Optical gas analysis apparatus, system and method

Abstract

The invention relates to an optical gas analysis apparatus, system and method. The invention relates to an optical gas analysis device (1), comprising: a gas cell (10) for measuring a gas (11); a first light source (21) wherein the first light source is a coherent light source, and wherein the first light source is adapted to emit a first light (41) towards the gas cell; a second light source (22) wherein the second light source is a non-coherent light source, and wherein the second light source is adapted to emit a second light (42) towards the gas cell; a first light sensor (31), wherein the first light sensor is adapted and arranged to receive a first light after passing through the measurement gas (11) in the gas cell (10) on a first optical path; and a second light sensor (32), wherein the second light sensor is adapted and arranged to receive a second light after passing through the same measurement gas (11) in the same gas cell (10) on a second optical path. The invention further relates to a corresponding optical gas analysis system (2) and method (100).

Description

Field of the Invention

[0001] The present invention relates to an optical gas analysis device. Further, the present invention also relates to a corresponding optical gas analysis system and method. Background Art

[0002] Optical gas analyzers can use the absorption of light by gases to determine the concentration of the gases. Measuring gases can also include a combination of one or more different gases. Gas analyzers can be used in several applications, such as industrial, environmental monitoring, and / or medical applications.

[0003] Several gases absorb light in a few narrow absorption lines or (in the case of larger molecules) absorption bands. The spectral position of the absorption lines or absorption bands is characteristic of each gas. Thus, gases can often be identified based on their absorption lines or absorption bands. The proportion of light absorbed by a gas depends on the gas itself, the gas concentration, and the optical path length over which the light can be absorbed. The Beer-Lambert or Lambert-Beer law is commonly applied in chemical analysis measurements to determine the concentration of the light-absorbing chemical substance. Thus, the Beer-Lambert law can provide a simplified model in the following way:

[0004] I(c,λ) = I 0 e -α(λ，c)L

[0005] a(λ,c) = ∈(λ)c,

[0006] where I is the light of wavelength λ transmitted through the gas cell, I 0 is the light intensity of wavelength λ in front of the gas cell, c is the gas volume concentration, α(λ,c) is the absorption coefficient of the gas, and L is the optical path length. Given the proportion of light absorbed, the wavelength, the specific absorption rate ∈(λ) of the gas, and the optical path length, the gas concentration can be determined.

[0007] Although the basic principle is known in the art, there are numerous different practical implementations for determining gas concentration by optical measurement. Some exemplary measurement principles of different gas analyzers are NDIR (non-dispersive infrared), TDLS (tunable diode laser spectroscopy), and, in a few cases, NDUV (non-dispersive ultraviolet).

[0008] NDIR can use a thermal emitter (such as a light bulb, a heated filament, or a heated film) as an infrared light source. Such sources can be labeled as blackbody emitters because their spectra can approximate the blackbody spectrum as described by Planck's law. Exemplary light sources for NDIR can emit infrared radiation with a wavelength range, for example, from 2 micrometers to 5 micrometers, or even to longer wavelengths, such as up to 14 micrometers. Target gas molecules can strongly absorb light at specific wavelengths within the emission wavelength range. For detection, one or more narrowband pass filters can be used to select the wavelength of interest and a reference wavelength. Generally, NDIR systems are considered to have a relatively low cost but can provide medium to low performance, which may be sufficient for many applications.

[0009] Tunable diode laser absorption spectroscopy (TDLS, also known as TDLAS, TLS, or TLAS) is a technique for measuring the concentration of certain substances (such as methane, water vapor, etc.) in a gas mixture using a tunable diode laser and laser absorption spectrometry. Compared with other concentration measurement techniques, the advantage of TDLS is that it can achieve very low detection limits (in the ppb range). In a TDLS system, a tunable laser diode is used as the light source. The wavelength emitted by the laser can be modified within a range of several nanometers. Typically, the wavelength can be tuned within a range that covers one or several gas absorption lines. Typically, a laser emitting in the near infrared is used, usually with a wavelength shorter than about 2 micrometers. In some cases, more expensive mid-infrared lasers can be used. However, the limitation of TDLS is that the wavelength tunable range of a single laser is limited. Therefore, a TDLS system may be able to measure a single gas. In some cases, if the absorption lines are within the tunable range, it may be possible to measure two gases using a single laser. TDLS systems are generally more expensive than NDIR but have better performance.

[0010] Regarding the background of optical gas sensing, for example, Hodgkinson et al. provided an overview, “Optical gas sensing: a review.”, Measurement Science and Technology 24, 2013, which is incorporated herein by reference.

[0011] Furthermore, DE 10 2010 050 626 B4 discloses a gas analyzer including a Herriott cell and its operating method. DE 10 2009 025 147 B3 discloses a method for operating a gas analysis spectrometer and a spectrometer. Summary of the Invention

[0012] An object of various aspects of the present invention is to provide a further improved optical gas analysis device, system and method. In particular, it is desirable to provide a compact optical gas analysis device, especially one in which multiple gases can be measured using one optical gas analysis device. Further, it is desirable to further improve the measurement accuracy and / or sensitivity.

[0013] In a first aspect of the present invention, there is provided an optical gas analysis device, which includes: a gas cell for measuring a gas; a first light source, wherein the first light source is a coherent light source and is adapted to emit first light to the gas cell; a second light source, wherein the second light source is an incoherent light source and is adapted to emit second light to the gas cell; a first light sensor, wherein the first light sensor is adapted to (and is arranged to) receive the first light that has passed through the measured gas in the gas cell on a first optical path; and a second light sensor, wherein the second light sensor is adapted to (and is arranged to) receive the second light that has passed through the same (identical) measured gas in the same gas cell on a second optical path.

[0014] In another aspect of the present invention, there is provided a system for optical gas analysis, which includes: the device as described above; and an analyzer, wherein the analyzer is adapted to determine at least one gas concentration of at least one component of the measured gas based on the light detected by the first light sensor and the light detected by the second light sensor.

[0015] In another aspect of the present invention, there is provided a method for optical gas analysis, which includes the following steps: providing a measured gas to the gas cell; emitting first light from a coherent first light source to the gas cell; emitting second light from an incoherent second light source to the same gas cell; detecting the first light that has passed through the measured gas in the gas cell on a first optical path using the first light sensor; and detecting the second light that has passed through the same (identical) measured gas in the same gas cell on a second optical path using the second light sensor.

