Limited range infrared illuminator and method for illuminating scene with infrared radiation

Abstract

The invention relates to a limited range infrared illuminator (1). The illuminator (1) comprises a radiation source (10) arranged to emit a beam (B) of infrared radiation within a predetermined wavelength range in the near-infrared range. The illuminator (1) comprises a beam splitter (20) arranged to divide the beam (B) into an illumination beam (IB) and a control beam (CB). The illuminator comprises an analyser (30) arranged to analyse absorption of the control beam (CB) in a reference medium. The illuminator comprises a controller (40) connected to the analyser (30) and to the radiation source (10) and arranged to adjust the wavelength of the beam (B) based on the analysis by the analyser (30). The invention relates also to a method for illuminating a scene with infrared radiation.

Description

[0001] LIMITED RANGE INFRARED ILLUMINATOR AND METHOD FOR ILLUMINATING SCENE WITH INFRARED RADIATION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an infrared illuminator and more particularly to a limited range infrared illuminator. The present invention further concerns a method for illuminating a scene with infrared radiation.

[0004] BACKGROUND OF THE INVENTION

[0005] Infrared radiation can be used to illuminate a scene, for example a view of an infrared surveillance camera, without light visible to the naked eye. Normally such illumination is done from the direction of the camera to avoid shadows in the illumination. In other words, the infrared illuminator is normally placed next to the infrared camera. However, while such infrared radiation does not cause light pollution in the traditional sense, it may still disturb another infrared camera pointing towards the infrared illuminator at a distance.

[0006] The aforementioned problem can be overcome by limiting the range of the infrared illuminator by using an infrared illuminator that emits infrared radiation only at a wavelength that corresponds to an absorption band wavelength of a constituent of air. For example, the constituent is oxygen, ozone or water vapour, all of which have an absorption band wavelength in the infrared range. This causes the ambient air to absorb the infrared radiation in such a way that the visibility range of the infrared radiation is limited. Depending on the power of the infrared illuminator, the infrared radiation may be absorbed within a few hundred meters to such a level that it cannot be distinguished from noise by a distant infrared camera while still providing sufficient illumination near the illuminator.

[0007] A problem with emitting radiation at such a limited wavelength is that the actual wavelength emitted by an infrared radiation source of the illuminator may vary slightly, for example due to the temperature of the infrared radiation source. This slight variation may take the radiated wavelength out of the desired absorption band wavelength. Therefore the radiation emitted by the infrared radiation source is to be controlled continuously to keep it tuned to the desired absorption band wavelength. Controlling the emitted radiation requires measuring the emitted radiation. A problem is that the measurement consumes a significant part of the output, or the photon flux, of the infrared radiation emitted by the radiation source, resulting in the output from the illuminator being significantly less than the output obtained from the radiation source of the illuminator.

[0008] BRIEF DESCRIPTION OF THE INVENTION

[0009] An object of the present invention is to provide a limited range infrared illuminator and a method for illuminating a scene with infrared radiation alleviating the problem above.

[0010] The object of the invention is achieved by a limited range infrared illuminator and a method for illuminating a scene with infrared radiation which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

[0011] The invention is based on the idea of providing a limited range infrared illuminator, wherein the illuminator comprises a radiation source arranged to emit a beam of infrared radiation within a predetermined wavelength range in the near-infrared range, a beam splitter arranged to divide the beam into an illumination beam and a control beam, an analyser arranged to analyse absorption of the control beam in a reference medium, and a controller connected to the analyser and to the radiation source and arranged to adjust the wavelength of the beam based on the analysis by the analyser.

[0012] The invention is also based on the idea of providing a method for illuminating a scene with infrared radiation, wherein the method comprises emitting a beam of infrared radiation within a predetermined wavelength in the near-infrared range, dividing the beam into an illumination beam and a control beam, analysing absorption of the control beam in a reference medium, adjusting the wavelength of the beam based on the analysis of the absorption of the control beam, and illuminating a scene using the illumination beam. An advantage of the invention is that because the control beam is separated from the illumination beam, the measurement of the control beam does not affect the power of the illumination beam. Therefore, only the power needed for a measurement reliable enough can be used for the control beam, while the rest of the power of the beam can be used in the illumination beam.

