SYSTEMS AND METHODS USING ACTIVE FTIR SPECTROSCOPY FOR DETECTION OF CHEMICAL TARGETS

Abstract

Active FTIR spectroscopy systems and methods for quantitative measurements of concentrations of chemical targets, such as gas, liquid and solid chemical targets, in an open-path measuring arrangement and a method of extracting an effective illumination spectrum of IR light illuminating chemical targets arranged in an open-path measuring arrangement.

Claims

1-31. (canceled)

32. An active FTIR spectroscopy system for quantitative measurements of concentrations of chemical targets in an open-path measuring arrangement, the system comprising: an illumination source comprising an optical parametric oscillator configured to generate broadband IR light, a calibration source configured to generate calibration light, a scanning interferometer arranged to synchronously receive each of the broadband IR light and the calibration light, and then modulate each of the received broadband IR light and calibration light over a scanning period, with the received calibration light being configured to deduce optical path differences introduced by the scanning interferometer over the scanning period for each of the modulated broadband IR light and the modulated calibration light, a launch system for illuminating the chemical targets with the modulated broadband IR light, a collector system for receiving broadband IR light spectrally-modulated by the chemical targets, wherein the launch system is configured so that an optical axis of a launching path is substantially co-aligned with an optical axis of the collector system, a detector system for detecting and then recording interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light, and a non-transitory computer readable medium encoded with a computer program, wherein the computer program comprises instructions stored therein for causing a computer processor to perform a plurality of functions to simultaneously compute concentrations of each of the chemical targets from spectra of the recorded interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light.

33. The system of claim 32, wherein the optical parametric oscillator comprises a nonlinear crystal tunable to generate broadband IR light with wavelengths from 1 μm to 16 μm, preferably from 2.5 μm to 4.5 μm, more preferably from 2.8 μm to 3.9 μm, and even more preferably from 3.1 μm to 3.5 μm.

34. The system of claim 32, wherein the optical parametric oscillator is configured to generate broadband IR light with an average output power of at least 10 mW, preferably at least 50 mW, more preferably at least 250 mW, and even more preferably at least 500 mW; or wherein the optical parametric oscillator is configured to generate broadband IR light with a repetition rate from 1 MHz to 20 GHz, preferably from 10 MHz to 10 GHz, more preferably from 70 MHz to 1 GHz, and even more preferably from 90 MHz to 500 MHz.

35. The system of claim 32, wherein the calibration source comprises any one of a wavelength stabilized laser source or a narrow-line diode laser source.

36. The system of claim 32, wherein the scanning interferometer is configured to modulate the received broadband IR light with a resolution of less than 0.5 cm.sup.−1, preferably less than 0.1 cm.sup.−1, more preferably less than 0.07 cm.sup.−1, and even more preferably less than 0.05 cm.sup.−1; or wherein the scanning interferometer is configured to modulate the received broadband IR light at scanning rates from 0.1 Hz to 1000 Hz, preferably from 0.5 Hz to 100 Hz, more preferably from 0.7 Hz to 10 Hz; or wherein the scanning interferometer is configured to modulate the received broadband IR light with an average power of at least 1 mW, preferably at least 20 mW, more preferably at least 70 mW, and even more preferably at least 100 mW.

37. The system of claim 32, wherein the scanning interferometer comprises at least one moving retroreflector, the moving retroreflector being arranged to increase the optical paths introduced by the scanning interferometer over the scanning period for each of the modulated broadband IR light and the modulated calibration light.

38. The system of claim 32, wherein the launch system comprises a steering arrangement, the steering arrangement comprising a steering mirror or a steering prism, wherein the steering mirror is arranged at 45° with respect to the optical axis of the launching path.

39. The system of claim 32, wherein the collector system comprises a telescope, and wherein the telescope comprises a reflecting telescope or a refracting telescope.

40. The system of claim 39, wherein the collector system further comprises an optical relay arrangement, such as one or more of a mirror, a lens or a prism, or a combination thereof, for relaying the received spectrally-modulated broadband IR light onto the detector system.

41. The system of claim 40, wherein the steering arrangement of the launch system is configured to be arranged adjacent the optical relay arrangement of the collector system such that the optical axis of the launching path is co-aligned with the optical axis of the collector system.

42. The system of claim 32, wherein the detector system comprises a combination of an IR detector for detecting spectra of interference fringes generated by the spectrally-modulated broadband IR light and a photodiode or a phototransistor for detecting spectra of interference fringes generated by the modulated calibration light.

