Spectroscopic gas sensor

Abstract

A spectroscopic gas sensor (1) comprising a multipass absorption cell (4) is described. The multipass absorption cell (4) comprises a component reflector unit (5) defining a concave first surface (6') with a first centre of curvature (19) and comprising at least one first reflector (6), and a main reflector unit (7) comprising a concave second reflector (8) defining a second centre of curvature (17) and a concave third reflector (9) defining a third centre of curvature (18). The component reflector unit (5), the second reflector (8), and the third reflector (9), are arranged in a White cell configuration. A light source (11) is configured to emit and direct light towards the concave second reflector (8). A first detector (15) is arranged in the component reflector unit (5) such that it is illuminated by a light spot during operation of the gas sensor (1). The component reflector unit (5) comprises an optical return unit (20) comprising a light input (LI) at a first distance from the longitudinal optical plane (C).

Description

[0001] SPECTROSCOPIC GAS SENSOR

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a spectroscopic gas sensor comprising a multipass absorption cell. In particular, the disclosure relates to a gas sensor for obtaining absorption information in at least two different wavelength bands.

[0004] BACKGROUND ART

[0005] Gas sensors for different applications are becoming increasingly common. An example of an application is the use of gas sensors for environmental air pollution monitoring. In air pollution monitoring, it is desirable to be able to detect a number of different gases simultaneously. A cost-effective non-dispersive infrared (NDIR) multi-gas sensor aimed at environmental air pollution monitoring is described in Atmosphere 2022, 13, 1789. The described sensor enables the detection of detection of up to three different gases simultaneously, including CO2, CH4, N2O, and H2O. It would be possible to add additional detectors for detection of all four of said gases. Other examples on applications include the detection of specific gases in industries, such as the detection of carbon dioxide. In breweries, carbon dioxide is used to carbonize beverages. To prevent accidents it is important to monitor the concentration of carbon dioxide. Another example of an application are gas sensors for breath analyzation. The latter example has received increased application in vehicles as a measure to detect and prevent driving under the influence of intoxicating substances. The breath analyzing equipment may be a stand-alone, even handheld, unit that gives a measured value of the content of a substance or substances in the driver’s breath.

[0006] In WO 2020 / 263168 a gas sensor operating in the 3.3-3.6 pm range is described, and has the capability of distinguishing between, e.g., ethyl alcohol and acetone by means of at least two narrow-band filters in the wavelength range between 3.3 and 3.6 pm. The gas sensors described in Atmosphere 2022, 13, 1789 and in WO 2020 / 263168 comprise a multi-pass absorption cell in which the three reflector surfaces are arranged in a White cell configuration. This is an advantageous arrangement of a multi-pass cell as it provides the possibility for a multi-channel gas sensor with a long path length for some channels, while the outer casing of the multi-pass absorption cell may be kept small. In WO 2021 / 173055 a similar gas sensor comprising a multi-pass absorption cell is described, which has dome shaped reflectors to enable the collimation of the light source to be omitted.

[0007] SUMMARY OF THE INVENTION

[0008] To keep the cost of the gas sensor low and to keep the signal-to-noise ratio high, it is desirable to have a small light sensor. However, a small light sensor requires more precise alignment of the components of the gas sensor, which increases the cost of the gas sensor.

[0009] An object of the present disclosure is to provide a spectroscopic gas sensor comprising a multi-pass absorption cell with reflectors in a White cell configuration, which can be manufactured at a lower cost than gas sensors according to the prior art.

[0010] Another object of the present disclosure is to provide a spectroscopic gas sensor comprising a multi-pass absorption cell with reflectors in a White cell configuration, which can be manufactured with lower tolerances for the alignment of the reflectors.

[0011] At least one of the above objects is achieved with a spectroscopic gas sensor comprising a multi-pass absorption cell, according to claim 1.

[0012] Further advantages are provided with the features of the dependent claims.

[0013] According to a first aspect a spectroscopic gas sensor comprising a multipass absorption cell configured for gas exchange is provided. The multipass absorption cell comprises a component reflector unit defining a concave first surface with a first centre of curvature and comprising at least one first reflector. The gas sensor also comprises a main reflector unit comprising a concave second reflector defining a second centre of curvature and a concave third reflector defining a third centre of curvature. The second reflector and the third reflector are facing the component reflector unit, wherein the component reflector unit, the second reflector, and the third reflector, are arranged in a White cell configuration with their respective centres of curvature defining a longitudinal optical plane extending through the component reflector unit, the second reflector, and the third reflector. The gas sensor also comprises a light source configured to emit and direct light towards the concave second reflector, through an illumination area along the concave first surface, wherein the centre of the illumination area is on a first side of the longitudinal optical plane, wherein a plurality of light spots are incident on the component reflector unit during operation of the gas sensor. The gas sensor also comprises a first detector arranged in the component reflector unit such that it is illuminated by a light spot during operation of the gas sensor. The gas sensor is characterised in that the component reflector unit comprises an optical return unit comprising

[0014] - a light input at a first distance from the longitudinal optical plane and arranged to be illuminated by light from the main reflector unit as a first return light spot during operation of the gas sensor, and

[0015] - a light output at a second distance from the longitudinal optical plane on the same side of the longitudinal optical plane as the first return light spot (Ra) and arranged to return the light, illuminating the light input (LI), as a second return light spot towards the main reflector unit, wherein the first detector is arranged to be illuminated by a light spot subsequent to the second return light spot at the second distance from the longitudinal optical plane.

