AU2007220811B2

AU2007220811B2 - Spectrograph calibration using known light source and raman scattering

Info

Publication number: AU2007220811B2
Application number: AU2007220811A
Authority: AU
Inventors: Thomas H. Chyba; Jeromy P. Rezac
Original assignee: Harris IT Services Corp
Current assignee: Peraton Inc
Priority date: 2006-02-23
Filing date: 2007-02-23
Publication date: 2010-09-30
Anticipated expiration: 2027-02-23
Also published as: EP1991843A2; WO2007101092A3; AU2007220811A1; WO2007101092A2; EP1991843A4

Description

SPECTROGRAPH CALIBRATION USING KNOWN LIGHT SOURCE AND RAMAN SCATTERING 5 BACKGROUND OF THE INVENTION The present invention relates to spectrometers, and more particularly to a technique for spectrograph calibration. Spectrometer devices are useful in monitoring for the presence of certain types of chemicals. For example, a spectrometer device may use principles of Raman scattering to L0 analyze a liquid, solid or gas material for certain chemicals. In one type of spectrometer device, a light dispersing element (also referred to as a spectrograph or diffraction grating) is provided to disperse light from a sample and the dispersed light is detected by a detector, such as a charge-coupled camera device. Pixels on the detector are activated depending on the nature of the dispersed light and data representing the spectrum 15 produced by the detector is analyzed to determine the presence of one or more chemicals. The performance of a spectrometer depends on the mechanical alignment of its components, on temperature and pressure conditions in and around the device, and in particular on the light dispersing element. To ensure accuracy of the measurements and analysis made by a spectrometer, it is necessary to adjust or calibrate for such variations. 20 SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a method for calibrating a spectrometer device, comprising: a. activating a calibration light source to emit light that is coupled to a light dispersing element; b. detecting dispersed light from the light dispersing element to produce first 25 spectrum data; c. modifying a dispersion curve for the light dispersing element based on said first spectrum data to produce a modified dispersion curve; d. detecting dispersed light produced as a result of activation of a light source that causes Raman. scattering to produce second spectrum data; 30 e. determining spectral line peak positions for a first chemical and for a second chemical in said second spectrum data; and f. computing calibration data for the spectrometer device from the spectral line peak positions for said first chemical, the spectral line peak positions for said second chemical, the known Raman wavenumber shifts for said first chemical and for said second chemical and said modified dispersion curve. 5 In one form, (c) modifying comprises producing the modified dispersion curve in wavelength space. In one form, (c) modifying comprises shifting and expanding or contracting the dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for the calibration light source. 10 In one form, (f) computing comprises computing said calibration data in wavenumber space based further on a relationship between wavelengths and wavenumber, thereby fitting the modified dispersion curve to an operating wavelength of the light source that causes Raman Scattering. In one form, (e) determining comprises determining spectral line peak positions 15 for nitrogen as said first chemical and spectral line positions for oxygen as said second chemical, In one form, the first chemical is nitrogen and the second chemical is oxygen, and further comprising monitoring spectrum data when the spectrometer device is in normal use; and determining whether the line peak positions for nitrogen has drifted more than a 20 predetermined number of pixels from calibrated peak positions for nitrogen determined from said monitoring performed over a period of time, and further comprising repeating (a) through (f) when the spectral line peak positions for nitrogen are more than said predetermined number of pixels from said calibrated peak positions for nitrogen. In one form, the method further comprises adjusting said calibration data so as to 25 keep the spectral line peak positions of nitrogen in the second spectrum data within a predetermined number of pixels of the calibrated line peak positions for nitrogen. In one form, (a) activating comprises activating a calibration light source that is part of said spectrometer device. In one form, (e) comprising determining spectral line peak positions of nitrogen as 30 said first chemical. In one form, (d) detecting comprises detecting Raman scattered light associated with a reference sample, and wherein said reference sample is a solid, liquid or gas. In one form, (d) detecting comprises detecting Raman scattered light associated with air. la Cc q (t According to a second aspect of the present invention there is provided a method for calibrating a spectrometer device used for Raman scattering analysis, comprising: a. modifying a dispersion curve for a light