PHOTOACOUSTIC GAS SENSOR DEVICE

Abstract

A photoacoustic gas sensor device for deter-mining a value indicative of a presence or a concentration of a component in a gas comprises a substrate and a measurement cell body, the substrate and the measurement cell body defining a measurement cell enclosing a measurement volume. A reflective shield divides the measurement volume into a first volume and a second volume. An opening in the measurement cell is provided for a gas to enter the measurement volume. In the first volume and on a front side of the substrate are arranged: An electromagnetic radiation source for emitting electromagnetic radiation through an aperture in the reflective shield into the second volume; and a pressure transducer communicatively coupled to the second volume for measuring a sound wave generated by the component in response to an absorption of electromagnetic radiation by the component. At least a portion of a surface of the reflective shield facing the second volume is made of a material reflecting electromagnetic radiation.

Claims

1. Photoacoustic gas sensor device, for determining a value indicative of a presence or a concentration of a component in a gas, the photoacoustic gas sensor device comprising: a substrate, a measurement cell body, the substrate and the measurement cell body defining a measurement cell enclosing a measurement volume, a reflective shield dividing the measurement volume into a first volume and a second volume, an opening in the measurement cell for a gas to enter the measurement volume, arranged in the first volume and on a front side of the substrate: an electromagnetic radiation source for emitting electromagnetic radiation through an aperture in the reflective shield into the second volume, a pressure transducer communicatively coupled to the second volume for measuring a sound wave generated by the component in response to an absorption of electromagnetic radiation by the component, wherein at least a portion of a surface of the reflective shield facing the second volume is made of a material reflecting electromagnetic radiation.

2. Photoacoustic gas sensor device according to claim 1, wherein at least the major portion of the surface of the reflective shield facing the second volume is made of the reflective material, in particular wherein the entire surface of the reflective shield facing the second volume is made of the reflective material, in particular wherein the reflective material is coated on a core of the reflective shield, in particular wherein the reflective shield is made of the reflective material, in particular wherein an inner surface of the measurement cell body facing the second volume is made of the or another reflective material, in particular wherein the reflective material is coated on a core of the measurement cell body, in particular wherein the measurement cell body is made of the reflective material, in particular wherein the reflective material is a metal or a metal-filled polymer, in particular wherein the inner surface of the measurement cell body facing the second volume and the surface of the reflective shield facing the second volume each have a reflectivity of more than 70%, in particular wherein a ratio of inner surfaces defining the second volume with a reflectivity of above 70% to inner surfaces defining the second volume with a reflectivity of below 70% is above 20.

3. Photoacoustic gas sensor device according to claim 1, wherein a ratio of the second volume to the first volume is at least 1.5, in particular wherein a thickness of the reflective shield is between 30 μm and 1 mm.

4. The photoacoustic gas sensor device according to claim 1, wherein a plane extension of the reflective shield and a plane extension of the substrate are aligned in parallel with each other, wherein the aperture is arranged in the reflective shield in vertical alignment with the electromagnetic radiation source arranged on the substrate, wherein the electromagnetic radiation source and the pressure transducer face the reflective shield, in particular wherein a distance between the aperture in the reflective shield and the electromagnetic radiation source is between 10 μm and 1 mm, in particular wherein the aperture is configured to enable a pressure equilibrium between the first volume and the second volume resulting in the communicatively coupling of the second volume and the pressure transducer to enable the pressure transducer to measure the sound wave generated by the component in the second volume.

5. Photoacoustic gas sensor device according to claim 1, comprising one or more additional apertures in the reflective shield connecting the first volume and the second volume, and/or comprising one or more gaps between the reflective shield and the measurement cell body or the substrate, the one or more gaps connecting the first volume and the second volume, in particular wherein the one or more additional apertures and/or the one or more gaps are configured to enable a pressure equilibrium between the first volume and the second volume resulting in the communicative coupling of the second volume and the pressure transducer to enable the pressure transducer to measure the sound wave generated by the component in the second volume.

