EP2834607A1

EP2834607A1 - Device and method for fast recording of an absorption spectrum of a fluid

Info

Publication number: EP2834607A1
Application number: EP13716250.9A
Authority: EP
Inventors: Ralf Buchtal; Peter Dreyer; Livio Fornasiero
Original assignee: Draeger Medical GmbH
Current assignee: Draegerwerk AG and Co KGaA
Priority date: 2012-04-05
Filing date: 2013-04-04
Publication date: 2015-02-11
Also published as: DE102012007030A1; JP6290173B2; WO2013150102A1; DE102012007030C5; US9939374B2; CN104412080B; JP2015512521A; JP6370442B2; US20150300948A1; DE102012007030B4; JP2017187503A; CN104412080A

Abstract

The invention relates to a device for recording an absorption spectrum of a fluid, with a first radiation source (1) that emits a radiation in a first spectral range along a first beam path (11), with a first measuring path (5), along which the radiation passes through the fluid and which is arranged in the first beam path (11), with a tunable Fabry-Perot interferometer (7), which is arranged in the first beam path (11) and can transmit radiation in the first spectral range as a displaceable bandpass filter, and with a first detector (9, 35) for measuring the intensity of the radiation in the first spectral range. According to the invention, the device has a first etalon (3), which is arranged for spectral modulation of radiation in the first beam path (11) and which has a plurality of transmission maxima (17) in the first spectral range, and the Fabry-Perot interferometer (7) is designed such that the bandpass filter formed by the Fabry-Perot interferometer (7) can be displaced across the first spectral range in such a manner that the spectral modulation of the radiation by the first etalon (3) can be measured by the first detector (9, 35) as a modulation over time of the radiation intensity.

Description

Device and method for rapid recording of a

Absorption spectrum of a fluid

The present invention relates to an apparatus for on ^¬ taking an absorption spectrum of a fluid having a first radiation source which emits radiation in a first spectral region along a first beam path, having disposed in said first beam path first measurement path along which the radiation through the fluid enters , having disposed in said first beam path tunable Fabry-Perot interferometer, which can transmit as a displaceable Bandpassfil ^¬ ter radiation in the first spectral range and a first detector for measuring the intensity of the radiation in the first spectral range and a method for receiving a Absorption spectrum of a fluid.

In medical technology, it is often necessary to determine the concentra ^¬ tion of selected components of the air we breathe or monitor. Thus, the concentration of the anesthetic gases used in the breath or in the control of drivers of the alcohol content in the respiratory air is measured, for example, in anesthetized Pati ^¬ often ducks in each breath. To determine the concentration of the respective components spectrometric methods are used. As a rule, signals are recorded and compared with references using discrete filters in wavelength bands in which the gases exhibit absorptions. From the change of the signals at these wavelengths, which are charakteris ^¬ table for a particular gas, thus it can be concluded in the breathing air on the concentration of the gas.

A number of devices are known from the prior art with which the concentration of selected constituents of a fluid or of a fluid can be determined by absorption measurements at selected wavelengths or over a spectral range. of a gas, such as breathing gas. Such devices have a radiation source which emits radiation over a selected, continuous spectral range. Between the radiation source and a subsequently arranged ^¬ measuring path in which the radiation passes through the fluid, an optical bandpass filter, hereinafter referred to as a filter, arranged, which restricts the spectrum of the radiation to a wavelength for a component of the gas whose concentration is to be measured is characteristic. The intensity of the radiation after it has passed through the filter and the measurement distance measured by ei ^¬ nem suitable detector. From the attenuation of the intensity compared to a reference measurement in a reference fluid in which the concentration of the constituent is known, it is now possible to deduce the concentration of the constituent in the fluid.

If the concentration of a plurality of components to be measured, a private geeigne ^¬ tes filter must be used for each component. In order to determine the concentration of the various constituents in a short period of time-preferably in the same breath as so-called breath-resolved measurement-the various filters are arranged on a so-called filter wheel. A filter wheel is a rotating ^¬ de disk that moves the filter in quick succession in the beam path between the radiation source and detector. A disadvantage, however, that only a small number of Be ^¬ stood share can be examined, there must be present for each component its own filter and the size of the filter wheel is limited. In addition, the use of mechanical components is generally disadvantageous because they increase the maintenance ^¬ effort and are prone to errors.

A complete absorption spectrum can be recorded using a tunable Fabry-Perot interferometer be as described in DE 10 2006 045 253 B3. For this purpose, a tunable Fabry-Perot interferometer is arranged in the beam path between radiation source and detector instead of the filter. A Fabry-Perot interferometer is an arrangement that has two partially transparent mirrors arranged parallel to one another, whose reflective coated mirror surfaces are assigned to one another. The distance of the mirror surfaces from one another determines a narrow wavelength range, which is transmitted by the Fabry-Perot interferometer or for which the Fabry-Perot interferometer is permeable. The width of the light transmitted by the Fabry-Perot interferometer wavelength range is referred to as spectral on ^¬ solution and is dependent on the wavelength. In a tunable Fabry-Perot interferometer, the distance of the mirror surfaces can be changed and thus the wavelength range that is transmitted can be shifted. A Fabry-Perot interferometer therefore represents a sliding band pass filter ^¬, the width of the spectral resolution of the Fabry-Perot interferometer corresponds. If the absorption spectrum of a fluid over a selected spectral region are fully recorded, the Fabry-Perot interferometer must be tuned le ^¬ diglich over the selected spectral range.