[0016] The above described plurality of light spots incident on the component reflector unit during operation of the gas sensor, comprises the first return light spot and the second return light spot. However, in contrast to other light spots the first return light spot is returned towards the main reflector unit from the second return light spot. In other words, the optical return unit receives light and returns light at different positions.

[0017] The light that is illuminating the light input is returned from the light output as a second return light spot directed towards the main reflector unit. The light illuminating the light input is from one of the second or third reflector, preferably the third reflector. The light returned from the light output is directed towards the same reflector as the light illuminating the light input originated from. Thus, if the light illuminating the light input is from the third reflector, the light returned from the light output is directed towards the third reflector. As mentioned above, the long path length detector is arranged to be illuminated by a light spot subsequent to the second return light spot. This means that the light that illuminates the long path length detector has travelled from the second return light spot.

[0018] The gas sensor may be configured such that the light is reflected at least two times, preferably at least three times, and most preferred four times in the main reflector unit before the light is incident on the light input. This would correspond to one, two and three intermediate reflections in the component reflector unit. According to the first aspect, the light source is configured to emit and direct light towards the concave second reflector. The light that is reflected from the second reflector will be focussed due to the concave shape of the second reflector. By configuration of the component reflector unit, the second reflector, the third reflector and the light source in a White cell arrangement and with the centre of the illumination area on a first side of the longitudinal optical plane, a number of light spots will be formed on the component reflector unit during operation, with every second light spot on the same side of the longitudinal optical plane as the light source and every second light spot on the opposite side of the longitudinal optical plane.

[0019] Any errors in the alignment of the angle between the second reflector and the third reflector will result in an increasing error in the position of the light spots in relation to their intended positions as the number of the light spot increases. However, after the optical return unit, the error in the position of the light spots will decrease. Thus, by arranging the light detector after the optical return unit the error has decreased.

[0020] Preferably, the light sensor is arranged at a position such that the number of light spots between the light source and the optical return unit is as close as possible to the number of light spots between the optical return unit and the light sensor.

[0021] The first detector may be arranged to be illuminated by a light spot on the same side of the longitudinal optical plane as the illumination area. The inventor has made the surprising discovery that the light spots are of higher quality at the first side of the longitudinal optical plane, i.e., the same side of the longitudinal optical plane as the area along the concave first surface, through which the light source configured to emit and direct light towards the concave second reflector. The higher quality of the light spots will enable a better focussing of the light spot on the corresponding detector, which in turn will enable lower losses and / or a smaller sensor to be used. Furthermore, when the component reflector unit moves in relation to the main reflector unit due to, e.g., vibrations, the movement of the light spots on the same side of the longitudinal optical plane as the illumination area will be considerably smaller than the movement of the light spots on the opposite side of the longitudinal optical plane in relation to the illumination area. The smaller movement will result in an improvement in the detection efficiency.

[0022] The optical return unit may be on the same side of the longitudinal optical plane as the illumination area. This provides an improvement in the detection efficiency for the same reasons as described above.

[0023] A number of improvements may be made to improve the quality of the light spots and thereby the detection efficiency. A first improvement is that the concave first surface, the second reflector, and the third reflector may have essentially equal radii of curvature. This improves the quality of the light spots.

[0024] A second improvement is that the component reflector unit may be arranged at a distance from the main reflector unit being equal to the radii of curvature.

[0025] A third improvement is that the concave first surface may define a spherical surface, and the second reflector and the third reflector may be spherical reflectors.

[0026] It is desirable that the distance from the component reflector unit to each one of the second reflector and the third reflector corresponds to the radius of curvature of the second reflector and the third reflector. In this way, the light will be focussed by the second reflector to a small light spot on the component reflector unit. The distance from the component reflector unit to each one of the second reflector and the third reflector does not have to be equal to the radius of curvature of the second reflector and the third reflector, but a small deviation is possible while still providing the desired light spots. The radius of curvature of the second reflector and the third reflector may for example be 85-115 % of the distance from the component reflector unit to each one of the second reflector and the third reflector, and still provide acceptable light spots. This is true at least for a limited number of passes in the multipass absorption cell. The quality of subsequent light spots is improved if the radius of curvature of the second reflector and the third reflector is in the interval 95-105 % of the distance from the component reflector unit to each one of the second reflector and the third reflector. It is most preferred that the radius of curvature of the second reflector and the third reflector is in the interval 99-101 % of the distance from the component reflector unit to each one of the second reflector and the third reflector. In case the concave first surface, the surface of the second reflector, and the surface of the third reflector, are not spherical surfaces their radii of curvature varies with no more than 10% over their surfaces, preferably by no more than 5% and most preferred by no more than 1 %.

[0027] The spectroscopic gas sensor may comprise a second detector arranged to be illuminated by a light spot prior to the first return light spot at the first distance from the longitudinal optical plane on the same side of the longitudinal optical plane as the illumination area. Preferably, the second detector is arranged to be illuminated by the first light spot on the same side of the longitudinal optical plane as the illumination area. Said light spot is the one that is affected the least by errors in the angle between the second and third reflectors.

[0028] When having a second detector sensor arranged to be illuminated by a light spot at the first distance from the longitudinal optical plane on the same side of the longitudinal optical plane as the illumination area it is more or less necessary to have at least one interference filter in front of either the first length detector or the second detector. With such an interference filter, it is provided that different wavelength regions are measured with the different detectors.

[0029] A second interference filter configured to transmit a second wavelength portion of the light, may be arranged in front of the second detector. The second interference filter will function as a reflector for light outside the second wavelength portion. This will allow the light outside the second wavelength portion to travel to the first detector. Interference filters are almost exclusively made planar and the component reflector unit is concave.

