FT spectrometer array with additional detector for frequency axis calibration and associated measuring method

Abstract

The invention relates to a FT spectrometer assembly (1), comprising: - a light source (2) for light (3), - an interferometer (5) having at least one beam splitter (6) and two interferometer arms (9, 10), by which the light (3) of the light source (2) is guided, each of the interferometer arms (9, 10) having a reflector (7, 8), - a measurement sample (12a) at a measurement position (12), - a detector (13) for the light (3), the measurement position (12) being disposed in the beam path of the light (3) between the interferometer (5) and the detector (13), - a device (11) for changing an optical path difference (GU) between the two interferometers (9, 10), - a reference laser (22), by which reference laser light (23) is guided through the interferometer (5), and - at least one reference detector (26) for measuring the reference laser light (23), the reference detector (26) being disposed in the beam path of the reference laser light (23) behind the interferometer (5). The spectrometer assembly is characterised in that an additional detector (14) is also provided, by means of which it is possible to measure a sub-beam (17) of the light (3) that has passed the interferometer (5) but has not passed the measurement position (12), and in that a further sub-beam (18) of the light (3) that has passed the interferometer (5) and has passed the measurement position (12) can be measured by the detector (13) concurrently with a measurement of the sub-beam (17) at the additional detector (14). The invention allows a high degree of measurement accuracy to be achieved in a simple manner with a high degree of availability of the FR spectrometer assembly.

Description

[0001] FT spectrometer arrangement with additional detector for calibration of the frequency axis and associated measurement method

[0002] The invention relates to an FT spectrometer arrangement comprising

[0003] - a light source for light,

[0004] - an interferometer with at least one beam splitter and two interferometer arms through which the light from the light source is guided, the interferometer arms each having a reflector,

[0005] - a measuring sample at a measuring position,

[0006] - a detector for the light, wherein the measuring position is arranged in the beam path of the light between the interferometer and the detector,

[0007] - a device for changing an optical path difference between the two interferometer arms, in particular for moving one or both of the reflectors,

[0008] - a reference laser, with which reference laser light is passed through the interferometer, and

[0009] - at least one reference detector for measuring the reference laser light, wherein the reference detector is arranged in the beam path of the reference laser light behind the interferometer. Such an FT spectrometer arrangement is known, for example, from DE 10 2004 025 448 A1. In FT (= Fourier transform) spectroscopy, broadband light, for example infrared (=IR) light, is split into two partial beams in an interferometer and a path difference is imposed between the partial beams. A detector is then read out onto which the superimposed partial beams fall after interaction with a sample to be examined. The detector readout is repeated for different path differences ("sampling points"). The superimposition of the partial beams results in interference, which, depending on the path difference and the frequency of the light, leads to a reduction or increase in the irradiance on the detector.The detector intensity data, which depend on the path difference, are subjected to a Fourier transformation in combination with other data processing steps, whereby a spectrum of the sample under investigation is obtained.

[0010] For measurements, it is important to be able to precisely specify or know the path difference of the partial beams at the scanning points. For this purpose, it is known to use a reference laser in addition to the typically broadband light used for the actual measurement of the sample. Its narrowband reference laser light also passes through the interferometer. The path difference of the interferometer arms is determined from its constructive and destructive interference at a reference detector (also called a scanning detector).

[0011] To correctly determine the path difference, it is advantageous to know the laser wavelength of the reference laser precisely. For a long time, HeNe (helium-neon) lasers were used as reference lasers, as they emit laser light with a very stable, known laser wavelength. However, HeNe lasers are comparatively large and expensive to operate due to their limited lifetime. Laser diodes, on the other hand, are more durable and have a more compact design. However, laser diodes have significantly poorer wavelength stability than HeNe lasers. In particular, laser diodes change the laser wavelength depending on the operating temperature and operating current, as well as due to aging.

[0012] From DE 10 2004 025 448 A1 it has become known to allow a wavelength drift due to fluctuating temperatures in an FTIR spectrometer with a VCSEL type laser diode as a reference laser and to correct the spectrum by measuring the temperature.

[0013] From DE 10 2014 226 487 A1 an FTIR spectrometer has become known in which light emanating from the reference laser source is guided through an absorption medium to a reference detector, and with the aid of electronics the laser wavelength of the reference laser is adjusted to the absorption line of the absorption medium via the known operating behavior of the FTIR spectrometer by means of a signal from the reference detector.

[0014] By taking the above measures, a wavelength drift of the reference laser can be largely compensated or a significant improvement in the wavelength stability of the reference laser can be achieved.

[0015] In addition to instability of the wavelength of the reference laser per se, there are other circumstances that can lead to a falsification of the spectrum of the measurement sample, for example, oblique incidence of the reference laser light into the interferometer. Compared to paraxial incidence, this results in a smaller optical path difference in the interferometer, which leads to the same reference signal as paraxially incident reference laser light of a longer wavelength. With oblique incidence, the frequency axis of the spectrum is therefore detuned, as would be the case with a drift of the wavelength of the reference laser. It is also known from DE 10 2004 025 448 A1 that the operating behavior of the FTIR spectrometer is occasionally determined using a reference sample with a known spectrum. However, performing measurements on the reference sample generally leads to an interruption of the measurement process.

[0016] From US 5 309 217 A an FTIR spectrometer is known in which two retroreflectors are arranged on arms of a double pendulum that are offset by 180°.

[0017] Object of the invention

[0018] It is an object of the invention to provide an FT spectrometer arrangement and an associated measuring method in which or with which a high measurement accuracy can be achieved in a simple manner with high availability of the FT spectrometer arrangement.

[0019] Description of the invention

[0020] This object is achieved according to the invention by an FT spectrometer arrangement of the type mentioned at the outset, which is characterized in that an additional detector is further provided with which a partial beam of light can be measured which has passed the interferometer but not the measuring position, and in that a further partial beam of light which has passed the interferometer and the measuring position can be measured with the detector at the same time as the partial beam is measured at the additional detector. The FT spectrometer arrangement according to the invention has two detectors, namely the detector for measuring the measurement sample and an additional detector. The latter serves to determine calibration information about a calibration substance with which the spectrum of the measurement sample can be corrected.

[0021] Data can be acquired simultaneously with the additional detector and the detector, in particular to simultaneously acquire sample information at the detector and information for calibration at the additional detector. The invention thus fundamentally enables calibration during the measurement of a sample.

[0022] The measurement position is not included in the light beam path to the additional detector, so that the measurement sample arranged at the measurement position does not interfere with the determination of the calibration information in any way. In particular, it is not necessary to remove a measurement sample (or measurement substance) from the measurement position in order to obtain the calibration information. Any industrial production processes that continuously pass a substance to be measured through a flow cell at the measurement position do not need to be interrupted to obtain the calibration information. Conversely, a desired calibration substance can be arranged in the light beam path between the interferometer and the additional detector without this interfering with the measurement of the measurement sample.In particular, within the scope of the invention, it can be arranged that only radiation is used for calibration which would not reach the measuring position with the measuring sample anyway, so that the signal-to-noise ratio is not deteriorated by the simultaneous measurement with the additional detector.

[0023] The calibration information obtained via the additional detector is used to determine a calibration factor that can be applied in the determination or calibration of spectra of measurement samples, in particular for calculating an effective laser wavelength of the reference laser (see more below).

[0024] Connecting the additional detector generally does not require an additional beam splitter (beyond the interferometer's beam splitter) or dichroic mirror (dichroit). Furthermore, connecting the additional detector generally does not require any moving parts; the additional detector is usually permanently connected to the measurement setup.

[0025] The measured spectral ranges of the detector and the additional detector can overlap completely or partially.

[0026] Essentially the same optical setup can be used for the measurement recordings on the detector and the calibration recordings of the additional detector.

[0027] In principle, all methods and measurement arrangements known for conventional FT spectrometers can be used at the measurement position, i.e., arrangements for sample preparation, sample illumination, and radiation detection. For example, a flow cell can be used for a gaseous or liquid sample. The reflectors of the two interferometer arms can, in particular, be designed as retroreflectors (cube-corner mirrors).

[0028] In the context of the description of the present invention, the electromagnetic radiation of different spectral ranges used to examine the measurement sample is uniformly referred to as "light." Typically, UV light, visible light, infrared light, or longer-wave radiation up to wavelengths of 5 mm are used in connection with the present invention.

[0029] Preferred embodiments

[0030] In a preferred embodiment of the FT spectrometer arrangement according to the invention, it is provided that the reflectors are designed as retroreflectors, that a first aperture is provided in the beam path of the light between the light source and the interferometer, with which part, in particular one half, of the light is shaded, and / or a second aperture is provided in the beam path of the light between the interferometer and the measuring position, with which part, in particular one half, of the light is shaded, and that the partial beam of light which can be measured with the additional detector is directed back from the beam splitter in the direction of the light source towards the first aperture or is directed from the beam splitter in the direction of the sample position towards the second aperture.The first aperture prevents back reflections of light entering the interferometer from the light source, while the second aperture blocks the path of light from the sample to the interferometer and back to the sample. The auxiliary detector measures light that would not reach the sample position anyway. The light measured by the auxiliary detector would be blocked by the second aperture anyway or would return to the light source (and be blocked by the first aperture), which is particularly efficient.