With a suitable coating of the mirror surfaces, a Fabry-Perot interferometer can also be permeable simultaneously for two different wavelength ranges. It is thus possible to simultaneously record the absorption spectra of the fluid in the two wavelength ranges. A device for recording an absorption spectrum of a fluid using such a Fabry-Perot interferometer is known from DE 10 2009 011 421 B3.

To determine the concentration of the individual components of the fluid, the spectrum recorded by the detector must be be compared with a reference measurement. For this purpose, lock-in amplifiers are used regularly. In a lock-in amplifier, a measured signal is multiplied by a reference signal and the result of the multiplication integrated in a low-pass filter. The lock-in amplifier thus forms the cross-correlation between the measured signal and the reference signal.

In order to use a lock-in amplifier, the signal outputted from the detector so that the radiation emitted by the radiation source Strah ^¬ radiation must, however, be time-modulated. A temporal modulation is to be understood here as a change in the intensity of the radiation with time. In the simplest case, this can be done by switching on and off the radiation source or a downstream chopper, which repeatedly interrupts the beam path between the radiation source and the detector.

Absorption of selected constituents in the respiratory air is preferably at wavelengths between 2 and 15 ym ym ge ^¬ measure. The available broadband radiation sources in this wavelength range are usually thermal radiators. If these electrically modulated in time with a sniff necessary for frequency-resolved measurements of more than 100 Hz, the relative modulation depends In the ^¬ intensity greatly from the considered wavelength from, the modulation of short to long wavelength decreases. In the spectral range between 2 ym and 15 ym, the intensity modulation at frequencies of 100 Hz and more at the longer wavelengths decreases so much that it is no longer sufficient for use with a lock-in amplifier.

Although the use of a mechanical chopper is possible in principle, this is again a mechanical component which is hard to miniaturize the complex in the maintenance and ^¬.

A similar problem arises when a photoacoustic or a pyroelectric sensor is to be used as the detector, since these two sensor types can only be used if the radiation has a temporal change in intensity.

It is thus the task to temporally modulate the radiation emitted by the radiation source over the entire selected spectral range so quickly that a photoacoustic sensor or a pyroelectric sensor arranged in the measurement section can be used as the detector. Furthermore, it is an object of the present invention to modulate the radiation so quickly in time that a subsequently arranged lock-in amplifier can be operated to compare the measured absorption spectrum with a reference spectrum.

The object is achieved by a device which has a first etalon for the spectral modulation of the radiation, which is arranged in the first beam path and which has a plurality of transmission maxima in the first spectral range. Further, the Fabry-Perot is configured such that the formed by the Fabry-Perot interferometer Bandpassfil ^¬ ter can be moved in such a way via the first spectral range, the spectral modulation of the radiation through the first etalon as a temporal modulation of the intensity of the radiation from the first detector can be measured.

The device initially has a first radiation source which emits radiation in a first spectral range with a preferably continuous spectrum. To determine the concentration of constituents of the respiratory air are suitable as a first radiation source, for example membrane ^¬ radiator, which have a continuous spectrum in a wavelength range between 2 ym and 20 ym. Alternatively, powerful helical radiators or other thermal radiators can be used, which also emit in this spectral range. The radiation emitted by the radiation source is preferably largely collimated by a lens arrangement or a reflector and guided along a first beam path through the device.

Along the first beam path, the first measurement distance, the Fabry-Perot interferometer and the first etalon are arranged in the propagation direction of the radiation prior to the detector, wherein the order in which the radiation, the first measuring ^¬ range, the Fabry-Perot interferometer and the first etalon happens, is arbitrary.

The first measuring path arranged in the first beam path may, for example, be a cuvette in which the fluid to be examined is arranged.

An etalon is to be understood here as a Fabry-Perot interferometer in which the distance between the mirror surfaces can not be changed or is not changed during a measurement. As an etalon, for example, a blank ^¬ coated, polished thin disk made of silicon or Ger ^¬ Manium or mainly comprises silicon or germanium may be used. Alternatively, as the etalon, for example, metallic or dielectric coated discs which are optically transparent or two coated or uncoated parallel spaced plates may be used. It is also conceivable, instead of using an etalon, to use optical components which have a comb-like transmission or reflection, for example certain plastic components. foils or cuvettes filled with a gas, the gas having a pronounced comb-like absorption.

The first etalon has a plurality of transmission maxima in the first spectral range. A broadband Strah ^¬ development, which passes through the first etalon, thus has referred to a spectral modulation, which is characterized by a change from the Etalon specific intensity minima and maxima. By spectral modulation is meant herein the dependence of the intensity of a radiation on the wavelength or the frequency. For example, in the spectral range between 4 μm and 5 μm, a silicon wafer with a thickness of 100 μm has 35 transmission maxima and just as many transmission minima. In the spectral range between 8 ym and 11 ym, the same disc has 25 transmission maxima.

It is conceivable that the first etalon is formed such that at least some of the transmission maxima of the first etalon comprise wavelengths which are charac teristic ^¬ for the components whose concentration is to be analyzed in the fluid. In this way, the Konzentra ^¬ tion of components of the fluid can be determined optimized, which have only narrow characteristic absorption lines.