[0016] Preferred embodiments of the present invention are defined in the dependent claims. It should be understood that the claimed system and method may have preferred embodiments similar and / or identical to those of the claimed optical gas analysis device, especially as defined in the dependent claims and disclosed herein.

[0017] Aspects of the solution proposed herein provide the possibility of further improving optical gas analysis, in particular obtaining a compact device capable of measuring several gases with medium to high performance. As used herein, performance may refer to the ability to detect low gas concentrations and / or measure gas concentrations with high accuracy. For example, some gases can be measured using the NDIR measurement principle, while some gases can be measured using the TDLS measurement principle. Aspects of the present invention are based on the idea of providing a device in which a common gas cell for measuring gases is provided. The gas cell can be filled with the gas to be analyzed. However, a gas cell with an inlet and an outlet can also be provided, in which a gas flows from the inlet to the outlet, so that this gas can be analyzed continuously.

[0018] The device according to aspects of the present disclosure is provided with both a first light source and a second light source, the first light source being a coherent light source adapted to emit a first coherent light into the gas cell; and the second light source being an incoherent light source adapted to emit a second incoherent light into the same gas cell. For example, the incoherent light source can be (i) an infrared light source, in particular a planar infrared emitter, a MEMS infrared emitter or an emitter adapted to emit infrared light by thermal emission; or it can be (ii) a UV light source, in particular a UV LED or other UV emitter. The coherent light source can be a laser, in particular a vertical cavity surface emitting laser (VCSEL), a distributed feedback laser (DFB), a quantum cascade laser (QCL) or an interband cascade laser (ICL). The first light source can include a collimator adapted to collimate the first light and emit the collimated first light into the gas cell. Additionally or alternatively, the second light source can include a reflector adapted to collect the second light and adapt the emission angle of the second light to the gas cell. However, the second light source can be adapted to emit at least a first portion of the second light having a divergence angle such that at least some of the second light passes through the measurement gas multiple times before reaching the second light sensor. Thereby, the absorption length can be increased, and for the second incoherent light, the measurement sensitivity can also be further improved. Optionally, at least a second portion of the second light can be provided to the second light sensor by passing through the measurement gas in a single pass. By combining both the single-pass and multi-pass optical paths from the second light source to the second light sensor, the sensitivity can be improved while still providing a reliable setup, which can allow for a larger alignment tolerance for the second light source. Another advantage of combining both the single-pass and multi-pass optical paths from the second light source to the second light sensor can be that the dynamic range of the device can be improved, for example, single-pass for high concentrations and three-pass for low concentrations.

[0019] The device further includes a first optical sensor and a second optical sensor. The first optical sensor is adapted and arranged to receive first light that has passed through the gas to be measured in a first optical path through the gas cell. The second optical sensor is adapted and arranged to receive second light that has passed through the same gas to be measured in a second optical path through the same gas cell. As described above, the second optical path can be a combined optical path, where a first part is provided on a single pass through the gas to be measured, and a second part is provided on a multi-pass optical path through the gas to be measured. As used herein, an optical sensor can include one or more detectors. For example, an optical sensor can include a number of detector pixels. Different detectors of the optical sensor can optionally be provided with at least some different wavelength selection filters. For example, the second sensor can be a four-thermopile detector or a four-thermopile detector, e.g., four individual detectors, each with a specific bandpass filter, and all four detectors integrated into a TO package. Other sensor types are possible. The number of detectors (or "channels") of the sensor can determine the number of gases that can be measured, and it is also possible that at least one channel is used as a reference while another or more channels are used as measurement channels. Commercial sensors include single-detector sensors, dual-detector sensors (two channels), four-detector sensors (four channels), and even 8-channel detector sensors. It should be appreciated that the present disclosure is not limited to a specific number of detectors per sensor.

[0020] Advantages of aspects of the proposed solution can be that several and / or different gases can be measured using a single system. An additional advantage of aspects of the proposed solution compared to two separate systems can be a lower cost for each gas or gas component.

[0021] Advantageously, no moving parts may be required. An advantage of such an implementation can be an increase in reliability. For example, depending on the desired measurement scheme, incoherent light modulation, laser modulation, and laser wavelength tuning can be performed without moving parts, e.g., entirely electrically. For example, a second light source for incoherent light can be modulated at a frequency below 1 kHz, particularly in the range between 1 and 100 Hz, particularly between 1 and 30 Hz. For example, a first light source for coherent light or laser can be modulated at a frequency above 1 kHz, particularly at least 20 kHz or higher. A DC offset can be used for both types of light. Optionally, for the first light and / or the second light, the light can be detected in a phase-sensitive manner, particularly using a lock-in principle.

[0022] In the following, some terms used throughout the application will be briefly explained and defined. The term "gas cell" can refer to the measurement volume of the gas to be measured. The gas cell can have different shapes. For example, a cylindrical gas cell can be provided with an inlet and an outlet for the measurement gas to be analyzed. The term "measurement gas" can refer to a measurement gas comprising one or more gases to be analyzed, in particular a mixture in which an optical gas analysis system can be used to determine the composition and / or concentration.

[0023] In an embodiment, the optical gas analysis device can be adapted to perform tunable diode laser spectroscopy (TDLS) measurements using a first light source and a first light sensor; and to perform non-dispersive infrared (NDIR) or non-dispersive ultraviolet (NDUV) measurements using a second light source and a second light sensor; and wherein both (i) TDLS measurements and (ii) NDIR or NDUV measurements are performed on the same measurement gas in the same gas cell. Both types of measurements can be achieved using a single gas cell and sharing at least some of the optical components. In an improvement, the device can be adapted to measure a first component of the measurement gas with high accuracy using TDLS, and to measure a second component of the measurement gas with lower accuracy using NDIR or NDUV. Thus, the first light source can be particularly adapted to the absorption line of the first component to be measured with high accuracy. For example, NDIR or NDUV can be used to determine whether there are impurities or contaminants in the measurement gas, while TDLS can be used to perform high-accuracy measurements on the desired target component. Thus, NDIR or NDUV can well complement TDLS measurements, rather than providing additional TDLS measurements at different wavelengths, which may potentially involve the higher cost of additional tunable lasers.