6. Photoacoustic gas sensor device according to claim 1, wherein the reflective shield is made of or comprises a material of a thermal diffusivity of less than 20 mm2/s, in particular wherein the reflective shield is made of or comprises plastic material or stainless steel, in particular wherein the reflective shield is configured and arranged to reduce or shield a temperature modulation of the gas in the second volume evoked by an operation of the electromagnetic radiation source, in particular wherein the reflective shield is thermally connected to a heatsink of the substrate, in particular to a ground contact of the substrate, in particular by means of one or more legs of the reflective shield, in particular wherein the electromagnetic radiation source is in contact with the reflective shield, in particular wherein the electromagnetic radiation source comprises an emitter and an optical band pass filter between the emitter and the reflective shield, wherein the reflective shield is in contact with the optical band pass filter.

7. Photoacoustic gas sensor device according to claim 1, comprising wiring including a ground contact, wherein the reflective shield is made of or comprises an electrically conducting material, wherein the reflective shield is electrically connected to the ground contact, in particular wherein the electrically conducting material is coated on a core of the reflective shield, in particular wherein the reflective shield is made of the electrically conducting material, in particular wherein the electrically conducting material is metal or a metal-filled polymer, in particular wherein the reflective shield is an electrostatic discharge protection element for protecting the electromagnetic radiation source and/or the pressure transducer from an electrostatic discharge, in particular wherein the substrate supports the wiring including the ground contact, in particular wherein the reflective shield comprises one or more electrically conducting legs mounted on the substrate and soldered or conductively adhered to the ground contact, in particular wherein the reflective shield and the one or more legs are formed integrally, in particular wherein the reflective shield and the measurement cell body are spaced apart from each other.

8. The photoacoustic gas sensor device according to claim 1, further comprising an integrated circuit configured to receive a measurement signal from the pressure transducer and to determine the value indicative of a presence or a concentration of the component dependent on the measurement signal, in particular dependent on an amplitude of the measurement signal, in particular wherein the measurement signal is bandpass-filtered around the modulation frequency, in particular wherein the integrated circuit is arranged in the first volume together with the electromagnetic radiation source and the pressure transducer, and is arranged on the front side of the substrate, in particular wherein the integrated circuit is configured to control the electromagnetic radiation source, in particular wherein the integrated circuit is configured to control an intensity of the electromagnetic radiation to modulate with a modulation frequency, which modulation frequency is between 1 Hz and 100 kHz.

9. The photoacoustic gas sensor device according to claim 1, further comprising another transducer for sensing one or more of temperature, humidity, pressure, one or more different components in a gas, in particular wherein the other transducer is arranged in the first volume together with the electromagnetic radiation source and the pressure transducer, and is arranged on or integrated in the front side of the substrate, in particular wherein the integrated circuit is configured to compensate the value indicative of a presence or a concentration of the component dependent on measurement values of the other transducer.

10. Photoacoustic gas sensor device according to claim 1, wherein the measurement cell body is mounted on the front side of the substrate, wherein the reflective shield is supported by the measurement cell body, in particular wherein the reflective shield is attached to the measurement cell body, in particular wherein the reflective shield and the measurement cell body are formed integrally, in particular wherein the measurement cell body comprises an overhang defining a compartment outside the measurement volume between the measurement cell body and the substrate on which front side of the substrate one or more electrical components are arranged in the compartment, in particular wherein the measurement cell body comprises the opening arranged for the gas to enter the second volume.

11. Photoacoustic gas sensor device according to claim 1, wherein the measurement cell body is mounted on the front side of the substrate, wherein the reflective shield is embodied as a cap mounted on the front side of the substrate, wherein the reflective shield and the measurement cell body are spaced apart from each other, in particular wherein the measurement cell body comprises the opening arranged for the gas to enter the second volume.

12. Photoacoustic gas sensor device according to claim 1, wherein the measurement cell body includes a first cap mounted on the front side of the substrate, wherein the measurement cell body comprises a second cap mounted on top of the first cap, wherein a ceiling of the first cap represents or includes the reflective shield, in particular wherein the measurement cell body comprises the opening arranged for the gas to enter the second volume.