[0031] A preferred embodiment is one in which a focusing lens or a focusing mirror is arranged between the interferometer and the additional detector in the partial beam of light that can be measured with the additional detector. This allows the intensity of light detected at the additional detector to be increased. A preferred embodiment is also one in which a container with a calibration substance is arranged between the interferometer and the additional detector in the partial beam of light that can be measured with the additional detector. This allows any calibration substance to be used in virtually any quantity or concentration to obtain the calibration information without disrupting the measurement of a sample at the measurement position. In particular, strong absorption lines of the calibration substance can be easily generated in the additional spectrum, which can be detected with great accuracy.Alternatively, a substance that is omnipresent (already present) in the beam path can be used as the calibration substance, for example, water vapor present due to atmospheric humidity. A preferred development of this embodiment is a calibration gas, preferably water vapor, methane, or acetylene. These substances have easily detectable absorption lines that can be used for calibration in the infrared, since the line positions (wavelengths) are known with high accuracy. For example, water as an atmospheric molecule is the subject of numerous studies, and a transition from methane is used by the BIPM as a frequency standard. Furthermore, these substances are inexpensive to obtain. Measurement methods according to the invention.

[0032] The present invention further includes a method for measuring a spectrum of a measurement sample by means of an FT spectrometer arrangement, in particular with an FT spectrometer arrangement according to the invention as described above, wherein the measurement comprises one or more measurement recordings, wherein in a respective measurement recording

[0033] - Light from a light source is passed through an interferometer, interacts with the sample at a measuring position behind the interferometer and is measured at a detector,

[0034] - furthermore, reference laser light from a reference laser, in particular a laser diode, is passed through the interferometer and measured on at least one reference detector,

[0035] - wherein, inter alia, a signal from the detector of the one or more measurement recordings and a calibration factor for calibrating a frequency axis are used to generate a spectrum of the measurement sample, characterized in that the measurement comprises one or more calibration recordings, wherein in a respective calibration recording - light from the light source is passed through the interferometer, and from the

[0036] Beam path of the light, a partial beam of the light is measured at an additional detector after passing the interferometer without passing the measuring position, - furthermore, reference laser light from the reference laser is passed through the interferometer and is measured at the at least one reference detector, that among other things a signal from the additional detector of the one or more calibration recordings is used to generate an additional spectrum, and that a reference spectrum or partial information therefrom of a calibration substance, in particular a calibration gas, which is contained in the beam path of the light between the light source and the additional detector, is compared with the additional spectrum or partial information therefrom in order to determine the calibration factor.

[0037] By measuring the partial light beam at the additional detector during the calibration recording(s), calibration information can be obtained for the actual measurement recording(s) on the measurement samples. A measurement sample at the measurement position does not interfere with the calibration recordings. Therefore, the measurement position does not need to be "empty" during a calibration recording; rather, a measurement sample can still be positioned or already positioned at the measurement position without interfering with the calibration recording. Conversely, if desired, a calibration substance can be placed between the interferometer and the additional detector without interfering with the measurement recordings.

[0038] By comparing the additional spectrum with the reference spectrum, a calibration factor can be determined, which can be used to calibrate a spectrum of a measurement sample with respect to the frequency axis. Calibration with the calibration factor determined according to the invention can essentially compensate for all sources of detuning of the frequency axis (wavelength scale) of the spectrum of a measurement sample, in particular any wavelength instability of the reference laser or an oblique incidence of the reference laser beam into the interferometer. Calibration can be performed as frequently as desired or necessary. The availability of the FT spectrometer equipment is practically unaffected by calibration recordings, since measurement recordings and calibration recordings can essentially take place simultaneously.Accordingly, within the scope of the invention, it is generally provided that measurement recordings and calibration recordings take place simultaneously for at least part of the operation of the FT spectrometer arrangement. The calibration factor, also referred to here as S, can be expressed in an effective laser frequency F. eff of the reference laser, which is used after the Fourier transformation to assign the frequencies or wavelengths corresponding to the data points of the spectrum (via the readout positions / path differences). The calibration factor S then correlates an effective laser frequency F eff of the reference laser with the (fixed) nominal laser frequency F no of the reference laser over F eff =F no *S. Note that the calibration factor S can be less than 1 or greater than 1. The nominal laser frequency is an approximate frequency specification, which a manufacturer, for example, specifies for a laser diode.

[0039] Note that the calibration factor S is constantly updated during operation of the FT spectrometer arrangement, for example by changing a previous calibration factor S ait is multiplied by an update factor UF, i.e. S=UF*S ait - A calibration factor S ait The spectrum generated from a measurement sample can then be corrected to a calibrated spectrum by correcting the frequency axis with the update factor UF, or a new calibrated spectrum can be generated directly with the current calibration factor S from at least one interferogram (raw data of the detector signal).

[0040] The additional spectrum can be an uncorrected additional spectrum (without correction with a calibration factor, or calibration factor "1", for example by adding the nominal laser frequency F to the Fourier transformation when determining the path differences. nois used as a basis), or one with a (previous) calibration factor S ait corrected additional spectrum.

[0041] For a measurement or calibration recording, the optical path difference is changed by moving one or both reflectors ("scan") and tracked, and the detector or the additional detector are read out at known path differences. For each measurement or calibration recording, one or more passes are generally made through the available or selected range of the optical path difference (travel path of the reflector(s) in the interferometer arms). One or more interferograms or additional interferograms are obtained as the signal or data of the detector or additional detector. A spectrum or additional spectrum can be obtained in each case by Fourier transformation.

[0042] The readout of the detector and the additional detector can be carried out at equidistant path differences (determined, for example, by the zero crossings of the AC voltage component of the detector signal for the reference laser light) (conventional method), but can also be carried out at equidistant times (where the path differences at the readout times are calculated via the measured time-dependent intensity of the interfering reference light, if necessary via interpolation).

[0043] The reference laser light is approximately monochromatic. The reference laser is typically designed as a low-cost laser diode; special frequency stability is generally not required; it is sufficient if the frequency stability is maintained for the validity period of a calibration factor, which, if necessary, can be shortened to the duration of a measurement recording within the scope of the invention.

[0044] Preferred variants of the measuring method according to the invention

[0045] A particularly preferred variant of the method according to the invention is one in which one or more of the measurement recordings and one or more of the calibration recordings are performed simultaneously as one or more joint recordings. During the joint recordings, the passes through the available travel path of the interferometer arms are used both for reading the detector and for reading the additional detector; thus, the interferometer arrangement remains fully available for measurement recordings during the calibration recordings.

[0046] A further development of this variant is also preferred, in which all measurement and calibration recordings are performed as a single recording. This maximizes the availability of the FT spectrometer setup for measurement operations, and furthermore, the calibration can be adjusted or checked quasi-continuously, maximizing the accuracy of the acquired spectra and the resulting measurement results, e.g., concentrations in a mixture.

[0047] In an advantageous further development, the calibrated spectrum of the measurement sample and the calibration factor used to generate the calibrated spectrum are based on one or more common recordings. In other words, the one or more calibration measurements used to determine the calibration factor, which are used to generate the calibrated spectrum of the measurement sample, are simultaneous with the one or more measurement recordings for this spectrum and are therefore based on the same passes through the available travel path of the interferometer arms. The reference laser light of the calibration recordings and the measurement recordings, which is used to determine the underlying path differences, is the same, so that no detuning can occur in this regard. This achieves a very high calibration accuracy.

[0048] In an alternative variant, the calibration factor applied when generating the calibrated spectrum of the measurement sample is based on one or more calibration images taken prior to the one or more measurement images on which the calibrated spectrum is based. This allows for the generation of the calibrated spectrum of the measurement sample to be carried out particularly quickly. To generate the spectrum or correct it with the correction factor, there is no need to wait for the correction factor to be determined from current joint images; instead, a previously calculated calibration factor can be used.

[0049] Variants with measurement of multiple spectra on multiple samples

[0050] A variant is also preferred in which, within the scope of the method, a measurement of several spectra is carried out on several measurement samples, whereby a calibrated spectrum of the respective measurement sample is generated in each case.

[0051] In this variant, several (different, new) measurement samples are measured sequentially. Within the scope of the invention, the same sample substance is also considered a new measurement sample if it has or could have changed since the last measurement, for example, if it has a different temperature or pressure, or has simply undergone chemical changes over time. Different measurements on a flow cell through which sample substance is continuously passed are considered to be performed on different measurement samples.

[0052] Preferably, the current calibration factor S is determined for a measurement sample during one or more joint recordings based on the one or more calibration recordings, and this is immediately applied to determine the calibrated spectrum from the one or more simultaneously recorded measurement recordings of the current measurement sample.

[0053] Alternatively, during one or more joint recordings, a calibration factor S ait calibrated spectrum of the current sample and a determination of a new calibration factor S for the next sample to be measured.

