Radiation source with stable temperature and radiant intensity

Abstract

A visible / IR light source comprising a plate 30 (e.g. a micro-hotplate) and one or more heaters 34 arranged to heat the plate to an incandescent state such that an emitting surface of the plate emits thermal radiation. The source includes on the plate, or on the heaters that heat and support the plate, a material 40a-d exhibiting a change of phase in an operating temperature range of the light source. The transition is detectable electrically or optically and may be used to stabilise the operating point of the source.

Description

Radiation source with stable temperature and radiant intensityTechnical domain

[0001] The invention relates to the field of microelectromechanical devices (MEMS) and especially to an incandescent light source that may be used as a source of infrared and / or visible light.

[0002] The invention also relates, in embodiments, to reference light sources that are intended to define and propagate a metrological standard.Related art

[0003] Often there is a need for radiation sources in the infrared or visible 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.

[0004] It is known to accomplish this with a source of blackbody radiation whose temperature is strictly controlled. The spectral radiance of such a source is entirely determined, through Planck's law, by its temperature and its coefficient of optical emissivity.

[0005] Blackbody cavities in thermal contact with a substance at its fixed freezing temperature, for example gold, are used as standard sources in radiometry and photometry. Examples of such realizations can be found in US 3077539 A, DE 10110131 A1 and US 7837382 B2, among others.

[0006] MEMS micro-hotplates are used as thermal emitters, especially in the infrared and for applications such as spectroscopy, gas sensing, and the like, as described, for example, by WO 2021 / 144463.

[0007] Document GB 2060246 A discloses a thermionic cathode for an electron gun with a porous element of sintered tungsten filled with barium oxide. During operation Barium or Barium oxide evaporate away, which degrades the emissivity.

[0008] US 2006 / 076868 A1 discloses an incandescent emitter with a micro-structured surface and a coating of gold, or another low-melting material, that is operated above the melting point, for the sake of an improved emissivity in the visible part of the spectrum.

[0009] The paper "programmable solid state atom sources for nanofabtication", by Han Han et al., Nanoscale 2015, 7, 10735, is authored, among others, by an inventor of the present application. It discloses a solid state atom source for nanofabrication. The source has a micro heating plate loaded with a layer of a material suitable for thermal evaporation. Because of its small size, the atomic flux can be controlled by pulse width modulation techniques. The device is not used as a light source. This paper shows that the resistance of a silver-loaded heater driven in pulse mode exhibits a flat segment corresponding to the melting of silver, and proposes to use this to quantify the latent heat of melting.Short disclosure of the invention

[0010] An aim of the present invention is the provision of a radiation emitter that overcomes the shortcomings and limitations of the state of the art.

[0011] Another aim of the invention is to propose a radiation emitter whose spectral radiance is well-characterized and stable, for use as standard source, calibration, and metrology applications.

[0012] Advantageously, the sources of embodiments of the invention can link the hotplate temperature to a phase transition of a substance, stabilizing in this way the emission intensity.

[0013] In operations, the hotplate is heated or cooled rapidly such that its instantaneous temperature crosses the transition temperature of the phase change material chosen. The resistance, or another temperature- related quantity, is observed as a function of time during this transient. "Rapidly" denotes in this context a transient phenomenon happening at a time scale shorter than the thermalization time of the emitter device.

[0014] Ideally, as postulated by the Gibb's phase rule, the heat capacity during the phase change will appear momentarily infinite. The resistance of the emitter device, which is directly linked to tis temperature, exhibits then a horizontal plateau where dT / dt « 0 and dR / dt « 0. In practice, thermalization effects and other phenomena may alter the ideal scenario, but the transition will still manifest itself by a change of slope or some remarkable feature in the temperature T(t) considered as a function of time. This change of slope will be referred to as a "plateau" in the following, for conciseness. The plateau may appear in the heater's resistance / ?(t) or in another suitable temperature-dependent quantity, for example the heating current / (t) the voltage drop across the heaters 7(t) or the luminous intensity L(t). The following disclosure will deal especially with heating transients, during which a material melts, sublimates, or evaporates, but the invention applies equally well to cooling transients, accompanied by freezing of a material.