[0013] Another advantage of the invention is that the illumination beam can be modified independently from the control beam. Thus, any modification in the illumination beam causes no disturbance in the measurement of the control beam.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Below, the invention is described in detail with reference to the enclosed drawings, in which

[0016] Figure 1 is a schematic drawing of an illuminator according to some embodiments of the invention;

[0017] Figure 2 is a schematic drawing of an illuminator according to some embodiments of the invention;

[0018] Figure 3 is a schematic drawing of a radiation source according to some embodiments of the invention;

[0019] Figure 4 is a schematic drawing of a radiation source according to some embodiments of the invention;

[0020] Figure 5 is a schematic drawing of an analyser according to some embodiments of the invention;

[0021] Figure 6 is a schematic drawing of a beam modifier according to some embodiments of the invention;

[0022] Figure 7 is a schematic drawing of a beam modifier according to some embodiments of the invention; Figure 8 is a schematic drawing of a beam modifier according to some embodiments of the invention; and

[0023] Figure 9 is a schematic drawing of a beam modifier according to some embodiments of the invention.

[0024] DETAILED DESCRIPTION OF THE INVENTION

[0025] Illuminator

[0026] The invention relates to a limited range infrared illuminator 1. In other words, the illuminator 1 has a limited visibility range, i.e. the distance from which the infrared radiation emitted by the illuminator can be detected is limited compared to a traditional infrared illuminator.

[0027] For example, the illuminator 1 can be used to illuminate a scene monitored by an infrared camera.

[0028] Radiation source

[0029] The illuminator 1 comprises a radiation source 10. The radiation source 10 is arranged to emit a beam B of infrared radiation within a predetermined wavelength range in the near-infrared range. Preferably, the radiation source 10 is arranged to emit the beam B only within the predetermined wavelength range. In this context, term "near-infrared range” means electromagnetic radiation having a wavelength of 750 to 1400 nm.

[0030] According to some embodiments, the predetermined wavelength range is in the range of 759 to 763 nm, preferably 760 to 762 nm, or 808 to 812 nm, preferably 809 to 811 nm, or 918 to 922 nm, preferably 919 to 921 nm, or 958 to 962, preferably 959 to 961 nm, or 1063 to 1067 nm, preferably 1064 to 1066 nm, or 1136 to 1140 nm, preferably 1137 to 1139 nm, or 1268 to 1272 nm, preferably 1269 to 1271 nm, or 1380 to 1384 nm, preferably 1381 to 1383 nm, or 1386 to 1390 nm, preferably 1387 to 1389 nm, or 1390 to 1394 nm, preferably 1391 to 1393 nm. Oxygen gas (O2) has an absorption band in vacuum at a wavelength around 760 nm, 810 nm, 920 nm, 1065 nm, and 1270 nm. Ozone (O3) has an absorption band in vacuum at wavelengths around 960 nm and 1138 nm. Water (H2O) vapour has an absorption band in vacuum at wavelengths around 1382.8 nm, 1388.1 nm, and 1392.4 nm. Preferably, the predetermined wavelength range corresponds to an absorption band in vacuum wavelength of oxygen gas, ozone or water vapour.

[0031] According to some embodiments, the width of the band of the infrared radiation emitted by the radiation source 10 is 0.01 to 1 nm, such as 0.05 to 0.5 nm, preferably 0.1 to 0.25 nm. In other words, the radiation source is arranged to emit a beam B of infrared radiation having a certain bandwidth within the predetermined wavelength range. This allows adjusting the infrared radiation emitted by the radiation source 10 while staying within the predetermined wavelength range.

[0032] The radiation source 10 is arranged to allow adjusting the wavelength of the emitted infrared radiation. In other words, the wavelength emitted by the radiation source 10 can be adjusted or fine-tuned.

[0033] Broadband infrared emitter

[0034] According to some embodiments, the radiation source 10 comprises a broadband infrared emitter 11 and an adjustable filter element 12, as exemplified in Figure 3.

[0035] In this context, term "broadband infrared emitted’ means an emitter that emits electromagnetic radiation in the infrared range, and that the range is broader than 10 nm.

[0036] The adjustable filter element 12 is configured to filter the infrared radiation emitted by the broadband infrared emitter 11 to the predetermined wavelength range. In other words, the broad wavelength range emitted by the broadband infrared emitter 11 is limited to the predetermined wavelength range by the adjustable filter element 12.

