Assembly and method for measuring a substance concentration in a gaseous medium by means of absorption spectroscopy

Abstract

An assembly and a method for measuring a gas concentration by means of absorption spectroscopy, in particular for capnometric measurement of the proportion of CO.sub.2 in breathing air in which IR light from a thermal light source is guided through a measuring cell with a gas mixture to be analyzed, and the concentration of the gas to be measured that is contained in the gas mixture is determined by measuring an attenuation of the light introduced into the measuring cell caused by absorption by the gas to be measured. The thermal light source is designed as an encapsulated micro-incandescent lamp with a light-generating coil.

Claims

1. An assembly for measuring a gas concentration by means of absorption spectroscopy, the assembly comprising; an IR light emitting thermal light source, a measuring cell with a gas mixture to be analyzed, the measuring cell having a gas inlet and a gas outlet configured flor flowing the measuring gas therebetween, the measuring cell defining a measuring path in which the IR light crosses the gas to be measured, one or more sensors, one or more bandpass filters upstream from the one or more sensors, an optical beam path comprising the thermal light source, the measuring cell including the measuring path, the one or more bandpass filters and the one or more sensors. a gas concentration measuring evaluation apparatus in communication with the one or more sensors to determine a concentration of the measuring gas as a result of attenuation of the IR light in the measuring cell, wherein the one or more bandpass filters comprises at least one measuring wavelength bandpass filter configured to transmit the IR light within a measuring wavelength range in which the gas to be measured absorbs IR light, and at least one reference wavelength bandpass filter configured to transmit IR light in a reference wavelength range in which the gas to be measured does not absorb IR light or only absorbs a slight amount in comparison to the measuring wavelength range, further wherein the thermal light source comprises an encapsulated micro-incandescent lamp with a light-generating coil disposed is in a substantially transparent capsule, the capsule being evacuated or filled with an inert gas, and wherein the measuring gas concentration is capnometric measurement of the proportion of CO.sub.2 in breathing air.

2. The assembly according to claim 1, wherein the encapsulation of the micro-incandescent lamp has a diameter of less than 2 mm, less than 1.5 mm, or less than 1 mm.

3. The assembly according to claim 1, wherein a greatest linear distance between two points of the coil is less than 1 mm, or less than 0.5 mm.

4. The assembly according to claim 1, wherein an envelope of the coil in a direction of projection in which the envelope assumes a maximum envelope projection surface has a maximum envelope projection surface of less than 0.1 mm.sup.2, or less than 0.02 mm.sup.2.

5. The assembly according to claim 1, wherein at least one sensor is an infrared-sensitive photodiode comprising a sensitive surface of less than 1 mm.sup.2, or less than 0.15 mm.sup.2.

6. The assembly according to claim 1, further comprising a control apparatus for power-controlled driving and/or modulation of the micro-incandescent lamp, wherein the control apparatus forms a product from current measured at the micro-incandescent lamp and measured voltage in order to determine an actual value of the emitted power, and/or is signal-linked to a photodiode arranged in the micro-incandescent lamp that receives a part of the light generated by the micro-incandescent lamp.

7. The assembly according to claim 1, wherein the IR light is guided bundled through the measuring cell and distributed to two or more sensors after passing through the measuring cell by means of a spectrally neutral optical plane-parallel or curved transmission or reflection lattice, wherein the transmission lattice or reflection lattice has a lattice constant that is less by a factor of 30 or more, or by a factor of 50 and more than a diameter of a light spot on the transmission and reflection lattice.

8. The assembly according to claim 1, wherein the measuring cell comprises a tube, the inside of which is diffuse or has a high-gloss reflection on one end of which the micro-incandescent lamp is arranged, and on the other end of which the sensor or sensors with the upstream bandpass filters are arranged.

9. The assembly according to claim 1, wherein the at least one bandpass filter is designed as a double bandpass filter that lets IR light pass through both in the measuring wavelength range as well as in the reference wavelength range, wherein the double bandpass filter is upstream from an individual sensor, wherein the control apparatus is designed and configured to modulate the micro-incandescent lamp between an operating point with a lower output and an operating point with a higher output in which the respective emission spectrum has different component ratios in the measuring wavelength range and in the reference wavelength range.

