Spectral gas sensor

Abstract

A spectral gas sensor (1) comprising a multi-path absorption cell (4) is described. The multi-path absorption cell (4) comprises: a component reflector unit (5) defining a concave first surface (6 ') having a first center 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 center of curvature (17) and a concave third reflector (9) defining a third center of curvature (18). The component reflector unit (5), the second reflector (8) and the third reflector (9) are arranged in a White cell configuration. The 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 the first detector 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

Technical Field

[0001] This disclosure relates to a spectral gas sensor including a multi-path absorption cell. Specifically, this disclosure relates to a gas sensor for obtaining absorption information in at least two different wavelength bands. Background Technology

[0002] Gas sensors are becoming increasingly common for various applications. One example is the use of gas sensors for ambient air pollution monitoring. In air pollution monitoring, it is desirable to detect multiple different gases simultaneously. A low-cost non-dispersive infrared (NDIR) multi-gas sensor for ambient air pollution monitoring is described in Atmosphere 2022, 13, 1789. The described sensor can simultaneously detect up to three different gases, including CO2, CH4, N2O, and H2O. By adding additional detectors, it is possible to detect all four of these gases. Other examples of applications include the detection of specific gases in industry, such as carbon dioxide. In breweries, carbon dioxide is used to carbonate beverages. Monitoring carbon dioxide concentration is important to prevent accidents. Another example of an application is gas sensors for breath analysis. The latter is increasingly used in vehicles as a measure to detect and prevent driving under the influence of alcohol or other intoxicating substances. Breath analysis devices can be stand-alone or even handheld units that provide measurements of the substances in a driver's breath.

[0003] WO 2020 / 263168 describes a gas sensor operating in the 3.3–3.6 µm range, capable of distinguishing, for example, ethanol and acetone using at least two narrowband filters in the 3.3 µm to 3.6 µm wavelength range. Gas sensors described in Atmosphere 2022, 13, 1789 and WO 2020 / 263168 include a multi-pass absorption cell with three reflector surfaces arranged in a White cell configuration. This is an advantageous configuration for multi-pass cells because it allows for long optical paths in some channels of a multi-channel gas sensor, while the housing of the multi-pass absorption cell can remain small. WO 2021 / 173055 describes a similar gas sensor including a multi-pass absorption cell with a dome-shaped reflector to allow for the omission of light source collimation. Summary of the Invention

[0004] To reduce the cost of gas sensors while maintaining a high signal-to-noise ratio, it is desirable to use miniaturized optical sensors. However, miniaturized optical sensors require more precise alignment of the gas sensor components, which increases the cost of the gas sensor.

[0005] The purpose of this disclosure is to provide a spectral gas sensor comprising a multi-path absorption cell having a reflector employing a White cell configuration, which can be manufactured at a lower cost than gas sensors according to the prior art.

[0006] Another object of this disclosure is to provide a spectral gas sensor comprising a multi-path absorption cell having a reflector in a White cell configuration, the spectral gas sensor being manufactured with lower reflector alignment tolerances.

[0007] At least one of the above objectives is achieved using a spectral gas sensor comprising a multi-path absorption cell according to claim 1.

[0008] The features of the dependent claims provide additional advantages.

[0009] According to a first aspect, a spectral gas sensor is provided, comprising a multi-path absorption cell configured for gas exchange. The multi-path absorption cell includes a component reflector unit defining a concave first surface having a first center of curvature and including at least one first reflector. The gas sensor also includes a main reflector unit including a second reflector defining a concave center of curvature and a third reflector defining a concave center of curvature. The second and third reflectors face the component reflector unit, wherein the component reflector unit, the second reflector, and the third reflector are arranged in a White cell configuration, wherein their respective centers of curvature define a longitudinal optical plane extending through the component reflector unit, the second reflector, and the third reflector. The gas sensor also includes a light source configured to emit and guide light toward the concave second reflector through an illumination region along the concave first surface, wherein the center of the illumination region is located on a first side of the longitudinal optical plane, and wherein, during operation of the gas sensor, multiple light spots are incident on the component reflector unit. The gas sensor also includes a first detector arranged in the component reflector unit such that the first detector is illuminated by a light spot during operation of the gas sensor. The gas sensor is characterized in that the component reflector unit includes an optical return unit comprising: - A light input, which is positioned a first distance from the longitudinal optical plane, and this light input is arranged to be illuminated by light from the main reflector unit as a first returned light spot during operation of the gas sensor, and - A light output, which is a second distance from the longitudinal optical plane, and the light output and the first returning light spot (Ra) are located on the same side of the longitudinal optical plane, and the light output is arranged to return the illumination light input (LI) as the second returning light spot to the main reflector unit. The first detector is arranged to be illuminated by a spot that is behind the second returned spot and is a second distance away from the longitudinal optical plane.

[0010] The multiple light spots incident on the component reflector unit during the operation of the gas sensor include a first return light spot and a second return light spot. However, unlike the other light spots, the first return light spot returns from the second return light spot towards the main reflector unit. In other words, the optical return unit receives light and returns it at different locations.

