Gas sensor for detecting gas components using two detectors and a controller providing a drive current in an incandescent light bulb

Abstract

A gas sensor (100) is described, which comprises a gas absorption cell (6), configured to house a gas to be detected, an incandescent light bulb (1) comprising a glass bulb (2) and a filament (3) enclosed in the glass bulb (2). The gas sensor (100) also comprises a first detector (4), configured to detect radiation in a first wavelength range of 1-5 um, and a control unit (7), configured to provide a drive current through the filament (3) of the incandescent light bulb (1) during operation of the gas sensor (100). The gas sensor comprises a second detector (5), configured to detect radiation in a second wavelength range of 6-15 µm radiation. The second detector (5) is arranged such that the second detector (5) is illuminated by radiation from the incandescent light bulb (1) during operation of the gas sensor (100).

Description

[0001] GAS SENSOR

[0002] TECHNICAL FIELD

[0003] The present application relates to a gas sensor comprising a gas absorption cell, configured to house a gas to be detected, an incandescent light bulb comprising a filament and a glass bulb, wherein the filament is enclosed in the glass bulb, a first detector, configured to detect radiation in a first wavelength range of 1-5 urn, and a second detector, configured to detect radiation in a second wavelength range of 6-15 urn radiation.

[0004] BACKGROUND ART

[0005] Refrigerants are used in many different kinds of cooling equipment such as air conditioning units and refrigerators. Since the ratification of the Montreal Protocol hydrofluorocarbons (HFCs), such as trifluoromethane or fluoroform, have replaced chlorofluorocarbons (CFCs) as refrigerants. Hydrofluorocarbons are also used in insulating foams, aerosol propellants, as solvents and for fire protection. The use for fire protection is possible thanks to hydrofluorocarbons being an asphyxiant. Hydrofluorocarbons does not harm the ozone layer as much as the chlorofluorocarbons they replace, but hydrofluorocarbons still contribute to global warming, with some like trifluoromethane having 10000 times the warming potential of carbon dioxide.

[0006] When used as a refrigerant in cooling equipment any leaks will present a hazard not only to the environment but also to any persons in the premises in which the cooling equipment is arranged. Thus, it is important to monitor cooling equipment so that any leaks of refrigerant may be detected as early as possible. For a reliable early detection of leaks, it is advantageous to have at least one sensor close to each piece of cooling equipment. It may be sufficient to have one sensor in a room in which the cooling equipment is arranged in case the refrigerant is free to move from each cooling equipment. Alternatively, if a person regularly moves in the room the sensor may be a portable sensor carried by said person. Optical gas sensors which could be used for the detection of HFC are known in the art. One example of such a gas sensor comprises an optical detector for detection at a wavelength of 3.4 pm. At this wavelength, the HC binding in HFC has an absorption peak. A drawback with detecting HFCs as 3.4 pm is that hydrocarbons may disturb the detection as hydrocarbons comprise the same binding. One way of solving this problem is to use a detector which can detect radiation only in the wavelength band above 5 pm and which specifically can detect radiation at 8.0 pm, as the FC binding has an absorption peak at 8.0 pm. Such detectors may be used to detect the FC binding in HFCs without the problem of detecting also HC bindings. However, detectors which are sensitive at 8.0 pm are usually not sensitive in terms of required light and requires a strong radiation source at 8.0 pm. In the prior art the radiation at 8.0 pm has been generated using for example micro hot plates, which have been heated to emit at and around 8.0 pm. A drawback with micro hot plates is that they are expensive if they are to have high intensity at 8.0 pm. Incandescent light bulbs are not used in the prior art gas sensors using far infrared, FIR, detectors. This is because far FIR radiation, from a filament in the light bulbs, is not transmitted through the glass of incandescent light bulbs.

[0007] SUMMARY OF THE INVENTION

[0008] An objective of the present disclosure is to provide a gas sensor, which alleviates at least one of the problems with prior art gas sensors.

