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

Description

[0001] The invention relates to an FT spectrometer arrangement comprising a light source, an interferometer with at least one beam splitter and two interferometer arms through which the light from the light source is passed, each interferometer arm having a reflector, a measurement sample at a measurement position, a detector for the light, the measurement position being arranged in the beam path of the light between the interferometer and the detector, a device for changing an optical path difference between the two interferometer arms, in particular for moving one or both of the reflectors, a reference laser with which the reference laser light is passed through the interferometer, and at least one reference detector for measuring the reference laser light, the reference detector being arranged in the beam path of the reference laser light behind the interferometer. wherein an additional detector is provided with which a partial beam of light can be measured which has passed the interferometer but not the measurement position, wherein with the detector another partial beam of light which has passed the interferometer and the measurement position can be measured simultaneously with a measurement of the partial beam at the additional detector.

[0002] Such an FT spectrometer arrangement is known from US 2019 / 017930 A1.

[0003] In Fourier transform (FT) spectroscopy, broadband light, for example infrared (IR) light, is split into two partial beams in an interferometer. A path difference is applied between the partial beams, and then a detector is read out. The superimposed partial beams, after interacting with a sample under investigation, strike the detector. The detector readout is repeated for different path differences (sample points). The superposition of the partial beams causes 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 intensity data of the detector, which depends on the path difference, is subjected to a Fourier transform in combination with other data processing steps, resulting in a spectrum of the sample under investigation.

[0004] For the measurements, it is important to be able to precisely define or know the path difference of the partial beams at the sampling points. For this purpose, it is known to use, in addition to the typically broadband light for the actual measurement of the sample, a reference laser whose narrowband reference laser light also passes through the interferometer, and to determine the path difference of the interferometer arms from its constructive and destructive interference at a reference detector (also called a scanning detector).

[0005] To accurately determine the path difference, it is advantageous to know the exact laser wavelength of the reference laser.

[0006] For a long time, HeNe (helium-neon) lasers were used as reference lasers, as they emit laser light with a very stable, well-known laser wavelength. However, HeNe lasers are comparatively large and expensive to operate due to their limited lifespan.

[0007] Laser diodes, on the other hand, are more durable and have a compact design. However, laser diodes exhibit significantly poorer wavelength stability than HeNe lasers. In particular, the laser wavelength of laser diodes changes depending on the operating temperature and current, and also due to aging.

[0008] 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.

[0009] From DE 10 2014 226 487 A1, an FTIR spectrometer is known in which light emanating from the reference laser source is guided through an absorption medium to a reference detector, and the laser wavelength of the reference laser is adjusted to the absorption line of the absorption medium by means of an electronics system via the known operating behavior of the FTIR spectrometer by means of a signal from the reference detector.

[0010] The above measures can largely compensate for wavelength drift of the reference laser or achieve a significant improvement in the wavelength stability of the reference laser.

[0011] Besides instability in the wavelength of the reference laser itself, other factors can distort the spectrum of the measurement sample, such as 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, leading to the same reference signal as paraxially incident reference laser light of a longer wavelength. Therefore, with oblique incidence, the frequency axis of the spectrum is detuned, similar to a drift in the wavelength of the reference laser.

[0012] From DE 10 2004 025 448 A1 it is further known that the operating behavior of the FTIR spectrometer is occasionally determined using a reference sample with a known spectrum.

[0013] However, performing measurements on the reference sample generally leads to an interruption of the measurement operation.

[0014] US Patent 5 309 217 A discloses an FTIR spectrometer in which two retroreflectors are arranged on arms of a double pendulum offset by 180°.

[0015] US Patent 2019 / 017930 A1 describes a spectrometer system in which infrared light is focused behind an interferometer. A Jacquinot aperture is positioned at the focus point, directing a portion of the infrared light to an auxiliary detector, while another portion of the infrared radiation passes through the Jacquinot aperture, continues through a sample, and is detected by an infrared detector.

[0016] US patent 2003 / 189709 A1 discloses an infrared spectrometer with an interferometer comprising two retroreflectors. A VCSEL laser is used as the reference laser.

[0017] US 2020 / 217786 A1 describes an FTIR spectrometer in which measurements are taken with samples, and the frequency axis is calibrated using a calibration factor. The calibration factor is determined through separate calibration measurements using air as the sample.

[0018] EP 3 413 021 A1 describes a method for measuring a spectrum using an FTIR spectrometer. A measurement with a reference sample to determine a wavelength shift takes place before a measurement with a sample.

[0019] From US 2019 / 301939 A1, an FTIR spectrometer is known in which a beam splitter is arranged in the beam path behind an interferometer, with which the IR light can be split so that a first part can be directed via a measurement sample to a detector, and a second part can be directed via a reference sample to another detector.

[0020] WO 2011 / 154035 A1 describes a spectrometer for the UV and IR range. In a first embodiment, light from a radiation source is passed partly through a sample and partly through a fiber Bragg grating, and both portions are fed to an interferometer; downstream of the interferometer, both portions are measured by a detector array. In a second embodiment, light from a radiation source is fed to a monochromator, and downstream of the monochromator, part of the radiation is passed through a sample to a first detector, and another portion of the radiation is passed through a fiber Bragg grating to a second detector.

[0021] Furthermore, an FTIR spectroscopy method and an associated system have been disclosed in WO 2005 / 111560 A1. Object of the invention

[0022] The object of the invention is to provide an improved FT spectrometer arrangement and an associated measuring method in which, or with which, high measuring accuracy can be achieved in a simple manner with high availability of the FT spectrometer arrangement. Description of the invention

[0023] This problem is solved according to the invention by an FT spectrometer arrangement of the type mentioned above, which is characterized in that that the reflectors are designed as retro-reflectors, that a first aperture is provided in the beam path of the light between the light source and the interferometer, with which a part, in particular half, of the light is shaded, and that the partial beam of light, which can be measured with the additional detector, is directed from the beam splitter towards the light source back to the first aperture or from the beam splitter towards the sample position towards the second aperture.

[0024] The FT spectrometer arrangement according to the invention has two detectors, namely the detector for measuring the sample and an additional detector. The latter serves to determine calibration information for a calibration substance with which the spectrum of the sample can be corrected.

[0025] The additional detector and the main detector allow for simultaneous data acquisition, particularly for capturing sample information at the main detector and calibration information at the additional detector at the same time. The invention thus fundamentally enables calibration during the measurement of a sample.

[0026] The measurement position is not included in the light beam path to the auxiliary detector, so the sample located 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 sample (or substance) from the measurement position 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 positioned in the light beam path between the interferometer and the auxiliary detector without interfering with the measurement of the sample.In particular, the invention allows for the use of radiation for calibration that would not reach the measurement position with the measurement sample anyway, so that the signal-to-noise ratio is not worsened by the simultaneous measurement with the additional detector.

[0027] The calibration information obtained via the additional detector is used to determine a calibration factor that can be applied when determining or calibrating spectra of measurement samples, in particular for calculating an effective laser wavelength of the reference laser (see below for more details).

[0028] The first aperture prevents back reflections of light entering the interferometer from the light source back to the light source. The additional detector measures light that would not reach the position of the sample anyway.

[0029] For the integration of the additional detector, no further beam splitter (beyond the interferometer's beam splitter) or dichroic mirror (dichroite) is usually required. Furthermore, no moving parts are typically needed for the integration of the additional detector; the additional detector is generally permanently integrated into the measurement setup.