dispersing element of the spectrometer device based on first spectrum data associated with dispersed 5 light detected by a detection device to produce a modified dispersion curve; b. determining line peak positions for a first chemical and a second chemical in second spectrum data associated with Raman scattered light detected by said detection device; and 10 c. computing calibration data for the spectrometer device from the line peak positions for said first chemical and for said second chemical, known Raman wavenumber shifts for said first chemical and for second chemical and said modified dispersion curve. In one form, (a) modifying comprises shifting, expanding or contracting the 15 dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for a calibration light source that is used to create the first spectrum data from which the modified dispersion curve is produced. In one form, (c) computing comprises computing said calibration data in wavenumber space based further on a relationship between wavelengths and 20 wavenumber, thereby fitting the modified dispersion curve to a wavelength of the light source that causes the Raman scattered light. In one form, (b) determining comprises determining spectral line peak positions for nitrogen as said first chemical and oxygen as said second cherhical. In one form, the first chemical is nitrogen and the second chemical is oxygen, and 25 further comprising monitoring spectrum data when the spectrometer device is in normal use; and determining whether the line peak positions for nitrogen has drifted more than a predetermined number of pixels from calibrated peak positions for nitrogen determined from monitoring over a period of time, and further comprising repeating (a) through (c) when the spectral line peak positions for nitrogen are more than said predetermined 30 number of pixels from said calibrated peak positions for nitrogen. In one form, the method further comprises adjusting said calibration data so as to keep the spectral line peak positions of nitrogen in the second spectrum data within a predetermined number of pixels from said calibrated peak positions for nitrogen. lb In one form, (b) determining comprises detecting Raman scattered light associated with a reference sample, and wherein said reference sample is a solid, liquid or gas. In one form, detecting comprises detecting Raman scattered light associated with air. 5 According to a third aspect of the present invention there is provided a spectrometer device, comprising: a. a light dispersing element; b. a detector that detects light dispersed by said light dispersing element; c. a calibration light source; t0 d. a light source suitable for causing Raman scattering; and e. a control unit coupled to said detector, wherein said control unit calibrates the spectrometer device by: 1. modifying a dispersion curve for the light dispersing element based on first spectrum data associated with detected dispersed light from [5 said calibration light source, thereby producing a modified dispersion curve; ii. determining spectral line peak positions for a first chemical and for a second chemical in second spectrum data produced by the detector as result of activation of said light source suitable for 20 causing Raman scattering; and in. computing calibration data for the spectrometer device from the line peak positions for said first chemical and for said second chemical, known Raman wavenumber shifts for said first chemical and said second chemical and said modified dispersion curve. 25 In one form, said control unit modifies said dispersion curve of the light dispersing element by shifting, expanding or contracting data representing the dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for a calibration light source that is used to create the first spectrum 30 data. In one form, said control unit computes said calibration data in waverniumber space based further on a relationship between wavelengths and wavenumber thereby fitting the modified dispersion curve to a wavelength of the light source suitable for causing Raman scattering. 10 In one form, said control unit determines spectral line peak positions for nitrogen as said first chemical and spectral line peak positions for oxygen as said second chemical. In one form, the first chemical is nitrogen and the second chemical is oxygen, and wherein the control unit monitors spectrum data when the spectrometer device is in use 5 and determines whether the line peak positions for nitrogen has drifted more than a predetermined number of pixels from calibrated peak positions for nitrogen determined from monitoring spectrum data over a period of time, and further comprising repeating said (i) modifying, (ii) determining and (iii) computing when the spectral line peak positions for nitrogen are more than said predetermined number of pixels from said 10 calibrated peak positions for nitrogen. According to a fourth aspect of the present invention there is provided a method for calibrating a spectrometer device substantially as herein described with reference to and as illustrated in the accompanying representations. 