Preferably, the first etalon is such that the maxima of the transmission rate from ^¬ stand is greater than the spectral resolution of the Fabry-Perot interferometer in the first spectral range ^¬. If the spectral resolution of the Fabry-Perot interferometer is wider than the distance of the transmission maxima, this leads to an undesired smoothing of the spectrum. In other words, the intensity maxima generated by the first etalon appear broader and shallower when sampled with the Fabry-Perot interferometer. Preferably, the surfaces of the first etalon are not aligned pa rallel ^¬ to the surfaces of the elements in the propagation direction of the radiation immediately before and arranged behind the first etalon in order to prevent additional undesirable Etaionef ^¬ fect.

Furthermore, a Fabry-Perot interferometer is arranged in the first beam path, in which the distance between the mirror surfaces can be adjusted so that the Fabry-Perot interferometer represents a displaceable bandpass filter in the first spectral range. In this case, the bandpass filter formed by the Fabry-Perot interferometer can be displaced over the first spectral range, preferably continuously, such that the spectral modulation by the first etalon is measured as a temporal modulation of the intensity of the radiation from the first detector. In other words, the spectral modulation of the radiation through the first etalon with the tuning of the Fabry-Perot interferometer and the associated displacement of the band-pass filter in a zeitli ^¬ che intensity modulation of the radiation is transmitted. In this case, due to the known width of the Fabry-Perot interferometer at any time on the transmitted wavelength and da ^{¬ be concluded} with the wavelength of the radiation.

If, for example, in the case of a device having a first etalon of a silicon wafer with a thickness of 100 μm, which has 35 transmission maxima and minima in a spectral range between 4 μm and 5 μm, the Fabry-Perot interferometer travels five times per second over the spectral range of the smallest to the largest wavelength and then tuned back ent ^¬ this indicates an intensity modulation of the radiation at a frequency of 350 Hz. In a spectral region that includes the wavelengths between 8 ym and 11 ym, has the sliding ^¬ che silicon wafer, for example, 25 Transmissionsmaxima- and minima. Thus, a five-time tuning of the Fabry-Perot interferometers across this spectral range in both directions correspond to an intensity modulation of the radiation with a frequency of 250 Hz.

Furthermore, the device has a first detector which is arranged so that it can measure the intensity of the radiation after it has passed the first etalon, the Fabry-Perot interferometer and the first measuring path. The first detector may, for example, be a semiconductor sensor, a thermopile, a thermoresistor, a pyroelectric sensor or a photoacoustic gas sensor. The latter is preferably arranged in the first measuring section.

Such a device is advantageous because it has no macro-mechanical elements and it is nevertheless possible to modulate the intensity of the radiation with a sufficiently high frequency for a breath-resolved measurement over a broad spectral range. As will be omitted macro mechanical components, the apparatus operates almost ver ^¬ from wear, making the maintenance compared to known prior art devices can be reduced and the service life is increased.

In a preferred embodiment for the determination of the absorption spectrum of the fluid from the measured with the first Detek ^¬ tor intensities of the radiation a first lock-in amplifier is provided. The lock-in amplifier forms the cross-correlation function between the output from the first sensor measurement signal and a reference signal and represents a particularly narrow band-pass filter, which allows even with strongly noisy signals, already minor Ab ^¬ deviations between the measurement signal and the reference signal. Therefore, the lock-in amplifier allows you to particularly advantageous way to record an absorption spectrum of the fluid.

In a further preferred embodiment the device comprises a second radiation source having a radiation in a second areas of the spectrum ^¬ rich emitted along a two ^¬ th ray path, a second measuring path along which the light emitted from the second radiation source passes through the fluid, and a second Detector for measuring the intensity of the radiation in the second spectral range. Here, the first and the second beam path are such forms ^¬ out that the Fabry-Perot interferometer is disposed in the first and the second beam path. The Fabry-Perot interferometers In ^¬ can transmit band-pass filter as a moveable radiation in the second spectral range. Furthermore, the device has a second etalon for the spectral modulation of the radiation, which is arranged in the second beam path and which has a plurality of transmission maxima in the second spectral range. In addition, the Fabry-Perot is adapted to that the of the Fabry-Perot interferometer ge ^¬ formed band-pass filter can be shifted such via the second spectral range, the spectral modulation of the radiation through the second etalon as a temporal Modulati ^¬ on the intensity of the radiation can be measured by the second detector.

This preferred embodiment has a second radiation ^¬ source, emits radiation in a second spectral range, wherein the second spectral range, preferably re walls ^¬ includes wavelengths than the first spectral range. For example, the first spectral range could comprise the wavelengths between 2 ym and 6 ym - preferably between 4 ym and 5 ym - and the second spectral range the wavelengths between 7 ym and 15 ym - preferably between 8 ym and 11 ym. It is also conceivable that the first radiation source and the second radiation source to emit radiation over the same spectral region, and are used by respective band-pass filter to the respective spectral ranges turned ^¬ limits.

The apparatus further comprises a second etalon which is arranged in the second beam path and modulates the radiation in the second spectral range in a similar manner as the ers ^¬ te etalon radiation in the first spectral range. In particular, the second etalon in the second spectral range comprises a plurality of transmission maxima whose distance preference ^¬ example is greater than the bandwidth or spectral resolu ^¬ solution of the Fabry-Perot interferometer in the second areas of the spectrum ^¬ rich. It is further conceivable that the second etalon is configured in such a way that at least some of the transmission maxima of the second etalon comprise wavelengths that are characteristic of the constituents whose concentration in the fluid is to be investigated.

