U.S. patent application number 12/468847 was filed with the patent office on 2009-10-15 for optical absorption spectrometer and method for measuring concentration of a substance.
This patent application is currently assigned to BAH Holdings LLC. Invention is credited to Michael Tkachuk.
Application Number | 20090257064 12/468847 |
Document ID | / |
Family ID | 40910172 |
Filed Date | 2009-10-15 |
United States Patent
Application |
20090257064 |
Kind Code |
A1 |
Tkachuk; Michael |
October 15, 2009 |
Optical Absorption Spectrometer and Method for Measuring
Concentration of a Substance
Abstract
An optical absorption spectrometer is provided for determining
the concentration of a substance within a sample. The optical
absorption spectrometer comprises a first radiation source for
supplying radiation to the sample to be measured; at least one
cavity for containing the sample during measurement; and a detector
assembly for detecting radiation transmitted along first and second
optical paths through the sample, the length of the first optical
path being greater than that of the second optical path.
Inventors: |
Tkachuk; Michael; (St.
Petersburg, RU) |
Correspondence
Address: |
NEAL, GERBER, & EISENBERG
SUITE 1700, 2 NORTH LASALLE STREET
CHICAGO
IL
60602
US
|
Assignee: |
BAH Holdings LLC
Glen Cove
NY
|
Family ID: |
40910172 |
Appl. No.: |
12/468847 |
Filed: |
May 19, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11670280 |
Feb 1, 2007 |
7570360 |
|
|
12468847 |
|
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Current U.S.
Class: |
356/453 ;
356/437 |
Current CPC
Class: |
G01N 2021/317 20130101;
G01N 21/31 20130101; G01J 3/42 20130101; G01N 21/0303 20130101;
G01J 3/457 20130101 |
Class at
Publication: |
356/453 ;
356/437 |
International
Class: |
G01J 3/45 20060101
G01J003/45; G01N 21/00 20060101 G01N021/00 |
Claims
1. An optical absorption spectrometer for determining the
concentration: of a substance within a sample, comprising: a first
radiation source for supplying radiation to the sample to be
measured; at least one cavity for containing the sample during
measurement; a detector assembly for simultaneously detecting
radiation transmitted along first and second optical paths through
the sample, the length of the first optical path within the sample
being greater than that of the second optical path within the
sample, whereby the radiation detector receives the sum of the
radiation transmitted along each of the first and second optical
paths, and outputs a radiation signal corresponding thereto; and a
processor adapted to generate a measurement of the concentration of
the substance in the sample from the radiation signal output by the
detector.
2. An optical absorption spectrometer according to claim 1 wherein
the at least one cavity has a dimension co-axial with the first
optical path which is greater than a dimension of the at least one
cavity co-axial with the second optical path.
3. An optical absorption spectrometer according to claim 2 wherein
the first and second optical paths are parallel, and the dimension
of the cavity parallel to the first and second optical paths varies
in a direction perpendicular to the axes of the first and second
optical paths.
4. An optical absorption spectrometer according to claim 3 wherein
the dimension of the cavity parallel to the first and second
optical paths varies in a step-wise manner.
5. An optical absorption spectrometer according to claim 3 wherein
the dimension of the cavity parallel to the first and second
optical paths varies continuously.
6. An optical absorption spectrometer according to claim 2 wherein
the cavity is defined within a chamber, the chamber comprising
cavity varying means for varying the cavity dimensions.
7. An optical absorption spectrometer according to claim 6 wherein
the cavity varying means comprise at least one chamber wall for
dividing the cavity from a sample-free volume.
8. An optical absorption spectrometer according to claim 7 wherein
the sample-free volume contains a radiation-transparent
material.
9. An optical absorption spectrometer according to claim 6 wherein
the cavity varying means comprise chamber walls for defining the
cavity therewithin, the chamber walls varying in thickness.
10. An optical absorption spectrometer according to claim 1 wherein
the cavity is provided with a first radiation guiding assembly for
guiding radiation along the first optical path, and a second
radiation guiding assembly for guiding radiation along the second
optical path.
11. An optical absorption spectrometer according to claim 10
wherein the first and second radiation guiding assemblies comprise
a plurality of reflective elements.
12. An optical absorption spectrometer according to claim 11
wherein the first and second radiation guiding assemblies further
comprise one or more beam splitters for dividing radiation between
the first and second optical paths.
13. An optical absorption. spectrometer according to claim 10
wherein the cavity dimensions are adjustable such that the length
of the first and/or second optical paths can be adjusted.
14. An optical absorption spectrometer according to claim 1 further
comprising a second radiation source for supplying radiation to the
sample to be measured, the first radiation source being arranged to
supply radiation along the first optical path, and the second
radiation source being arranged to supply radiation along the
second optical path.
15. An optical absorption spectrometer according to claim 1 wherein
the detector assembly comprises first and second detector elements
for detecting radiation transmitted through the sample, the first
detector element being arranged to detect radiation on the first
optical path, and the second detector element being arranged to
detect radiation on the second optical path.
16. An optical absorption spectrometer according to claim 1 adapted
for the detection of methane.
17. An optical absorption spectrometer according to claim 1 wherein
the optical absorption spectrometer is a correlated interference
polarization spectrometer and further comprises: a filter for
filtering radiation transmitted by the sample, the filter having a
number of pass bands at wavelengths corresponding to absorption
peaks in the absorption spectrum of the substance to be detected,
the filter being responsive to an applied signal to modulate the
wavelengths of the pass bands; wherein the detector assembly
comprises a processor for determining the difference in the minimum
and maximum intensities of the detected radiation to thereby
determine the concentration of the substance in the sample.
18. A method of determining the concentration of a substance within
a sample, comprising: transmitting radiation through the sample to
be measured along a first optical path and a second optical path,
the length of the first optical path within the sample being
greater than that of the second optical path within the sample;
filtering the radiation transmitted through the sample using a
filter having a number of pass bands at wavelengths corresponding
to absorption peaks in the absorption spectrum of the substance to
be detected, the filter being responsive to an applied signal to
modulate the wavelengths of the pass bands; simultaneously
detecting the sum of the filtered radiation on each of the first
and second optical paths to determine the difference in its maximum
and minimum intensities to thereby determine the concentration of
the substance in the sample.