[0030] The first detector is arranged to be illuminated by light that has been emitted from the second return light spot, i.e. , the light spot illuminating the first detector is subsequent to the second return light spot. The second detector is arranged to be illuminated by light prior to the first return light spot. A second interference filter configured to transmit a second wavelength portion of the light, may be arranged in front of the second detector. The second interference filter will function as a reflector for light outside the second wavelength portion. This will allow the light outside the second wavelength portion to travel to the long path length detector. In other words, light that illuminates the light input has been emitted from the second interference filter.

[0031] The names long path length detector, the long path length interference filter, the super short path length detector, the super short path length interference filter could also be used instead of numbering. The order of the components in the light path from the light source is different from the order of importance. As described above the most relevant detector is the first detector, which corresponds to a long path length detector, but after introduction of the second detector, i.e. the super short path length detector, the long path length detector is no longer the first detector in the light path. By using the names long path length detector and super short path length detector the confusion when we introduce intermediate detectors, as described below, is avoided. The intermediate detectors are called third and fourth detectors below.

[0032] The fact that the component reflector unit defines a concave first surface does not necessarily mean that each part of the surface of the component reflector unit is concave. As described above the interference filter is arranged in the component reflector. When the component reflector unit defines a concave first surface the first interference filter may be arranged along said concave first surface. Interference filters are normally planar as it is difficult to fabricate curved interference filters. When arranging a planar interference filter in the component reflector unit it is preferably arranged with its centre point coinciding with the concave first surface and with its normal at its centre point coinciding with the normal of the concave first surface. As the light spots on the component reflector unit have a small extension, such a planar interference filter will result in acceptable results. For interference filters having the double task of transmitting a part of the light and to reflect other light to a subsequent light spot, it is preferred to have such an interference filter arranged along the concave first surface.

[0033] The spectroscopic gas sensor may comprise a first interference filter configured to transmit a first wavelength portion of the light and to reflect light outside of the first wavelength portion, and arranged in the component reflector unit, wherein the first detector is arranged to detect light that has been transmitted through the first interference filter. The first interference filter provides the possibility to select the wavelength band at which the first detector detects light.

[0034] It is possible to arrange an optical filter in front of the light source or to use a monochrome light source instead of or in addition to the filter in front of the first detector.

[0035] The second distance may be larger than the first distance. This results in that the light spots are closer to the longitudinal optical plane prior to the optical return unit.

[0036] The optical return unit may comprise a first mirror arranged with its centre at the first distance from the longitudinal optical plane and a second mirror arranged with its centre at the second distance from the longitudinal optical plane, wherein the first mirror is arranged to reflect the light incident on the first mirror towards the second mirror, and wherein the second mirror is arranged to reflect the light from the first mirror in the opposite direction as the light incident on the first mirror. This is a first implementation of the optical return unit.

[0037] It is possible to have more than two mirrors in the optical return unit wherein the light is deviated at a smaller angle than 45° at each mirror.

[0038] According to an alternative, the optical return unit may comprise a cube corner reflector. Such a cube corner reflector should be arranged aligned off centre to provide the proper reflection.

[0039] According to an alternative, the optical return unit may comprise an ellipsoid mirror having its vertex located close to the surface of the component block mirror, but positioned so that the lower half of the ellipsoid is illuminated by the incoming rays that consequently are being focused on the ellipsoidal focal point. Thereafter, the rays progress towards the upper half of the ellipsoidal mirror, which redirects the rays in the opposite direction compared to the incoming rays.

[0040] According to an alternative, the optical return unit may comprise a light guide. The light guide may be based on, e.g., optical fibres.

[0041] The interference filters may have a transmission wavelength in the wavelength region 1-20 pm. When the spectroscopic gas sensor comprises two detectors the second detector and the first detector may be arranged on a common electrical circuit board, wherein the second detector and the first detector are fixed to the circuit board and wherein the first lens is fixed to the circuit board above the second detector and the second lens is fixed to the electrical circuit board above the first detector. By having the detectors arranged in this way close to each other, the fabrication of the gas sensor is simplified as the detectors and the necessary electronics may be arranged on a common electrical circuit board.

[0042] In the following, preferred embodiments will be described with reference to the appended drawings.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 shows schematically a spectroscopic gas sensor according to an embodiment.

[0045] Figure 2 shows schematically in a view as shown in Figure 1 , a multipass absorption cell according to a slightly different embodiment.

[0046] Figure 3 shows schematically the multipass absorption cell of Figure 2 in a view perpendicular to the view of Figure 2.

[0047] Figure 4 shows a component reflector unit according to the prior art in a view towards the component reflector unit.

[0048] Figure 5a shows a component reflector unit 5 according to an embodiment in a view towards the component reflector unit 5. Figure 5b shows the component reflector unit 5 in cross section in a side view.

[0049] Figure 6 is a more detailed figure of the component reflector unit 5 of Figure 5.

[0050] Figure 7 shows schematically a spectroscopic gas sensor with an ellipsoidal retroreflector arranged at the component reflector unit. DETAILED DESCRIPTION

[0051] In the following description of preferred embodiments, the same reference numeral is used for similar features in the different drawings.

