WO1996029582A1 - Procede de mesure de la concentration de gaz par interferometrie a correlation - Google Patents
Procede de mesure de la concentration de gaz par interferometrie a correlationInfo
- Publication number
- WO1996029582A1 WO1996029582A1 PCT/CA1996/000162 CA9600162W WO9629582A1 WO 1996029582 A1 WO1996029582 A1 WO 1996029582A1 CA 9600162 W CA9600162 W CA 9600162W WO 9629582 A1 WO9629582 A1 WO 9629582A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- gas
- interferometric
- concentration
- target
- spectral region
- Prior art date
Links
- 238000000034 method Methods 0.000 title claims abstract description 38
- 238000005259 measurement Methods 0.000 title claims abstract description 20
- 238000005305 interferometry Methods 0.000 title claims abstract description 14
- 230000003595 spectral effect Effects 0.000 claims abstract description 25
- 238000001228 spectrum Methods 0.000 claims abstract description 12
- 230000035945 sensitivity Effects 0.000 claims abstract description 11
- 230000001419 dependent effect Effects 0.000 claims abstract description 8
- 238000005070 sampling Methods 0.000 claims abstract description 5
- 230000005855 radiation Effects 0.000 claims abstract description 3
- 238000001914 filtration Methods 0.000 claims description 9
- 238000000862 absorption spectrum Methods 0.000 claims description 7
- 230000002452 interceptive effect Effects 0.000 claims description 4
- 239000007789 gas Substances 0.000 abstract description 57
- 238000004611 spectroscopical analysis Methods 0.000 abstract description 5
- 238000010521 absorption reaction Methods 0.000 description 8
- 238000009615 fourier-transform spectroscopy Methods 0.000 description 3
- 230000000737 periodic effect Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 238000004868 gas analysis Methods 0.000 description 2
- 230000014509 gene expression Effects 0.000 description 2
- 238000012544 monitoring process Methods 0.000 description 2
- 230000010287 polarization Effects 0.000 description 2
- 241000894007 species Species 0.000 description 2
- 238000004847 absorption spectroscopy Methods 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- BJQHLKABXJIVAM-UHFFFAOYSA-N bis(2-ethylhexyl) phthalate Chemical compound CCCCC(CC)COC(=O)C1=CC=CC=C1C(=O)OCC(CC)CCCC BJQHLKABXJIVAM-UHFFFAOYSA-N 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000007405 data analysis Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
Definitions
- the present invention relates to methods of measurements of gas constituents of the atmosphere and environmental monitoring, and is particularly concerned with methods of gas concentration measurements in gas media by absorption spectroscopy method.
- the problem addressed here is to determine the path integrated concentration of a specific gas along a given line of sight.
- a specific example of an application of this technique is in open path monitoring, where the average concentration of the gas is inferred from this path integrated concentration by knowing the path length between a transmitter and receiver. The absorption of the gas is measured by either a spectrograph, spectrometer, or an interferometer.
- a typical way of determining the path integrated concentration using interferometric instrumentation is by doing Fourier transform spectroscopy. As described by John Meaburn in Detection and Spectrometry of Faint Light, Dordrecht/Boston, 1976, Chapter 9, (prototype), where the absorption spectra of the gas is fit to the data using a numerical analysis technique such a least squares algorithm. This involves taking an interferogram with an interferometer, which in this example, is of a Michelson type design. The response of an interferometer, E( ⁇ ), to an absorption in the spectral domain, e(v), is given as follows:
- Michelson interferometers split light into two beams with differing path lengths, and then recombine the beams to determine how they interfere.
- ⁇ is proportional to the path length difference between the two paths that the light travels.
- the interferogram is a plot of light intensity as a function of path length difference.
- An interferogram may be converted into a conventional absorption spectrum (intensity as a function of frequency) by applying an inverse Fourier transform to the data.
- an inverse Fourier transform to the data.
- This technique is ideally suited to the identification of gas species in an unknown sample due to its resolution of spectral features, but may be poorly suited to gas measurement due to its lack of sensitivity.
- An object of the present invention is to provide an improved method of gas concentration measurement by correlation interferometry.
- the present invention optimizes the sensitivity of gas concentration measurements of a given species present in a gaseous medium without greatly sacrificing selectivity.
- a method of gas concentration measurements by correlation interferometry where an interferometric response, E( ⁇ j), to an absorption spectrum, e(v), of a target gas in a gas media is expressed as
- a method of correlation interferometry analyzing only radiation in a predetermined spectral region where target gasses absorb to determine concentrations comprising the steps of passing light through an investigated gas to determine concentrations of target gasses therein, filtering the light to for a spectral region, collecting data in the spectral region in an interferometric domain, and analyzing the data to determine concentration of a target gas, an interferometric response being functionally dependent on the concentration of the target gas.
- a method of gas concentration measurement by correlation interometry comprising the steps of passing light through a gas to be analyzed, bandpass filtering the light in a spectral domain to provide a predetermined spectral region, sampling the predetermined spectral region in an interferometric domain to collect data, generating an interferogram from the collected data, and comparing the interferogram to predetermined interferograms to determine gas concentration for a target gas.
