Thermal source of infrared and visible light

Abstract

A MEMS thermal radiation source (150) comprising, in a package, a plate (30) and one heater or a plurality of heaters (342) configured to heat the plate (30) to an emission temperature such that an emitted radiation (I(λ)) of the plate and is available outside the package, characterised by a reference radiation detector (110) receiving a part of the emitted radiation (I(λ0)) and providing a reference intensity signal indicative of an intensity of the emitted radiation (I(λ)). (Fig. 6)

Description

Thermal source of Infrared and visible light Technical domain

[0001] The invention relates to the field of infrared emitters, andespecially to microelectromechanical devices (MEMS) that emit infraredand / or visible light through a thermal process.Related art

[0002] Often there is a need for radiation sources in the infrared orvisible spectrum that are constant and reproducible in intensity. Such references are useful, for example, in calibrating of optical thermometers, pyrometers, radiometers, spectrometers, and in many other applications.

[0003] Spectral measurement techniques are based on the principle that,when light with a certain range of wavelengths interacts with a sample, theradiation transmitted or reflected or scattered from the sample carries, inits spectrum, a signature of the material in the sample itself. Infrared radiation has the right energy to excite vibration and rotation states of many significant molecules and can be used to detect and analyse quantitatively these chemical species.

[0004] A common concern of spectral measurements is that of thestability and reproducibility of the radiation source. Many known emitters of infrared radiation have been used in this context in the art. LED emitters are readily available, are reliable, and provide excellent efficiency. On the other hand, LED sources have an emission spectrum that depends on junction temperature, drive current, voltage, and age. The production spread of their spectra can also be considerable. 4KMEMS-6-PCT

[0005] Phosphor-based LED sources are somewhat more consistent intheir emission spectra, but still present the same shortcomings. Age-relateddrift is difficult to quantify and compensate.

[0006] Thermal sources like tungsten light bulbs have long been used asemitters of visible and infrared radiation. Filament aging is a highly non-linear process, and these sources have their own special problems suchevaporation of tungsten onto windows.

[0007] MEMS thermal emitters are a known alternative to conventionallight bulbs. Yet, they may also show ageing effect and are sensitive to environmental changes. The spectral radiance of such a source is entirely determined, through Planck’s law, by its temperature and its coefficient of optical emissivity.

[0008] Advantageously, thermal sources emit broadband radiationcovering the infrared spectrum between λ=1 µm and λ=5 µm, where manysignificative molecular lines are found. Spectral measurement can be taken at one single wavelength, for example tuned to a peak in the absorbanceor the reflectance of a substance that is sought—for example the 4.3 µmabsorbance peak of CO2—or over a range of different wavelengths. In thelatter case one distinguishes also between dispersive measurements, in which the radiation to be analysed is decomposed into a continuous spectrum by a dispersive element, and nondispersive methods that rely on one or a few monochromatic or narrowband components. Detectors forinfrared light in the region between λ=1 µm and λ=5 µm include leadselenide (BPS) and lead sulphide (PbS) devices.

[0009] It is known to measure a transmittance of a sample at awavelength tuned to a characteristic transition of a sought substance andat a nearby wavelength where no special interaction is expected, and touse the information from the latter as a reference. For example, knownnondispersive CO2 measurement methods take a measurement oftransmittivity of a cell filled with the gas under study at 4.3 µm,4KMEMS-6-PCTcorresponding to a vibration mode of the CO2 molecule, and at 3.9 µm. The intensity ratio can be used to determine CO2 concentration. This method can compensate for uncontrolled changes in the overall radiance of the source but is not effective in presence of changes in the spectrum shape.

[0010] Photoacoustic sensing has received much attention in the field ofgas analysis. Infrared radiation absorbed by the sample is converted intoacoustic waves that are detected with a microphone. This technique canachieve favourable signal-to-noise ratios through use of acoustic resonance,synchronous detection, and high-performance MEMS microphones; nevertheless, the stability of the infrared source directly affects the accuracy and reliability of the measurement.

[0011] MEMS micro-hotplates in applications such as spectroscopy, gassensing and the like are described, for example, by WO 2021 / 144463,US 2022214272.Short disclosure of the invention

[0012] An aim of the present invention is the provision of a radiationsource and a method of using a thermal radiation source to measure asample overcoming the shortcomings and limitations of the state of the art.

[0013] According to the invention, these aims are attained by the objectof the attached claims, and in particular by a thermal radiation source comprising a plate and one heater or a plurality of heaters configured to heat the plate to an emission temperature such that an emitted radiation is emitted from the plate and, characterised by a reference radiation detector receiving a part of the emitted radiation and providing a reference intensity signal indicative of an intensity of the emitted radiation.