[0011] The light illuminating the input light returns from the output light, serving as a second returning spot guided to the main reflector unit. The input light originates from either the second or third reflector, preferably the third reflector. The light returning from the output light is guided to the same reflector from which the input light originated. Therefore, if the input light originates from the third reflector, the light returning from the output light is guided to the third reflector.

[0012] As described above, the long-path detector is arranged to be illuminated by the spot after the second returned spot. This means that the light illuminating the long-path detector originates from the second returned spot.

[0013] The gas sensor can be configured such that light is reflected at least twice, preferably at least three times, and most preferably four times in the main reflector unit before it is incident on the light input. This corresponds 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 guide light toward a concave second reflector. The light reflected from the second reflector is focused due to the concave shape of the second reflector. By arranging the component reflector unit, the second reflector, the third reflector, and the light source in a White cell arrangement, and with the center of the illumination area on a first side of the longitudinal optical plane, multiple light spots are formed on the component reflector unit during operation, wherein every other light spot is located on the same side of the longitudinal optical plane as the light source, and every other light spot is on the opposite side of the longitudinal optical plane.

[0014] Any angular alignment error between the second and third reflectors will cause the error in the position of the light spot relative to its expected position to increase with the number of light spots. However, the error in the position of the light spot will decrease after the optical return unit. Therefore, the error can be reduced by placing the photodetector after the optical return unit.

[0015] Preferably, the light sensor is positioned 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.

[0016] The first detector can be arranged to be illuminated by a light spot located on the same side of the longitudinal optical plane as the illumination area. The inventors have unexpectedly discovered that the light spot has higher quality on the first side of the longitudinal optical plane, i.e., the same side as the region along the concave first surface of the longitudinal optical plane, through which the light source is configured to emit and guide light toward the concave second reflector. A higher quality light spot will enable better focusing onto the corresponding detector, thereby reducing losses and / or allowing the use of a smaller sensor. Furthermore, when the component reflector unit moves relative to the main reflector unit due to, for example, vibration, the movement of the light spot on the same side of the longitudinal optical plane as the illumination area will be significantly less than the movement of the light spot on the opposite side of the longitudinal optical plane relative to the illumination area. This smaller movement will improve detection efficiency.

[0017] The optical reflection unit can be located on the same side of the longitudinal optical plane as the illumination area. This will improve detection efficiency for the same reasons mentioned above.

[0018] Many improvements can be made to enhance the quality of the light spot and thus improve detection efficiency. The first improvement is that the concave first surface, second reflector, and third reflector can have substantially equal radii of curvature. This improves the quality of the light spot.

[0019] The second improvement is that the component reflector unit can be arranged at a distance from the main reflector unit equal to the radius of curvature.

[0020] The third improvement is that the concave first surface can define a sphere, and the second and third reflectors can be spherical reflectors.

[0021] It is desirable that the distance from the component reflector unit to each of the second and third reflectors corresponds to the radius of curvature of the second and third reflectors, respectively. In this way, light is focused by the second reflector into a small spot on the component reflector unit. The distance from the component reflector unit to each of the second and third reflectors does not necessarily need to be equal to the radius of curvature of the second and third reflectors, but a small deviation is allowed as long as the desired spot size is still provided. The radii of curvature of the second and third reflectors can, for example, be 85%–115% of the distance from the component reflector unit to each of the second and third reflectors, respectively, and still provide an acceptable spot size. This is certainly true, at least for a finite number of optical paths in a multi-path absorption cell. If the radii of curvature of the second and third reflectors are within the range of 95%–105% of the distance from the component reflector unit to each of the second and third reflectors, respectively, the quality of the subsequent spot size is improved. Most preferably, the radii of curvature of the second and third reflectors are within the range of 99%-101% of the distance from the component reflector unit to each of the second and third reflectors, respectively. When the concave first surface, the surface of the second reflector, and the surface of the third reflector are not spherical, their radii of curvature vary on their surfaces by no more than 10%, preferably no more than 5%, and most preferably no more than 1%.

[0022] The spectral gas sensor may include a second detector arranged to be illuminated by a spot of light preceding the first reflected light spot, which is located on the same side of the longitudinal optical plane as the illumination area and at a first distance from the longitudinal optical plane. Preferably, the second detector is arranged to be illuminated by the first spot of light, which is located on the same side of the longitudinal optical plane as the illumination area. The spot of light is the one least affected by the angular error between the second and third reflectors.

[0023] When the second detector sensor is arranged to be illuminated by a light spot located on the same side of the longitudinal optical plane as the illumination area and at a first distance from the longitudinal optical plane, it is more or less necessary to place at least one interference filter in front of the first or second detector. By using such an interference filter, it is possible to measure different wavelength regions using different detectors.

[0024] A second interference filter, configured to transmit light at the second wavelength, can be positioned in front of the second detector. The second interference filter will act as a reflector for light outside the second wavelength range. This will allow light outside the second wavelength range to propagate to the first detector. The interference filter is almost entirely planar, and the reflector element is concave.