[0009] Another objective of the present invention is to provide a gas sensor which may detect a component in gas using an absorption peak of the component in the mid infrared, MIR, wavelength band, as well as an absorption peak of the component in the far infrared, FIR, wavelength band, and which may be produced at a lower cost than prior art gas sensors.

[0010] At least one of these objectives is fulfilled with a gas sensor according to the independent claim.

[0011] Additional advantages are provided with the features of the dependent claims. According to a first aspect, a gas sensor is provided. The gas sensor comprises a gas absorption cell configured to house a gas to be detected. The gas sensor comprises an incandescent light bulb comprising a glass bulb and a filament enclosed in the glass bulb. The gas sensor comprises a first detector, configured to detect radiation in a first wavelength range of 1-5 urn, and a control unit, configured to provide a drive current through the filament of the incandescent light bulb during operation of the gas sensor. The gas absorption cell, the incandescent light bulb, and the first detector are arranged such that the first detector is illuminated by radiation from the incandescent light bulb during operation of the gas sensor, which radiation has been transmitted through the gas absorption cell. The gas sensor is characterised in that it comprises a second detector, configured to detect radiation in a second wavelength range of 6-15 pm, wherein the second detector is arranged such that the second detector is illuminated by radiation from the incandescent light bulb during operation of the gas sensor.

[0012] The incandescent light bulb may be the only light source of the gas sensor. Preferably, the incandescent light bulb is the only light source of the gas sensor.

[0013] By driving a current through the filament of the incandescent light bulb the filament is heated such that radiation is emitted. The filament of the incandescent light bulb radiates IR radiation similar to a black body radiator. When the filament is sufficiently hot it will emit IR radiation with a peak in the first wavelength range. The current through the filament may be sufficiently strong to induce IR radiation from the filament with a peak in the first wavelength range.

[0014] With a gas sensor according to the first aspect the first detector detects light in the first wavelength range. Such an arrangement of an incandescent light bulb and a detector is known from the prior art. The gas sensor may be configured for detection in a certain wavelength range with the first detector. Such configuration may be done in many different ways. One option is to configure the first detector for detection only in the certain wavelength range. Another option is to arrange an optical filter in front of the first detector such that only light which has passed the optical filter reaches the first detector.

[0015] The arrangement of the second detector enables detection of light in the wavelength range of 6-15 pm. The gas sensor may be configured for detection in a certain wavelength range with the first detector. Such configuration may be done in many different ways. One option is to configure the first detector for detection only in the certain wavelength range. Another option is to arrange an optical filter in front of the first detector such that only light which has passed the optical filter reaches the first detector. The inventors have realized that the glass bulb in the incandescent light bulb is heated by the radiation from the filament and that FIR radiation is emitted from the heated glass bulb. The drive current should be sufficiently strong and the second detector should be sufficiently sensitive to be able to detect radiation in the second wavelength range emitted from the glass bulb. This is fulfilled during normal operation of the incandescent light bulb. However, the intensity of the FIR radiation from the glass bulb is not sufficiently strong for detection of low concentrations of the target gas.

[0016] The main objective with the gas sensor according to the first aspect is to be able to detect the target gas from a concentrated gas source in the meaning of geometrical extension of the gas source. The gas source from which the gas to be detected originates typically has a geometrical extension comparable to the size of the gas sensor or smaller.

[0017] One example of a target gas is HFC as was mentioned in the description of background art. A common situation where HFC detection is desired is when a leak in a cooling equipment is to be detected. HC should normally not be present around a cooling equipment. The detection of HC around a cooling equipment is therefore a strong indicator of a possible leak of HFC. The first detector may be used to detect the HC binding in HFC by detection at 3.4 pm. When HC is detected with the gas sensor, the gas sensor may be moved around to find the possible leak. When the gas sensor is sufficiently close to a leak the concentration of HFC might be sufficiently high so that the FC binding may be detected with the second detector. The FC binding has an absorption peak at 8.0 pm. The leak of HFC is a concentrated gas source in that a leak is usually a crack in a pipe or similar. This means that the scale of the gas source is usually on the order of millimetres. The size of the gas sensor is in the order of centimetres.