[0052] Figure 1 shows schematically a spectroscopic gas sensor 1 comprising a casing 2 with an opening 3 for gas exchange in and out of the casing. The gas sensor 1 comprises a multipass absorption cell 4 configured for gas exchange through the opening 3. The multipass absorption cell 4 comprises a component reflector unit 5 defining a concave first surface 6’ with a first centre of curvature 19. The component reflector unit comprises a first reflector 6. The multipass absorption cell 4 also comprises a main reflector unit 7 comprising a concave second reflector 8 defining a second centre of curvature 17 and a concave third reflector 9 defining a third centre of curvature 18, each facing the component reflector unit 5, and thus also facing the first reflector 6. Each one of the second reflector 8 and third reflector 9 may consist of a plurality of flat surfaces, which together form a concave surface. Such flat surfaces on a curved surface are sometimes called facets. It is of course also possible to have the second reflector 8 and the third reflector configured as uniformly curved surfaces, i.e. , as curved surfaces without facets and edges. It is also possible to add additional reflectors or to divide the shown reflectors into a number of separate reflectors. In the embodiment shown in Figure 1 the first reflector 6 is arranged along a first plane A. The second reflector 8 and the third reflector 9 are arranged on a common support 10.

[0053] The multipass absorption cell 4 also comprises a light source 11 configured to emit and direct light towards the concave second reflector 8 through an illumination area 25 along the concave first surface 6’. The light source may be implemented in many different ways. It is possible to use any infrared, IR, light source. Examples of IR light sources include, e.g., an LED, a MEMS emitter, a laser, and an incandescent IR lamp. The light source preferably illuminates only the second reflector.

[0054] The multipass absorption cell 4 also comprises an optional first interference filter 14, configured to transmit a first wavelength portion, of the light and to reflect light outside of the first wavelength portion, and arranged in the component reflector unit along the first concave surface 6’ such that it is illuminated by light from the light source 11 during operation of the gas sensor. The multipass absorption cell 4 also comprises a first detector 15 arranged in the component reflector unit 5 such that it is illuminated by a light spot during operation of the gas sensor 1 arranged to detect light that has been transmitted through the first interference filter 14. An optional first positive lens 16, is arranged between the first interference filter 14 and the first detector 15, for focusing the light that has been transmitted through the first interference filter 14. By having such a positive lens 16, the transmitted light is focussed to a smaller area than the illuminated area on the interference filter. This makes it possible to have a much smaller detector surface, which lowers the cost of the detector, as well as increases the detected signal-to-noise ratio since inherent detector noise commonly increases with detector active surface area.

[0055] Figure 2 shows schematically a multipass absorption cell 4, which is slightly different from that of Figure 1 in a cross sectional view along a-a as shown in Figure 1 . The view in Figure 2 is in the zy-plane as indicated in Figure 2, which is perpendicular to the xy-plany of Figure 1. Figure 3 shows schematically the multipass absorption cell 4 of Figure 1 and 2 in a view perpendicular to the view of Figure 2 along b-b as shown in Figure 2. The component reflector unit 5, the second reflector 8, and the third reflector 9, are arranged in a White cell configuration with their respective centres of curvature defining a longitudinal optical plane C extending through the component reflector unit, the second reflector 8, and the third reflector 9.

[0056] As is shown in Figure 2 the light source 11 is configured such that the centre of the illumination area 25 is on a first side of the longitudinal optical plane C. As will be described below a number of light spots are incident on the component reflector unit 5 during operation of the gas sensor 1 .

[0057] As is shown in Figure 1 and Figure 2 the component reflector unit 5 comprises an optical return unit 20.

[0058] As is shown in Figure 2 the optical return unit comprises a light input LI at a first distance from the longitudinal optical plane C, which during operation of the gas sensor 1 is arranged to be illuminated by a first return light spot Ra, and a light output LO at a second distance from the longitudinal optical plane C on the same side of the longitudinal optical plane C, which during operation of the gas sensor 1 is configured to return the light from a second return light spot Rb, wherein the first detector 15 is arranged to be illuminated by a light spot at the second distance from the longitudinal optical plane. In the embodiment of the multipass absorption cell 4 shown in Figure 2 the component reflector unit 5 comprises a first concave reflector 6 with a first centre of curvature 19 which is situated at plane B, the concave second reflector 8 defines a second centre of curvature 17 and the concave third reflector 9 defines a third centre of curvature 18 which are both situated in plane P at the component reflector unit 5. As is shown in Figure 3 the second centre of curvature 17 and the third centre of curvature 18 are at a distance from each other and defines a first axis 21 , wherein the first, second and third centres of curvature are arranged in a same longitudinal optical plane, C, extending through the component reflector unit and the main reflector unit 7. The first centre of curvature 19 is arranged between the second centre of curvature 17 and the third centre of curvature 18 in a direction parallel to the first axis 21 . The distance between a reflector and its corresponding centre of curvature is its radius of curvature. The first centre of curvature is between the second reflector 8 and the third reflector 9. The centre of the component reflector unit 5 and the first centre of curvature defines a symmetry line 22. The centre of the second reflector 8 and the second centre of curvature 17 are both situated on the same side of the symmetry line 22 and the centre of the third reflector 9 and the third centre of curvature 17 are both situated on the opposite side of the symmetry line 22 as is shown in Figure 3. The angle a between the second reflector 8 and the third reflector 9 is defined by the distance between the second centre of curvature 17 and third centre of curvature 18, and the radius of curvature CR.