[0025] The first detector is arranged to be illuminated by light emitted from the second returned light spot, i.e., the light spot illuminating the first detector is after the second returned light spot. The second detector is arranged to be illuminated by light before the first returned light spot. A second interference filter, configured to transmit the second wavelength portion of the light, can be arranged in front of the second detector. The second interference filter will act as a reflector for light outside the second wavelength portion. This will allow light outside the second wavelength portion to propagate to the long-path detector. In other words, the light illuminating the input light is emitted from the second interference filter.

[0026] Alternatively, the names long-path detector, long-path interference filter, ultra-short-path detector, and ultra-short-path interference filter can be used instead of the numbering. The order of components in the optical path from the light source differs from the order of importance. As mentioned above, the most relevant detector is the first detector, which corresponds to the long-path detector, but after the introduction of the second detector, the long-path detector is no longer the first detector in the optical path. By using the names long-path detector and ultra-short-path detector, confusion is avoided when we introduce intermediate detectors as described below. Intermediate detectors are referred to as the third and fourth detectors below.

[0027] The fact that the component reflector unit defines a concave first surface does not necessarily mean that every part of the surface of the component reflector unit is concave. As described above, an interference filter is arranged in the component reflector. When the component reflector unit defines a concave first surface, the first interference filter can be arranged along the concave first surface. The interference filter is usually planar because it is difficult to manufacture a curved interference filter. When a planar interference filter is arranged in the component reflector unit, it is preferable to arrange it such that its center point coincides with the concave first surface, and the normal at its center point coincides with the normal of the concave first surface. Such a planar interference filter will therefore produce acceptable results because the light spot on the component reflector unit has a smaller spread. For interference filters with a dual function, i.e., transmitting a portion of the light and reflecting the rest to a subsequent light spot, it is preferable to arrange such interference filters along the concave first surface.

[0028] The spectral gas sensor may include a first interference filter configured to transmit a first wavelength portion of light and reflect light outside the first wavelength portion, and the first interference filter is arranged in a component reflector unit, wherein a first detector is arranged to detect light that has been transmitted through the first interference filter. The first interference filter provides the possibility of selecting the wavelength band of light detected by the first detector.

[0029] Optical filters can be placed in front of the light source, or a monochromatic light source can be used to replace or add to the filter in front of the first detector.

[0030] The second distance can be greater than the first distance. This allows the light spot to be closer to the longitudinal optical plane before reaching the optical return unit.

[0031] The optical return unit may include: a first reflector arranged such that its center is a first distance from the longitudinal optical plane; and a second reflector arranged such that its center is a second distance from the longitudinal optical plane, wherein the first reflector is arranged to reflect light incident on the first reflector toward the second reflector, and wherein the second reflector is arranged to reflect light from the first reflector in the opposite direction to the light incident on the first reflector. This is a first implementation of the optical return unit.

[0032] The optical return unit may have more than two mirrors, in which the light is deflected at an angle of less than 45° at each mirror.

[0033] According to an alternative, the optical return unit may include a cubic corner reflector. This cubic corner reflector should be arranged off-center to provide adequate reflection.

[0034] According to an alternative, the optical return unit may include an ellipsoidal mirror with its vertex located on a surface close to the component block mirror. This ellipsoidal mirror is positioned such that the lower half of the ellipsoid is illuminated by incident light rays, which are ultimately focused at the ellipsoidal focal point. The light then travels towards the upper half of the ellipsoidal mirror, which redirects the light rays in a direction opposite to that of the incident light.

[0035] According to an alternative, the optical return unit may include an optical guide. The optical guide may be based on, for example, an optical fiber.

[0036] Interference filters can have transmission wavelengths in the wavelength range of 1-20 µm.

[0037] When a spectral gas sensor includes two detectors, the second and first detectors can be arranged on a shared circuit board, wherein the second and first detectors are fixed to this circuit board, and a first lens is fixed to the circuit board above the second detector, and a second lens is fixed to the circuit board above the first detector. By arranging the detectors in this way close to each other, the manufacture of the gas sensor is simplified because the detectors and necessary electronics can be arranged on a shared circuit board.

[0038] In the following description, preferred embodiments will be described with reference to the accompanying drawings. Attached Figure Description

[0039] Figure 1 A spectral gas sensor according to an embodiment is schematically shown.

[0040] Figure 2 For example Figure 1 The view shown schematically illustrates a multi-path absorption cell according to a slightly different implementation.

[0041] Figure 3 Perpendicular to Figure 2 The perspective of the viewpoint schematically shows Figure 2 Multi-path absorption cell.

[0042] Figure 4 The component reflector unit according to the prior art is shown from a view toward the component reflector unit.

[0043] Figure 5 A view of the component reflector unit 5 according to the embodiment is shown from a perspective facing the component reflector unit 5. Figure 5 b shows a cross-section of component reflector unit 5 in a side view.

[0044] Figure 6 yes Figure 5 A more detailed diagram of component reflector unit 5.

[0045] Figure 7 A spectral gas sensor with an ellipsoidal retroreflector arranged in the component reflector unit is schematically shown. Detailed Implementation

[0046] In the following description of preferred embodiments, the same reference numerals are used for similar features in different figures.