Preferably, the surfaces of the second etalon are also not aligned parallel to the surfaces of the elements arranged in the propagation direction of the radiation directly in front of and behind the second etalon, in order to avoid additional undesirable etalon effects.

This preferred embodiment further includes a second measurement path along which the light emitted from the second radiation source ^¬ radiation passes through the fluid. It is conceivable that the first and the second measurement path have the same length, of the length of a Messstre ^¬ blocks the route is to be understood that traverses the radiation along the measurement path through the fluid. In a Favor ^¬ th embodiment, the first measurement section is integrally formed with the second measurement path, ie the first and the second measuring section coincide or extend on the same Way through the fluid. Such a device is particularly advantageous since the absorption spectra in the fluid can be measured in two spectral ranges in the same measuring path and thus it is ensured that the concentration measurement is not influenced by spatial differences in the concentration of the constituents.

However, it is also conceivable that the first and the second measuring section have different lengths. Last ^¬ res is particularly advantageous when the fluid in the ers ^¬ th and in the second spectral range absorbed to different extents. So it is for example conceivable that a Be ^¬ standing part of the fluid whose concentration is to be determined, absorbs only very weak in the first spectral range. Nevertheless, in order to achieve measurable changes in the absorption for the first detector, a longer first measuring section is used. Conversely, it is also conceivable that, for example, the concentration of a constituent of the fluid determines who should ^¬, the strongly absorbed already in low concentrations in the second spectral range. In this case, the second measuring section could be selected to be correspondingly short, so that measurable changes in the absorption occur even at high concentrations of the second detector.

In this preferred embodiment of the invention, the Fabry-Perot interferometer is arranged in the first and the second beam path. In other words, both the radiation emitted by the first radiation source and the radiation emitted by the second radiation source passes through the Fabry-Perot interferometer, wherein the first and the second optical path extend parallel, approximately parallel or one above the other through the Fabry-Perot interferometer.

For example, it is conceivable that the first and the second beam path, after the radiation, the respective Eta lon happened to hit the same beam splitter. In this case, the beam splitter is arranged, for example, such that the portion of the radiation emitted by the first radiation source in the Fabry-Perot interferometer transmits the first beam path while the portion of the radiation emitted by the second radiation source is reflected there by the beam splitter forms the second beam path.

The device further comprises a second detector adapted to measure the intensity of the radiation in the second spectral region after passing through the second etalon, the second measurement path and the Fabry-Perot interferometer.

Furthermore, the Fabry-Perot interferometer is ^{ausgebil ¬} det, that the distance of the mirror surfaces can be adjusted so that the Fabry-Perot interferometer is a displaceable bandpass filter in the second spectral range. Here, the band-pass filter formed by the Fabry-Perot interferometer can such via the second spectral region - are shifted, that the spectral modulation is measured by the second etalon as a temporal Mo ^¬ dulation the intensity of the radiation from the second detector - preferably continuously. In other words, the spectral Modulati ^¬ on the radiation through the second etalon with the tuning of the Fabry-Perot interferometer and the associated displacement of the band-pass filter is transmitted to a temporal intensity ^¬ modulation of the radiation.

In a preferred embodiment, the Fabry-Perot interferometer is designed such that it can transmit radiation in the first and in the second spectral range at the same time and that the light emitted by the Fabry-Perot interferometer can be transmitted. bandpass filter can be shifted over the first and the second spectral range at the same time that the spectral modulation of the radiation by the first and the second etalon can be measured as a temporal modulation of the intensity of the radiation from the first and the second detector. In other words, the distance of the mirror surfaces of the Fabry-Perot interferometer can be adjusted so that it simultaneously forms a band pass in the first and the second spectral range. Thus, the first and second spectral regions can be scanned simultaneously, and the absorption spectrum of the fluid can be recorded in both spectral regions in a particularly short time, making it possible to measure the concentration of constituents in a fluid whose composition changes in very short periods of time ,

In a further preferred embodiment for the determination of the absorption spectrum of the fluid from the measured with the two ^¬ th detector intensities of the radiation a second lock-in amplifier is provided.

It is further preferred to form the first etalon integrally with the second etalon. In other words, instead of the first and the second etalon, only one etalon is provided which has a plurality of transmission maxima both in the first and in the second spectral range. For example, a 100 ym thick slice of silicon could be used which has 35 transmission maxima in the spectral range between 4 ym and 5 ym and 25 transmission maxima in the spectral range between 8 ym and 11 ym. As a result, the size of the device can be significantly reduced.

It is particularly preferred if the first etalon is arranged in such a way in the first and in the second beam path as a beam splitter, that the first beam path and the second beam path. lengang run parallel in the propagation direction of the radiation behind the first etalon. In other words, the first etalon and the second etalon to be replaced by a common Eta ^¬ lon, which is also designed as a beam splitter and the first and the second beam path brings together such that the Fabry-Perot interferometer in Ausbrei ^¬ power direction of the radiation behind the etalon, is passed in parallel by the radiation emitted by the first and the second radiation source. Such a device is also particularly advantageous, since in this way a particularly compact design can be realized.