[0024] The gas cell can be a multipass cell, which includes a first reflective surface and a second reflective surface, where the first reflective surface and the second reflective surface face each other. In particular, the multipass gas cell can include a first spherical mirror providing the first reflective surface and a second spherical mirror providing the second reflective surface, where the first spherical mirror and the second spherical mirror face each other. Advantageously, one or more optomechanical components can be shared between the two measurement methods for measuring the first light source and the second light source. In particular, a spherical mirror with an opening can be shared between the two methods. The gas cell can be adapted to provide a multipass gas cell for the first optical light from the first light source to the first optical sensor. In addition, the gas cell can be further adapted to also provide a multipass gas cell for at least a portion of the second optical light from the second light source to the second optical sensor. As used herein, the multipass gas cell is adapted to increase the path length of the light from the light source to the optical sensor, such that the path length through the gas being measured in the gas cell is increased. In other words, in an improved embodiment, it is proposed to provide a specific arrangement, where the gas cell is adapted to provide a multipass gas cell for the coherent first light and (at least a portion of) the incoherent second light. The device can be adapted such that at least another portion of the incoherent second light can reach the second optical sensor on a direct optical path, i.e., without multipass. As described above, the advantage of combining both the single-pass and multipass optical paths from the second light source to the second optical sensor can be that the dynamic range of the device can be improved. For example, single-pass can be used for high concentrations and three-pass for low concentrations.

[0025] In a further improved embodiment, the multipass gas cell can be adapted to provide at least a first portion of the incoherent second light on a second optical path from the second light source to the second optical sensor, where the second optical path is a multipass beam path; and to provide at least a second portion of the incoherent second light on a third optical path from the second light source to the second optical sensor, where the third optical path is a direct (or single-pass) optical path from the second light source to the second optical sensor.

[0026] In an improved embodiment, the first reflective surface and the second reflective surface can be shared by the first optical path and the second optical path. Thus, the first reflective surface and the second reflective surface can be adapted to provide a first multipass optical path from the first light source to the first optical sensor, and a second multipass optical path (for at least a portion of the light) from the second light source to the second optical sensor. Thus, a multipass optical path can be provided for both the coherent first light and the incoherent second light. Thereby, the sensitivity of both measurements using coherent light and incoherent light can be further improved while providing a compact optical gas analysis device.

[0027] In an improved embodiment, the first optical path may include multiple reflections at the first reflective surface and multiple reflections at the second reflective surface. The second optical path (for at least a portion of the second light) may include exactly one or at least one reflection at the first reflective surface and exactly one or at least one reflection at the second reflective surface. At least another portion of the second light may be provided directly to the second light sensor without being reflected at the first reflective surface and / or the second reflective surface. In particular, the first optical path may include more reflections at the first reflective surface than the second optical path. Similarly, the first optical path may include more reflections at the second reflective surface than the second optical path. The inventors have recognized that choosing different numbers of reflections for the first light and the second light can facilitate implementation. In other words, the first coherent light undergoes more reflections compared to the non-coherent second light, which may originate from a more divergent light source such as a thermal emitter.

[0028] The first reflective surface may include a first aperture adapted to allow the first light from the first light source to pass through and a second aperture adapted to allow the second light from the second light source to pass through. The second aperture may be disposed at the center of the first reflective surface. The first aperture may be disposed eccentrically with respect to the first reflective surface. Similarly, the second reflective surface may include a third aperture adapted to allow the first light from the first light source to pass through and a fourth aperture adapted to allow the second light from the second light source to pass through. The fourth aperture may be disposed at the center of the second reflective surface. The third aperture may be disposed eccentrically with respect to the second reflective surface. An aperture may refer to a portion of the first reflective surface and / or the second reflective surface for receiving light from the light source or coupling light out to the light sensor. In particular, an aperture may be an opening for light on each respective reflective surface. The first aperture and the second aperture may optionally be connected, for example, such that the first aperture and the second aperture are connected to form a pear-shaped aperture. For example, the larger second aperture and the smaller first aperture may be combined, and for example, the cusp between the circles of the first circular aperture and the second circular aperture may be removed. Additionally or alternatively, this may equally apply to the third aperture and the fourth aperture which may also be connected. An aperture may include a filter, also referred to as a window. An aperture may allow light to pass through, but may still provide a barrier for measuring the gas. The window may be disposed not only in direct contact with the respective reflective surface, but also in front of or behind the respective aperture, for example, in the wall of the gas cell.

[0029] In an improved embodiment, the first aperture and / or the third aperture may include a window that is transparent to the first light and reflective to the second light. Similarly, the second aperture and / or the fourth aperture may include a window that is transparent to the second light and reflective to the first light. The advantage of this embodiment is that the apertures can still serve as reflectors for a portion of the light, thereby allowing further reflection within the multipass gas cell. Additionally, unnecessary interference with the optical sensors can be reduced. For example, the window can be used to remove a certain range of wavelengths. Typically, if some of the gases present absorb longer wavelengths, but the interference filter of the detector of the optical sensor cannot filter out those longer wavelengths, then, for example, sapphire will be used to absorb wavelengths greater than 5.5 microns. As described above, the window of the aperture can be arranged not only in direct contact with the corresponding reflective surface, but also in front of or behind the corresponding aperture, for example, in the wall of the gas cell. For example, the window can be adhered to the gas cell, typically made of metal or plastic, rather than to the mirror. This can facilitate manufacturing.

[0030] Regarding the arrangement of the first light source and the second light source, and the first optical sensor and the second optical sensor, generally speaking, the light sources can be arranged on the same side or on different sides of the gas cell. Similarly, the first optical sensor and the second optical sensor can be arranged on the same side or on different sides of the gas cell. One or more of the optical sensors can be arranged on the same side of the gas cell as one or more of the light sources, or on a different side of the gas cell from one or more of the light sources. In an improved embodiment, the first light source can be arranged on the first side of the gas cell, while the second light source can be arranged on the second side of the gas cell. Thus, the first light source and the second light source can be arranged on opposite sides of the gas cell. Additionally or alternatively, the second optical sensor can be arranged on the first side of the gas cell, while the first optical sensor can be arranged on the second side of the gas cell. Thus, the first optical sensor and the second optical sensor can be arranged on opposite sides of the gas cell. The advantage of providing the corresponding light sources and / or optical sensors on different sides is that crosstalk can be reduced. Additionally, the advantage of an arrangement that is at least partially on opposite sides can be a more compact design. For example, crosstalk can be reduced by not having the TDLS optical sensor face the NDIR source, otherwise the TDLS optical sensor might detect a portion of the broadband incoherent light. The TDLS optical sensor as the first optical sensor can be, for example, a photodiode adapted to (and arranged to) detect the light emitted by the first coherent light source. Further, the space constraints can be relaxed. The sources are often larger in volume than the optical sensors, so combining one source and one optical sensor on each side can make the assembly more compact. The source (and optical sensor) for one measurement principle does not necessarily have to be on the same side, but can advantageously be provided on opposite sides of the gas cell.

