Multi-pass spectral absorption cell and gas sensor comprising same

Abstract

The present invention relates to a multi-pass spectral absorption cell (200) and in particular to a White-type cell operating as a non-dispersive near-infrared gas sensor. The invention also relates to a gas sensor (300) and an alcohol sensor. A component reflector unit (245; 245 ') of a multi-pass spectral absorption cell (200) is provided. 445) comprises a first detector (250) and a second detector (260) and a light source (210) arranged to emit a first portion of light to first emit light in the first concave mirror (220; 420) and forms a first light path terminating in the first detector (250), and a second portion of the light is emitted to first pass through the second concave mirror (230; 430), forming a second optical path terminating in a second detector (260).

Description

TECHNICAL FIELD

[0001] The present invention relates to a multi-pass optical spectral absorption cell operating as a non-dispersive near infrared gas sensor. The present invention further relates to an alcohol sensor and a sensor system. BACKGROUND

[0002] Multi-pass optical spectral absorption cells are commonly used in various types of gas sensors, at least not as non-dispersive near infrared gas sensors (NDIR sensors). The term "multi-pass" refers to letting electromagnetic radiation reflect, preferably multiple times, within a measurement cell in order to increase the optical path and thereby the sensitivity of the sensor or measurement system of which the cell is a part. A particularly useful implementation of a multi-pass cell is the so-called White-cell, which was introduced by John U White already in 1942 (Journal of the Optical Society of America, 1942) and has been used since then. Figure 1a A prior art White-cell is schematically illustrated in Fig. 1. The prior art White-cell structure 100 is configured by three concave mirrors having the same radius of curvature, a light source 110 and a detector 150. On the opposite surface of the light source 110, a first reflector 120 and a second reflector 130 are positioned in parallel and a third reflector 140 is positioned facing the first reflector 120 and the second reflector 130. The light source 110 irradiates infrared light 115 at a predetermined angle at any point at the side of the third reflector 140. The infrared light 115 is then reflected by the first reflector 120, repeatedly reflected between the third reflector 140 and the first reflector 120 and the second reflector 130, and then incident to the detector 150. A gas to be analyzed is held in the cavity and when the infrared light 150 passes through the gas, the output voltage of the detector 150 gives a measure of the concentration of a specific substance present in the gas. The separation of the centers of curvature of the first reflector 120 and the second reflector 130, the angle between the incident optical axis and the longitudinal plane determines the number of reflections of the incident light and thus the effective optical path length of the device. The White-cell according to Fig. 1 provides eight reflections. However, White-cells giving a significantly higher number of reflections are known in the art. One way of increasing the number of reflections known in the art is to arrange the light source 110 with an offset in the transverse direction from the longitudinal optical plane of the measurement unit, thereby providing a 3D White-cell geometry giving two rows of light spots (upper row 146 light spots and lower row 147 light spots) on the assembly reflector unit 140, and 16 reflections as illustrated in Fig. 2. Figure 1b

[0003] ​One limitation with gas sensors used in NDIR applications and based on White cells is that they are adapted for one wavelength or a rather limited wavelength range, which only corresponds to a limited number of substances that it is possible to detect with one setup. This causes problems in certain applications, such as in breath analysis for detecting and determining the breath concentration of illegal or harmful substances, such as alcohol, by means of a tracer substance, such as carbon dioxide, since it will not be possible to detect both simultaneously with the same mirror / detector combination.

[0004] US9823237 discloses an integrated breath alcohol sensor system providing simultaneous measurement of alcohol concentration and tracer substance concentration, typically CO2. The measurement cavity according to US9823237 comprises two detector / mirror setups, one adapted for alcohol determination and one adapted for CO2. The optical axes of the two setups are perpendicular to each other and arranged with their radii of curvature centered in the same plane. Disadvantages include a complex manufacturing process and replication of mirror components, which increases production costs.

[0005] WO0181901 discloses White cells with complex geometry compared to the original White cell. The disclosed cells are 2d cells and can be equipped with multiple detectors related to different ray paths in the cell. Disadvantages include a very complex geometry with key components scattered in the cell, which makes the cell difficult to mechanically and thermally stabilize and costly. SUMMARY

[0006] It is an object of the present invention to overcome the disadvantages associated with prior art lock-in devices. This is achieved by a multi-pass optical absorption cell, a gas sensor and an alcohol meter as defined by the independent claims.

[0007] According to an aspect of the present invention, a multi-pass optical absorption cell is provided. The multi-pass optical absorption cell comprises:

[0008] - a measurement cavity provided with at least one gas inlet and at least one gas outlet, and wherein comprising in the measurement cavity:

[0009] - a main reflector unit comprising a first concave mirror and a second concave mirror;

[0010] - an assembly reflector unit comprising a third mirror 240 facing the first and second concave mirrors, and wherein the first and second concave mirrors are spherical mirrors and have the same concave radius of curvature and are arranged with their radii of curvature centers aligned in the same longitudinal optical plane A;

[0011] - a light source provided in the vicinity of or in the assembly reflector unit and arranged to direct emitted light towards the main reflector unit, the light source being positioned with an offset from the longitudinal optical plane A in the transversal direction;

[0012] - a first detector provided in the vicinity of or in the assembly reflector unit,

[0013] - a second detector provided in the vicinity of or in the assembly reflector unit; and in that the light source is arranged to emit a first portion of light to first reflect in the first concave mirror and form a first light path ending in the first detector, and a second portion of light to first reflect in the second concave mirror, forming a second light path ending in the second detector.

