WO2003067720A2

WO2003067720A2 - Laser device comprising a quantum cascade laser

Info

Publication number: WO2003067720A2
Application number: PCT/EP2002/014017
Authority: WO
Inventors: Armin Lambrecht; Thomas Beyer; Marcus Braun
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2002-02-08
Filing date: 2002-12-10
Publication date: 2003-08-14
Also published as: WO2003067720A3; DE10205310B4; DE10205310A1

Abstract

The invention relates to a laser device and corresponding applications. Said device comprises a quantum cascade laser (7) and a pulse generator for the pulsed operation of the quantum cascade laser. The invention is characterised in that the quantum cascade laser (7) can be operated by the pulse generator by means of pulse packets, which use frequencies ranging between 1 Hz and 100 kHz, and that a heat dissipation device (8) is connected to the quantum cascade laser (7). Cost-effective Fabry-Perot lasers can be used as the quantum cascade laser. A special control method enables a partially coherent, LED-type emission.

Description

Laser device with a quantum cascade laser

The present invention relates to a laser device with a quantum cascade laser (QCL) according to the preamble of claim 1, a method for operating a quantum cascade laser according to the preamble of claim 18, a gas measuring device according to the preamble of claim 21, a lighting device according to the preamble of claim 25 and a method for detection of light according to the preamble of claim 29. Such laser devices are used, for example, in infrared measurement technology or as an infrared light source.

Laser devices with a quantum cascade laser are generally known from the prior art. The market for infrared optical gas measuring devices is currently predominantly non-dispersive devices. Broadband thermal emitters are usually used as infrared light sources. The radiation detection is preferably carried out using thermal detectors such as thermopiles, pyrodetectors or else using photoacoustic detection methods. The NDIR method (nondisperse IR absorption) generally requires spectral filtering of the radiation, which can be achieved with interference filters, with micromechanical Fabry-Perot resonators or with gas filters. On the one hand, the low modulability of the thermal radiators has a limiting effect on the performance data of these measuring devices, so that in general it is only possible to modulate with frequencies in the 10 to 100 Hz range. Above all, the devices are limited by the physically predetermined, too low spectral power density of a Planck radiator.

Higher outputs would be achieved with large-area emitters, but these cannot be easily mapped onto small detection elements.

Another disadvantage of thermal radiators is that their radiation can only be collimated with a considerable loss in performance, and measurements over long distances are therefore only possible with considerable effort.

Thermal light sources are generally also used in the more costly, dispersive devices, so that many of the aforementioned disadvantages remain. Laser spectroscopic measurement methods are generally only used for special applications and for highly sensitive laboratory measurements. This is partly due to the high component prices and the highly sensitive and often maintenance-intensive measuring technology.

These processes require lasers that only emit in one mode. However, this type of laser can generally only be obtained by complex selection or additional structuring of the laser, such as, for example, by “distributed feedback (DFB) structuring”. This increases the costs of such a laser.

However, other compact light sources such as IR LEDs or other non-thermal incoherent sources in the long-wave mid-infrared range (MIR) are not available.

The development of new gas and liquid measuring devices is currently moving towards the use of non-thermal light sources. Above all, so-called quantum cascade lasers (QCL) are used, which operate at room temperature as a pulsed laser source with high power density.

The development of new lighting devices, for example for motor vehicles, ships or airplanes, is currently in the direction of the use of infrared light sources. Infrared light penetrates fog much better, so that a motor vehicle, ship or plane equipped with an infrared viewer and infrared headlights can also be controlled much more safely in fog than with the usual equipment. Passive infrared sensors and infrared camera systems are now used in the automotive sector to avoid collisions with people and animals on the road. Security would be increased by additional infrared lighting.

The invention is therefore based on the object of specifying a laser device of the type mentioned at the outset and a method for operating a quantum cascade laser in which the mean spectral power density is increased compared to previous devices and methods. Furthermore, a faster and more sensitive gas measuring device should be specified. In addition, a lighting device is said to the visibility in bad weather conditions, such as B. Fog increased.

With regard to the laser device, this object is achieved according to the invention by a laser device with a quantum cascade laser with the features of claim 1.

With regard to the method, the object is achieved according to the invention by a method for operating a quantum cascade laser with the features of claim 18.

With regard to the gas measuring device, the object is achieved according to the invention by a gas measuring device having the features of claim 21.