- the present invention allows one to increase the sensitivity of measurements of the concentrations of target gases with known spectrum contained in the investigated gas medium without greatly sacrificing selectivity, while sampling over only a subset of intervals required for Fourier spectroscopy.
- the present invention measurements of gas concentration are made using a correlation interferometry method. Utilizing this technique limits the spectral range to the absorption of interest with a band pass filter, and only samples a subset of the interferogram range required for Fourier spectroscopy. Data analysis is performed exclusively in the interferometric domain without performing a Fourier transform.
- the selection criteria for points in the interferogram, d has the advantage that the measured intensity is strongly dependent on the concentration of the target gas in the observation region.
- the selectivity (ability to differentiate between a target and non-target gasses) of the correlation interferometry method will be poorer than with the Fourier transform method.
- the selectivity of the correlation interferometry method is optimized by selecting an appropriate spectral region with the filter, that is the spectral region where the gas of interest absorbs, and where the resulting interferometric region least affected by the presence of non-target gasses.
- Fig. 1 illustrates,, in a flow chart, the method of correlation interferometry in accordance with an embodiment of the present invention
- Fig. 2a graphically illustrates a mathematical representation of spectral regions of two gasses in accordance with an embodiment of the present invention
- Fig. 2b graphically illustrates a mathematical representation of interferometric region for the two gasses of Fig. 2a in accordance with an embodiment of the present invention
- Fig. 3a graphically illustrates an exemplary representation of spectral regions of two gasses in accordance with an embodiment of the present invention
- the apparatus of Fig. 2 the apparatus of Fig. 2;
- Fig. 3b graphically illustrates an exemplary representation of interferometric region for the two gasses of Fig. 3a in accordance with an embodiment of the present invention.
- Fig. 4 illustrates in a block diagram an example of an gas analysis set ⁇ up for implementing the method of the present invention.
- f c The Nyquist critical frequency
- the technique of correlation interferometry involves first filtering the incoming light in the spectral domain with a bandpass filter, predetermined for a target gas, as represented by a block 2. Then, sampling the intensity in the resulting interferometric domain, where strong modulation is produced by the presence of the target gas, as represented by a block 4. Generating an interferogram for the target gas, as represented by block 6. Finally, correlating the interferogram for the target gas to a predetermined interferogram to determine gas concentration, as represented by block 8.
- ⁇ is a path length difference in the interferometer
- ⁇ is a calibration coefficient
- 1 is the path length through the gas.
- the method of Fig. 1 automatically eliminates the influence of any gas that does not have an interferometric signature in this region of the interferogram or has a spectral absorption outside the range of the filter, even though only a small number of data points are taken.
- the selectivity of the instrument, operated in accordance with this method while not as good as Fourier spectroscopy, can be optimized by an appropriate selection of data point positions.
- the advantage of this method is sensitivity. Because nearly all of the observation time is spent in the areas of the interferogram that are dependent on the presence of the target gas, as opposed to spending time at areas where there is no interferometric signature, or where the signature of the filter is present, a greatly enhanced signal to noise ratio is obtained.
- this method can be visualized as characterizing the target gas entirely in terms of its center wavelength and its periodicity.
- the chosen filter selects gasses that absorb only in the predetermined range. While tuning of the interferometer selects only gasses which have the desired periodicity. Because all of the measurement time is spent characterizing the gasses by these criteria and is not spent measuring relatively unimportant characteristics such as the filter function, the sensitivity is optimized.
- Fig. 2a there is graphically illustrated a mathematical representation of spectral regions of two gasses in accordance with an embodiment of the present invention.
- the reason both sensitivity and selectivity can be optimized is illustrated in the following example.
- a measured gas medium contains two gases: a target gas and interfering gas.
- the target gas absorption spectrum, P ⁇ (v) can be simulated by the function:
- v ⁇ and v 2 are centers of the gas absorption bands, ⁇ , and ⁇ 2 are parameters which define absorption band widths, ⁇ ] and ⁇ 2 determine the spacing of the periodic absorption features which normally occur in molecular spectra.
- Fig. 2b there is graphically illustrated a mathematical representation of interferometric region for the two gasses of Fig. 2a in accordance with an embodiment of the present invention.
- the output of an interferometer measuring these gasses are cosine Fourier transforms of the corresponding functions P ⁇ v) and P 2 (.v). Using known properties of the cosine Fourier transform, as described in: Korn G., Korn T., Mathematics
- the first terms of these expressions are Fourier spectra of the periodical structures of the gas spectra; the second terms are due to the filter and are independent of the gas concentration.
- Fig. 2b shows plots of these functions.
- the structure due to the filter is similar in both plots, but the structure due to the gas is displaced along the ⁇ axis a distance dependent on the spacing of the gasses periodical structure ( ⁇ j and ⁇ 2 respectively).
- ⁇ j and ⁇ 2 respectively the spacing of the gasses periodical structure
- Usually different gasses have different periodical structures even though they may overlap spectrally. This means that this method is selective because the filter provides discrimination in the spectral domain, and d e interferometer provides discrimination by the periodicity of the features.