19. An optical absorption spectrometer according to claim 5 wherein
the dimension of the cavity parallel to the first and second
optical paths varies linearly.
20. An optical absorption spectrometer according to claim 8 wherein
the radiation-transparent material is nitrogen.
21. An optical absorption spectrometer according to claim 11
wherein the plurality of reflective elements are selected from a
group consisting of mirrors and prisms.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical absorption
spectrometer for determining the concentration of a substance
within a sample, and in particular for the optical detection of
gases.
BACKGROUND OF THE INVENTION
[0002] A number of optical gas detection techniques exist and are
based on the measurement of the absorption of incident radiation by
the gas molecules. For example, non-dispersive infrared (NDIR)
spectroscopy involves using a detector to monitor radiation
transmitted by a sample when exposed to a radiation source. By
measuring the radiation absorbed by the sample within a particular
wavelength range, the concentration of a target gas in the sample
can be determined.
[0003] As explained in WO-A-01/65219 and WO-A-01/40748, another
known type of optical absorption gas detector is the correlated
interference polarisation spectrometer (CIPS). A CIPS works on the
principle that for any given wavelength incident radiation, the
quantity that is absorbed by the gas is a function of the
cross-section (.sigma.(.lamda.)) of any particular molecule of the
gas. If the dependence of the cross-section on the wavelength is
very pronounced, then the gas molecules will absorb radiation over
a very narrow waveband. Accordingly, the spectral displacement
between the maximum and minimum intensities of the transmitted
radiation is very small. By measuring the difference between the
maximum and minimum intensities of the transmitted radiation, it is
possible to calculate the concentration of the gas.
[0004] The CIPS uses the quasi- periodical structure of the
electronic absorption spectrum of the gas molecules, which occurs
due to their vibrational-rotational properties. FIG. 1 shows, as an
example, the absorption spectrum of methane in the vicinity of 3.25
micrometers. The spectrum consists of a number of very narrow
(approximately 1 nm wide) quasi-periodic absorption bands, labelled
2, that are detected by the CIPS.
[0005] In order to detect the quasi-periodic structure, the CIPS
filters radiation transmitted by the sample gas using a comb filter
which is generated by a controlled interference polarisation filter
(cIPF). The cIPF is formed from a modified interference
polarisation filter (IPF) which uses the phenomenon of
birefringence in certain crystals to obtain a transmission spectrum
which is characterised by a quasi-periodic sequence of spectral
passbands.
[0006] In order to be able to use the IPF in the detection of
gases, the IPF must provide a transmission spectrum that closely
matches the quasi-periodic absorption spectra of the gas to be
detected (i.e. the bandwidth between adjacent peaks in the
absorption spectrum of the gas to be detected must correspond to
the bandwidth between adjacent transmission peaks in the IPF
transmission spectrum). Furthermore, the IPF must be able to shift
this spectrum in time so that one can detect the intensity of the
radiation transmitted at both the absorption and non-absorption
bands of the absorption spectrum of the target gas. Various ways of
achieving this are disclosed in WO-A-01/65219.
[0007] It will be appreciated that, whilst this description will
generally refer to the detection of gases, the techniques and
apparatus disclosed herein could be used for the measurement of any
substance.
[0008] One problem frequently encountered in many types of optical
absorption spectrometer, including NDIR devices and CIPS apparatus,
is that the measurement range of the instrument is inherently
limited. For example, a CIPS of the sort described in WO-A-01/65219
and WO-A-01/4702 has a measurement range typically limited to three
or four orders of magnitude. For concentrations below its desired
range, the CIPS does not have the sensitivity (i.e. signal/noise
ratio) to measure the concentration accurately. For concentrations
above its designed range, the signal output by the CIPS saturates
and will not respond properly.
[0009] One factor which contributes to this problem is the design
of the cavity used to. contain the gas sample during test. For high
sensitivity, a long gas cavity having for example an optical path
between 20 cm and 30 cm is required in order to ensure that there
is a sufficient number of gas molecules in the chamber to achieve
detectable absorption at low gas concentration levels and obtain a
reasonable signal in the presence of noise. The disadvantage of
using a long optical path is that, in the presence of higher gas
concentrations, substantially all the radiation is absorbed by the
gas molecules before it can reach the detector (in accordance with
Beer's law). As such, it is not presently possible to use a single
apparatus for the accurate measurement of high gas concentrations
as well as low gas concentrations.
[0010] The problem is also made worse by the dependence of the
measured radiation signal on uncontrollable measurement conditions.
For example, factors such as dust occurring in the optical system
or radiation beam misalignment, etc., can decrease the incident
radiation intensity, leading to an erroneous concentration
measurement.
[0011] What is needed is an optical absorption spectrometer which
can operate across a wide range of target gas concentrations. It
would also be advantageous is the measurement signal were less
effected by changes in the lighting conditions.
SUMMARY OF INVENTION
[0012] In accordance with a first aspect of the invention, an
optical absorption spectrometer for determining the concentration
of a substance within a sample comprises
[0013] a first radiation source for supplying radiation to the
sample to be measured;
[0014] at least one cavity for containing the sample during
measurement; and
[0015] a detector assembly for detecting radiation transmitted
along first and second optical paths through the sample, the length
of the first optical path being greater than that of the second
optical path.
[0016] The first aspect of the invention further provides a method
of determining the concentration of the substance within the
sample, comprising transmitting radiation through the sample to be
measured along a first optical path and a second optical path, the
length of the first optical path being greater than that of the
second optical path;
[0017] filtering the radiation transmitted through the sample using
a filter having a number of pass bands at wavelengths corresponding
to absorption peaks in the absorption spectrum of the substance to
be detected, the filter being responsive to an applied signal to
modulate the wavelengths of the pass bands;
[0018] detecting the filtered radiation to determine the difference
in its maximum and minimum intensities to thereby determine the
concentration of the substance in the sample.