[0054] Within the scope of this variant, it is also possible to use a calibration factor, determined from one or more calibration recordings, for multiple measurement samples and the generation of their respective calibrated spectrum. A preferred further development of this variant is to determine a separate calibration factor for each calibrated spectrum of the multiple measurement samples, which is used to generate the calibrated spectrum of a respective measurement sample. This enables a particularly high accuracy of the calibration factor; the calibration factor applies only to one spectrum at a time (without prejudice to the possibility that the next calibration factor coincidentally matches the previous calibration factor).

[0055] In a sub-variant of this further development, where one or more of the measurement recordings and one or more calibration recordings are carried out simultaneously as one or more joint recordings, it is provided that the respective calibration factor and the respective calibrated spectrum of the respective measurement sample are based on joint recordings. In other words, the one or more measurement recordings for determining the calibration factor S for a respective measurement sample and the one or more measurement recordings of this measurement sample to which the calibration factor S is applied are recorded simultaneously. The reference laser light of the calibration recording(s) and the measurement recording(s) for a respective measurement sample is the same, so that no detuning can occur in this regard. This achieves a particularly high calibration accuracy.

[0056] An alternative development proposes applying a calibration factor across samples, determined based on one or more calibration images, to generate multiple calibrated spectra from multiple measurement samples. This can accelerate the generation of calibrated spectra for the different measurement samples and reduce computational effort.

[0057] In a sub-variant of this further development, the calibration factor across samples is determined based on one or more calibration images that have a higher spectral resolution than at least some of the measurement images on which the multiple calibrated spectra of the multiple measurement samples are based. This allows for a significant improvement in calibration accuracy with minimal effort, particularly by requiring a slightly longer measurement time during the calibration image(s) with higher resolution.

[0058] Another preferred sub-variant provides that at least some of the measurement recordings, on which a calibrated spectrum of a measurement sample is based, which is generated with the cross-sample calibration factor, are carried out together with monitor recordings, wherein the monitor recordings correspond to calibration recordings with which an auxiliary calibration factor is determined, and that a decision is made on the basis of a comparison between the auxiliary calibration factor and the cross-sample calibration factor whether

[0059] - the cross-sample calibration factor is still usable and / or the respective calibrated spectrum calculated with the cross-sample calibration factor is usable, or

[0060] - a new cross-sample calibration factor must be determined and / or the respective calibrated spectrum calculated using the cross-sample calibration factor must be discarded. This procedure can ensure, with minimal effort, that the cross-sample calibration factor or a calibrated spectrum based on it maintains a certain minimum accuracy.

[0061] If a new, cross-sample calibration factor needs to be determined, this can be done subsequently, for example, with one or more calibration images with increased spectral resolution, and applied in the future. Alternatively, the auxiliary calibration factor can be calculated for the next cross-sample calibration factor and applied in the future.

[0062] Typically, a limit value or interval of an absolute or relative deviation of the auxiliary calibration factor from the (previous) cross-sample calibration factor is defined (stored), which is used to determine whether the cross-sample calibration factor is still usable. For example, it can be specified that the auxiliary calibration factor may deviate by a maximum of 0.01% from the cross-sample calibration factor to still be considered usable.A further development is advantageous, wherein one or more of the measurement recordings and one or more of the calibration recordings are carried out simultaneously as one or more common recordings, which provides that the measurement comprises several calibration recordings which are common recordings and are distributed over the several measurement samples, that an averaged additional spectrum is calculated from the signals of the additional detector or the additional spectra which belong to these several calibration recordings, and that the reference spectrum or a partial information therefrom is compared with the averaged additional spectrum or a partial information therefrom in order to determine the calibration factor, wherein this calibration factor is applied.

[0063] - for the generation of the calibrated spectra of the last measurement sample of the measurement samples to which the calibration images are distributed,

[0064] - and / or for generating a calibrated spectrum of at least one next measurement sample, in particular wherein the plurality of measurement samples comprises a predefined number of most recently measured measurement samples.

[0065] The additional spectrum averaged (over the measurement of multiple samples) generally exhibits a better S / N ratio than a single additional spectrum (acquired during the measurement of a single sample). This can improve the accuracy of the calibration.

[0066] Variants for determining the calibration factor

[0067] In a preferred variant, the comparison of the reference spectrum or the partial information thereof with the additional spectrum or the partial information thereof is carried out by a wavenumber position P ref an absorption line of the calibration substance in the reference spectrum and a corresponding wavenumber position P zusthe absorption line of the calibration substance in the additional spectrum, in particular where the calibration factor S is determined as S = (P r ef / Pzus)*S ait , with Sai t : previous calibration factor used in generating the additional spectrum, where S ait = 1 if no calibration factor was previously applied when generating the additional spectrum. This procedure is particularly simple.

[0068] If the additional spectrum contains an absorption line of sufficient strength without significant overlap with other absorption lines, so that the wavenumber position P refThis variant is particularly simple since the wavelength range can be easily determined. As a rule, a suitable absorption line can be ensured by placing a container containing a suitable calibration substance in the light beam path between the interferometer and the additional detector. Within the scope of this variant, for example, an absolute difference or a quotient of the wavenumber positions can be used to quantitatively determine the calibration factor. If the additional spectrum is a (with a previous correction factor S ait ) corrected additional spectrum, the difference should be 0 or the quotient 1 (within the desired accuracy) if the (previous) calibration factor is to be correct. Typically, an interval is defined within which the difference or the quotient must lie in order to confirm the (previous) calibration as usable. In the case of S ait = 1, the nominal laser frequency F nobe used to generate the additional spectrum; the nominal laser frequency F no defines the frequency axis of the additional spectrum.

[0069] A variant is advantageous which provides that the comparison of the reference spectrum or the partial information thereof with the additional spectrum or a partial information thereof is carried out by calculating the cross-correlation of a selected spectral range of the reference spectrum and a corresponding spectral range of the additional spectrum, ie the integral Kl with is calculated, with so: mean wavenumber position of a characteristic absorption range of the calibration substance, 5 : integration width, s: wavenumber, t: wavenumber shift variable, f: reference spectrum; g: additional spectrum, and for the maximum of K1(t) the corresponding value tq of the wavenumber shift variable t is determined, and the calibration factor S is S = [(s0 / (so+tq)] *S aitis determined with Sai t : previous calibration factor used in generating the additional spectrum, where S ait=l if no previous calibration factor was applied when generating the additional spectrum. With this variant, the calibration factor S can be determined quite accurately, even if the detection of individual peaks in the additional spectrum is difficult, for example due to overlap. Typically, in preparation for calculating the cross-correlation, the additional spectrum (and, if necessary, the reference spectrum) is interpolated to a higher data point density. This is preferably done by adding "zeros" to the respective interferogram (so-called "zerofilling"). Note that the reference spectrum of the calibration substance can be determined experimentally or based on a model calculation using either a high-resolution spectrum obtained using molecular data, e.g., as contained in databases such as HITRAN, or a measured spectrum with higher spectral resolution.The model calculation can then contain a model for the spectrometer, in particular the so-called instrument line shape (ILS).

[0070] Equally advantageous is a variant which provides that the comparison of the reference spectrum or the partial information thereof with the additional spectrum or a partial information thereof is carried out by calculating the integral K2 with is calculated, with so: mean wavenumber position of a characteristic absorption range of the calibration substance, 5: integration width, s: wavenumber, s: calibration factor variable, f: reference spectrum; g: additional spectrum, and for the maximum of K2(s) the corresponding value sO of the calibration factor variable s is determined, and as calibration factor S=sO*S ait is determined with Sai t: previous calibration factor used in generating the additional spectrum, where S ait =1 if no previous calibration factor was applied when generating the additional spectrum. Even with this variant, the calibration factor S can be determined quite accurately, even if the detection of individual peaks in the additional spectrum is difficult, for example, due to overlap. Note that the reference spectrum of the calibration substance can be determined experimentally or based on a model calculation (see above).

[0071] A further advantageous variant is one in which the comparison of the reference spectrum or a partial piece of information therefrom with the at least one additional spectrum or a partial piece of information therefrom is carried out by iteratively modeling a plurality of model spectra starting from the reference spectrum, with the model spectra each being compared with the experimentally obtained additional spectrum. For iterative modeling of the model spectra ("fit routine"), the Levenberg-Marquardt algorithm, for example, can be used. This variant allows for a particularly precise determination of the calibration factor S.

[0072] A preferred further development of this variant provides that at least one instrument-related line shape and an interim calibration factor are taken into account as parameters in the modeling, and that the interim calibration factor value IKW, which was used for the model spectrum with the best agreement with the experimentally obtained additional spectrum, is determined and used as the calibration factor S=IKW*S ait is determined with Sai t : previous calibration factor used in generating the additional spectrum, where S ait=1 if no previous calibration factor was applied when generating the additional spectrum. The determination of the calibration factor is particularly precise here; in particular, distortions in the additional spectrum due to the measuring instruments used can be compensated. Note that other variables can also be obtained as a result during modeling (fitting routine), for example, aperture sizes that contribute to stretching or compressing the frequency axis.