[0014] The invention comprises also a method of analysing a samplewith infrared radiation, comprising: providing a thermal radiation source, heating a plate of the thermal radiation source at an emission temperature4KMEMS-6-PCTsuch that an emitting surface of the plate radiates infrared radiation on the sample, determining a desired characteristic of the sample based on aninteraction of the sample with part of the infrared radiation havingwavelengths in an interaction band, characterised in that part of the thermal radiation is directed to a reference radiation detector that is responsive to wavelengths outside the interaction band, and in that the reference radiation detector provides a reference intensity signal indicative of an intensity of the radiation.

[0015] Dependent claims relate to features of the invention that, whileimportant and useful, are not essential, such as the choice, for the reference radiation detector of a sensitivity limited or extending wavelengths shorter than a peak wavelength of the emission spectrum. Thesensitivity of the reference detector may be limited, for example, to therange λ < 1700 nm or else λ < 1200 nm. In these wavelength ranges, manysolid-state radiation detectors are available including, among others, Siliconphotodiodes, Germanium photodiodes and InGaAs photodiodes. All theseare applicable to the invention.

[0016] The radiation source of the invention may assume any suitableform or nature but, in favourable embodiments, it may be a hot plate in a MEMS device enclosed in a package with a window at least partlytransparent to the emitted radiation. It may be advantageous, inembodiments, to suspend the hot plate to the heaters themselves, and back it with a reflector that increases the overall efficiency.

[0017] The reference detector may be placed in different positions withrespect to the radiation source. In some embodiments, the radiation source sits above the reference detector, and the latter sees a part of the radiation emitted from a lower surface of the plate. The reflector, if present, may have a small aperture to this effect. In other embodiments, the reference detector may be placed on a side and receive stray radiation diffused from an edge of the radiation source.4KMEMS-6-PCT

[0018] The radiation source of the invention may comprise controlcircuit configured to stabilise the reference intensity signal to a predetermined level by controlling a heating power, and in this modestabilising the emission temperature, the intensity, and the spectraldistribution of the emitted radiation.

[0019] A method of analysing a sample (120, 125) with infraredradiation, comprising: providing a thermal radiation source (150), heating a plate (30) of the thermal radiation source (150) at an emission temperature such that an emitting surface of the plate radiates infrared radiation on the sample (120, 125), determining a desired characteristic of the sample based on an interaction of the sample with the infrared radiation, characterised in that part of the thermal radiation is directed to a reference radiation detector (110) without interacting with the sample (120, 125), and in that the reference radiation detector (110) provides a reference intensity signal indicative of an intensity of the radiation.

[0020] Concerning the method, the sample may be characterisedthrough an analysis of a radiation (reflected, transmitted or scattered) emerging from the sample, by one or more infrared radiation detectors and, optionally bandpass filters. In alternative, the characterisation may use a photoacoustic effect, in which the incident radiation is converted into a sound signal.

[0021] The invention uses the fact that the emission spectrum ofblackbody emitters is accurately described by the Planck’s law that dependson a single parameter: the temperature, corrected to account for thegeometric etendue of the emitter, which can be characterised quite nicely.

[0022] In a perfect blackbody emitter of known temperature, theintensity of the radiation at one arbitrary wavelength is sufficient to determine the spectral intensity at any other wavelength of the spectrum.Even if the temperature is not precisely known, it is sufficient to keep the4KMEMS-6-PCTspectral intensity constant at any one given wavelength to ensure that the intensity everywhere in the emission spectrum.

[0023] The same conclusion applies to real thermal sources having anemissivity ^(λ,^T)^lower than one, provided the temperature of the source is essentially uniform. Under these conditions, the spectrum is again describedby Planck’s law with the wavelength-dependent emissivity as a correctionfactor. The consequence is that, if the temperature T and the emissivity areknown, the emission spectrum can be entirely determined by the intensity at any given wavelength λ0. Moreover, under reasonable hypothesis on thedependence of ^ on the temperature, the spectral shape and the spectralintensity can again be kept constant simply by keeping constant theintensity at one wavelength I(λ0).

[0024] If the goal is that of stabilising the spectrum and the intensity, anarrowband measurement of the reference intensity I(λ0) is not required.The same result can be achieved by keeping constant the integrated intensity in a band of wavelengths, provided the responsivity of the reference detector, as well as the absorbance in the optical path betweenthe source and the reference detector in this band can be consideredconstant.