15 According to a fifth aspect of the present invention there is provided a method for calibrating a spectrometer device used for Raman scattering analysis substantially as herein described with reference to and as illustrated in the accompanying representations. 20 According to a sixth aspect of the present invention there is provided a spectrometer device substantially as herein described with reference to and as illustrated in the accompanying representations. 25 BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a block diagram of a spectrometer device having calibration capability according to an embodiment of the invention. Id WO 2007/101092 PCT/US2007/062665 FIG. 2 is a generalized flow diagram depicting two basic computations performed during calibration according to an embodiment of the invention. FIG. 3 is a flow chart depicting in more detail the calibration procedure according to an embodiment of the invention. 5 FIG. 4 is a plot showing an example of a spectral line produced from a calibration light source according to an embodiment of the invention. FIG. 5 is a diagram showing how the dispersion curve for the light dispersing element is modified based on the spectrum obtained by activation of the calibration light source according to an embodiment of the invention. 10 FIG. 6 is a plot showing an example of the peak spectral line for oxygen in a spectrum produced when the light source is activated and the light is dispersed onto the detector according to an embodiment of the invention. FIG. 7 is a plot showing an example of the peak spectral line for nitrogen in a spectrum produced when the light source is activated and the light is dispersed onto the 15 detector according to an embodiment of the invention. FIG. 8 is a plot showing calibration data in wavenumbers space produced from the modified dispersion curve and shifted spectral lines for nitrogen and oxygen according to an exemplary embodiment of the invention. DETAILED DESCRIPTION 20 Referring first to FIG. 1, a spectrometer system or device according to an embodiment of the present invention is generally shown at reference numeral 10. The spectrometer system 10 comprises a calibration light source 20 and a Raman light source 30. A light collection system 40 collects light and is coupled to a spectrograph 60 which serves as the light dispersing element. In one embodiment, the light collection system 40 25 may comprise an auto-focused telescope and an optical fiber 50 that couples the light collected by the telescope to the spectrograph 60. An intensified charge-coupled device (ICCD) camera 70 is optically coupled to the spectrograph 60 and is in turn coupled to a signal processor (processor) 80. The processor 80 executes a calibration algorithm 100 (described hereinafter) and a spectral analysis algorithm 200, such as a Raman scattering 30 algorithm. The processor 80 may be more generally referred to as a control unit since it may also perform other control functions of the spectrometer 10. The desired Raman scattering effect is measured when the Raman light source 30 is activated and the spectrometer 10 is in a measurement mode. The Raman light source 2 WO 2007/101092 PCT/US2007/062665 30 can be any narrow band light source used for Raman excitation. In an exemplary embodiment, the light source 30 is an ultraviolet (UV) excimer laser. The calibration light source 20 may be any stable wavelength source and may include an undetermined number of spectral lines within the spectral region of interest. In an exemplary 5 embodiment, the calibration light source 20 is a mercury (Hg) lamp. Alternatively, the calibration light source 20 may be an Argon (Ag) lamp or any fixed wavelength light source (laser) having a desired and known spectrum. As is known in the art, a diffraction grating, such as the spectrograph 60, has a theoretical curve of dispersion vs. wavelength number expressed in terms of ICCD pixels 10 and can be approximated as a polynomial curve, in an exemplary embodiment it is a cubic polynomial curve. The manufacturer of the grating typically provides data describing this curve. Thus, the dispersion curve function is previously determined either by measurement or theory. According to an embodiment of the present invention, the calibration algorithm 100 calibrates the dispersion curve of the spectrograph 60. This 15 calibration algorithm 100 may be invoked at any time to verify proper operation of the spectrometer and may be used with a calibration source that is built into the spectrographic system 10 or with a calibration light source that is external to the system 10. Moreover, this technique can be used for in-field measurements or bench-top measurements (under controlled or test conditions). 20 On the ICCD camera 70, the pixels are a certain width across, but the wavelength to pixel mapping is non-linear as a result of the light dispersing function of the spectrograph 60 that spreads out the wavelengths at different angles. This is a basic aspect of a spectrograph. Because the light dispersing function is non-linear, it is necessary to have knowledge about how that dispersion works, through calibration. 