[0037] The adjustable filter element 12 is arranged to allow the adjustment of the filtered wavelength range. In other words, the wavelength range passed through by the adjustable filter element 12 can be adjusted. This allows fine-tuning of the wavelength range that is passed through, for example depending on the prevailing conditions. Laser

[0038] According to some embodiments, the radiation source 10 comprises a laser 13, as exemplified in Figure 4. Laser is able to emit infrared radiation having a specific wavelength without having to use filters and therefore it has an improved efficiency.

[0039] Preferably, the laser 13 is a semiconductor laser or a diode laser. For example, the laser 13 is a single-mode laser, distributed feedback laser, external cavity laser, a gas laser, a solid-state laser, or a single wavelength laser.

[0040] The wavelength of the infrared radiation emitted by the laser 13 can be adjusted by adjusting the operation temperature of the laser 13 and the electrical current provided to the laser 13. Preferably, the radiation source 10 comprises a heater and / or a cooler (not shown in the figures) for adjusting the operation temperature of the laser 13.

[0041] According to some embodiments, the radiation source 10 comprises a first lens 14 for collimating the infrared radiation emitted by the laser 13 to obtain the beam B. In other words, the first lens 14 is used to collimate the infrared radiation emitted by the laser 13 to a consistent beam in which divergence and convergence is minimized.

[0042] Beam splitter

[0043] The illuminator 1 comprises a beam splitter 20. The beam splitter 20 is arranged to divide the beam B into an illumination beam IB and a control beam CB.

[0044] Preferably, the beam splitter 20 is arranged immediately downstream of the radiation source 10. In other words, there is no element affecting the beam B between the radiation source 10 and the beam splitter 20. This is to ensure that the control beam CB is obtained in as early stage as possible from the beam B to keep it clean from any disturbances.

[0045] According to some embodiments, the beam splitter 20 comprises a semi-permeable mirror. For example, the beam splitter 20 comprises a partially silvered mirror.

[0046] According to some embodiments, the beam splitter 20 is arranged to divide the beam B in such a way that the photon flux of the control beam CB is at least 1 %, such as 1 to 10 %, preferably 2 to 7 %, of the photon flux of the beam B. This ensures that the control beam CB is strong enough but that power of the beam B is not wasted into the control beam CB but is used in the illumination beam IB.

[0047] The illuminator 1 comprises an analyser 30. The analyser 30 is arranged to analyse absorption of the control beam CB in a reference medium, such as a reference gas. In other words, the analyser 30 is arranged to analyse, what is the photon flux of the control beam CB not absorbed in a reference medium.

[0048] The control beam CB is directed from the beam splitter 20 to the analyser 30. The control beam CB may be guided using one or more mirrors between the beam splitter 20 and the analyser 30.

[0049] The absorption of the control beam CB in the reference medium corresponds to the absorption of the illumination beam IB in the corresponding medium in the atmosphere. Therefore, the reference medium is selected from the constituents of atmosphere having an absorption band in the near-infrared range. According to some embodiments, the reference medium is water vapour, ozone or oxygen. Both water vapour and oxygen have a sufficient absorbency in the infrared range. While the concentration of water vapour in the atmosphere may change, for example, depending on weather conditions, the concentration of oxygen is relatively constant and therefore the absorbency of infrared radiation in oxygen in the atmosphere stays steadier. Due to low concentration of ozone in the atmosphere near the surface of the Earth, the absorbency of infrared radiation in ozone is lower than in water vapour or oxygen, but it might still be sufficient for some applications.

[0050] Absorber and beam detector

[0051] According to some embodiments, the analyser 30 comprises an absorber 31 and a beam detector 32, as exemplified in Figure 5.

[0052] The reference medium is contained by the absorber 31. For example, the absorber 31 is a vessel containing the reference medium, or an optical fibre comprising an array of holes filled with the reference medium running along the length of the fibre, i.e. photonic bandgap (PEG) fibre.

[0053] According to some embodiments, the concentration of the reference medium in the absorber 31 is higher than the concentration of the reference medium in the atmosphere. In other words, the concentration of the reference medium in the absorber 31 is higher than the concentration of the reference medium in the environment surrounding the absorber 31. For example, the concentration of the reference medium in the absorber 31 is at least 200 mole-% of the concentration of the reference medium in the atmosphere. Preferably, the concentration of the reference medium in the absorber 31 is 50 to 100 % by mole, more preferably 75 to 99 % by mole, most preferably 95 to 99 % by mole. This is to increase the absorbency in the absorber 31 so that either the absorber 31 can be made shorter or the analysis result is improved.