[0018] Another example is the detection of HC bindings. The detection of HC bindings is of interest in health studies through human breath analysis. The HC binding may be detected with the sensor according to the first aspect described above. The sensor can be used to check if there is an abnormal amount of hydrocarbons in the breath. The first detector may be used to detect the HC binding in HFC by detection at 3.4 pm. Two Far-IR channels at 8.3 urn and 11 urn wavelength can be used to distinguish if a detected deviation origins from an access of acetone or isoprene, both indicators of different diseases related to lung- and metabolic issues. Presence of acetone may be detected at low concentrations with the second detector at 8.3 pm. A third detector may be used for detection of the absorption peak at 11 pm for isoprene. The gas source in this example is the mouth, which has a size on the order of centimetres.

[0019] In both of the above-described examples, the gas sensor might be relatively insensitive for detection in the FIR range. The gas sensor according to the first aspect is less expensive than the FIR gas sensors according to the prior art.

[0020] A problem with many FIR detectors is that they are prone to drift. To avoid this problem the control unit may be configured to provide a drive current as an alternating current, AC. With such a drive current the heating of the glass bulb will vary and thereby the intensity of the FIR. This will provide a base level for the FIR radiation and improve the measurements.

[0021] The control unit may be configured to provide the drive current with a frequency in the range of 0.01 Hz to 10 Hz, most preferably in the range of 0.1 Hz to 5 Hz, and most preferred in the range of 0.1 Hz to 1 Hz. The drive frequency should be sufficiently low to allow the glass bulb to cool between the peaks in the drive current, but sufficiently high to avoid drift problems.

[0022] The glass bulb of the incandescent light bulb will radiate FIR radiation similar to a black body radiator. The intensity of the radiation from a black body radiator is strongly dependent on the temperature. This means that a temperature variation of a certain magnitude will affect the intensity more if the variation is at a higher peak temperature. The control unit may be configured to provide the drive current with a magnitude such that the peak temperature of the glass bulb during operation is at least 50°C, preferably at least 80°C and most preferred at least 100°C. At 100°C, the intensity of the FIR radiation and the variation in FIR radiation is considerably higher than at 50°C.

[0023] The gas sensor may also comprise a first optical filter configured between the gas absorption cell and the first detector, such that the radiation from the incandescent light bulb that reaches the first detector has been transmitted through the first interference filter. This is an efficient way of filtering out the relevant wavelengths, which are to be detected, and improves the signa Ito-noise ratio. The first optical filter may be an interference filter. Interference filters may be configured to filter out a very narrow wavelength band.

[0024] The gas sensor may also comprise a second optical filter configured between the gas absorption cell and the second detector, such that the radiation from the incandescent light bulb that reaches second detector has been transmitted through the second interference filter. This is an efficient way of filtering out the relevant wavelengths, which are to be detected, and improves the signal-to-noise ratio. The second optical filter must be made of a material, which transmits FIR radiation.

[0025] The second optical filter may be an interference filter. Interference filters may be configured to filter out a very narrow wavelength band.

[0026] The gas absorption cell may be arranged as a White cell. By configuring the gas absorption cell as a White cell it is possible to provide a very long absorption path length in a very compact gas sensor. A White cell is a multipass absorption cell. In a White cell the arrangement of opposing reflective surfaces is specified.

[0027] The detectors should preferably be low cost sensors. At least one of the first detector and the second detector may be a photodiode. Photodiodes may be small and may be produced at a low cost.

[0028] Preferably, the light bulb is a miniature or sub-miniature lamp. Such lamps are characterised by their dimensions. The incandescent light bulb may have a length of 3-30 mm, preferably 4-15 mm. The incandescent light bulb may have a width, which is smaller than the length. The defined dimensions are typical for miniature or sub-miniature lamps. It is preferable to have a light bulb with as thin glass as possible as the thinner glass will enable the temperature of the glass to be varied more quickly. The thickness of the glass in the light bulb is preferably less than 0.5 mm and most preferred less than 0.2 mm.