25 Calibration for the spectrograph 60 involves determining the mapping of wavelengths to pixel numbers on the ICCD camera 70. Part of calibration involves mapping out wavelength vs. pixel number, and then factoring in the Raman signature of interest fixed in wavenumber space. As is known in the art, wavenumber space is in units (1/cm) and is the difference in wavenumbers 30 between the Raman light source 30 and the Raman shifted light. To do this, it is necessary to determine the Raman light source wavelength on the ICCD camera 70 because although the Raman wavenumber signatures are fixed with respect to the Raman light source, the Raman wavelength signatures are dependent on the Raman light source 3 WO 2007/101092 PCT/US2007/062665 wavelength. Factors that can change the wavelength calibration include temperature, pressure and small mechanical changes in the alignment. The wavelength to wavenumber calibration can also be changed by a shift in the Raman light source wavelength. With reference to FIG. 2 in conjunction with FIG. 1, the calibration algorithm 100 5 is generally described. The calibration algorithm 100 may be viewed as a two stage process comprising obtaining the calibration spectrum at 102 and then obtaining the Raman air spectrum at 104. The starting point is the predetermined dispersion curve for the spectrograph. The (onboard or external) calibration light source 20 is activated and the resulting spectrum is obtained. The peak position of the calibration light source lines 10 in the captured spectrum and known expected wavelengths of these lines are used to modify the dispersion curve for the spectrograph 60 to best fit the calibration light source lines. In particular, the dispersion curve is shifted along with contracting or expanding it, along the wavelength axis, which has the effect of changing the coefficients of the terms in the dispersion curve. The calibration light source line center positions are determined 15 by fitting the spectral peaks near their peaks, and avoiding effects such as side mode structure at the bases of the peaks. The calibration light source 20 allows for a mapping of the spectrograph wavelengths to the ICCD pixels based on the dispersion curve and known line peak positions for the calibration light source 20. Next, the spectrum is obtained for air by activating the Raman light source 30 and 20 collecting the Raman scattered light associated with air. Peak pixel positions for first and second chemicals in the reference sample spectrum data are determined. In one embodiment, the first chemical is nitrogen (N 2 ) and the second chemical is oxygen (02) and their fractional peak pixel locations are determined by a fit to a Lorentzian lineshape function. Using the 02 and N 2 peak pixel positions in the reference sample (e.g., air) 25 spectrum data, known Raman wavenumber shift of 02 and N 2 , together with the modified dispersion curve and relationship between wavelengths and wavenumbers, the best fit for the Raman light source wavelength is determined. This in turn provides spectrographic calibration information in wavenumber space. It should be understand that while air is used as the reference sample when activating the Raman light source, the reference 30 sample used may be a liquid, solid or gas. Air is only an example of a reference or calibration sample for the Raman light source that may be convenient for certain applications according to one embodiment. 4 WO 2007/101092 PCT/US2007/062665 The nitrogen and oxygen peak line positions and the modified dispersion curve allow one to determine the wavelength of the Raman light source 30 and to do a mapping in wavenumber space. The position of the nitrogen and oxygen lines on the ICCD's pixel array depend on the wavelength of the Raman light source, but the wavelength calibration 5 light source peak line positions on the ICCD's pixel array do not depend on that wavelength. The line peak positions for calibration light source 20 provide a mapping of the wavelengths to the pixels. The nitrogen and oxygen peak position lines provide information about any shift in the Raman light source 30. The nitrogen and oxygen peak line positions are always the same number of wavenumbers away from that of the Raman 10 light source 30. Turning now to FIG. 3, in conjunction with FIGs. I and 4-8, the calibration algorithm 100 will be described in greater detail. At 110, the (onboard or external) calibration light source 20 is activated and the light is detected and coupled to the spectrograph 60 by the light collection system 40. At 120, the ICCD camera 70 detects 15 the dispersed light from the spectrograph 60 and produces pixel spectrum data. FIG. 4 is a plot showing an example of a spectral line produced from a calibration light source according to an embodiment of the invention. At 130, the processor 80 modifies the dispersion curve for the spectrograph 60 based on the pixel spectrum data for the calibration light source 20 to produce a calibrated 20 or modified dispersion curve