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FTIR modulator for first responder applications
James R. Engel* , Julia H. Rentz, David L. Carlson
OPTRA, Inc., 461 Boston Street, Topsfield, MA, USA 01983
ABSTRACT
This paper describes the evolution and capabilities of our Fourier transform infrared (FTIR) modulator in multiple
configurations for various applications concerning Homeland Security. The heart of our system is the extremely
compact and rugged Michelson interferometer we originally developed during our involvement with the JSLSCAD
program. (1) Our “J-Series” modulator is capable of resolving the 7 to 14 µm spectral region to 4 or 16 cm-1 with a
measured radiometric sensitivity of 2×10-9 W/(cm2 ster·cm-1 ). This system has successfully undergone rigorous testing
for operation over a temperature range of -40 to +65ºC and vibration levels associated with a spectrum of military
ground, air, and water vehicles. The following describes the design and characterization of our J-Series modulator as
well as the subsequent evolutions of the instrument in the forms of an active open-path FTIR and an imaging
(hyperspectral) FTIR. We present this system in passive and active configurations with cooled, uncooled, and imaging
detectors. We also project sensitivity limits for each configuration and measurement for some common chemical agents
as well as industrial compounds.
Key Words: FTIR spectrometer, chemical agent detection, first response, industrial monitoring
1.0 INTRODUCTION
1.1
Description of the opportunity
There exists a long family history of chemical agent detectors based on FTIR in this spectral region. The modulator
described in this paper is the latest manifestation of FTS design concepts that have been applied to chemical agent
detection for nearly three decades. The COIN system (Correlation Interferometer, designed by Engel et al at Block
Engineering) was developed in the early 1970’s to demonstrate that ruggedized, low-power FTS systems were practical
for use in demanding field environments. COIN also served as a platform for the development of some of the earliest
detection algorithms based on processing in interferogram space using correlation techniques. This activity was
separated in time by the development of chemical agent detection systems using circular variable filters before a return to
FTS technology in the XM-21 (Honeywell Electro-Optics Center). The XM-21 system design was later type-classified
as the M-21 (Intellitec, now part of General Dynamics). Engel and Carlson (previously at Block Engineering) were
involved in these latter FTS efforts. A new era in the application of FTS technology to chemical agent detection was
launched with OPTRA’s work, under subcontract to BAIRD Corporation, for the LSCAD (Lightweight Standoff
Chemical Agent Detector). This program consisted of a major redesign of the modulator with an emphasis on a
lightweight, low power design. The I LSCAD modulator design served as the basis of the Improved-LSCAD (work
done by Block Engineering) and a series of FTS systems now offered by Block Engineering, including the basic
modulator employed in the MCAD (Mobile Chemical Agent Detector) system. And with even more improvements
developed by OPTRA, this modulator design is now incorporated into the JSLSCAD (Joint Standoff Chemical Agent
Detector).
While the JSLSCAD design effort counted the LSCAD/I LSCAD design as a solid baseline from which to build, two
specific areas of improvement were identified at the onset:
1.
2.
Increasing the retardation rate to accommodate more demanding search timelines
Increasing the operation range of the instrument with respect to vibration and temperature
The following table lists the system requirements against which OPTRA’s J-Series modulator was designed.
*
[email protected]
Copyright
2003 Society of Photo-Optical Instrumentation Engineers.
This paper will be published in The Proceedings of Optical Technologies for Industrial, Environmental, and Biological Sensing and is
made available as an electronic preprint with permission of SPIE. One print or electronic copy may be made for personal use only.
Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any
material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
Table 1: JSLSCAD system requirements
Requirement
Value
Units
Spectral Range
Spectral Resolutions
NESR
Scan Rates
Retardation Rate
Field of view
Entrance aperture
Temperature Range
2.0
7 – 14
4 & 16
Better than LSCAD
18 & 72
5.2
1.5
2.54
-40 to +65
µm
cm-1
W/(cm2ster·cm-1)
igrams/second
cm/s
degrees (circular)
cm
C
THE J-SERIES MODULATOR
The J-Series modulator is a rugged Michelson interferometer employing a voice coil motor driven flexure assembly to
translate the moving mirror and a laser diode reference system based on spatial fringes instead of the more traditional
fringe counters. The flexure assembly was designed to specifically meet the environmental requirements of the system
and translates the moving mirror with less than 15 asec of tilt over the 4 cm-1 stroke length. The reference system
includes a tilted facet on the edge of the stationary mirror which results in spatial fringes as the two beams recombine;
we sample the fringes with our OPTRA-patented Triphase Detector (TPD) which is a linear silicon array. The
interferometer has a zinc selenide beamsplitter/compensator pair with a special low chromatic phase dispersion
beamsplitter coating and visible/near IR (NIR) channels on the top and bottom for the laser reference as well as a white
light reference for use during alignment. The 1.27 cm diameter moving and stationary mirrors are diamond-turned
aluminum and are mounted to the flexure assembly and an adjustable ball mount respectively. The interferometer with
no fore or exit optics measures a compact 1.765×1.825×2.725 inches.