[0029] The gas sensor may comprise a circuit board. The incandescent light bulb may have two connection pins, which electrically and mechanically connects the light bulb to the circuit board. Such a connection enables the light bulb to be arranged at a distance from the circuit board. This enables the minimization of the heat transfer from the light bulb to the circuit board. The length of the connection pins between the incandescent light bulb and the circuit board may be 15-50 mm to minimize the heat transfer.

[0030] As the modulation efficiency is increased at higher temperatures, it is advantageous to have the light bulb at a high temperature. It is preferable to keep the current to the light bulb low. In order to meet these contradictory demands the gas sensor may comprise a heater configured for heating of the circuit board. To maximize the heat transfer the light bulb should be arranged close to the circuit board.

[0031] The gas absorption cell is in fluid communication with the environment surrounding the gas sensor.

[0032] In the following preferred embodiments will be described with reference to the appended drawings.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 shows schematically a gas sensor according to a first embodiment.

[0035] Figure 2 shows schematically a gas sensor according to a second embodiment.

[0036] Figure 3 shows schematically the component reflector unit of the gas sensor in Figure 2, in a view towards the component reflector unit from the main reflector unit.

[0037] Figure 4 shows the radiance as a function of the wavelength for three different temperatures of the light source.

[0038] Figure 5 shows the modulation efficiency as a function of the wavelength for three different temperatures of the light source.

[0039] Figure 6a-b show an incandescent light bulb connected to a circuit board. DETAILED DESCRIPTION

[0040] In the following description of preferred embodiments, the same reference numeral will be used for the same feature in the different drawings. The drawings are not drawn to scale.

[0041] Figure 1 shows schematically a gas sensor 100 according to a first embodiment. The gas sensor 100 comprises an incandescent light bulb 1 comprising a glass bulb 2 and a filament 3 enclosed in the glass bulb 2. The gas sensor 100 comprises a first detector 4, configured to detect radiation in a first wavelength range of 1-5 urn, and a second detector 5, configured to detect radiation in a second wavelength range of 6-15 pm radiation. The space between the incandescent light bulb 1 and the detectors 4, 5, comprises a gas to be detected and constitutes a gas absorption cell 6, configured to house a gas to be detected. As shown by the dashed line 32 in the embodiment shown in Figure 1 the gas absorption cell may be enclosed by walls 32. The gas to be detected may flow into the gas absorption cell through an opening 33. Alternatively, the walls 32 may be omitted and the gas absorption cell 6 may be in free communication with the environment surrounding the gas sensor 100. It is of course possible to partly or fully enclose the gas absorption cell 6 as will be described below. The gas sensor 100 also comprises a control unit 7, configured to provide a drive current through the incandescent light bulb during operation of the gas sensor 100, such that radiation in the first wavelength range is emitted from the filament 3. The gas absorption cell 6, the incandescent light bulb 1, and the first detector 4 are arranged such that the first detector 4 is illuminated by radiation from the incandescent light bulb 1 during operation of the gas sensor 100, which radiation has been transmitted through the gas absorption cell 6. The second detector 5 is arranged such that the second detector 5 is illuminated by radiation from the incandescent light bulb 1 during operation of the gas sensor 100. In the embodiment of Figure 1 the control unit is also configured receive the signals from the first detector 4 and the second detector 5.

[0042] The second detector 5 detects light in the second wavelength range of 6-15 pm. Radiation in this wavelength region is not transmitted through glass. The radiation in the second wavelength region is emitted from the glass in the glass bulb 2. The emission of radiation in the second wavelength range is black body radiation from the glass bulb due to heating of the glass by the radiation from the filament. The intensity of black body radiation in the second wavelength range is strongly temperature dependent. Low cost detectors for the wavelength range of 6-15 ^m, typically, have a low sensitivity. To achieve a sufficiently strong signal from the second detector 5 it is preferable to have a high temperature on the glass bulb 2. The temperature of the glass is controlled by the drive current from the control unit. The magnitude of a sufficient drive current may be obtained through trial and error during design of the gas sensor. Apart from the drive current, the temperature of the glass bulb depends on many factors such as the physical design of the incandescent light bulb, i.e., dimensions of filament 3 and dimensions of glass bulb. The temperature is also dependent on the physical design of the gas sensor in the vicinity of the incandescent light bulb such as, e.g., distance to enclosing walls.