[0031] The first optical path and the second optical path can be adapted to cross each other in the gas cell. In other words, the first optical path from the first coherent light source to the first light sensor and the second optical path from the second incoherent light source to the second light sensor can cross each other or have at least one intersection point. Thus, the first optical path and the second optical path can be adapted to at least partially cover the same or a consistent measurement volume of the gas cell. The advantage can be that measurements are made using both the coherent light source and the incoherent light source, thereby providing a more meaningful and / or more accurate measurement result for the measurement gas in the gas cell.

[0032] The length of the first optical path can be at least 3 times, in particular at least 5 times, especially at least 8 times that of the second optical path. The length of the first optical path can be at least 5 times, in particular at least 10 times, especially at least 15 times the length of the gas cell. The length of the second optical path can be at least 2 times, in particular at least 3 times the length of the gas cell. For example, the first optical path can be a first multi-pass optical path with 17 passes, for example for TDLS; while the second optical path can be a second multi-pass optical path with 3 passes, for example for NDIR. Similarly, a part of the second light can reach the second light sensor on a direct path, while another part of the second light can reach the second light sensor on a multi-pass optical path. As used herein, the length of the gas cell can refer to the interval or distance between the first reflective surface and the second reflective surface of the gas cell facing each other.

[0033] The gas cell can be adapted to provide a Herriott (Herriott-type) cell for a first optical path from a first light source to a first light sensor. The gas cell can be further adapted to provide a Pfund (Pfund-type) cell for a second optical path from a second light source to a second light sensor. In other words, it is proposed to provide a multi-pass cell design that combines Pfund and Herriott. The inventors recognized that, particularly for optical gas analysis with a coherent first light source and a divergent incoherent second light source, it can be advantageous to combine these two types. In particular, for Pfund-type behavior, the gas cell can be a multi-pass gas cell including a first reflecting surface and a second reflecting surface facing each other, particularly two spherical mirrors, which can even be the same. The reflecting surfaces can each have a central aperture. For Pfund-type behavior of the second light, the gas cell can be configured such that light rays from the second light source can enter at an aperture (e.g., the second aperture as described above) at the center of the first reflecting surface or the second reflecting surface, at least a portion of the second light is redirected twice at two reflection points, and then exits the cell at an aperture (e.g., the fourth aperture as described above) at the center of the respective other of the first reflecting surface and the second reflecting surface on the third pass. Further light rays from the second light source (which can be a divergent light source) can exhibit corresponding behavior when passing through the gas cell such a number of times. Further, some light rays from the second light source can reach the second light sensor directly on a third optical path without undergoing a multi-pass through the gas cell. The second light source can be adapted to emit a (divergent) light cone. As an example of the third optical path, light rays within a narrow central angle range may not be reflected and may only undergo a single pass through the measurement gas. As an example of the second optical path, light rays emitted at a larger angle may be reflected and may undergo three passes. At even larger angles, the light rays may be scattered, absorbed, or otherwise lost. Similar to the Pfund-type cell, the Herriott-type cell can be a multi-pass cell including a first reflecting surface and a second reflecting surface facing each other, particularly two spherical mirrors, which can even be the same. For Herriott-type behavior, at least one aperture can be provided in one of the reflecting surfaces to allow the input beam and the output beam to enter and exit the cavity. Alternatively, the beam can exit through an aperture on the opposite reflecting surface.

[0034] In an improved embodiment, the Pfund cell may include a first reflective surface and a second reflective surface facing each other, where the Pfund cell may be configured such that the focus of at least one of the reflective surfaces forming the Pfund cell is behind the surface of the opposing reflective surface forming the Pfund cell, particularly where the focus is located outside the gas cell. At first glance, positioning the focus outside the gas cell may seem counterintuitive, as this may require a larger aperture to couple light into the Pfund cell. However, an advantage of this improved embodiment may be that an incoherent light source disposed behind the reflective surface of the gas cell and adapted to emit divergent second light may couple light into the gas cell more efficiently. Another advantage may be that this arrangement provides design freedom. For example, the placement of the second light source is more flexible. It also becomes possible to place the output window of the gas cell behind the reflective surface on the sidewall of the gas cell. This may facilitate the sealing of the gas cell and / or reduce manufacturing costs.

[0035] The second light source may include an optical filter adapted to reduce one or more predetermined spectral components in the second light. The optical filter may include a gas adapted to absorb these predetermined spectral components. An advantage may be that the spectrum of the second light may be adapted to the desired application.

[0036] In a further improved embodiment of the system, the analyzer may be adapted to correct the gas concentration determined based on the light detected by the first light sensor or the second light sensor by means of the light detected by a respective other light sensor, so as to determine at least one gas concentration of at least one component in the measurement gas. An advantage of this aspect may be a further improvement in sensitivity. Description of the Drawings

[0037] With reference to the (one or more) embodiments described below, these and other aspects of the invention will become apparent and be elucidated. In the following drawings:

[0038] Figure 1 A schematic diagram of a first embodiment of an optical gas analysis device according to an aspect of the invention is shown;

[0039] Figure 2 A schematic diagram of a second embodiment of an optical gas analysis device according to an aspect of the invention is shown;

[0040] Figure 3 A front view of a mirror of the gas cell of the optical gas analysis device is shown;

[0041] Figure 4 Shows Figure 3 a perspective view of the mirror of;

[0042] Figure 5 Another embodiment of a mirror of the gas cell of the optical gas analysis device is shown;

[0043] Figure 6 Shows a perspective view of the first optical path and the second optical path in the gas cell of an optical gas analysis device illustrating aspects of the present invention;

[0044] Figure 7 Shows an illustration of Figure 6 A side view of the first optical path and the second optical path in the gas cell of the optical gas analysis device;

[0045] Figure 8 Shows having Figure 6 A perspective view of an independent illustration of the first optical path in the gas cell of the optical gas analysis device;