[0019] The provision of first and second optical paths in this way
enables the spectrometer to be used for accurate measurement of
both low and high gas concentrations. At low gas concentrations,
the longer optical path provides high sensitivity, whereas at high
concentrations the shorter optical path contains relatively few gas
molecules such that a measurement can still be taken. The signal
received by the detector assembly is effectively a superposition of
the radiation received along each optical path. There may of course
be any number of different optical paths through the sample,
provided there are at least two.
[0020] In a preferred embodiment, the dimension of the at least one
cavity coaxial with the first optical path is greater than the
dimension of the at least one cavity coaxial with the second
optical path. That is, it is the dimensions of the cavity itself
which are used to provide at least two alternative optical paths
through the sample of differing lengths. Conveniently, the first
and second optical paths are parallel to one another, although this
need not be the case.
[0021] Where the first and second optical paths are parallel, it is
preferred that the dimension of the cavity parallel to the first
and second optical paths varies in a direction perpendicular to the
axes of the first and second optical paths. In this way,
manufacture and control of the cavity dimensions is simplified
since the profile of the cavity need only vary across one of its
faces.
[0022] The variation of the cavity dimension could take any form,
e.g. curved, but preferably, in a first example, the dimension of
the cavity parallel to the first and second optical paths varies in
a stepwise manner. In other cases, it is preferable for the
dimension of the cavity parallel to the first and second optical
paths to vary continuously, preferably linearly.
[0023] Variation of the cavity dimensions can be achieved in
numerous ways. In one particularly preferred embodiment, the cavity
is defined within a chamber, the chamber comprising cavity varying
means for varying the cavity dimensions. In most cases, the cavity
dimensions are varied spatially, but could be varied temporally as
an alternative or in addition.
[0024] Advantageously, the cavity varying means comprise at least
one chamber wall for dividing the cavity from a sample-free volume.
In this way, the cavity can be readily defined with an appropriate
dimension profile and the sample excluded from the remainder of the
chamber. Preferably, the sample-free volume contains a radiation
transparent material, preferably nitrogen. Solid materials such as
transparent polymers are convenient alternatives.
[0025] In other examples, the cavity varying means advantageously
comprise chamber walls for defining the cavity therewithin, the
chamber walls varying in their thickness. This conveniently
provides a single `gas cell` component having a regular external
shape, if desired, yet a internal cavity of varying dimension. In
the locality of the optical paths, the chamber walls are preferably
radiation transparent.
[0026] In an alternative embodiment, the cavity is advantageously
provided with a first radiation guiding assembly for guiding
radiation along the first optical path, and a second radiation
guiding assembly for guiding radiation along the second optical
path.
[0027] In this way, a cavity of regular dimension can be employed
and adapted to provide the first and second optical paths by
appropriate arrangement of the guiding assemblies, which may
comprise components defining at least part of the cavity walls.
[0028] Conveniently, the first and second guiding assemblies
comprise a plurality of reflective elements, preferably mirrors or
prisms (and which may form part of the cavity walls). In some
examples, it is preferable that the first and second guiding
assemblies further comprise one or more beam splitters for dividing
radiation between the first and second optical paths.
[0029] In addition, it may be advantageous if the cavity dimensions
themselves are adjustable such that the length of the first and/or
second optical paths can be adjusted. This allows the spectrometer
to be tuned according to the gas concentration range
encountered.
[0030] In some embodiments, it is preferred that the optical
absorption spectrometer further comprises a second radiation source
for supplying radiation to the sample to be measured, the first
radiation source being arranged to supply radiation along the first
optical path, and the second radiation source being arranged to
supply radiation along the second optical path.
[0031] It may further be advantageous if the detector assembly
comprises first and second detector elements for detecting
radiation transmitted through the sample, the first detector
element being arranged to detect radiation on the first optical
path, and the second detector element being arranged to detect
radiation on the second optical path.
[0032] Either of these arrangements increases the number of
configurations, and therefore different path lengths achievable,
using the apparatus.
[0033] Preferably, the detector assembly further comprises a
processor adapted to generate a measurement of the concentration of
the substance in a sample from a radiation signal output by the
detector. Advantageous processing techniques which could be used
are discussed below.
[0034] In a particularly preferred embodiment, the optical
absorption spectrometer is adapted for the detection of methane. It
is especially preferred that the optical absorption spectrometer is
a correlated interference polarization spectrometer and further
comprises:
[0035] a filter for filtering radiation transmitted by the sample,
the filter having a number of pass bands at wavelengths
corresponding to absorption peaks in the absorption spectrum of the
substance to be detected, the filter being responsive to an applied
signal to modulate the wavelengths of the pass bands; wherein
[0036] the detector assembly comprises a processor for determining
the difference in the minimum and maximum intensities of the
detected radiation to thereby determine the concentration of the
substance in the sample.
[0037] In accordance with a second aspect of the invention, a
detector assembly for use in an optical absorption spectrometer for
determining the concentration of the substance within a sample
comprises:
[0038] a radiation detector for detecting radiation transmitted by
the sample to be measured and generating a signal in accordance
with the detected radiation; and
[0039] a processing circuit coupled to the radiation detector and
comprising averaging means adapted to generate a first signal
representing the average intensity of the detected radiation,
L.sub.AV, and comparison means adapted to generate a second signal
representing the relative intensities of the maxima and minima of
the detected radiation, L.sub.CIPS, wherein the processing circuit
is adapted to generate a normalised output signal, S.sub.NORM,
based on a ratio of the second signal to the first signal, to
thereby determine the concentration of the substance.
[0040] By outputting a normalised signal in this way, it has been
found that the concentration range across which the assembly can
generate accurate measurements is increased. Moreover, the
dependence of the concentration signal on incident radiation
intensity is reduced. In particular, unintentional reductions in
radiation intensity for reasons such as dust in the system and
radiation beam misalignment will not affect the normalised
value.