[0025] Micro-plate MEMS emitter exhibit a remarkable uniformity oftemperature and, thanks to their simple structure, do not noticeably age;therefore, they can be controlled and stabilised effectively by the invention.

[0026] Another remarkable aspect of the invention is that, thanks to theuniversality of Planck’s law, the reference wavelength λ0 and can be quite far away from the peak emission of the spectrum and from the measurement wavelength. To provide an example, a thermal detector at aT ≈ 2000 K would be a perfectly valid source of λ=4.3 µm IR radiation, forCO2 detection; however, it can be controlled and stabilised by keepingconstant the emission at λ≈0.8^µm, between NIR and the visible spectrum.4KMEMS-6-PCTThis opens the possibility of using a silicon photodiode or a detector of visible light to monitor the emission I(λ0).

[0027] Highly sensitive silicon photodetectors are available at rather lowcosts and using a silicon photodiode in the reference measurement has undeniable economic advantages. It has also unexpected technical positive effects, however. The emission of a blackbody at 2000 K and the responsivity of silicon photodiode overlaps in a wavelength range 0.85-1.1 µm roughly. This interval is at the extreme longwave limit of the sensitivity of silicon photodetectors and is way below the peak of a blackbody at 2000 K. The intensity in this region of spectrum is roughly proportional to T6, which gives us an excellent control on the emitter’s temperature. Concretely, if the signal from the photodiode is stabilised to 0.1%—a realistic target—the temperature variation interval would be 0.0017%. This translates into a temperature of 2000 ± 0.3 K, which is remarkably stable. The invention may, in embodiment, take advantage of the nonlinear relationship between temperature and intensity at short wavelength to achieve a very high sensitivity of the control loop and an excellent temperature stability.

[0028] The emission of the same thermal source at 4.3 µm approximatelyvaries proportionally to T2. Any temperature fluctuation will reflect on thereference channel at 0.8 µm much more than at the measurement channel at 4.3 µm. This means that the intensity at the wavelength of interest can be stabilised very effectively.

[0029] Since, as mentioned above, the responsivity of conventionalsilicon photodetectors overlaps only slightly with the emission of a thermal source at 2000 K. This combination provides naturally a measurement ofthe reference intensity in a rather narrow band 0.85 < λ < 1.1 µm. Anarrower bandwidth could be obtained by suitable optical filters. The principle of the invention does not require a narrow-band measurement, however, and the reference detector could also be a Ge photodiode, or a InGaAs photodetector, whose responsivity overlaps more with the Planck’s4KMEMS-6-PCTemission spectrum at 2000 K. In this case also, optical filters could be used to restrict the bandwidth of the reference measurement. This may be advantageous, for example, to exclude parts of the spectrum that may be affected by environmental factors.

[0030] In other words, a thermal infrared source used in an experimentor in a measurement in a determined interaction band of wavelengths can be stabilised effectively through a reference signal taken at a different band of wavelengths. The interaction band and the band used to generate the reference signal may overlap in part or be totally separate. In many important use cases, the short-wavelength part of the Planck’s distributionshows the strongest temperature dependence and can be used veryeffectively to stabilise the emission in the spectral band relevant for the interaction that is studied. Short description of the drawings

[0031] Exemplar embodiments of the invention are disclosed in thedescription and illustrated by the drawings in which: Figure 1 illustrates schematically a measurement process using an infrared source according to the invention. Figure 2 illustrates schematically a photoacoustic detection processusing the inventive source and methods. Figure 3 illustrates a variant of the invention in which a reference sensor is placed after the sample cell. Figure 4 illustrates a possible structure of a MEMS hot plate used inthe invention, and4KMEMS-6-PCTFigure 5 illustrates another structure of the same with flexures in theheating support arms.Figure 6 shows, in cross section, a possible arrangement of a sourceaccording to the invention, andFigure 7 illustrates an alternative arrangement.Figure 8 plots Planck distributions at 1500 K and at 2000 K as well astheir temperature derivativesFigure 9 plots the product between the responsivity and the Planckdistributions at 1500 K and 2000 K along with the temperaturederivatives of the Planck distribution.Figure 10 is a plot of the ratio between the temperature derivative ofthe Planck distribution at two selected wavelengths, as a function of the temperature. Figure 11 is a simplified schematic illustrating the general structure of a control unit used in the invention.Figure 12 is a bidimensional representation of the relativetemperature sensitivity of the Planck distribution as a function of the reference wavelength and temperature, for a measurement atλ = 4.3 µm.Figure 13 is a bidimensional plot of the relative temperaturesensitivity of the Planck as a function of the reference wavelengthλref and of the measurement wavelength λ, for T = 2500 K.4KMEMS-6-PCTFigure 14 is a simplified representation of a multichannel spectralemitter based on the invention. Figure 15 is a variant of the emitter of figure 14 with a commonmultiplexed reference measurement. Examples of embodiments of the present invention