[0059] With the light source 11 , the component reflector unit 5, the second reflector 8 and the third reflector 9 arranged as described light emitted from the light source 11 is reflected and concentrated to a light spot 12 on the component reflector unit and more precisely on the first reflector 6. The first light spot is on the other side of the longitudinal optical plane C in relation to the illumination area 25. A subsequent light spot 13 on the component reflector unit, originating from the previous light spot 12 on the component reflector unit which has been reflected and concentrated by the main reflector unit 7 to the subsequent second light spot 13, is on a different location in relation to the previous light spot 12 and on the same side of the longitudinal optical plane as the light source 11. A subsequent third light spot may correspond to the first return light spot Ra. However, in the embodiment of Figure 2 the first return light spot Ra corresponds to a subsequent fourth light spot, which is on the same side of the longitudinal optical plane as the light source 11 . The second return light spot Rb is at a second larger distance from the longitudinal optical plane C, but on the same side of the longitudinal optical plane as the light source 11 . A light spot subsequent to the second return light spot Rb will illuminate the first detector 15. The first detector 15 is arranged on the same side of the longitudinal optical plane C as the light source at the second distance from the longitudinal optical plane C.

[0060] The light source preferably illuminates only the second reflector 8. It is only light that illuminates the second reflector that will be reflected in the desired way and be useful in the detection. Light from the light source 11 that illuminates the third reflector 9 will be reflected along a different path and is not useful for the detection. Light which does not illuminate the second reflector will be lost. To avoid energy losses it is, therefore, preferred that all light from the light source illuminate only the second reflector. The light source 11 may be collimated or non-collimated, but the light source 11 is preferably configured such that it illuminates only the second reflector 8 and not the third reflector 9. In figure 1 , the light has been illustrated as coming from a point source. If the light source is an has an extension in space the situation becomes more complicated. The light from the light source may be collimated. If the light from a light source having an extension in space is collimated the light on the second reflector can be seen as an enlarged image of the light source. The light on the component reflector unit 5 will in turn be a reduced image of the light source.

[0061] The embodiment described with reference to Figure 1 may be optimised in many ways. In the embodiment of Figure 1 the second reflector 8 has a second centre of curvature 17, which is situated slightly behind the first plane A, and the third reflector 9 has a third centre of curvature 18, which is situated slightly in front of the first plane A. For an improved quality of the subsequent light spot 13 on the first interference filter 14, it is advantageous if the second centre of curvature 17 and the third centre of curvature 18 is on the first plane A. It is also advantageous if the first concave surface 6’ of the component reflector unit 5 has a centre of curvature 19. The second plane B is at the surface 8’ of the second reflector 8 and at the surface 9’ of the third reflector 9. The optional first interference filter 14 may also be arranged along the first concave surface. Preferably, the concave first surface 6’, the surface 8’ of the second reflector 8, and the surface 9’ of the third reflector 9, are spherical surfaces. In case the concave first surface 6’, the surface 8’ of the second reflector 8, and the surface 9’ of the third reflector 9, are spherical surfaces are not spherical surfaces their radii of curvature varies with no more than 10% over their surfaces, preferably by no more than 5% and most preferred by no more than 1 %.

[0062] In a gas sensor according to the present disclosure the distance from the component reflector unit to each one of the second reflector and the third reflector does not have to be equal to the radius of curvature of the component reflector unit 6, the second reflector 8 and the third reflector 9, but a small deviation is possible while still providing the desired light spots. The radius of curvature of the second reflector and the third reflector may for example be 85-115 % of the distance from the component reflector unit to each one of the second reflector and the third reflector, and still provide the desired light spots. This is true at least for a limited number of passes in the multipass absorption cell. The quality of subsequent light spots is improved if the radius of curvature of the first concave surface 6’, the second reflector 8 and the third reflector 9 is in the interval 95-105 % of the distance from the component reflector unit to each one of the second reflector 8 and the third reflector 9. It is most preferred that the radius of curvature of the first concave surface 6’, the second reflector 8 and the third reflector 9 is in the interval 99-101 % of the distance from the component reflector unit to each one of the second reflector 8 and the third reflector 9.

[0063] An arrangement as shown in Figure 3 in which the component reflector unit 5, the second reflector 8, and the third reflector 9 are spherical reflectors, have essentially equal radii of curvature and which are arranged as described in relation to Figure 2 and Figure 3, is called a White cell. In the embodiment of Figure 1 , not all requirements for a White cell are fulfilled. Even if the radii of curvature of the different reflectors, 5, 8, and 9, are slightly changed or if the positions of the centres of curvature are moved slightly, the reflections may still be located favourably. In the present disclosure, the term White cell configuration is used to describe a multipass cell with the above described variations in radii of curvature and distances.

[0064] Figure 4 shows a component reflector unit 5 according to the prior art in a view towards the component reflector unit 5. The component reflector unit 5 comprises a first reflector 6 which defines a concave first surface 6’. A light source 11 which in this example comprises a light emitting diode, LED, 23 and a mirror 24 is configured behind the component reflector unit. As described above the LED 23 may be replaced with any other IR light source. Examples of IR light sources include, e.g., an LED 23, a MEMS emitter, a laser, and an incandescent IR lamp. The light from the LED may be collimated. Collimation is a fuzzy term. The degree, or quality, of collimation, depends on the spatial size of the emitter. With a close to point source, like a single mode laser, it is easy to produce a well collimated beam travelling a long way without any spatial divergence. The spectroscopic gas sensor described herein will work with such a light source, but the benefit of the configuration of the multipass absorption cell described herein is that it works also for larger area light sources, as long as the divergence is limited by optical design to hit the second reflector and preferably only the second reflector. Collimation from a certain sized emitter actually means that an enlarged image is formed at some distance, which in our case is at the distance of the opposing mirror. A collimation lens in front of the emitter will form an image on the opposing mirror. If the size of this enlarged image is larger than the area of the opposing mirror, the light coming outside the mirror area will get lost for the system. Thus, as mentioned above it is preferable if only the second reflector 8 is illuminated by the light source, as the light outside the second reflector 8 is lost.

