Note: Descriptions are shown in the official language in which they were submitted.
~ W092/09877 2 ~ ~ 7~3 ~ PCT/US911095~4
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F~IR REMOTE SENSOR APPA~ATUS.
AND METHOD
BACKG~OUND OF THE INVENTION
The present invention relates to an apparatus
and method for environmental monitoring of one or more
gases in a specific target area. In particular, the
present invention is an apparatus and method for
determining the identity and quantity of airborne
emissions at plants or other geographically defined
locations, for example in the vicinity of chemical or
wastewater plants or nuclear facilities.
Present environmental regulations by Federal,
state and local governmental agencies impose certain
environmental monitoring requirements on operators of
plants that produce airborne emissions regarded as
pollutants. It is anticipated that in the future
regulations of this type will require even more stringent
monitoring. Therefore, improved methods to conduct
environmental monitoring of plants for airborne emissions
can result in decreasing the burden of such an effort on
the plant operator. In addition, improved monitoring
capa~ility benefits the regulatory agencies involved and
ultimately the public in terms of better data on actual
emissions so that appropriate emission control systems
and programs can be implemented.
Present methods for monitoring for gaseous
airborne components include both canister and open long
path techniques. With canister techniques, the identity
and the concentration of airborne pollutants in a
specific geographical area can be determined by obtaining
one or more samples of air from the area and performing
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an analysis of the samples. This technique consists of
collecting an air sample in an evacuated canister at a
location where the identities of the gases are re~uired
to be determined (i.e. the target area). The air sample
may be subsequently taken to a laboratory where an
analysis is performed.
Spectroscopic analysis is one method that may
be used to identify gases present (and the concentration
thereof) in a gaseous sample. Spectroscopic analysis is
based on the determination that matter absorbs light at
characterlstic wavelengths. Accordingly, to identify
components of a gaseous sample, a beam of light is
transmitted through the sample and then the light beam is
collected after it has passed through the sample. An
absorption spectrum is then generated from the collected
light beam. By comparing the sample's absorption
spectrum with known reference spectra, the identities
(and concentrations) of the components, such as
pollutants, present in the gaseous sample can be
determined.
As applied to the canister method described
above, spectrographic analysis may be used to identify
the gases present in a sample as well as the
concentration. While being a reliable techni~ue, the
canister method is essentially only a "point monitor" of
gases and the data obtained are not necessarily
representative of the concentrations of gases over large
distances. In order to overcome this disadvantage,
multitudes of canisters can be placed in the target area
and the results averaged in order to determine the
overall compositions of the gases in the target area.
Thus, even where automated, this method can be unduly
burdensome. Moreover, the canister method is essentially
historical in the sense that it provides a detPrmin~tion
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of the identity and concentration of gaseous components
at only the point of time at which the sample is taken
and thus ~s not necessarily a "real-time" reading.
In contrast to the canister technique, an
analysis of the components in the atmosphere can be made
directly in the environment across a portion of the
target area, (i.e. the "open long path" technique).
~nown open long path techniques essentially are optical
systems that transmit an optical beam across the target
area and treat the volume of atmosphere through which the
beam is transmitted as the "sample". The system is
"open" in that the sampling is not done on a portion of
the atmosphere contained in a canister or other closed
sampling medium as such, but rather is open in the
environment. Systems of thls type can be utilized to
identify and measure the concentrations of gases over
large distances (such as over several hundred meters or
more and even as far as l kilometer). Thus, with an open
long path system, which may comprise a relatively small
number of components, an analysis can be performed that
otherwise would require numerous canisters and a separate
laboratory facility. The open long path technique has
the potential to be more readily manageable than the
canister techni~ue. Xnown open long path techniques are
not without their own shortcomings however.
One known open long path system employs
spectroscopic analysis in a bistatic configuration. A
bistatic configuration is characterized by separate light
transmission and reception units placed on opposite sides
of the target area. The transmission and reception units
must be aligned so that light transmitted from the
transmission unit will enter the reception unit on the
opposite side of the target area without any appreciable
~ loss in light energy. This configuration presents
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difficulties associated with alignment especially when
the analysis is required to be performed over long
distances such as several hundred meters or more.