[0043] A common problem with low cost detectors for radiation in the second wavelength range of 6- 15 pm is drift of the output signal, i.e., the output signal varies even if the intensity of the radiation incident on the detector is constant. One solution to avoid this problem is to have a periodically varying intensity of the light from the glass bulb. The periods with low intensity may be used as a base level and the periods with high intensity may be used for the measurement. The intensity in the second wavelength range is dependent on the temperature of the glass bulb 2, which in turn is dependent on the intensity of the radiation from the filament 3. Thus, the intensity of the radiation in the second wavelength range may be varied by varying the drive current as an alternating current, AC. When the intensity of the radiation from the filament 3 is low, the temperature of the glass bulb 2 decreases. The cooling of the glass bulb is due to radiation and possibly also due to convection cooling. The frequency of the drive current should be sufficiently low to allow the glass bulb to cool when the drive current is low, but high enough to avoid drifting. In the described embodiment, the frequency of the drive current is in the range of 0.01 Hz to 10 Hz, most preferably in the range of 0.1 Hz to 5 Hz, and most preferred in the range of 0.1 Hz to 1 Hz. It is preferred to have zero drive current when the drive current is low. The alternating drive current could be a sinus wave, which varies between zero and a top value.

[0044] As said above the intensity of the black body radiation from the glass bulb is strongly dependent on the temperature. The control unit is configured to provide a drive current which has a magnitude such that the peak temperature of the glass bulb during operation is at least 50°C, preferably at least 80°C and most preferred at least 100°C. If the glass bulb is allowed to cool 10°C degrees between the peaks in drive current, this decrease in temperature will result in a larger decrease in radiation intensity the higher the peak temperature is. In other words, a temperature variation between 90°C and 100°C will result in a larger variation in radiation intensity than a temperature variation between 70°C and 80°C. Also, the time for cooling from 100°C to 90°C is shorter than the time for cooling from 80°C to 70°C. Thus, a higher peak temperature allows for a higher frequency of the drive current and provides a larger variation in radiation intensity. The peak temperature must be kept sufficiently low to avoid damage to other components in the gas sensor.

[0045] The first detector and / or the second detector may be a photodiode. The first detector 4 and the second detector 5 are sensitive in different wavelength ranges as described above. However, to be able to filter out more specific wavelengths some additional filtering is required. Such additional filtering may be integrated in the detectors. Alternatively, the gas sensor 100 may comprise an interference filters. In the embodiment in Figure 1, an optional interference filter 8 is arranged between the gas absorption cell 6 and the first detector 4, such that the radiation from the incandescent light bulb 1 that reaches the first detector 4 has been transmitted through the first interference filter 8. Similarly, a second interference filter 9 may be arranged between the gas absorption cell 6 and the second detector 5. The second interference filter 9 must be made of a material, which is transparent to the radiation in the second wavelength band.

[0046] Figure 2 shows a gas sensor 100 according to a second embodiment. The gas sensor 100 is arranged in a White cell configuration. The gas sensor 100 comprises a component reflector unit 13 defining a concave first surface 14 with a first centre of curvature 15 and comprising at least one first reflector 16. The incandescent light bulb 1 is arranged in an opening 24 in the first reflector 16. In the embodiment of Figure 2 the first reflector 16 is a spherical reflector in which a first interference filter 8, a second interference filter 9, and a third interference filter 11 are arranged. A first detector 4 is arranged behind the first interference filter 8 such that the first detector 4 is illuminated by light that has been transmitted through the first interference filter 8. A second detector 5 is arranged behind the second interference filter 9 such that the second detector 5 is illuminated by light that has been transmitted through the second interference filter 9. A third optional detector 10 is arranged behind the third interference filter 11 such that the third detector 10 is illuminated by light that has been transmitted through the third interference filter 11. In the embodiment of Figure 2 the interference filters 8, 9, 11, are planar filters. The light that has been transmitted through the interference filters 8, 9, 11, is focussed by the conical mirrors 12.