[0014] The distance between the main reflector unit and the assembly reflector unit can be 95% to 105% of the radius of curvature of the first mirror and the second mirror. If the input beam is focused in the plane of the assembly reflector unit, it will also be focused in the plane of the assembly reflector unit each round trip, i.e. after reflection in the main reflector unit.

[0015] The third mirror can also be a concave mirror, but can also comprise a plurality of facets arranged in an overall concave configuration, where each facet is a flat mirror. Such a configuration of the third mirror gives good results when the distance between the main reflector unit and the assembly reflector unit is approximately the same as the radius of curvature of the first reflector and the second reflector, and can also give acceptable results for slightly different distances between the main reflector unit and the assembly reflector unit.

[0016] Depending on the configuration of the absorption cell and the number of reflections in the third mirror, the area of the third mirror can be so small that it can be configured as a single flat mirror.

[0017] The third mirror can be a spherical mirror and have the same radius of curvature as the first mirror and the second mirror. The first mirror, the second mirror and the third mirror can be arranged with their centers of curvature aligned in the same longitudinal optical plane A.

[0018] The main reflector unit and the assembly reflector unit can be arranged at a distance from each other that is substantially the same as the radius of curvature of the first mirror, the second mirror and the third mirror. Such a multi-pass optical absorption cell is often referred to as a White cell.

[0019] According to an embodiment of the present application, the first detector is provided in a first detector opening in the third mirror and / or the second detector is provided in a second detector opening in the third mirror. By providing the first detector and / or the second detector in an opening in the third mirror, a more rigid attachment of the detectors and the light source relative to each other and relative to the third mirror can be achieved.

[0020] Preferably, the first detector is provided in the assembly reflector unit and the second detector is provided in the assembly reflector unit.

[0021] Preferably, the first detector is fixed relative to the third mirror. Preferably, the second detector is fixed relative to the third mirror. The fixation of the first and second detectors relative to the third mirror can be achieved in many different ways. The first detector and the second detector can be directly attached to the third mirror. The first detector and the second detector can be fixed to an intermediate element, such as a printed circuit board, wherein the intermediate element is attached to the third mirror. In addition to the direct attachment of the first detector and the second detector to the third mirror, it is also possible by fixation to an intermediate element. By fixing the detectors relative to the third mirror, a stable imaging of the light source on the detectors is achieved. In this way, the amount of light on the detectors will not vary due to vibrations or the like. This should be more and more important when the number of reflections increases, as this will require a longer third mirror and a larger distance between the light source and the detectors.

[0022] The light source can be provided in a light source opening in the third concave mirror. The light source can be fixed relative to the third mirror. The light source can be directly attached and fixed to the third mirror. The light source can be attached and fixed to the third mirror by means of an intermediate element, such as a printed circuit board. The attachment and fixation of the light source by means of an intermediate element can be in addition to the direct attachment of the light source to the third mirror. By fixing the light source relative to the third mirror, the vibrational movements of the light source relative to the third mirror are minimized.

[0023] According to an embodiment of the present application, the second light path comprises only one reflection in the primary reflector unit and in the second detector opening, which second detector opening is located in the third mirror at a position corresponding to the emitted light being reflected only once by the second concave mirror.

[0024] According to an embodiment of the present application, the light source is provided in a first half of the assembly reflector unit and the first detector is provided in a second half of the assembly reflector unit, and wherein the second detector is provided in the same half of the assembly reflector unit as the first detector.

[0025] According to an embodiment of the present application, the first part of the light arranged to be reflected first in the first concave mirror and along the first light path represents a major part of the light emitted by the light source.

[0026] According to one embodiment of the present application, the first light path comprises a predetermined number of reflections between the primary reflector unit and the assembly reflector unit. The first light path can be arranged to be longer than the second light path, preferably at least twice as long, and even more preferably at least four times as long.

[0027] According to one embodiment of the present application, the main portion of the light beam arranged to be reflected in the first mirror represents at least 75% of the emitted light, preferably at least 85% of the emitted light, and even more preferably at least 95% of the emitted light.

[0028] The desired percentage of the light beam to be reflected in the first mirror can be achieved by collimating, directing and / or blocking light from the light source. The blocking of light can be achieved by shielding the light source such that the desired percentage of light is incident on the first mirror. The shielding can be provided by the light source opening when the light source is arranged in the light source opening in the third concave mirror. Mirrors and / or lenses can be used to direct the light to achieve the desired percentage of light in the first light path. The arrangement of the light source and the detector in the openings in the third mirror facilitates the necessary alignment to achieve the desired percentage of light in the first and second light paths.

[0029] According to one embodiment of the present application, the first detector is arranged to measure the light intensity in a first wavelength range, and the second detector is arranged to measure the light intensity in a second wavelength range, the second wavelength range being separate from the first wavelength range.

[0030] According to one embodiment of the present application, the first concave mirror and the second concave mirror are provided as a continuous structure comprised by the primary reflector unit, forming a continuous mirror surface with two separate curvatures.