A bistatic open long path system, such as
described above, can utilize quantitative Fourier
transform infrared (FTIR) spectroscopic techniques for
the analysis of the gaseous components therein. An FTIR
system i5 described in "Remote and Cross-stack
Measurement of Stack Gas Concentrations Using a Mobile
FTIR System" Herget, Applied Optics, 21;635 (1982).
Another open long path method is described in
U.S. Pat. No. 4,426,640 issued to Becconsall et al. The
method described by this patent is not based upon
spectroscopic analysis. The '640 patent discloses a
laser scanning apparatus in which two laser beams (a
detection beam and a reference beam) are used to
determine the concentration of a selected gas in a
specific area, such a plant site. The detection beam is
tuned to the specific absorption wavelength of the gas to
be monitored and the reference beam is ad~usted to a
wavelength that is significantly less strongly absorbed
by the gas. The beams are combined and transmitted
generally downwards from a tall tower overlooking the
plant site. The combined beams are scattered by the
ground, buildings, pipework, trees, etc. A portion of
the combined laser light beams is scattered back in the
direction of the sc~nning apparatus. Then, the device
described in the '640 patent collects the portion of the
scattered, reflected light and a detector produces
electrical signals corresponding to the intensity of the
collected light (electromagnetic radiation). The
concentration of the selected gas present in the
monitored area is determined from the ratio of the
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electrical signals of the detection beam and the
reference beam.
A limitation on the usefulness of the 640
apparatus is that only one selected gas can be monitored
at a time because the transmission wavelengths of the two
lasers are specifically adapted for the one ~ust the one
selected gas. In order for the device described in the
640 to monitor additional gases, either the transmission
wavelengths of the two lasers must be recalibrated or
additional lasers must be incorporated. Essentially, the
devlce described in the '640 patent requires an
identification in advance of the specific gas to be
monitored and then enables a concentration of that gas to
be determined.
Accordingly, it is an object of the present
invention to provide a monitoring system capable of
readily measuring more than one gas in a target area.
It is a further object of the present invention
to provide an open long path system that is readily
alignable for the monitoring of one or more atmospheric
components in a target area.
It is a further object of the present invention
to provide for real-time, or near real time monitoring of
gases in a target area.
SUMMARY OF THE l~V~NllON
According to a first aspect of the present
invention, there is provided an apparatus for analyzing
gases in a target area that has at least one cooperative
reflective element located on one side thereof. The
apparatus comprises an infrared light source for
producing a modulated infrared beam capable of being
absorbed at wavelengths characteristic of the gases being
analyzed and an optical arrangement for transmitting the
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infrared beam from the light source across the target
area and receiving the infrared beam returned by the
cooperative reflective element from across the target
area. Based upon the absorption spectrum of the received
beam, an determination can be made of the gases present
and the concentrations thereof in the target area.
According to a second aspect of the present
invention, there is provided a unistatic open long path
system for analyzing gases in a target area. The system
comprises an infrared light source located at one side of
the target area for producing a modulated infrared beam
capable of being absorbed at wavelengths characteristic
of the gases being analyzed and an optical arrangement
for transmitting the infrared beam from the light source
across the target area. The system further includes at
least one cooperative reflective element located on
another side of the target area an positioned to receive
the beam transmitted from the optical arrangement and to
return it back along the path of incidence. The optical
arrangement is capable of receiving the infrared beam
returned by the cooperative reflective element from
across the target area. The identity and concentration
of one or more gases in the target area can be determined
by the absorption spectrum of the received beam.
According to a third aspect of the present
invention, there is provided a method for analyzing gases
in a target area comprising the steps of generating a
modulated infrared beam that may be absorbed at different
wavelengths by the gases, transmitting the infrared beam
in a first path across the target area, reflecting the
transmitted infrared beam at a location across the target
area, receiving the reflected infrared beam, and
comparing the absorption spectrum of the received beam to
a library of absorption spectra characteristlc of the
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gases to be analyzed in a storage library memory whereby
one or more gases present in the target area can be
identified.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated by reference to
several embodiments thereof and a detailed description of
the preferred embodiment, in which:
FIG. l shows a schematic of a first preferred
embodiment of the present invention.