13. Photoacoustic gas sensor device according to claim 1, further comprising a gas permeable membrane covering the opening, wherein the gas permeable membrane is permeable for a gas exchange between the measurement volume and surroundings of the measurement cell, in particular wherein the gas permeable membrane is made of one or more of the following materials: sintered metal, ceramic, plastic.

14. Photoacoustic gas sensor device according to claim 1, wherein the substrate comprises the opening, in particular wherein the membrane is attached to the substrate, in particular wherein the membrane is attached to the front side of the substrate, in particular wherein the opening in the substrate is offset from the aperture in the reflective shield.

15. Photoacoustic gas sensor device according to claim 1, wherein the opening is formed by a gap between the measurement cell body and the substrate, in particular wherein the membrane is arranged between the measurement cell body and the substrate, in particular in the gap.

16. Photoacoustic gas sensor device according to claim 1, wherein the electromagnetic radiation source comprises an emitter including an active area for emitting the electromagnetic radiation, wherein a diameter of the aperture in the reflective shield is between 100% and 400% of a diameter of the active area of the emitter, in particular wherein a spacing between the reflective shield and the active area of the emitter is between 20% and 200% of the diameter of the active area, in particular wherein the aperture in the reflective shield is an optical aperture shielding from radiation of a wavelength or band outside a desired wavelength or band entering the second volume.

17. Photoacoustic gas sensor device according to claim 1, wherein the electromagnetic radiation source comprises: an emitter including an active area for emitting the electromagnetic radiation, a package for the emitter, the package comprising an access opening enabling the active area of the electromagnetic radiation source to to emit the electromagnetic radiation, an optical bandpass filter covering the access opening of the package, wherein the reflective shield is arranged to cover edges of the optical band pass filter, and/or wherein a diameter of the aperture in the reflective shield is between 1 and 2.5 times a diameter of the access opening in the package, in particular wherein the reflective shield is arranged at a distance between 0 μm and 200 μm from a top surface of the optical bandpass filter, in particular wherein a diameter of the aperture in the reflective shield is between 100% and 400% of a diameter of the active area of the emitter, in particular wherein the aperture in the reflective shield and the access opening in the package are optical apertures shielding the second volume from radiation of a wavelength or band outside a desired wavelength or band.

18. Photoacoustic gas sensor device according to claim 1, wherein the measurement cell body is mounted to the substrate by means of a snap fit, in particular wherein the measurement cell body comprises one or more snap arms and the substrate comprises one or more corresponding holes for the one or more snap arms to reach through, in particular wherein the snap fit is designed to mount the measurement cell body acoustically tight to the substrate, in particular wherein a footprint of the substrate and a footprint of the measurement cell body match by a tolerance of at most 10% for each dimension defining the planar extension thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Embodiments of the present invention, aspects and advantages will become apparent from the following detailed description thereof. The detailed description makes reference to the annexed drawings, wherein the figures show:

[0083] FIG. 1 to FIG. 3, and FIG. 5 to FIG. 10, each, a cut view of a photoacoustic gas sensor device according to an embodiment of the invention,

[0084] FIG. 4 perspective views a) and b) from top and from below of a photoacoustic gas sensor device according to an embodiment of the invention, and an open cut view c) of a slightly different embodiment of a photoacoustic gas sensor device.

DETAILED DESCRIPTION OF THE DRAWINGS

[0085] Same elements are referred to by same reference numerals across all figures.

[0086] FIG. 1 shows a schematic cut view of a photoacoustic gas sensor device according to an embodiment of the present invention.

[0087] The device comprises a substrate 1, e.g. a printed circuit board (PCB), with a front side 11 and a back side 12 opposite the front side 11. A measurement cell body 2 is mounted on the front side 11 of the substrate 1, which substrate 1 and measurement cell body 2 together form a measurement cell enclosing a measurement volume 3. The measurement cell has an opening 4 to allow an exchange of gas between the measurement volume 3 and surroundings of the device. In FIG. 1, the opening 4 is located in the measurement cell body 2. The opening 4 is preferably covered by a membrane 5 which is gas permeable to allow for a gas exchange such that a concentration of the component of interest in the gas is similar as in the surroundings.