[0030] The measured spectral ranges of the detector and the auxiliary detector can overlap completely or partially.

[0031] Essentially the same optical setup can be used for the measurement images on the detector and the calibration images of the additional detector.

[0032] In principle, all methods and measurement setups known for conventional FT spectrometers can be used at the measurement position, i.e., setups 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 be designed as retroreflectors (cube corner mirrors).

[0033] In the context of the description of the present invention, the electromagnetic radiation of different spectral ranges used to examine the sample is uniformly referred to as "light". Typically, UV light, visible light, infrared light, or longer-wavelength radiation up to wavelengths of 5 mm is used in connection with the present invention. Preferred embodiments

[0034] In a preferred embodiment of the FT spectrometer arrangement according to the invention, a second aperture is provided in the light beam path between the interferometer and the measurement position, by which a portion, in particular half, of the light is blocked. The light measured by the additional 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.

[0035] A preferred embodiment includes a focusing lens or mirror positioned between the interferometer and the additional detector in the partial beam of light that can be measured by the additional detector. This increases the intensity of the light detected by the additional detector.

[0036] A preferred embodiment also includes a container with a calibration substance arranged between the interferometer and the auxiliary detector in the partial beam of light that can be measured with the auxiliary detector. This allows any calibration substance in virtually any quantity or concentration to be used for obtaining the calibration information without interfering with the measurement of a sample at the measurement position. In particular, this method allows for the simple generation of strong absorption lines of the calibration substance in the auxiliary spectrum, which can be detected with high accuracy. Alternatively, a substance that is omnipresent (already present) in the beam path, such as water vapor present due to humidity, can also be used as the calibration substance.

[0037] A preferred further development of this embodiment is in which the calibration substance is a calibration gas, preferably steam or methane or acetylene.

[0038] These substances possess easily detectable absorption lines that are well-suited for infrared calibration, as their line positions (wavelengths) are known with high accuracy. For example, water, as an atmospheric molecule, is the subject of numerous studies, and a methane transition is used by the BIPM as a frequency standard. Furthermore, these substances are readily available at low cost. Inventive measuring methods

[0039] The present invention also encompasses a method for measuring the spectrum of a sample using an FT spectrometer arrangement with an FT spectrometer arrangement according to the invention as described above. wherein the measurement comprises one or more measurement recordings, wherein in each measurement recording light from a light source is passed through an interferometer, interacts with the sample at a measurement position behind the interferometer and is measured at a detector, furthermore reference laser light from a reference laser, in particular a laser diode, is passed through the interferometer and is measured at at least one reference detector, wherein, among other things, 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 sample, wherein the measurement comprises one or more calibration recordings, wherein in each calibration recording light from the light source is passed through the interferometer,and a partial beam of light from the light path is measured at an additional detector after passing through the interferometer without passing the measurement position, furthermore, reference laser light from the reference laser is passed through the interferometer and measured at at least one reference detector, wherein, among other things, a signal from the additional detector of one or more calibration images is used to generate an additional spectrum, and wherein a reference spectrum or a partial piece of information therefrom of a calibration substance, in particular a calibration gas, which is contained in the light path between the light source and the additional detector, is compared with the additional spectrum or a partial piece of information therefrom in order to determine the calibration factor.

[0040] By measuring the partial beam of light at the auxiliary detector during the calibration recording(s), calibration information for the actual measurement recording(s) on the sample(s) can be obtained. A 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 sample can be already or still be positioned at the measurement position without obstructing the calibration recording. Conversely, if desired, a calibration substance can be placed between the interferometer and the auxiliary detector without interfering with the measurement recordings.

[0041] Using the additional spectrum, a calibration factor can be determined by comparison with the reference spectrum. This factor can then be used to calibrate the spectrum of a measurement sample with respect to the frequency axis. Calibration with the calibration factor determined according to the invention can, in principle, compensate for all sources of frequency axis (wavelength scale) distortion 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 required. The availability of the FT spectrometer apparatus is practically unaffected by calibration recordings, since measurement and calibration recordings can generally be performed simultaneously.Accordingly, the invention generally provides that measurement recordings and calibration recordings take place simultaneously, at least over part of the operation of the FT spectrometer arrangement.

[0042] The calibration factor, also denoted here by S, can be included in the effective laser frequency Feff of the reference laser. After the Fourier transform, this frequency is used 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 the effective laser frequency Feff of the reference laser with the (fixed) nominal laser frequency Fno of the reference laser via Feff = Fno * S. Note that the calibration factor S can be less than 1 or greater than 1. The nominal laser frequency, in this context, refers to an approximate frequency specification, such as that given by a manufacturer for a laser diode.

[0043] Note that the calibration factor S is continuously updated during operation of the FT spectrometer setup, for example by multiplying a previous calibration factor Sold by an update factor UF, i.e., S = UF * Sold. A spectrum of a measurement sample generated with a previous (outdated) calibration factor Sold 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 from at least one interferogram (raw data of the detector signal) using the current calibration factor S.

[0044] The additional spectrum can be an uncorrected additional spectrum (without correction with a calibration factor, or calibration factor "1", for example by using the nominal laser frequency F no as the basis for the Fourier transform when determining the path differences), or an additional spectrum corrected with a (previous) calibration factor S alt.

[0045] For a measurement or calibration scan, the optical path difference is changed ("scan") by moving one or both reflectors and monitored. The detector or auxiliary detector is read out at known path differences. Each measurement or calibration scan involves one or more passes through the available or selected range of the optical path difference (travel of the reflector(s) in the interferometer arms). One or more interferograms or auxiliary interferograms are obtained as signals or data from the detector or auxiliary detector. A spectrum or auxiliary spectrum can be obtained by Fourier transformation.

[0046] The detector and the auxiliary detector can be read out at equidistant path differences (e.g., determined by the zero crossings of the AC component of the detector signal for the reference laser light) (conventional method), but can also be read 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, possibly via interpolation).

[0047] The reference laser light is approximately monochromatic. The reference laser is typically designed as a cost-effective laser diode; particular frequency stability is not generally required; it suffices if the frequency stability is given for the validity period of a calibration factor, which, within the scope of the invention, can be shortened to the duration of a measurement if necessary. Preferred variants of the measuring method according to the invention

[0048] 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 distance of the interferometer arms are used for both reading out the detector and for reading out the auxiliary detector; thus, the interferometer arrangement remains fully available for measurement recordings during the calibration recordings.

[0049] A further development of this variant is also preferred, whereby all measurement and calibration recordings are performed as a single, combined recording. This maximizes the availability of the FT spectrometer setup for measurement operations, and furthermore, the calibration can be adjusted or checked quasi-continuously, thus maximizing the accuracy of the acquired spectra and the resulting measurement data, e.g., concentrations in a mixture.

[0050] In an advantageous further development, the calibrated spectrum of the sample and the calibration factor applied in generating the calibrated spectrum are based on one or more common images. 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 sample, are simultaneous with the one or more measurement images for this spectrum and are therefore based on the same passes through the available travel range of the interferometer arms. The reference laser light for the calibration images and the measurement images, which is used to determine the underlying path differences, is the same, so no detuning is possible in this respect. This results in very high calibration accuracy.