FIG. 2 shows a detailed schematic of the remote
sensor portion of the embodiment of Figure l.
FIG. 3 shows a detailed schematic of a second
preferred embodiment of the present invention.
FIG. 4 shows a detailed schematic of another
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED
EMBODIMENTS OF THE INV~N'1'10N
~ igure l is a schematic of an arrangement
utilizing a preferred embodiment of the present
invention. As depicted in Figure l, the preferred
embodiment of the present invention is an open long path
spectroscopic monitoring system having a unistatic
configuration. A target area l0 may possess any shape
and may be irregular, but is generally geographically
contiguous. The target area l0 may have located upon it
a facility that emits or has the potential for emitting
components to the atmosphere the identity and
concentration of which it an o~ject of the present
invention to determine. The facility may be a chemical
plant, a manufacturing plant, a wastewater treatment
site, a hazardous waste site, or a power generating
plant. ~owever, the present invention is understood not
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. . .
to be limited to these applications, and may be utilized
for the monitoring of the gaseous components present in
areas having other types of facilities located thereupon
as well. When the target area 10 is a plant of the type
mentioned above, the distance across the target area 10
may range up to a kilometer although distances even
greater than a kilometer may be monitored as long as a
path for optical transmission exists across the area.
Furthermore, it is understood that the target area need
not be contiguous with the entire plant property and it
may be desired to define the target area to be a portion
of the plant property or even to extend outside the plant
property depending upon a determination 8S to where
representative emissions from the plant may be monitored.
Figure 1 shows a remote sensor 16 located at
one side 18 of the target area 10. The remote sensor 16
transmits an infrared light beam 20 that travels across
the target area 10 in a first path 22. Located at
another side 24 of the target area is a reflector element
26. The reflector element 26 is positioned and adapted
to return the transmitted infrared beam 20 directly back
along the first path 22 to the remote sensor 16 so that
the reflected beam 28 returns to the remote sensor 16.
The reflector element 26 is preferably a cooperative
reflector, i.e. a component having high reflectivity back
to the source. In the preferred embodiment, the
reflector element is a cube corner retroreflector array.
A light beam that impinges upon a cube corner
retroreflector array will be reflected back along its
path of incidence if it strikes within the
retroreflector's beam acceptance angle (typically up to
45 degrees). Thus, the use of a cube corner
retroreflector array facilitates alignment of the
reflector element with respect to the remote sen~or.
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_ g
~owever, other components may be used for the reflector
element instead of a retroreflector array, such as a flat
mirror or a Cassagrain telescope with a flat mirror at
its focal plane.
Referring to Figure 2, there is depicted a
schematic of the remote sensor 16 of Figure 1. The
remote sensor 16 includes a optical arrangement portion
30 and an F~IR (Fourier Transform Infrared spectrometer)
portion 32. The optical arrangement portion 30 includes
an infrared light source 40 capable of producing a broad
band infrared light beam 42. In a preferred embodiment,
the infrared light source 40 is an incandescent filament,
which produces an infrared beam encompassing wavelengths
of 600 cm1 to 6000 cm~l. The infrared light source 40
directs the infrared beam 42 to a beamsplitter 44 where
the infrared beam 42 from the light source 40 is
reflected along a path 50 through an aperture 54 and into
a telescope portion 56 of the optical arrangement portion
30. The telescope can be of focal system design, such as
Newtonian, Cassagrain, or Gregorian; or of off-axis
design, such as off-axis Dall Kirkham, off-axis
Newtonian, or off-axis focusing Dall Kirkham. (In the
preferred embodiment, the telescope is a 14.5 inch
Cassagrain telescope). Within the Cassagrain focal
system telescope shown 56, the infrared beam in the path
50 is reflected by a mirror arrangement 58 (preferably a
parabolic mirror arrangement). The mirror arrangement 58
includes mirrors 60 and 62. The infrared beam in the
path 50 is reflected by the mirror 60 to the mirror 62
and then transmitted through a telescope opening 64 (as
the beam 20) into the target area 10 ~shown in Figure 1).