[0088] A pressure transducer 6 such as a MEMS microphone or a pressure sensor, and an electromagnetic radiation source 7, which in this example is an infrared source, are both located on the front side 11 of the substrate inside the measurement cell. The electromagnetic radiation source includes an active area 711 emitting the electromagnetic radiation, i.e. the infrared radiation in this example, indicated by arrows 8. The infrared source emits infrared radiation of the band, wherein the intensity of the infrared radiation is modulated as described above. The infrared radiation is selectively absorbed by molecules of the gas component of interest.

[0089] A reflective shield 17 is provided in the measurement cell. The reflective shield 17 presently extends in a plane parallel to a planar extension of the substrate 1. The reflective shield 17 is attached to or formed integrally with the measurement cell body 2. The reflective shield 17 divides the measurement volume 3 into a first volume 31 between the substrate 1 and the shield 17, and a second volume 32 between the shield 17 and the measurement cell body 2. The reflective shield 17 comprises an aperture 18 which presently is aligned with the infrared source 7, such that infrared radiation 8 can emit from the infrared source 7 through the aperture 18 into the second volume 32.

[0090] It is preferred that a surface 171 of the shield 17 facing the second volume 32 is made of a material reflecting the electromagnetic radiation emitted by the electromagnetic radiation source 7. This is indicated by the various arrows representing the electromagnetic radiation 8 reflected in the second volume 32 after being emitted from the infrared source 7. A ratio of infrared radiation 8 absorbed is increased by increasing a mean optical path length of the infrared radiation 8 within the measurement volume 3. This is achieved by a material of at least the inner surface 21 of the measurement cell body 2 being chosen to be reflective. In case of a coating, the reflective coating may be made from a metal such as gold, aluminum, nickel, copper. In this way, the overall reflectivity inside the second volume 32 is increased, which leads to more accurate measurements of the concentration of the component. The increase of the mean optical path length, in particular in contrast to the linear optical path in conventional photoacoustic gas sensors, is illustrated by multiple reflections of the infrared radiation 8 in the various Figures. Here, the photoacoustic effect comes into play: Molecules of the gas component of interest, e.g. CO.sub.2, absorb the electromagnetic radiation in the second volume 32 leading to the generation of heat and hence an increase of pressure. By modulating an intensity of the electromagnetic radiation with a modulation frequency in the infrared source 7, a modulation of pressure may be achieved.

[0091] Such pressure modulation or pressure variations, i.e. sound waves, may be measured by the pressure transducer 6. In this example, the aperture 18 in the reflective shield 17 allows such sound waves generated in the second volume 32 to reach into the first volume 31 and hence to reach the pressure transducer 6. For this reason, a gap is provided between the reflective shield 17 and the electromagnetic radiation source 7. The sound waves are indicated by reference numeral 9 in FIG. 1. Accordingly, by means of the aperture 18 in the shield 17, the second volume 32, in which the absorption and sound wave generation predominantly takes place, is communicatively coupled to the first volume 31 and the pressure transducer 6. Accordingly, in the present example, not only the electromagnetic radiation enters the second volume 32 through the aperture 18, but also the sound waves propagate from the second volume 32 into the first volume 31 to the pressure transducer 6.

[0092] An integrated circuit 14 is arranged on the front side 11 of the substrate 1, which integrated circuit 14 may e.g. be an ASIC. In FIG. 1, the integrated circuit 14 is located outside the measurement cell; in a different embodiment, however, it may as well be located inside the measurement cell. The integrated circuit 14 is configured to control the electromagnetic radiation source 7, e.g. by imposing an intensity modulation on e.g. the infrared radiation emitted with a modulation frequency. The modulation frequency may be within the audible spectrum, e.g. between 20 Hz and 20 kHz, or it may be up to 100 kHz, or it may even be down to 5 Hz. The integrated circuit 14 is further configured to receive measurement values from the pressure transducer 6, as well as for determining a value of the gas component concentration from those measurement values, e.g. by using a predefined or a resettable calibration function linking the measurement values to concentration value of the gas component. The value of the gas component concentration may be output via a digital interface, e.g. an I2C interface, as may be values of one or more other transducers if any.