43. The system of claim 32, wherein the detector system further comprises a digital signal acquisition system configured to record at least one spectrum of interference fringes generated by the spectrally-modulated broadband IR light in synchronism with at least one spectrum of interference fringes generated by the modulated calibration light such that the interference fringes of the modulated calibration light are configured to provide an accurate timebase calibration for the interference fringes of the spectrally-modulated broadband IR light to enable calculating an accurate wavelength scale for the at least one spectrum of interference fringes generated by the spectrally-modulated broadband IR light.

44. The system of claim 32, wherein the broadband IR light spectrally-modulated by the chemical targets comprises spectral-modulation by backscattering or absorption or diffuse reflectance of the broadband IR light.

45. The system of claim 32, wherein the plurality of functions comprises simultaneously computing concentrations of each of the chemical targets by performing calibration and Fourier transformation of the recorded spectra of interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light to produce at least one absorption spectrum for the chemical targets, fitting library absorption spectra for each of the chemical targets and an extracted illumination spectrum of the generated broadband IR light to the at least one absorption spectrum, and then calculating concentrations of each of the chemical targets from said produced and said fitted spectrum.

46. The system of claim 32, wherein the chemical targets comprise IR-absorbing chemical species.

47. The system of claim 32, wherein the open-path of the open-path measuring arrangement is at least 0.1 meters long, preferably at least 10 meters long, more preferably at least 30 meters long, and even more preferably at least 70 meters long.

48. The system of claim 32, further comprising a scattering aid having a convex or a plane surface, wherein the scattering aid comprises a pane of paper, concrete, laminate, wood, brick, stone, painted surface, metal or plastic.

49. The system of claim 48, wherein the chemical targets are configured to be arranged in the open-path measuring arrangement between the launch system and the scattering aid such that an optical axis of the scattering aid is co-aligned with the optical axis of the launching path.

50. The system of claim 32, wherein the system is configured for quantitative measurements of concentrations of IR-absorbing gaseous, liquid, aerosol or powder chemical targets in an open-path measuring arrangement.

51. A method for quantitative measurements of concentrations of chemical targets in an open-path measuring arrangement, the method comprising: providing an active FTIR spectroscopy system according to claim 32.

52. A method for quantitative measurements of concentrations of chemical targets in an open-path measuring arrangement, the method comprising: generating broadband IR light, generating calibration light, synchronously receiving and then modulating, by means of a scanning interferometer, the generated broadband IR light and the generated calibration light, providing a launch system for illuminating the chemical targets with the modulated broadband IR light and a collector system for receiving the modulated broadband IR light spectrally-modulated by the chemical targets such that an optical axis of a launching path of the launch system is substantially co-aligned with an optical axis of the collector system, providing a detector system for detecting and then recording spectra of interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light, and providing a non-transitory computer readable medium encoded with a computer program, wherein the computer program comprises instructions stored therein for causing a computer processor to perform a plurality of functions to simultaneously compute concentrations of each of the chemical targets from spectra of the recorded interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light.

53. The method of claim 52, wherein the determining by the plurality of functions comprises simultaneously computing concentrations of each of the chemical targets by performing calibration and Fourier transformation of the recorded spectra of interference fringes generated by the spectrally-modulated broadband IR light and by the modulated calibration light to produce at least one absorption spectrum for the chemical targets, fitting library absorption spectra for each of the chemical targets and an extracted illumination spectrum of the generated broadband IR light to the at least one absorption spectrum, and then calculating concentrations of each of the chemical targets from said produced and said fitted spectrum.

54. The method of claim 52, further comprising providing a scattering aid having a convex or a plane surface, wherein the chemical targets are configured to be arranged in the open-path measuring arrangement between the launch system and the scattering aid such that an optical axis of the scattering aid is co-aligned with the optical axis of the launching path; and wherein the method is configured for quantitative measurements of concentrations of IR-absorbing gaseous, liquid, aerosol or powder chemical targets in an open-path measuring arrangement.