[0054] The absorber 31 is at least partially transparent to allow infrared radiation to penetrate the absorber 31 so that it reaches the beam detector 32.

[0055] The beam detector 32 is arranged to measure the control beam CB led through the absorber 31. In other words, the beam detector 32 is arranged to measure the photon flux of the control beam CB led through the absorber 31.

[0056] Controller

[0057] The illuminator 1 comprises a controller 40. The controller 40 is connected to the analyser 30 and to the radiation source 10. For example, the controller 40 is connected electrically to the analyser 30 and to the radiation source 10. The controller 40 is arranged to adjust the wavelength of the beam B based on the analysis by the analyser 30. In other words, the controller 40 adjusts the radiation source 10 based on the measurement data from the analyser 30. The actual wavelength of the beam B may vary depending on conditions. The purpose of the controller 10 is to maintain the wavelength of the beam B at the predetermined wavelength range, i.e. the absorption band wavelength of the reference medium. According to some embodiments, the controller 40 is arranged to adjust the wavelength of the beam B in such a way that the absorption of the control beam CB in the reference medium is maximized. In other words, the controller 40 is arranged to find the lowest transmittance of the control beam CB in the reference medium. The purpose of the controller 40 is to avoid a situation in which the illuminator emits infrared radiation at a frequency that is not absorbed by the reference medium because the wavelength of the emitted infrared radiation is slightly different from the absorption band of the reference medium.

[0058] According to some embodiments, the controller 40 is arranged to adjust the wavelength of the beam B by adjusting the adjustable filter element 12. In other words, the controller 40 is arranged to adjust the wavelength range filtered by the adjustable filter element 12.

[0059] According to some embodiments, the controller 40 is arranged to adjust the wavelength of the beam B by adjusting the operation temperature of the laser 13, the electrical current provided to the laser 13, or both. For example, the operation temperature of the laser 13 is adjusted by heating and / or cooling the laser 13 using the heater and / or the cooler.

[0060] Beam modifier

[0061] According to some embodiments, the illuminator 1 comprises a beam modifier 50, as exemplified in Figure 2. The beam modifier 50 is arranged to modify the illumination beam IB. The purpose of the beam modifier is to modify the illumination beam IB to suit better for illuminating the scene.

[0062] According to some embodiments, the beam modifier 50 comprises an aperture 51 for limiting the illumination beam IB, as exemplified in Figures 6, 7, and 9. The purpose of the aperture 51 is to limit the illumination beam IB to avoid scattered light exiting the illuminator 1 pointing in different direction than the illumination beam IB. Preferably, the aperture 51 is arranged to limit the illumination beam IB outside the brightest part of the illumination beam IB. According to some embodiments, the beam modifier 50 comprises a second lens 52 for dispersing the illumination beam IB, as exemplified in Figures 6, 8, and 9. The purpose of the second lens 52 is to make the illumination beam IB to illuminate a broader area.

[0063] According to some embodiments, the beam modifier 50 comprises a diffuser 53 for diffusing the illumination beam IB, as exemplified in Figures 7, 8, and 9. The purpose of the diffuser 53 is to improve laser safety, if laser 13 is used, and to reduce interference patterns in the illumination beam IB.

[0064] According to some embodiments, the beam modifier 50 comprises two or more of the aperture 51, the second lens 52 and the diffuser 53.

[0065] Power source

[0066] The illuminator 1 comprises a power source (not shown in the figures) connected to the radiation source 10, to the analyser 30 and to the controller 40 to provide them with electrical power. For example, the power source comprises a battery. The power source may also or alternatively comprise a connector for connecting the illuminator 1 to an external power supply.

[0067] Method

[0068] The invention relates to a method for illuminating a scene with infrared radiation. For example, the scene is illuminated for monitoring the scene with an infrared surveillance camera.

[0069] According to some embodiments, the method is conducted by the limited range infrared illuminator 1 as described above.

[0070] Emitting beam

[0071] The method comprises emitting a beam B of infrared radiation within a predetermined wavelength range in the near-infrared range. Preferably, the beam is emitted B only at the predetermined wavelength range. For example, the beam is emitted by the radiation source 10 as described above. In this context, term "near- infrared range” means electromagnetic radiation having a wavelength of 750 to 1400 nm.