[0051] An alternative approach proposes that the calibration factor applied when generating the calibrated spectrum of the measurement sample be based on one or more calibration images taken prior to the measurement images on which the calibrated spectrum is based. This allows for particularly fast generation of the calibrated spectrum of the measurement sample. Instead of waiting for the correction factor to be determined from current, combined images, a pre-calculated calibration factor can be used to generate the spectrum or correct it with the correction factor.

[0052] Variants with measurement of multiple spectra on multiple samples

[0053] A preferred variant is one in which the method involves measuring multiple spectra on multiple samples, generating a calibrated spectrum for each sample. In this variant, several (different, new) samples are measured sequentially. For the purposes of this invention, the same sample substance is also considered a new sample if it has changed or could have changed since the last measurement, for example, by exhibiting a different temperature or pressure, or simply by undergoing chemical changes over time. Different measurements performed on a flow cell through which sample substance is continuously passed are considered to be performed on different samples.

[0054] Preferably, for a measurement sample, the current calibration factor S is determined based on one or more calibration images during one or more joint recordings, and this is immediately applied to determine the calibrated spectrum from the one or more simultaneously recorded measurement images of the current measurement sample.

[0055] Alternatively, during one or more joint recordings, a spectrum of the current measurement sample calibrated with a previous calibration factor S can be taken and a new calibration factor S can be determined for the next measurement sample.

[0056] Within this variant, it is also possible to use a calibration factor, determined on the basis of one or more calibration images, for several measurement samples and the generation of their respective calibrated spectrum.

[0057] A preferred method is a further development of this variant in which a separate calibration factor is determined for each calibrated spectrum of the multiple measurement samples, with which the calibrated spectrum of the respective measurement sample is generated. This allows for a particularly high accuracy of the calibration factor; the calibration factor is only valid for one spectrum at a time (without prejudice to the possibility that the next calibration factor happens to coincide with the previous one).

[0058] In a sub-variant of this development, where one or more of the measurement images and one or more of the calibration images are acquired simultaneously as one or more joint images, the respective calibration factor and the respective calibrated spectrum of the respective measurement sample are based on these shared images. In other words, the one or more measurement images used to determine the calibration factor S for a given measurement sample and the one or more measurement images of this sample to which the calibration factor S is applied are acquired simultaneously. The reference laser light of the calibration image(s) and the measurement image(s) for a given measurement sample is the same, so no detuning is possible in this respect. This results in a particularly high calibration accuracy.

[0059] An alternative development proposes applying a cross-sample calibration factor, determined based on one or more calibration images, to generate multiple calibrated spectra from several samples. This would accelerate the generation of calibrated spectra for the different samples and reduce computational effort.

[0060] In a sub-variant of this development, the cross-sample calibration factor 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 the calibration accuracy to be significantly improved with minimal effort, particularly by requiring only a slightly longer measurement time during the higher-resolution calibration image(s).

[0061] Another preferred sub-variant provides for, that at least some of the measurement recordings, on which a calibrated spectrum of a measurement sample is based (generated with the cross-sample calibration factor), are performed together with monitor recordings, where the monitor recordings correspond to calibration recordings used to determine an auxiliary calibration factor, and that a comparison between the auxiliary calibration factor and the cross-sample calibration factor is used to decide whether the cross-sample calibration factor is still usable and / or the respective calibrated spectrum calculated with the cross-sample calibration factor is usable, or whether a new cross-sample calibration factor must be determined and / or the respective calibrated spectrum calculated with the cross-sample calibration factor must be discarded. This procedure ensures, with minimal effort, that the cross-sample calibration factor orA calibrated spectrum based on this maintains a certain minimum accuracy.

[0062] If a new, cross-sample calibration factor needs to be determined, this can be done subsequently, for example with one or more calibration images of increased spectral resolution, and applied in the future. Alternatively, the auxiliary calibration factor for the next cross-sample calibration factor can also be determined and applied in the future.

[0063] Typically, a limit or interval for the absolute or relative deviation of the auxiliary calibration factor from the (previous) cross-sample calibration factor is defined (stored) to determine whether the cross-sample calibration factor is still usable. For example, it may be specified that the auxiliary calibration factor may deviate from the cross-sample calibration factor by a maximum of 0.01% to still be considered usable.

[0064] An advantageous further development involves performing one or more of the measurement shots and one or more of the calibration shots simultaneously as one or more joint shots, providing that the measurement includes several calibration shots, which are joint shots, and are distributed across the several measurement samples. that an averaged additional spectrum is calculated from the signals of the additional detector or the additional spectra belonging to these multiple calibration recordings, and that the reference spectrum or a partial piece of information therefrom is compared with the averaged additional spectrum or a partial piece of information therefrom to determine the calibration factor, wherein this calibration factor is applied for the generation of the calibrated spectra of the last measurement sample of the measurement samples on which the calibration recordings are distributed, and / or for the generation of a calibrated spectrum of at least one subsequent measurement sample, in particular wherein the multiple measurement samples comprise a predefined number of each of the most recently measured measurement samples.

[0065] The additional spectrum averaged from the measurement of multiple samples generally exhibits a better signal-to-noise ratio than a single additional spectrum recorded during the measurement of a single sample. This can improve the accuracy of the calibration. Variants for determining the calibration factor

[0066] In a preferred embodiment, the comparison of the reference spectrum or partial information thereof with the supplementary spectrum or partial information thereof is carried out by determining a wavenumber position Pref of an absorption line of the calibration substance in the reference spectrum and a corresponding wavenumber position Pzus of the absorption line of the calibration substance in the supplementary spectrum, in particular where the calibration factor S is determined as S = (Pref / Pzus) * Salt, where Salt is the previous calibration factor applied when generating the supplementary spectrum, and Salt = 1 if no previous calibration factor was applied when generating the supplementary spectrum. This procedure is particularly simple. If the supplementary spectrum contains an absorption line of sufficient strength without significant overlap with other absorption lines, such that the wavenumber position Pref can be easily determined, this embodiment is particularly simple.A suitable absorption line can generally be ensured by placing a container with a suitable calibration substance in the light path between the interferometer and the auxiliary detector. In this scenario, for example, an absolute difference or a ratio of the wavenumber positions can be used to quantitatively determine the calibration factor. If the auxiliary spectrum is an auxiliary spectrum corrected with a previous correction factor Salt, the difference should be 0 or the ratio 1 (within the desired accuracy) if the (previous) calibration factor is to be correct. Typically, an interval is defined within which the difference or ratio must lie to confirm the (previous) calibration as usable.In the case of S alt =1, the nominal laser frequency F no can be used as a basis for generating the additional spectrum; the nominal laser frequency F no then defines the frequency axis of the additional spectrum.

[0067] An advantageous variant involves comparing the reference spectrum or partial information from it with the additional spectrum or partial information from it by calculating the cross-correlation of a selected spectral range of the reference spectrum and a corresponding spectral range of the additional spectrum, i.e., the integral K1 with ∫ σ 0 − δ σ 0 + δ f σ ∗ g σ + τ dσ = K 1 τ calculated, with σ 0 : mean wavenumber position of a characteristic absorption region of the calibration substance, δ : integration width, σ : wavenumber, τ : wavenumber shift variable, f : reference spectrum; g: Additional spectrum, and for the maximum of K1(τ) the corresponding value τ0 of the wavenumber shift variable τ is determined, and the calibration factor S is calculated as S=[(σ 0 / (σ 0 +τ0)]*S alt, where S alt is the previous calibration factor applied when generating the additional spectrum, and S alt = 1 if no previous calibration factor was applied when generating the additional spectrum. With this method, 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 superposition. Typically, to prepare for the calculation of the cross-correlation, the additional spectrum (and, if necessary, the reference spectrum) is interpolated to a higher data point density.This is preferably achieved 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, which may be based on either a high-resolution spectrum generated using molecular data, e.g., such as that contained in databases like HITRAN, or a measured spectrum with higher spectral resolution. The model calculation can then include a model for the spectrometer, in particular the so-called instrument function, also known as the instrument line shape (ILS).