With regard to the lighting device, the object is achieved according to the invention by a lighting device having the features of claim 25.

A laser device according to the invention with a quantum cascade laser and a pulse generator for pulsed operation of the quantum cascade laser is characterized in that the pulse generator enables the quantum cascade laser to be operated with pulse sequences from fast pulses in the range from 1 ns to 200 ns and pulse repetition frequencies in the range from 1 Hz to 100 kHz and a heat dissipation device which passively dissipates the heat is connected to the quantum cascade laser.

Such a laser device is inexpensive and has a compact design. Therefore, it can be installed in a gas measuring device or a lighting device. Fabry-Perot lasers or ridge lasers without mode-selective structuring are preferably used as QCL.

With such a laser device, much higher average spectral power densities can be achieved than with available IR LEDs. With these high spectral power densities, faster and more sensitive infrared measuring devices are possible. Here, the better collimability compared to thermal emitters can be exploited. The power can be modulated without problems using the operating current and is better than with thermal emitters. Furthermore, such a laser device in an existing arrangement, for. B. a headlight installed and operated from the 12V / 24V electrical system of a motor vehicle.

It can be advantageous if the frequency is variable. In this way, the frequency can be selected according to the respective requirement.

Furthermore, it is advantageous if the pulse height and / or pulse duration of an individual pulse and / or the pulse intervals of individual pulses can be varied in the course of the pulse packet and / or in different pulse packets. This enables a good adaptation to different work processes.

In an advantageous further development of the invention, the quantum cascade laser can be located in a preferably standardized, standard housing with at least one wall section that is transparent to the laser radiation. This wall section can be formed, for example, from a partially IR (infrared) transparent polymer (e.g. polyethylene). This results in a compact design and the laser device can also be easily installed in other devices, thereby reducing costs.

It can be advantageous if the housing with a gas, such as. B. nitrogen or argon is filled and sealed. This protects the quantum cascade laser and the assembly technology (solder connections etc.), which can be sensitive to oxidizing gases.

It can be favorable if the wall section which is transparent to the laser radiation has a Fresnel lens. This allows the radiation to be shaped depending on the application. This Fresnel lens can be formed, for example, by hot pressing from a plastic material (e.g. polyethylene).

It can be advantageous if the Fresnel lens is segmented, since it is then used for light scattering, e.g. B. in a headlight, is suitable. It can also be advantageous if the heat dissipation device is located in the housing, since this provides effective heat dissipation and a compact design.

Furthermore, it can be favorable if the heat dissipation device is designed as a passive cooling device, since this reduces the costs

Furthermore, it can be advantageous if the radiation is coupled into an infrared optical fiber. This makes it possible to redirect the radiation.

It can also be advantageous if the distance between the quantum cascade laser and the lens can be changed. This enables various lighting options to be implemented. This change in distance can be achieved, for example, by an electromagnetic device integrated in the housing, e.g. a pot coil or a piezo adjuster.

In the method according to the invention for operating a quantum cascade laser, the quantum cascade laser is operated with pulse trains consisting of individual, very short pulses in the range from 1 ns to 200 ns. The pulse trains are switched on and off with frequencies in the 1 Hz to 100 kHz range. This creates the effect of a broadband, incoherent LED-like light source.

It can be advantageous if a voltage supply with a DC / DC conversion is used to generate the pulse trains. This makes universal use of the laser device, eg. B. possible in motor vehicles or aircraft. Here, for. B. the power supply from a 12V or a 24V electrical system of a motor vehicle can be used.

A gas measuring device according to the invention is characterized in that it has a laser device according to the invention. With such a gas measuring device, a faster and more sensitive infrared measurement is possible than with conventional NDIR techniques. In addition, it may be advantageous if the gas measuring device has an inexpensive thermal detector for detecting the radiation, e.g. B. thermopile, pyrode- detector, microbolometer or photoacoustic detector.

It can be advantageous if the quantum cascade laser and the detection unit are fastened on a common substrate, since this reduces the space requirement and also the costs. For example, optical components can be saved as a result.

An illumination device according to the invention has a laser device according to at least one of claims 1 to 17. With such a lighting device, good scene lighting is possible.

It can be favorable if a further lighting device, different from the laser device, is provided. This enables independent operation.

It can also be advantageous if parts of the headlight can be used for visible and infrared light. This saves components and further reduces costs.

