other objects, advantages and novel features of the invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying drawings wherein:

PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 represents a schematic diagram of a conventional forward looking 
infrared (FLIR) thermal imager, modified for the remote detection of 
chemical warfare nerve agents in an air-released suspect thermal cloud 10. 
An afocal system 14, in the preferred embodiment a Galilean telescope, 
collects the infrared radiation 12 emitted by the suspect thermal cloud 
10. The collected infrared radiation 12 is reflected off a silver-backed 
scanning mirror 16 to a lens system 18; the scanning mirror 16 provides 
both horizontal sweep and vertical interlacing to give 320 lines of 
vertical resolution. This optical-mechanical scan mechanism permits the 
instantaneous field of view of the vertical detector array 24 to scan the 
spatial distribution of the collected infrared radiation 12 in the object 
plane. The lens system 18 focuses the reflected infrared radiation as it 
passes through the spectral filter wheel 20. The spectral filter wheel 20 
is mechanically rotated to pass infrared radiation in selected passbands, 
these passbands being selected according to the infrared radiation 
emissivity characteristics of chemical warfare nerve agents in the 8 to 14 
micron atmospheric window. The lens system 18 is designed such that the 
image plane of the filtered infrared radiation 22 lies on the surface of 
the vertical detector array 24. The vertical detector array 24 is composed 
of 160 mercury-cadmium-telluride photon detectors; in the alternative, the 
photon detectors could be fabricated from other materials, such as 
mercury-doped germanium, which have a useful infrared radiation detection 
range from 8 to 12 microns. Any of the known closed-cycle refrigerators, 
such as the Stirling refrigerator, a Joule-Thompson refrigerator, or a 
Claude refrigerator, may be used as a cooling means 26 to maintain the 
detector array 24 at the design operating temperature. The image plane 
irradiance modulations caused by horizontal scanning are converted by the 
vertical detector array 24 to voltage fluctuations 28. The voltage 
fluctuations 28 are amplified by 160 sets of preamplifiers 30; the 
amplified voltages 32 from the preamplifiers 30 drive light-emitting 
diodes (LEDs) 34, each amplifier driving a single LED. The LEDs 34 are 
arrayed in a form identical to that of the vertical detector array 24; 
this has the effect of converting the 8 to 12 micron infrared radiation 
fluctuations into modulations in the visible spectrum. A scanning 
mechanism 36 sweeps the LEDs 34 to focus a visible image 38 onto a vidicon 
40. The vidicon signal 42 is then converted into a thermogram 44 which is 
displayed in real-time on a cathode-ray tube 46. Thermal imaging devices, 
sans spectral filter wheel 20, as used in the present invention are known 
in the prior art and their fabrication, operating parameters and 
characteristics are well known to those skilled in the art of thermal 
imaging; a representative example is Texas Instruments Model AN/AAS-28. 
The spectral filter wheel 20 is schematically illustrated in FIG. 2 to show 
the face view of the preferred embodiment. The spectral filter wheel 20 is 
essentially separated into quadrants, each quadrant containing a spectral 
filter element having an infrared radiation passband transparent to a 
specific wavelength band of infrared energy emitted by the suspect thermal 
cloud 10. The detection of chemical warfare nerve agents in a suspect 
thermal cloud 10 with a FLIR modified with a spectral filter wheel 20 is 
dependent upon the image contrast of the generated thermogram 44, i.e., 
the discernible difference in the shades of gray between the suspect 
thermal cloud 10 and its background. The image contrast of the thermogram 
44 is a function of several variables: 
EQU Detection Contrast=f{.DELTA.T', .intg.R.sub..lambda. d.sub..lambda., C, L, 
a.sub..lambda. } 
where .DELTA.T' is the apparent temperature between the suspect thermal 
cloud 10 and its background, .intg.R.sub.80 d.sub..lambda. is the 
integrated spectral response of the FLIR, C is the concentration of the 
chemical warfare nerve agents in the suspect thermal cloud 10, L is the 
optical path through the suspect thermal cloud 10 and a.sub..lambda. is 
the spectral absorption coefficient. An interferometric spectrometer was 
used to obtain the absolute spectral signatures of simulants, i.e., 
compounds whose infrared radiation emissivity signatures are substantially 
equivalent to chemical warfare nerve agents, and interferents air-released 
in varying naval environments. This device provided the absolute 
radiometric spectral signatures of the simulants and interferents and 
their backgrounds; it also provided the difference between these two 
values, the apparent temperature, which is the signal to which a FLIR 
responds. A computer optimization program was developed which specified 
spectral filters that maximized or minimized the ratio of simulant and 
interferent apparent temperatures with respect to specific naval 
backgrounds. This computer optimization program is described by S. R. 