in wavelength space. Modification of the dispersion curve is shown in FIG 5 according to one embodiment, where the dispersion curve from the spectrograph is shifted and expanded or contracted based on the spectrum data generated when the calibration light source 20 is activated. Next, at 140, the Raman light source 30 is activated into a reference sample, e.g., 25 air, and the Raman shifted (scattered) light is collected and coupled to the spectrograph 60. At 150, the ICCD camera 70 detects dispersed light from the spectrograph 60 and produces spectrum data for the reference sample, e.g., air. FIG. 6 is a plot showing an example of the peak spectral line for oxygen derived from the air spectrum produced when the Raman light source is activated and the Raman scattered light is coupled into 30 the spectrograph 60, dispersed, and detected with the ICCD camera 70 according to one embodiment. FIG. 7 is a plot showing an example of the peak spectral line for nitrogen in the air spectrum produced when the Raman light source 30 is activated and the Raman 5 WO 2007/101092 PCT/US2007/062665 scattered light is coupled into the spectrograph 60, dispersed, and detected with an ICCD camera 70 according to one embodiment. At 160, the oxygen and nitrogen line peak pixel positions in the air spectrum data are used, together with the known Raman wavenumber shift of oxygen and nitrogen and 5 the modified dispersion curve, to best fit the Raman light source wavelength, and in so doing, produce spectrographic calibration information in wavenumber space. FIG. 8 is a plot showing exemplary calibration data in wavenumbers space produced from the modified dispersion curve and Raman shifted spectral lines for nitrogen and oxygen and the fitted Raman light source wavelength according to one embodiment. In this example, 10 the Raman light source wavenumber value is zero. The calibration technique described above may be used in the field to verify proper operation of the spectrometer. For example, during a test session, air spectrum data is obtained by activating the Raman light source into the air and collecting the Raman air spectrum. If the line peak positions for nitrogen and oxygen are acceptable, 15 then it may not be necessary to activate the onboard calibration light source and execute the calibration algorithm 100. However, if the nitrogen and oxygen line peak positions are not acceptable, then the algorithm 100 is invoked using the onboard calibration light source and performing the curve fitting procedure using the measurements described above. 20 The calibration algorithm 100 in the embodiments described herein is very precise. However, it requires time to perform. Therefore, according to an embodiment of the invention, the spectrometer can monitor a real-time data stream (containing spectrum data) for the location of the nitrogen peak and automatically adjusts the calibration data (shift the data pixel number) so as to always keep the nitrogen peak in the data stream 25 within a certain number of pixels (e.g., one pixel) of the position of the nitrogen spectral peak pixel number in the (previously computed) calibration data. This achieves fast recalibration albeit with less accuracy then the complete calibration algorithm 100. If the nitrogen peak drifts too far (greater than a predetermined threshold in pixels) from the calibration nitrogen peak pixel number over time, the processor may generate an alert 30 (audible and/or visual) to the user so the user can initiate the more precise calibration algorithm 100. Thus, the spectrometer may constantly monitor the spectrum and initiate this "fast" calibration in real-time when appropriate and decide when the complete calibration algorithm 100 is needed and invoked. 6 In certain spectrometer applications many of the target chemicals and sample surfaces may have features that overlap the oxygen line, potentially confusing real-time monitoring. However, due to the physics of Raman spectra (i.e. the energy of chemical vibrational bonds), there are no (or extremely few) features interfering with the nitrogen 5 line. Consequently, the nitrogen line is nearly always available with little interference from a surface being scanned or from chemicals on the surface. As a result, it is not necessary to change the mode of operation to locate the position of the nitrogen line. The foregoing techniques are applicable to other types of spectrographic systems, such as those that operate in the infrared and UV, and in systems that use a narrowband 10 light source other than a laser for creating the Raman scattering. Moreover, any spectral lamp with emission lines in the wavelength region of interest may be used as the calibration light source. The system and methods described herein may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing 15 embodiments are therefore to be considered in all respects illustrative and not meant to be limiting. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the statement of invention and description of the invention which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 7