[0068] Equally advantageous is a variant which provides that the comparison of the reference spectrum or partial information from it with the additional spectrum or partial information from it is carried out by calculating the integral K2 in a selected spectral range of the reference spectrum and a corresponding spectral range of the additional spectrum. ∫ σ 0 − δ σ 0 + δ f σ ∗ g s + σ dσ = K 2 s The calibration factor S is calculated using the following parameters: σ0: mean wavenumber position of a characteristic absorption region of the calibration substance, δ: integration width, σ: wavenumber, s: calibration factor variable, f: reference spectrum, g: additional spectrum. For the maximum of K2(s), the corresponding value s0 of the calibration factor variable s is determined, and the calibration factor S = s0 * Sold is calculated, where Sold is the previous calibration factor applied when generating the additional spectrum, and Sold = 1 if no previous calibration factor was applied when generating the additional spectrum. This method also allows for a fairly accurate determination of the calibration factor S, although identifying individual peaks in the additional spectrum can be difficult, for example, due to superposition. Note that the reference spectrum of the calibration substance can be determined experimentally or based on a model calculation (see above).

[0069] Another advantageous approach involves comparing the reference spectrum, or a portion thereof, with at least one additional spectrum, or a portion thereof. This is achieved by iteratively modeling a multitude of model spectra based on the reference spectrum, with each model spectra being compared to the experimentally obtained additional spectrum. The Levenberg-Marquardt algorithm, for example, can be used for the iterative modeling of the model spectra (the "fit routine"). This approach allows for a particularly accurate determination of the calibration factor S.

[0070] A preferred further development of this variant envisages that at least an instrument-related line shape and an interim calibration factor be considered 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 supplementary spectrum, is determined and calculated as the calibration factor S = IKW * Salt, where Salt is the previous calibration factor applied when generating the supplementary spectrum, and Salt = 1 if no previous calibration factor was applied when generating the supplementary spectrum. The determination of the calibration factor is particularly accurate here; in particular, distortions in the supplementary spectrum due to the measuring instruments used can be compensated for. Note that other quantities can also be obtained as results during the modeling (fit routine), for example, aperture sizes that contribute to a stretching or compression of the frequency axis. Other variants

[0071] A preferred variant involves generating a pre-corrected additional spectrum from the signal of the additional detector using a Fourier transform and a previous calibration factor Salt. This pre-corrected additional spectrum, or a portion thereof, is then compared with the reference spectrum or a portion thereof. By comparing the pre-corrected additional spectrum (based on Salt) or a portion thereof with the reference spectrum or a portion thereof, the update factor UF can be determined. This factor can then be used to calculate the current calibration factor S = UF * Salt. This variant is primarily used to verify the previous calibration factor Salt (especially across multiple samples) and, if necessary, to confirm or reject a calibrated spectrum of the measured sample calculated with it.To initiate a recalibration (calculation of a new calibration factor, especially across multiple samples). The pre-corrected additional spectrum should, with a usable / correct calibration factor, correspond to the reference spectrum (within the desired accuracy) with respect to the position of the lines of the calibration substance.

[0072] An alternative approach involves generating an uncorrected additional spectrum from the signal of the additional detector using a Fourier transform. This uncorrected additional spectrum, or a portion thereof, is then compared with the reference spectrum, or a portion thereof. This approach is primarily used to directly determine a "complete" calibration factor S from the current calibration recording(s) (especially for a currently measured sample or for at least one subsequent sample to be measured). For generating the uncorrected additional spectrum, no (previous) correction factor Salt is applied; rather, the previous correction factor Salt is set to "1", which is equivalent.However, in this variant, a verification of a previous calibration factor Salt can also be carried out by determining a "current" calibration factor (target calibration factor) Salt using the uncorrected additional spectrum and comparing it with the previous calibration factor Salt, which was temporarily applied to generate the calibrated spectrum of the sample. If the "current" calibration factor Salt does not deviate from the previous calibration factor Salt or deviates only slightly, the calibrated spectrum of the sample calculated with the previous calibration factor can be confirmed as usable.

[0073] One advantageous variant involves placing a container holding the calibration substance in the light path between the interferometer and the auxiliary detector. This allows virtually any calibration substance, in any concentration, to be introduced into the light path to generate a distinct absorption. Positioning the container "behind" the interferometer (between the interferometer and the auxiliary detector) has the advantage that the calibration substance does not interfere with the measurement of the sample's spectrum. Alternatively, the container can also be positioned in the common light path to both the interferometer and the auxiliary detector, i.e., "inside" or "in front" of the interferometer.

[0074] In an alternative approach, a substance omnipresent in the FT spectrometer setup is chosen as the calibration substance. This simplifies the measurement setup. The omnipresent substance can, in particular, be a component of air if the FT spectrometer setup is exposed to air.

[0075] A preferred variant uses water vapor, methane, or acetylene as the calibration substance. These substances are readily available at low cost.

[0076] Within the scope of the present invention, an FT spectrometer arrangement according to the invention, as described above, is used in a method according to the invention, as described above. This allows highly accurate calibrated spectra to be obtained with high availability of the FT spectrometer arrangement.

[0077] In a preferred variant, the calibration factor is determined and the calibrated spectrum is generated in situ by an evaluation unit of the FT spectrometer arrangement. This makes the calibrated spectra immediately available.

[0078] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those described in more detail below can each be used individually or in any combination according to the invention. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention. Detailed description of the invention and drawing

[0079] Fig. 1 schematically shows the setup of an exemplary embodiment of an FT spectrometer arrangement according to the invention; Fig. 2 schematically shows an exemplary course of the amplitude of the measurement signal at the detector or auxiliary detector and the amplitude of the measurement signal at the reference detector as a function of the path difference of the interferometer arms; Fig. 3 schematically shows, by way of example, the Fourier-transformed measurement signal of an auxiliary detector as a function of the wavenumber ("auxiliary spectrum"); Fig. 4a schematically shows an exemplary section of an auxiliary spectrum to illustrate the determination of the calibration factor; Fig. 4b schematically shows one of the Fig. 4aFig. 5 shows a corresponding section from a reference spectrum to illustrate the determination of the calibration factor; Fig. 5 schematically shows a diagram of an exemplary integral K1 of a cross-correlation for determining the calibration factor; Fig. 6 schematically shows a diagram of an exemplary apparatus function as a function of the wavenumber; Fig. 7 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 interferogram of the measurement sample; Fig. 7 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 pre-calibrated spectrum of the measurement sample; Fig.Fig. 7 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; Fig. 8 schematically illustrates a variant of a measurement method according to the invention, with common recordings, wherein the calibration factor for these measurement recordings is determined with calibration recordings that take place simultaneously with measurement recordings; Fig. 8 schematically illustrates a variant of a measurement method according to the invention, with common recordings, wherein the calibration factor for the measurement recordings is determined with calibration recordings that take place before measurement recordings; Fig. 8 schematically illustrates a variant of a measurement method according to the invention, wherein a calibration factor is determined from an averaged additional spectrum as a moving average; Fig.Fig. 8d schematically illustrates a variant of a measurement method according to the invention, wherein a cross-sample calibration factor is applied based on a calibration image with increased spectral resolution; Fig. 8e schematically illustrates a variant of a measurement method according to the invention, wherein 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 with monitor images. Inventive FT spectrometer arrangement

[0080] The Fig. 1 Figure 1 shows a schematic view of the structure of an exemplary embodiment of an FT spectrometer arrangement according to the invention.