[0041] Nonetheless, for long radiation paths, the normalised signal
still suffers as a result of increased radiation absorption
(particularly at higher concentrations), and it has therefore been
found advantageous if the processing circuit is further adapted to
generate an average output signal, S.sub.AV, corresponding to the
first signal, L.sub.AV, to thereby determine the concentration of
the substance.
[0042] The average output signal S.sub.AV, represents the total
intensity of radiation transmitted through the sample. Where the
radiation is infrared, this corresponds to NDIR detection.
Therefore, the average output signal S.sub.AV provides a second way
of calculating the concentration of the substance in the sample.
Whilst the normalised CIPS output signal, S.sub.NORM, gives
accurate results at low concentrations, it has been found that this
average output signal S.sub.AV provides good measurement results at
higher concentrations.
[0043] Therefore, in a particularly preferred embodiment, the
processing circuit is adapted to generate the normalised output
signal S.sub.NORM when the concentration of the substance is below
a predetermined threshold, and to generate the average output
signal S.sub.AV when the concentration of the substance is above
the predetermined threshold.
[0044] In effect, the assembly can operate as a CIPS below the
threshold and as an NDIR unit (or similar) above the threshold.
Thus a broad concentration range can be measured.
[0045] Preferably, the predetermined threshold is defined as a
proportion of the maximum L.sub.CIPS signal, preferably 70% of the
maximum L.sub.CIPS signal. In other cases, the threshold could
simply be defined as a set concentration level. For example, using
the example of methane, the predetermined threshold may be 5%
concentration by volume.
[0046] Advantageously the optical absorption spectrometer comprises
a filter for filtering radiation transmitted by the sample, the
filter having a number of pass bands at wavelengths corresponding
to absorption peaks in the absorption spectrum of the substance to
be detected, the filter being responsive to an applied signal to
modulate the wavelengths of the pass bands, and the comparison
means of the processing circuit is responsive to the applied signal
to generate the second signal, L.sub.CIPS.
[0047] Preferably, the comparison means comprises a lock-in
amplifier referenced to the applied signal. Advantageously, the
comparison means further comprises a bandpass filter.
[0048] This aspect of the invention further provides a correlated
interference polarisation spectrometer for determining the
concentration of a substance within a sample, the spectrometer
comprising:
[0049] a radiation source for supplying radiation to the sample to
be measured;
[0050] a filter for filtering radiation transmitted by the sample,
the filter having a number of pass bands at wavelengths
corresponding to absorption peaks in the absorption spectrum of the
substance to be detected, the filter being responsive to an applied
signal to modulate the wavelengths of the pass bands;
[0051] a detector assembly comprising a radiation detector for
detecting the filtered radiation and generating a signal in
accordance with the detected radiation, and a processing circuit
coupled to the radiation detector, comprising averaging means
adapted to generate a first signal representing the average
intensity of the detected radiation, L.sub.AV, and comparison means
adapted to generate a second signal representing the relative
intensities of the maxima and minima of the detected radiation,
L.sub.CIPS, the processing circuit generating a normalised output
signal, S.sub.NORM, based on a ratio of the second signal to the
first signal, to thereby determine the concentration of the
substance.
[0052] Preferably, the sample to be measured is contained within a
cavity, at least one radiation path passing through the cavity. It
is further advantageous to provide first and second radiation paths
as specified by the first aspect of the invention in order to
further broaden the concentration range of the instrument.
[0053] Preferably, the at least one radiation path is 30 cm or less
in length, preferably 20 cm or less.
[0054] This aspect of the invention further provides a method of
determining the concentration of a substance within a sample,
comprising:
[0055] transmitting radiation through the sample to be
measured;
[0056] filtering the radiation transmitted through the sample using
a filter having a number of pass bands at wavelengths corresponding
to absorption peaks in the absorption spectrum of the substance to
be detected, the filter being responsive to an applied signal to
modulate the wavelengths of the pass bands;
[0057] detecting the filtered radiation;
[0058] processing the detected radiation to generate a first signal
representing the average intensity of the detected radiation,
L.sub.AV, and a second signal representing the relative intensities
of the maxima and minima of the detected radiation, L.sub.CIPS;
and
[0059] generating a normalised output signal, S.sub.NORM, based on
a ratio of the second signal to the first signal, to thereby
determine the concentration of the substance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] Examples of optical absorption spectrometers and methods for
measuring the concentration of a substance within a sample in
accordance with the present invention will now be described and
contrasted with conventional systems with reference to the
accompanying drawings, in which:
[0061] FIG. 1 is a graph showing the absorption spectrum of methane
around 3.25 microns;
[0062] FIG. 2 is a schematic diagram of an example of a known CIPS
apparatus;
[0063] FIG. 3 is a schematic diagram of a CIPS apparatus in
accordance with a first embodiment of the invention;
[0064] FIG. 4 shows a schematic diagram of a sample cavity
arrangement for use in an optical absorption spectrometer in
accordance with a second embodiment of the invention;
[0065] FIG. 5 shows a second sample cavity arrangement for use with
an optical absorption spectrometer according to a third embodiment
of the invention;
[0066] FIG. 6 shows a third sample cavity arrangement for use with
a optical absorption spectrometer in accordance with a fourth
embodiment of the invention;
[0067] FIG. 7 shows a fourth sample cavity arrangement for use with
a optical absorption spectrometer in accordance with a fifth
embodiment of the design;
[0068] FIG. 8 shows a fifth sample cavity arrangement for use with
an optical absorption spectrometer in accordance with a sixth
embodiment of the invention;
[0069] FIG. 9 shows a schematic circuit diagram forming part of a
detector assembly in accordance with a seventh embodiment of the
present invention;
[0070] FIG. 10 is a graph showing the output of a CIPS and its
dependency on concentration, using the example of methane;
[0071] FIG. 11 is a graph showing the average radiation intensity
signal and its dependence on methane concentration;
[0072] FIG. 12 is a graph showing the normalised output signal and
its dependency on methane concentration;
[0073] FIG. 13 is a graph showing the output of a CIPS having a
longer optical path, and its dependency on methane
concentration;
[0074] FIG. 14 is a graph showing the average radiation intensity
signal of the same system; and,
[0075] FIG. 15 is a graph showing the normalised output signal in
the same system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0076] An example of a conventional CIPS adapted to detect the
quasi-periodic structure of the gas molecules' absorption spectrum
is shown in FIG. 2.