10. The assembly according to claim 1, further comprising a pump and/or one or more switchable valves that are configured for temporarily increasing the pressure and/or reducing the pressure of the gas mixture in the measuring cell.

11. A method for measuring a gas concentration by means of absorption spectroscopy according to claim 1 in which the IR light is guided from the thermal light source through the measuring cell with the gas mixture to be analyzed, and the gas concentration of the gas to be measured that is contained in the gas mixture is determined by measuring the attenuation of the IR light introduced into the measuring cell caused by absorption by the gas to be measured, wherein the thermal light source is designed as an encapsulated micro-incandescent lamp with a light-generating coil, wherein a sensor or several sensors are designed as infrared-sensitive photodiodes with a sensitive surface that is less than 1 mm.sup.2, or less than 0.15 mm.sup.2.

12. The method according to claim 11, wherein the micro-incandescent lamp is modulated with a measurement repetition frequency f.sub.Mess that is greater than 10 Hz or greater than 25 Hz, wherein a temperature of the coil is greater than 400° C. during measurement, and has a temperature modulation rise of at least 300° C., or at least 500° C., or exceeds 1000° C. at a maximum.

13. The method according to claim 11, wherein the micro-incandescent lamp is operated with power control.

14. The method according to claim 11, wherein over the course of measuring, the gas mixture pressure in the measuring cell is increased and/or lowered sequentially over intervals in time and the absorption is measured depending on the pressure, wherein to change the pressure, in particular, an outflow of the gas mixture is interrupted, and/or an inflow or an outflow of the gas mixture is supported and increased by a pump.

15. The method for measuring a gas concentration by means of absorption spectroscopy according to claim 11, in the assembly in which the IR light is conducted from the thermal light source through the measuring cell with the gas mixture to be analyzed, and the gas concentrations of the gas to be measured that is contained in the gas mixture is determined by measuring an attenuation of the light introduced into the measuring cell caused by absorption by the gas to be measured, wherein over the course of measuring, the gas mixture pressure in the measuring cell is increased and/or lowered sequentially over intervals in time, or fluctuations in the gas mixture pressure are measured, and the absorption is measured depending on the pressure, wherein a pressure-dependent measuring series is analyzed with respect to components that are linearly and nonlinearly dependent on the pressure, and the component that is nonlinearly dependent on the pressure is used to measure the gas concentration of the gas to be measured, or to correct and/or calibrate a measurement of the gas concentration of the gas to be measured.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described below, without restricting the general idea of the invention, based on exemplary embodiments in reference to the drawings, whereby we expressly refer to the drawings with regard to the disclosure of all details according to the invention that are not explained in greater detail in the text. In the following:

(2) FIG. 1 shows a schematic depiction of a first exemplary embodiment of an apparatus according to the invention for the side stream,

(3) FIG. 2 a)-c) show schematic diagrams of beam paths for capnometry

(4) FIG. 3 a), b) show schematic depictions of apparatuses according to the invention with optically neutral transmission lattices,

(5) FIG. 4 a)-f) show schematic depictions of optically neutral reflection lattices that can be used according to the invention,

(6) FIG. 5 shows a schematic depiction of a second exemplary embodiment of an apparatus according to the invention,

(7) FIG. 6 shows spectral characteristics of two operating points of a light source as well as a filter and absorption characteristic for a capnometry application,

(8) FIG. 7 shows a schematic depiction of a third exemplary embodiment of an apparatus according to the invention,

(9) FIG. 8 a), b) shows schematic depictions of power control circuits that can be used according to the invention,

(10) FIG. 9 shows a schematic depiction of a fourth exemplary embodiment of an apparatus according to the invention,

(11) FIG. 10 shows a depiction of high-resolution spectral characteristics of the absorption band of CO.sub.2 and a bandpass filter that can be used according to the invention, and

(12) FIG. 11 a)-d) show a schematic depiction of a micro-incandescent lamp.

(13) In the drawings, the same or similar elements and/or parts are always provided with the same reference numbers; a reintroduction will therefore always be omitted.