[0073] Further variants A further preferred variant is one in which a Fourier transformation is used from the signal of the additional detector using a previous calibration factor S ait a pre-corrected additional spectrum is generated, and then the pre-corrected additional spectrum or a partial information thereof is compared with the reference spectrum or the partial information thereof. From the comparison of the pre-corrected additional spectrum (based on Sait ) or the partial information from it with the reference spectrum or the partial information from it, the update factor UF can be determined, with which the current calibration factor S=UF*S ait can be calculated. This variant is primarily used to check the existing calibration factor Sait (especially across samples) and, if necessary, to confirm or reject a calibrated spectrum of the measurement sample calculated with it and, if necessary, to initiate a new calibration (calculation of a new calibration factor, especially across samples). The pre-corrected additional spectrum should agree with the reference spectrum (within the desired accuracy) with regard to the position of the calibration substance lines, assuming a usable / correct calibration factor.

[0074] An alternative variant provides for an uncorrected additional spectrum to be generated from the signal of the additional detector using a Fourier transformation, and then the uncorrected additional spectrum or a partial information thereof is compared with the reference spectrum or the partial information thereof. This variant is primarily used to directly determine a "total" calibration factor S from the current calibration recording(s) (in particular for a currently measured sample or for at least one subsequent sample to be measured). No (previous) correction factor S is required to generate the uncorrected additional spectrum. ait applied, or the previous correction factor S ait was assumed to be "1", which is the same. However, in this variant, a review of a previous calibration factor S aitby determining a "current" calibration factor (target calibration factor) S using the uncorrected additional spectrum and comparing it with the previous calibration factor S ait , which was previously used to generate the calibrated spectrum of the measurement sample. If the "current" calibration factor S does not deviate or deviates only slightly from the previous calibration factor Sait, the calibrated spectrum of the measurement sample calculated with the previous calibration factor can be confirmed as usable.

[0075] An advantageous variant involves placing a container containing a calibration substance in the light beam path between the interferometer and the additional detector. This allows virtually any calibration substance in any concentration to be introduced into the light beam path to generate a significant absorption. Positioning the container "behind" the interferometer (between the interferometer and the additional detector) has the advantage that the calibration substance does not interfere with the measurement of the sample's spectrum. Alternatively, the container can be placed in the shared light beam path to the detector and the additional detector, i.e., "inside" or "in front of" the interferometer.

[0076] In an alternative variant, a substance that is omnipresent in the FT spectrometer setup is chosen as the calibration substance. This simplifies the measurement setup. The omnipresent substance can be a component of the air, especially if the FT spectrometer setup is exposed to air.

[0077] A preferred variant is one in which the calibration substance is steam, methane, or acetylene. These substances are available inexpensively.

[0078] Uses according to the invention

[0079] Finally, the scope of the present invention also includes the use of an FT spectrometer arrangement according to the invention, as described above, in a method according to the invention, as described above. This allows calibrated spectra to be obtained with high accuracy while maintaining high availability of the FT spectrometer arrangement. In a preferred variant of the use according to the invention, the determination of the calibration factor and the generation of the calibrated spectrum are carried out in situ by an evaluation unit of the FT spectrometer arrangement. This makes the calibrated spectra immediately available.

[0080] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further detailed features can be used individually or in combinations according to the invention. The embodiments shown and described are not to be understood as an exhaustive list, but rather are exemplary in nature for describing the invention.

[0081] Detailed description of the invention and drawing

[0082] Fig. 1 shows schematically the structure of an exemplary embodiment of an FT spectrometer arrangement according to the invention;

[0083] Fig. 2 shows schematically an exemplary course of the amplitude of the measuring signal at the detector or additional detector and the amplitude of the measuring signal at the reference detector as a function of the path difference of the interferometer arms;

[0084] Fig. 3 shows schematically, as an example, the Fourier-transformed measurement signal of an additional detector as a function of the wavenumber (“additional spectrum”);

[0085] Fig. 4a shows schematically an exemplary section of an additional spectrum to illustrate the determination of the calibration factor;

[0086] Fig. 4b schematically shows a section of a reference spectrum corresponding to Fig. 4a, illustrating the determination of the calibration factor; Fig. 5 schematically shows a diagram of an exemplary integral Kl of a cross-correlation for determining the calibration factor;

[0087] Fig. 6 shows a schematic diagram of an exemplary apparatus function as a function of wavenumber;

[0088] Fig. 7a schematically illustrates the generation of a spectrum of a measurement sample corrected with a calibration factor, in a variant with determination of an update factor and direct determination of the corrected spectrum from the interference pattern of the measurement sample;

[0089] Fig. 7b schematically illustrates the generation of a spectrum of a measurement sample corrected with a calibration factor, in a variant with determination of an update factor and conversion of the frequency axis of a precalibrated spectrum of the measurement sample;

[0090] Fig. 7c schematically illustrates the generation of a spectrum of a measurement sample corrected with a calibration factor, in a variant with direct determination of the correction factor from an uncorrected additional spectrum;

[0091] Fig. 8a schematically illustrates a variant of a measuring method according to the invention, with joint recordings, wherein the calibration factor for these measurement recordings is determined by means of calibration recordings that take place simultaneously with these measurement recordings;

[0092] Fig. 8b schematically illustrates a variant of a measuring method according to the invention, with joint recordings, wherein the calibration factor for the measurement recordings is determined with calibration recordings that take place before measurement recordings;

[0093] Fig. 8c schematically illustrates a variant of a measuring method according to the invention, wherein a calibration factor is determined from an averaged additional spectrum as a moving average;

[0094] Fig. 8d schematically illustrates a variant of a measuring method according to the invention, wherein a cross-sample calibration factor is applied based on a calibration image with increased spectral resolution;

[0095] Fig. 8e schematically illustrates a variant of a measurement method according to the invention, in which a cross-sample calibration factor is applied based on a calibration image with increased spectral resolution, and the usability of the cross-sample calibration factor is monitored using monitor images. FT spectrometer arrangement according to the invention

[0096] Fig. 1 shows a schematic view of the structure of an exemplary embodiment of an FT spectrometer arrangement 1 according to the invention.

[0097] The FT spectrometer arrangement 1 comprises a light source 2 that emits broadband light 3, infrared (=IR) light in the embodiment shown. The light 3 is guided to an interferometer 5 by means of a collimating arrangement, in the embodiment shown a collimating mirror 4.

[0098] The interferometer 5 comprises a beam splitter 6 and two reflectors 7, 8, which are designed here as retroreflectors (cube corner mirrors) 7a, 8a, thereby forming two interferometer arms 9, 10. By means of a device 11 for changing the optical path difference, the length of the interferometer arm 9 can be changed, and thereby the optical path difference between the interferometer arms 9, 10 can be changed or adjusted. The device 11 can, for example, comprise an electric drive for the reflector 7, with which the reflector 7 can be displaced along the optical axis 27 of the interferometer arm 9. Typically, the drive 11 causes the reflector 7 to constantly oscillate along the direction of the axis 27. The reflector 8 is rigid (stationary) in the embodiment shown; Alternatively, the reflector 8 can also be movable (not shown in detail), see for example US 5 309 217 A.At the beam splitter 6, the light 3 arriving from the light source 2 is partially transmitted to the reflector 7 and from there reflected back to the beam splitter 6, and partially reflected to the reflector 8 and from there reflected back to the beam splitter 6. Light 3 from the reflector 7 is partially reflected at the beam splitter 6 in the direction of the measuring position 12 and the detector 13, and partially transmitted in the direction of an additional detector 14. Light 3 from the reflector 8 is partially transmitted at the beam splitter 6 in the direction of the measuring position 12 and the detector 13, and partially reflected in the direction of the additional detector 14. Note that a first aperture 15 is arranged in the beam path of the light 3 between the light source 2 and the interferometer 5, with which part of the light 3 (here approximately half) is shaded.Likewise, a second aperture 16 is arranged in the beam path of the light 3 between the interferometer 5 and the measuring position 12, with which a part of the light (here approximately half) is also shaded.

[0099] A partial beam 17 of light 3 thus travels from the interferometer 5 to the additional detector 14, and a partial beam 18 of light 3 travels to the sample position 12 and to the detector 13. The partial beam 17 is focused by a focusing lens 14a into an entrance aperture of the additional detector 14. A container 19 containing a calibration substance 19a (e.g., methane gas) can also be arranged in the partial beam 17. This container is essentially transparent to the light 3 in a spectral range in which the calibration substance has signatures used for calibration. The partial beam 18 is focused by means of a focusing arrangement, which in the embodiment shown is designed as a focusing mirror 20, onto the measuring position 12, at which a measuring sample 12a is arranged (for example in a container which is essentially transparent to light in the spectral range used).The radiation 3, which has passed the measuring position 12, is focused into an entrance aperture of the detector 13 by an imaging arrangement, which is designed here as an imaging mirror 21.