[0077] This type of spectrometer can be operated either as an
active device using an emitter, such as an incandescent lamp, or as
a passive device, using the sun as an emitter. The design shown in
FIG. 2 is for an active CIPS system.
[0078] The CIPS includes a radiation source 11 such as an LED, a
laser or the like positioned next to a collimator 12. The
collimator 12 operates to convert the incident radiation into a
collinear beam which then passes through a cavity 13. The cavity 13
contains the gas to be detected and includes an input aperture 13A
and an output aperture 13B. Radiation transmitted through the
cavity 13 is then filtered using a narrow band filter 14 which
allows only wavelengths in the vicinity of the desired absorption
spectra to pass. Thus for example to detect methane then the
bandpass filter would allow radiation in the 3.25 micrometer region
to pass.
[0079] Following the filter 14 is a controlled interference
polarisation filter (cIPF) section 15. The cIPF 15 generates a
transmission spectrum that consists of very narrow pass bands
coinciding with the quasi- periodic absorption spectrum of the
target gas. The cIPF 15 is controlled to cause the transmission
spectrum to be scanned back and forth within the selected working
range. The operation of the cIPF 15 is controlled by an alternating
voltage supply 17 which guides the cIPF and the detection system.
The detection system consists of a detector 16 coupled to an
amplifier 18 which operates to amplify a signal detected by the
detector 16. The output of the amplifier 18 is coupled to a
microprocessor 19 and is also coupled to the alternating voltage
supply 17 so as to synchronise the processing of the detected
signal with the operation of the cIPF 15. The signals obtained from
the microprocessor 19 are then output to a display 20 which
displays an indication of the concentration of the detected
gas.
[0080] Any form of IPF such as a SOLC or Lyot filter can be used.
However, it is preferred to use either a Woods filter including
input and output polarisers and an etalon formed from a
birefringent crystal, as described in WO-A-01/40748, or a
photoelastic modulator of polarisation (PMP) as described in
WO-A-01/65219.
[0081] As explained in the aforementioned publications, the
transmission spectrum of the cIPF 15 includes a number of
transmission peaks such that it acts as a comb filter. By
appropriate choice of components, the transmission peaks can be
selected to correspond to absorption peaks in the
vibrational-rotational spectrum of the target gas. Accordingly,
with the transmission and absorption peaks aligned, the radiation
transmitted by the cIPF 15 will be the radiation having wavelengths
which are absorbed by the target gas. The state of the cIPF can
then be changed, for example by applying a stress, causing its
transmission spectrum to alter. Accordingly, the comb filter
effectively scans from left to right and then back again across the
wavelength spectrum.
[0082] When the transmission peaks are aligned with troughs in the
vibrational-rotational spectrum then the radiation transmitted by
the cIPF 15 represents wavelengths which are transmitted by the
target gas. This allows the maximum and minimum transmission
intensities in the absorption spectra of the gas to be determined
(i.e. the height of the peaks can be determined). Thus by choosing
an appropriate amplitude driving signal, the cIPF 15 can act as a
modulator, scanning the vibrational-rotational spectrum of the
sample gas from one maxima to the next and back again.
[0083] To measure the concentration of gas, the microprocessor 19
is adapted to determine the intensity of radiation received at the
detector 16 at the time when the peaks of the cIPF transmission
spectrum are aligned with the maxima of the vibrational-rotational
spectrum of the target gas, and then at the time when the peaks of
the cIPF transmitter spectrum are aligned with the troughs of the
target gas spectrum. By correlating this information with an
indication of the relative transmission intensities of the cIPF,
this allows the microprocessor 19 to determine the relative
intensities and the spectral displacement of the peaks and troughs
in the vibration or rotational spectrum. This in turn allows
identification of the target gas, and the measurement of its
concentration, to be achieved.
[0084] However, as already described, in common with other types of
optical absorption spectrometers, CIPS of the type shown in FIG. 2
suffer from the limitation that they can only take concentration
measurements over a narrow range of concentrations, typically
limited to three or four orders of magnitude. For high sensitivity
measurements, the design of the spectrometer requires a long gas
cavity 13 having typically an optical path of between 20 and 30 cm,
potentially through the use of multi-pass chambers. It is necessary
to increase the absolute number of absorbing molecules in order to
get sufficient absorption at very low gas concentration levels to
obtain a reasonable signal in the presence of noise.
[0085] The disadvantage of using a long optical path through the
sample cavity 13 is that in the presence of higher gas
concentrations, essentially all of the radiation is absorbed by the
gas molecules. This reduces the flexibility of the system to
simultaneously detect both very low gas concentrations and high gas
concentrations.
[0086] FIG. 3 shows a first embodiment of the present invention,
which is a CIPS having a modified gas cavity 23. The remaining
components shown in FIG. 3 largely correspond to the FIG. 2
example.
[0087] A radiation source 21 such as a lamp, LED or laser,
generates a radiation beam which is collimated by collimator 22 to
pass through cavity 23 containing gas G which is the sample to be
measured. Radiation transmitted by the gas G passes to the
controlled interference polarization filter (cIPF) section 25.
Here, the cIPF is shown to comprise two polariser elements 25a and
25e either side of an etalon 25b and a crystal 25c which exhibits
induced birefringence. The birefringence is induced by a
piezoelectric driver 25d supplied from an AC voltage 27 and
synchronised with the processor 29.