Horman and W. J. Taczak in "OPTRAN: A Computer Code for Optimization of 
Electro-Optical Sensor Spectral Response," Naval Surface Weapons 
Center/Dahlgren Laboratory Technical Report 3909, 1978. This computer 
optimization model not only generated chemical warfare nerve agent 
spectral signatures, but also calculated background, atmospheric 
transmission and atmospheric emission effects. This computer program was 
used to generate performance specifications for the spectral filter wheel 
20 bandpass interference filters. Spectral interference filter 48 is 
comprised of a germanium substrate with multi-layered interference coats 
such that it has an infrared radiation passband that extends from 
approximately 8.0 microns to approximately 12.0 microns. Spectral 
interference filter 48 is spectrally optimized to pass approximately all 
infrared radiation emitted by a gray body in the 8 to 14 micron 
atmospheric window. Spectral interference filter 50 is comprised of a 
germanium substrate with multi-layered interference coats such that it has 
an infrared radiation passband that extends from approximately 10.25 
microns to approximately 10.75 microns. Spectral interference filter 50 is 
optimized to pass a wavelength band characteristic of chemical warfare 
nerve agents that has a relatively low infrared radiation emissivity; for 
gray bodies this wavelength passband has a relatively high infrared 
radiation emissivity. Spectral interference filter 52 is comprised of a 
germanium substrate with multi-layered interference coats such that it has 
an infrared radiation passband from approximately 9.60 microns to 
approximately 10.0 microns. Spectral interference filter 52 is spectrally 
optimized to a first major infrared radiation emission waveband 
characteristic of chemical warfare nerve agents and which is characterized 
by relatively high infrared radiation emissivity. Spectral interference 
filter 54 is comprised of a germanium substrate with multi-layered 
interference coats such that it has an infrared radiation passband from 
approximately 10.7 microns to approximately 12.0 microns. Spectral 
interference filter 54 is spectrally optimized to a second major infrared 
radiation emission waveband characteristic of chemical warfare nerve 
agents and which is characterized by relatively high infrared radiation 
emissivity. 
In operation, the FLIR imager is orientated so that the suspect thermal 
cloud 10 is within the field-of-view of the imager. The suspect thermal 
cloud 10 is generated by the air-release of a gaseous composition by means 
of an aircraft or an exploding projectile. Incident infrared radiation 12 
from the suspect thermal cloud 10 is collected by the afocal system 14. 
The scanning mirror 16 and the lens system 18 transmit the infrared 
radiation 12 to the spectral filter wheel 20. The spectral filter wheel 20 
is initially configured so that spectral interference filter 48 is in the 
path of the infrared radiation 12. The filtered infrared radiation 22 in 
the 8 to 12 micron waveband is detected by the vertical detector array 24 
which converts irradiance modulations to voltage fluctuations 28, the 
voltage fluctuations 28 are amplified to drive LEDs 34, the visible image 
38 of the LEDs 34 is focused onto a vidicon 40 and the vidicon signal 42 
is converted into a thermogram 44 which is displayed on the CRT 46. With 
spectral interference filter 48 aligned in the path of the incident 
infrared radiation 12, a thermogram 44 of the suspect thermal cloud 10, 
T.sub.1, having an image contrast with respect to its background of 
IC.sub.1, is generated (FIG. 5a). The infrared radiation 12 emitted by the 
suspect thermal cloud 10 is sequentially interrogated by means of spectral 
interference filters 50, 52, and 54, by rotation of the spectral filter 
wheel 20 to generate thermograms 44 of the suspect thermal cloud 10, 
T.sub.2, T.sub.3, and T.sub.4, having image contrasts with respect to 
their backgrounds of IC.sub.2, IC.sub.3, and IC.sub.4, respectively (FIGS. 
5b, 5c, and 5d). A comparative analyses of the relative image contrasts 
among the four thermograms will determine whether the air-released suspect 
thermal cloud 10 is composed of chemical warfare nerve agent constituents. 
Whether the suspect thermal cloud 10 is composed of chemical warfare nerve 
agents, or interferents, the relative image contrast IC.sub.1 of 
thermogram T.sub.1 will be higher than the image contrasts IC.sub.2, 
IC.sub.3 and IC.sub.4, of thermograms T.sub.2, T.sub.3 and T.sub.4. The 
passband of spectral interference filter 48 corresponds approximately to 
the 8 to 12 micron atmospheric window; the spatial distribution of the 
infrared radiation emissivity of the suspect thermal cloud 10 (FIGS. 3 or 
4) will generate strong signals at all passband wavelengths so that 
thermogram T.sub.1 will have a high image contrast, IC.sub.1, with respect 
to its background. With spectral interference filter 50 in the infrared 
radiation pathway of the FLIR, a thermogram T.sub.2, with a relatively 
weak image contrast with respect to its background, IC.sub.2, will be 
generated if the suspect thermal cloud 10 is composed of chemical warfare 
nerve agent constituents. This result is reached, because as shown in FIG. 
3, chemical warfare nerve agents have a relatively weak infrared radiation 
emissivity in this passband. By contrast, if the composition of the 
suspect thermal cloud 10 is an interferent, a thermogram T.sub.2 with a 
relatively strong image contrast with respect to its background, IC.sub.2, 
will be generated because of the relatively strong infrared radiation 
emissivity of an interferent in this passband (FIG. 4). With either 
spectral interference filters 52 or 54 in the infrared radiation pathway 
of the FLIR, thermograms T.sub.3 and T.sub.4 with relatively strong image 
contrasts with respect to their background, IC.sub.3 and IC.sub.4 
respectively, will be generated. As seen in FIGS. 3 and 4 the infrared 
radiation emissivity of either an interferent or a chemical warfare nerve 
agent is relatively high at either of these passbands. Therefore, if the 
relative image contrasts with respect to the background, IC.sub.1, 
IC.sub.2, IC.sub.3 and IC.sub.4, of thermograms T.sub.1, T.sub.2, T.sub.3 
and T.sub.4, respectively, are approximately equal, the composition of the 
suspect thermal cloud 10 is an interferent. If the suspect thermal cloud 
10 is composed of chemical warfare nerve agents, the relative image 
contrasts with respect to the background, IC.sub.1, IC.sub.3 and IC.sub.4 
of thermograms T.sub.1, T.sub.3 and T.sub.4, respectively, will be 
approximately equal; the relative image contrast with respect to the 
background, IC.sub.2 of thermogram T.sub.2, however, will be much weaker 
than IC.sub.1, IC.sub.3 or IC.sub.4.