[0100] The FT spectrometer arrangement 1 further comprises a reference laser 22, which emits a reference light 23 in the form of a laser beam; the laser beam is preferably selected in the visible spectral range. The reference light 23 is guided parallel to the light into the interferometer 5 by means of a mirror 24, where, analogously to light 3, it is partially transmitted and partially reflected at the beam splitter 6, and then reflected by the reflectors 7, 8. In the beam path behind the interferometer 5, i.e., between the interferometer 5 and the detector 13, the reference light 23 is guided by a further mirror 25 into a reference detector 26, where it is registered. The reference light 23 thus essentially travels the same optical path in the interferometer as light 3.Using the reference detector 26, the constructive and destructive interference of the reference light as a result of the changing optical path difference of the interferometer arms 9, 10 can be measured. If the (effective) laser wavelength of the reference light 23 is known, the path difference can be determined as a function of time, particularly for the times at which the detector 13 is read. The effective laser wavelength can be determined within the scope of the invention by using the additional detector 14 to generate an additional spectrum containing absorption lines of the calibration substance 19a (contained in the container 19 or omnipresent in the FT spectrometer arrangement 1) and comparing it with a reference spectrum of the calibration substance. This allows a calibration factor to be determined, with which the effective laser wavelength can be calibrated.The calibrated, effective laser wavelength can be used to generate an interferogram of the sample using the detector signals, which is used to generate a calibrated, i.e. highly accurate, spectrum of the sample using Fourier transformation (FT).

[0101] It should be noted that the FT spectrometer arrangement 1 is an exemplary embodiment that can be modified in many ways. For example, the curved / imaging mirrors 4, 20, and 21 can also be replaced by lens arrangements with corresponding imaging properties, possibly in combination with plane mirrors. Likewise, the additional detector 14 and the container 19 can also be arranged in front of the second aperture 16 (instead of the first aperture 15). Measurement methods according to the invention

[0102] Within the scope of the measurement method according to the invention, a detector and an additional detector are read out at defined path differences. Fig. 2 illustrates, by way of example, in the upper half, a section of the measurement signal A at a detector or additional detector as a function of the path difference GU (in arbitrary units αu) of an FT spectrometer arrangement according to the invention, such as that shown in Fig. 1. Note that with a uniform change in the path difference or uniform movement of the reflector (reference 7 in Fig. 1), the path difference is a linear function of time. The lower half also shows the curve (in arbitrary units) of a measurement signal A. refof a reference detector as a function of the (same) path difference. The DC component is removed by a high-pass filter. The corresponding reference laser light passes through the same interferometer as the light. At the reference detector, constructive and destructive interference alternate as a function of the path difference GU. In the variant shown, at each zero crossing of the measurement signal A refAt the reference detector, a readout of the detector or additional detector is provided ("sampling" at the zero crossings); these zero crossings are spaced apart by the optical path difference of l / 2, where l is the wavelength of the reference laser light (note that o=l / , with F: laser frequency and c: speed of light). Accordingly, the readout positions of the detector and the additional detector are (in the best case) known as precisely as the laser wavelength of the reference laser. The readout positions are incorporated via the Fourier transformation into the accuracy of the frequency axis of the spectra and additional spectra, which are obtained based on the measurement signals of the detector and the additional detector.

[0103] When using laser diodes in the reference laser, fluctuations in the laser frequency can occur, particularly due to aging or temperature fluctuations. An oblique incidence of the reference laser beam into the interferometer can also lead to a change in the apparent laser frequency. Effects of this kind can lead to an "effective" laser frequency F eff The deviation of the effective laser frequency F eff from the (fixed) nominal laser frequency F no described by the calibration factor S, with Fe ff =F no*S. If the calibration factor S for an FT spectrometer arrangement is known, this arrangement can be used to generate precise spectra of measurement samples corrected by means of the calibration factor. Note that in practice the current calibration factor S is usually stored via a current, effective laser frequency of the reference laser. Within the scope of the invention, the value of the calibration factor S is advantageously determined via the additional detector (see further below). In the method according to the invention, one or more measurement recordings are carried out to generate a spectrum of a measurement sample; a single measurement recording corresponds to at least one pass through the available range of the optical path difference of the interferometer (by means of the device for changing the optical path difference). One or more interferograms can then be determined from the signal of the detector of the one or more measurement recordings.Note that an interferogram can be generated for each pass through the path difference, for each acquisition, or for all acquisitions together, depending on how summing / averaging is performed. The one or more interferograms can then be subjected to a Fourier transformation. If only one interferogram is provided, its Fourier transform can be considered the spectrum of the sample; if multiple interferograms are provided, their Fourier transforms are summed / averaged to obtain the spectrum of the sample.

[0104] In a corresponding manner, one or more calibration recordings are carried out to generate the additional spectrum of a calibration substance, whereby the additional spectrum of the calibration substance is obtained from the signal of the additional detector or the associated one or more additional interferograms by Fourier transformation.

[0105] Figure 3 shows an example of an additional spectrum obtained in this way, i.e., a diagram plotting the intensity I of the Fourier transform of (here) an additional interferogram determined using the signal from the additional detector as a function of the wavenumber s (note that s=1 / l, where l is the wavelength of the light). The additional spectrum contains, on the one hand, the spectral distribution of the emission of the light source, which here has its spectral maximum in the range between 1000 and 2000 cm 1 On the other hand, the calibration substance (and possibly other substances) in the beam path causes a characteristic local absorption. In this example, the absorption of water is particularly sharp in the ranges 1400-1900 cm 1 and 3500-4000 cm 1, which can be used well for calibration. Note that a "raw" spectrum obtained directly from the Fourier transform can be divided by a background spectrum (recorded without the substance of interest, here the calibration substance); in a "cleaned" spectrum obtained in this way (also called a transmission spectrum), the spectral lines of the substance of interest can be identified in isolation (not shown in detail). Both "raw" and "cleaned" spectra can be used within the scope of the invention.

[0106] Fig. 4a shows a schematic enlarged view of the wavenumber range from approximately 5503 to 5512 1 / cm of an additional spectrum (see, for example, the additional spectrum from Fig. 3). In this wavenumber range, the calibration substance produces various absorption lines, in particular the strongest absorption peak at P ZU s at approx. 5507.3 cm 1(more precisely: 5507.321). The wavenumber axis contains a reference laser frequency (or a corresponding reference laser wavelength) used in the Fourier transformation. The wavenumber axis (or the reference laser frequency) is to be calibrated.

[0107] For comparison, Fig. 4b also shows a section of a reference spectrum of the calibration substance, which also covers the wavenumber range from approximately 5503 to 5512 1 / cm; the wavenumber axis of the reference spectrum is assumed to be exactly correct. The absorption spectrum of Fig. 4b is very similar to the absorption spectrum of Fig. 4a; however, depending on the degree of previous miscalibration of the spectrum of Fig. 4a, the spectrum of Fig. 4a is stretched or compressed compared to the spectrum of Fig. 4b, which is evident from the shift in the absorption peaks. In the example shown, it is clear that the strongest absorption peak P ref(=s0) here at about 5507.5 cm 1 (exactly: 5507.5137) and opposite the peak P zus in the spectrum recorded with the additional detector (peak position of P zus in the spectrum of the additional detector in Fig. 4b (marked by a dashed line). Using this detected shift, calibration can be carried out immediately. The calibration factor S is given by S=(P r ef / Pzus)*S ait , where S ait the Caliph ¬ factor on which the additional spectrum was based. If the additional spectrum was uncorrected (i.e., at the unchanged, nominal laser frequency F no based), S ait =l. If an effective laser frequency of F eff (old) = F no *S ait (with S ait U.N ¬ equal to 1), the effective laser frequency F eff with the factor P r ef / Pzus=UF (with UF: update factor), so Fe ff (new) = F eff (old)*UF or F eff (new) = F no *S or F eff (new) = F no *S ait *UF. In the example above, UF=P ref / P ZU s=5507, 5137 / 5507, 321 = 1, 0000349898. Note that the layers P ref and P ZU s are typical partial information from the reference spectrum and the additional spectrum, which are used here to determine the calibration factor S.

[0108] Using the (new) calibration factor S or the (new) effective laser frequency Fe ff =F no *S the generation of a spectrum of a measurement sample can then be carried out with high precision.

[0109] If determining the position of individual peaks in the additional spectrum (and possibly also in the reference spectrum) is difficult, the calibration factor can also be determined by cross-correlating the additional spectrum and the reference spectrum over a selected (characteristic) spectral range (absorption range). For this purpose, the integral Kl is calculated using where s 0 : mean wavenumber position of the characteristic absorption range of the calibration substance, s : integration width, s : wavenumber, x : wavenumber shift variable, f : reference spectrum; g : additional spectrum. A shift of the wavenumber axis, expressed by the wavenumber shift variable t, is investigated using the integral Kl.

[0110] Fig. 5 shows an example diagram of the value of an integral Kl as a function of the displacement variable x; note that x has the dimension 1 / cm. At tq, the integral Kl has a maximum, ie, at this displacement between the additional spectrum g and the reference spectrum f, the greatest agreement occurs. The (new) calibration factor S is then given by S = [(ao / (ao+xO)]*S ait .