[0047] Figure 1 A spectroscopic gas sensor 1 is schematically shown, comprising a housing 2 having an opening 3 for gas exchange between the inside and outside of the housing. The gas sensor 1 includes a multi-path absorption cell 4 configured for gas exchange through the opening 3. The multi-path absorption cell 4 includes a component reflector unit 5 defining a concave first surface 6' having a first center of curvature 19. The component reflector unit includes a first reflector 6. The multi-path absorption cell 4 also includes a main reflector unit 7 comprising a concave second reflector 8 defining a second center of curvature 17 and a concave third reflector 9 defining a third center of curvature 18, each facing the component reflector unit 5 and therefore also facing the first reflector 6. Each of the second reflector 8 and the third reflector 9 may consist of multiple planes that together form the concave surface. Such planes on a curved surface are sometimes referred to as facets. Alternatively, the second reflector 8 and the third reflector may be configured as uniform curved surfaces, i.e., curved surfaces without facets and edges. Additional reflectors can be added, or the shown reflector can be divided into multiple individual reflectors. Figure 1In the illustrated embodiment, the first reflector 6 is arranged along the first plane A. The second reflector 8 and the third reflector 9 are arranged on a common support 10.

[0048] The multi-path absorption cell 4 also includes a light source 11 configured to emit and guide light toward the concave second reflector 8 through an illumination region 25 along the concave first surface 6'. The light source can be implemented in many different ways. Any infrared (IR) light source can be used. Examples of IR light sources include, for example, LEDs, MEMS emitters, lasers, and incandescent infrared lamps. The light source preferably illuminates only the second reflector.

[0049] The multi-path absorption cell 4 also includes an optional first interference filter 14, configured to transmit a first wavelength portion of light and reflect light beyond that wavelength portion, and arranged along the concave first surface 6' in the component reflector unit, such that the filter is illuminated by light from the light source 11 during operation of the gas sensor. The multi-path absorption cell 4 also includes a first detector 15, arranged in the component reflector unit 5, such that the first detector is illuminated by a light spot during operation of the gas sensor 1, which is 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 to focus the light that has been transmitted through the first interference filter 14. By providing such a positive lens 16, the transmitted light is focused onto an area smaller than the illumination area on the interference filter. This allows for a much smaller detector surface, reducing detector cost and improving the detection signal-to-noise ratio, as the inherent noise of the detector typically increases with increasing effective surface area.

[0050] Figure 2 The multi-path absorption cell 4 is schematically shown, which is related to... Figure 1 The multi-path absorption cell in the middle is slightly different, it presents a pattern along the optical path. Figure 1 The cross-sectional view of line aa shown. Figure 2 The view in such Figure 2 In the zy plane shown, the zy plane is perpendicular to Figure 1 The xy plane. Figure 3 schematically shown Figure 1 and Figure 2 The multi-path absorption cell 4 in the middle has an angle along the Figure 2 The bb line shown is consistent with... Figure 2 The angles are perpendicular to each other. The component reflector unit 5, the second reflector 8 and the third reflector 9 are arranged in a White Pool configuration, wherein their respective centers of curvature define a longitudinal optical plane C extending through the component reflector unit, the second reflector 8 and the third reflector 9.

[0051] like Figure 2 As shown, the light source 11 is configured such that the center of the illumination area 25 is located on the first side of the longitudinal optical plane C. As will be described later, during the operation of the gas sensor 1, multiple light spots are incident on the component reflector unit 5.

[0052] like Figure 1 and Figure 2 As shown, component reflector unit 5 includes optical return unit 20.

[0053] like Figure 2 As shown, the optical return unit includes an optical input LI and an optical output LO. The optical input is a first distance away from the longitudinal optical plane C. During the operation of the gas sensor 1, the optical input is arranged to be illuminated by a first return light spot Ra. The optical output is a second distance away from the longitudinal optical plane C on the same side. During the operation of the gas sensor 1, the optical output is configured to return light from a second return light spot Rb. The first detector 15 is arranged to be illuminated by a light spot a second distance away from the longitudinal optical plane.

[0054] exist Figure 2 In the embodiment of the multi-path absorption cell 4 shown, the component reflector unit 5 includes a first concave reflector 6 having a first curvature center 19 located on plane B, a concave second reflector 8 defining a second curvature center 17, and a concave third reflector 9 defining a third curvature center 18, both of which are located in plane P at the component reflector unit 5. Figure 3 As shown, the second curvature center 17 and the third curvature center 18 are spaced apart from each other and define a first axis 21. The first, second, and third curvature centers are arranged in the same longitudinal optical plane C extending through the component reflector unit and the main reflector unit 7. The first curvature center 19 is arranged between the second curvature center 17 and the third curvature center 18 in a direction parallel to the first axis 21. The distance between a reflector and its corresponding curvature center is its radius of curvature. The first curvature center is located between the second reflector 8 and the third reflector 9. The center of the component reflector unit 5 and the first curvature center define a line of symmetry 22. Figure 3 As shown, the center of the second reflector 8 and the second curvature center 17 are both located on the same side of the symmetry line 22, and the center of the third reflector 9 and the third curvature center 17 are both located on opposite sides of the symmetry line 22. The angle α between the second reflector 8 and the third reflector 9 is defined by the distance between the second curvature center 17 and the third curvature center 18 and the radius of curvature CR.