[0046] Figure 9 Shows an illustration of Figure 8 A side view of the first optical path;

[0047] Figure 10 Shows having Figure 6 A perspective view of an independent illustration of the second optical path in the gas cell of the optical gas analysis device;

[0048] Figure 11 Shows an illustration of Figure 10 A side view of the second optical path;

[0049] Figure 12 Shows a flowchart of a method according to aspects of the present invention. Detailed Description

[0050] Figure 1 Schematically shows a first embodiment of an optical gas analysis device. The device is generally designated by reference numeral 1 in the figures. The device 1 includes a gas cell 10 for measuring a gas 11. The gas cell may include one or more gas connectors to allow gas exchange in the gas cell. In the example shown, an inlet 12 and an outlet 13 are provided for the measurement gas. Thus, the measurement gas 11 can be introduced into the gas cell 10 via the inlet 12 and leave the gas cell via the outlet 13. The gas cell 10 can be filled with the measurement gas to be analyzed. However, a gas flow from the inlet 12 to the outlet 13 can also be provided so that a gas flow can be continuously analyzed. The inlet 12 and the outlet 13 can also be arranged at different positions, for example, on opposite side surfaces of the gas cell 10 such that the measurement gas 11 passes through the gas cell 10.

[0051] The optical gas analysis device 1 further includes a first light source 21, a second light source 22, a first light sensor 31, and a second light sensor 32. The first light source 21 is a coherent light source and is adapted to emit a first light 41 into the gas cell 10. For illustration, in Figure 1 and Figure 2In the figure, the first light is illustrated by a dashed line. The second light source 22 is a non - coherent light source and is adapted to emit a second light 42 into the gas cell. For illustration purposes, in Figure 1 and Figure 2 in the figure, the second light is illustrated by a solid line. The first light sensor 31 is adapted and arranged to receive the first light 41 after it has passed through the gas cell 10 on the first optical path 43. The second light sensor 32 is adapted and arranged to receive the second light 42 after it has passed through the same measurement gas 11 in the same gas cell 10 on the second optical path 44.

[0052] The optical gas analysis device 10 can be adapted to perform tunable diode laser spectroscopy (TDLS) measurements using the first light source 21 and the first light sensor 31; and perform non - dispersive infrared (NDIR) or non - dispersive ultraviolet (NDUV) measurements using the second light source 22 and the second light sensor 32. Both (i) TDLS measurements and (ii) NDIR or NDUV measurements are performed on the same measurement gas 11 using the same gas cell 10.

[0053] The first light source 21 can be, for example, a tunable single - mode laser, such as a NIR VCSEL or DFB laser or a mid - IR QCL or ICL laser. The first light 41 can be a light beam that is inserted into the gas cell 10 through a small opening or window. After optionally being reflected several times and thus passing through the measurement gas 11 in the gas cell 10 several times, the first light leaves the gas cell 10.

[0054] The second light source 22 is a non - coherent light source adapted to emit a second non - coherent light into the same gas cell and can be, for example, (i) an infrared light source, in particular a planar infrared emitter, a MEMS infrared emitter or an emitter adapted to emit infrared light by thermal emission; or can be (ii) a UV light source, in particular a UV LED or other UV emitter.

[0055] The second light source 22 for non - coherent radiation can optionally include a reflector 23 to reduce the divergence of the emitted second light 42. Reflectors of several shapes are possible, such as spherical, parabolic, Winston cone, elliptical or free - form. The reflector shape, its distance to the reflecting surface or mirror in the gas cell, the reflector diameter and the opening of the window or aperture on the reflecting surface can be optimized to improve system performance. Reflector optimization can be combined with mirror optimization, in particular such that the reflector matches the mirror, in particular such that the collimated light fully illuminates the mirror and losses are minimized.

[0056] Although the light sources 21, 22 and the light sensors 31, 32 can be integrated into the gas cell, at least some of them are preferably arranged outside the gas cell in order to protect the gas cell 10 and facilitate the sealing of the gas cell 10. The gas cell 10 should be sealed. Gas connectors (in the given example, the inlet 12 and the outlet 13) allow the exchange of gas in the cell. For the first light and the second light, the gas cell may include one or more windows through which the first light 41 and the second light 42 can enter and leave the gas cell 10.

[0057] In Figure 1 the example shown, a first window 15 is provided for allowing the coherent first light 41 from the coherent first light source 21 to enter the gas cell 10. The first window 15 may also be referred to as the laser incident window. A second window 16 is provided for allowing the incoherent second light 42 from the incoherent second light source 22 to enter the gas cell 10. The second window may also be referred to as the IR or UV incident window. A third window 17 is provided for allowing the coherent first light 41 to leave the gas cell 10 towards the first light sensor 31. The third window 17 may also be referred to as the laser exit window. A fourth window 18 is provided for allowing the incoherent second light 42 to leave the gas cell 10 towards the second light sensor 32. The fourth window may also be referred to as the IR or UV exit window. It should be appreciated that the use of separate windows is optional and, depending on the beam path within the gas cell, a common incident window and exit window may be used. Similarly, the use of separate windows for the first light and the second light may be optional. The incident window and the exit window may be arranged on the same side or different sides of the gas cell 10. Similarly, the light source and the light sensor may be arranged on the same side or different sides of the gas cell.

[0058] The respective windows 15, 16, 17, 18 of the gas cell 10 are adapted to the wavelength range of interest. The gas cell 10 may include the first window 15 and / or the third window 17 adapted to perform TDLS measurements. The gas cell 10 may include the second window 16 and / or the fourth window 18 adapted to perform NDIR measurements or NDUV measurements. For example, for performing NDIR, typical examples are sapphire (transparent up to about 5 microns), calcium fluoride (transparent up to about 9 microns), barium fluoride (transparent up to about 12 microns), zinc sulfide (transparent up to about 12.5 microns), silicon (transparent up to about 15 microns), zinc selenide (transparent up to about 18 microns), germanium (transparent up to about 18 microns). Other materials are also possible, including but not limited to KRS-5, AMTIR and related materials.

[0059] One or more of the windows 15, 16, 17, 18 of the gas cell 10 may be provided with an anti-reflection coating. For example, in order to perform TDLS, the first window 15 and / or the third window 17 may be anti-reflection coated windows optimized for the typical NIR wavelengths used. Windows for TDLS are typically not compatible with NDIR and vice versa, and separate windows of at least two different materials and / or windows with different coatings may be used.