Figure 1 shows the J-Series modulator with all of the components labeled. The following sections detail the sub-system
designs.
Figure 1: J-Series Modulator
Laser diode
reference
assembly
Moving mirror /
flexure
assembly
1.825”
2.725”
Reference
detector
assembly
Stationary
mirror
assembly
1.765”
Entrance
aperture
Exit
aperture
Vibration
isolation
mounts
2.1
Optical design
The optical design of the J-Series in the passive configuration with the mechanically cooled mercury cadmium telluride
(MCT) detector (i.e. the JSLSCAD configuration) is shown in figure 2. We employ a germanium (Ge) refractive
telescope, exit lens, and field lens inside the detector assembly. The 1.5º full field on the 2.54 cm diameter entrance
aperture is converted to a 3º full field inside the 1.27 cm diameter interferometer. Note that this is roughly half the field
value permitted by the obliquity limit imposed by 4 cm-1 spectral resolution at 1428 cm-1 (i.e. the highest optical
frequency).
Figure 2: J-Series optical design
Fold
mirror
Primary
Primary
telescope
Telescope
Lens
lens
Secondary
Secondary
telescope
Telescope
Lens
lens
Cold
Cold
Optics
optics
Exit
ExitLens
lens
Scanning
Moving
Mirror
mirror
Compensator
Beamsplitter/
compensator
Stationary
Stationary
mirror
Mirror
2.2
Figure 2 shows the optical
layout of the J-Series
modulator in passive
configuration with the cooled
MCT detector (i.e. the
JSLSCAD configuration).
We employ a Ge refractive
telescope which converts a
1.5º field of view to a 3º field
inside the interferometer.
The exit optics assembly
includes a Ge exit lens, an
aluminum fold mirror, a Ge
window, a field stop, and a
Ge field lens immediately in
front of the detector.
Laser diode reference assembly
The laser diode reference is a wavelength stabilized distributed feedback (DFB) source at 810 nm. The reference system
is similar to a standard laser interferometer except for a tilted facet on the edge of the stationary mirror which induces an
incidence angle in the beam traversing the stationary mirror arm (figure 3). The result is spatial fringes when the two
beams recombine at the TPD. The TPD is a silicon array with every third element wired in parallel. The tilt angle and
wavelength of the laser diode result in one fringe for every three elements; the three channels are thus exactly 120º apart
in phase. We interpolate this phase measurement and yield an absolute position measurement of the optical path
difference (OPD) with significantly higher resolution than is achievable by standard fringe-counting laser reference
systems. This capability allows for considerable flexibility and configurability for both rapid scanning and step scanning
applications.
Figure 3: Laser diode reference assembly
Triphase
detector
(with fold
mirror)
Moving
mirror
Laser
diode
module
(with fold
mirror)
Stationary
mirror
with tilted
facet
VIS/NIR
patch of
BS/comp
Figure 3 shows our laser diode
reference system. We impose
spatial fringes on the TPD as a
result of the small tilt in the
visible/NIR channel on the
stationary mirror. As the
moving mirror scans, the
spatial fringes move across the
TPD. We interpolate the phase
of the fringes to yield a high
resolution absolute position
measurement which we use to
servo control the moving
mirror and clock the
interferogram.
2.3
Moving mirror assembly
The moving mirror assembly is the key to the environmental performance of the J-Series modulator. Significant design
and empirical effort produced a moving mirror which is able to maintain modulation efficiency over the temperature and
vibration level ranges imposed by our system requirements. The final design employs membrane flexures with
constrained layer damping (CLD) driven by a voice coil motor to achieve the required 1.25 mm (physical) stroke length
for 4 cm-1 resolution with less than 15 asec of dynamic tilt. Figure 4a is a photo of the motor assembly; 4b shows a
typical tilt measurement (taken using an electronic autocollimator) and 4c the results of vibration testing demonstrating
the utility of the CLD.
