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 correlation

Info

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
Application number
PCT/CA1996/000162
Other languages
English (en)
Inventor
Evgeniy Vladimirovich Ivanov
Original Assignee
Sci-Tec Instruments Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sci-Tec Instruments Inc. filed Critical Sci-Tec Instruments Inc.
Priority to AU49347/96A priority Critical patent/AU4934796A/en
Publication of WO1996029582A1 publication Critical patent/WO1996029582A1/fr

Links

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

La présente invention concerne un procédé d'interférométrie à corrélation, consistant à analyser les rayonnements seulement dans la région spectrale spécifique où les gaz concernés absorbent ces rayonnements. Les données sont traitées entièrement dans le domaine interférométrique. La réaction interférométrique à ces valeurs dépend fonctionnellement de la concentration du gaz étudié. Sans grand préjudice pour la sélectivité, la présente invention permet d'augmenter la sensibilité des mesures de concentrations des gaz concernés, le spectre connu étant celui du milieu gazeux étudié, l'échantillonnage n'intervenant que pour un sous-ensemble d'intervalles nécessaire à la spectroscopie de Fourier.
PCT/CA1996/000162 1995-03-21 1996-03-20 Procede de mesure de la concentration de gaz par interferometrie a correlation WO1996029582A1 (fr)

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)

* Cited by examiner, † Cited by third party
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)

* Cited by examiner, † Cited by third party
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

Patent Citations (4)

* Cited by examiner, † Cited by third party
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)

* Cited by examiner, † Cited by third party
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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