[0060] It is also possible that the optical filters of the first optical sensor 31 and / or the second optical sensor 32 can serve as windows of the gas cell 10. For example, the first optical sensor 31 and / or the second optical sensor 32 may be coupled to the gas cell 10 in a sealed manner. Similarly, the first light source 21 and / or the second light source 22 may be coupled to the gas cell 10 in a sealed manner.

[0061] In Figure 1 the illustrative example shown, the second optical sensor 32 may include one or more detectors, for example, two detectors 33A and 33B. The detectors 33A, 33B may be sensitive to different wavelengths of interest in the emission spectrum of the second light source 22 for analyzing the measurement gas in the gas cell 10. Exemplary detectors 33A, 33B for the second light in the infrared wavelength range may be thermal detectors such as bolometers, thermopiles or pyroelectric detectors, but semiconductor-based detectors or other detectors may also be used. Each IR detector may be equipped with a separate IR filter for identifying the gas of interest. The second optical sensor 32 may, for example, include a dual detector, a quad detector or even an octal detector tightly integrated into a single package. For the first optical sensor 31, it may be sufficient to provide, for example, a single photodiode as a detector. Optionally, a filter may be provided that may be adapted to the tuning range of the desired TDLS measurement.

[0062] Referring again to Figure 1 , the gas cell 10 may be a multipass cell including a first reflecting surface 51 and a second reflecting surface 52. The first reflecting surface 51 and the second reflecting surface 52 face each other. In Figure 1 the example shown, the multipass gas cell includes a first spherical mirror 53 providing the first reflecting surface 51 and a second spherical mirror 54 providing the second reflecting surface 52. The first spherical mirror 53 and the second spherical mirror 54 face each other. In Figure 1 the first reflecting surface 51 and the second reflecting surface 52 are illustrated by separate elements, however, the mirrors or the first and second reflecting surfaces may also be provided by the sidewalls of the gas cell 10.

[0063] As Figure 1As shown, the first reflective surface 51 and the second reflective surface 52 are shared by the first optical path 43 of the first light 41 and the second optical path 44 of the second light 42. The first optical path 43 includes multiple reflections at the first reflective surface 51 and multiple reflections at the second reflective surface 52. The second optical path 44 includes at least one, particularly exactly one, reflection at the first reflective surface 51 and at least one, particularly exactly one, reflection at the second reflective surface 52.

[0064] In the illustrated example, the gas cell 10 is adapted to provide a Herriott-type cell for the first optical path 43 from the first light source 21 to the first light sensor 31. The gas cell 10 is further adapted to provide a Pfund-type cell for the second optical path 44 from the second light source 22 to the second light sensor 32. The Pfund cell includes a first reflective surface 51 and a second reflective surface 52 facing each other. In an improved embodiment, the Pfund cell is configured such that the focus of at least one of the reflective surfaces 51, 52 forming the Pfund cell is behind the surface of the opposite reflective surface 51, 52 forming the Pfund cell, and in particular, where the focus is located outside the gas cell 10. In Figures 3 to 5 an exemplary configuration of the reflective surface or mirror is shown. In Figures 6 to 11 an exemplary optical path is illustrated. Accordingly, it is proposed to provide an optical gas analysis device that includes an advantageous Pfund-Herriott combination.

[0065] Figure 3 and Figure 4 shows a first exemplary embodiment of a mirror 53 for a Pfund-Herriott combination, which includes a reflective surface 51. Advantageously, the first mirror 53 and the second mirror 54 can be the same. This can simplify manufacturing and reduce costs due to reduced inventory. In the illustrated embodiment, the mirrors 53, 54 are spherical mirrors. The first reflective surface 51 includes a first aperture 55 adapted to allow the first light 41 from the first light source 31 to pass through and a second aperture 56 adapted to allow the second light 42 from the second light source 32 to pass through. The second aperture 56 is arranged at the center of the first reflective surface 51. The first aperture 55 is arranged eccentrically with respect to the first reflective surface 51. Correspondingly, the second reflective surface 52 includes a third aperture 57 adapted to allow the first light 41 from the first light source 31 to pass through and a fourth aperture 58 adapted to allow the second light 42 from the second light source 22 to pass through. The fourth aperture 58 is arranged at the center of the second reflective surface 52. The third aperture 57 is arranged eccentrically with respect to the second reflective surface 52. The corresponding apertures are also illustrated in Figure 1 and Figures 6 to 11 and

[0066] In other words, using the NDIR as an example, an opening or aperture 56 is used on axis to allow mid-IR light to pass through. It has been found that working in the infrared band, for example, up to 12 microns in wavelength, may impose restrictions on the use of partially reflective materials or only partial coatings, which may not be possible or too expensive. Therefore, aperture 56 provides a feasible implementation for allowing a second light to enter and leave the gas cell. An additional opening or aperture 55 is placed off-axis to allow a laser as an exemplary first light to enter the cell. Similarly, an additional opening or aperture 57 is placed off-axis to allow a laser as an exemplary first light to leave the cell. Apertures 55, 57 should be small so as to still allow reflection in a Herriott cell design. Similarly, apertures 56, 58 should be small enough to allow efficient passage in a Pfund cell design. If 55, 57 are too large, the maximum number of passes will be reduced. If 56, 58 are too large, a Herriott cell cannot be implemented and the Pfund cell will not reflect enough light. As described above, the optical design of the reflector and mirror should be matched or optimized. In Figure 3 In FIG. 5 , the points on the reflecting surface 53 represent possible reflection points of the first light on the first optical path 43 during several passes through the Herriot cell.

[0067] Using this approach, the reflector carrier material of the first reflector 53 and / or the second reflector 54 can be a common reflector substrate material, so that common manufacturing methods can be used and the manufacturing cost is moderate. The reflector can then be uniformly coated with a metal coating (typically gold) to cover a wide wavelength range as required. In principle, other coatings can be used. In the pool assembly, two nominally identical reflectors 53, 54 can be used, with the central openings 56, 58 aligned with the central axis of the pool, and the focus of each reflector can be placed at least on the back of the facing reflector or reflecting surface. This is in Figure 7 , where the focus is placed, for example, at the output of the second light source 22 and close to the second light sensor 32. The small openings 55, 57 for the laser light as the first light are not necessarily aligned along the central axis of the gas cell, or do not necessarily face each other, because the relative radial position of the openings 55, 57 determines the number of passes in the Herriot cell. The angle θ is measured as the angle from the cell axis to the laser entrance opening 55 and the laser exit opening 57 (see Figure 3 57', and Figure 8 The angle θ is defined by rotating one of the mirrors 53, 54 around the cell axis to achieve the desired angle and thus the number of reflections.