It is also preferable to form the first detector integrally with the second detector. In other words, instead of a first and a second detector, only one detector is provided, which measures the intensity of the radiation in both the first and in the second spectral range, whereby a particularly compact design of the device is possible.

It is further preferred that the first radiation source is identical to the second radiation source. In other words, instead of two radiation sources, only one radiation source is provided, which emits radiation in the first and in the second spectral range. This also makes it possible to realize a particularly compact design.

In a preferred embodiment, the first radiation source is a thermal radiator whose intensity can be modulated in time so rapidly that the relative change in the intensity of the radiation is more pronounced in one of the two spectral ranges than in the other spectral range. If only one radiation source is present and that formed as a ther ^¬ mixer radiator, so it is advantageous to the intensity ^¬ In the radiation source to modulate in time quickly. If the modulation frequency is sufficiently high, then the intensity modulation affects only the shorter wavelength spectral range, while the longer wavelength one of the two spectral ranges is essentially not modulated. Significant net ^¬ a detector an overlay of the absorption spectra in the first and second spectral range, this additional modulation of shorter wavelength spectral range can be used in a particularly advantageous manner, to separate the first and second absorption spectrum from one another.

The device can also be configured such that the absorption of the fluid in further spectral ranges can be determined. To this end the device for each additional spectral region a radiation source that emits in the Spekt ^¬ ralbereich radiation along a beam path. Along each beam path, a measurement path, an etalon and a detector are arranged, the radiation passing through the fluid along the measurement path, the etalon for modulating the radiation in the spectral range having a plurality of transmission maxima and the detector for measuring the intensity of the radiation is set up in the spectral range. The beam paths are designed such that the Fabry-Perot interferometer is arranged in each beam path. The Fabry-Perot interferometer is adapted to transmit radiation in each of the spectral regions as a displaceable bandpass filter, wherein the bandpass filter can be shifted over the spectral regions such that the spectral modulation of the radiation through the etalons as a temporal modulation of the intensity of the radiation from the Detectors can be measured. In such a device, it is conceivable that all the radiation sources, all etalons, all measuring sections and / or all detectors are integrally formed with each other, whereby a broadband ^device for recording absorption spectra can be realized in a particularly compact construction. The object is further achieved by a method for using a device according to the invention, in which the Fabry-Perot interferometer is tuned so that the bandpass filter formed by the Fabry-Perot interferometer is shifted over the first spectral range such that the spectral modulation of the radiation the first etalon is measured as a temporal modulation of the intensity of the radiation from the first detector.

In a preferred embodiment, to determine the absorption spectrum in the first spectral range, a measurement signal output by the second detector is compared with the first lock-in amplifier with a reference signal.

The reference signal required for this purpose can be determined in a variety of ways. The intensity maxima of the radiation produced by the first etalon and the Fabry-Perot interferometer usually do not occur at a constant time interval. Therefore, an external reference signal with a constant frequency can not be used as a rule. It is conceivable, however, such a match, the control of the Fabry-Perot interferometer to ^¬ that the maxima of intensity-modulated radiation in a temporally equidistant occur. In this case, it would be possible to use a constant frequency reference signal.

Alternatively, it is also possible first to record an entire absorption spectrum for the first spectral range and to transform it with an evaluation unit in such a way that the intensity maxima are temporally equally spaced. The measuring signal produced in this manner can also be compared with an ex ^¬ tern generated reference signal. It is also conceivable first to record an absorption spectrum in which the first or second measurement path is filled or evacuated with a reference fluid of known composition, and to use the measurement signal output by the detector for further measurements as a reference signal.

If no lock-in amplifier is used, it is also conceivable to determine an absorption spectrum in which the intensity maxima measured by the detector are integrated. From the time at which an intensity maximum occurs, at a known width of the Fabry-Perot interferometer are closed at the time of the wavelength that is transmitted at the ^¬ ser setting of the Fabry-Perot interferometer and thus on the wavelength to which the respective intensity maximum occurs. Is now over the entire maximum intensity, that is integrated by an intensity minimum to the next, it can from the comparison of the value of the integral with a reference measurement of the relative absorption at the wavelength to be closed and a complete Absorpti ^¬ onsspektrum are created in the first spectral range, without that a lock-in amplifier is necessary.

In a further preferred embodiment, the Fabry-Perot interferometer is tuned in such a way that the bandpass filter formed by the Fabry-Perot interferometer is shifted over the second spectral range such that the spectral modulation of the radiation by the second etalon is a temporal modulation of the intensity the radiation is measured by the second detector. In this embodiment, too, it is further preferred if, for determining the absorption spectrum in the second spectral range, a measurement signal output by the second detector is compared with the second lock-in amplifier with a reference signal. In addition, it is particularly preferred that the Fabry-Perot interferometer is tuned in such a way that time ^¬ equal to the absorption spectrum of the fluid in the first and in the second frequency range can be determined. This embodiment is particularly advantageous because it allows to record an absorption spectrum of the fluid in the first and in the second spectral range in a particularly short time.