A method for detecting light from an illumination device according to one of claims 25 to 28 is characterized in that an infrared receiver system is used for the detection. Further processing of the infrared light emitted by the lighting device is thereby possible.

It can also be advantageous if the infrared receiver system has a thermal imaging device, since this shows an image of the area illuminated by the lighting device. The infrared receiver system can also consist of discrete independent infrared sensors.

Advantageous further developments are the subject of the respective subclaims.

The invention is explained in more detail below using an exemplary embodiment and associated drawings. These show:

Figure 1 shows a quantum cascade laser in a housing.

2 shows the control of a quantum cascade laser;

3 shows a schematic illustration of an illumination device with a quantum cascade laser and a conventional illumination device;

4 shows a schematic illustration of the laser device for measurements over long distances;

5 shows a schematic representation of the laser device for short measuring distances and large detectors;

6 shows a schematic structure of a measuring system with a pulsed quantum cascade laser, a gas cuvette and a detector;

Fig. 7 shows the pulse packets.

A laser device with a pulsed quantum cascade laser 7, as z. B. can be used in a gas measuring device 15 or an illumination device 16 is explained in more detail with reference to Figure 1. The laser device is a quantum cascade laser LED 1 according to claim 1.

The laser device has a quantum cascade laser 7 which can be operated with pulse packets which are switched at frequencies in the range from 1 Hz to 100 kHz. Since the quantum cascade laser 7 requires no special cooling, it can be via a heat dissipation device 8, for. B. a passive heat sink, by means of a fastening device 20 to a housing 4, which then z. B. is cooled by air cooling. The attachment takes place on the rear of the housing 23. The quantum cascade laser 7, the heat dissipation device 8 and the fastening device 20 are located in the housing 4. The housing 4 is usually a standardized, standard housing. Fabry-Perot lasers or ridge lasers without mode-selective structuring are preferably used as quantum cascade lasers.

The front of the housing 22 has a wall section 6 which is transparent to the laser radiation and which is arranged in a housing opening 18 on the radiation side. The wall section 6 can, for. B. from a partially IR-permeable polymer, such as. B. polyethylene formed.

Since the quantum cascade laser 7 or the assembly technique (solder connections etc.) can be sensitive to oxidizing gases, the housing 4 is, for example, with dry nitrogen 5 or another gas 5, z. B. argon, filled and sealed.

A lens 6, preferably a Fresnel lens 6, is inserted into the radiation-side housing opening 18 on the housing front side 22 for beam shaping. A segmented design of the Fresnel lens 6 can be used to implement a “fan of light” as in vehicle headlights. The Fresnel lens 6 can be formed, for example, by hot pressing from a plastic material, such as polyethylene.

A plurality of housing bushings 17 are attached to the rear of the housing 23. A negative contact 21 is introduced into the housing 4 through the housing bushing 2 and connected to the quantum cascade laser 7. A positive contact is introduced into the housing 4 through the housing bushing 3 via the heat dissipation device 8 and the holder 2. Through the remaining housing bushings 17, further contacts, for. B. temperature sensor, introduced into the housing or the gas 5 can be exchanged. Furthermore, bushings 17 for an active heat dissipation device, such as. B. a Peltier cooler or for a distance control between quantum cascade laser 7 and the side facing the quantum cascade laser 7 19 of the Fresnel lenses, such as. B. a piezo actuator is provided.

The control of the quantum cascade laser LED 1 is shown in FIG. To operate the quantum cascade laser LED 1, pulse trains from individual pulses in the range from 1 ns to 200 ns are emitted by a pulse generator 9. The fast pulse sequences are keyed in and out at a frequency in the 1 Hz to 100 kHz range, so that the quantum cascade laser LED 1 behaves like a current-modulated LED. The tactile The ratio between the fast pulse sequences and the time-outs is chosen such that the quantum cascade laser 7 relaxes thermally again to the initial temperature value during the time-out.

Between the pulse generator 9 and the quantum cascade LED 1 there is a circuit breaker 10 with which the forwarding of the pulse trains from the pulse generator 9 to the quantum cascade laser LED 1 can be switched on / off or controlled.

The use of a voltage supply (not shown) with a DC / DC conversion for generating the pulse trains is favorable since the quantum cascade laser LED 1 can thereby be used universally.

The fact that a 12-volt or a 24-volt electrical system can be used for the power supply, an application z. B. possible in a motor vehicle.