[0032] Figure 1 shows a measurement method in which a sample isanalysed spectrally by infrared radiation. The figure relates to a transmission experiment, and the present disclosure will deal with this kind of measurement, mostly, for brevity. It should be noted that the inventionapplies as well to measures where the infrared radiation is reflected orscattered or interact with the sample in other ways. The example relates to the analysis of a fluid sample, for example a gas analysis, but the invention applies also to measures of solid and liquid samples, and can be used to analyse spectrally complex structures, such as multilayer samples, biological samples and so on.

[0033] In figure 1, 100 represents a MEMS micro-plate infrared sourceemitting radiation (arrow 161) with spectral intensity I0(λ) that, startingfrom the source 100 enters the cell 120 containing and a sample withtransmissivity T(λ). The radiation emerging from the cell impinges (arrow162) on the infrared detector 130 with a modified spectral intensity IT(λ).This measurement may use a filter to allow only the radiation in a selectedspectral band to reach the detector 130, for example a passband filter (notshown) centred at 4.3 µm for CO2.

[0034] The source also comprises a reference detector 110 that receives apart of the source’s radiation (arrow 165) and records its intensity. It is not required that the spectral responsivity of the reference detector matchesthat of the infrared detector 130 and, indeed, the responsivity of thereference detector may be limited to a different part of the spectrum without overlap with the measurement made in the infrared detector 130. 4KMEMS-6-PCTFor example, the infrared detector may be responsive to wavelengths atλ = 4.3 µm while the reference detector is responsive to shorterwavelengths. Possibly, the reference detector 110 may be a siliconphotodetector with a peak sensitivity at λ = 0.8 µm and essentially noresponsivity below λ = 1.1 µm or a germanium photodetector that candetect radiation up to λ = 1.7 µm, or a InGaAs photodetector with aresponsivity extending to λ = 2.5 µm, or any other suitable radiationdetector. Optionally, but not necessarily, a filter 138 may be used to reducethe band of the spectrum reaching the reference detector. The output of the reference detector is a sensitive indication of the temperature of the MEMS micro-plate and of the intensity of the radiation at all the wavelength in the spectrum, including the measurement wavelength. It can be used to stabilise the temperature, or in any other way.

[0035] Figure 2 is a similar arrangement where a concentration of CO2 ismeasured by photoacoustic principles. The gas sample is contained in a cell125 that, preferably, is tuned to present an acoustic resonance at apredetermined frequency. The source 100 emits an IR radiation modulatedin amplitude at the same predetermined frequency, and the filter 124 select a specific wavelength that is selectively absorbed by CO2, or any other gas that is searched. An acoustic oscillation in the cell is established, the amplitude being proportional to the intensity of radiation at the absorption wavelength and to the concentration of CO2. This oscillation ismeasured by the microphone 147 and processed by lock-in amplifier 144 orby any other suitable circuit and produces a concentration signal. As in theprevious examples, the reference detector 110 is used to stabilise theemission.

[0036] The arrangement of figures 1 and 2 illustrate measurementmethods in which the thermal radiation reaching the reference detector does not interact with the sample. This is not an essential feature of the invention, however, and the invention includes also variants as depicted infigure 3 in which the reference sensor 110 is placed after the sample cell120 and receives radiation that has interacted with the sample. Preferably,filters 138, 144 select different spectral bands for the measurement4KMEMS-6-PCTdetector 130 and the reference detector 110 used to stabilise the emission.If the responsivity of the reference detector 110 and that of the infrareddetector 130 are different, one or both filters 144, 138 may be omitted,however.

[0037] With reference to figures 4 and 5, the source of the invention hasa plate 30 that is designed to be heated resistively to an incandescent state.The heating action is obtained by connecting the source to a power sourcesuch that the plate, and especially the support arms 342, 344 carry asuitable current. Importantly, since the cross section of the arms is considerably less than that of the plate itself, the heating power is localised in the arms, mostly, and the heat is conducted from the arms to the plate 30, whose temperature in operation is sensibly uniform.