[0031] According to one embodiment of the present application, the multi-pass optical absorption cell comprises:

[0032] - a primary reflector unit comprising a first concave mirror and a second concave mirror, the first concave mirror and the second concave mirror being arranged such that a primary reflector unit back plane B is a common symmetry back plane of the first concave mirror and the second concave mirror;

[0033] - an assembly reflector unit comprising a third mirror facing the first concave mirror and the second concave mirror, the assembly reflector unit having a back plane C symmetrical to the third mirror, wherein

[0034] - the first concave mirror and the second concave mirror are spherical mirrors and have the same concave radius of curvature, and are arranged with their centers of curvature aligned in the same longitudinal optical plane A; and wherein

[0035] - the assembly reflector unit back plane C is arranged transversely to the common longitudinal optical plane A, and the primary reflector unit back plane B and the assembly reflector unit back plane C are arranged relative to each other such that in the longitudinal optical plane A a normal to the primary reflector unit back plane B and a normal to the assembly reflector unit back plane C form an angle a.

[0036] The third mirror can be configured as described above.

[0037] The third mirror can be a concave mirror. The first, second and third concave mirrors can be spherical mirrors, and have the same concave radius of curvature, and are arranged with their centers of curvature aligned in the same longitudinal optical plane A.

[0038] According to one embodiment of the present invention, the multi-pass optical absorption cell is arranged to provide an upper row of light spots and a lower row of light spots on the assembly reflector unit, and the angle a is chosen to cause a displacement of the light spots in the lower row such that at least a majority of the light spots in the lower row will be in the same longitudinal position as the light spots of the upper row. The terms "upper" and "lower" only refer to the geometry of the embodiment of the present invention shown in the figures and / or used during normal operation or mounting of the one or more devices, and are not intended to limit the present invention in any way.

[0039] According to one aspect of the present invention, a gas sensor is provided comprising the above described multi-pass optical absorption cell. The first wavelength range can relate to a first substance, and the second wavelength range substance relates to a second substance.

[0040] According to one embodiment of the present invention relating to a gas sensor, the first detector is arranged to provide a measurement relating to the concentration of the first and second substance, wherein the signal provided by the first detector relating to the concentration of the first substance relates to a target substance, and the signal provided by the second detector relating to the concentration of the second substance relates to a tracer substance, and wherein the concentration of the target substance is expected to be substantially lower than the concentration of the tracer substance under normal measurement conditions.

[0041] According to one aspect of the present invention, an alcohol meter is provided comprising the above described gas sensor, wherein the first substance is alcohol, and the second substance is carbon dioxide or water.

[0042] Due to the present invention, a compact multi-pass optical absorption cell can be provided and thus a gas sensor and an alcohol meter can be provided which are arranged to measure in two different wavelength ranges simultaneously, and thereby detect two different substances.

[0043] One advantage provided by the present invention is that the design is robust and relatively simple to manufacture.

[0044] One advantage provided by embodiments of the present application is that the second detector can be incorporated in the assembly reflector unit without significantly increasing the size of the assembly reflector unit.

[0045] Many additional benefits and advantages of the present application will be readily understood by persons skilled in the art upon consideration of the detailed description and drawings below. BRIEF DESCRIPTION OF DRAWINGS

[0046] The present application will now be described in more detail with reference to the accompanying drawings, in which

[0047] Figure 1a - b schematically illustrates a prior art White-type multi-pass optical absorption cell;

[0048] Figure 2a - g schematically illustrates a multi-pass optical absorption cell according to the present application;

[0049] Figure 3 - schematically illustrates a gas sensor device according to the present application; and

[0050] Figure 4a - g schematically illustrates one embodiment of a multi-pass optical absorption cell according to the present application;

[0051] All drawings are schematic and not necessarily to scale, and generally only show the parts that are necessary for the elucidation of the respective embodiments, while other parts can be omitted or suggested only. Any reference signs appearing in the various drawings serve identical objects or features throughout the drawings, unless indicated otherwise. DETAILED DESCRIPTION

[0052] Terms such as "top", "bottom", "upper", "lower", "under", "over", etc. merely refer to the geometry of the embodiments of the present application as shown in the drawings and / or during normal operation or installation of the device(s) and are not intended to limit the present application in any way.

[0053] In Figure 2a - h schematically depicts a multi-pass optical absorption cell 200 according to the present application. As Figure 2aThe multi-pass optical spectral absorption cell 200 is depicted in a top view and comprises a measurement cavity 211 provided with at least one gas inlet 212 and at least one gas outlet 213. Disposed in the measurement cavity 211 are a primary reflector unit 235 comprising a first concave mirror 220 and a second concave mirror 230 and an assembly reflector unit 245 comprising a third concave mirror 240 facing the first concave mirror 220 and the second concave mirror 230. The first concave mirror 220, the second concave mirror 230 and the third concave mirror 240 are spherical mirrors and have the same concave radius of curvature. The radius of curvature is typically in the order of 2-15 cm, determining the minimum physical length of the system as well as the achievable optical path length. The first concave mirror 220, the second concave mirror 230 and the third concave mirror 240 are arranged with their centers of curvature aligned in the same longitudinal optical plane A, as is schematically illustrated in a top view in Figure 2b Figure 2c The first concave mirror 220 and the second concave mirror 230 are arranged substantially adjacent and parallel to each other in the primary reflector unit 235. This can be seen as the first concave mirror 220 and the second concave mirror 230 having a common back plane transverse to the longitudinal optical plane, the primary reflector unit back plane B, passing through the periphery of the spheres associated with the first concave mirror 220 and the second concave mirror 230, respectively. Similarly, an assembly reflector unit back plane C can also be defined comprising the periphery of the sphere associated with the third concave mirror 240 and symmetrical with respect to the third concave mirror 240, also transverse to the longitudinal optical plane. The primary reflector unit back plane B and the assembly reflector unit back plane C are perpendicular to the longitudinal optical plane A, and the primary reflector unit back plane B is parallel to the assembly reflector unit back plane C.