[0088] In this example, the last component of cIPF 25 is a bandpass
filter 24 which corresponds in function to filter 14 shown in FIG.
2.
[0089] Radiation transmitted by the cIPF 25 is detected by detector
assembly 31 which comprises a photodetector 26, amplifying means 28
and processor 29. In this example, the amplifying means 28 is shown
to produce two outputs as will be described in further detail
below. However, a single amplifier such as component 18 shown in
FIG. 2 could be used instead. As in the prior art system, processor
29 can be adapted to detect the difference in intensities between
the peaks and troughs and thereby generate a concentration
measurement which is output on display 30.
[0090] Gas cavity 23 differs from prior art systems in that, due to
its shape, radiation passing through the cavity at one position
will traverse a path through the gas sample G which differs in
length to that traversed by radiation passing through the cavity 23
at a different position. In particular, in the example of FIG. 3,
the cavity 23 has a triangular cross-section which is arranged such
that, in the direction of radiation propagation, the dimension of
the cavity is greater towards its upper surface (as shown in the
diagram) than at its lower extremity. A first exemplary optical
path through the cavity adjacent its top surface therefore has a
greater length than a second exemplary optical path passing through
the cavity close to its lower extremity. The longer path provides
for high sensitivity measurements, and the short path provides for
measurement at high concentrations. The radiation received by the
detector 26 via the cIPF 25 is essentially a superposition of
Beer's law for more than one path.
[0091] A simpler example of a suitable gas cavity used in a second
embodiment of the invention is shown in FIG. 4. Here, the cavity 32
is defined between walls 33a and 33b of a chamber which comprises
means for varying the cavity dimensions in the form of a partition
34 dividing off a volume containing radiation-transparent material
T. The walls 33a and 33b and the partition 34 are all substantially
transparent to the radiation, at least in the vicinity of the
radiation beam(s) passing therethrough. The radiation transparent
material T may be a solid material (e.g. it could be a continuation
of the wall 33B or the partition 34), or it could be an inert gas
such as nitrogen, i.e. one which does not absorb the radiation. The
presence of volume T effectively causes the cavity dimension
parallel to the radiation beam to vary along a perpendicular
direction in a stepwise manner.
[0092] For clarity, FIG. 4 does not show the surrounding components
of the spectrometer (which could be a CIPS or NDIR spectrometer,
for example), and instead simply illustrates a radiation source 35
and detector 38 together with lenses 36 and 37 which provide a
collimated radiation beam through the sample gas G and directly
transmits radiation on to detector 38.
[0093] Three exemplary radiation rays, AA', BB', and CC' are
illustrated. The uppermost rays, AA' and BB' pass through the upper
portion of the cavity 32 whereas the lower radiation ray CC' passes
through the lower portion of the cavity 32 as well as the partition
34 and the radiation transparent material T. The dimension of the
cavity 32 coaxial with radiation ray AA' is greater than the
dimension of the cavity coaxial with the radiation ray CC'. Thus
the radiation on path AA' may be said to follow an optical path
through the sample G which is longer than that on a second optical
path CC'. Radiation on the first optical path AA' traverses a
distance L, through the gas sample whereas radiation on the optical
path CC' traverses a distance L2 through the gas sample before it
reaches the partition 34 and is then passed to the detector 38
without encountering any further gas molecules.
[0094] The radiation detected by the photodetector 38 is the sum of
the radiation received along each path. The long path AA' provides
for high sensitivity and can measure low gas concentrations since
sufficient gas molecules are present along the path to cause enough
absorption to be detected. The short path CC', on the other hand,
provides for the detection of high gas concentrations since the
relatively small number of molecules which can be accommodated
across the distance L.sub.2 means that some radiation still arrives
at the detector 38.
[0095] The radiation detected by the photodetector 38 is a
superposition of Beer's law for each path. Taking the two paths AA'
and CC', the detected radiation is:
I=I.sub.1 exp(-aCL.sub.1)+I.sub.2 exp(-aCL.sub.2)
where a is an extinction coefficient, C is the concentration of the
gas and I.sub.1 and I.sub.2 are the intensities of the radiation
passing along optical paths L.sub.1 and L.sub.2 respectively. Since
L.sub.1 is greater than L.sub.2, at low gas concentrations the
first term is dominant, whereas at high gas concentrations, the
second term dominates the resultant radiation.
[0096] In another example, the cavity of FIG. 4 could be used
without the partitioned volume T, eg by providing two radiation
sources and two detectors, and arranging the two optical paths to
pass through the cavity in perpendicular directions, one crossing
the long dimension of the cavity and the other the short.
[0097] FIG. 5 shows another cavity for use in a third embodiment of
an absorption spectrometer. As in the embodiment of FIG. 3, here
the cavity 40 has a triangular cross-section.
[0098] The cavity 40 is defined within a chamber having walls 41a
and b and a partition 42 creating a sample-free volume of radiation
transparent material T. As in the example of FIG. 4, any radiation
transparent material could be used, but nitrogen is particularly
preferred. The partition 40 could be fixed or could be movable by a
user such that the dimensions of the cavity 40 could be adjusted
according to the measurement conditions.
[0099] As in FIG. 4, the three radiation rays AA', BB' and CC' are
depicted. In this example, each of the paths traverses a different
distance through the sample gas G since the dimension of the gas
cavity 40 varies continuously from top to bottom. In this case, the
variation is linear whereas in other cases any other function e.g.
curved, sinusoidal or parabolic could be used instead.
[0100] Again, this arrangement allows for the sensitive detection
of low gas concentrations via the longer optical paths AA' and BB',
and for detection of high gas concentrations via the shorter
optical path CC'. The radiation detected by the photodetector 46
can be approximated by the superposition of Beer's Law for each of
the three paths:
I=I.sub.1 exp(-aCL.sub.1)+I.sub.2 exp(-aCL.sub.2)+I.sub.3
exp(-aCL.sub.3)
where a is an extinction coefficient, C is the concentration of the
gas, and I.sub.1 to I.sub.3 are the intensities of radiation on
optical paths AA', BB', and CC' through the sample having optical
path lengths of L.sub.1 to L.sub.3 respectively.