[0065] Light emitted from the LED 23 illuminates the mirror 24 as can be seen by the light source reflection SO. The light source reflection SO is directed towards the concave second reflector 8 through an illumination area 25 along the first concave surface. The centre of the illumination area 25 is on a first side of the longitudinal optical plane A. With the first reflector 6, the second reflector 8 and the third reflector 9 arranged as described above the light from the light source reflection SO will be reflected and concentrated by the second reflector 8 to a first light spot S1 on the opposite side of the longitudinal optical plane C and on the opposite side of the symmetry line 22. The first light spot will illuminate the third reflector 9 in which the light will be reflected and concentrated to a second light spot S2. Subsequent light spots are shown as S3-S6. If the area 25 would have been centred on the longitudinal optical plane C, all reflections would have been centred on the longitudinal optical plane C. By having the area 25 displaced, every second light spot S1 , S3, and S5, is situated below the longitudinal optical plane C and every second spot S2, S4, and S6 is situated above the longitudinal optical plane C. The displacement from the longitudinal optical plane C will for the odd numbers of spots be exactly the same as for the even number of spots, but in opposite direction from plane C, provided that the opposing reflector unit 7, housing the reflectors 8, 9, are perfectly aligned. Any tilt deviation of the main reflector unit 7 from this alignment will move the positions of the odd numbers of spots S1 , S3, S5 illuminating the component block 5, but not the even numbers of spots S2, S4, S6 illuminating the component block 5. This is because any additional reflector 8, 9 tilt will redefine the plane C so that the even spot number of reflections will end up to the same level as the light source reflection SO.

[0066] A light detector 15 is arranged at the position of the fourth light spot S4. As can be seen in Figure 4 the second light spot S2 is centred on the same level as the first detector 15. The main reflector unit 7 with the second reflector 8 and the third reflector 9, may be manufactured in one piece by molding in plastic. However, if there are large tolerances in the manufacturing the angle a between the second reflector 8 and the third reflector 9, may vary from one main reflector unit 7 to another and / or be different than intended. This will affect the distance 21 . The dotted circles shown at the light spots S1-S6, shows the effect of a larger than intended angle a between the second reflector 8 and the third reflector 9. As can be seen in Figure 3 a larger angle a result in a larger distance between the light spots increase. This results in a far from optimal illumination of the detector 15 where a large portion of the light is lost. Another effect to take into consideration is that vibrations and thermo-mechanical tensions may cause the main reflector 7 unit to move in relation to the component reflector unit 5 as is illustrated by the arrows 31 for the row of light spots S1 , S3, S5, on the lower side of the longitudinal optical plane C and potentially by the arrows 3T for the upper row of light spots S2, S4, S6. However, the movement of the light spots S2, S4, S6 on the same side of the longitudinal optical plane C as the illumination area 25, is considerably smaller than the movement of the light spots S1 , S3, S5 on the opposite side of the longitudinal optical plane C in relation to the illumination area 25.

[0067] Figure 5a shows a component reflector unit 5 according to an embodiment in a view towards the component reflector unit 5. Figure 5b shows the component reflector unit 5 in cross section in a side view. The component reflector unit 5 comprises a first reflector 6 which defines a concave first surface 6’. A light source 11 is configured behind the component reflector unit 5 and emits light from a light spot denoted SO in Figure 5. The component reflector unit 5 comprises a first interference filter 14, which corresponds to a long path length interference filter, a second interference filter 14’, which corresponds to a super short path length interference filter, a third interference filter 14”, which corresponds to a medium path length interference filter and a fourth interference filter 14”’, which corresponds to a short path length interference filter, . A detector 15, 15’, 15”, 15’”, is arranged behind each one of the interference filters 14, 14’, 14”, 14’”. The light will be reflected between the main reflector unit (Figure 1-3) and the component reflector unit 5, and will result in consecutive light spots S1-S14 on the component reflector unit 5. At the position of the eighth light spot, an optical return unit 20 is arranged. The optical return unit the optical return unit comprises a light input LI at a first distance from the longitudinal optical plane C and arranged to be illuminated by a first return light spot Ra during operation of the gas sensor. In the embodiment of Figure 5 the first return light spot corresponds to the light spot denoted Ra. The optical return unit also comprises a light output LO at a second distance from the longitudinal optical plane on the same side of the longitudinal optical plane and arranged to return the light as a second return light spot Rb. In the embodiment of Figure 5, the first return light spot corresponds to the light spot denoted Ra. In the embodiment of Figure 5 the light input LI is a mirror angled towards the light output LO which is a mirror angled towards the light input LI.

[0068] In the embodiment of Figure 5 the light that is incident on the light input LI has been reflected in the main reflector unit 7 eight times and has been reflected in the component reflector unit 5 seven times.