[0054] The distance between the primary reflector unit back plane B and the assembly reflector unit back plane C is a function of the Whit unit geometry in proximity to the radius of curvature of the first concave mirror 220, the second concave mirror 230 and the third concave mirror 240.

[0055] Alternatively, the third mirror can comprise a plurality of facets arranged in an overall concave configuration, wherein each facet is a flat mirror. Such a configuration of the third mirror gives good results when the distance between the primary reflector unit and the assembly reflector unit is approximately the same as the radius of curvature of the first reflector and the second reflector. In the following, the third mirror will be referred to as a concave mirror.

[0056] ​Both the assembly reflector unit 245 and the primary reflector unit 235 are generally rectangular in shape, extending further in the longitudinal direction than in the transverse direction. The optimal minimum shape of the active area of the primary reflector unit 235 depends on the actual active emitter area, which ideally will be magnified by the light collector onto the primary reflector unit 235. Typically, this image will be 10 to 40 times the emitter source. The third concave mirror 240 can have dimensions of 10 x 35 mm, and the first and second concave mirrors 220 and 230 are each about 10 x 15 mm.

[0057] The first concave mirror 220 provided adjacent to the second concave mirror 230 is to be understood such that the first concave mirror 220 and the first concave mirror 220 can be contiguous. However, they can also be arranged with a small distance therebetween. According to one embodiment, the first concave mirror 220 and the first concave mirror 220 are provided as a continuous structure comprised by the primary reflector unit 235, essentially forming a mirror unit with two separate curvatures. Such a single piece double mirror design can be advantageous as it will firmly fix the relative position of the centers of the two radii of curvature, which is a very sensitive parameter for the ray propagation in a multi-pass cell. Any small change d of this parameter value will move the final output position by d*N (N = number of single passes in the cell) with a subsequent transmission signal loss at the beam exit.

[0058] The light source 210 is arranged in the vicinity of or in the assembly reflector unit 245 and is directed such that the emitted light is directed essentially towards the primary reflector unit 235. The light source 210 is positioned with an offset in the transverse direction from the longitudinal optical plane A, thereby providing a 3D White cell geometry, which gives at least two rows of light spots, an upper row 246 and a lower row 247 on the assembly reflector unit 245, as shown in Figure 2c

[0059] According to one embodiment of the present application, the light source 210 is arranged with its light emitting portion coinciding with an imaginary sphere associated with the third concave mirror 240. According to one embodiment, the light source 210 is provided outside the third concave mirror 240, although preferably integrated in the assembly reflector unit 245. According to one embodiment, the light source 210 is provided in a light source opening 221 in the third concave mirror 240. Typically and preferably, the light source 210 is provided closer to one edge in the elongated direction than to the center of the third concave mirror 240.

[0060] ​In the multi-pass optical absorption cell 200 according to the application, the assembly reflector unit 245 is provided with a first detector 250 and a second detector 260 adapted to measure the intensity of the incoming light. According to an embodiment, at least one of the first detector 250 and the second detector 260 is provided in an opening in the third concave mirror 240. According to an embodiment, the first detector 250 is provided in a first detector opening 251 and the second detector 260 is provided in a second detector opening 261. Preferably, the first detector 250 and the second detector 260 are aligned with the imaginary sphere associated with the third concave mirror 240.

[0061] In the multi-pass optical absorption cell 200 according to the application, the light source 210 is arranged to emit a first portion of the light to be reflected first in the first concave mirror 220 and a second portion of the light to be reflected first in the second concave mirror 230, as Figure 2e - in a top view (e) and a bird's eye view (f) schematically illustrated in f. The light reflected first in the first concave mirror 220 is arranged to form a first light path 222 comprising a plurality of reflections between the main reflector unit 235 and the assembly reflector unit 245 and terminating in the first detector 250. In the figure, only the first reflection is illustrated for simplicity. The plurality of reflections is typically and preferably a predetermined number of reflections, for example 16 reflections, which gives an optical length of about 1 m. The light reflected first in the second concave mirror 230 is arranged to form a second light path 223 terminating in the second detector 260, as Figure 2g - in a top view (g) and a bird's eye view (h) schematically illustrated in h.

[0062] According to an embodiment, the second light path 223 comprises only one reflection in the main reflector unit 235 and the second detector opening 261 and the second detector 260 is positioned in the third concave mirror 240 at a position corresponding to the second portion of the light being reflected only once by the second concave mirror 230.

[0063] According to an embodiment, the second light path 223 comprises a predetermined number of reflections between the main reflector unit 235 and the assembly reflector unit 245. Thus, the second detector opening 261 and the second detector 260 are located at a position related to the predetermined number of reflections.

[0064] The curvature of the concave mirrors has been described as spherical, but can have a slight ellipsoidal shape in order to correct for the astigmatism defect. Such corrections are well known in the art and the term spherical mirror as used here encompasses such variations and corrections.

[0065] According to Figure 2cIn the embodiment illustrated simultaneously in Figs. 2 and 3, the light source 210 is provided in the first half 246 of the assembly reflector unit 245, and the first detector 250 is provided in the second half 247 of the assembly reflector unit 245. The second detector 260 is provided in the same half of the assembly reflector unit 245 as the light source 210.