[0101] In practice, there are of course an infinite number of light
paths AA', BB' etc passing though the cavity, which make up the
radiation beam. The cavity dimension changes continuously across
the beam's cross section. Hence, the total radiation detected by
the photodetector 46 can be more accurately represented by:
I = i I i exp ( - aCL i ) ##EQU00001##
where I.sub.i, is the intensity of the light on optical path i,
having a path length L.sub.i through the cavity, and a and C are as
previously defined. The optical paths i are defined between the two
outermost paths, M' and CC'.
[0102] FIG. 6 shows a fourth embodiment of a cavity 50 containing
the gas sample G. In this case, the cavity 50 itself follows a
convoluted path between radiation-transparent end plates 50a and
50b. At each corner, a reflective element 51a to 51d is provided to
guide radiation along the cavity. The reflective elements could
comprise mirrors or prisms, and could be integral with the walls of
the cavity 50.
[0103] A first radiation source 52 is provided adjacent the first
end 50A of the cavity 50. Radiation from the source 52 passes
through a collimating lens 53 into the cavity 50 where it is guided
along the convoluted length of the cavity 50 by reflective elements
51a to d toward end plate 50b where radiation is focused by lens 56
onto detector 57. This constitutes a first optical path through the
gas sample G.
[0104] A second, shorter optical path is configured by providing a
transparent window in reflective element 51d through which
radiation from a second radiation source 54, collimated by lens 55,
enters the gas cavity 50. Having entered the gas cavity 50 a
significant distance after the radiation from the first radiation
source 52, radiation on the second optical path has a relatively
short distance to travel before exiting the cavity via plate 50b
onto detector 57.
[0105] Optionally, the cavity dimensions can be made adjustable in
which case end plate 50b may be movable along the axis of the
cavity to change the length of the two optical paths. This is
particularly useful for very high concentration gas measurements
since the second optical path can be made very short. Reference
numeral 50c in FIG. 6 denotes the end plate moved to a position in
which the optical paths are shortened.
[0106] As in the previous embodiments, here the detected radiation
is a sum of that received from each optical path. It will be noted
that the embodiment of FIG. 6 differs from the first three
embodiments in that, in the main, the different optical path
lengths are achieved not by varying dimensions of the cavity but by
arranging for radiation to be guided along two separate paths
through the cavity. In this way, reflective elements 51a to d can
be thought of as constituting a first radiation guiding assembly
for passing radiation along the first optical path, and the
transparent mirror 51d alone as constituting a second radiation
guiding assembly passing radiation along the second optical path
through the cavity 50.
[0107] FIG. 7 illustrates a fifth embodiment which works on similar
principles. Here, the gas cavity 60 comprises a main section 60a
and, optionally, an extension section 60b. Like the FIG. 6
embodiment, the cavity 60a follows a convoluted path. The radiation
entering the cavity from radiation source 63 via collimator lens 64
and cavity end wall 62a is directed around the corners of the
cavity by reflective elements 61a to f. Radiation reflected along
the entire length of the cavity 60a follows the longest path before
exiting the cavity via end wall 62b and focusing lens 66 on to
detector 65. The reflective elements 61a to f make up a first
radiation guiding assembly.
[0108] To provide a second, shorter, optical path through cavity
60a, reflective elements 61b and 61e are half-silvered or provided
with a transparent window. Thus, when the radiation reaches
reflective element 61b, the beam is split such that one portion
follows the first optical path whereas a second portion passes
through the reflective element 61b and reenters the cavity through
reflective element 61e. The beam that rejoins the first optical
path and is reflected by element 61f into detector 65. The second
radiation guiding assembly comprises elements 61a, 61b, 61e and
61f. Thus, this second optical path is shorter than the first by
virtue of cutting out the central "bend" of the cavity. These two
optical paths can be used to detect high and low gas concentrations
as previously described.
[0109] In some cases, it may be advantageous to include one or more
further optical paths for higher accuracy. This could be achieved
by making more elements of the radiation guiding arrangement
semi-transparent, for example element 61c, so that radiation can
exit at different points along the cavity. Alternatively, an
extension cavity 60b can be provided which receives radiation from
one such semi-transparent element, 61f, passes it through a further
sample of gas (using additional reflective elements 61g and 61h)
before reaching a second detector 67 through focusing lens 68. The
output from detectors 65 and 67 can be used in conjunction to
determine the total radiation transmitted by the sample as a whole.
The cavities 60A and 60B may or may not be in direct fluid
communication with one another.
[0110] By knowledge of the distance each optical path travels
within the cavity 60a and, where used, 60b, similar equations to
those mentioned above can be used to deduce the gas
concentration.
[0111] FIG. 8 shows a sixth embodiment which uses a cavity 70
having a comparatively regular shape. The cavity 70 is defined by
end plates 71a and 71b together with walls having an at least
partially reflective internal surface. Radiation from a source 72
passes via a collimator lens 73 into the cavity 70 through first
end plate 71a. Radiation is reflected off each internal wall of the
cavity, undergoing multiple passes of the cavity's width, until it
reaches end plate 71b where it exits the cavity via focussing lens
76 to detector 77. This constitutes a first optical path through
the cavity 70, and the internal reflective walls constitute a first
radiation guiding arrangement.
[0112] To establish a second radiation path through the cavity 70,
reflective elements 74 and 75 are disposed within the cavity 70
adjacent to the first end wall 71a and second end wall 71b
respectively. Reflective element 74 diverts a portion of the
radiation received from source 72 such that it bypasses the
internal walls of the cavity 70 and strikes reflective element 75
which guides the radiation directly out of the cavity through wall
71b and toward detector 77. This second optical path in the cavity
is therefore significantly shorter than the first. Reflective
elements 74 and 75 make up the second radiation guiding
assembly.