55. A method of extracting, using a non-transitory computer readable medium encoded with a computer program, an illumination spectrum I.sub.o of a broadband IR light generated by an optical parametric oscillator in order to illuminate chemical targets arranged in an open-path measuring arrangement for the purpose of computing concentrations of each of the chemical targets, the method comprising the steps of: storing to the non-transitory computer readable medium spectra I of interference fringes generated by spectrally-modulated broadband IR light upon illumination of the chemical targets, storing on the non-transitory computer readable medium library absorption spectra for at least one known concentration of each of the chemical targets, and encoding the medium with a computer program comprising instructions stored therein for causing a computer processor to perform an algorithm which allows the illumination spectrum I.sub.o to be modelled as a many-point spline function and to become a free parameter when fitting the library absorption spectra for each of the chemical targets to the spectra I of interference fringes generated by spectrally-modulated broadband IR light upon illumination of the chemical targets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0114] Various aspects of the disclosure will now be described by way of example only and with reference to the accompanying drawings, of which:

[0115] FIG. 1 shows the idler spectra produced by fan-out-grating tuning of the MgO:PPLN nonlinear crystal of the OPO;

[0116] FIG. 2 shows the layout of an embodiment of the active FTIR spectroscopy system of the present disclosure;

[0117] FIG. 3 shows the layout of another embodiment of the active FTIR spectroscopy system of the present disclosure;

[0118] FIG. 4A shows an example spectrum from a set of 45 spectra collected at meters range using the system of FIG. 2;

[0119] FIG. 4B shows measured concentrations at 10, 20, 25 and 30 meters, with data points showing the average values from approximately 45 spectra each, and the error bars showing the ±1 standard deviation range (the spectra was collected using the system of FIG. 2);

[0120] FIG. 5A shows an example spectrum from a set of 51 spectra collected at 70 meters range using the system of FIG. 2;

[0121] FIG. 5B shows measured concentrations of methane (lower curve) and water (upper curve) measured before, during and after the methane release using the system of FIG. 2;

[0122] FIG. 6 shows measured data of the average of 1250 spectra acquired (using the system of FIG. 2) over a 35 meters indoor range (shaded area), and a comparison with the best-fit HITRAN data (dashed black line) for water vapor and methane;

[0123] FIG. 7 shows the results of fitting to 1063 individual spectra acquired (using the system of FIG. 2) over a 35 meters indoor range, showing concentrations for water vapor (a), methane (b) and the Allan deviation analysis (c);

[0124] FIG. 8 shows the algorithm for the method of extracting the Illumination spectrum of the broadband IR light generated by the OPO.

DETAILED DESCRIPTION

[0125] In a preferred embodiment of the active FTIR spectroscopy system, the IR light source was an ultrafast OPO (Chromacity Ltd.) based on a fan-out-grating MgO:PPLN nonlinear crystal, which provided 100-MHz pulses with tunability from 2.6 μm-4.2 μm and broad spectra as shown in FIG. 1. To access the strongest absorptions in methane and ethane, the inventors selected a grating position that provided instantaneous coverage from 3.1-3.5 μm, with an average power >300 mW and a 1-cm-diameter beam with a measured beam factor M2 value of 1.05.

[0126] In the layout of the active FTIR spectroscopy system shown in FIG. 2, the OPO light was first coupled into a scanning Michelson interferometer before being launched into free-space and subsequently collected by a 6-inch f/4 Newtonian telescope after scattering from a remote chemical target. The HeNe light was also coupled into the scanning Michelson interferometer before being collected by a silicon photodiode detector.

[0127] The returned light (i.e., the spectrally-modulated light) was detected using an InSb liquid-nitrogen-cooled photodiode situated at the telescope focus. Light from the OPO was launched along an optical axis co-aligned with the telescope's field of view using a small 45° steering mirror situated directly before the secondary mirror of the telescope. The scanning Michaelson interferometer operated at 1 Hz and achieved a typical resolution of 0.05 cm.sup.−1, which is sufficient to resolve the narrow and complex absorption-line structure of light molecules, such as water, methane and ethane. The entire system was constructed on a 60×90 cm breadboard and mounted on a trolley.

Simultaneous Methane, Ethane and Water Measurement at 30-m Range

[0128] To establish the ability of the system to measure multiple spectrally-overlapping species simultaneously, the inventors performed indoor measurements at a range of up to 30 meters in which the IR light launched from the OPO entered a 20-cm-long gas cell containing a 1.5±0.15% ethane in air mixture and situated directly after the launch mirror, which was situated immediately before the entrance aperture of the collection telescope.