[0055] With the light source 11, component reflector unit 5, second reflector 8, and third reflector 9 arranged as described above, light emitted from the light source 11 is reflected and converged onto the component reflector unit to form a light spot 12, more specifically, onto the first reflector 6. The first light spot is located on the opposite side of the longitudinal optical plane C relative 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 by the main reflector unit 7 and converged into a subsequent second light spot 13) is located at a different position relative to the previous light spot 12 and on the same side of the longitudinal optical plane as the light source 11. The subsequent third light spot may correspond to the first returned light spot Ra. However, in Figure 2 In this embodiment, the first returning light spot Ra corresponds to the subsequent fourth light spot, which is located on the same side of the longitudinal optical plane as the light source 11. The second returning light spot Rb is at a greater second distance from the longitudinal optical plane C, but is still located on the same side of the longitudinal optical plane as the light source 11. The light spot following the second returning 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 and at a second distance from the longitudinal optical plane C.

[0056] The light source preferably illuminates only the second reflector 8. Only light illuminating the second reflector is reflected in the desired manner and is useful for detection. Light from the light source 11 illuminating the third reflector 9 will be reflected along a different path and is useless for detection. Light that does not illuminate the second reflector is lost. Therefore, to avoid energy loss, it is preferable that all light from the light source illuminates only the second reflector. The light source 11 can be collimated or decollimated, but it is preferably configured such that it illuminates only the second reflector 8 and not the third reflector 9. Figure 1 In the diagram, light has been shown as originating from a point source. If the source has spatial extension, the situation becomes more complex. Light from the source can be collimated. If light from a source with spatial extension is collimated, the light on the second reflector can be considered a magnified image of the source. The light on component reflector unit 5 will then be a reduced image of the source.

[0057] See Figure 1 The described implementation methods can be optimized in many ways. Figure 1In this embodiment, the second reflector 8 has a second center of curvature 17 slightly behind the first plane A, and the third reflector 9 has a third center of curvature 18 slightly in front of the first plane A. To improve the quality of the subsequent light spot 13 on the first interference filter 14, it is advantageous that the second center of curvature 17 and the third center of curvature 18 are on the first plane A. It is also advantageous that the concave first surface 6' of the component reflector unit 5 has a center of curvature 19. The second plane B is located on the surface 8' of the second reflector 8 and the surface 9' of the third reflector 9. Optionally, the first interference filter 14 can also be arranged along the concave first 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. If the concave first surface 6', the surface 8' of the second reflector 8, and the surface 9' of the third reflector 9, whether spherical or aspherical, the variation in the radius of curvature of their entire surfaces should not exceed 10%, preferably not more than 5%, and most preferably not more than 1%.

[0058] In the gas sensor according to this disclosure, the distance from the component reflector unit to each of the second and third reflectors does not necessarily 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 permissible while still providing the desired light spot. The radius of curvature of the second and third reflectors can be, for example, 85%-115% of the distance from the component reflector unit to each of the second and third reflectors, and still provide the desired light spot. This is certainly true, at least for a finite number of optical paths in a multi-path absorption cell. If the radius of curvature of the concave first surface 6', the second reflector 8, and the third reflector 9 is in the range of 95%-105% of the distance from the component reflector unit to each of the second reflector 8 and the third reflector 9, the quality of the subsequent light spot is improved. Most preferably, the radius of curvature of the concave first surface 6', the second reflector 8, and the third reflector 9 is in the range of 99%-101% of the distance from the component reflector unit to each of the second reflector 8 and the third reflector 9.

[0059] like Figure 3 The arrangement shown is called the White Pool, in which component reflector unit 5, second reflector 8, and third reflector 9 are all spherical reflectors with substantially equal radii of curvature, and simultaneously arranged according to... Figure 2 and Figure 3 Arranged in a manner described in the relevant descriptions. Figure 1 The implementation described does not meet all the requirements of the White cell. Even with slight changes in the radii of curvature of the different reflectors 5, 8, and 9, or a slight shift in the position of the center of curvature, reflection can still be well positioned. In this disclosure, the term "White cell configuration" is used to describe a multi-path cell with the aforementioned variations in radii of curvature and distance.