Figure 4a: Moving mirror assembly
Figure 4b: Dynamic tilt data
10
8
6
4
2
0
-2
-4
-6
-8
-10
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
Motor Displacement (mm)
Motor Response
Curves
FigureJSLSCAD
4c: J -Series
motor
response
CLD vs No CLD
10 4.0
CLD
No CLD
Optical Displacement (µm )
10 3.0
10 2.0
CLD
10
1.0
No CLD
10 0.0
10 -1.0
0
100
200
300
400
500
600
Frequency (Hz)
2.4
700
800
900
1000
Figure 4 summarizes the moving
mirror assembly. Figure 4a is a
photo of the flexure, diamondturned aluminum mirror, and
housing which encloses the voice
coil motor. The tabs on the top and
bottom of the mirror are the two
visible/NIR channels. Figure 4b
shows typical data from a tilt
measurement over the stroke length.
Figure 4c shows the motor response
as a function of frequency with and
without the CLD. The servo
controls the primary resonance at 50
Hz and the CLD is responsible for
minimizing the secondary resonance
at 350 Hz.
Performance evaluation
The following shows a typical NESR plot for search (16 cm-1 ) and confirm (4 cm-1 ) mode over the spectral range of the
instrument. Also provided are LSCAD and ILSCAD values for the same.
NESR SN005
Mode mode
(4 co-added
scans)
Figure
5b: Confirm
Confirm
NESR
NESR, SN003,
Search
Mode
Figure 5a: Search
mode
NESR
.
LSCAD
I-LSCAD
JSLSCAD
LSCAD
I-LSCAD
JSLSCAD
1.E-07
W/(cm2 ster cm-1)
W/(ster cm2 cm-1)
(4 coadded scans)
1.E-06
1.E-06
1.E-08
1.E-09
600
800
1000
1200
1400
1.E-07
1.E-08
1.E-09
600
1600
800
Optical Frequency (wavenumber)
1000
1200
1400
1600
Optical Frequency (wavenumber)
Figures 6a and 6b show the clean (i.e. no spectral satellites) instrument function in search and confirm mode.
CO2
Line, System
1, Confirm mode
Mode
Figure
6a: Confirm
CO2 Line, System 2, Search Mode
FigureFWHH
6a:= 15.08
Search
mode
cm-1
Instrument Function
FWHH = 3.75 cm-1
Instrument Function
30
30
25
25
20
20
15
AU
FWHH = 15.08 cm -1
10
10
5
5
0
0
-5
800
-5
850
900
950
1000
1050
FWHH = 3.75 cm -1
AU
15
1100
800
850
900
950
1000
1050
1100
Frequency (cm-1)
Frequency (cm-1)
3.0 EVOLUTIONS OF THE J-SERIES
This section is intended to describe some actual and theoretical evolutions of the J-Series modulator in a number of
different configurations.
3.1
Passive standoff detection
Passive standoff detection by definition is a single ended system that relies on a slight temperature contrast between the
background and the agent we wish to detect. Figure 7 shows the set up.
Figure 7: Passive chemical detection configuration
Fore-Optics
Exit-Optics
with MCT or
DLATGS
T background
Absorbing / Emitting
Medium, Tchemical
J-Series
Modulator
Figure 7 depicts the passive
configuration. This instrument
stares out over a spatial area of
interest, continuously collecting
IR radiation. If an agent or
chemical plume enters into the
FOV of the instrument with
sufficient temperature contrast
relative to the background, a
positive detection can be made.
The instrument in passive detection mode employs simple fore-optics composed of either of a refractive telescope (as
shown) with germanium lenses or a reflective telescope. The exit optics module consists of an exit lens (also
germanium) and either a mechanically cooled mercury cadmium telluride (MCT) detector or an uncooled deuterated Lalanine tri-glycine sulfate (DLATGS) pyroelectic detector, depending on the required sensitivity, expected concentration
levels, temperature contrast, and integration time permitted. The DLATGS represents a 15x reduction in price relative to
the MCT at the cost of a 35x reduction in sensitivity. The DLATGS will provide a broader spectral response (over the
400 to 4000 cm-1 range). The intended installation will likely dictate the most appropriate detector choice.