[0038] Preferably the plate 30 and the arms 342, 344 are fabricated outof a conductive and refractory material that can stand the temperature at which the plate is designed to operate. In most cases of practical interest, the operating temperature will be above 800 K, often above 2000 K. Materials capable of operating at these temperatures include refractory metals such as tungsten, tantalum, molybdenum, niobium, rhenium, conducting refractory ceramics such as tungsten carbide, hafnium carbide, tantalum carbide, Hafnium tantalum carbide, graphite, and many other.

[0039] The source of the invention comprises in general a housing, notrepresented, to protect the incandescent emitter. Most materials, including tungsten, react readily with atmospheric gases (O2, N2, CO2) at high temperature. To prevent this, the emitter may be in an evacuated space, which also minimise thermal losses. The housing could also be filled with alow-pressure gas composition based on an inert gas, such as argon orxenon.

[0040] In the depicted embodiment, the plate is suspended above areflector 20 by the arms 342, 344, such that the back surface of the plate isfacing the reflector 20. This optional reflector may comprise a layer of a4KMEMS-6-PCTsubstance that reflects most of the IR or visible radiation emitted by the plate 30. Gold-metallised mirrors have been used with good success in this application. The reflector is especially advantageous when the source is meant to operate at very high temperatures but may be dispensed with atlower temperatures, or when the radiation of interest is at longerwavelengths for example at wavelengths shorter than three microns.

[0041] Preferably, a certain degree of elasticity and / or compliance in thearms 342, 344 provides an elastic buffer against the expansion andcontraction over the large temperature changes that the device mustwithstand and increases the useful life of the device. In figure 4, theflexibility is provided by slender arms 342 that have an arcuate form andmeet the plate 30 at an angle, rather than radially. When heated to theoperating temperature, thermal expansion is absorbed by a deformation ofthe arms 342 and a rotation of the whole plate 30 around the central axis.

[0042] Figure 5 presents a different structure that allows to achieve thesame goal. Here compliant structures on the arms 344 take care of thethermal expansion. Many other flexible and compliant structures are possible and included in the scope of the invention. The examples shown have arms that are co-planar with the plate 30, which eases the fabrication, but this is not a necessary limitation.

[0043] Figure 6 show, in cross section, a possible arrangement of aMEMS thermal radiation source 150 according to the invention. The hotplate 30 is suspended and heated by the elastic arms and is placed above areflector 20 that mirrors most of the radiation emitted by the lower face ofthe plate back on the emitter plate 30. The infrared radiation emitted by the upper face of the plate goes across the window 50, the optional filter 51, and is directed towards a sample or used in any other way.

[0044] The reflector 20 allows a small but significant amount ofradiation 165 to pass across and reach the underlying photodetector 110.This may be obtained by a pinhole 23 in the metallised coating of the 4KMEMS-6-PCTreflector, or in any other way. In the example, the MEMS emitter is bondeddirectly above the photodiode, which is soldered on a PCB 115, but theinvention also includes embodiment that have the source and thephotodiode soldered on opposite sides of a thin PCB. or be contained in asame package.

[0045] In this example, the radiation 165 reaches the referencephotodetector 110 through the substrate 10, which is transmissive, orpartially transmissive to the reference wavelength. In alternativerealizations, the substrate 10 may have an opening to let the referenceradiation 165 through. The lid 50 is transmissive to the wavelength or bandof interest 161, but not necessarily to the reference wavelength. In anexample optimised for CO2 detection, the lid 50 is silicon that is transmissiveto radiation at 4300 nm, which is the wavelength of interest, while the reference measurement is centred at 900 nm, which cannot pass through silicon, but can cross the glass substrate 10.

[0046] The source 150 of the invention may include a control circuitand / or a controlled power supply heating the source in a controlledmanner such that the temperature and emission of the source are stabilised, as it will be explained later. These elements may also be external, however, and are not represented.

[0047] Figure 7 shows another variant of the invention that uses strayradiation escaping the source from the edge to illuminate the referencedetector 110. The radiation reaching the reference detector 110 comesfrom the upper or lower surface of the plate 30 directly, or throughreflections on the lid 50 and / or any other structure in the source. As in theexample of figure 6, the lid 50 does not need to be transparent to thereference radiation. In some cases, the source may include a narrowbandfilter (not shown) the wavelength of interest from the broadband thermal emission of the plate. This filter is in general not transmissive to the reference radiation. 4KMEMS-6-PCT

[0048] In this example, as well as in the previous one, the source and thephotodiode are soldered on a PCB, but other dispositions and packaging may be envisioned without leaving the scope of the invention.

[0049] In another embodiment, not represented in the drawings, areference detector could be placed far from the emitter, possibly close to the main infrared detector.