[0066] According to an embodiment, the first portion of light arranged to be first reflected in the first concave mirror 220 and along the first light path 222 represents a major portion of the light emitted by the light source 210. Accordingly, the second portion of light arranged to be first reflected in the second concave mirror 230 and along the second light path 222 represents a minor portion of the light emitted by the light source 210. The relation between the major light portion and the minor light portion should be chosen according to the requested S / N requirements of the two detectors. The major portion of the emitted light, i.e. the light in the first light path, can for example represent at least 75% of the emitted light, such as at least 85% of the emitted light, or such as at least 95% of the emitted light. The desired percentage of the emitted light in the first light path can be controlled by the positioning of the light source 210 relative to the light source opening 221 and the size and shape of the light source opening 221.

[0067] According to an embodiment, the first detector 250 is arranged to measure the light intensity in a first wavelength range, and the second detector 260 is arranged to measure the light intensity in a second wavelength range, the second wavelength range being separate from the first wavelength range. The first wavelength range can relate to a first substance, and the second wavelength range can relate to a second substance. The first detector 250 can be arranged to provide a signal related to a target substance, and the second detector 260 is arranged to provide a signal related to a tracer substance. The term "target substance" refers to a substance for which the concentration is of primary interest. The term "tracer substance" refers to a substance for which the concentration is of primary interest in facilitating the measurement of the concentration of the target substance, e.g. in order to remove the influence of interfering factors or to relate the measurement to a specific condition. Typically, the concentration of the target substance is expected to be significantly lower than the concentration of the tracer substance. In the case where the multi-pass optical absorption cell 200 is used in a breath alcohol sensor system, the first substance / target substance is alcohol, and the second substance / tracer substance is carbon dioxide or water.

[0068] According to one embodiment, the first detector 250 and the second detector 260 are arranged to measure light intensity in the same wavelength range, and thereby to provide measurements of the concentration of the same substance. In such a case, the arrangement with two separate light paths and two detectors can be used to extend the sensitivity range of the multi-pass optical absorption cell 200, as the first light path 222 (the longer light path) provides accurate measurements for a lower concentration range, while the second light path (the shorter light path) provides accurate measurements for a higher concentration range of the substance. Alternatively or in combination, the setup in case the first detector 250 and the second detector 260 are arranged to measure light intensity in the same wavelength range is mainly used to provide redundancy and / or facilitate functional control or calibration.

[0069] The multi-pass optical absorption cell 200 can be incorporated in a variety of gas sensor devices. A gas sensor device 300 according to the present application is schematically illustrated in Figure 3 The gas sensor device 300 is adapted for use as, but is not limited to, a breath analysis device, in particular to measure the alcohol content in a breath sample by means of a tracer substance such as water vapor or carbon dioxide. A breath analysis device will be described in the following as a non-limiting example. Other gas sensor devices in which the multi-pass optical absorption cell 200 is a central part include, but are not limited to, devices arranged for measurements in which there is an overlap between the target substance and water vapor absorption. Examples of such devices are gas sensors for measuring nitrous oxide, methane and carbon dioxide.

[0070] The gas sensor device 300 comprises the multi-pass optical absorption cell 200 with a measurement cavity 211, a gas inlet 212 and a gas outlet 213, which measurement cavity 211 is comprised in a housing 301. The gas inlet 212 and the gas outlet 213 of the measurement cavity 211 are connected with an inlet 302 and an outlet 303 of the housing 301. A gas sample 304, e.g. a breath sample, is drawn into the measurement cavity 211 during use for analysis of its tracer and other substance, e.g. intoxicant, content. The inlet 302 can comprise a heater 311 arranged to heat the gas sample. Heating the gas sample can be important due to temperature dependence of the measurement, and also to avoid condensation on parts of the measurement cavity 211. A fan 312 is typically provided to provide a uniform air flow through the measurement cavity 211. The fan 312 can be provided at the outlet 304 of the housing 301.

[0071] The sensor signals according to one embodiment of the present application are produced by non-dispersive infrared (NDIR) spectroscopy, in which case the light source 210 is an IR source and the first and second detectors 250 are IR sensitive detectors. Suitable light sources and detectors are commercially available. According to one embodiment, the first detector 250 is tuned to the absorption spectrum of the intoxicating substance, typically ethanol, and the second detector 260 is tuned to the absorption spectrum of CO2or water. CO2has a strong absorption peak at a wavelength of 4.26 pm, while H2O has relatively broad peaks at 2.5-2.8 and 5.3-7.6 pm. Ethanol has a specific peak at 9.5 pm, which is not shared by any of the most common interfering substances, but has a small cross-sensitivity to CO2.

[0072] The IR source, the light source 210 and the first and second detectors 250, 260 are preferably operated synchronously using a repetition and sampling rate that exceeds the frequency bandwidth required for analyzing the breath signal. Synchronous operation using phase-locked techniques is preferred from the point of view of noise and interference suppression. A repetition and sampling rate of 5 Hz can be considered a lower limit compatible with the response times of MEMS (Micro Electro Mechanical System) based IR emitters and photovoltaic or thermopile IR detectors.

[0073] The interface electronic circuit modules 321, 322, 323, 324, 325 control the heater 311, the light source 210, the first detector 250, the second detector 260 and the fan 312, respectively. Each of these subsystems comprises electronic drive and power supply control means, which adapt the different functionalities to be manageable by a central processing unit CPU 310, which is a general purpose digital microcontroller. Also comprised in the gas sensor device 300 are memory means 332, 333 for permanent and temporary storage of information.