- Fig. 3a graphically illustrates an exemplary representation of spectral regions of two gasses in accordance with an embodiment of the present invention.
- Fig. 3a shows absorption spectrum for SOj and CHOH, which overlap quite strongly, but has different periodic structures for
- FIG. 3b there is graphically illustrated, an exemplary representation of interferometric region for the two gasses of Fig. 3a in accordance with an embodiment of the present invention.
- Fig. 3b illustrates part of their corresponding Fourier spectra, found by means of ultraviolet Fourier spectrometer based on the modulated interference polarization filter with the birefringent wedge of the above incorporated copending application. The differing periodic structure of the gasses is apparent in the interferogram.
- Fig. 4 illustrates in a block diagram an example of an gas analysis set ⁇ up for implementing the method of the present invention.
- the gas analyzer includes a bandpass filter 20, an interferometer 22 and a photodetector 24.
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- General Physics & Mathematics (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
- Spectrometry And Color Measurement (AREA)
Abstract
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU49347/96A AU4934796A (en) | 1995-03-21 | 1996-03-20 | A method of gas concentration measurement by correlation interferometry |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
RU95103988 | 1995-03-21 | ||
RU95103988A RU2083959C1 (ru) | 1995-03-21 | 1995-03-21 | Способ измерения концентрации газов методом корреляционной фурье-спектроскопии |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1996029582A1 true WO1996029582A1 (fr) | 1996-09-26 |
Family
ID=20165798
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/CA1996/000162 WO1996029582A1 (fr) | 1995-03-21 | 1996-03-20 | Procede de mesure de la concentration de gaz par interferometrie a correlation |
Country Status (3)
Country | Link |
---|---|
AU (1) | AU4934796A (fr) |
RU (1) | RU2083959C1 (fr) |
WO (1) | WO1996029582A1 (fr) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2923905A1 (fr) * | 2007-11-19 | 2009-05-22 | Cnes Epic | Procede et dispositif pour l'inversion interferometrique a echantillonnage libre |
GB2606385A (en) * | 2021-05-06 | 2022-11-09 | Univ Heriot Watt | Hyperspectral imaging method |
Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3939348A (en) * | 1974-06-11 | 1976-02-17 | Allied Chemical Corporation | Infrared gas analysis |
US5247343A (en) * | 1991-11-01 | 1993-09-21 | Nicolet Instrument Corporation | Raman spectrometer having interferometer with optical substraction filters |
WO1994011713A1 (fr) * | 1992-11-18 | 1994-05-26 | Norsk Hydro A. S. | Appareil de mesure spectroscopique d'un gaz |
US5357340A (en) * | 1989-10-12 | 1994-10-18 | Hartmann & Braun | Method for spectroscopy using two Fabry-Perot interference filters |
-
1995
- 1995-03-21 RU RU95103988A patent/RU2083959C1/ru active
-
1996
- 1996-03-20 AU AU49347/96A patent/AU4934796A/en not_active Abandoned
- 1996-03-20 WO PCT/CA1996/000162 patent/WO1996029582A1/fr active Application Filing
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3939348A (en) * | 1974-06-11 | 1976-02-17 | Allied Chemical Corporation | Infrared gas analysis |
US5357340A (en) * | 1989-10-12 | 1994-10-18 | Hartmann & Braun | Method for spectroscopy using two Fabry-Perot interference filters |
US5247343A (en) * | 1991-11-01 | 1993-09-21 | Nicolet Instrument Corporation | Raman spectrometer having interferometer with optical substraction filters |
WO1994011713A1 (fr) * | 1992-11-18 | 1994-05-26 | Norsk Hydro A. S. | Appareil de mesure spectroscopique d'un gaz |
Cited By (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2923905A1 (fr) * | 2007-11-19 | 2009-05-22 | Cnes Epic | Procede et dispositif pour l'inversion interferometrique a echantillonnage libre |
WO2009065736A1 (fr) * | 2007-11-19 | 2009-05-28 | Centre National D'etudes Spatiales | Procede et dispositif pour l'inversion interferometrique a echantillonnage libre |
JP2011503596A (ja) * | 2007-11-19 | 2011-01-27 | サントル・ナシオナル・デテュード・スパシアル(セ・エヌ・ウ・エス) | 自由サンプリングを用いる干渉逆変換のための方法及びデバイス |
US9234799B2 (en) | 2007-11-19 | 2016-01-12 | Centre National D'etudes Spatiales | Method and device for interferometric inversion with free sampling |
GB2606385A (en) * | 2021-05-06 | 2022-11-09 | Univ Heriot Watt | Hyperspectral imaging method |
WO2022234289A1 (fr) * | 2021-05-06 | 2022-11-10 | Heriot-Watt University | Procédé d'imagerie hyperspectrale |
Also Published As
Publication number | Publication date |
---|---|
RU2083959C1 (ru) | 1997-07-10 |
RU95103988A (ru) | 1997-04-27 |
AU4934796A (en) | 1996-10-08 |
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