FIG. 3 schematically shows a headlamp 16 with a quantum cascade laser LED 1 and a conventional lighting device 33.

In a headlight housing 34 there are a quantum cascade laser LED 1 and a further lighting device 33 that differs from the quantum cascade laser LED 1. This can be, for example, a glow emission filament, a halogen lamp or a gas discharge lamp.

The two lighting devices 1 and 33 are mounted behind a common headlight cover 35.

For scene lighting, the distance d of the quantum cascade laser 7 from the side 19 of the Fresnel lens 6 facing the quantum cascade laser can be selected to be smaller than the lens focal length, as a result of which a divergent beam 11 is generated. By segmented design of the Fresnel lens 6 headlamps may be a "light fan" as in vehicle ^'can be realized. The fact that parts of the headlamp 16 such as. For example, an unillustrated reflector or headlight cover 35 for both visible and infrared light can be used, such a headlight 16 can be produced easily and inexpensively. For the detection of infrared light, e.g. B. from a lighting device, as described in Figure 3, an infrared receiver system can be used, for. B. has a thermal imaging device. Infrared detectors 14 based on thermopiles, bolometers or pyrodetectors and a phase-synchronous detection method are also conceivable. The infrared receiver system can also consist of discrete independent infrared sensors. With such a system z. B. in fog the visibility of a driver can be significantly improved. Such a system thus contributes significantly to increasing security.

The shape of the beam 11 can be changed by changing the distance d between the quantum cascade laser 7 and the side 19 of the lens 6 facing the quantum cascade laser. The change in distance d can e.g. B. be made by changing the size of the heat dissipation device 8. The change in the distance d can also be achieved, for example, by a piezo adjuster or an electromagnetic device (pot coil) integrated in the housing.

A quantum cascade laser LED 1 for measurements over large distances is shown schematically in FIG. In order to be able to measure over large distances, the beam 11 must be collimated. For this purpose, the distance d of the quantum cascade laser 7 from the Fresnel lens 6 is selected to be equal to the lens focal length.

FIG. 5 shows a quantum cascade laser 1 for direct imaging of the beam 11 on a large-area detector, not shown. For short measuring distances and large detectors (e.g. pyrodetectors), the beam 11 can be focused again directly on the focus 12 by the Fresnel lens 6 by the distance d of the quantum cascade laser 7 from the side 19 of the Fresnel lens facing the quantum cascade laser 6 is chosen larger than the focal length of the lens.

FIG. 6 shows the schematic structure of a gas measuring device 15 with a quantum cascade laser LED 1, a gas cuvette 13 and a detector 14. The gas cuvette 13 has a gas cuvette housing 24, a side wall for light entry 25, through which the beam 11 emitted by the quantum cascade laser 7 can enter the gas cuvette interior 27 and a side wall for light exit 26 through which the Beam 11 can exit the gas cell interior 27 again. The gas cuvette interior 27 is filled with gas. Alternatively, the gas cell interior 27 can also be filled with a liquid. Then the gas measuring device 15 can also be used as a liquid measuring device.

The detector 14 has a detector housing 28 and a sensor 29 which is attached to the rear of the detector 36 by means of a sensor holder 30. The sensor 29 and the sensor holder 30 are located in the detector housing 28. The detector housing 28 has a radiation entry opening 31 on the detector front side 37, through which the beam 11 can enter the detector 14. On the back of the detector 36 housing bushings 32 are arranged through which z. B. lines, not shown, can be introduced into the detector housing 28.

It is conceivable that the gas measuring device 15 has a thermal detector 14 for detecting the radiation 11. Such detectors 14 are inexpensive and well suited for the detection of infrared light.

It is also conceivable that the quantum cascade laser 7 and the detector unit 14 are attached to a common substrate. A compact design of the gas measuring device 15 can thereby be achieved and optical components can be saved.

In the case of a gas measuring device 15 in particular, it can be advantageous if the radiation 11 emanating from the quantum cascade laser 7 is coupled into an infrared optical fiber (not shown). As a result, the radiation 11 of the quantum cascade laser 7 can be redirected and the arrangement of the gas cuvette 13 can be carried out according to the particular requirements of the measurement.