This is especially possible if only one detector is used to measure the intensity of radiation in the first and second spectral range, since the Intensitätsmodu ^¬ lation of the radiation through a first and a second etalon, which is also integrally (ie, as an element) be formed Kgs ^¬ NEN, takes place. First, a single detector measures a superposition of the absorption spectra in the first and second spectral range. However, since the intensity maxima of the radiation after passing through the etalon or have different, dependent on the spectral range distances, the absorption spectra can be separated after recording in a simple manner. This can be done electronically, via a Fourier analysis or via two lock-in amplifiers, which use different reference signals characteristic of the respective spectral range. In this context, in addition to the amplitude modulation for the lock-in method, the frequency modulation can also be used for a Fourier analysis of the signal sequence.

The present invention will be explained below with reference to a drawing illustrating four embodiments. The drawing shows in

1 shows a first embodiment of an inventive device Shen ^¬ Fig. 2 shows a second embodiment of an inventive device ^¬ SEN,

Fig. 3 shows a third embodiment of an inventive ^¬ Shen device.

Fig. 4 shows a fourth embodiment of an inventive device ^¬ SEN,

Fig. 5 shows a fifth embodiment of an inventive device ^¬ SEN,

6 shows a method for determining an absorption spectrum from a measuring signal output by a detector,

7 shows a transmission spectrum of an etalon of the first

Embodiment in a first spectral range,

8 shows a transmission spectrum of an etalon of the first

Embodiment in a second spectral range and

9 shows a superimposition of the transmission spectra shown in FIGS. 7 and 8.

1 shows a first embodiment of an OF INVENTION ^¬ to the invention apparatus is shown having a first radiation source 1, a first etalon 3, a first measuring section 5, a Fabry-Perot interferometer 7 and a detector 9, which are arranged along a first beam path 11 , The first radiation source 1 is a thermal radiator (eg membrane radiator, ^helical radiator or Nernst pin) which extends over a spectral range from 2 μm to .mu.m 20 ym has a continuous spectrum whose maximum is about 5 ym.

The etalon 3 arranged in the direction of propagation of the radiation behind the radiation source 1 consists of a silicon wafer having a thickness of 100 μm, which transmits radiation in a first spectral range and in a second spectral range and has a plurality of transmission maxima in both spectral ranges. The calculated transmission of the etalon 3 in the first spectral region ym wavelengths of 4 comprises up to 5 ym, is shown in Figure 7, wherein on the Radofs ^¬ senachse 13, the wavelength is shown in m and the relative transmission on the ordinate axis 15 °. In the first Spekt ^¬ ralbereich, the etalon transmission maxima at 3 35 17 and as many transmission minima 19th 8 shows the calculated transmission of the etalon 3 in the second areas of the spectrum is rich ^¬ comprising wavelengths between 8 and 11 ym ym, shown, wherein for identical elements of all Zeichnun ^¬ gene, the same reference numerals have been used. On the abscissa axis 13 in Figure 8 is the wavelength in m represents ^¬ Darge and on the ordinate axis 15, the relative transmission of the etalon 3. In the second spectral region, the etalon 25 transmission peaks 17 3 and as many transmission minima 19.

In the direction of propagation of the radiation along the first beam path 11, a first measuring section 5 is referred to ^¬ sorted, which is, for example, is a column filled with a respiratory gas cuvette. The etalon 3 is such to ^¬ arranged that the top surface 21 does not extend ^¬ facing the surface of the cuvette parallel to this, unwanted Etaioneffekte between the cuvette and the etalon 3 to vermei ^¬. Subsequently, in the first beam path 11, the Fabry-Perot interferometer 7 is arranged, which has a first and a second mirror surface 23, 25, wherein the first mirror ^¬ surface 23 parallel to the second mirror surface 25 ver ^¬ runs and assigns to them. The spacing of the Spiegeloberflä ^¬ surfaces 23, 25 of the Fabry-Perot interferometer 7 can be turned so ^¬ assumed that the band-pass filter formed by the Fabry-Perot interferometer 7 transmits radiation in the first and second spectral range simultaneously.

Once again in the propagation direction of the radiation, a detector 9 is arranged, which measures the intensity of the radiation after it has crossed the etalon 3, the first measuring section 5 and the Fabry-Perot interferometer 7. In the De ^¬ Tektor 9 may be for example, a quantum detector or a thermal detector such as a py roelektrischen sensor.

To record an absorption spectrum of the gas arranged in the first measuring section 5, the distance between the mirror ^surfaces 23, 25 is continuously changed so that the bandpass filter formed by the Fabry-Perot interferometer 7 simultaneously scans the first and second spectral ranges. At any time, the detector 9 records a Intensi ^¬ ty of the radiation at this time, the wavelength can be assigned to the distance of the mirror surfaces 23, 25 of the Fabry-Perot interferometer 7, the band-pass filter formed by the Fabry-Perot interferometer 7 transmits or passes at this distance. The detector 9 consequently measures a superposition of the radiation transmitted in the first and the second spectral range.

In Figure 9, an appropriate overlay of the TRANSMISSI ^¬ onsspektren of the first etalon 3 is illustrated in the first and the second spectral range, with the upper axis of abscissa 27 represents the wavelength in the first spectral range in m, the lower abscissa axis 29, the wavelength in the second areas of the spectrum ^¬ rich and the ordinate axis 31, the relative intensity of the measured radiation at the detector 9.