[0111] Instead of an integral Kl of a cross-correlation, the integral K2 can also be determined with where s 0 is the mean wavenumber position of the characteristic absorption range of the calibration substance, s is the integration width, s is the wavenumber, s is the calibration factor variable, f is the reference spectrum; g is the additional spectrum. The integral K2 is used to investigate a stretching of the wavenumber axis, expressed by the calibration factor variable s.

[0112] The integral K2 as a function of the calibration factor variable s (which is not shown separately here, but looks similar to the integral Kl shown in Fig. 5) then has a maximum to be determined at sO; note that the calibration factor variable s is dimensionless, and so is sO. The (new) calibration factor S is determined as S=sO*S ait -

[0113] The areas of the functions f and g encompassed by the respective integral Kl, K2 over the integration width d represent typical partial information of the reference spectrum and the additional spectrum, which are used to determine the calibration factor S.

[0114] Alternatively, it is also possible to iteratively model model spectra starting from the reference spectrum or partial information from it (e.g., a section as shown in Fig. 4b) and to determine the model spectrum that exhibits the best match with the additional spectrum. At least one line shape determined by the FT spectrometer arrangement used ("instrument") is taken into account ("instrument line shape" ILS); an example of such a line shape or instrument function is shown in Fig. 6. Fig. 6 shows how, depending on the instrument, an infinitely sharp absorption line ("delta function") would appear in a measured (additional) spectrum. The instrument usually causes both a certain line broadening and a certain asymmetry of the measured absorption line.Furthermore, an interim calibration factor value (IKW) is included in the modeling, which is assumed to be the frequency error in the measurement of the additional spectrum for the respective model spectrum. The IKW of the model spectrum with the best agreement with the experimental additional spectrum can then be used to determine the calibration factor S, where S=IKW*S. ait · In addition, further parameters of the FT spectrometer setup can be determined using the model spectrum.

[0115] Figures 7a to 7c briefly explain, by way of example, how the determination of a corrected spectrum of a measurement sample can be carried out within the scope of the invention. In the examples in Figs. 7a to 7c, one or more measurement recordings used to generate a spectrum of a measurement sample are represented by a square with sharp corners. The signal from one or more measurement recordings is converted into an interferogram (or possibly a set of interferograms). pr The same applies to calibration images obtained by an additional interferogram (or a set of additional interferograms) IN ZU s. Obtained spectra can be calibrated with the calibration factor S (KS, calibrated spectrum) or based on a previous calibration factor S aitbe precalibrated (VS, precalibrated spectrum) or uncalibrated (US, uncalibrated spectrum); the subscript pr or zus indicates the assignment to the measurement sample / measurement recordings on the detector (pr) or to the calibration substance / calibration recordings on the additional detector (zus). The reference spectrum of the calibration substance (not subject to calibration) is designated SP ref marked.

[0116] In the examples of Fig. 7a to 7c, the measurement recordings and calibration recordings are made as joint recordings simultaneously with respect to time t. In the variant of Fig. 7a, based on the at least one calibration recording I Nzus with the previous calibration factor S ait by Fourier transformation a pre-calibrated additional spectrum VS ZU s is generated. This is compared with the reference spectrum, and an update factor UF is determined. From the previous calibration factor S aitand the update factor UF, the (current) calibration factor S is determined. With this calibration factor S, the at least one measurement IN pr the calibrated spectrum KS pr of the sample by Fourier transformation. In practice, the current effective laser frequency, which is set to S ait and in the production of VS zus was applied, multiplied by the update factor UF, so that the new effective laser frequency is then based on the updated calibration factor S and is used to generate KS pr can be applied.

[0117] In the variant shown in Fig. 7b, the at least one calibration recording IN ZUS with the previous calibration factor S ait by Fourier transformation a precalibrated additional spectrum VS zusThis is then compared with the reference spectrum, and an update factor UF is determined. In addition, based on at least one measurement IN pr with the previous calibration factor S ait by Fourier transformation a precalibrated spectrum VS pr of the sample. Its frequency axis is then corrected using the update factor UF. Accordingly, in this case, the corrected spectrum KS pr the measurement sample the updated calibration factor S=S ait *UF included. In the variant shown in Fig. 7c, the at least one calibration image IN zus without calibration by Fourier transformation an uncalibrated additional spectrum US zus This is compared with the reference spectrum and the current calibration factor S is determined directly. Using this calibration factor S, the at least one measurement IN pr the calibrated spectrum KSpr of the measurement sample by Fourier transformation.

[0118] Figures 8a to 8e illustrate various variants of the measurement method according to the invention, explaining the chronological sequence of measurement recording(s) for a measurement sample and calibration recording(s) for calibrating the spectra of the measurement sample; note that in each case, a measurement of a plurality of measurement samples is provided. The designations and symbols largely correspond to the designations in Figs. 7a-7c (see above). In addition, different measurement samples are numbered PI, P2, P3. Furthermore, monitor recordings are also provided, to which the subscript mn belongs, and with which auxiliary calibration factors HS are determined; the comparisons V with the (final) calibration factor S are shown in circles. The possibilities for determining S explained in Figs. 7a-7c are no longer broken down individually; accordingly, in summary, only an additional spectrum SP is generally used for the determination of S. ZUs assigned to at least one calibration measurement, again represented by IN ZU s· Furthermore, in one variant, an averaged (additional) spectrum GS zus , which is based on calibration images taken during the measurement of different samples. Within individual time slots t0, t1, t2, etc. of the respective measurement, no further breakdown of time t is provided, which is plotted to the right. The planes of measurement images M, calibration images K, and monitor images MN are each plotted one above the other (upward).

[0119] Fig. 8a shows a variant in which several measurement samples PI, P2 are measured consecutively during the measurement; the measurement of two measurement samples is shown as an example. For each measurement sample PI, P2, at least one measurement IN is taken simultaneously in a respective time slot t1, t2. pr and at least one calibration recording IN ZUSas at least one common recording. The calibration factor S determined in the respective time slot t1, t2 is applied to the at least one measurement recording IN pr the same time slot tl, t2 to obtain the corrected spectrum KS pr to obtain.

[0120] In this variant, the best possible accuracy of the calibration factor S for the respective measurement sample PI, P2 is achieved. Fig. 8b shows a variant in which at least one calibration recording IN is made in a first time slot tO. ZU s, with which the calibration factor S is determined via the additional spectrum SPzus. In a subsequent time slot tl, at least one measurement recording IN pr on a first sample PI, and the corresponding corrected spectrum KS pr of the first measurement sample PI is generated with the calibration factor S from the time slot tO. Simultaneously with at least one measurement recording IN prAt the first sample PI, at least one further calibration recording IN takes place in the time slot tl ZU s takes place (joint recording), and the resulting calibration factor S can then be applied in the next time slot t2 for the sample P2 measured in this time slot t2. Any number of samples can be measured in a similar manner.

[0121] In this variant, joint recordings (i.e. simultaneous measurement recordings and calibration recordings) can be used, whereby in a single time slot (in the example tl) to generate the corrected spectrum KS pr the measurement sample does not have to wait for the calculation of the calibration factor S of the current time slot (tl), but the (almost as current and therefore almost as accurate) calibration factor S of the previous time slot (tO) can be used.

[0122] In the variant of Fig. 8c, four time slots t1, t2, t3, t4 of the measurement are shown as examples. During each time slot, at least one measurement recording IN pr on a measurement sample (shown here for the sake of simplicity only for the time slots t3 and t4); for example, in time slot t3 at least one measurement recording IN pr of the sample PI. The correction factor S applied to generate the corresponding corrected spectrum KSpr is based on an averaged additional spectrum GS zus , which is based on the calibration recordings IN ZUS from the last three time slots tl, t2 and t3. In the time slots tl and t2, at least one measurement recording was made on another sample at the same time as the calibration recordings IN ZUS (not shown in detail). In this variant, the measurements are also taken IN pr and the calibration recordings IN zusas joint recordings. To determine the next calibration factor S for sample P2 in the next time slot t4, the calibration recordings IN ZU s from the time slots t2, t3 and t4.

[0123] In this variant, the calibration factor S is based on a moving average of the calibration measurements IN ZU s from several (here three) recent time slots or corresponding measured samples. This allows the calibration factor S to be determined with greater reliability and accuracy (through averaging) while still being kept up-to-date (through temporal sliding).

[0124] Note that alternatively, the calibration factor S determined over a number of time slots (e.g. tl, t2, t3) can be applied only at the next time slot (in the example t4) to calculate the corrected spectrum KS prto accelerate the measuring sample (not shown in detail, but see Fig. 8b in an analogous manner).