[0068] Figure 5A further embodiment of a reflector 53, 54 in a gas cell 10 of an optical gas analysis device 1 is shown. The first aperture 55 and the second aperture 56 can optionally be connected, for example, so that the first aperture and the second aperture are connected to form a pear-shaped aperture 59. For example, the larger second aperture 56 and the smaller first aperture 55 can be merged, and for example, the sharp point between the circles of the first circular aperture and the second circular aperture can be removed. Additionally or alternatively, this can also apply to the third aperture 57 and the fourth aperture 58, which can also be connected. In other words, a combined opening can thus be achieved on the reflector, for example, with a pear shape, without the need to retain material between the two openings.

[0069] Considering Figures 3 to 5 , again refer to Figure 1 , the first aperture 55 may be aligned with the first window 15 . The second aperture 56 may be aligned with the second window 16 . The third aperture 57 may be aligned with the third window 17 . The fourth aperture 58 may be aligned with the fourth window 18 .

[0070] Figures 6 to 11 The figure shows the configuration and optical paths in a combined Pfund-Heriot cell described in more detail herein. Elements corresponding to those previously described are denoted by the same reference numerals.

[0071] Figure 6 and Figure 7 The figure shows a perspective view and a side view of a first optical path 43 and a second optical path 44 in a gas cell. For simplicity, the side walls of the gas cell are not shown in the figures.

[0072] Figure 8 and Figure 9 The figure shows an isolated perspective view and a side view of the first optical path 43 in the gas cell (i.e., without the second optical path 44 for better understanding). For the first light 41 of the first light source 21, the reflection surface 51 of the first reflector 53 and the reflection surface 52 of the second reflector 54 provide a Herriot-type reflection in the gas cell. In other words, a collimated (single) laser beam as the first light 41 enters the cavity via the aperture 55, is reflected multiple times at different angular positions between the first reflection surface 51 and the second reflection surface 52, and leaves the cavity toward the first light sensor 31 via the aperture 56.

[0073] Figure 10 and Figure 11The figure shows an independent perspective view and a side view of the second optical path 44 in the gas cell (i.e., for better understanding, without the first optical path 43). For the second light 42 of the second light source 22, the reflecting surface 51 of the first mirror 53 and the reflecting surface 52 of the second mirror 54 provide Pfund-type reflection in the gas cell. In other words, a diverging light cone is provided as the second light 42 by the second light source 21 and enters the cavity via the aperture 56. A part of the second light 21 can directly reach the second light sensor 32 without being reflected by the first reflecting surface 51 or the second reflecting surface 52. Such a direct or single-path light beam path can also be referred to as the third optical path 45. Another part of the second light 21 can undergo a first reflection at the second reflecting surface 52, a second reflection at the first reflecting surface 51, and leave the cavity towards the second light sensor 32 via the aperture 58. Although the second light 21 can generally be a light cone, exemplary light rays on such a light beam path are illustrated by the thick solid line 46.

[0074] Figure 2 A schematic diagram of a second embodiment of the optical gas analysis device 1 is shown. Hereinafter, the differences from the Figure 1 illustrated embodiment will be briefly described.

[0075] As Figure 2 shown, the first light source 21 and the second light source 22 can also be arranged on different sides of the gas cell 10. Similarly, the first light sensor 31 and the second light sensor 32 can be arranged on different sides of the gas cell 10. As described above, the advantage of providing the corresponding light sources and / or light sensors on different sides is that crosstalk can be reduced. In addition, the advantage of at least partially arranging them on opposite sides can be a more compact design. For example, crosstalk can be reduced by not having the TDLS light sensor (e.g., the first light sensor 31) face the NDIR source (e.g., the second light source 22), otherwise the TDLS light sensor might detect a part of the broadband incoherent light. The TDLS light sensor as the first light sensor can be, for example, a photodiode adapted to (and arranged to) detect the light emitted by the first coherent light source. Further, the space constraints can be relaxed. The source is often larger in volume than the light sensor, so combining one source and one light sensor on each side can make the assembly more compact. The source (and light sensor) for one measurement principle does not necessarily have to be on the same side, but can advantageously be provided on opposite sides of the gas cell.

[0076] Furthermore, Figure 2 an optional modification of the second light source 22 and the corresponding incident window 16 is shown. Optionally, a filter can be used to reduce the influence of one or more unwanted absorption lines from the second light source 22. In Figure 2In the example shown, a gas-filled cell 60 is provided in front of the second light source 22. The gas-filled cell may be filled with a light-absorbing gas adapted to filter out one or more unwanted absorption lines by negative filtering. The windows of the gas cell are designated by reference numerals 61 and 62. It is also possible that the second light source 22 is provided filled with a light-absorbing gas adapted to filter out one or more unwanted absorption lines by negative filtering. Thus, unwanted absorption lines can be removed.

[0077] The optical gas analysis device 1 described in the present disclosure, for example, as Figure 1 or Figure 2 shown, can be part of a system 2 for optical gas analysis. Figure 2 The exemplary map in shows further system elements. The system 2 further includes an analyzer 81, where the analyzer is adapted to determine at least one gas concentration of at least one component of the measurement gas 11 based on the light detected by the first light sensor 31 and the light detected by the second light sensor 32. The analyzer 81 can be provided as a separate entity or, for example, integrated into the controller 80, as Figure 2 shown. The analyzer 81 is adapted to receive a first information indicating the first light after passing through the measurement gas and a second information indicating the second light after passing through the measurement gas. The controller 80 can optionally be connected to the first light source 21 and / or the second light source 22 and is adapted to control them, for example, by setting the emission timing, emission frequency, wavelength, or other emission characteristics. The controller can be connected to the first light sensor 31 and / or the second light sensor 32 to receive information about the first light and / or the second light detected after passing through the measurement gas 11.