DETAILED DESCRIPTION

(14) In FIG. 1, a first exemplary embodiment is schematically depicted of an assembly 10 according to the invention for the side stream. It can be a capnometer that comprises a measuring cell 12 which is supplied through a gas inlet 14 and a gas outlet 16 with a constant flow of breathing air by a pump (not depicted) that has been branched from a main flow of breathing gas of a patient. For this, the measuring cell 12 also has a gas inlet opening 15 and a gas outlet opening 17 on its opposing side. The gas outlet opening 17 is preferably arranged entirely around the measuring cell 12 so that the required cross-section of the gas outlet can be realized with a minimum distance between the measuring cell and filter. The measuring cell 12 is designed cylindrical with a diameter that is small relative to the length of the measuring cell 12. Accordingly, the volume of the measuring cell 12 is minimized relative to the available absorption length, i.e., the length of the measuring cell 12. Minimizing the volume has the advantage that the measuring gas only has a short retention time in the measuring cell 12, and therefore enables very precise, high-resolution measurement over time of the concentration of a target gas in the measuring gas.

(15) For measurement, the assembly 10 comprises a quasi-punctiform filament source in the form of a micro-incandescent lamp 20 that is arranged in an evacuated glass bulb. This makes it possible for the light source to shine at a very high power and very high temperature without negatively influencing its service life. The main portion of the emitted light lies in the infrared range for capnometry, in particular in the middle infrared range. The micro-incandescent lamp 20 lies within the focal point of a spherical or parabolic reflector 22 that renders the light beams largely parallel so that the light shines through the measuring cell 12 as evenly as possible. The measuring cell 12 can be reflective on the inside.

(16) An assembly consisting of two detectors 25, 27 with two upstream filters, 24, 26 and that are illuminated as evenly as possible by the light that shines through the measuring cell 12 is located at the output of the measuring cell 12. A filter 24 for a gas channel is located upstream of the detector 25 and has a narrow bandpass for the absorption bands of the target gas, whereas the filter 26 is designed as a bandpass filter for a reference channel where the target gas has no or only slight absorption. A control and evaluation unit is not depicted. The use of a quasi-punctiform light source in the form of a micro-incandescent lamp 20 makes it possible in this case to realize very high measuring precision with very little light loss, and also to achieve very fast and precise power and temperature control of the light source.

(17) Likewise, an evaluation apparatus 18 is symbolically depicted that receives signals from the detectors 25, 27 and ascertains the concentration of the target gas in the measuring cell 12 according to internal calculation rules, look-up tables, etc. and a corresponding calibration.

(18) For the sake of illustration, FIG. 2 a)-c) depict schematic diagrams of beam paths for capnometry. FIG. 2 a) shows the basic shape of a channel in which a quasi-punctiform infrared light source, in particular a micro-incandescent lamp 20, is arranged in the focal point of a parabolic reflector 22 that renders light beams emitted from the focal point of the reflector 22 a parallel light beam bundle. This parallel light beam bundle is in turn focused by a reflector 32, which is also designed as a parabolic reflector, onto the focal point of the reflector 32 in which a sensor 30 is arranged, which preferably also has a small volume, such as a photodiode. Only very little light is lost in this light transmission so that a good signal-to-noise ratio (SNR) can be achieved with a relatively low initial intensity.

(19) In contrast to this, the instance is depicted in FIG. 2 b) in which the light source 20′ is not quasi-punctiform. In this case, part of the light source 20′ lies outside of the focal point of the reflector 22 so that the emitted light is not entirely transmitted as a parallel light bundle to the opposing reflector 32, but rather passes the reflector 32 in a nonparallel manner and is accordingly lost. Because of this light loss, the amount and intensity of emitted light must be greater than in the instance according to the invention, which leads to inefficiency and greater power consumption.

(20) FIG. 2 c) shows another example in which the light is also generated in the same way as depicted in FIG. 2 a), however the parallel light bundle contacts a schematically depicted, spectrally neutral transmission lattice 40 that divides the light bundle without dispersive effects, i.e., spectrally neutral, into two different light bundles which are distributed by the corresponding filters 24, 26 for a gas channel and a reference channel 20 to two corresponding reflectors 32 and the corresponding sensors 25, 27 for a gas channel and a reference channel.