[0125] In the variant shown in Fig. 8d, at least one calibration recording IN ZUS H with an increased resolution, compared to the resolution of at least one measurement recording IN pr in the time slots tl, t2, t3 for the measurement samples PI, P2, P3. For illustration purposes, the calibration recording IN ZUS H with a high index H and shown in a rectangle that is twice as wide as the squares of the measurement images IN pr . From at least one calibration image IN ZUS H With increased resolution, an additional spectrum is determined with which the calibration factor S is determined, which is used in several subsequent time slots, here the time slots tl, t2, t3, to obtain the corrected spectra KS prseveral test samples, here the test samples PI, P2,

[0126] P3. The calibration factor S applied across all samples is due to the higher measurement resolution of the underlying at least one calibration image IN ZUS H particularly precise.

[0127] In the simplest case, in this variant, the calibration factor S is valid for a certain number of subsequent time slots or measurement samples (e.g. three) or for a certain period of time (e.g. 10 minutes) before a new, at least one calibration recording IN ZU s H with an increased resolution to update the calibration factor S.

[0128] In the variant of Fig. 8e, which is based on the variant of Fig. 8d (see above), the validity of the calibration factor S, which is calculated in the time slot tO on the basis of the at least one calibration recording IN ZU s H with increased resolution, using monitor recordings INmn checked. In subsequent time slots t1, t2, t3, at least one monitor recording INmn is performed with the additional detector, typically with the same resolution as the measurement recordings INp r of the measurement samples. This results in an additional monitor spectrum of the calibration substance (not shown in detail), on the basis of which (as with calibration recordings) a calibration factor is determined, which is referred to as the auxiliary calibration factor HS in the context of the monitor recordings.

[0129] The determination of the auxiliary calibration factors HS is less accurate than the determination of the calibration factor S; nevertheless, by comparing HS and S, it can be determined whether the last determined calibration factor S is still usable and should remain valid. For example, it can be defined that the calibration factor S is no longer considered usable in the future as soon as an auxiliary calibration factor HS deviates from the calibration factor S by more than a limit value of 0.01%.

[0130] In the example shown, it is determined in time slot t3 that the calibration factor S is no longer usable. In the variant shown, the calibration factor S from time slot t0 is applied for the last time for the sample P3, and then in time slot t4, another calibration recording, at least one IN ZUS Hincreased resolution, with which a new calibration factor is determined for subsequent time slots (the latter not shown in detail). Alternatively, it would also be possible, for example, to use the auxiliary calibration factor HS determined there in time slot t3 to generate the calibrated spectrum KS pr of sample P3. Note that time slots for calibration recordings with increased resolution (t0 in Fig. 8d, and t0, t4 in Fig. 8e) typically last longer than time slots for monitor recordings (t1, t2, t3 in Fig. 8d, 8e). Note also that simultaneously with the calibration recordings IN ZU s H also measurement recordings IN pr on measurement samples, if desired (joint recordings) (not shown in detail).

[0131] Reference symbol list

[0132] 1 FT spectrometer arrangement

[0133] 2 Light source 3 Light

[0134] 4 collimating mirrors

[0135] 5 interferometers

[0136] 6 beam splitters

[0137] 7 reflector 7a retro reflector

[0138] 8 Reflector

[0139] 8a Retro-Reflector

[0140] 9 (first) interferometer arm

[0141] 10 (second) interferometer arm 11 Device for changing the optical path difference

[0142] 12 Measuring position 12a Measuring sample

[0143] 13 Detector

[0144] 14 Additional detector 14a Focusing lens

[0145] 15 first aperture

[0146] 16 Second aperture 17 Partial beam (in the beam path behind the interferometer to the additional detector) 18 Partial beam (in the beam path behind the interferometer to the measuring position)

[0147] 19 Container 19a Calibration substance

[0148] 20 focusing mirror 21 imaging mirror

[0149] 22 reference lasers

[0150] 23 Reference Light

[0151] 24 mirrors 25 mirrors

[0152] 26 Reference detector

[0153] 27 optical axis of the interferometer arm 9

[0154] A measurement signal (detector or additional detector)

[0155] Aref measurement signal (reference detector) f function of the reference spectrum

[0156] F eff effective laser frequency F no nominal laser frequency g function of the additional spectrum

[0157] H Index high resolution

[0158] HS auxiliary calibration factor

[0159] I Intensity

[0160] IN Interferograms / signal representing images according to index

[0161] K range calibration recordings

[0162] KS corrected spectrum according to index

[0163] Kl Integral of the cross-correlation

[0164] M Area measurement recordings mn Index monitor recordings

[0165] MN area monitor recordings pr index measurement samples / spectrum

[0166] Pref Wavenumber position (peak) in the reference spectrum

[0167] Pzus wavenumber position (peak) in the additional spectrum

[0168] P1,P2,P3... samples

[0169] S (current) calibration factor

[0170] Salt previous calibration factor

[0171] SP spectrum according to index t0,tl,t2... time slots t time

[0172] UF Update Factor

[0173] US uncorrected spectrum according to index

[0174] V Comparison (HS and S)

[0175] VS pre-corrected spectrum according to index zus index calibration substance / additional spectrum d integration length s wavenumber so mean wavenumber position t wavenumber shift variable tq wavenumber shift variable at the maximum of Kl

Claims

Patent claims 1. FT spectrometer arrangement (1), comprising - a light source (2) for light (3), - an interferometer (5) with at least one beam splitter (6) and two interferometer arms (9, 10) through which the light (3) of the light source (2) is guided, wherein the interferometer arms (9, 10) each have a reflector (7, 8), - a measurement sample (12a) at a measurement position (12), - a detector (13) for the light (3), wherein the measuring position (12) is arranged in the beam path of the light (3) between the interferometer (5) and the detector (13), - a device (11) for changing an optical path difference (GU) between the two interferometer arms (9, 10), in particular for moving one or both of the reflectors (7, 8), - a reference laser (22), with the reference laser light (23) through the interferometer (5) is guided, and - at least one reference detector (26) for measuring the reference laser light (23), wherein the reference detector (26) is arranged in the beam path of the reference laser light (23) behind the interferometer (5), characterized in that an additional detector (14) is further provided, with which a partial beam (17) of the light (3) can be measured which has passed the interferometer (5) but has not passed the measurement position (12), and that with the detector (13) a further partial beam (18) of the light (3) which has passed the interferometer (5) and has passed the measurement position (12) can be measured simultaneously with a measurement of the partial beam (17) at the additional detector (14).

2. FT spectrometer arrangement (1) according to claim 1, characterized in that the reflectors (7, 8) are designed as retroreflectors (7a, 8a), that a first aperture (15) in the beam path of the light (3) between the light source (2) and the interferometer (5), with which a part, in particular half, of the light (3) is blocked, and / or a second aperture (16) in the beam path of the light (3) between the interferometer (5) and the measuring position (12), with which a part, in particular half, of the light (3) is shaded, and that the partial beam (17) of the light (3), which can be measured with the additional detector (14), is directed from the beam splitter (6) towards the light source (2) back to the first aperture (15) or from the beam splitter (6) towards the sample position (12) towards the second aperture (16).

3. FT spectrometer arrangement (1) according to claim 1 or 2, characterized in that in the partial beam (14) of the light (3) which can be measured with the additional detector (14) a focusing lens (14a) or a focusing mirror is arranged between the interferometer (5) and the additional detector (14).

4. FT spectrometer arrangement (1) according to any one of claims 1 to 3, characterized in that a container (19) with a calibration substance (19a) is arranged between the interferometer (5) and the additional detector (14) in the partial beam (17) of the light (3) which can be measured with the additional detector (14).

5. FT spectrometer arrangement (1) according to claim 4, characterized in that the calibration substance (19a) is a calibration gas, preferably water vapor or methane or acetylene.

6. Method for measuring a spectrum (KS) pr) a measurement sample (12a) by means of an FT spectrometer arrangement (1), in particular with an FT spectrometer arrangement (1) according to one of the preceding claims, wherein the measurement comprises one or more measurement recordings (IN pr ) includes, in each measurement recording (IN pr) - Light (3) from a light source (3) is passed through an interferometer (5), interacts with the sample (12a) at a measuring position (12) behind the interferometer (5) and is measured at a detector (13), - furthermore, reference laser light (23) from a reference laser (22), in particular a laser diode, is passed through the interferometer (5) and measured at at least one reference detector (26), - wherein, among other things, a signal from the detector (13) of one or more measurement recordings (IN pr ) and a calibration factor (S) can be used to calibrate a frequency axis to obtain a spectrum (KS pr) of the measurement sample (12), characterized in that the measurement includes one or more calibration images (IN ZU s) includes, in each calibration recording (IN ZU s) - Light (3) from the light source (2) is passed through the interferometer (5), and a partial beam (17) of the light (3) is measured at an additional detector (14) after passing through the interferometer (5) without passing the measuring position (12), - further reference laser light (23) from the reference laser (22) is passed through the interferometer (5) and at least one reference detector (26) is measured to show that, among other things, a signal from the additional detector (14) of one or more calibration images (IN ZUS ) is used to create an additional spectrum (SP Z to generate us) and that a reference spectrum (SP ref) or partial information thereof of a calibration substance (19a), in particular a calibration gas, which is contained in the beam path of the light (3) between the light source (2) and the additional detector (14), with the additional spectrum (SP zus ) or a partial piece of information from it is compared to determine the calibration factor (S).