[0060] Figure 4 The component reflector unit 5 according to the prior art is shown from a view toward the component reflector unit 5. The component reflector unit 5 includes a first reflector 6 defining a concave first surface 6'. In this example, a light source 11 including a light-emitting diode, an LED 23, and a reflector 24 is configured behind the component reflector unit. As mentioned above, the LED 23 can be replaced by any other IR light source. Examples of IR light sources include, for example, the LED 23, a MEMS emitter, a laser, and an incandescent infrared lamp. The light from the LED can be collimated. Collimation is a vague term. The degree or quality of collimation depends on the spatial size of the emitter. For near-point light sources such as single-mode lasers, it is easy to produce a well-collimated beam that can propagate over long distances without any spatial divergence. The spectral gas sensor described herein is suitable for such light sources, but the advantage of the multi-path absorption cell configuration described herein is that it is equally applicable to light sources with larger areas, provided that beam divergence is limited by optical design to ensure that the beam hits the second reflector, and preferably only the second reflector. Collimation of a transmitter of a specific size essentially means forming a magnified image at a certain distance; in this case, this distance is the position of the opposing mirror. A collimating lens in front of the transmitter will form an image on the opposing mirror. If the size of the magnified image is larger than the area of ​​the opposing mirror, light incident outside the mirror's area will escape the system. Therefore, as mentioned earlier, it is preferable to allow the light source to illuminate only the second reflector 8, because light outside the range of the second reflector 8 will be lost.

[0061] As can be seen from the light source reflection S0, the light emitted from LED 23 illuminates the reflector 24. The light source reflection S0 is guided along the first concave illumination area 25 to the concave second reflector 8. The center of the illumination area 25 is located on the 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 S0 is reflected by the second reflector 8 and converged into a first light spot S1, which is located on the opposite side of the longitudinal optical plane C and the opposite side of the line of symmetry 22. The first light spot illuminates the third reflector 9, where the light is reflected and converged into a second light spot S2. Subsequent light spots are shown as S3-S6. If area 25 is centered on the longitudinal optical plane C, then all reflections will be centered on the longitudinal optical plane C. By shifting area 25, every other light spot (i.e., S1, S3, S5) will be located below the longitudinal optical plane C, while another set of every other light spots (i.e., S2, S4, S6) will be located above the longitudinal optical plane C. Provided that the opposing reflector units 7 housing reflectors 8 and 9 are perfectly aligned, the offset of odd-numbered light spots relative to the longitudinal optical plane C will be exactly the same as the offset of even-numbered light spots relative to the same plane C, but in the opposite direction. Any tilt deviation of the main reflector unit 7 relative to this alignment will cause the positions of the odd-numbered light spots (S1, S3, S5) of the irradiation component block 5 to shift, but will not affect the positions of the even-numbered light spots (S2, S4, S6) of the irradiation component block 5. This is because any additional tilt of reflectors 8 and 9 will redefine the plane C, so that the reflection of even-numbered light spots will ultimately be at the same height as the reflection of the light source S0.

[0062] A photodetector 15 is positioned at the location of the fourth light spot S4. For example... Figure 4 As shown, the center of the second light spot S2 is at the same height as the first detector 15. The main reflector unit 7, with the second reflector 8 and the third reflector 9, can be manufactured by single-piece injection molding of plastic. However, if there are large tolerances in the manufacturing process, the angle α between the second reflector 8 and the third reflector 9 may differ between different main reflector units 7, and / or this angle α may not match the expected angle. This will affect the distance 21. The dashed circles shown at light spots S1-S6 illustrate the effect when the angle α between the second reflector 8 and the third reflector 9 is greater than the expected angle α. Figure 3As shown, a larger included angle α results in a greater distance between the light spots. This leads to the illumination of detector 15 being far from optimal, with most of the light being lost. Another effect to consider is that vibration and thermomechanical tension may cause the primary reflector unit 7 to move relative to the component reflector unit 5. For the row of light spots S1, S3, and S5 on the lower side of the longitudinal optical plane C, this movement is shown as arrow 31, while for the row of light spots S2, S4, and S6 on the upper side, this movement may be shown as arrow 31'. However, the movement of light spots S2, S4, and S6 on the same side of the illumination area 25 as the longitudinal optical plane C is significantly less than the movement of light spots S1, S3, and S5 on the opposite side of the illumination area 25 relative to the longitudinal optical plane C.

[0063] Figure 5 A view of the component reflector unit 5 according to the embodiment is shown from a perspective facing the component reflector unit 5. Figure 5 b shows a cross-section of the component reflector unit 5 in a side view. The component reflector unit 5 includes a first reflector 6 defining a concave first surface 6'. A light source 11 is positioned behind the component reflector unit 5 and from... Figure 5 The light spot, denoted as S0, emits light. The component reflector unit 5 includes a first interference filter 14 corresponding to a long-path interference filter, a second interference filter 14' corresponding to an ultra-short-path interference filter, a third interference filter 14'' corresponding to a medium-path interference filter, and a fourth interference filter 14''' corresponding to a short-path interference filter. Detectors 15, 15', 15'', and 15''' are respectively arranged behind each of the interference filters 14, 14', 14'', and 14'''. Light will be emitted from the main reflector unit ( Figures 1 to 3 The light is reflected between the component reflector unit 5 and the component reflector unit 5, and will generate continuous light spots S1 to S14 on the component reflector unit 5. An optical return unit 20 is provided at the position of the eighth light spot. The optical return unit includes a light input LI, which is located a first distance away from the longitudinal optical plane C, and is arranged to be illuminated by the first returned light spot Ra during the operation of the gas sensor. Figure 5 In this embodiment, the first returning light spot corresponds to a light spot denoted as Ra. The optical return unit also includes a light output LO, which is located at a second distance from the longitudinal optical plane and on the same side of the longitudinal optical plane as the light input, and is arranged to return light as a second returning light spot Rb. Figure 5 In this implementation, the first returning spot corresponds to a spot denoted as Ra. Figure 5 In the implementation method, the light input LI is a reflector at an angle to the light output LO, and the light output LO is a reflector at an angle to the light input LI.