(21) FIG. 3 a), b) show schematic depictions of the optical conditions of apparatuses according to the invention with spectrally neutral transmission lattices. FIG. 3 a) shows an instance in which the light is generated as in FIGS. 2 a) and 2 c). After passing through the measuring cell 12, the parallel light bundle reaches a spectrally neutral transmission lattice 41 that also focuses by means of a corresponding curvature. In the shown embodiment, the parallel light bundle comes to be by focusing the spectrally neutral transmission lattice 41 on three different small-surface sensors 28, 28′, 28″ that for example can be designed for one reference channel and two different target gases with different absorption ranges, alternatively also for three different target gases, or also for one gas and two reference wavelengths which then enables dispersion compensation.

(22) In the portrayed case, the light passes through the measuring cell 12 perpendicular to the direction of flow of the gas mixture. This can be used both in a main stream as well as in a side stream. For use in a side stream, the coupling into, and respectively out of, the measuring cell 12 can however also be configured to be collinear with the main stream direction in the measuring cell 12.

(23) FIG. 4 a)-f) show schematic depictions of spectrally neutral reflection lattices that can be used according to the invention. The structure 50 from FIG. 4 a) shown in a cross-section serves to distribute the light to two detectors; the structures 51, 52 from FIGS. 4 b) and 4 c) serve to split the light toward three detectors. FIG. 4 d) shows a three-dimensional, specific depiction of a reflection lattice 53 that, by its gable roof structure, serves to distribute to two detectors similar to the structure 50 from FIG. 4 a). FIG. 4 e) shows a perspective depiction, and FIG. 4 f) shows a front, plan view of a surface structure of a reflection lattice 54 that is based on a pyramid structure with a hexagonal basic structure. This reflective lattice 54 is suitable for distributing the light to six detectors.

(24) FIG. 5 shows a schematic depiction of a second exemplary embodiment of an assembly 110 according to the invention with a left partial image in a side view and with a right part in a frontal plan view following plane A:A from the left partial image. The micro-incandescent lamp 20 is enclosed in a holder that has a parabolic reflector 22 which converts the emitted light into a parallel beam bundle and shines through the measuring cell 12 of the assembly 110 through which measuring gas flows from the left (in the direction of the arrow). This assembly therefore corresponds more to a measurement in a main flow; however, it can also be suitably used for a side stream measurement, or can be optimized for a side stream measurement by correspondingly coupling the light in and out in the measuring cell 12.

(25) After exiting the measuring cell 12, the light bundle is deflected 90° in the shown view by a reflection lattice 53 corresponding to FIG. 4 b), wherein in this case a group consisting of a filter 24 for the gas channel and a detector 25 for the gas channel of the target gas is depicted as the detector unit, wherein the small-surface detector 25 is arranged in the focal point of the parabolic reflector 32. To protect the individual components, three additional unidentified, optically transparent and spectrally neutral entry and exit windows are also depicted.

(26) In the right depiction in FIG. 5, it is discernible how the reflection lattice 53 divides the light bundle drawn as a circle into two different light bundles for two equivalently designed detector units for the absorption channel and the reference channel. The reference numbers correspond to those from FIG. 2 c) and FIG. 3 b).

(27) An advantage of this design in the event of contaminants in the optical path is also shown in FIG. 5. Interference 112 is formed in the measuring cell, for example a drop of condensation or a dirt particle that is arranged fixed or movable in the measuring cell 12. The reflection lattice 53 ensures that this interference is similarly in both channels, i.e., in the absorption channel and the reference channel, ensuring that such interference does not impair measurement. This can be seen in the right depiction in FIG. 5 where the images 114, 114′ of the interference 112 appear in both channels in the same manner and cause an attenuation of the signal in the same manner. Since the concentration is measured by the absorption, i.e., the comparison of intensities in the absorption channel and in the reference channel with each other, both channels are affected in the same way. The structure of the images 114, 114′ is a result of distributing all the light to both detectors 25, 27 by the gable roof structure of the reflection a lattice 53. Since both channels are affected, this input substantially cancels itself out without substantially impairing the relationship between the measured intensities.