Claims

1. A method for calibrating a spectrometer device, comprising: a. activating a calibration light source to emit light that is coupled to a light dispersing element; b, detecting dispersed light from the light dispersing element to produce first spectrum data; c. modifying a dispersion curve for the light dispersing element based on said first spectrum data to produce a modified dispersion curve; d, detecting dispersed light produced as a result of activation of a light source that causes Raman scattering to produce second spectrum data; e. determining spectral line peak positions for a first chemical and for a second chemical in said second spectrum data; and f. computing calibration data for the spectrometer device from the spectral line peak positions for said first chemical, the spectral line peak positions for said second chemical, the known Raman wavenumber shifts for said first chemical and for said second chemical and said modified dispersion curve.

2. The method of claim 1, wherein (c) modifying comprises producing the modified dispersion curve in wavelength space.

3. The method of claim I or 2, wherein (c) modifying comprises shifting and expanding or contracting the dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for the calibration light source.

4. The method of claim 3, wherein (f) computing comprises computing said calibration data in wavenumber space based further on a relationship between wavelengths and wavenumber, thereby fitting the modified dispersion curve to an operating wavelength of the light source that causes Raman Scattering.

5. The method of any one of claims I to 4, wherein (e) determining comprises determining spectral line peak positions for nitrogen as said first chemical and spectral line positions for oxygen as said second chemical. [8] 0cAG

6. The method of any one of claims I to 5, wherein the first chemical is nitrogen and the second chemical is oxygen, and further comprising monitoring spectrum data when the spectrometer device is in normal use; and determining whether the line peak positions for nitrogen has drifted more than a predetermined number of pixels from calibrated peak positions for nitrogen determined from said monitoring performed over a period of time, and further comprising repeating (a) through (f) when the spectral line peak positions for nitrogen are more than said predetermined number of pixels from said calibrated peak positions for nitrogen.

7. The method of claim 6, and further comprising adjusting said calibration data so as to keep the spectral line peak positions of nitrogen in the second spectrum data within a predetermined number of pixels of the calibrated line peak positions for nitrogen.

8. The method of any one of claims I to 7, wherein (a) activating comprises activating a calibration light source that is part of said spectrometer device.

9. The method of any one of claims 1 to 8, wherein (e) comprising determining spectral line peak positions of nitrogen as said first chemical.

10. The method of any one of claims 1 to 9, wherein (d) detecting comprises detecting Raman scattered light associated with a reference sample, and wherein said reference sample is a solid, liquid or gas.

11. The method of claim 10, wherein (d) detecting comprises detecting Raman scattered light associated with air.

12. A method for calibrating a spectrometer device used for Raman scattering analysis, comprising; a. modifying a dispersion curve for a light dispersing element of the spectrometer device based on first spectrum data associated with dispersed light detected by a detection device to produce a modified dispersion curve; b. determining line peak positions for a first chemical and a second chemical in second spectrum data associated with Raman scattered light detected by said detection device; and c. computing calibration data for the spectrometer device from the line peak positions for said first chemical and for said second chemical, known [9] Raman wavenumber shifts for said first chemical and for second chemical and said modified dispersion curve.