The first embodiment of the invention is particularly advantageous since it provides a particularly compact apparatus ^¬ which allows to record the absorbance spectrum of a fluid simultaneously with intensity modulated radiation in two spectral regions without the macro mechanical components must be used. Such devices are low maintenance, durable and inexpensive to manufacture.

In order to obtain the absorption spectra in the first and in the second spectral range, the measurement signal 33 output by the detector 9 has to be separated. For this, the difference ^¬ Liche frequency of the transmission maxima can be 17 used in the first and second spectral range, the position of which also is known. By way of example, the device may have a first and a second lock-in amplifier (not shown) with which the measuring signal 33 is compared with a reference signal suitable for the spectral range. For example, the measurement signal 33 of a reference measurement in the respective spectral range can be used as the reference signal.

It is also possible to separate the first and the second spectral range by means of a Fourier analysis, wherein likewise the different frequency of the transmission maxima in the two spectral ranges is utilized.

2 shows a second embodiment of a ^¬ OF INVENTION apparatus is shown to the invention, in which the order of the Fabry-Perot interferometer 7 and the first measuring section 5 are reversed. In addition, the detector 35 is at a broadband photoacoustic sensor (eg microphone, cantilever, tuning fork-shaped oscillating quartz or the like), which is arranged in the first measuring path 5 forming cuvette 37. The use of photoacoustic sensors is advantageous because it allows for a detection of the absorption spectra over a wide wavelength and dynamic range. On the other hand, a signal is only generated if an absorbing fluid is actually present. Even small signals, ie only weak absorptions, can therefore still be detected with good contrast.

A third embodiment is shown in Figure 3, wherein a second Strah ^¬ radiation source 39 is provided adjacent to the first radiation source. 1 The first and second Strah ^¬ radiation source 1, 39 are both wide-band thermal emitter, the radiation of which are restricted by suitable bandpass filters 41, 43 on the first and the second spectral range. The radiation emitted by the second radiation source 39 radiation is guided along a second beam path 45 through the Vorrich ^¬ processing.

Along the first and second beam path 11, 45 is a first etalon 3, and a second etalon 47 are arranged in the propagation direction behind the band-pass filter having a multi ^¬ plurality of transmission peaks in the first and second spectral range.

The light emitted from the first and the second radiation source radiation incident on the same beam splitter 49 which is arranged so that subsequently the first and the second beam path above the other or in parallel by the first measuring section 5 forming ^¬ integrally with the second measuring section (coincides with it), and the Fabry-Perot Interferometer 7 run and hit the same detector 9. The embodiment illustrated in Figure 3 is advantageous ^¬ way, since the first and the second radiation source 1, 39 inde pendent ^¬ can be switched from each other and it is thus possible in a simple manner to record absorption spectra in only one of the two spectral ranges.

Also, different thickness etalon 3 and 47 can be used, which allows to adapt the modulations in the first and the second spectral range separately to the absorption ^¬ spectrum of the fluid.

FIG. 4 shows a fourth exemplary embodiment, which is a modification of the third exemplary embodiment shown in FIG. Instead of the first etalon 3, a ^two-¬ th etalon 47 and a beam splitter 49, a first etalon 3 is merely used, which also serves as a beam splitter and as an etalon in the first and second spectral range. This exploits that an etalon also has the function ei ^¬ nes bandpass filter when it reflects radiation. The device according to the invention shown in FIG. 4 is particularly advantageous since it has two independent radiation sources and nevertheless a particularly compact construction.

FIG. 5 shows a fifth exemplary embodiment, which shows the use of a first and a second etalon 3, 47 in front of a cuvette 37 through which a gas flows, in which the radiation passes through the gas along a first and a second measuring section 5, 5 ' , wherein the first and the second measuring section 5, 5 'have different lengths. These con ^¬ figuration has the advantage that on the longer path gases with low absorption cross-section, on the shorter route but gases with a high absorption cross section can be detected gleichzei ^¬ tig over a wide concentration range. Recently a method for the determination of an absorption spectrum of a measurement signal is schematically Darge ^¬ represents in Fig. 6. In the upper diagram in FIG. 6, the abscissa 51 represents the wavelength and the ordinate axis 53 the transmission of the etalon and the Fabry-Perot interferometer, the dashed curve 55 representing the transmission of the etalon and the solid curve 57 representing the Transmission of the Fabry-Perot interferometer, which is shifted from small to large wavelengths. To determine the absorption spectrum of Messsig ^¬ Nalles on the intensity maxima and minima is integrated during recording. This is schematically shown in the lower diagram in Fig. 6, in which on the abscissa axis 51 is also the wavelength and the ordinate axis 59, the integrated measuring signal 61 is Darge ^¬ represents. From the value of the integral over a minimum or maximum, the position of which is known, an absorption spectrum and thus a statement about the concentration of the constituents in a fluid can be made from the comparison with a reference measurement. This method is particularly advantageous because the absorption spectrum of a fluid can be determined without the need to use a lock-in enhancer.