3.2
Active standoff detection
Active standoff detection requires the introduction of an IR source module which is placed in front of the FTIR
spectrometer as shown in figure 8a or integration into the modulator as shown in figure 8b. The former is called bistatic
and the latter monostatic. The later also requires a large mirror or retroreflector array to return the open-path beam to the
spectrometer for detection. While the passive measurements may be either molecular absorption or emission (depending
on the relative temperatures of the compound and background), active measurements are always molecular absorption
which rely on the large temperature difference between the compound and a hot (T ˜ 1273 K) IR source seen as
background.
Figure 8a: Bistatic active chemical detection configuration
Exit-Optics
with MCT or
DLATGS
IR-Source, Tsource
Absorbing Medium,
Tchemical
J-Series
Modulator
Figure 8 b: Monostatic active chemical detection configuration
IR-Source, T source
Absorbing Medium,
Tchemical
Exit-Optics
with MCT or
DLATGS
J-Series
Modulator
Figure 8a shows our system in bistatic active mode. We place an IR source with collimating optics (such as a parabolic
reflector) in front of the instrument which imposes the needed temperature contrast between the chemical and the new
“background”. Figure 8b shows our system in monostatic active mode where the IR source is modulated by the interferometer
prior to being sent through the absorbing medium. The monostatic configuration requires a mirror or retroreflector array to
return the open-path beam to the spectrometer for detection. In each case, the spectrum is measured in absorption and adds
considerably more sensitivity to the measurement relative to passive mode.
In active mode, our system may or may not require fore-optics, depending on the range of the open path; this
configuration can also be used with a sampling chamber. Active configuration may include either the DLATGS or MCT
detector but may require a special (> 16 bit) A to D to cover the dynamic range.
OPTRA has realized this J-Series evolution under another development effort to design and build a low-cost open-path
FTIR for industrial monitoring (figure 9a). (2) Our monostatic OP-FTIR operates over the same wavelength range with
8 cm-1 resolution; this system employs the uncooled DLATGS detector. This effort also produced a novel plastic
injection molded retroreflector array (patent pending) to return the open-path beam (figure 9b).
Figure 9a: OPTRA’s OP-FTIR spectrometer
Figure 9 b: Plastic retroreflector array
Figure 9a shows an
evolution of the J-Series
modulator in active mode.
We designed and
constructed our low-cost
OP-FTIR spectrometer
using the J-Series
modulator as the base. The
spectrometer module
measures a compact
7.25×8.5×10.5”. Figure 9b
shows the plastic injection
molded retroreflector array
we developed as part of this
effort.
3.3
Hyperspectral passive standoff detection
Our system in hyperspectral mode simultaneously provides two-dimensional spatial (image) information and spectral
information (across the entire image). Figure 10 shows the sys tem in hyperspectral mode.
Figure 10: Hyperspectral (passive) chemical detection
Absorbing / Emitting
Medium, Tchemical
Exit-Optics
with FPA
Spectral Output
X
Y
T background
J-Series
Modulator
λ
Hyperspectral Data Cube
Figure 10 shows our system in hyperspectral configuration. For this mode we step scan the moving mirror, acquiring a
frame from the FPA at each step across the stroke of the mirror. This alleviates the computational complexity of frames
acquired while the mirror is in motion. The FFT is performed on each pixel, producing a spectrum of each spatial
resolution element across the field of view of the image (i.e. the hyperspectral cube). This configuration is ideal for a
scenario where we wish to monitor a large area but want the capability to spatially locate the area containing the chemical
or biological agent or organic compound.
A point of interest with respect to our system in its hyperspectral configuration stems from our spatial fringe reference
and our high-resolution spatial control of the scanning mirror. This allows us to do two things. First of all, we can step
scan the mirror through the stroke length, which means that the entire FPA is readout at the same retardation location or
optical path difference (OPD). This may dramatically simplify computation, as no post processing is necessary to line
the samples up within each interferogram. Second of all, our high spatial resolution on our servo control of the scanning
mirror allows us to hold the mirror at zero OPD where we record a simple broadband IR image no different from an IR
camera. This capability may be of significant use within detection and discrimination to simplify computation. Other
hyperspectral systems based on Fabry-Perots or acousto-optic modulators can not do this.