[0113] As before, the radiation detected at detector 77 is the sum
transmitted along each of the optical paths.
[0114] Each of the above embodiments can be used in otherwise
conventional optical absorption spectrometers such as the CIPS
shown in FIG. 2 or in NDIR systems. In a CIPS system, further
advantages can be achieved by replacing the conventional detected
signal algorithm with a detector assembly operating on the
principle schematically illustrated in FIG. 9. It should be noted
that this detector assembly can also be used in CIPS assemblies
employing conventional gas cavities.
[0115] In a CIPS, as described above, radiation transmitted by the
moving comb filter is received by a detector 81 (FIG. 9). In
conventional systems, this signal is amplified and synchronised to
the modulator generator signal resulting in a sinusoidal output
signal whose amplitude is related to the difference in intensity
between the maximum and minimum of the vibrational-rotational gas
spectrum. This signal is hereinafter referred to as
"L.sub.CIPS".
[0116] FIG. 10 is a graph showing the typical response of a
conventional L.sub.CIPS signal to changes in gas concentration
(here using the example of methane as the substance under test). It
will be seen that at low concentrations the difference between the
absorption band and the transmission band signals increases with
the concentration of the gas. For higher concentrations, the
absorption band signal saturates and the difference is no longer
increased. For still higher concentrations the overall transmission
band signal begins to drop, making it difficult to produce an
accurate measurement.
[0117] Using the detection assembly proposed according to a seventh
embodiment of the invention, as shown in FIG. 9, in addition to
generating the L.sub.CIPS (using band pass filter 83 and lock in
amplifier 84 to synchronise with control signal 85), the average
radiation intensity, L.sub.AV, transmitted by the sample is also
calculated. Radiation incident on detector 81 is amplified by
amplifier 82 and split into two signals by an arrangement of
resistors R1 and R2 and capacitors C1 and C2. The configuration of
resistor R2 and the capacitor C1 constitutes an averaging circuit
which averages the signal received from the comb filter over time.
To achieve this, the components should meet the following
condition:
R 2 C 1 >> 1 2 .pi. f 0 ##EQU00002##
where f.sub.0 is the frequency of the CIPS signal modulation on
line 85 (FIG. 9). The average radiation intensity signal L.sub.AV
follows Beer's law. Its dependence on concentration is shown in
FIG. 11, again using the example of methane.
[0118] By dividing the modulated CIPS signal L.sub.CIPS by the
average photodetector output L.sub.AV, the CIPS signal is
normalised ("S.sub.NORM"). This is depicted in FIG. 12. As is
evident from FIG. 12, the normalised output signal S.sub.NORM
demonstrates a good signal response verses concentration over a
very wide range. Moreover, as well as improving the concentration
range, the use of this normalised signal also reduces the
dependence of the output on the intensity of incident radiation.
For example, any changes in the output of the radiation source or
residual factors such as dust in the optical system will not effect
the value of the normalised signal.
[0119] Such detection techniques have been found to be particularly
useful for optical absorption spectrometers with relatively short
radiation paths no more than about 20 cm. In more sensitive
analysers (1 ppm and better) with longer radiation paths (30 cm and
more) the effects of absorption are still so significant that at
high concentrations it is difficult to obtain an accurate
measurement.
[0120] To address this, the detector assembly shown in FIG. 9
preferably provides two outputs: the normalised CIPS signal,
S.sub.NORM, and an average photodetector output S.sub.AV which is
derived directly from the average photodetector signal L.sub.AV
described above.
[0121] FIG. 13 shows the modulated photodetector output L.sub.CIPS
signal for a conventional CIPS apparatus having a long optical path
(35 cm in this example). It will be seen that the signal drops
sharply for concentrations greater than approximately 20%. FIG. 14
shows the average photodetector output for the same CIPS apparatus.
As before, this follows Beer's Law.
[0122] FIG. 15 shows the normalised photodetector output S.sub.NORM
for the same apparatus having a 35 cm optical path. At gas
concentrations higher than approximately 50%, it is clear that the
signal becomes saturated and cannot be used for accurate
measurements of such gas concentrations. However, the present
inventors have recognised that the average photodetector output
signal S.sub.AV shows significant concentration dependence in this
region and can therefore be used to provide a measurement of gas
concentration with reasonable accuracy (about 2%). The average
photodetector output signal S.sub.AV corresponds to the signal
which would be obtained using NDIR technology (assuming the
apparatus utilises infra red radiation). Effectively, the CIPS
apparatus can be used as an NDIR unit for the measurement of high
gas concentrations.
[0123] It is therefore proposed to provide the detector assembly
with additional processing for outputting the S.sub.NORM signal
when the detected gas concentration is below a predetermined
threshold, and output the average photodetector signal S.sub.AV
when the detected gas concentration exceeds that threshold. It may
also be advantageous to output both signals across an intermediate
range of concentrations. For example, the processor may output the
S.sub.NORM signal when the gas concentration is below a first
threshold, then output both the S.sub.NORM and S.sub.AV signals
between the first threshold and a second, higher threshold, and
output only the S.sub.AV signal above the second threshold.
[0124] The value of the predetermined threshold can be selected
according to the measurements being undertaken. Typically, taking
methane as an example, the normalised CIPS output (S.sub.NORM) is
accurate in the approximate range 0 to 5% vol (100% lower explosion
limit of methane), and the S.sub.AV output can be used for gas
concentrations from 5% to 100% vol. Alternatively, the threshold
value could be linked to a proportion of the maximum L.sub.CIPS
signal. For example, referring to FIG. 13, if the predetermined
threshold is defined as 70% of the maximum L.sub.CIPS signal
(approximately 1.8V), this corresponds to a methane concentration
of approximately 4% volume.
[0125] These detection algorithms can be used in combination with a
conventional gas cavity, but gas cavities providing first and
second optical paths, as described with reference to the first to
sixth embodiments are preferred in order to obtain maximum
benefits.
* * * * *