[0050] Figure 8 is a plot of the spectral radiance of an ideal black body,, which is a good approximation of the emission of MEMS hot platesource, at 2000 K (81) and 1500 K (83). The wavelength λ = 4.3 µm that ismost used for CO2 measurements is specially marked in the plot. Themathematical expression of is the Planck’s equation:where h denotes the Planck’s constant, c the speed of light and kB theBoltzmann constant and is the spectral radiance (energy per unit area per unit solid angle) of an ideal blackbody. The radiance of the sources of the invention can be described roughly by this formula, with a correcting factor that, under reasonable hypothesis, can be considered as a constant value.

[0051] The derivative of the Planck’s distribution isexpressed by4KMEMS-6-PCTCurves 86, 89 plot as a function of the wavelength at T = 2000 Kand T = 1500 K. When the temperature of the source changes by a smallamount , the change in the spectral radiance can be expressed by a first order Taylor approximationRegions in the spectrum where the derivative is higher exhibita stronger dependence from the temperature T of the source, in the sense that any fluctuation in the temperature will yield a change in the spectralintensity larger than in other regions of the spectrum where islower. Accordingly, if one manages to stabilise the spectral intensity in a band of the spectrum where is close to its maximum, the intensity will be even more stable everywhere in the spectrum.

[0052] Figure 9 plots the productwhere denotes theresponsivity of a Silicon photodiode for T=2000 K (curve 87) and T=1500 K(88). Curves 86, 89 are the derivative as in the previous figure. Itappears that the reference signal available at the photodiode’s output isdominated by the radiation in the interval 850 nm-1050 nm. The signalprovided by a Silicon photodiode is much more strongly dependent on the source’s temperature than the output of an infrared detector, because the response of the photodiode is closer to the maximum of . Theeffect is particularly remarkable at T ≈ 2000 / 2500 K, where the matchbetween the curves is optimal, but is present in a broad range of temperatures.

[0053] The ratio of derivatives4KMEMS-6-PCTis an expression of the temperature sensitivity of the intensity of a thermalradiation source at a given wavelength relative to the temperature sensitivity of the intensity of the same source at another wavelength. It can be used to understand the roles of the temperature and of thereference wavelength when controlling a measurement at wavelength.

[0054] Figure 10 plots the temperature sensitivity z as a function of thetemperature. The wavelengths and are chosen to represent examples of practical interest:is the wavelength of a prominent peak in the absorption spectrum of CO2. Curve 91 is relative to a typical Silicon detector with a responsivity centred around , while curve 93 may apply to photodetector with a responsivity centred around for example a Germanium photodiode or an InGaAs / InP photodiode. When a thermal emitter is used to measure CO2 at , it is advantageous to stabilize the source based on the spectral intensity at shorter wavelengths, which are more sensitive to temperature fluctuations. Whenthe temperature of the source is about T = 2000 K, Silicon photodiodes havea responsivity centred around and are especially suitable. Atlower temperatures, other photodiodes capable of measuring the spectral intensity at longer wavelengths may be advantageous. The wavelength has been chosen in this disclosure because it is relevant to the measurement of CO2 that is a use case of special interest, but it does not limit the invention, which is applicable to any other molecular transition or significant process in the infrared spectrum of radiation.

[0055] Figure 12 is a two-dimensional contour plot of the relativetemperature sensitivity as a function ofthe temperature T and the reference wavelength with .Figure 13 is a contour plot of the same sensitivity z as a function of themeasurement wavelength and of the reference wavelength withT = 2500 K. The reference wavelength and the temperature can be chosento achieve high sensitivity, for example z^>^5, z^>^10, z^>^20, or even z^>^50. 4KMEMS-6-PCT

[0056] In many significant use cases, thermal sources like those of theinvention are used for their ability to provide radiation in a desiredemission band situated, according to the needs, between λ = 2 µm andλ = 25 µm, preferably between λ = 2 µm and λ = 12 µm. This region, forexample, includes the fundamental vibration modes of many chemical substance or functional groups, and is especially important in spectroscopicmeasurements. The same is true, however, for many other usefulapplications of the invention, for example in telecommunication, imaging, and other. The inventors have found that, by matching the responsivity of the reference detector with the spectral region where the Planck’sdistribution exhibits a higher temperature sensitivity, the stability of thesource at the wavelength of interest is improved. A reference measurement at a shorter wavelength provides, in many important use cases, a stronger temperature sensitivity, more signal, less thermal and electronic noise than a measurement at the interaction wavelength. Moreover, the intensity at shorter wavelength can be measured with Silicon, Germanium InGaAs, InGaAs / InP photodiodes that are cheaper, smaller, and exhibit less noise.