[0081] The FT spectrometer arrangement 1 comprises a light source 2 that emits broadband light 3, in the illustrated embodiment infrared (=IR) light. The light 3 is directed to an interferometer 5 by means of a collimating arrangement, in the illustrated embodiment a collimating mirror 4.

[0082] The interferometer 5 comprises a beam splitter 6 and two reflectors 7, 8, which are configured 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, include an electric drive for the reflector 7, with which the reflector 7 can be moved along the optical axis 27 of the interferometer arm 9. Typically, the drive 11 sets the reflector 7 into a continuous oscillation along the direction of the axis 27. In the embodiment shown, the reflector 8 is rigidly (stationarily fixed) in position. Alternatively, the reflector 8 can also be movable (not shown in detail), see, for example, US 5 309 217 A.

[0083] At the beam splitter 6, the light 3 arriving from the light source 2 is partially transmitted to the reflector 7 and reflected back to the beam splitter 6, and partially reflected to the reflector 8 and reflected back to the beam splitter 6. At the beam splitter 6, the light 3 from the reflector 7 is partially reflected towards the measuring position 12 and the detector 13, and partially transmitted towards an additional detector 14. At the beam splitter 6, the light 3 from the reflector 8 is partially transmitted towards the measuring position 12 and the detector 13, and partially reflected towards 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, which blocks a portion of the light 3 (approximately half).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 about half) is also shaded.

[0084] A partial beam 17 of the light 3 thus travels from the interferometer 5 to the auxiliary detector 14, and a partial beam 18 of the light 3 travels to the sample position 12 and the detector 13. The partial beam 17 is focused by a focusing lens 14a into an entrance aperture of the auxiliary 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 where the calibration substance has signatures that are used for calibration. The partial beam 18 is focused onto the measurement position 12, where a measurement sample 12a (for example, in a container that is essentially transparent to light in the used spectral range) is arranged by means of a focusing arrangement, which in the illustrated embodiment is designed as a focusing mirror 20.The radiation 3, which has passed the measuring position 12, is focused by an imaging arrangement, which here is designed as an imaging mirror 21, into an input aperture of the detector 13.

[0085] Furthermore, the FT spectrometer arrangement 1 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 by means of a mirror 24 into the interferometer 5, where, analogous to the light 3, it is partly transmitted and partly reflected at the beam splitter 6, and is reflected at the reflectors 7, 8. In the beam path downstream of the interferometer 5, i.e., between the interferometer 5 and the detector 13, the reference light 23 is guided by another mirror 25 into a reference detector 26 and registered there. The reference light 23 thus travels essentially the same optical path in the interferometer as the light 3. Using the reference detector 26, the constructive and destructive interference of the reference light can be measured as a result of the changing optical path difference of the interferometer arms 9, 10.Given a known (effective) laser wavelength of the reference light 23, the path difference can be determined as a function of time, particularly for the times at which the detector 13 is read out. Within the scope of the invention, the effective laser wavelength can be determined by generating an additional spectrum using the additional detector 14. This additional spectrum contains absorption lines of the calibration substance 19a (which is contained in the container 19 or is ubiquitous in the FT spectrometer arrangement 1) and is compared with a reference spectrum of the calibration substance. This allows a calibration factor to be determined, which can then be used to calibrate the effective laser wavelength. The calibrated effective laser wavelength can be used to generate an interferogram of the sample using the detector signals. This interferogram is then used to generate a calibrated, and therefore highly accurate, spectrum of the sample by Fourier transform (FT).

[0086] Note 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 be replaced by lens arrangements with corresponding imaging properties, possibly in combination with plane mirrors. Likewise, the auxiliary detector 14 and the container 19 can also be arranged in front of the second aperture 16 (instead of in front of the first aperture 15). Inventive measuring methods

[0087] In the measurement method according to the invention, a detector and an additional detector are read out at defined path differences; note that the measurement method in the invention is carried out with an FTIR spectrometer arrangement according to the invention. Fig. 2The upper half illustrates, by way of example, a section of the measurement signal A at a detector or auxiliary detector as a function of the path difference GU (in arbitrary units au) of an FT spectrometer arrangement according to the invention, as in Fig. 1 illustrated; note that with a uniform change in the path difference or uniform movement of the reflector (Bzz. 7 in Fig. 1The path difference is a linear function of time. The lower half shows the waveform (in arbitrary units) of a measurement signal Aref from 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 depicted configuration, at each zero crossing of the measurement signal Aref at the reference detector, a readout of the detector or auxiliary detector is performed ("scanning" at the zero crossings); these zero crossings are spaced λ / 2 apart by the optical path difference, where λ is the wavelength of the reference laser light (note that c = λ / F, where F is the laser frequency and c is the speed of light).Accordingly, the readout positions of the detector and the auxiliary detector are (ideally) known as precisely as the laser wavelength of the reference laser. The readout positions are incorporated into the accuracy of the frequency axis of the spectra and auxiliary spectra, which are obtained based on the measurement signals of the detector and the auxiliary detector, via the Fourier transform.

[0088] When using laser diodes in the reference laser, fluctuations in the laser frequency can occur, particularly due to aging or temperature variations. 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 be summarized as an "effective" laser frequency Feff. The deviation of the effective laser frequency Feff from the (fixed) nominal laser frequency Fno is described by the calibration factor S, where Feff = Fno * S. If the calibration factor S for an FT spectrometer setup is known, accurate spectra of samples can be generated using this setup, corrected by the calibration factor. Note that in practice, the current calibration factor S is usually stored via the 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 below).

[0089] In the method according to the invention, one or more measurement recordings are carried out to generate a spectrum of a sample; a single measurement recording corresponds to at least one pass through the range of the optical path difference of the interferometer available (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 one interferogram can be generated per pass through the path difference, or per measurement recording, or for all measurement recordings together, depending on how the summation / averaging is performed. The one or more interferograms can then be subjected to a Fourier transform.If only one interferogram was provided, its Fourier transform can be considered the spectrum of the measurement sample; if several interferograms were provided, their Fourier transforms are added / averaged to obtain the spectrum of the measurement sample.

[0090] 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 associated one or more additional interferograms by Fourier transformation.

[0091] The Fig. 3Figure 1 shows an example of such a supplementary spectrum, i.e., a diagram plotting the intensity I of the Fourier transform of (here) a supplementary interferogram determined using the signal of the supplementary detector as a function of the wavenumber σ (note that σ = 1 / λ, where λ is the wavelength of the light). The supplementary spectrum contains, firstly, the spectral distribution of the emission from the light source, which here has its spectral maximum in the range between 1000 and 2000 cm⁻¹. Secondly, the calibration substance (and possibly other substances) in the beam path exhibits 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 well used for calibration.