[0093] In the present example, another transducer 13 is arranged on the front side 11 of the substrate 1. In FIG. 1, the other transducer 13 is located outside the measurement cell; in a different embodiment, however, it may as well be located inside the measurement cell. Such other transducer 13 advantageously is one or more of the following: a temperature sensor, a humidity sensor, a combined temperature/humidity sensor, a pressure sensor, in particular a barometric pressure sensor, another microphone, another gas sensor, e.g. of metal oxide type or of electrochemical type. Through measurement values of temperature and/or humidity and/or any of the other parameters measured by such other transducer, a gas concentration value may be compensated, e.g. for effects of temperature and/or humidity, e.g. by the integrated circuit 14. Hence, effects of ambient conditions on the measurement of the component can be reduced or eliminated.

[0094] Further electrical components 15 may be arranged on the front side 11 of the substrate 1, preferably outside the measurement cell. Such further electrical components 15 may include passive components or auxiliary electronics, e.g. capacitors and resistors, as required.

[0095] On the back side 12 of the substrate 1, land grid array (LGA) pads 16 are arranged for SMD assembly and reflow soldering by a customer. Other contacts such as DFN, QFN or castellated holes are possible.

[0096] In one example, the component to be measured is CO.sub.2. For CO.sub.2, measurements in the range between 0 and 10,000 ppm, or between 0 and 40,000 ppm, or between 0 and 60,000 ppm CO.sub.2 are possible.

[0097] The proposed photoacoustic gas sensor device, as e.g. shown in FIG. 1, may be built with a small form factor, such that it has an overall size of e.g. 1×1×0.7 cm.sup.3. Thus it is significantly smaller and also cheaper to manufacture than conventional photoacoustic or NDIR-based gas sensors.

[0098] In the embodiment of FIG. 2, the electromagnetic radiation source 7 is embodied different from the one used in FIG. 1. The electromagnetic radiation source 7 of FIG. 2 comprises an emitter 71 packaged into a package 73, and an optical bandpass filter 72 on top of the package 73. The package 73 has an access opening 731 for radiation emitted by the emitter 71 to reach the second volume 32. The emitter 71 may be a broadband infrared emitter, e.g. emitting radiation over the entire infrared spectrum. The optical bandpass filter 72 allows to exclusively pass radiation of a band that is set according to the gas component of interest. For a detection of CO.sub.2, the band is for instance centered around 4.3 μm, and has a typical band width of 0.5 μm, or smaller, e.g. 0.2 μm or 0.1 μm, such that a measured value is actually selective on CO.sub.2. The optical aperture 18 and a further optical aperture represented by the properly designed access opening 731 may in combination prevent radiation of different bands to enter the second volume 32.

[0099] It is noted that the optical bandpass filter 72 is in contact with the reflective shield 17, e.g. via an O-ring or other sealing means. While such arrangement may be beneficial for several purposes such as thermal shielding etc., the acoustic coupling between the second volume 32 and the first volume 31, and the pressure transducer 6 respectively now no longer can be granted through the aperture 18 in the reflective shield 17. For this reason, gaps are provided between the reflective shield and the substrate 1 or the measurement cell body 2, for acoustically coupling the second volume 32 to the pressure transducer 6. Such gaps may better be seen from subsequent diagram 4c).