3.4
Raman capability
The final configuration of the FTIR modulator exploits the fact that there are two separate visible/NIR channels to the
beamsplitter/compensator, only one of which is in use during normal operation (i.e. the laser reference channel). We
propose that the other channel may be used to modulate Raman scattered light, the transform of which yields the spectra.
While this concept has yet to be demonstrated, its inclusion is entirely feasible and has the potential to further increase
the utility of the FTIR modulator system.
4.0 PROJECTED SENSITIVITIES
Having described a number of configurations of the FTIR modulator, here we present the projected sensitivities of each
configuration in the format of minimum detectable concentration pathlengths of a list of common chemical agents and a
second list of industrial organic compounds. For each we project a noise equivalent spectral radiance which is calculated
according to
NESR =
AD
D * ⋅η ⋅ Θ ⋅ ∆σ ⋅ ∆t
[ =]
W
cm ⋅ ster ⋅ cm −1
2
(1)
A D is the detector area, D* is the detector detectivity, η ?is the radiometric efficiency, Θ is the etendue or throughput, ∆σ
is the spectral resolution, and ∆t is the integration time. For all configurations, we take η to be 0.1, and Θ is dictated by
the obliquity limit and the 1.27 cm interferometer mirrors. We project for spectral resolutions of 4 and 16 cm-1 , and ∆t is
one second unless otherwise noted (for hyperspectral mode). We present the single channel configurations, active and
passive, with both the cooled, MCT detector and the uncooled, DLATGS detectors of 2 mm in diameter in each case.
For passive mode we assume a 3 K temperature contrast between the agent/compound and the background of 300 K; for
active mode, we assume a 1273K IR source and room-temperature agent/compound. For active mode we also provide
short and long range estimates, the former for which the obliquity limits the throughput, and the latter for which the
pathlength limits the throughput. Table 2a gives the minimum detectable concentration pathlengths in the standard units
of mg/m2 for seven common chemical agents; table 2b gives the same in standard units of ppm·m for seven common
organic industrial compounds. The limits are based on a single line analysis using published reference spectra. (3, 4) In
each configuration we also provide the calculated NESR in units of W/(cm2 ster·cm-1 ). The hyperspectral internal field of
view (per pixel) is 0.11º.
Table 2a: Chemical agent minimum detectable concentration pathlengths
Tabun
Sarin
Soman
GF
VX
D. Mustard Lewsite
α: 9.30E-04 1.50E-03 1.12E-03 1.10E-03 4.00E-04
1.30E-04 5.60E-04
NESR
CLmin
CLmin
CLmin
CLmin
CLmin
CLmin
Clmin
mg/m2 mg/m2 mg/m2 mg/m2 mg/m2
mg/m2
mg/m2
7.95E-08
200.2
121.3
162.4
167.3
461.1
1810.9
289.0
1.98E-08
46.5
28.2
37.8
38.9
107.1
412.7
67.7
9.94E-10
2.3
1.4
1.9
1.9
5.3
20.2
3.3
2.48E-10
0.6
0.3
0.5
0.5
1.3
5.0
0.8
Configuration
Passive
DLATGS, 4 cm-1
DLATGS, 16 cm-1
MCT, 4 cm-1
MCT, 16 cm-1
Active, Short Range
DLATGS, 4 cm-1
7.95E-08
0.14
DLATGS, 16 cm-1
1.98E-08
0.04
MCT, 4 cm-1
9.94E-10 1.8E-03
MCT, 16 cm-1
2.48E-10 4.5E-04
Active, Long Range
DLATGS, 4 cm-1
9.59E-07
1.75
DLATGS, 16 cm-1
2.40E-07
0.44
MCT, 4 cm-1