[0092] 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 such a "cleaned" spectrum (also called a transmission spectrum), the spectral lines of the substance of interest can be isolated (not shown in detail). Within the scope of the invention, both "raw" and "cleaned" spectra can be used.

[0093] In Fig. 4a The wavenumber range of approximately 5503 to 5512 1 / cm of an additional spectrum is shown schematically (see, for example, the additional spectrum from Fig. 3The image is shown enlarged. In this wavenumber range, the calibration substance generates various absorption lines, in particular the strongest absorption peak at Pzus at approximately 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 transform. The wavenumber axis (or the reference laser frequency) is to be calibrated.

[0094] For comparison, it is still in Fig. 4b A section of a reference spectrum of the calibration substance is shown, which also covers the wavenumber range of 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 the absorption spectrum of Fig. 4a very similar; depending on the degree of previous miscalibration of the spectrum of Fig. 4a is the spectrum of Fig. 4ahowever compared to the spectrum of Fig. 4b Stretched or compressed, which is recognizable by the shift of the absorption peaks. In the example shown, it can be seen that the strongest absorption peak Pref (=σ0) is located at approximately 5507.5 cm-1 (exactly: 5507.5137) and is shifted compared to the peak Pzus in the spectrum recorded with the additional detector (peak position of Pzus in the spectrum of the additional detector of Fig. 4b (indicated by a dashed line).

[0095] This detected shift allows for immediate calibration. The calibration factor S is calculated as S = (Pref / Pzus) * Salt, where Salt denotes the calibration factor underlying the additional spectrum. If the additional spectrum was uncorrected (i.e., based on the unchanged, nominal laser frequency Fno), Salt can be set to 1. If an effective laser frequency Feff (alt) = Fno * Salt (where Salt is not equal to 1) was already assumed for the additional spectrum, the effective laser frequency Feff can be updated by the factor Pref / Pzus = UF (where UF is the update factor), i.e., Feff (new) = Feff (alt) * UF, or Feff (new) = Fno * S, or Feff (new) = Fno * Salt * UF. In the example above, UF=P ref / P zus =5507.5137 / 5507.321=1.0000349898. Note that the positions P ref and P zus are typical partial pieces of information from the reference spectrum and the additional spectrum, which are used here to determine the calibration factor S.

[0096] Using the (new) calibration factor S or the (new) effective laser frequency F eff =F no *S, the generation of a spectrum of a measurement sample can then be carried out with high accuracy.

[0097] If determining the position of individual peaks in the supplementary spectrum (and possibly also in the reference spectrum) proves difficult, the calibration factor can also be calculated via cross-correlation of the supplementary and reference spectra over a selected (characteristic) spectral range (absorption range). For this purpose, the integral K1 is calculated using... ∫ σ 0 − δ σ 0 + δ f σ ∗ g σ + τ dσ = K 1 τ with σ₀: mean wavenumber position of the characteristic absorption region of the calibration substance, δ: integration width, σ: wavenumber, τ: wavenumber shift variable, f: reference spectrum; g: additional spectrum. The integral K₁ is used to investigate a shift of the wavenumber axis, expressed by the wavenumber shift variable τ.

[0098] The Fig. 5The diagram shows, as an example, the value of an integral K1 as a function of the shift variable τ; note that τ has the dimension 1 / cm. The integral K1 has a maximum at τ0, meaning that at this shift, the greatest agreement occurs between the additional spectrum g and the reference spectrum f. The (new) calibration factor S is then obtained with S = [(σ0 / (σ0 + τ0)] * Salt].

[0099] Instead of an integral K1 of a cross-correlation, the integral K2 can also be determined using ∫ σ 0 − δ σ 0 + δ f σ ∗ g s + σ dσ = K 2 s with σ₀: mean wavenumber position of the characteristic absorption region of the calibration substance, δ: integration width, σ: wavenumber, s: calibration factor variable, f: reference spectrum; g: additional spectrum. The integral K₂ is used to investigate a stretching of the wavenumber axis, expressed by the calibration factor variable s.

[0100] The integral K2 as a function of the calibration factor variable s (which is not shown separately here, but looks similar to the one in Fig. 5 The integral K1 shown then has a maximum at s0, which needs to be determined; note that the calibration factor variable s is dimensionless, and s0 is also dimensionless. The (new) calibration factor S is determined as S = s0 * S old.

[0101] The regions of the functions f and g encompassed by the respective integrals K1, K2 over the integration width δ around σ 0 represent typical partial information of the reference spectrum and the additional spectrum, which are used to determine the calibration factor S.

[0102] Alternatively, it is also possible to start from the reference spectrum or a partial piece of information from it (such as a section as in Fig. 4b (shown)

[0103] The aim is to iteratively model the model spectra and determine the model spectrum that shows the best agreement with the additional spectrum. At least one line shape dictated by the FT spectrometer configuration ("instrument") used is taken into account ("instrument line shape" ILS); an example of such a line shape or instrument function is shown in Fig. 6 posed. In Fig. 6This illustrates how an infinitely sharp absorption line ("delta function") would appear in a measured (additional) spectrum due to the instrument's characteristics. The instrument typically causes both a certain line broadening and a certain asymmetry of the measured absorption line. Furthermore, the model incorporates an interim calibration factor (ICF), which is assumed to be the frequency error in the measurement of the additional spectrum for the respective model spectrum. The ICF 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 = ICF * Salt. In addition, further parameters of the FT spectrometer setup can also be determined using the model spectrum.

[0104] In the Figures 7a to 7cThe following is a brief example illustrating how the determination of a corrected spectrum of a measurement sample can be carried out within the scope of the invention. The examples of Figs. 7a to 7cEach measurement image, used to generate a spectrum of a sample, is represented by a square with pointed corners. The signal of the one or more measurement images is represented by an interferogram (or possibly a set of interferograms) IN pr. The same applies to calibration images, which are represented by an additional interferogram (or a set of additional interferograms) IN zus. Obtained spectra can be calibrated with the calibration factor S (KS, calibrated spectrum), pre-calibrated using a previous calibration factor S alt (VS, pre-calibrated spectrum), or uncalibrated (US, uncalibrated spectrum). The subscript pr or zus indicates the assignment to the sample / measurement images at the detector (pr) or to the calibration substance / calibration images at the additional detector (zus). The reference spectrum of the calibration substance (not subject to calibration) is marked SP ref.

[0105] In the examples of Figs. 7a to 7c The measurement and calibration recordings are performed simultaneously as joint recordings with respect to time t.

[0106] In the version of Fig. 7aA pre-calibrated additional spectrum, VSzus, is generated using at least one calibration image INzus and the existing calibration factor Salt by Fourier transformation. This is compared with the reference spectrum, and an update factor UF is determined. The (current) calibration factor S is then determined from the existing calibration factor Salt and the update factor UF. Using this calibration factor S, the calibrated spectrum KSpr of the measurement sample is then generated from at least one measurement image INpr by Fourier transformation. In practice, the existing effective laser frequency, which was based on Salt and used to generate VSzus, can be multiplied by the update factor UF, so that the new effective laser frequency is then based on the updated calibration factor S and can be used to generate KSpr.