[0074] The CPU 310 and the memory means 332 and 333 are arranged to control the transmission and storage of data including the sensor signals during the analysis described below, and to control the method steps and to perform the mathematical operations described below in real time. Other configurations are possible to provide the control and calculation functions indicated herein, as appreciated by the skilled person, and the above should be considered as illustrative examples and one embodiment. One alternative embodiment is that the breath analysis system is integrated with other measurement and / or control systems in the vehicle, and the functions of the CPU 331 are provided by a main CPU in the vehicle that also handles other tasks, and the detectors and other units of the gas sensor device 300 communicate via the vehicle bus system or the like.

[0075] The gas sensor device 300 can comprise or be connected to a human / machine interface (HMI) unit 334 for audiovisual communication between the system and the user. The HMI unit 334 typically comprises communication means via a microphone / speaker, a touch screen or other input / output means. It has the ability to visually, verbally or symbolically communicate specific requests and classification results to the subject. Alternatively, the gas sensor device 300 can be connected to and make use of an existing infotainment system in the vehicle for the human / machine interface.

[0076] In automotive applications, the system can comprise a vehicle driving performance control unit 335 connected directly to the vehicle control system, providing "alcohol lock" functionality. In vehicles driven by fossil fuels, the unit 335 can control the ignition, and in other types of vehicles it will control other essential driving mechanisms.

[0077] A communication unit 336 can be provided for wireless exchange of information between the gas sensor device 300 and other external units, preferably over the Internet, and is useful in a wide range of applications.

[0078] In one embodiment of the present application, the above-mentioned dual-wavelength setup is provided without a significant increase in the size of the White cell, in particular without an increase in the size of the mirrors. In Figure 4a One implementation of a White-type multi-pass optical absorption cell 200 according to such an embodiment is schematically depicted in Fig. -g. As Figure 4a As depicted in a top view in Fig. -c, the multi-pass optical absorption cell 200 comprises a measurement cavity 211 provided with at least one gas inlet 212 and at least one gas outlet 213. Arranged in the measurement cavity 211 are a main reflector unit 435 comprising a first concave mirror 420 and a second concave mirror 430 and an assembly reflector unit 445 comprising a third concave mirror 440 facing the first concave mirror 420 and the second concave mirror 430. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are spherical mirrors and have the same concave radius of curvature. The radius of curvature is typically in the order of 2-15 cm, determining the minimum physical length of the system as well as the achievable optical path length. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are arranged with their centers of curvature aligned in a common longitudinal optical plane A, as schematically depicted in a side view of the assembly reflector unit 445 in Fig. -d and in a top view in Fig. -e, and Figure 4b As depicted in a top view in Fig. -c, the multi-pass optical absorption cell 200 comprises a measurement cavity 211 provided with at least one gas inlet 212 and at least one gas outlet 213. Arranged in the measurement cavity 211 are a main reflector unit 435 comprising a first concave mirror 420 and a second concave mirror 430 and an assembly reflector unit 445 comprising a third concave mirror 440 facing the first concave mirror 420 and the second concave mirror 430. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are spherical mirrors and have the same concave radius of curvature. The radius of curvature is typically in the order of 2-15 cm, determining the minimum physical length of the system as well as the achievable optical path length. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are arranged with their centers of curvature aligned in a common longitudinal optical plane A, as schematically depicted in a side view of the assembly reflector unit 445 in Fig. -d and in a top view in Fig. -e, and Figure 4c As depicted in a top view in Fig. -c, the multi-pass optical absorption cell 200 comprises a measurement cavity 211 provided with at least one gas inlet 212 and at least one gas outlet 213. Arranged in the measurement cavity 211 are a main reflector unit 435 comprising a first concave mirror 420 and a second concave mirror 430 and an assembly reflector unit 445 comprising a third concave mirror 440 facing the first concave mirror 420 and the second concave mirror 430. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are spherical mirrors and have the same concave radius of curvature. The radius of curvature is typically in the order of 2-15 cm, determining the minimum physical length of the system as well as the achievable optical path length. The first concave mirror 420, the second concave mirror 430 and the third concave mirror 440 are arranged with their centers of curvature aligned in a common longitudinal optical plane A, as schematically depicted in a side view of the assembly reflector unit 445 in Fig. -d and in a top view in Fig. -e, and Figure 4dIn -e a top view of the primary reflector unit 435 is schematically illustrated. The second concave mirror 430 and the second concave mirror 430 are arranged substantially adjacent and parallel to each other in the primary reflector unit 435. This can be seen as the first concave mirror 420 and the second concave mirror 430 have a common back plane going through the periphery of the spheres associated with the first concave mirror 420 and the second concave mirror 430 respectively, the primary reflector unit back plane B. Similarly, a component reflector unit back plane C can be defined comprising the periphery of the sphere associated with the third concave mirror 440 and symmetric about the third concave mirror 440. The component reflector unit back plane C is arranged transverse to the common longitudinal optical plane A. The primary reflector unit back plane B and the component reflector unit back plane C are arranged at an angle a relative to each other such that only in the longitudinal optical plane A the normal to the primary reflector unit back plane B and the normal to the component reflector unit back plane C form the angle a. This tilt of the primary reflector unit 435 relative to the component reflector unit 445 is in -e illustrated, where the primary reflector unit back plane B forms an angle a with the component reflector unit back plane C, as seen from above, where an imaginary plane C' is indicated close to plane B. In -e the projection of the planes B and C are parallel and both perpendicular to the longitudinal optical plane A, in a side view. In -e the normal to the primary reflector unit back plane B and the component reflector unit back plane C and the angle a are indicated. The effect of the tilted primary reflector unit 435 is further illustrated in -e, where the primary reflector unit 435 is tilted relative to the component reflector unit 445, and -e illustrates the relationship between the optical axes of the first concave mirror 420, the second concave mirror 430 and the third concave mirror 440, where d) is a prior art mirror arrangement without tilt, and b) is a tilted optical arrangement according to an embodiment. Figure 4a Figure 4b Figure 4c Figure 4d Figure 4d

[0079] The distance between the primary reflector unit back plane B and the component reflector unit back plane C is according to the Whitel unit geometry close to the radii of curvature of the first concave mirror 420, the second concave mirror 430 and the third concave mirror 440.