7. Method according to claim 6, characterized in that one or more of the measurement recordings (IN pr ) and one or more of the calibration images (INzus) are performed simultaneously as one or more joint images.

8. Method according to claim 7, characterized in that all measurement recordings (INp r ) and calibration images (IN ZU s) as joint recordings.

9. Method according to claim 7 or 8, characterized in that the ca-librated spectrum (KS) pr) the measurement sample (12) and the calibration factor (S) used in the generation of the calibrated spectrum (KS) pr ) is applied, based on one or more common recordings.

10. Method according to one of claims 6 to 8, characterized in that the calibration factor (S) used in the generation of the calibrated spectrum (KS) Pr ) of the measurement sample (12) is applied to one or more calibration images (IN ZU s) based, which are temporally prior to one or more measurement recordings (IN pr ) were recorded, on which the ca-librated spectrum (KS) pr ) is based.

11. Method according to one of claims 6 to 10, characterized in that the method involves a measurement of several spectra (KS Pr ) on several measurement samples (12; PI, P2, P3), each a calibrated spectrum (KS) pr ) of the respective measurement sample (12; PI, P2, P3) is generated.

12. Method according to claim 11, characterized in that for each calibrated spectrum (KS) pr ) of the several measurement samples (12; PI, P2, P3) a separate calibration factor (S) is determined, with which the calibrated spectrum (KS) Pr ) of a respective measurement sample (12; PI, P2, P3) is generated.

13. Method according to claim 12 in conjunction with claim 7, characterized in that the respective own calibration factor (S) and the respective calibrated spectrum (KS) pr ) of the respective measurement sample (12; PI, P2, P3) are based on common recordings.

14. Method according to claim 11, characterized in that a sample-spanning calibration factor (S), which is determined on the basis of one or more calibration images (INzus), is used for the generation of several calibrated spectra (KS). pr ) is applied to several measurement samples (12; PI, P2, P3).

15. Method according to claim 14, characterized in that the sample-spanning calibration factor (S) is based on one or more calibration images (I NZUS). h ) is determined, which have a higher spectral resolution than at least some of the measurement recordings (IN pr ), on which the several calibrated spectra (KS pr ) of the multiple measurement samples (12; PI, P2, P3).

16. Method according to claim 14 or 15, characterized in that at least a part of the measurement recordings (IN pr ), each displaying a calibrated spectrum (KS pr ) of a measurement sample (12; PI, P2, P3) which is generated with the cross-sample calibration factor (S), together with monitor images (IN nm ) is performed, whereby the monitor recordings (IN mn ) Calibration images (IN ZU s) corresponding- chen, with which an auxiliary calibration factor (HS) is determined, and that based on a comparison (V) between the auxiliary calibration factor (HS) and the sample-spanning calibration factor (S) determine whether - the cross-sample calibration factor (S) is still usable and / or the respective calibrated spectrum (KS) calculated with the cross-sample calibration factor (S) pr ) is usable, or - a new cross-sample calibration factor (S) must be determined and / or the respective calibrated spectrum (KS) calculated with the cross-sample calibration factor (S) pr ) must be discarded.

17. Method according to one of claims 11 to 16 in conjunction with claim 7, characterized in that the measurement comprises several calibration recordings (INzus) which are common recordings and are distributed over the several measurement samples (12; PI, P2, P3) such that an averaged additional spectrum (GS) zus) from the signals of the additional detector (14) or the additional spectra (SP zus ), which are related to these several calibration shots (IN ZUS ) belong, is calculated, and that the reference spectrum (SP ref ) or a partial piece of information from it with the averaged additional spectrum (GS) zus ) or a partial piece of information from it is compared to determine the calibration factor (S), whereby the calibration factor (S) is applied - for the generation of the calibrated spectra (KS pr ) of the last measurement sample (12; PI, P2, P3) of the measurement samples (12; PI, P2, P3) to which the calibration images (IN) refer zus ) distribute, - and / or for generating a calibrated spectrum (KS) pr ) at least one next measurement sample (12; PI, P2, P3), in particular wherein the multiple measurement samples (12; PI, P2, P3) comprise a predefined number of each of the most recently measured measurement samples (12; PI, P2, P3).

18. Method according to one of claims 6 to 17, characterized in that the comparison of the reference spectrum (SP ref ) or the partial information from this with the additional spectrum (SP ref ) or the partial information from it is obtained by assigning a wavenumber position P ref an absorption line of the calibration substance (19a) in the reference spectrum (SP ref ) and a corresponding wavenumber position P zus the absorption line of the calibration substance (19a) in the additive spectrum (12; PI, P2, P3) are determined, in particular where the calibration factor S is determined as S = (Pref / Pzus)* Sait, with S ait : during the generation of the additional spectrum (SP) zus ) applied previous calibration factor, where S ait = 1 if during the generation of the additional spectrum (SP) zus ) no previous calibration factor was applied.

19. Method according to one of claims 6 to 17, characterized in that the comparison of the reference spectrum (SP ref ) or the partial information from it with the additional spectrum (SP) zus ) or a partial piece of information from it is obtained by cross-correlating a selected spectral range of the reference spectrum (SP) ref ) and a corresponding spectral range of the additional spectrum (SP zus ) is calculated, i.e., the integral Kl with is calculated as follows: mean wavenumber position of a characteristic absorption region of the calibration substance (19a), 5: integration width, s: wavenumber, x: wavenumber shift variable, f: reference spectrum; g: additional spectrum, and for the maximum of K1(t) the corresponding value tq of the wavenumber shift variable t is determined, and the calibration factor S is S = [(ao / (ao+xO)]*S ait is determined with Sai t : during the generation of the additional spectrum (SP)zus ) applied previous calibration factor, where S ait = 1 if no previous calibration factor was applied when generating the additional spectrum (SPzus).

20. Method according to one of claims 6 to 17, characterized in that the comparison of the reference spectrum (SP ref ) or the partial information from this with the additional spectrum (SP ZU s) or a partial piece of information from it is obtained by calculating the integral K2 in a selected spectral range of the reference spectrum (SPref) and a corresponding spectral range of the additional spectrum (SPzus) with is calculated as follows: mean wavenumber position of a characteristic absorption region of the calibration substance (19a), 5: integration width, s: wavenumber, s: calibration factor variable, f: reference spectrum; g: set spectrum, and for the maximum of K2(s) the corresponding value sO of the calibration factor variable s is determined, and as calibration factor S=s0*S ait is determined with S a n: during the generation of the additional spectrum (SP) zus ) applied previous calibration factor, where S ait = 1 if no previous calibration factor was applied when generating the additional spectrum (SPzus).

21. Method according to one of claims 6 to 17, characterized in that the comparison of the reference spectrum (SP re f) or a partial piece of information therefrom with at least one additional spectrum (SP zus) or partial information from it is obtained by iteratively modeling a large number of model spectra starting from the reference spectrum (SPref), with each model spectra being compared with the experimentally obtained additional spectrum (SPzus).

22. Method according to claim 21, characterized in that at least one instrument-related line shape and an interim calibration factor are taken into account as parameters in the modeling, and that the interim calibration factor value IKW, which is the model spectrum with the best agreement with the experimentally obtained additional spectrum (SP), zus ) was used, is determined, and as a calibration factor S=IKW*S ait is determined with S ait : during the generation of the additional spectrum (SP) zus ) applied previous calibration factor, where S ait = 1 if no previous calibration factor was applied when generating the additional spectrum (SPzus).

23. Method according to any one of claims 6 to 22, characterized in that a Fourier transform is applied to the signal of the additional detector (14) using a previously used calibration factor Sai t a pre-corrected additional spectrum (VS) zus ) is generated, and then the pre-corrected additional spectrum (VS ZU s) or a partial piece of information derived from it using the reference spectrum (SP) ref ) or the partial information from it is compared.

24. Method according to one of claims 6 to 22, characterized in that an uncorrected additional spectrum (US) is obtained from the signal of the additional detector (14) by applying a Fourier transform. zu s) is generated, and then the uncorrected additional spectrum (US ZU s) or a partial piece of information from it with the reference spectrum (SP ref ) or the partial information derived from it is compared.

25. Method according to one of claims 6 to 24, characterized in that a container (19) containing calibration substance (19a) is arranged in the beam path of the light (3) between the interferometer (5) and the additional detector (14).

26. Method according to one of claims 6 to 24, characterized in that the calibration substance (19a) is a substance that is omnipresent in the FT spectrometer arrangement (1).

27. Method according to any one of claims 6 to 26, characterized in that the calibration substance (19a) is water vapor or methane or acetylene.

28. Use of an FT spectrometer arrangement (1) according to one of claims 1 to 5 in a method according to one of claims 6 to 27.

29. Use according to claim 28, characterized in that the determination of the calibration factor (S) and the generation of the calibrated spectrum (KSp r) by an evaluation unit of the FT spectrometer arrangement (1) in situ.