[0069] The optical return unit will make the light turn around and travel in the opposite direction. The light spot denoted Ra has come from the third reflector 9. The light spot denoted Rb will return to the third reflector 9 of the main reflector unit 7 and result in the light spot denoted S9. In case the angle a is different from the intended angle the light spots will come closer and closer to their intended position on the return path. In the embodiment of Figure 5, the light spot S2 is light that has travelled the shortest distance from the light source and the light spot denoted S14 is light that has travelled the longest distance from the light source. The light spots denoted S2 and S14 are the light spots that are least affected by an error in the angle a between the second reflector 8 and the third reflector 9 of the main reflector unit 7. The light spot denoted S4 is light that has travelled slightly longer than the light in the light spot denoted S2. The light spot denoted S12 is light that has travelled slightly shorter than the light in the light spot denoted S14. The light spots denoted S4 and S12 are affected slightly more by an error in the angle a between the second reflector 8 and the third reflector 9, than the light spots denoted S2 and S14.

[0070] It is possible to arrange the optical return unit 20 on the lower part of the component reflector unit such as at the positions of the light spots denoted S7 and S9. It is also possible to position detectors on the lower part of the component reflector unit such as at the positions of the light spots denoted S7 and S9. However, due to the higher sensitivity to vibrations it is less preferred to arrange the detectors or the component reflector unit 20 on the lower part of the component reflector unit, i.e. , on the opposite side of the longitudinal optical plane in relation to the position of the illumination area 25.

[0071] Figure 6 is a more detailed perspective view of the component reflector unit 5 of Figure 5. The reflector 6 is concave and is arranged partly above the longitudinal optical plane C such that the light spots denoted S6 and S10 are on the reflector 6. The light source 11 illuminates the mirror 24, which directs the light towards the second reflector (Figures 1-3). In Figure 6 the first interference filter 14, which corresponds to a long path length interference filter, the second interference filter 14’, which corresponds to a super short path length interference filter, the third interference filter 14”, which corresponds to a medium path length interference filter, the fourth interference filter 14”’, which corresponds to a short path length interference filter, and the optical return unit 20 with the light input LI and the light output LO, are shown. A detector as well as a lens is arranged behind each interference filter 14, 14’, 14”, 14’”. In Figure 6 the third lens 16” behind the third interference filter 14” and the first lens 16 behind the first interference filter 14 are shown. Behind the third lens 16” is a third detector 15” (Figure 5), i.e., a medium path length, MPL, detector, and behind the first lens 16 is a first detector 15, i.e., a long path length, , detector. Also shown in Figure 6 is the optical return unit 20 with the light input LI in the form of an angled mirror and the light output LO in the form of an angled mirror. A second detector 15’, i.e., a super short path length detector, (Figure 5) is arranged behind the second interference filter 14’. A fourth detector 15’”, i.e., a medium path length detector, is arranged behind the fourth interference filter 14’”. The detectors 15, 15’, 15”, 15’”, are arranged on a common circuit board 32 and the lenses 16”, 16’” are arranged in lens holders , 33”, 33’”, on the circuit board 32. The lens holders 33”, 33’”, ensures a correct positioning of the lenses 16”, 16’”, in relation to the detectors 15”, 15’”. Only two lens holders 33”, 33’”, are shown in Figure 6, but it goes without saying that all lenses 16, 16’, 16”, 16”’ (Figure 5), may be arranged in lens holders. The positional error of the light spots due to an error in the angle a between the second reflector 8 and the third reflector 9 as shown in Figure 3, will increase towards the optical return unit and will decrease after the optical return unit.

[0072] The transmission wavelengths of the interference filters are adapted to the absorption peaks of the different gases to be measured. The positions of the interference filters 14, 14’, 14”, 14’”, in the gas sensor 1 are chosen in dependence of the expected concentrations of the gases to be measured. The medium path length and the long path length, i.e. , the third and first interference filters 14”, 14, are chosen for detection of the gases having a low absorption cross section and / or an expected low concentration. The super short path length and the short path length are chosen for detection of the gases having a high absorption cross section and / or an expected high concentration. The strength of these absorption lines allows for the quantification of the concentration of a gas species.

[0073] The relationship between optical absorption and gas concentration at a specific wavelength A follows the Lambert-Beer law,

[0074] Abs(A)=s(A)xCxL where Abs(A) is the absorption, s is the molar attenuation coefficient of the gas species for the incoming wavelength A, C is the molar concentration of the gas to be sensed, and L is the length of the optical pass.

[0075] The infrared (IR) spectral region, specifically the 2-20 pm region, corresponds to the presence of strong and distinct fundamental rotational-vibrational energy transitions of molecular gases and is, therefore, often referred to the ‘fingerprint’ region suitable for the identification and quantification of components in gas mixtures.

[0076] If the gas sensor is to be used for environmental air pollution monitoring the interference filters may be chosen to have transmission wavelengths at 4.3 pm for the detection of CO2, 4.5 pm for the detection of N2O, 3.3 pm for the detection of CH4, and 2.6 pm for the detection of H2O. The number of interference filters and corresponding detectors is dependent on the number of gases that are to be detected.

[0077] The lenses may be formed of many different materials. Examples of suitable materials for light in the region of 2 pm to 10 pm, are silicon, germanium and chalcogenide glass.

[0078] The lenses used in the above described gas sensor typically has a length along their optical axes and a dimension perpendicular to their optical axis of in the range of 1 -20 mm.

[0079] The optical return unit can be implemented as a cube corner reflector if positioned off centre in relation to the first return light spot Ra and the second return light spot Rb. Alternatively, the optical return unit may comprise an ellipsoid mirror having its vertex located close to the surface of the component block reflector 6, but positioned so that the lower half of the ellipsoid is illuminated by the incoming rays. Such an optical return unit 20 is shown schematically in Figure 7. The optical return unit 20 comprises an ellipsoidal retro-reflector arranged at the component reflector unit 5. The first focal point F1 of the ellipsoidal retro-reflector is at the component reflector unit, and the second focal point of the ellipsoidal retro-reflector is at the main reflector unit 7.