1.20E-08 2.2E-02
MCT, 16 cm-1
3.00E-09 5.5E-03
Imaging, Per Pixel, 60s integration
FPA, 16 cm-1
1.44E-06
544
Imaging, 1 Superpixel, 60s integration
FPA SP, 4
6.38E-07
208
FPA SP, 16 cm-1
1.60E-07
49
0.09
0.02
1.2E-03
2.9E-04
0.12
0.03
1.5E-03
3.9E-04
0.12
0.03
1.6E-03
3.9E-04
0.34
0.08
4.3E-03
1.1E-03
0.86
0.21
1.1E-02
2.7E-03
0.34
0.09
4.3E-03
1.1E-03
1.12
0.28
1.4E-02
3.5E-03
1.50
0.37
1.9E-02
4.7E-03
1.50
0.37
1.9E-02
4.7E-03
4.12
1.03
5.1E-02
1.3E-02
10.32
2.58
1.3E-01
3.2E-02
4.16
1.04
5.2E-02
1.3E-02
328
440
454
1252
5187
768
126
29
169
39
174
41
479
112
1885
431
300
71
Table 2b: Organic compound minimum detectable concentration pathlengths
MEK
meth Cl toluene
phenol
xylenes formald
TCE
α: 4.00E-04 1.54E-03 3.38E-04 5.68E-04 2.90E-04 7.68E-04 1.27E-03
NESR
CLmin
CLmin
CLmin
CLmin
CLmin
CLmin
CLmin
ppm m
ppm m
ppm m
ppm m
ppm m
ppm m
ppm m
7.95E-08
552.4
106.0
487.8
396.8
558.1
532.1
128.0
1.98E-08
126.6
24.8
114.2
90.8
130.8
113.6
30.0
9.94E-10
6.2
1.2
5.6
4.4
6.4
5.5
1.5
2.48E-10
1.5
0.3
1.4
1.1
1.6
1.4
0.4
Configuration
Passive
DLATGS, 4 cm-1
DLATGS, 16 cm-1
MCT, 4 cm-1
MCT, 16 cm-1
Active, Short Range
DLATGS, 4 cm-1
7.95E-08
0.29
DLATGS, 16 cm-1
1.98E-08
0.07
MCT, 4 cm-1
9.94E-10
3.6E-03
MCT, 16 cm-1
2.48E-10
9.0E-04
Active, Long Range
DLATGS, 4 cm-1
9.59E-07
3.50
DLATGS, 16 cm-1
2.40E-07
0.87
MCT, 4 cm-1
1.20E-08
4.4E-02
MCT, 16 cm-1
3.00E-09
1.1E-02
Imaging, Per Pixel, 60s integration
FPA, 16 cm-1
1.44E-06
1557
Imaging, 1 Superpixel, 60s integration
FPA SP, 4
6.38E-07
575
FPA SP, 16 cm-1
1.60E-07
132
0.14
0.04
1.8E-03
4.4E-04
0.69
0.17
8.6E-03
2.2E-03
0.20
0.05
2.5E-03
6.3E-04
0.70
0.18
8.8E-03
2.2E-03
0.12
0.03
1.5E-03
3.7E-04
0.14
0.04
1.8E-03
4.5E-04
1.72
0.43
2.2E-02
5.4E-03
8.36
2.09
1.0E-01
2.6E-02
2.43
0.61
3.0E-02
7.6E-03
8.49
2.12
1.1E-01
2.7E-02
1.41
0.35
1.8E-02
4.4E-03
1.73
0.43
2.2E-02
5.4E-03
282
1299
1124
1482
1998
340
110
26
507
119
413
95
580
137
556
119
133
31
5.0 CONCLUSION
In conclusion, OPTRA has developed an extraordinarily versatile FTIR modulator of which we are in the process of
exploring configurations and applications. The rugged Michelson has demonstrated conservation of modulation
efficiency across an extremely harsh range of temperature and vibration levels. We have demonstrated the capability of
the system in both passive and active mode and look forward to demonstrating hyperspectral mode. In addition to the
considerable operational capacity of this instrument, it also holds strong promise of low cost and portability. We feel
that these factors will open up a considerable number of new applications for which FTIR spectroscopy has historically
been dismissed as too complicated, large, expensive, and fragile.
REFERENCES
1. Joint Services Lightweight Standoff Chemical Agent Detector (JSLSCAD).
2. United States Air Force SBIR Phase II Contract No. F4265001C 0185: Low-Cost OP-FTIR Spectrometer with
NanoScale Reference for Industrial Monitoring
3. D.F. Flanigan, "Infrared Spectra and Absorption Coefficients for GA, GB, GD, VM, VX, and the G analog", EATM
321-6 (August 1966)
4. World Wide Web < http://www.epa.gov/ttnemc01/ftir/refnam.html#v> EPA - TTN EMC - Spectral Database Fourier Transform Infrared (FTIR) Reference Spectra