Reference sign list:

I First radiation source

3 First Etalon

5 first test section

7 Fabry-Perot interferometer

9 detector

II first ray path

13 abscissa axis

15 ordinates

17 transmission maxima

21 Cuvette-facing surface of the first etalon

23 Mirrored surface

25 mirrored surface

27 Upper abscissa axis

29 Lower abscissa axis

31 ordinate axis measuring signal

detector

cuvette

Second radiation source

Bandpass filter

Second ray path

Second Etalon

beamsplitter

abscissa

axis of ordinates

Transmission curve of the etalon

Transmission curve of the Fabry-Perot interferometer ordinate axis

Integrated measuring signal

Claims

claims

Device for recording an absorption spectrum of a fluid

with a first radiation source (1) which emits radiation along a first beam path (11) in a first spectral range,

with a first measuring path (5) arranged in the first beam path (11), along which the radiation passes through the fluid,

with a tunable in the first beam path (11) Fabry-Perot interferometer (7), which can transmit as a slidable bandpass filter radiation in the first spectral range, and

with a first detector (9, 35) for measuring the intensity of the radiation in the first spectral range,

characterized,

in that the device has a first etalon (3) for the spectral modulation of the radiation, which is arranged in the first beam path (11) and which has a plurality of transmission maxima (17) in the first spectral range, and

that the Fabry-Perot (7) is adapted to that of the Fabry-Perot interferometer (7) formed tape ^¬ pass filter can be shifted in such a way via the first spectral range, the spectral modulation of the radiation through the first etalon (3) as a temporal Modula ^¬ tion of the intensity of the radiation from the first detector (9, 35) can be measured.

Apparatus according to claim 1, characterized in that for determining the absorption spectrum of the fluid from the first detector (9, 35) measured intensities of the radiation, a first lock-in amplifier is provided.

Apparatus according to claim 1 or 2, characterized in that a second radiation source (39) along a second beam path (45) emits radiation in a second spectral range,

in that the device has a second detector for measuring the intensity of the radiation in the second spectral range,

in that a second measuring path (5 ') is arranged in the second beam path (45), along which the radiation emitted by the second radiation source (39) passes through the fluid,

the first and the second beam path (11, 45) are formed such that the Fabry-Perot interferometer (7) is arranged in the first and the second beam path (11, 45),

that the Fabry-Perot interferometer (7) can transmit a ver ^¬ slideable bandpass filter radiation in the second spectral region,

in that the device has a second etalon (47) for the spectral modulation of the radiation, which is arranged in the second beam path (45) and which has a plurality of transmission maxima (17) in the second spectral range, and

is that the Fabry-Perot interferometer (7) to be ^¬ oriented such that the of the Fabry-Perot interferometer (7) band-pass filter formed can be displaced in such via the second spectral range, the spectral modulation of the radiation through the second etalon (47) as a temporal modulation of the intensity of the radiation from the second detector can be measured.

Device according to Claim 3, characterized in that the Fabry-Perot interferometer (7) is designed in such a way that that it can simultaneously transmit radiation in the first and in the second spectral range, and

that of the Fabry-Perot interferometer (7) band-pass filter formed can be displaced in such a simultaneously via the first and the second spectral range, the spectral modulation of the radiation through the first and second etalon (3, 47) as a time modu ^¬ lation the intensity of the radiation from the first and the second detector (9, 35) can be measured.

5. Apparatus according to claim 3 or 4, characterized in that for determining the absorption spectrum of the Flu ^¬ ids from the measured with the second detector intensities of the radiation, a second lock-in amplifier is provided.

6. Device according to one of claims 3 to 5, characterized in that the first etalon (3) is formed integrally with the second etalon (47).

7. Device according to one of claims 3 to 6, characterized in that the first measuring section (5) coincides with the second ^¬ th measuring section (5 ').

8. Device according to one of claims 3 to 7, characterized in that the first detector (9, 35) is integrally formed with the second detector.

9. Device according to one of claims 3 to 8, characterized in that the first radiation source (1) is formed integrally with the second radiation source (39).

10. The device according to claim 9, characterized in that the first radiation source (1) is a thermal radiator and that the intensity of the first radiation source (1) can be modulated in time so fast that the rela tive ^¬ change in the intensity of the radiation in one of the two spectral regions is more pronounced than in the other spectral range.

11. A method for recording an absorption spectrum of a fluid in a first spectral range with a device according to one of claims 1 to 10,

characterized,

that the Fabry-Perot interferometer (7) is so tuned by ^¬ that of the Fabry-Perot interferometer (7) band-pass filter is formed is shifted so via the first spectral range, the spectral modulation of the radiation through the first etalon (3) as a zeitli ^¬ che modulation of the intensity of the radiation from the first detector (9, 35) is measured.

12. The method of claim 11, if dependent on claim 2, characterized in that for determining the absorption spectrum in the first spectral range of the first detector (9, 35) output measuring signal (33) with the first lock-in amplifier with a reference signal is compared.

13. The method of claim 11 or 12, when dependent on claim 3, characterized in that the Fabry-Perot interferometer (7) is tuned such that formed from said Fabry-Perot interferometer (7) Bandpassfil ^¬ ter second in such a way over the Spectral range is shifted, that the spectral modulation of the radiation by the second etalon (47) is measured as a temporal modulation of the intensity of the radiation from the second detector.

14. The method according to claim 13, if dependent on claim 5, characterized in that for determining the absorption spectrum in the second spectral range, a measurement signal (33) output by the second detector is compared with the second lock-in amplifier with a reference signal.

15. The method of claim 13 or 14, if dependent on claim 4, characterized in that the Fabry-Perot interferometer (7) is tuned so that time ^¬ equal to the absorption spectrum of the fluid in the first and in the second frequency range can be determined ,