[0080] As another alternative, a light guide may be used as optical return unit. The light guide may be based on, e.g., optical fibres.

[0081] The described embodiments may be amended in many ways without departing from the scope of the invention, which is limited only by the appended claims. It is for example possible to add reflectors to the multipass absorption cell.

Claims

CLAIMS1 . A spectroscopic gas sensor (1 ) comprising a multipass absorption cell (4) configured for gas exchange, wherein the multipass absorption cell (4) comprises- a component reflector unit (5) defining a concave first surface (6’) with a first centre of curvature (19) and comprising at least one first reflector (6), and- a main reflector unit (7) comprising a concave second reflector (8) defining a second centre of curvature (17) and a concave third reflector (9) defining a third centre of curvature (18), wherein the second reflector (8) and the third reflector (9) are facing the component reflector unit (5), wherein the component reflector unit (5), the second reflector (8), and the third reflector (9), are arranged in a White cell configuration with their respective centres of curvature (17, 18, 19) defining a longitudinal optical plane (C) extending through the component reflector unit (5), the second reflector (8), and the third reflector (9),- a light source (11 ) configured to emit and direct light towards the concave second reflector (8), through an illumination area (25) along the concave first surface (6’), wherein the centre of the illumination area (25) is on a first side of the longitudinal optical plane (C), wherein a plurality of light spots are incident on the component reflector unit (5) during operation of the gas sensor (1 ), and- a first detector (15) arranged in the component reflector unit (5) such that it is illuminated by a light spot during operation of the gas sensor (1 ), characterised in that the component reflector unit (5) comprises an optical return unit (20) comprising- a light input (LI) at a first distance from the longitudinal optical plane (C) and arranged to be illuminated by light from the main reflector unit as a first return light spot (Ra) during operation of the gas sensor, and- a light output (LO) at a second distance from the longitudinal optical plane (C) on the same side of the longitudinal optical plane as the first return light spot (Ra) and arranged to return the light, illuminating the light input (LI), as a second return light spot (Rb) towards the main reflector unit, wherein the first detector (15) is arranged to be illuminated by a light spot subsequent to the second return light spot (RB) at the second distance from the longitudinal optical plane (C).

2. The spectroscopic gas sensor (1 ) according to claim 1 , wherein the first detector (15) is arranged to be illuminated by a light spot on the same side of the longitudinal optical plane (C) as the illumination area (25).

3. The spectroscopic gas sensor (1 ) according to claim 1 or 2, wherein the optical return unit is on the same side of the longitudinal optical plane (C) as the illumination area (25).

4. The spectroscopic gas sensor (1) according to claim 1 , 2 or 3, wherein the concave first surface (6’), the second reflector (8), and the third reflector (9) have essentially equal radii of curvature.

5. The spectroscopic gas sensor (1 ) according to claim 4, wherein the component reflector unit (5) is arranged at a distance from the main reflector unit being equal to the radii of curvature.

6. The spectroscopic gas sensor (1 ) according to anyone of claims 1-4, wherein the concave first surface (6’) define a spherical surface, and wherein the second reflector (8), and the third reflector (9), are spherical reflectors.

7. The spectroscopic gas sensor (1 ) according to anyone of the preceding claims, comprising a second detector (15’) arranged to be illuminated by a light spot prior to the first return light spot at the first distance from the longitudinal optical plane (C) on the same side of the longitudinal optical plane (C) as the illumination area.

8. The spectroscopic gas sensor (1 ) according to anyone of the preceding claims, comprising a second interference filter (14’) configured to transmit a second wavelength portion of the light and to reflect light outside of the second wavelength portion, and arranged in the component reflector unit (5), wherein the second detector (15’) is arranged to detect light that has been transmitted through the second interference filter (14’).

9. The spectroscopic gas sensor (1 ) according to anyone of claims 1-8, comprising a first interference filter (14) configured to transmit a first wavelength portion of the light and to reflect light outside of the first wavelength portion, and arranged in the component reflector unit (5), wherein the first detector 15 is arranged to detect light that has been transmitted through the first interference filter (14).

10. The spectroscopic gas sensor (1 ) according to anyone of the preceding claims, wherein the second distance is larger than the first distance.11 . The spectroscopic gas sensor (1 ) according to anyone of the preceding claims, wherein the optical return unit (20) comprises a light input (LI) arranged with its centre at the first distance from the longitudinal optical plane (C) and a light output (LO) arranged with its centre at the second distance from the longitudinal optical plane (C).

12. The spectroscopic gas sensor (1 ) according to claim 11 , wherein the light input (LI) comprises a first mirror (34) and the light output (LO) comprises a second mirror (35), wherein the first mirror (34) is arranged to reflect the light incident on the first mirror (34) towards the second mirror (35), and wherein the second mirror (35) is arranged to reflect the light from the first mirror (34) in the opposite direction to the light incident on the first mirror (34).

13. The spectroscopic gas sensor (1 ) according to anyone of claims 1-11 , wherein the optical return unit (20) comprises a cube corner reflector.

14. The spectroscopic gas sensor (1 ) according to anyone of claims 1-11 , wherein the optical return unit (20) comprises an ellipsoidal retro-reflector.

15. The spectroscopic gas sensor (1 ) according to anyone of claims 1-11 , wherein the optical return unit (20) comprises a light guide, wherein the light input is a first end of the light guide the light output is a second end of the light guide.

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