[0107] In the Fig. 7bIn the depicted variant, a pre-calibrated additional spectrum VSzus is also generated from at least one calibration recording INzus using the existing calibration factor Salt via Fourier transformation. This is then compared with the reference spectrum, and an update factor UF is determined. Furthermore, a pre-calibrated spectrum VSpr of the measurement sample is generated from at least one measurement recording INpr using the existing calibration factor Salt via Fourier transformation. Its frequency axis is then corrected using the update factor UF. Accordingly, the updated calibration factor S = Salt * UF is also included in the corrected spectrum KSpr of the measurement sample in this case as well.

[0108] In the Fig. 7cIn the depicted variant, an uncalibrated additional spectrum USzus is generated from at least one calibration image INzus without calibration via Fourier transformation. This is compared with the reference spectrum, and the current calibration factor S is thereby directly determined. Using this calibration factor S, the calibrated spectrum KSpr of the measurement sample is then generated from at least one measurement image INpr via Fourier transformation.

[0109] In the Figures 8a to 8e Various variants of the measurement method according to the invention are explained, wherein the temporal sequence of measurement recording(s) for a measurement sample and calibration recording(s) for calibrating the spectra of the measurement sample are explained; note that in each case a measurement of a plurality of measurement samples is provided.

[0110] The names and symbols largely correspond to the names of Fig. 7a-7calike (see above). Additionally, different measurement samples are numbered P1, P2, and P3. Monitor recordings are also provided, which include the subscript mn, and with which auxiliary calibration factors HS are determined; the comparisons V with the (final) calibration factor S are shown in circles. The In Fig. 7a-7cThe described methods for determining S are no longer broken down individually; accordingly, when determining S, only a general additional spectrum SPzus is assigned to at least one calibration measurement, again represented by INzus. Furthermore, one variant provides an averaged (additional) spectrum GSzus, 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 the time t, which is plotted to the right, is provided. The planes of measurement images M, calibration images K, and monitor images MN are each plotted one above the other (upwards).

[0111] The Fig. 8aFigure 1 shows a variant in which several samples P1, P2 are measured sequentially; the measurement of two samples is shown as an example. For each sample P1, P2, at least one measurement IN pr and at least one calibration measurement IN zus are taken simultaneously in their respective time slots t1, t2, as at least one combined measurement. The calibration factor S determined in the respective time slot t1, t2 is applied to the at least one measurement IN pr from the same time slot t1, t2 to obtain the corrected spectrum KS pr.

[0112] In this variant, the best possible accuracy of the calibration factor S is achieved for the respective measurement sample P1, P2.

[0113] The Fig. 8bFigure 1 shows a variant in which at least one calibration image INzus is taken in the first time slot t0, and the calibration factor S is determined using the additional spectrum SPzus. In the next time slot t1, at least one measurement image INpr is then taken of a first sample P1, and the corresponding corrected spectrum KSpr of the first sample P1 is generated using the calibration factor S from time slot t0. Simultaneously with the at least one measurement image INpr of the first sample P1, at least one further calibration image INzus is taken in time slot t1 (a joint image), and the calibration factor S determined from this can then be applied in the next time slot t2 to the sample P2 measured in that time slot t2. Any number of samples can be measured in this manner.

[0114] 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 t1) to generate the corrected spectrum KS pr of the measurement sample, it is not necessary to wait for the calculation of the calibration factor S of the current time slot (t1), but the (almost equally current and therefore almost equally accurate) calibration factor S of the previous time slot (t0) can be used.

[0115] In the version of Fig. 8cFour time slots t1, t2, t3, and t4 of the measurement are shown as examples. During each time slot, at least one measurement IN pr is taken on a sample (shown here only for time slots t3 and t4 for simplicity); for example, at least one measurement IN pr of sample P1 is taken in time slot t3. 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 images IN zus from the last three time slots t1, t2, and t3. In time slots t1 and t2, at least one measurement on a different sample was taken simultaneously with the calibration images IN zus (not shown in detail). Thus, in this variant as well, the measurement images IN pr and the calibration images IN zus are taken simultaneously.

[0116] To determine the next calibration factor S for sample P2 in the next time slot t4, the calibration images IN tos from time slots t2, t3 and t4 are then used.

[0117] In this variant, the calibration factor S is based on a moving average of the calibration measurements IN from several (here three) recent time slots or associated measured samples. This allows the calibration factor S to be determined with greater reliability and accuracy (through averaging) while still remaining up-to-date (through the time-based moving average).

[0118] Note that, alternatively, the calibration factor S, determined over a number of time slots (e.g., t1, t2, t3), can only be applied at the next time slot (t4 in the example) to accelerate the calculation of the corrected spectrum KS pr of the measurement sample (not shown in detail, but see below). Fig. 8b (in an analogous manner).

[0119] In the Fig. 8d In the variant shown, at least one calibration image IN zus H< is acquired in time slot t0 with an increased resolution compared to the resolution of the at least one measurement image IN pr in each of time slots t1, t2, t3 for the samples P1, P2, P3. For illustration, the calibration image IN zus H< is therefore given a high index H and represented in a rectangle twice as wide as the squares of the measurement images IN pr. From the at least one high-resolution calibration image IN zus H<, an additional spectrum is determined, which is used to calculate the calibration factor S. This factor is then applied in several subsequent time slots, here time slots t1, t2, t3, to generate the corrected spectra KS pr of several samples, here samples P1, P2, P3. The calibration factor S, applied across samples, is particularly accurate due to the higher measurement resolution of the underlying at least one calibration image IN zus H<.

[0120] 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 duration (e.g., 10 minutes) before a new, at least one calibration recording IN tos H< with an increased resolution is carried out to update the calibration factor S.

[0121] In the version of Fig. 8e , based on the variant of Fig. 8dThe calibration process (see above) is then used to verify the validity of the calibration factor S, which was determined in time slot t0 based on at least one calibration image INzus with increased resolution, using monitor images INmn. In subsequent time slots t1, t2, and t3, at least one monitor image INmn is taken with the additional detector, typically with the same resolution as the measurement images INpr of the samples. This yields an additional monitor spectrum of the calibration substance (not shown in detail), based on which (as with calibration images) a calibration factor is determined. This factor is referred to as the auxiliary calibration factor HS in the context of the monitor images.

[0122] Determining the auxiliary calibration factors HS has a lower accuracy than determining the calibration factor S; nevertheless, a comparison V of HS and S can reveal whether the most recently determined calibration factor S is still usable and should remain valid. For example, it can be defined that the calibration factor S will no longer be considered usable in the future as soon as an auxiliary calibration factor HS deviates from the calibration factor S by more than a threshold of 0.01%.

[0123] In the example shown, it is determined in time slot t3 that the calibration factor S is no longer usable. In the depicted variant, the calibration factor S from time slot t0 is applied one last time to sample P3, and then in time slot t4, a further calibration acquisition, IN zus H<, with increased resolution, is started, with which a new calibration factor for subsequent time slots is determined (the latter not shown in detail). Alternatively, it would also be possible, for example, to apply the auxiliary calibration factor HS determined in time slot t3 to generate the calibrated spectrum KS pr of measurement sample P3.