[0057] Figure 11 is a highly simplified representation of a control circuitthat could be used to stabilise a radiation source according to theinvention. The source 200 is a controlled voltage source that is connected tothe heaters 35, represented as a resistive equivalent load. The photodiode110 receives a part of the thermal emission IR(λ) from the radiation sourceand generates a reference signal 113 for the electronic controller 250 whichin turn controls the source 200 such that the reference signal 113 isconstant. This result can be achieved by a proportional linear controller, a PID controller, or in any other suitable manner. The level of the reference signal 113 is linked to the temperature of theplate 30 of the source in a manner that depends on the spectralresponsivity of the reference detector 110 and the on the transmissivity ofany intervening filters and media, if present. This relationship can be better understood referring to Figure 10. 4KMEMS-6-PCT

[0058] In particular, for the same change in the emitter’s surfacetemperature, the reference signal 113 may change, in proportion to itsnominal value, five, ten or even by a higher ratio, more than the spectral radiance in the emission band; thus, by using different wavelength for the emission band and the reference measurement, and choosing the reference detector suitably, the spectral radiance of the emitted signal can be stabilised with high accuracy.

[0059] The stability of the source is of paramount importance in manyapplications and has a direct influence of the measurement that can be done when the source of the invention is used, for example, in spectroscopic measurements.

[0060] Figure 14 illustrates a variant of the invention that is especiallysuitable for spectroscopic measurements. This device includes a plurality ofthermal sources 110a-110c that may present different features. Forexample, sources 110a-110c may have emitter plates of different sizes or beconfigured to operate at different temperatures such that the spectra and the intensity of the emitted radiation are inherently variable between thesources. In variant, however, the sources 110a-c may be identical, but drivenat different voltages, or with different duty cycles, or modulated indifferent manners such that their emission differs. In many cases, theindividual thermal sources 110a-110c are identical in structure and nominalpower, the filters 124a-c selecting different band of the emission spectrum.Importantly, the invention implements a reference measurement thanks to which the emission of the sources is controlled such that they do not drift with respect to each other in any way.

[0061] The sources are individually addressable by the programmablepower source 350 in such a way that their emissions can be controlled andmultiplexed in time. Part of the radiation is received by the referencedetectors 110a-110c that are used to stabilise the output of each of thesources, as disclosed above. The emitter may include individual filters 124a- c, or other means for selecting the emission band for each of the individual 4KMEMS-6-PCTsource. The individual filters may be replaced, for example, by a single graded filter or by a dispersive element such as a grating or a prism. Thedetector 130 receives the radiation that, starting from any of the sources,has interacted in the sample cell 120 and could provide a single output signal that is the superposition of all these individual components;however, in preferred embodiments, the sources 100a-c are driven by thesource 350 in time-division multiplexed fashion, such that the contributionof the individual sources can be resolved by analysing the output of thedetector 130 synchronously with the source 350.

[0062] Figure 15 shows a variation of the multi-source emitter where theindividual reference detectors 110a-c are replaced by a common referencedetector 110. As disclosed above, the source 350 (not represented in this figure) drives the source in time-division multiplexed fashion such that the reference signals for each individual source can be resolved and each source is individually controllable and stabilised. Reference symbols in the figures

[0063] 10 substrate20 reflector23 pinhole30 micro hot plate32 contact, bonding pad35 heater’s resistance50 window51 filter layer81 Planck distribution B(T, λ): T=2100 K82 Planck distribution B(T, λ): T=2500 K83 Planck distribution B(T, λ): T=1600 K84 Planck distribution B(T, λ): T=2000 K85 responsivity4KMEMS-6-PCT86 derivative ∂B(T, λ) / ∂T: T=2000 K87 product, 2100 K88 product, 1600 K89 derivative ∂B(T, λ) / ∂T: T=2500 K91 ∂B(T, 0.9 µm) / ∂T / ∂B(T, 4.3 µm) / ∂T93 ∂B(T, 1.5 µm) / ∂T / ∂B(T, 4.3 µm) / ∂T100 MEMS source100a-c MEMS source110a-c reference detector110 reference detector, silicon photodiode113 reference signal115 PCB120 sample cell124 filter124a-c filter125 sample cell130 IR photodetector131 IR photodetector132 IR photodetector136 filter137 filter38 filter144 lock-in detector147 microphone150 MEMS thermal radiation source61 emitted radiation to the sample cell162 radiation emerging from the sample cell65 fraction of the emitted radiation to the reference detector200 controlled voltage source250 controller300 multi-emitter source342 arm344 flexure350 multiplexer4KMEMS-6-PCT