(28) FIG. 6 shows spectral characteristics of a light source at two operating points as well as a filter and absorption characteristic for a capnometry application. The wavelength is depicted on the horizontal axis within a range of 1 to 5 μm, i.e., the infrared range; on the vertical axis, normalized values are depicted for the emission, absorption and transmission between 0 and 1. According to the legend at the bottom left, the CO.sub.2 absorption of a 5% concentration and an absorption length of 0.9 cm at about 4.26 μm are portrayed as a solid line. A range a) of a double bandpass filter is drawn around this absorption band that lets infrared light within a range of about 4.1 to 4.4 μm pass through so that the entire range that is absorbed in CO.sub.2 falls within this range a) of the double bandpass filter. A second range b) of the double bandpass filter serves as a reference filter that lets infrared light pass through approximately between 2.5 and 2.8 μm. Here, CO.sub.2 does not possess any absorption.

(29) The emission of a thermal emitter is depicted by a dot-dashed line and a dashed line at a first and second operating point (AP) at 1300 K and 450 K, respectively. At the higher temperature, the maximum of the light intensity is at about 2.25 μm, whereas the portrayed characteristic has not yet reached its maximum with a thermal emitter temperature of 450 K. It should be noted that the emitted power is also greater at a higher temperature so that the two emission spectra in the depiction in FIG. 6 have very different scaling factors from each other.

(30) It is clearly discernible that at the lower temperature at operating point 1, there is very little light intensity in reference band b), although there is much more intensity in absorption band a). At a higher operating point 2 at 1300 K, the light intensity in the reference band b) is however higher than in the absorption band a) so that, given a knowledge of the respective emission spectrum, a clear distinction between the components of the reference and the gas absorption can be made by means of a corresponding temperature modulation. This can be very precisely adjusted within the framework of a calibration.

(31) FIG. 7 shows a schematic depiction of a third exemplary embodiment of an assembly 210 according to the invention in a double bandpass assembly so that the spectrum conditions from FIG. 6 apply in this case. The optical assembly of one channel with which the measuring cell 12 is irradiated corresponds to the one from FIG. 2 a). This assembly can also be used in a main stream or in a side stream and, by suitably coupling in and coupling out the light in the measuring cell 12, the light can be guided in the direction of flow the measuring cell.

(32) In the instance shown in FIG. 7, the light emitted by the micro-incandescent lamp 20 is filtered after passing through the measuring cell 12 by a double bandpass filter 212 with the spectral characteristic from FIG. 6 and focused on the individual sensor 30. For this, a control apparatus 60 with a performance control is provided that very precisely controls the power output and temperature of the quasi-punctiform thermal emitter, i.e., the micro-incandescent lamp 20, in order to adjust the corresponding emission spectra from FIG. 6 to modulate very quickly, or suitably select other temperatures with the corresponding emission spectra.

(33) FIG. 8 a), b) schematically depict power control circuits according to the invention that can be used for this. A multiplication circuit is realized in FIG. 8 a) in which a momentary current and a momentary voltage drop over the micro-incandescent lamp 20 are measured by two amplifiers 68, 69, the current and voltage are multiplied with each other in a multiplier 70 and entered into a control unit 62 as an actual value that is compared with a target value, and controls the light source via a series resistor as an output value (Out). The measurement of current and resistance and the multiplication can be either analog or digital.

(34) Alternatively according to FIG. 8 b), the light power emitted by the light source 20 can also be measured by a photodiode 64 whose output current is in turn fed as an actual value via an amplifier 66 to the control unit 62 that then correspondingly regulates the power of the micro-incandescent lamp 20 to the variable target value.

(35) The solution according to FIG. 8 b) is simpler than that from FIG. 8 a), however it should be noted that a section of the spectrum of the light source is used for controlling the light source that does not serve to measure CO.sub.2, but is instead proportional thereto. In addition, it is useful to use a shortwave photodiode for control in comparison to the measuring wavelength, for example within the visual spectrum or in the near infrared since this enables much more stable control. These photodiodes normally have a spectrum with a narrower band in comparison to the emission spectrum of the micro-incandescent lamp 20 so that the target values must be correspondingly adapted. If applicable, nonlinearities should also be taken into account that arise from the overlap of the respective temperature-dependent emission spectrum with the sensitivity spectrum of the sensor.