FIG. 7 shows the pulse packets 38 as a function of time. The switching frequency 1 / time does not necessarily have to be constant. For lighting purposes, e.g. In vehicles, for example, it can be advantageous to have a "sleep mode with a relatively low switching frequency, for example 10 Hz. When observing an interesting object with an IR camera, the switching frequency can be increased, for a better one To ensure detection of moving objects or to achieve brighter lighting. With gas sensors, too, it can be advantageous if the switching frequency is not constant. With a low gas concentration, a relatively low switching frequency can be sufficient. If the gas concentration suddenly rises above a certain threshold, the switching frequency can be increased in order to ensure a rapid detection of the increased gas concentration.

Likewise, the pulse height of an individual pulse 39 shown in FIG. 7 need not remain constant, but can be varied in the course of the pulse packet 38 or also from pulse packet 38 to pulse packet 38. This also applies to the pulse duration and the spacing of individual pulses. This enables a good adaptation to different applications. In Fig. 7 the laser drive signal, e.g. Operating voltage on the laser (volt) or operating current (ampere), plotted on the ordinate.

With the quantum cascade laser LED 1, average spectral power densities of> 1mW / (mm ² 'srψm) can be achieved. This is many times higher than with available IR LEDs, which are also only available in a wavelength range of 3 to 5 μm. This is also many times higher than can be achieved with thermal emitters.

Another advantage of the invention is that gas measuring devices 15 can be realized with significantly better properties than before. This also applies to liquid measuring devices. Furthermore, the effort for the control and detection electronics compared to the laser spectroscopic measuring systems is reduced, so that there are significant cost advantages. This means that for the first time, laser-based systems can also compete in price with non-dispersive infrared measuring devices without giving up important advantages of laser measuring technology.

With these high spectral power densities, faster and more sensitive infrared measuring devices with compact, but specific and highly sensitive sensors are possible, which can be used in production and process measurement technology, in the automotive sector, in security technology and in climate and environmental sensors. The better collimability compared to thermal emitters can also be exploited here. The quantum cascade laser LED 1 according to the invention can also be used for scene lighting in the infrared. Infrared light in the spectral range between 8 and 12 μm penetrates fog much better, so that a motor vehicle, ship or plane equipped with an infrared viewer and infrared headlights can also be controlled much more safely in fog than with the usual equipment. The lighting device 16 according to the invention provides additional security compared to previous passive infrared sensors. Additional infrared lighting increases the detection reliability of people and animals on the road and reduces malfunctions. In stationary use, the function of infrared detectors, such as. B. of motion detectors, object protection can be improved by additional infrared lighting.

The quantum cascade laser LED 1 according to the invention is an ideal light source for the aforementioned applications. You can in existing lighting devices such. B. headlights 16, can be integrated and can be operated via the usual 12V / 24V vehicle electrical system. The spectral range is also adapted to the range of the natural emission of objects near room temperature (10 μm), so that the same detection systems can be used as for passive detection.

In automotive applications, glare resistance when using such additional light sources is an important issue. The strongly polarized beam 11 of a quantum cascade laser LED 1 can be blocked on the one hand in direct reflection by means of suitable polarization filters in front of the corresponding IR sensors. The radiation is narrow-band, so that suitable filters can be used in front of broadband receivers. Above all, however, the good modulability of the quantum cascade laser LED 1 down to the 100 kHz range enables phase-locked lock-in detection. Scattering and glare from another vehicle can thus be effectively suppressed.

A sensitive measurement method, such as the Lockin method, is only possible for scene analysis because the power can be modulated without problems via the operating current and is better than with thermal emitters. The modulability is only limited by the heat balance of the quantum cascade laser 7. A low duty cycle <1% (ie pulse sequence on / off duty cycle) can be used for scene analysis. So that the thermal load of the component is low, so that this is also in existing arrangements such. B. a headlight 16 can be installed.

Claims

claims

1. Laser device with a quantum cascade laser (7) and a pulse generator (9) for pulsed operation of the quantum cascade laser (7), characterized in that the pulse generator (9) of the quantum cascade laser (7) with pulse packets that operate at frequencies in the 1 Hz to 100 kHz range can be switched, can be operated and a heat dissipation device (8) is connected to the quantum cascade laser (7).

2. Laser device according to claim 1, characterized in that the frequency is variable.

3. Laser device according to claim 1 or 2, characterized in that a single pulse (39) of a pulse packet (38) has a pulse width in the range from 1 ns to 200 ns.