[0100] In FIG. 3, as is in FIG. 2, the reflective shield 17 is not attached nor mounted to the measurement cell body 2, but instead is directly mounted onto the substrate 1, i.e. on the front side 11 of the substrate 1 such as all the other electrical components. For this purpose, the reflective shield 17 not only provides a planar extension, but also provides for one or more legs referred to by 172. In one embodiment, the reflective shield 17 and its legs 172 are made integrally, and, e.g. the legs 172 are bent in order to serve as support. Preferably, the reflective shield 17 including the legs 172 are spaced apart from the measurement cell body 2, i.e. a gap is provided between the legs 172 of the reflective shield 17 and the measurement cell body 2 as is shown in FIG. 3, in order to allow reflow soldering also of the reflective shield 17. In case the reflective shield 17 including its legs 172 is made from metal, the reflective shield 172 not only serves as reflecting element, but also as protection against electrostatic discharge. For this purpose, it is preferred that at least one of the legs 172 is electrically connected to a ground contact 10 on the front side 11 of the substrate 1. This ground contact 10 may, via additional wiring on or in the substrate 1, be electrically connected to one of the contact pads 16, and serve as a ground connection for the entire photoacoustic sensor device.

[0101] FIG. 4 illustrates another embodiment of a photoacoustic gas sensor according to the present invention in two perspective views from top in a), and from bottom in b). In addition, diagram c) shows the photoacoustic gas sensor in a perspective view from top without the measurement cell body 2. As can be derived from FIGS. 4a) and 4b), the photoacoustic gas sensor of this embodiment may be considered as a variation of the photoacoustic gas sensor of the embodiment of FIG. 3. In the embodiment of FIGS. 4a) and 4b), the components 13, 14 and 15 are assembled at different locations on the substrate 1 outside the measurement cell. However, inside the measurement cell, as can be seen from the open cut view in FIG. 4c), the shield 17 again comprises legs 172 for mounting on the substrate 1. In particular, one of the mounting pads on the front side 11 of the substrate 1 serves as ground contact 10. One of the legs of the shield is electrically connected, e.g. soldered to the ground contact 10, for ESD purposes as laid out above. However, the cut open photoacoustic gas sensor from FIG. 4c) is slightly different from the embodiment shown in FIGS. 4a) and 4b) in that the components 13, 14 and 15 previously arranged outside the measurement cell are now arranged under the reflective shield 17 inside the measurement cell to be built by mounting the measurement cell body 2 onto the substrate 1.

[0102] In the embodiment of FIG. 5, the reflective shield 17 now is formed integrally with the measurement cell body 2. Here, the measurement cell body 2 comprises a frame 22, and a lid 21 acting as a cover. The opening 4 now is provided in the lid 21, and the membrane 5 is attached to a top side of the lid 21 facing the surroundings. In this embodiment, the frame 22, the lid 21 and the reflective shield 17 may all be made from the reflective material, e.g. from metal. However, in a different embodiment, one or more of the frame 22, the lid 21 and the reflective shield 17 may comprise a plastic core, and a reflective coating where desired. Again, the electromagnetic radiation 8 enters the second volume 32 through the aperture 18 in the reflective shield 17. By means of the dimensioning of the aperture 18 and a gap between the reflective shield 17 and the electromagnetic radiation source 7, sound waves generated in the second volume 32 in response to the reaction of the molecules of the component of the gas with the electromagnetic radiation 8 reach the pressure transducer 6 and are converted in an electrical signal there.

[0103] The embodiment of FIG. 6 resembles the embodiment of FIG. 5. Again, the reflective shield 17 is formed integrally with the measurement cell body 2, which additionally comprises a frame 22. A lid 21 manufactured separate from the frame 22 but attached thereto acts as a cover co-defining the second volume 32. In contrast to the embodiment of FIG. 5, the frame 22 now includes an overhang 221, also referred to as bulge. By means of the overhang 221, a compartment 19 is generated between the frame 22 and the substrate 1, which compartment is open to the left hand side. The compartment 19 is not part of the measurement volume 3 but is located outside. Still, electrical components such as the other transducer 13 arranged on the front side 11 of the substrate 1 in the compartment 19 are protected by the overhang 221, and thus may be less exposed not only to mechanical impact but also to moisture, dirt etc.