(36) FIG. 9 shows a schematic depiction of a fourth exemplary embodiment of an assembly 310 according to the invention that can be linked to any of the above exemplary embodiments, i.e., to those exemplary embodiments that only have one optical channel as well as to those exemplary embodiments in which the light bundle is divided into two separate optical channels after passing through the measuring cell 12. Merely as an example, an assembly 10 according to the first exemplary embodiment from FIG. 1 has been selected as the basis for the assembly 310. A modification exists in that the gas inlet 14 is equipped with a check valve 312 which opens in the gas inlet direction and blocks against this direction. At the output for the measuring cell 12 and the gas outlet opening 17, a gas outlet 16 is attached that expands to a gas reservoir 316 and is equipped with a pressure gauge 314 to measure the gas pressure in the gas reservoir 316, wherein the measured pressure also corresponds to the pressure in the measuring cell 12.

(37) Furthermore, a pump 318 is provided that can pump gases either into the narrow gas reservoir 316 and the measuring cell 12, or can be operated in the reverse pump direction in order to conduct gas out of the measuring cell 12 and thereby increase the pressure in one pump direction and lower it in the opposite pump direction. To do this, a controller 320 for the pump 318 is provided that controls the pump rotor via a motor 322 that can be designed as an actuator, or directly, and influences the direction and/or the strength of the pump 318. With this arrangement 310, it is possible to perform a nonlinearity analysis depending on the pressure available in the measuring cell 12 as described above, for example according to formulas (8) and (9). Accordingly, it is inter alia possible to achieve a cyclically recurring pressure change in the measuring cell 12 that can be used as an independent analysis in order to check whether the calibration parameters of the underlying continuous measurement are still correct or must be adapted since the concentration of the target gas can be isolated from interfering sources with the assistance of this method. Since this nonlinearity measurement has the best precision at high target gas concentrations, it is preferable to undertake pressure modulation when for example, the end expiration value, typically about 5% CO.sub.2, has been reached during expiration.

(38) The underlying spectral characteristics for this analysis are depicted in FIG. 10 in high resolution. The wavelength range between 4.18 and 4.26 μm depicted on the horizontal axis corresponds to a narrow section from the spectrum range depicted in FIG. 6. The band structure of CO.sub.2 absorption can be clearly seen, in this case a depiction of transmission. In contrast to the double bandpass filter from FIG. 6, a narrower bandpass filter is used in this case that cuts off the edge regions of the absorption of CO.sub.2 and is thus comparatively slightly narrower than the CO.sub.2 absorption band. A bandwidth that is too small would undesirably reduce the available light intensity; however, very strong nonlinearity in the pressure dependency is ensured with the shown configuration. The intensity arriving at the detector results, from a mathematical perspective, from an overlapping of the filter function with the wavelength-dependent function of the target gas absorption coefficient with its band structure. This overlap causes a nonlinear dependency of the detectable transmission on the pressure, or respectively the concentration of the target gas.

(39) Also drawn is an interfering gas absorption coefficient that is basically assumed to be constant in the depicted range and which also reduces the transmitted light power. Since however the absorption coefficient of the interfering gas is wavelength-independent, the overlapping of the filter function of the interfering gas transmission in this range only yields linear and no non-linear pressure-dependent terms.

(40) FIG. 11 a) schematically depicts a micro-incandescent lamp 20 as an example that can be used according to the invention. This comprises a coil 81 as a central light generating unit which is arranged in a capsule 84 that is evacuated or filled with an inert gas. The area around the coil 81 is provided with a vacuum 85. The capsule 84 itself is transparent, for example made of glass. The evacuated interior is sealed at the bottom by a solid base 87 which is penetrated by two supply conductors 86 that terminate in contact pins 88 which the coil 81 contacts. The capsule 84 has a basically cylindrical shape with a diameter 90 in relation to the longitudinal axis of the micro-incandescent lamp 20. This is advantageously less than 2 mm, wherein even smaller capsules 84 are even more advantageous for optimally exploiting power.