4. Laser device according to claim 3, characterized in that the pulse height and / or pulse duration of the individual pulse (39) and / or the pulse spacing of individual pulses (39) in the course of the pulse packet (38) and / or in different pulse packets (38) are varied can.

5. Laser device according to at least one of the preceding claims 1 to 4, characterized in that preferably Fabry-Perot laser or ridge laser without mode-selective structuring are used as the quantum cascade laser (7).

6. Laser device according to at least one of the preceding claims 1 to 5, characterized in that the quantum cascade laser (7) in a housing (4), preferably in a standardized, standard housing (4), with at least one wall section transparent to the laser radiation ( 6) is located.

7. Laser device according to claim 6, characterized in that the housing (4) is filled with a gas (5).

8. Laser device according to claim 6 or 7, characterized in that the housing (4) is filled with an inert gas (5), preferably nitrogen or argon.

9. Laser device according to at least one of the preceding claims 6 to 8, characterized in that the wall section (6) which is transparent to the laser radiation is formed from a partially IR-transparent polymer, preferably polyethylene.

10. Laser device according to at least one of the preceding claims 6 to 9, characterized in that the wall section which is transparent to the laser radiation has a Fresnel lens (6).

11. Laser device according to claim 10, characterized in that the Fresnel lens (6) is segmented.

12. Laser device according to claim 10 or 11, characterized in that the Fresnel lens (6) is formed by hot pressing from a plastic material, preferably polyethylene.

13. Laser device according to at least one of the preceding claims 6 to 12, characterized in that the heat dissipation device (8) is in the housing (4).

14. Laser device according to at least one of the preceding claims 1 to 13, characterized in that the heat dissipation device (8) is designed as a passive cooling device.

15. Laser device according to at least one of the preceding claims 1 to 14, characterized in that a radiation (11) is coupled into an infrared optical fiber.

16. Laser device according to at least one of the preceding claims 1 to 15, characterized in that the distance (d) between the quantum cascade laser (7) and the lens (6) is variable.

17. Laser device according to claim 16, characterized in that the change in the distance (d) is achieved by an electromagnetic device, preferably a pot coil or by a piezo adjuster.

18. Method for operating a quantum cascade laser (7) according to at least one of the preceding claims 1 to 17, characterized in that the quantum cascade laser (7) is operated with pulse packets (38) consisting of individual very short pulses in the range from 1 ns to 200 ns. wherein the pulse packets (38) are switched on and off with frequencies in the range from 1 Hz to 100 kHz.

19. The method according to claim 18, characterized in that a voltage supply with a DC / DC conversion is used to generate the pulse trains.

20. The method according to claim 19, characterized in that the voltage supply from the electrical system of a motor vehicle, in particular a 12V or a 24V electrical system, is used.

21. Gas measuring device with a laser device according to at least one of the preceding claims 1 to 17.

22. Gas measuring device according to claim 21, characterized in that the gas measuring device (15) has a gas cuvette (13).

23. Gas measuring device according to claim 21 or 22, characterized in that the gas measuring device (15) for detecting the radiation has a thermal detector (14).

24. Gas measuring device according to at least one of the preceding claims 21 to 23, characterized in that the quantum cascade laser (7) and the detection unit (14) are attached to a common substrate.

25. Illumination device with a laser device according to at least one of the preceding claims 1 to 17.

26. Lighting device, in particular headlights (16), with a lighting device according to claim 25, characterized in that a further lighting device (33) is provided which is different from the laser device.

27. Lighting device according to claim 26, characterized in that the lighting device (33) different from the laser device preferably has a glow emission filament, a halogen lamp or a gas discharge lamp.

28. Lighting device according to at least one of the preceding claims 25 to 27, characterized in that parts of the headlight (16) can be used for visible and infrared light.

29. A method for detecting light from an illumination device according to at least one of the preceding claims 25 to 28, characterized in that an infrared receiver system is used for the detection.

30. The method according to claim 29, characterized in that the infrared receiver system (14) has a thermal imaging device.

31. The method according to claim 29, characterized in that the infrared receiver system (14) has an arrangement of thermal infrared detectors, in particular thermopiles, pyrodetectors or bolometers.

32. The method according to claim 29, characterized in that a phase-synchronous detection method is used. Method according to at least one of the preceding claims 29 to 32, characterized in that the infrared receiver system (14) has a polarization filter for suppressing glare.