13. The method of claim 12, wherein (a) modifying comprises shifting, expanding or contracting the dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for a calibration light source that is used to create the first spectrum data from which the modified dispersion curve is produced.

14. The method of claims 12 or 13, wherein (c) computing comprises computing said calibration data in wavenumber space based further on a relationship between wavelengths and wavenumber, thereby fitting the modified dispersion curve to a wavelength of the light source that causes the Raman scattered light.

15. The method of any one of claims 12 to 15, wherein (b) determining comprises determining spectral line peak positions for nitrogen as said first chemical and oxygen as said second chemical.

16. The method of any one of claims 12 to 15, wherein the first chemical is nitrogen and the second chemical is oxygen, and further comprising monitoring spectrum data when the spectrometer device is in normal use; and determining whether the line peak positions for nitrogen has drifted more than a predetermined number of pixels from calibrated peak positions for nitrogen determined from monitoring over a period of time, and further comprising repeating (a) through (c) when the spectral line peak positions for nitrogen are more than said predetermined number of pixels from said calibrated peak positions for nitrogen.

17. The method of claim 16, and further comprising adjusting said calibration data so as to keep the spectral line peak positions of nitrogen in the second spectrum data within a predetermined number of pixels from said calibrated peak positions for nitrogen.

18. The method of any one of claims 12 to 17, wherein (b) determining comprises detecting Raman scattered light associated with a reference sample, and wherein said reference sample is a solid, liquid or gas.

19. The method of claim 18, wherein detecting comprises detecting Raman scattered light associated with air. [10] S(& I'

20. A spectrometer device, comprising: a. a light dispersing element; b. a detector that detects light dispersed by said light dispersing element; c. a calibration light source; d. a light source suitable for causing Raman scattering; and e. a control unit coupled to said detector, wherein said control unit calibrates the spectrometer device by: i. modifying a dispersion curve for the light dispersing element based on first spectrum data associated with detected dispersed light from said calibration light source, thereby producing a modified dispersion curve; ii. determining spectral line peak positions for a first chemical and for a second chemical in second spectrum data produced by the detector as result of activation of said light source suitable for causing Raman scattering; and iii. computing calibration data for the spectrometer device from the line peak positions for said first chemical and for said second chemical, known Raman wavenumber shifts for said first chemical and said second chemical and said modified dispersion curve.

21. The spectrometer device of claim 20, wherein said control unit modifies said dispersion curve of the light dispersing element by shifting, expanding or contracting data representing the dispersion curve for the light dispersing element to produce said modified dispersion curve based on peak pixel positions for a calibration light source that is used to create the first spectrum data.

22. The spectrometer device of claim 21, wherein said control unit computes said calibration data in wavenumber space based further on a relationship between wavelengths and wavenumber thereby fitting the modified dispersion curve to a wavelength of the light source suitable for causing Raman scattering.

23. The spectrometer device of any one of claims 20 to 22, wherein said control unit determines spectral line peak positions for nitrogen as said first chemical and spectral line peak positions for oxygen as said second chemicaL [11) ok

24. A method for calibrating a spectrometer device substantially as herein described with reference to and as illustrated in the accompanying representations.

25. A method for calibrating a spectrometer device used for Raman scattering analysis substantially as herein described with reference to and as illustrated in the accompanying representations.

26. A spectrometer device substantially as herein described with reference to and as illustrated in the accompanying representations.

27. The spectrometer device of any one of claims 20 to 23, wherein the first chemical is nitrogen and the second chemical is oxygen, and wherein the control unit monitors spectrum data when the spectrometer device is in use and determines whether the line peak positions for nitrogen has drifted more than a predetermined number of pixels from calibrated peak positions for nitrogen determined from monitoring spectrum data over a period of time, and further comprising repeating said (i) modifying, (ii) determining and (iii) computing when the spectral line peak positions for nitrogen are more than said predetermined number of pixels from said calibrated peak positions for nitrogen. [12] 02(oC