(41) The coil 81 has a curved shape so that a large amount of coil length is available in a relatively small space, and a high power density is accordingly achieved. FIG. 11 b) shows the coil 81 together with its envelope 83, wherein a distance is left in the schematic depiction between the envelope 83 and the coil 81 only for the sake of illustration. The envelope surrounds the volume 82 that the coil 81 assumes. The greatest linear extension of the envelope 92 runs between the two endpoints of the coil 81 in this case and hence is mostly outside of the envelope 83 of the coil 81. The greatest linear extension 92 however also describes the greatest, or respectively greatest possible linear distance between two points of the coil 81. Consequently, this greatest linear extension 92 is a linear measure of the compactness of the light-generating region of the micro-incandescent lamp 20. Especially in an imaging optical system, a very small greatest linear extension 92 is advantageous because a majority of the generated light can be arranged in the focal point of the imaging optical system so that light loss can be avoided. This translates to the benefit of the signal-to-noise ratio and the battery life.

(42) At the same time, a corresponding coil 81 has a very small thermal mass so that the coil is heated to the operating temperature within fractions of a second, and a modulation of several hundred ° C. is feasible given a sufficient frequency for a time-resolved measurement of the change of a measuring gas concentration, for example for tracking the concentration of CO.sub.2 in a breathing gas in the context of capnometry.

(43) FIG. 11 c) shows the coil 81 from the narrow side. In this case, the thickness of the coil is the same as the smallest linear extension 94 of the envelope 83. FIG. 11 d) shows an alternative coil 81′ that, in contrast to the coil 81, is straight and not bent. The envelope 83′ in this case is basically cylindrical, and the greatest linear extension of the envelope lies within the coil 81′. While the absolute length of the coils is the same, the linear extension of the coil 81′ is therefore greater than in the case of the coil 81 presented beforehand. A bend in the coil is therefore advantageous in the context of the present invention in order to generate high light intensity from a small overall volume.

(44) The above-presented exemplary embodiments each present the ideal case of an assembly for example with division by a transmission lattice or a reflection lattice that is always spectrally neutral, a double bandpass filter assembly, and an assembly for the nonlinear analysis. These assemblies can however also be combined with each other so that the nonlinear analysis can for example also be used in a double bandpass filter assembly, or an assembly with a spectrally neutral lattice and a plurality of receivers. Likewise, a double bandpass filter assembly can be combined with spectrally neutral transmission lattices or reflection lattices and a plurality of detectors in order for example to perform an analysis with respect to a plurality of target gases.

(45) All named features, including those taken from the drawings alone as well as individual features that are disclosed in combination with other features, are considered, alone and in combination, to be essential for the invention. Embodiments according to the invention can be fulfilled by individual features or a combination of several features. In the scope of the invention, features which are designated by “in particular” or “preferably” are understood to be optional features.

LIST OF REFERENCE SIGNS

(46) 10 Assembly 12 Measuring cell 14 Gas inlet 15 Gas inlet opening 16 Gas outlet 17 Gas outlet opening 18 Evaluation apparatus 20 Micro-incandescent lamp 20′ Expanded light source 21 Evacuated glass bulb 22 Reflector 24 Filter for gas channel 25 Detector for gas channel 26 Filter for reference channel 27 Detector for reference channel 28, 28′, 28″ Detectors 30 IR photodiode 32 Reflector 34 Lost light components 40 Spectrally neutral transmission lattice 41 Spectrally neutral focusing transmission lattice 42 Spectrally neutral transmission lattice 50-54 Spectrally neutral reflection lattice 60 Control apparatus 62 Control unit 64 Photodiode 66 Amplifier 68 Amplifier for current measurement 69 Amplifier for voltage measurement 70 Multiplier 81 Coil 82 Volume 83, 83′ Envelope 84 Capsule 85 Vacuum 86 Supply conductor 87 Base 88 Contact pin 90 Diameter of the encapsulation 92 Greatest linear extension of the envelope 94 Smallest linear extension of the envelope 110 Assembly 112 Interference 114, 114′ Image of the interference 210 Assembly 212 Double bandpass filter 310 Assembly 312 Check valve 314 Pressure gauge 316 Gas reservoir 318 Pump 320 Controller for the pump 322 Motor