[0072] According to some embodiments, the predetermined wavelength range is in the range of 759 to 763 nm, preferably 760 to 762 nm, or 808 to 812 nm, preferably 809 to 811 nm, or 918 to 922 nm, preferably 919 to 921 nm, or 958 to 962, preferably 959 to 961 nm, or 1063 to 1067 nm, preferably 1064 to 1066 nm, or 1136 to 1140 nm, preferably

[0073] 1137 to 1139 nm, or 1268 to 1272 nm, preferably 1269 to 1271 nm, or 1380 to 1384 nm, preferably 1381 to 1383 nm, or 1386 to 1390 nm, preferably 1387 to 1389 nm, or 1390 to 1394 nm, preferably 1391 to 1393 nm. Oxygen gas (O2) has an absorption band in vacuum at a wavelength around 760 nm, 810 nm, 920 nm, 1065 nm, and 1270 nm. Ozone (O3) has an absorption band in vacuum at wavelengths around 960 nm and

[0074] 1138 nm. Water (H2O) vapour has an absorption band in vacuum at wavelengths around 1382.8 nm, 1388.1 nm, and 1392.4 nm. Preferably, the predetermined wavelength range corresponds to an absorption band in vacuum wavelength of oxygen gas, ozone or water vapour.

[0075] According to some embodiments, the width of the band of emitted beam B of infrared radiation is 0.01 to 1 nm, such as 0.05 to 0.5 nm, preferably 0.1 to 0.25 nm. In other words, the beam B of infrared radiation has a certain bandwidth within the predetermined wavelength range. This allows adjusting the infrared radiation while staying within the predetermined wavelength range.

[0076] Dividing beam

[0077] The method comprises dividing the beam B into an illumination beam IB and a control beam CB. For example, the beam B is divided by the beam splitter 20 as described above.

[0078] According to some embodiments, the beam B is divided in such a way that the photon flux of the control beam CB is at least 1 %, such as 1 to 10 %, preferably 2 to 7 % of the photon flux of the beam B. This ensures that the control beam CB is strong enough but that power of the beam B is not wasted into the control beam CB but is used in the illumination beam IB.

[0079] The method comprises analysing absorption of the control beam CB in a reference medium, such as a reference gas. In other words, the method comprises determining the photon flux of the control beam CB not absorbed in a reference medium. For example, the absorption is analysed by the analyser 30 as described above.

[0080] According to some embodiments, the reference medium is water vapour, ozone or oxygen. Both water vapour and oxygen have a sufficient absorbency in the infrared range. While the concentration of water vapour in the atmosphere may change, for example, depending on weather conditions, the concentration of oxygen is relatively constant and therefore the absorbency of infrared radiation in oxygen stays steadier. Due to low concentration of ozone in the atmosphere near the surface of the Earth, the absorbency of infrared radiation in ozone is lower than in water vapour or oxygen, but it might still be sufficient for some applications.

[0081] The method comprises adjusting the wavelength of the beam B based on the analysis of the absorption of the control beam CB. For example, the wavelength of the beam B is adjusted by the controller 40 as described above. The actual wavelength of the beam B may vary depending on conditions. The purpose of adjusting the wavelength is to maintain the wavelength of the beam B at the predetermined wavelength range, i.e. the absorption band wavelength of the reference medium.

[0082] According to some embodiments, the wavelength of the beam B is adjusted in such a way that the absorption of the control beam CB in the reference medium is maximized. In other words, the wavelength of beam B is adjusted to find the lowest transmittance of the control beam CB in the reference medium. The purpose of the adjustment is to avoid a situation in which the illuminator emits infrared radiation at a frequency that is not absorbed by the reference medium because the wavelength of the emitted infrared radiation is slightly different from the absorption band of the reference medium. illumination beam

[0083] According to some embodiments, the method comprises modifying the illumination beam IB. The purpose of modifying the illumination beam IB is to make it suit better for illuminating the scene. For example, the illumination beam IB is modified by the beam modifier 50 as described above.

[0084] According to some embodiments, modifying the illumination beam IB comprises limiting the illumination beam IB. The purpose of limiting the illumination beam IB is to avoid scattered light when illuminating the scene. For example, the limiting is done by an aperture 51.

[0085] According to some embodiments, modifying the illumination beam IB comprises dispersing the illumination beam IB. The purpose of dispersing the illumination beam is to make the illumination beam IB to illuminate a broader area. For example, the dispersing is done by the second lens 52 as described above.