[0104] In the embodiment of FIG. 7 the measurement cell body 2 and the reflective shield 17 are embodied in a different setup. A first cap 23 is mounted on the front side 11 of the substrate 1, and a second cap 24 is mounted on top of the first cap 23. A ceiling 213 of the first cap 23 serves as reflective shield 17, while side walls of the first cap and the second cap 24 in combination contribute to the measurement cell body 2. The opening is arranged in the second cap 24, while the opening is arranged in the ceiling 231 of the first cap 23. This setup provides for an easy mounting of the caps 23 and 24.

[0105] First, the embodiment of FIG. 8 differs from the embodiment of FIG. 5 in that all electrical components are arranged inside the measurement volume 3, and specifically are commonly arranged in the first volume 31 on the front side 11 of the substrate 1. This may include, in addition to the electromagnetic radiation source 7 and the pressure transducer 6 as follows: The integrated circuit 14, the one or more other transducers 13, and any further electrical components 15. Subject to the number and footprint of electric components to be commonly arranged in the first volume 31, this configuration may increase the footprint of the sensor, while on the other hand all electrical components are now protected by the reflective shield 17 and the measurement cell body 2. The electrical components may in addition benefit from other functions performed by the reflective shield 17, such as electrostatic discharge protection or thermal management.

[0106] Second, the embodiment of FIG. 8 differs from the embodiment of FIG. 5 in that the opening 4 now is provided between the substrate 1 and the measurement cell body 2. Owed to the construction of the measurement cell body 2, and specifically the frame 22 thereof, when clipping the frame 22 to the substrate 1, a horizontal gap is generated between the front side 11 of the substrate 1 and a bottom surface of the frame. This gap preferably takes the shape of a ring around the measurement volume 3, and is filled by e.g. a ring of gas permeable membrane material. Accordingly, the gas to be measured enters the measurement volume 3 laterally through the opening 4 between the measurement cell body 2 and the substrate 1, and diffuses from the first volume 31 through the aperture 18 into the second volume 32 where it meets the electromagnetic radiation 8. This process is indicated in FIG. 8 by the dotted arrow. In this embodiment, the lid 21 is understood to seal the measurement volume 3 from the top. In this embodiment, the footprint of the substrate 1 matches the footprint of the measurement cell body 2 such that snap fits 25 can be used for easily attacking the measurement cell body 2 to the substrate 1.

[0107] The embodiment of FIG. 9 differs from the embodiment of FIG. 1 in that all electrical components are arranged inside the measurement volume 3, and specifically are commonly arranged in the first volume 31 on the front side 11 of the substrate 1. This may include, in addition to the electromagnetic radiation source 7 and the pressure transducer 6 as follows: The integrated circuit 14, the one or more other transducers 13, and any further electrical components 15. Subject to the number and footprint of electric components to be commonly arranged in the first volume 31, this configuration is advantageous in that all electrical components are now protected by the reflective shield 17 and the measurement cell body 2. The electrical components may in addition benefit from other functions performed by the reflective shield 17, such as electrostatic discharge protection or thermal management.

[0108] The embodiment of FIG. 10 resembles the embodiment of FIG. 9 with all electrical components being commonly arranged within the measurement volume 3, and specifically within the first volume 31. However, while the opening 4 in the embodiment of FIG. 9 is arranged in the measurement cell body 2, and specifically in the portion of the measurement cell body 2 defining the second volume 32, the opening 4 of the embodiment of FIG. 10 now is arranged in the substrate 1, in form of a through-hole in the substrate 1. Accordingly, the gas to be measured enters the measurement volume 3 through the opening 4 in the substrate 1, and diffuses from the first volume 31 through the aperture 18 into the second volume 32 where it meets the electromagnetic radiation 8. The gas permeable membrane 5 now is attached to the substrate 1, and preferably is attached to the front side 11 of the substrate 1 facing the first volume 31. In this embodiment, the lid 21 is understood to seal the measurement volume 3 from the top.

[0109] While above there are shown and described embodiments of the invention, it is to be understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

[0110] It is understood, that in particular the embodiments of FIG. 8 and FIG. 10 shall also be disclosed without the presence of the reflective shield 17.