After being reflected by a reflector element,
e.g. 26, as described above, the infrared beam 28 is
returned along the first path 22 back into the opening 64
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20 87 439
?,~ 0
of the telescope portion 56. In the preferred
embodiment, the telescope portion ~6 is used for both
transmitting and receiving the infrared beams 20 and 28,
respectively. This is enabled because the reflector
element, e.g. 26, is positioned and aligned to return a
beam back along its path of incidence. Inside the
telescope portion 56, the returned beam 28 travels along
the same path as the transmitted beam but in the reverse
direction back through the aperture 54 to the
beamsplitter 44.
The beamsplitter 44 transmits a portion 66 of
the reflected beam 28 through to a transfer optics group
68. The transfer optics group 68 includes mirrors 70 and
72 positioned to transmit the reflected beam, indicated
by the numeral 80, to the FTIR portion 32. The FTIR
portion 32 includes a mirror 82 that receives the beam 80
from the optical arrangement portion 38. The mirror 82
reflects the beam 80 into the Michelson design
interferometer 90. In the preferred embodiment, the
interferometer is a high speed, continuous-scan Michelson
design interferometer. The interferometer 90 generates
an interferometer output, e.g. interferogram signal 92,
from the beam 80. The interferogram signal 92 propagates
to another group of mirrors 94, 96, and 98 which focus
the signal 92 from the interferometer 90 onto a detector
100. The detector 100 measures the power or intensity of
the signal 92. In a preferred embodiment, the detector
is a liquid nitrogen cooled photoconductive detector or a
mercury cadmium telluride, an indium antimony, or a
pyroelectric detector. The detector 100 and the
Michelson interferometer 90 provide outputs 102 and 104
to a computer 106. The computer 106 identifies an
absorption spectrum based upon these outputs. The
computer 106 includes a memory 108 having a library of
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absorptlon spectra that have been developed under
controlled conditions for specific gases in known
concentrations. The computer 106 compares the absorption
spectrum obtained from the remote sensor 16 to the known
spectra in the memory library. In the preferred
embodiment, a peak identification or a linear least
s~uares fitting technique, or other quantitative data
analysis techniques may be employed. By this comparison,
the identity of the gases present in the target area 10
can be determined. In a preferred embodiment, the memory
library includes identifying spectra for over 200 gases,
however, the present invention may be adapted to analyze
fewer than or more than 200 gases depending upon the
needs of the user. Typical gases that may be monitored
by the present invention include: SF6, methylene
chloride, methanol, ammonia, methane, chlorobenzene p-
dichlorobenzene, CO, toluene, vinyl chloride. The
computer preferably operates continuously and depending
upon the speed of the computer, the analysis of the gases
proceeds simultaneously or nearly so thereby providing
real-time or near real-time monitoring.
The computer 106 can also be used to determine
an analysis of the concentration of any of the gases
identified as being present in the target area. The
spectrum data stored in the computer memory 108 for each
gas is based upon samples of each of the gases at a known
concentration. By comparison of the observed spectrum
with the library spectrum a determination of the
concentration of the particular gas in the target area
can be derived from Beer Lambert's law as is well known
in the art. The light source of the present invention is
readily capable of transmitting an infrared beam
accurately over target area distances of up to 1
kilometer or more. Since the nominal sensitivity of the
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.
present invention at an optical path of 1 kilometer is
approximately 1 part per billion, the identified gases
can be quantified accurately.
Referring again to Figure 2, in a preferred
embodiment of the present invention, a light source 110
such as a HeNe laser, is positioned inside in the FTIR
portion 32. The light source 110 is associated with a
mirror 112 for reflecting a beam of light from the light
source 110 in the path of interferometer output beam 92
but in the reverse direction. The light source 110 is
used for alignment of the internal optics as well as for
wavelength calibration of the interferometer.