[0047] The centres of the interference filters 8, 9, 11, are essentially on the concave first surface 14, i.e., the normal to the surface of the interference filters 8, 9, 11, at the centre is directed towards the centre of curvature of the concave first surface 14. It is not necessary that the concave first surface 14 is a perfect spherical surface. It is possible that the concave first surface 14 deviates slightly from a perfect spherical surface. The gas sensor of Figure 2 also comprises a main reflector unit 17 comprising a second reflector 18 with a concave second surface 19 defining a second centre of curvature 20 and a third reflector 21 with a concave third surface 22 defining a third centre of curvature 23. The concave first surface 14, the concave second surface 19, and the concave third surface 22 are reflective surfaces. The second reflector 18 and the third reflector 20 are facing the component reflector unit 13. The gas absorption cell 6 is between the component reflector unit 13 and the main reflector unit 17. Additional walls (not shown) parallel to the plane of the Figure may also enclose the gas absorption cell 6. At least one opening into the gas absorption cell 6 is provided, for example by omitting one or more of the walls enclosing the gas absorption cell 6.

[0048] Figure 3 shows the component reflector 13 unit in a view towards the component reflector unit from the main reflector unit 17. The component reflector unit 13, the second reflector 18, and the third reflector 9, are arranged in a White cell configuration with their respective centres of curvature 15, 20, 23, defining a longitudinal optical plane C extending through the component reflector unit 13, the second reflector 18, and the third reflector 21. As can be seen in Figure 2 the centre of curvature of the concave first surface 14, i.e., the surface of the concave first reflector 16, is at the surface of the second reflector 18 and the third reflector 21, at the point where the second reflector 18 is in contact with the third reflector 21.

[0049] Similarly, the centre of curvature 20 of the concave second surface 19, i.e., the surface of the second reflector 18, is at the concave first surface 14. The centre of curvature 23 of the concave third surface 22, i.e., the surface of the third reflector 21, is at the concave first surface 14. It is not necessary that the concave first surface 14, the concave second surface 19, and the concave third surface 22 are spherical surfaces. A small deviation from a spherical surface is acceptable. In case the concave first surface 14, the concave second surface 18, and the concave third surface 21, are not spherical surfaces their radii of curvature varies with no more than 10% over their surfaces, preferably by no more than 5% and most preferred by no more than 1%.

[0050] In Figure 3, the incandescent light bulb 1 is shown in the opening 24. The area of the incandescent light bulb that emits light is schematically shown as the area SO. As can be seen in Figure 3 the incandescent light bulb is arranged above the longitudinal optical plane C. The light from the incandescent light bulb 1 is emitted towards the reflective second concave surface 19 as is indicated by the dotted lines 24 in Figure 2. The light is reflected in the second concave surface towards the first concave surface 14 as is shown by the dash-dotted lines 25. The radiation is focused on the first concave surface as an image on the first reflector 14 as a first light spot SI as is shown in Figure 3. The light from the first light spot is reflected towards the third concave surface 22 as is shown by the dashed lines 26. The light us then reflected towards the first interference filter 8 along the dashed-double-dot lines T1 to a second light spot S2 as is shown in Figure 3. The light that is not transmitted through the first interference filter 8 will be reflected in the main reflector unit 17 to a third light spot S3 below the longitudinal optical plane C and to a fourth light spot S4 above the longitudinal optical plane C as is shown in Figure 3. The fourth light spot S4 is on the second interference filter 9. The light that is not transmitted through the second interference filter 9 will be reflected in the main reflector unit 17 to a fifth light spot S5 below the longitudinal optical plane C and to a sixth light spot S6 above the longitudinal optical plane C as is shown in Figure 3. The sixth light spot S6 is on the third interference filter 11.