[0064] exist Figure 5In this embodiment, the light incident on the light input LI has been reflected eight times in the main reflector unit and seven times in the component reflector unit 5.

[0065] The optical return unit redirects the light and propagates it in the opposite direction. The light spot, denoted as Ra, originates from the third reflector 9. The light spot, denoted as Rb, returns to the third reflector 9 of the main reflector unit 7 and produces a light spot denoted as S9. When the included angle α differs from the expected angle, the light spots will increasingly approach their expected positions on the return path. Figure 5 In this embodiment, spot S2 is the light that travels the shortest distance from the light source, and spot S14 is the light that travels the longest distance from the light source. Spots S2 and S14 are the spots least affected by the angle α error between the second reflector 8 and the third reflector 9 in the main reflector unit 7. Spot S4 is the light with a slightly longer propagation distance than the light in spot S2. Spot S12 is the light with a slightly shorter propagation distance than the light in spot S14. Spots S4 and S12 are slightly more affected by the angle α error between the second reflector 8 and the third reflector 9 than spots S2 and S14.

[0066] The optical return unit 20 can be arranged below the component reflector unit, for example, at the location of the light spots indicated as S7 and S9. The detector can also be positioned below the component reflector unit, such as at the location of the light spots indicated as S7 and S9. However, since the lower part is more sensitive to vibration, it is not preferable to arrange the detector or component reflector unit 20 below the component reflector unit (i.e., on the opposite side of the longitudinal optical plane relative to the illumination area 25).

[0067] Figure 6 yes Figure 5 A more detailed perspective view of component reflector unit 5. Reflector 6 is concave and partially arranged above the longitudinal optical plane C, such that light spots, indicated as S6 and S10, are on reflector 6. Light source 11 illuminates reflector 24, which directs the light to the second reflector ( Figures 1 to 3 ).exist Figure 6 The diagram shows a first interference filter 14 corresponding to a long optical path interference filter, a second interference filter 14' corresponding to an ultra-short optical path interference filter, a third interference filter 14'' corresponding to a medium optical path interference filter, a fourth interference filter 14''' corresponding to a short optical path interference filter, and an optical return unit 20 having an optical input LI and an optical output LO. Detectors and lenses are arranged behind each interference filter 14, 14', 14'', and 14'''. Figure 6The image shows a third lens 16'' behind the third interference filter 14'' and a first lens 16'' behind the first interference filter 14. Behind the third lens 16'' is a third detector 15''. Figure 5 That is, a medium optical path MPL detector, and behind the first lens 16 is the first detector 15, that is, a long optical path detector. Figure 6 The image also shows an optical return unit 20, which has a light input LI in the form of a tilted mirror and a light output LO in the form of a tilted mirror. A second detector 15' (i.e., an ultra-short optical path detector) is also shown. Figure 5 A fourth detector 15''' (i.e., an intermediate optical path detector) is positioned behind the fourth interference filter 14'''. Detectors 15, 15', 15'', and 15''' are arranged on a common circuit board 32, and lenses 16'' and 16''' are arranged in lens holders 33'' and 33''' on the circuit board 32. Lens holders 33'' and 33''' ensure that lenses 16'' and 16''' are correctly positioned relative to detectors 15'' and 15'''. Figure 6 Only two lens supports, 33'' and 33''', are shown, but it goes without saying that all lenses are 16', 16'', 16''', and 16''''. Figure 5 It can be arranged in a lens holder. For example... Figure 3 As shown, the spot position error caused by the angle α error between the second reflector 8 and the third reflector 9 increases as it approaches the optical return unit, and decreases after the optical return unit.

[0068] The transmission wavelength of the interference filters can be adapted to the absorption peaks of different gases to be measured. The positions of the interference filters 14, 14', 14'', and 14''' in the gas sensor 1 are selected according to the expected concentration of the gas to be measured. Medium and long optical paths (i.e., the third interference filter 14'' and the first interference filter 14) are selected for detecting gases with low absorption cross-sections and / or low expected concentrations. Ultra-short and short optical paths are selected for detecting gases with high absorption cross-sections and / or high expected concentrations. The intensity of these absorption lines allows for the quantification of the concentration of the gas species.

[0069] The relationship between light absorption at a specific wavelength λ and gas concentration follows the Lambert-Beer law. Abs(λ) = ε(λ) × C × L Where Abs(λ) is the absorbance, ε is the molar attenuation coefficient of the gas species with respect to the incident wavelength λ, C is the molar concentration of the gas to be detected, and L is the optical path length.