[0086] According to some embodiments, modifying the illumination beam IB comprises diffusing the illumination beam IB. The purpose of diffusing the illumination beam IB is to improve laser safety, if laser is used for emitting the beam B, and to reduce interference patterns in the illumination beam IB. For example, the diffusing is done by the diffuser 53 as described above. scene

[0087] The method comprises illuminating a scene using the illumination beam IB. In other words, the scene is illuminated with infrared radiation of the illumination beam IB. For example, the scene is illuminated for monitoring the scene with an infrared surveillance camera.

[0088] The invention has been described above with reference to the examples shown in the figures. However, the invention is in no way restricted to the above examples but may vary within the scope of the claims.

Claims

CLAIMS1. A limited range infrared illuminator (1), characterized in that the illuminator (1) comprises a radiation source (10) arranged to emit a beam (B) of infrared radiation within a predetermined wavelength range in the near-infrared range, a beam splitter (20) arranged to divide the beam (B) into an illumination beam (IB) and a control beam (CB),- an analyser (30) arranged to analyse absorption of the control beam (CB) in a reference medium, and a controller (40) connected to the analyser (30) and to the radiation source (10) and arranged to adjust the wavelength of the beam (B) based on the analysis by the analyser (30).

2. The limited range infrared illuminator (1) according to claim 1, characterized in that the radiation source (10) comprises a broadband infrared emitter (11) and an adjustable filter element (12) configured to filter the infrared radiation to the predetermined wavelength range, wherein the adjustable filter element (12) is arranged to allow the adjustment of the filtered wavelength range.

3. The limited range infrared illuminator (1) according to claim 1, characterized in that the radiation source (10) comprises a laser (13).

4. The limited range infrared illuminator (1) according to claim 3, characterized in that the radiation source (10) comprises a first lens (14) for collimating the infrared radiation emitted by the laser (13) to obtain the beam (B).

5. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that the predetermined wavelength range is in the range of 759 to 763 nm, preferably 760 to 762 nm, or 808 to 812 nm, preferably 809 to 811 nm, or 918 to 922 nm, preferably 919 to 921 nm, or 958to 962, preferably 959 to 961 nm, or 1063 to 1067 nm, preferably 1064 to 1066 nm, or 1136 to 1140 nm, preferably 1137 to 1139 nm, or 1268 to 1272 nm, preferably 1269 to 1271 nm, or 1380 to 1384 nm, preferably 1381 to 1383 nm, or 1386 to 1390 nm, preferably 1387 to 1389 nm, or 1390 to 1394 nm, preferably 1391 to 1393 nm.

6. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in thatthe beam splitter (20) is arranged to divide the beam (B) in such a way that the photon flux of the control beam (CB) is at least 0.5 %, preferably 2 to 10 %, of the photon flux of the beam (B).

7. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that- the analyser (30) comprises an absorber (31) and a beam detector (32); the reference medium is contained by the absorber (31); and the beam detector (32) is arranged to measure the control beam (CB) led through the absorber (31).

8. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that the reference medium is water vapour, ozone or oxygen.

9. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that the controller (40) is arranged to adjust the wavelength of the infrared radiation emitted by the radiation source (10) in sucha way thatthe absorption of the control beam (CB) in the reference medium is maximized.

10. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that the controller (40) is arranged to adjust the wavelength of the infrared radiation emitted by the radiation source (10) by adjusting the adjustable filter element (12).

11. The limited range infrared illuminator (1) according to any one of claims 1 to 10, characterized in that the controller (40) is arranged to adjust the wavelength of the infrared radiation emitted by the radiation source (10) by adjusting the operation temperature of the laser (13), the electrical current provided to the laser (13), or both.

12. The limited range infrared illuminator (1) according to any one of the preceding claims, characterized in that the limited wavelength range infrared illuminator (1) comprises a beam modifier (50) arranged to adjust the illumination beam (IB).

13. A method for illuminating a scene with infrared radiation, characterized in that the method comprises emitting a beam (B) of infrared radiation within a predetermined wavelength range in the near-infrared range, dividing the beam (B) into an illumination beam (IB) and a control beam (CB), analysing absorption of the control beam (CB) in a reference medium, adjusting the wavelength of the beam (B) based on the analysis of the absorption of the control beam (CB), and illuminating a scene using the illumination beam (IB).

14. The method according to claim 13, characterized in that the method is conducted by the limited wavelength range infrared illuminator (1) according to any one of claims 1 to 12.

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