[0129] FIG. 4A (solid area) displays an example of a single measured spectrum (no averaging) exhibiting densely packed absorption lines from water, methane and ethane, as well as continuum absorption from ethane, which suppresses the overall spectral intensity. FIG. 4A shows an example spectrum from a set of 45 collected at 30-m range from a rough aluminum-foil scattering aid. The upper plot shows the envelope of the illumination spectrum extracted from a fitting procedure which simultaneously minimized the rms error between the experimental spectrum (solid area) and a synthetic spectrum (black line at the top of the solid area) calculated from the envelope and a fitted mixture of PNNL absorbance data for water, methane and ethane. In this example, the best-fit concentrations determined were 1.15% (water), 1860 ppb (methane) and 1.37% (ethane). The lower plot shows the rms fitting residual.

[0130] Quantitative open-path spectroscopy requires either a reliable reference spectrum or a method of inferring the original illumination spectrum, and this problem has been treated in different ways in previously reported studies (G. B. Rieker, F. R. Giorgetta, W. C. Swann, J. Kofler, A. M. Zolot, L. C. Sinclair, E. Baumann, C. Cromer, G. Petron, C. Sweeney, P. P. Tans, I. Coddington, and N. R. Newbury, “Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths,” Optica 1, 290, 2014; L. Nugent-Glandorf, F. R. Giorgetta, and S. A. Diddams, “Open-air, broad-bandwidth trace gas sensing with a mid-infrared optical frequency comb,” Applied Physics B 119, 327-338, 2015).

[0131] The approach taken by the inventors (discussed further below with reference to FIG. 8) results in the retrieval of an illumination spectrum (dashed upper line in FIG. 4A) which represents the OPO output spectrum prior to undergoing atmospheric absorption. The black line at the top of the solid area in FIG. 4A is the best-fit absorption spectrum using 0.1-cm.sup.−1 PNNL library data as the fitting reference (S. W. Sharpe, T. J. Johnson, R. L. Sams, P. M. Chu, G. C. Rhoderick, and P. A. Johnson, “Gas-Phase Databases for Quantitative Infrared Spectroscopy,” Appl. Spectrosc. 58, 1452-1461, 2004). The residual (bottom spectra in FIG. 4A) shows some deviations of order similar to the rms noise near the spectral lines due to mismatch between the measured line shapes and the PNNL data. The inset in FIG. 4A shows the typical correspondence between the measured spectrum and the best-fit data.

[0132] FIG. 4B shows the extracted concentration data for water, methane and ethane, showing that it is possible to obtain environmental concentration values consistent with independent humidity measurements (water), established ambient levels (methane) or known control concentrations (ethane).

[0133] Measured concentrations at 10, 20, 25 and 30 m, with data points showing the average values from approximately 45 spectra each, and the error bars showing the ±1 standard deviation range are shown in FIG. 4B. The methane background value is consistent with reported ambient levels measured at 1885±15 ppb (E. J. Dlugokencky, A. M. Crotwell, P. M. Lang, J. W. Mund and, M. E. Rhodes (2018), “Atmospheric Methane Dry Air Mole Fractions from quasi-continuous measurements at Barrow, Ak. and Mauna Loa, Hi., 1986-2017,” Version: 2018-03-19, Path: ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4/in-situ/surface/). Respective water and ethane values were consistent with the ambient relative humidity measured in the building and the filling concentration of the ethane cell. The dashed lines show the average of all the measured values.

Real-Time Methane Emission Measurement at 70-m Range

[0134] Using a 78-m indoor corridor as a test site, the inventors simulated a point emission by releasing a 2% methane:air mix for 100 seconds at a rate of 103 μg s.sup.−1 at a distance of 65 m from the OPO. The signal was recorded from a simple target of rough aluminum foil situated 70 meters from the OPO, with the beam passing near the emission point. No ethane cell was present. The spectra recorded every seven seconds were fitted in the same way as described previously to provide concentrations of water, methane and ethane.

[0135] Although ethane concentration remained as an available fitting parameter, as expected, the resulting fitted concentration was negligible since ethane is not naturally present in the atmosphere. FIG. 5A shows an example of a spectrum recorded without averaging at 70-m range and at a moment close to the peak methane emission. The Q-branch of methane can be clearly seen near 3.3 μm. The inset of FIG. 5A shows the correspondence to the best-fit PNNL database in the 3.18-3.21 μm region. In contrast to ethane, methane and water show very little continuum absorption under these experimental conditions (20° C., 101800 Pa), so the inferred illumination spectrum closely follows the envelope of the measured spectrum.

[0136] FIG. 5B presents the measured water (upper line) and methane (lower line) concentrations over 400 seconds, showing the methane concentration rising from background levels (˜1900 ppb) to a peak of around 13000 ppb before returning to near the original value as the gas disperses. Prior to the methane release, the rms variation of the measured concentration of background methane at this range was <100 ppb. Water showed more variability, which is expected to be associated with convection effects and environmental variations.