[0070] The infrared (IR) spectral region, especially the 2-20 µm region, corresponds to the presence of strong and unique ground-state rotational-vibrational energy transitions in molecular gases, and is therefore often referred to as the "fingerprint" region, which is suitable for the identification and quantification of components in gas mixtures.

[0071] If the gas sensor will be used for ambient air pollution monitoring, interference filters with the following transmission wavelengths can be selected: 4.3 µm (for CO2 detection), 4.5 µm (for N2O detection), 3.3 µm (for CH4 detection), and 2.6 µm (for H2O detection).

[0072] The number of interference filters and their corresponding detectors depends on the amount of gas to be detected.

[0073] Lenses can be formed from many different materials. Examples of suitable materials for light in the 2 µm to 10 µm range are silicon, germanium, and chalcogenide glasses.

[0074] The length of the lens used in the aforementioned gas sensors along its optical axis and its dimension perpendicular to its optical axis are typically in the range of 1-20 mm.

[0075] If the optical return unit is positioned off-center relative to the first return spot Ra and the second return spot Rb, it can be implemented as a cubic corner reflector. Alternatively, the optical return unit may include an ellipsoidal mirror whose apex is positioned close to the surface of the component block reflector 6, but positioned such that the lower half of the ellipsoid is illuminated by the incident light. Figure 7 The optical return unit 20 is schematically shown in the figure. The optical return unit 20 includes an ellipsoidal retroreflector arranged at the component reflector unit 5. The first focus F1 of the ellipsoidal retroreflector is at the component reflector unit, and the second focus of the ellipsoidal retroreflector is at the main reflector unit 7.

[0076] As an alternative, an optical guide can be used as an optical return unit. The optical guide can be based on, for example, optical fiber.

[0077] The described embodiments can be modified in many ways without departing from the scope of the invention as defined only by the appended claims. For example, a reflector can be added to the multi-path absorption cell.

Claims

1. A spectroscopic gas sensor (1) comprising a multi-pass absorption cell (4) configured for gas exchange, wherein, The multi-optical-path absorption cell (4) comprises: - a component reflector unit (5) defining a concave first surface (6') having a first center 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 center of curvature (17) and a concave third reflector (9) defining a third center of curvature (18), wherein the second reflector (8) and the third reflector (9) face 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, wherein their respective centers of curvature (17, 18, 19) define 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 guide light towards the concave second reflector (8) through an illumination area (25) along the concave first surface (6'), wherein a center of the illumination area (25) is located on a first side of the longitudinal optical plane (C), wherein during operation of the gas sensor (1) a plurality of light spots is incident on the component reflector unit (5), and - a first detector (15) arranged in the component reflector unit (5) such that the first detector is illuminated by light spots during operation of the gas sensor (1), characterized 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 and located on the same side of the longitudinal optical plane as the first return light spot (Ra) and arranged to return 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 light spots after the second return light spot (RB) and at the second distance from the longitudinal optical plane (C).

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

3. The optical spectral 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 optical spectral 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 substantially equal radii of curvature.

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

6. The optical spectral gas sensor (1) according to any one of claims 1 to 4, wherein The concave first surface (6') defines a spherical surface, and wherein the second reflector (8) and the third reflector (9) are spherical reflectors.

7. The optical spectral gas sensor (1) according to any one of the preceding claims, comprising a second detector (15') arranged to be illuminated by a light spot before the first return light spot that is on the same side of the longitudinal optical plane (C) as the illumination region and at a distance of the first distance from the longitudinal optical plane (C).

8. The optical spectral gas sensor (1) according to any one of the preceding claims, comprising a second interference filter (14') configured to transmit a second wavelength portion of the light and to reflect light other than the second wavelength portion, and arranged in the component reflector unit (5), wherein, The second detector (15') is arranged to detect light that has transmitted through the second interference filter (14').

9. The optical spectral gas sensor (1) according to any one of claims 1 to 8, comprising a first interference filter (14) configured to transmit a first wavelength portion of the light and to reflect light other than the first wavelength portion, and arranged in the component reflector unit (5), wherein, The first detector 15 is arranged to detect light that has transmitted through the first interference filter (14).

10. The optical gas sensor (1) according to any one of the preceding claims, wherein The second distance is larger than the first distance.

11. The optical spectral gas sensor (1) according to any one of the preceding claims, wherein The optical return unit (20) comprises a light input (LI) arranged with its center at a distance of the first distance from the longitudinal optical plane (C) and a light output (LO) arranged with its center at a distance of the second distance from the longitudinal optical plane (C).

12. The optical 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 light incident on the first mirror (34) towards the second mirror (35), and wherein the second mirror (35) is arranged to reflect light from the first mirror (34) in a direction opposite to the light incident on the first mirror (34).

13. The optical spectral gas sensor (1) according to any one of claims 1 to 11, wherein The optical return unit (20) comprises a corner cube reflector.

14. The optical gas sensor (1) according to any one of claims 1 to 11, wherein The optical return unit (20) comprises an ellipsoidal retroreflector.

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

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