[0080] The component reflector unit 445 and the primary reflector unit 435 are both typically rectangular in shape extending further in the longitudinal direction than in the transverse direction. The optimal minimum shape of the active area of the primary reflector unit 435 depends on the actual active emitter area, which ideally will be magnified by the light collector onto the primary reflector unit 435. Typically, this image will be 10 to 40 times the emitter source. The third concave mirror 440 can have dimensions of 10 x 35 mm, and the first concave mirror 420 and the second concave mirror 430 are each about 10 x 15 mm.

[0081] ​​​​​The first concave mirror 420 provided adjacent to the second concave mirror 430 shall be understood such that the first concave mirror 420 and the first concave mirror 420 can be contiguous. However, they can also be arranged with a small distance in between. According to one embodiment, the first concave mirror 420 and the first concave mirror 420 are provided as a continuous structure comprised by the primary reflector unit 435, essentially forming a mirror unit with two separate curvatures. Such a single piece double mirror design can be advantageous as it will firmly fix the relative position of the centers of the two radii of curvature, which is a very sensitive parameter for the ray propagation in a multi-pass cell. Any slight change d of this parameter value will move the final output position by d*N (N = number of single passes in the cell) with a subsequent loss of the transmitted signal at the beam exit.

[0082] The light source 410 is arranged in the vicinity of or in the assembly reflector unit 445 and is directed such that the emitted light is directed essentially towards the primary reflector unit 435. The light source 410 is positioned with an offset in the lateral direction from the longitudinal optical plane A, thereby providing a 3D White cell geometry during use which gives at least two rows of light spots on the assembly reflector unit 445, an upper row 446 and a lower row 447, as illustrated in Figure 4d As a result of the angle a between the primary reflector unit 435 and the assembly reflector unit 445 according to the present application, the positions of the light spots in the lower row 447 in the longitudinal direction will be shifted compared to a comparable White cell where the primary reflector unit back plane B is parallel to the assembly reflector unit back plane C, while the light spots of the upper row 446 will not be shifted, as illustrated in Figure 1b In addition to the angle a, the shift of the light spots of the lower row 447 will depend on the mirror dimensions and the number of passes (optical length) between the mirrors by simple trigonometry. The person skilled in the art, having the knowledge of the geometry of a White cell and having gained an understanding of the present specification, will easily calculate the angle a corresponding to a certain shift. For a multi-pass spectral absorption cell 200 having a third concave mirror 440 with dimensions of 10 x 35 mm and first and second concave mirrors 420, 430 of about 10 x 15 mm and having an optical length of 100 mm and 2-5 passes, a suitable angle a ranges from 2° to 15°.

[0083] According to one embodiment of the present application, the angle a is chosen to cause a shift in the position of the light spots in the lower row 447, so that the position in the longitudinal direction will be essentially the same as for the light spots in the upper row 446, as illustrated in Figure 4gThe second detector 460 and the corresponding second detector opening 261 can thus be positioned below the first detector 450 and the corresponding first detector opening 251 and the assembly reflector unit 445 with two detectors can be provided in size without any or very limited increase. This corresponds to an angle a of 6° of the cell according to the above described example. As Figure 4a and Figure 4c The light source 410, the first detector 450 and the second detector 460 are arranged on a common printed circuit board 415 as shown in

[0084] According to one embodiment of the application, the light source 410 is arranged with its light emitting part coinciding with an imaginary sphere associated with the third concave mirror 440. According to one embodiment, the light source 410 is provided outside the third concave mirror 440, although preferably integrated in the assembly reflector unit 445. According to one embodiment, the light source 410 is provided in a light source opening 221 in the third concave mirror 440. Typically and preferably, the light source 410 is provided closer to one edge in the elongated direction than to the centre of the third concave mirror 440.

[0085] The curvature of the concave mirrors has been described as spherical, but can have a slight ellipsoidal shape in order to correct for stigmatisation defects. Such corrections are well known in the art and the term spherical mirror as used here encompasses such variations and corrections.

[0086] With reference to Figure 4a the embodiments described are readily implemented in a gas sensor device 300 as described with reference to Figure 3 above.

[0087] The above described embodiments should be understood as illustrative examples of the system and method of the present application. Those skilled in the art will understand that various modifications, combinations and changes can be made to the embodiments. In particular, different part solutions in different embodiments can be combined in other configurations, if technically possible.