Detection Sensitivity, Repeatability and Accuracy

[0137] Sensitivity to path integrated concentrations of a few ppb can be achieved by averaging multiple spectra. Using a 35-m-range aluminum-foil target, and with the OPO tuned away from the strongest methane and water absorption lines, the inventors averaged 1250 spectra over one hour to obtain the spectrum shown in FIG. 6. As before, the dashed line is the fitted illumination spectrum, the solid area depicts the (averaged) experimental data and the black line at the top of the solid area is the combined envelope and absorption-line fit, in this case using HITRAN data for methane, water-vapor and carbon dioxide (L. S. Rothman et al., “The HITRAN2012 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 130, 4-50, 2013).

[0138] The use of HITRAN avoided an artefact in the PNNL data which introduced a weak continuum absorption in methane, of minimal impact when fitting individual spectra, but observable when fitting a low-noise averaged spectrum. The quality of the fit can be seen from the inset of FIG. 6, which compares the fitted and average spectra, with a 0.1-cm.sup.−1 instrument response imposed on the HITRAN data. The best-fit concentrations were 0.977% water, 1830 ppb methane and no carbon dioxide (as expected in this wavelength band). Small differences between the experimental and HITRAN line shapes are observed in the residuals.

[0139] The rms error in absorption-free regions is 0.19%, and this figure can be used to infer a detection sensitivity of 97 ppb for methane at this range and in this wavelength band, where the absorbance is characteristically weak (see FIG. 6, inset). Tuning the OPO to 3.3 μm encounters the strongest methane absorption (the Q-branch), and at this wavelength, the equivalent detection sensitivity would be 17 ppb.

[0140] A comparison can be made with commercial open-path Fourier-transform spectrometers, for which a survey of 64 common gases (George M. Russwurm, Jeffrey W. Childers, “FT-IR Open-path Monitoring Guidance Document,” U.S. Environmental Protection Agency, Human Exposure and Atmospheric Sciences Division, National Exposure Research Laboratory, 1999) reported a sensitivity of 1597 ppb.Math.m for methane measurements near 3017 cm.sup.−1 (or 3.3 μm) (Q-branch absorption). A leading commercial system reports 2 ppb detection sensitivity for methane at 200 m range with 1 hour of averaging (Bruker, “D-fenceline™ and OPS”, www.brukeropenpath.com at the following path: /atmosfir-d-fenceline/) The equivalent performance of the FTIR system of the present disclosure is 595 ppb.Math.m or 1.5 ppb (1 hour average), but critically this is achieved without the need for a precision retroreflector-array target.

[0141] While averaging is advantageous from a noise point of view, the stability of the atmosphere ultimately limits the accuracy and repeatability of the measurements which can be obtained. All the measurements reported by the inventors were performed indoors, where convection and temperature changes were the principal causes of fluctuations in the concentrations and/or absorbances of ambient water vapor and methane.

[0142] Using the same dataset as in FIG. 6, the inventors present in FIG. 7 the results of individual fits to each spectrum, showing mean concentrations and standard deviations for methane and water vapor of 1861±349 ppb and 0.963±0.0725% respectively. Around 200 spectra for which the fitting error was exceptionally high were removed prior to the analysis. The mean concentration values agree closely with the best fit concentrations of the average spectrum (FIG. 6). The Allan deviation (graph (c) in FIG. 7) provides an insight into the stability and repeatability of the measurements, showing that averaging around 100 results (˜5 minutes of acquisition time) provides the greatest repeatability, but with longer acquisitions showing slightly higher fluctuations in the mean, particularly for water vapor, whose concentration changes sensitively with temperature. A trade-off between repeatability and sensitivity is therefore encountered at acquisition times above 5-10 minutes.

Extraction of the Illumination Spectrum of the Broadband IR Light

[0143] The Beer-Lambert law describes the absorbance in terms of the light intensity before (I.sub.o) and after (I) an absorbing medium, according to I=I.sub.o exp(−α), where I, I.sub.o and α are functions of wavelength. Quantitative spectroscopy relies on inverting the Beer-Lambert law to obtain the absorbance, α=−log(I/I.sub.o), which requires accurate knowledge of the illumination intensity before the sample.

[0144] In a laboratory measurement, a spectrum can be recorded without the sample and another spectrum with the sample present, but in a remote sensing context it is impossible to run a control experiment where the atmosphere is absent, so this option is unavailable.