[0051] In contrast to gas sensor according to the embodiment of Figure 1, the gas sensor according to the embodiment of Figure 2 and Figure 3 comprises three optical filter in the form of interference filters 8, 9, 11, and three detectors 4, 5, 10. To increase the signal from the detector for FIR radiation it is favourable to have the detector for FIR radiation as the second detector 5 or the third detector 10. In case the gas sensor is to detect isoprene and acetone, the first interference filter may be configured to transmit radiation with a wavelength of 3.4 pm for detection of the HC binding, and the second interference filter may be configured to transmit radiation with a wavelength of 8.3 pm for detection of acetone. The third interference filter 11 may be configured to transmit radiation with a wavelength of 11 pm for detection of isoprene. In case the gas sensor 100 is to detect HFC the first interference filter 8 may be configured to transmit 3.4 ^m for detection of the HC binding, and the second interference filter 9 may be configured to transmit 8.0pm for detection of the FC binding. The third interference filter 11 may be configured to transmit an optional wavelength for detection of another gas.

[0052] In a gas sensor according to the embodiment of Figure 2 and 3 the distance from the component reflector unit to each one of the second reflector and the third reflector does not have to be equal to the radius of curvature of the component reflector unit 13, the second reflector 18 and the third reflector 21, but a small deviation is possible while still providing the desired light spots. The radius of curvature of the second reflector and the third reflector may for example be 85-115 % of the distance from the component reflector unit to each one of the second reflector and the third reflector, and still provide the desired light spots. This is true at least for a limited number of passes in the multipass absorption cell. The quality of subsequent light spots is improved if the radius of curvature of the first concave surface 14, the second concave surface 19 and the third concave surface is in the interval 95-105 % of the distance from the component reflector unit to each one of the second reflector and the third reflector. It is most preferred that the radius of curvature of the first concave surface 14, the second concave surface 19 and the third concave surface 22 is in the interval 99-101 % of the distance from the component reflector unit 13 to each one of the second reflector 18 and the third reflector 21.

[0053] In Figure 2, the gas absorption cell is in fluid communication with the environment surrounding the gas sensor. This may be the through the side 28.

[0054] The gas sensor 100 only comprises one light source in the form of the incandescent light bulb 1.

[0055] Figure 4 shows the radiance, in milliwatt / cm2 / pm, as a function of the wavelength, in micrometres, for three different temperatures of the light source, i.e., the glass of the incandescent light bulb. The dash dotted line is for 40°C, the dashed line is for 70°C, and the solid line is for 100°C. Figure 5 shows the modulation efficiency, in microwatt / cm2 / pm / K, as a function of the wavelength, in micrometres, for three different temperatures of the light source. The dash dotted line is for 40°C, the dashed line is for 70°C, and the solid line is for 100°C. As can be seen from Figure 5, the modulation efficiency increases with temperature. Figure 6a and 6b shows the incandescent light bulb 1 mounted on a circuit board 29. Two pins 30 electrically and mechanically connects the light bulb 1 with the circuit board 29. Drive electronics (not shown) may be arranged on the circuit board 29. The light bulb is typically a miniature or sub-miniature lamp. The length of the light bulb LI is typically 3-30 mm, preferably 4-15 mm. The incandescent light bulb typically has a width W which is smaller than the length.

[0056] To allow the temperature of the light bulb to vary rapidly, the thickness of the glass in the light bulb should be kept small. Preferably, the thickness of the glass in the light bulb is less than 0.5 mm and preferably less than 0.2 mm.

[0057] The temperature of the light bulb should be limited to avoid excessive heating of the parts of the sensor surrounding the light bulb. To further reduce the heat transfer from the light bulb to the circuit board the connection pins 30 may be made long as shown in Figure 6a. The length L2 of the connection pins may be at least 15 mm, preferably at least 40 mm.

[0058] According to another aspect, the current to the light bulb should be kept as low as possible. To maximize the modulation efficiency while minimizing the current to the light bulb the gas sensor may comprise a heater 31 configured for heating of the circuit board 29, as is shown in Figure 6b. In this embodiment the length L2' of the connection pins should be less than 15 mm, preferably less than 5 mm.