[0078] Figure 12 A flowchart of a method 100 for optical gas analysis is shown. In a first step S101, a gas cell with a measurement gas is provided. In a second step S102, first light from a coherent first light source is emitted into the gas cell. In step S103, second light from an incoherent second light source is emitted into the same gas cell. In step S104, the first light is detected by a first light sensor after passing through the measurement gas in the gas cell along a first optical path. In step S105, the second light is detected by a second light sensor after passing through the same measurement gas in the same gas cell along a second optical path. The order of the method steps can be changed and at least some of the method steps can be performed in parallel. Based on the first light and the second light detected after passing through the measurement gas in the gas cell, in an optional step S106, one or more components and / or component concentrations in the measurement gas can be determined and provided as an output.

[0079] Although the present invention has been described in detail in the drawings and the foregoing description, such description and illustration should be regarded as illustrative or exemplary, rather than restrictive; the present invention is not limited to the disclosed embodiments. By studying the drawings, the disclosure and the appended claims, those skilled in the art can understand and implement other variations of the disclosed embodiments when practicing the claimed invention.

[0080] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may implement the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to achieve an advantage.

[0081] Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. An optical gas analysis device (1), comprising: - a gas cell (10) for measuring a gas (11); - a first light source (21), wherein the first light source is a coherent light source, and wherein the first light source is adapted to emit a first light (41) into the gas pool; - a second light source (22), wherein the second light source is an incoherent light source, and wherein the second light source is adapted to emit a second light (42) into the gas pool; - a first light sensor (31), wherein the first light sensor is adapted and arranged to receive the first light after passing through the measurement gas (11) in the gas cell (10) on a first optical path; as well as - a second light sensor (32), wherein the second light sensor is adapted and arranged to receive the second light after passing through the same measurement gas (11) in the same gas cell (10) on a second optical path.

2. The device according to claim 1, wherein the device is suitable for performing tunable diode laser spectroscopy (TDLS) measurement using the first light source (21) and the first light sensor (31); and performing non-dispersive infrared (NDIR) or non-dispersive ultraviolet (NDUV) measurements using the second light source (22) and the second light sensor (32); and Wherein both (i) the TDLS measurement and (ii) the NDIR or NDUV measurement are performed on the same measurement gas (11) in the same gas cell (10).

3. The device according to any one of the preceding claims, wherein: The gas pool (10) is a multi-pass pool, which includes a first reflecting surface (51) and a second reflecting surface (52), wherein the first reflecting surface and the second reflecting surface face each other; in particular, the multi-pass gas pool includes a first spherical reflecting mirror (53) providing the first reflecting surface (51) and a second spherical reflecting mirror (54) providing the second reflecting surface (52), wherein the first spherical reflecting mirror and the second spherical reflecting mirror face each other.

4. The device according to claim 3, wherein the first reflecting surface (51) and the second reflecting surface (52) are shared by the first optical path and the second optical path (43, 44).

5. An apparatus according to claim 3 or claim 4, wherein the first optical path (43) includes multiple reflections at the first reflecting surface (51) and multiple reflections at the second reflecting surface (52); and wherein the second optical path (44) includes at least one reflection at the first reflecting surface (51) and at least one reflection at the second reflecting surface (52).

6. The device according to any one of claims 3 to 5, wherein the first reflecting surface (51) comprises a first aperture (55) adapted to allow a first light (41) from the first light source (21) to pass therethrough and a second aperture (56) adapted to allow a second light (42) from the second light source (22) to pass therethrough; and wherein the second aperture (56) is arranged at the center of the first reflecting surface (51); and wherein the first aperture (55) is arranged eccentrically relative to the first reflecting surface (51); and / or wherein the second reflecting surface (52) comprises a third aperture (57) adapted to allow the first light (41) from the first light source (21) to pass therethrough and a fourth aperture (58) adapted to allow the second light (42) from the second light source (22) to pass therethrough; and wherein the fourth aperture (58) is arranged at the center of the second reflecting surface (52); and wherein the third aperture (57) is arranged eccentrically relative to the second reflecting surface (52).

7. The device according to claim 6, wherein the first aperture and / or the third aperture (55, 57) comprises a window (15, 17) which is transparent to the first light (41) and reflective to the second light (42).

8. An apparatus according to any one of the preceding claims, wherein the first light source (21) is arranged on a first side of the gas pool (10), and wherein the second light source (22) is arranged on a second side of the gas pool (10); in particular, wherein the second light sensor (32) is arranged on the first side of the gas pool, and wherein the first light sensor (31) is arranged on the second side of the gas pool.

9. The device according to any of the preceding claims, wherein the first optical path (43) and the second optical path (44) intersect each other in the gas cell (10).

10. The device according to any of the preceding claims, wherein the length of the first optical path (43) is at least 3 times, in particular at least 5 times, in particular at least 8 times, greater than the length of the second optical path (44).

11. An apparatus according to any of the preceding claims, wherein the gas cell (10) is adapted to provide a Herriott cell for the first optical path (43) from the first light source (21) to the first light sensor (31); and wherein the gas cell (10) is further adapted to provide a Pfund cell for the second optical path (44) from the second light source (22) to the second light sensor (32).

12. Apparatus according to claim 11, wherein the Pfund cell comprises a first reflecting surface (51) and a second reflecting surface (52) facing each other, and wherein the Pfund cell is configured such that a focal point of at least one of the reflecting surfaces forming the Pfund cell is behind a surface forming an opposite reflecting surface of the Pfund cell, in particular wherein the focal point is located outside the gas cell (10).

13. An optical gas analysis system (2), comprising: - Device (1) according to any one of the preceding claims; as well as - an analyzer (81), wherein the analyzer is adapted to determine at least one gas concentration of at least one component of the measurement gas (11) based on the light detected by the first light sensor (31) and the light detected by the second light sensor (32).

14. A system according to claim 13, wherein the analyzer is suitable for determining the at least one gas concentration of at least one component in the measurement gas (11) by correcting the gas concentration determined based on the light detected by the first light sensor (31) or the second light sensor (32) based on the light detected by the corresponding other light sensor.

15. An optical gas analysis method, the method comprising the following steps: - providing a gas cell (10) with a measurement gas (11) (S101); - emitting a first light (41) from a coherent first light source (21) towards the gas pool (S102); - emitting a second light (42) from an incoherent second light source (22) towards the same gas pool (S103); - detecting the first light (S104) after passing through the measurement gas (11) in the gas cell (10) on a first optical path using a first light sensor (31); and - detecting the second light (S105) after passing through the measurement gas (11) in the same gas cell (10) on a second optical path using a second light sensor (32).

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