[0145] An alternative laboratory approach employs a reference detector to record the instantaneous intensity of the illumination source (I.sub.o) in tandem with the intensity after the sample (I), so providing the desired I/I.sub.o ratio. In a free-space atmospheric measurement, this approach also fails because a local reference detector cannot account for systematic effects like unknown contributions to the spectral envelope of the light from the scattering target or the propagation path.

[0146] To learn the effective illumination spectrum the solution is to allow I.sub.o to be an additional free parameter when fitting the molecular absorbances to the measured spectrum, however fitting a structured spectral envelope from a broadband OPO is challenging, since it must be described by many more free points than the gas, liquid and solid absorbances, which (neglecting temperature and pressure corrections) need just one number per chemical species fitted. Performing a global multi-point optimisation from a naïve initial guess is slow and failure-prone because of many local minima in the optimization landscape.

[0147] Instead, the inventors first obtain a rapid, accurate estimate of I.sub.o and the absorbance values from a piecewise fitting of small fragments of the measured spectrum, then use these as a robust starting point to refine the values in a full-spectrum fit. This approach combines the baseline removal reported for free-space spectroscopy using dual combs at 1.55 μm (G. B. Rieker et al, “Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths,” Optica 1, 290, 2014) and the global optimization used in dual-comb spectroscopy with OPGaP OPOs (O. Kara, L. Maidment, T. Gardiner, P. G. Schunemann, and D. T. Reid, “Dual-comb spectroscopy in the spectral fingerprint region using OPGaP optical parametric oscillators,” Opt. Express 25, 32713-32721, 2017).

[0148] For species like water and methane that exhibit no band-continuum absorption the inventors note that another strategy is to reconstruct an envelope from the points between the absorption lines (L. Nugent-Glandorf, F. R. Giorgetta, and S. A. Diddams, “Open-air, broad-bandwidth trace gas sensing with a mid-infrared optical frequency comb,” Applied Physics B 119, 327-338, 2015), but this approach cannot deal with spectra from heavier alkanes like ethane and propane.

[0149] With reference to FIG. 8, the algorithm comprises a plurality of functions comprising: [0150] i. dividing the stored scattered broadband IR light spectrum, I into N spectral segments, I.sub.N (of approximately 1 cm.sup.−1), for each of which the illumination intensity can be considered constant [0151] ii. for n target chemicals, dividing the library spectrum of each into N spectral segments, matching in wavelength the segments of the IR light spectrum [0152] iii. for each segment of the IR light spectrum, estimating the concentrations of the n chemical targets by fitting n+1 free parameters, specifically the local illumination intensity co and n absorption coefficients {c.sub.1 . . . c.sub.n}, which together modify the library spectra to provide a simulated spectrum exhibiting the lowest root-mean-squared difference from the measured spectrum of the IR light over the spectral segment [0153] iv. combining N sets of coefficients {c.sub.1 . . . c.sub.n}.sup.1 . . . {c.sub.1 . . . c.sub.n}.sup.N using a weighting function F(x) (for example, F(x)=x.sup.3) to provide an initial estimate of the absorption coefficients for n target chemicals: {c.sub.1 . . . c.sub.n}=ΣF({c.sub.0})×{c.sub.1 . . . c.sub.n}/ΣF({c.sub.0}) [0154] v. concatenating the set of N values, {c.sub.o}, which describe the local illumination intensities for each spectral segment, to provide an initial estimate of the full illumination spectrum I.sub.0, where necessary using an interpolation function (e.g. a spline) to provide values of I.sub.o at all wavelengths contained in the measured spectrum of the IR light [0155] vi. combining, then iteratively optimizing, the estimated full illumination spectrum I.sub.o and the estimated absorption coefficients, {c.sub.1 . . . c.sub.n}, for the n target chemicals to modify the library spectra to provide a simulated spectrum exhibiting the lowest root-mean-squared difference from the complete measured spectrum of the IR light, and in so doing to obtain the best-fit concentrations and illumination envelope.

[0156] Although illustrative embodiments of the disclosure have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the disclosure is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. For example, FIG. 3 shows an active FTIR spectroscopy system based on that of FIG. 2, but with a different layout for the steering arrangement. In this case, the modulated IR light is coupled-out (to illuminate the chemical targets) via the telescope of the collector system itself, and so combining the functionality of the set of launch & collection systems. In such a layout, the modulated IR light would be coupled into the system confocally with the detector system (with a requirement of optical axis coalignment still be required).