Claims

Claims1. A thermal infrared radiation source (150) comprising a plate (30) andone heater or a plurality of heaters (342, 344) configured to heat the plate(30) to an emission temperature such that an emitted radiation (161) isemitted from the plate, further comprising a reference radiation detector(110) receiving a part (165) of the emitted radiation and providing areference intensity signal indicative of an intensity of the emitted radiation(161), characterised in that the reference detector (110) is a siliconphotodiode or a germanium photodiode, or an InGaAs photodiode, or anInGaAs / InP photodiode or a photodetector having a sensitivity limited tothe range λ < 1800 nm.

2. The infrared radiation source (150) of the preceding claim, wherein thereference radiation detector (110) has a sensitivity limited to wavelengths shorter than a peak wavelength of the emission spectrum.

3. The infrared radiation source (150) of any one of the preceding claims,wherein the plate (30) is a MEMS device enclosed in a package with awindow (50) for the emitted radiation (161).

4. The infrared radiation source (150) of any one of the preceding claims,the plate (30) being suspended and held in position by the heater or heaters.

5. The infrared radiation source (150) of any one of the preceding claims,wherein the plate (30) has an upper surface facing a window (50) of the package and a lower surface facing a reflector (20) reflecting the radiation emitted from the lower surface towards the plate (30) and transmitting a fraction of the radiation emitted from the lower surface towards thereference detector (110).4KMEMS-6-PCT6. The infrared radiation source (150) of any one of the claims 1-3, whereinthe plate faces a window (50) of the package, and the reference detector(110) is placed to receive a fraction of the thermal radiation scattered towards an edge of the window.

7. The infrared radiation source of any one of the preceding claims,comprising a control circuit (250) configured to stabilise the referenceintensity signal to a predetermined level by controlling a heating power, thereby stabilising the emission temperature.

8. A spectroscopic system for analysing a sample with infrared radiationcomprising an infrared radiation source according to the preceding claim.

9. The spectroscopic system of the preceding claim comprising a pluralityof infrared radiation sources (110a-110c) according to any one of thepreceding claims, and a programmable power source (350) for addressingeach radiation source individually.10.The spectroscopic system of the preceding claim, wherein the infraredradiation sources are configured to send a part (165) of the emitted radiation to a common reference detector (110) providing a reference intensity signal indicative of an intensity of the emitted radiation.11.The spectroscopic system of any one of claims 9-10, comprising an arrayof filters or a single graded filter or a dispersive element for selecting spectral bands of the radiation emitted by the radiation sources that is directed to the sample. 12.A method of providing infrared radiation in a predefined emission band of the spectrum comprising: providing a thermal radiation source (150), heating a plate (30) of the thermal radiation source (150) at an emission temperature such that an emitting surface of the plate emits infraredradiation in the emission band, wherein part of the thermal radiation is4KMEMS-6-PCTdirected to a reference radiation detector (110) responsive to wavelengthsin a reference band different from the emission band that provides areference signal (113) and the reference signal is used to control a heatingpower applied to the plate (30) such that a spectral radiance of theradiation in the emission band is stabilised, characterised in that theemission temperature is below 2500 K and the reference detector (110) is asilicon photodiode, or a Germanium photodiode, or an InGaAs photodiodeor a InGaAs / InP photodiode, or a PbS detector or a PbSe detector aphotodetector with a sensitivity limited to the range λ < 1800 nm.13.The method of the preceding claim, wherein a ratio (z) of the temperature sensitivity of the intensity of the thermal radiation source at afirst wavelength in the reference band above the temperature sensitivity ofthe intensity of the thermal radiation source at a second wavelength in theemission band is larger than, five, more preferably larger than ten, morepreferably larger than twenty.14.The method of any one of claims 12-13, comprising determining adesired characteristic of a sample (120, 125) based on an interaction of thesample with the radiation in the emission band and measuring an intensityof the radiation in the emission band after the interaction with the sample with a photodetector (130) having a responsivity different from that of the reference radiation detector, or comprising an active material different from an active material of the reference radiation detector. 15.The method of the preceding claim, wherein the part of the thermal radiation reaching the reference detector does not interact with the sample (120, 125). 16.The method of any one of claims 12-15, wherein the reference signal has a maximum spectral responsivity at a wavelength outside the interaction band, or shorter than wavelengths in the interaction band. 4KMEMS-6-PCT

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