[0124] Note that typically time slots are allocated for calibration images with increased resolution (t0 in Fig. 8d , and t0, t4 in Fig. 8e ) last longer than time slots for monitor recordings (t1, t2, t3 in Fig. 8d , 8eIt should also be noted that, if desired, measurement recordings IN pr on measurement samples can also take place simultaneously with the calibration recordings IN zus H< (joint recordings) (not shown in detail). Reference symbol list

[0125] 1FT spectrometer setup 2 Light source 3 Light 4 Collimating mirror 5 Interferometer 6 Beam splitter 7 Reflector 7a Retro-reflector 8 Reflector 8a Retro-reflector 9 (First) interferometer arm 10 (Second) interferometer arm 11 Device for changing the optical path difference 12 Measuring position 12a Sample 13 Detector 14 Auxiliary detector 14a Focusing lens 15 First aperture 16 Second aperture 17 Partial beam (in the beam path behind the interferometer to the auxiliary detector) 18 Partial beam (in the beam path behind the interferometer to the measuring position) 19 Container 19a Calibration substance 20 Focusing mirror 21 Imaging mirror 22 Reference laser 23 Reference light 24 Mirror 25 Mirror 26 Reference detector 27 Optical axis of the interferometer arm 9 A Measurement signal (detector or auxiliary detector) A ref Measurement signal (reference detector) f Function of the reference spectrum F eff Effective laser frequency F no Nominal laser frequency g Function of the auxiliary spectrum H Index High resolution HS Auxiliary calibration factor I Intensity IN Interferograms / signal representing images according to index K Range Calibration images K Corrected spectrum according to index K1 Integral of cross-correlation M Range Measurement images mn Index Monitor images MN Range Monitor images pr Index Measurement samples / spectrum P ref Wavenumber position (peak) in the reference spectrum P zus Wavenumber position (peak) in the auxiliary spectrum P1,P2,P3... Samples S (current) calibration factor S alt Previous calibration factor S P Spectrum according to index t0,t1,t2... Time slots t Time UF Update factor US Uncorrected spectrum according to index V Comparison (HS and S) V S Pre-corrected spectrum according to index zus Index Calibration substance / Add-on spectrum δIntegration width σWavenumber σ0 mean wavenumber position τWavenumber shift variable τ0Wavenumber shift variable at maximum of K1

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) from the light source (2) is guided, wherein the interferometer arms (9, 10) each comprise one reflector (7, 8), - a measurement sample (12a) at a measurement position (12), - a detector (13) for the light (3), wherein the measurement 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 which reference laser light (23) is guided through the interferometer (5), - 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) downstream of the interferometer (5), and - an additional detector (14) with which a partial beam (17) of the light (3), which has passed through the interferometer (5) but has not passed through the measurement position (12), can be measured, wherein the detector (13) can measure a further partial beam (18) of the light (3), which has passed through the interferometer (5) and has passed through the measurement position (12), simultaneously with a measurement of the partial beam (17) at the additional detector (14), characterized in that the reflectors (7, 8) are designed as retro-reflectors (7a, 8a), 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 a portion, in particular one 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), originating from the beam splitter (6) is directed back towards the light source (2) onto the first aperture (15).

2. FT spectrometer arrangement (1) according to claim 1, characterized in that a second aperture (16) is furthermore provided in the beam path of the light (3) between the interferometer (5) and the measurement position (12), with which a part, in particular one 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), originating from the beam splitter (6) is directed towards the sample position (12) onto 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 by 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 one of claims 1 to 3, characterized in that in the partial beam (17) of the light (3), which can be measured with the additional detector (14), a container (19) with a calibration substance (19a) is arranged between the interferometer (5) and 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 (KSpr) of a measurement sample (12a) using an FT spectrometer arrangement (1) according to one of the preceding claims, wherein the measurement comprises one or more measurement recordings (INpr), wherein in a respective measurement recording (INpr) - light (3) from a light source (3) is guided through an interferometer (5), interacts with the measurement sample (12a) at a measurement position (12) downstream of 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 guided through the interferometer (5) and is measured at at least one reference detector (26), - wherein, among other things, a signal from the detector (13) of the one or the plurality of measurement recordings (INpr) and a calibration factor (S) for calibrating a frequency axis are used to generate a spectrum (KSpr) of the measurement sample (12), wherein the measurement comprises one or a plurality of calibration recordings (INzus), wherein in a respective calibration recording (INzus) - light (3) from the light source (2) is guided through the interferometer (5), and a partial beam (17) of the light (3) from the beam path of the light (3) is measured at an additional detector (14) after passing through the interferometer (5) without passing through the measurement position (12), - furthermore, reference laser light (23) from the reference laser (22) is guided through the interferometer (5) and is measured at the at least one reference detector (26), wherein, among other things, a signal from the additional detector (14) of the one or the plurality of calibration recordings (INzus) is used to generate an additional spectrum (SPzus), and wherein a reference spectrum (SPref) or partial information therefrom 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), is compared with the additional spectrum (SPzus) or partial information therefrom in order to determine the calibration factor (S).

7. Method according to claim 6, characterized in that one or a plurality of the measurement recordings (INpr) and one or a plurality of the calibration recordings (INzus) are performed simultaneously as one or a plurality of joint recordings.

8. Method according to claim 7, characterized in that all measurement recordings (INpr) and calibration recordings (INzus) are performed as joint recordings.

9. Method according to claim 7 or 8, characterized in that the calibrated spectrum (KSpr) of the measurement sample (12) and the calibration factor (S), which is applied in the generation of the calibrated spectrum (KSpr), are based on one or a plurality of joint recordings.

10. Method according to one of claims 6 to 8, characterized in that the calibration factor (S), which is applied in the generation of the calibrated spectrum (KSpr) of the measurement sample (12), is based on one or a plurality of calibration recordings (INzus) that were recorded prior to the one or the plurality of measurement recordings (INpr) on which the calibrated spectrum (KSpr) is based.

11. Method according to one of claims 6 to 10, characterized in that, as part of the method, a measurement of several spectra (KSpr) is performed on a plurality of measurement samples (12; P1, P2, P3), wherein a calibrated spectrum (KSpr) of each respective measurement sample (12; P1, P2, P3) is generated.

12. Method according to claim 11, characterized in that for each calibrated spectrum (KSpr) of the plurality of measurement samples (12; P1, P2, P3), a separate calibration factor (S) is determined with which the calibrated spectrum (KSpr) of a respective measurement sample (12; P1, P2, P3) is generated.

13. Method according to claim 12 in conjunction with claim 7, characterized in that the respective separate calibration factor (S) and the respective calibrated spectrum (KSpr) of the respective measurement sample (12; P1, P2, P3) are based on joint recordings.

14. Method according to claim 11, characterized in that a cross-sample calibration factor (S), which is determined on the basis of one or a plurality of calibration recordings (INzus), is used to generate a plurality of calibrated spectra (KSpr) of a plurality of measurement samples (12; P1, P2, P3).

15. Method according to claim 14, characterized in that the cross-sample calibration factor (S) is determined on the basis of one or a plurality of calibration recordings (INzusH) which have a higher spectral resolution than at least part of the measurement recordings (INpr) on which the plurality of calibrated spectra (KSpr) of the plurality of measurement samples (12; P1, P2, P3) are based.

16. Method according to one of claims 6 to 15, 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).

17. Method according to one of claims 6 to 16, characterized in that a substance omnipresent in the FT spectrometer arrangement (1) is selected as the calibration substance (19a).

18. Method according to one of claims 6 to 17, characterized in that the calibration substance (19a) is water vapor or methane or acetylene.

19. Method according to one of claims 6 to 18, characterized in that the determination of the calibration factor (S) and the generation of the calibrated spectrum (KSpr) are performed in situ by an evaluation unit of the FT spectrometer arrangement (1).