Referring to Figure 3, in a preferred
embodiment of the present invention, a sc~nn~ng apparatus
116 is associated with the remote sensor 16. The
scanning apparatus 116 is adapted to direct the
transmitted beam 20 from the remote sensor 16 along one
or more paths 22', 22'' to one or more additional
reflector elements 26', 26'', respectively, located at
other positions on sides of the target area apart from
reflector element 26. The scanning apparatus 116 may
comprise an arrangement of mirrors that sequentially
directs the light from the remote sensor 16 along the
different paths. Alternatively, the scanning apparatus
may include a motor or other driving apparatus attached
to the remote sensor to turn it so that the telescope
portion thereof is essentially aligned with each of the
reflector elements. In the preferred embodiment in which
the reflector elements are cube corner retroreflectors, a
light beam transmitted to the retroreflector will be
reflected back to the remote sensor as long as the angle
of incidence is within 45 degrees of the perpendicular.
This allows a 45 degree tolerance in the alignment of the
retroreflector, thereby drastically reducing the time and
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effort required in conventional "open long path" systems
to align the equipment. Accordingly, the scanning
apparatus 116 allows a sampling of the atmosphere across
dlfferent optical paths across different portions o~ the
target area 10 to be made while utilizing a single remote
sensor 16 and a plurality of reflector elements 26, 26',
26''.
In another preferred embodiment of the present
invention, the remote sensor is adapted to transmit a
modulated infrared beam across the target area.
Referring to Figure 4, a remote sensor is depicted. The
remote sensor includes a combined optical arrangement and
an FTIR portion 120. The optical arrangement includes a
light source 122 that may be similar or identical to the
one disclosed in the previous embodiment. The optical
arrangement directs the beam 124 from the light source to
the interferometer module 126. In the interferometer
module 126, the beam is modulated. The now modulated
beam 128 is directed to the telescope portion 130 and is
directed out (e.g. 132) to one or more reflector elements
positioned across the target area, as described above.
The reflected modulated beam is 134 returned back into
the telescope portion 130 and through a beam splitter 136
which directs part 138 of the reflected modulated beam
132 into the detector module 140. The detector module
140 consists of a series of focusing mirrors and a
detector which measures the power or intensity of the
beam. As in the previous embodiment, the detector
provides an output (not shown) to a computer that
determines the presence and/or quantity (concentration)
of one or more gases in the target area. Also, as in the
previous embodiment, a laser 142 is provided for
alignment purposes (comparable to laser 110). In this
preferred embodiment, by transmitting a modulated
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2087439 1~'
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infrared beam, a substantial improvement in the signal to
noise ratio can be obtained. ~
As compared to prior bistatic open long path
systems, the present invention overcomes the problems of
alignment that are inherent in such systems. The ease of
alignment afforded by the use of retroreflectors readily
enables scans of the target area to be taken. Instead of
analyzing a portion of the target area and then
repositioning the apparatus at a different location, the
preferred embodiments allow target area portions to be
analyzed repetitively and/or sequentially without
relocating the transmission/receiving unit. This is
provided in part by the use of a single
transmission/receiving unit and a plurality of
retroreflector arrays. Since the preferred embodiment
utilizes a single transmission/receiving unit, the light
beam transmitted across the target area has only to be
reflected back along its path of incidence to be
collected and analyzed. The use of retroreflector arrays
in a preferred embodiment facilitates alignment.
The present invention possesses several
advantages over the '640 device. The present invention,
unlike the '640 device, is capable of detecting hundreds
of gases simultaneously, or nearly so, that have
absorption wavelengths within the infrared light region.
Also, because the present invention relies on
identification of an absorption spectrum for each gas,
the present invention eliminates ~he need for the
transmission of a separate reference beam required by the
method of the '640 patent.
It is intended that the foregoing detailed
description be regarded as illustrative rather than
limiting, and that it be understood that it is the
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following claims, including all equivalents, which are
intended to define the scope of the invention.
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