[0059] The described embodiments may be amended in many ways without departing from the scope of the invention, which is limited only by the appended claims.

Claims

CLAIMS1. A gas sensor (100) comprising:- a gas absorption cell (6), configured to house a gas to be detected,- an incandescent light bulb (1) comprising a glass bulb (2) and a filament (3) enclosed in the glass bulb (2),- a first detector (4), configured to detect radiation in a first wavelength range of 1-5 urn, and- a control unit (7), configured to provide a drive current through the filament (3) of the incandescent light bulb (1) during operation of the gas sensor (100), wherein the gas absorption cell (6), the incandescent light bulb (1), and the first detector (4) are arranged such that the first detector (4) is illuminated by radiation from the incandescent light bulb (1) during operation of the gas sensor (100), which radiation has been transmitted through the gas absorption cell (6), characterised in that it comprises- a second detector (5), configured to detect radiation in a second wavelength range of 6-15 pm radiation, wherein the second detector (5) is arranged such that the second detector (5) is illuminated by radiation from the incandescent light bulb (1) during operation of the gas sensor (100); wherein the control unit (7) is configured to provide the drive current with a magnitude such that the peak temperature of the glass bulb (2) during operation is at least 40°C.

2. The gas sensor (100) according to claim 1, wherein the control unit (7) is configured to provide a drive current as an alternating current, AC.

3. The gas sensor (100) according to claim 2, wherein the control unit (7) is configured to provide the drive current with a frequency is in the range of 0.01 Hz to 10 Hz, most preferably in the range of 0.1 Hz to 5 Hz, and most preferred in the range of 0.1 Hz to 1 Hz.

4. The gas sensor (100) according to anyone of claims 1-3, wherein control unit (7) is configured to provide the drive current with a magnitude such that the peak temperature of the glass bulb (2) during operation is at least 70°C and most preferred at least 100°C.

5. The gas sensor (100) according to anyone of claims 1-4, also comprising a first optical filter(8) configured between the gas absorption cell (6) and the first detector (4), such that the radiation from the incandescent light bulb that reaches first detector (4) has been transmitted through the first optical filter (8).

6. The gas sensor (100) according to claim 5, wherein the first optical filter (8) is a first interference filter (8)7. The gas sensor (100) according to anyone of claims 1-6, also comprising a second optical filter(9) configured between the gas absorption cell (6) and the first detector (4), such that the radiation from the incandescent light bulb that reaches first detector (4) has been transmitted through the second optical filter (9).

8. The gas sensor (100) according to claim 7, wherein the second optical filter (8) is a second interference filter (8)9. The gas sensor (100) according to anyone of the preceding claims, wherein the gas absorption cell (6) is arranged as a White cell.

10. The gas sensor (100) according to anyone of the preceding claims, wherein at least one of the first detector (4) and the second detector (5) is a photodiode.

11. The gas sensor (100) according to anyone of the preceding claims, wherein the gas absorption cell is in fluid communication with the environment surrounding the gas sensor.

12. The gas sensor (100) according to anyone of the preceding claims, wherein the gas sensor (100) only comprises one light source in the form of the incandescent light bulb (1).

13. The gas sensor (100) according to claim 12, wherein the incandescent light bulb (1) has a length of 3-30 mm, preferably 4-15 mm.

14. The gas sensor (100) according to claim 13, wherein the incandescent light bulb (1) has a width, which is smaller than the length.

15. The gas sensor (100) according to claim 12, 13, or 14, wherein the gas sensor (100) comprises a circuit board (29) and wherein the incandescent light bulb (1) has two connection pins, which electrically and mechanically connects the light bulb (1) to circuit board (29).

16. The gas sensor (100) according to claim 15, wherein the gas sensor comprises a heater (31) configured for heating of the circuit board (29).

17. The gas sensor (100) according to claim 15, wherein the length of the connection pins between the incandescent light bulb (1) and the circuit board (29) is 15-50 mm.

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