Claims

1. A multi-pass spectral absorption cell (200) comprising a measurement cavity (211) having at least one gas inlet (212) and at least one gas outlet (213), and wherein the measurement cavity (211) includes: - The main reflector unit (235; 435) includes a first concave mirror (220; 420) and a second concave mirror (230; 430); - Component reflector unit (245; 445) which includes a third reflector 240 facing the first concave mirror (220; 420) and the second concave mirror (230; 430), wherein the first concave mirror (220; 420) and the second concave mirror (230; 430) are spherical mirrors and have the same concave radius of curvature and are arranged with their radius of curvature centers aligned in the same longitudinal optical plane A; - A light source (210) provided near or in the component reflector unit (245; 445) and arranged to direct emitted light toward the main reflector unit (235; 435), the light source (210) being positioned in the lateral direction with an offset from the longitudinal optical plane A; as well as - A first detector (250) is provided near or within the component reflector unit (245; 445). The multi-pass spectral absorption cell is characterized by: - A second detector (260) is provided near or in the component reflector unit (245; 445); The light source (210) is arranged to emit a first portion of light to be reflected first in the first concave mirror (220; 420) and form a first optical path (222) terminating at the first detector (250), and to emit a second portion of light to be reflected first in the second concave mirror (230; 430) and form a second optical path (223) terminating at the second detector (260).

2. The multi-pass spectral absorption cell (200) according to claim 1, wherein, The first detector (250) is provided in a first detector opening (251) in the third reflector (240, 440) and / or the second detector (260) is provided in a second detector opening (261) in the third reflector (240, 440).

3. The multi-pass spectral absorption cell (200) according to claim 1 or 2, wherein, The second optical path (223) includes only one reflection in the main reflector unit (235; 435) and the second detector opening (261), the second detector opening (261) being located in the third mirror (240; 440) at a position corresponding to the emitted light being reflected only once by the second concave mirror (230; 430).

4. The multi-pass spectral absorption cell (200) according to any one of claims 1-3, wherein, The light source (210) is provided in the first half (246) of the component reflector unit (245; 445), and the first detector (250) is provided in the second half (247) of the component reflector unit (245; 445), wherein the second detector (250) is provided in the same half of the component reflector unit (245; 445) as the first detector (250).

5. The multi-pass spectral absorption cell (200) according to any one of claims 1-4, wherein, The first portion of the light, which is arranged to be reflected first in the first concave mirror (220; 420) and along the first light path (212), represents the main portion of the light emitted by the light source (210).

6. The multi-pass spectral absorption cell (200) according to any one of claims 1-5, wherein, The first optical path (212) includes multiple reflections between the main reflector unit (235; 435) and the component reflector unit (245, 445).

7. The multi-pass spectral absorption cell (200) according to claim 6, wherein, The first optical path (212) is longer than the second optical path (223), preferably at least twice as long, and even more preferably at least four times as long.

8. The multi-pass spectral absorption cell (200) according to claim 5, wherein, The main portion of the light beam (212) arranged to be reflected in the first mirror (220; 420) represents at least 75% of the emitted light, preferably at least 85% of the emitted light, and even more preferably at least 95% of the emitted light.

9. The multi-pass spectral absorption cell (200) according to any one of claims 1-8, wherein The first detector (250) is arranged to measure the light intensity in a first wavelength range, and the second detector (260) is arranged to measure the light intensity in a second wavelength range, which is separate from the first wavelength range.

10. The multi-pass spectral absorption cell (200) according to any one of claims 1-9, wherein, The first concave mirror (220; 420) and the second concave mirror (230, 430) are provided as a continuous structure included by the main reflector unit (235; 435), forming a continuous mirror surface with two separate curvatures.

11. The multi-pass spectral absorption cell (200) according to any one of claims 1-10, comprising: -The first concave mirror (420) and the second concave mirror (430) are arranged such that the back plane B of the main reflector unit is the common symmetrical back plane of the first concave mirror (420) and the second concave mirror (430); - wherein the third reflecting mirror (440) is concave, and wherein the component reflector unit has a back plane C symmetrical to the third concave mirror (440), wherein - The first concave mirror (420), the second concave mirror (430) and the third concave mirror (440) are spherical mirrors and have the same concave radius of curvature and are arranged with their radii of curvature centers aligned in the same longitudinal optical plane A; in - The back plane C of the component reflector unit is arranged transversely to the common longitudinal optical plane A, and the back plane B of the main reflector unit and the back plane C of the component reflector unit are arranged relative to each other such that the normal of the back plane B of the main reflector unit and the normal of the back plane C of the component reflector unit form an angle α in the longitudinal optical plane A.

12. The multi-pass spectral absorption unit (200) according to claim 11, wherein, The multi-pass spectral absorption unit (200) is arranged to provide an upper row of light spots (446) and a lower row of light spots (447) on the component reflector unit (445), and the angle α is selected to cause a shift of the light spots in the lower row (447) such that at least most of the light spots in the lower row will be in the same longitudinal position as the light spots in the upper row (446).

13. A gas sensor (300) comprising a multi-pass spectral absorption cell (200) according to any one of claims 1-12.

14. The gas sensor (300) according to claim 13, wherein, The first wavelength range relates to a first substance, and the second wavelength range relates to a second substance.

15. The gas sensor (300) according to claim 14, wherein, The first detector (250) is arranged to provide measurements relating to the concentrations of the first substance and the second substance, wherein the signal relating to the concentration of the first substance provided by the first detector (250) is related to the target substance, and the signal relating to the concentration of the second substance provided by the second detector (260) is related to the tracer substance, wherein the concentration of the target substance is expected to be substantially lower than the concentration of the tracer substance under normal measurement conditions.

16. An alcohol meter comprising a gas sensor (300) according to claim 13, wherein the first substance is alcohol and the second substance is carbon dioxide or water.

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