[0015] During the phase transition, the temperature of the heater will be stably anchored to the melting / freezing / sublimation point of the phase change substance, and the difference between the heating power and the power losses is consumed by the latent heat of the substance that undergoes the phase change. The larger the latent heat of the phase transformation relative to the emitter's heat capacity, the longer and the easier to observe the phase change will be.

[0016] The duration of the phase change will be influenced also by the separation between the transition temperature and the final equilibrium temperature. When these temperatures are close, the change of phase happens at a temperature close to the final equilibrium temperature at which the heating power is exactly balanced by the heat losses; therefore, the net power available for the phase change will be rather small and the transition will be longer in time, resulting in a longer interval where the temperature remains essentially constant. In typical implementations, the transition times will be on the order of the thermalization time of the system, for example in the interval 0.1-100 ms or, more often, in the interval 1-10 ms.

[0017] The aims of the invention are attained by the object of the attached claims, and especially by a light source comprising a plate and one heater or a plurality of heaters arranged to heat the plate to an incandescent state such that an emitting surface of the plate emits radiation in the infrared and / or visible spectrum and a phase-change material exhibiting a change of phase at a transition temperature that is in an operating temperature range of the light source, the phase-change material lying on a surface of the plate or on a surface of the heater or heaters, while a monitoring unit determines that the temperature of the plate is at the transition temperature by detecting a plateau of temperature during a heating or cooling phase.

[0018] Embodiments of the invention may add other advantageous features, according to the need, as presented in the dependent claims, these features, which despite their usefulness are not essential, include the use of a phase-change material that sublimates at the transition temperature without a liquid phase. This avoids the complications linked to the need to contain the liquid phase and prevent it from escaping or falling off. The phase-change material, be it sublimating or fusible, may be one between Al, Ti, Be, Rh, V, Cu, Au, Ag, Ni, Pd, Pt, Mo, Nb, Ir, Pt, Zn, Cr, Sb, Cd, As, Si. Alloys are also possible, insofar as they exhibit one phase change at the desired temperature. It is possible to find material and combinationsof materials that provide many useful reference points between 800 K and 3000 K.

[0019] Phase-change materials that, in the molten state, wet the surface on which they lie are more effectively contained and less liable of dripping or escaping.

[0020] The change of phase may be detected by observing the temperature of the plate during heating and is marked by an interval during which the temperature is essentially constant. Embodiment of the invention include resistive heaters that serve also as temperature sensors. Their resistance is correlated with the instantaneous temperature, and the phase transition is signalled by an interval during which the resistance remains constant. It is possible to control the heating power to maintain the temperature stable at the transition temperature, at least during a certain interval of time.

[0021] When the device of the invention determines that the source has precisely the temperature of the phase transition, the radiation of the source can be used directly, knowing that its intensity and spectrum are those characteristics for the transition temperature. Also, the device of the invention can be configured, in embodiments, to identify the phase change and calibrate a temperature sensor — possibly the resistance of the heating elements themselves — against the transition temperature, which is known a priori. Thanks to this, the operating temperature of the source can be set precisely at a desired value different from the transition temperature but linked thereto, independent from ageing of the emitter, environmental fluctuations, changes in the electronics drive, and other disturbing effects.

[0022] Preferably, the heaters are the same metallic arms holding the plate in place. This is advantageous because, even if the heating current flows also through the plate, most of the power is dissipated in the arms, where the cross-section is smaller and the resistance is higher, and the temperature on the surface of the plate is remarkably constant.

[0023] Since most of the materials that could be used to make the plate would react with the atmospheric gases at the working temperature, the light source comprising the plate and, the heaters, is in an evacuated space or in a container filled with inert gas at low pressure.

[0024] Optionally, a reflector facing the back side of the plate creates a cavity that is filled with radiation in thermal equilibrium with the incandescent plate. In embodiments, the emitting surface of the plate shadows the reflector completely or in large part.

[0025] The phase-change material may be deposited on the second surface of the plate, facing the substrate. In this manner, the emissivity of the emitting surface remains that of the base material of the plate (for example, tungsten) independently from the amount of material left.

[0026] If the emissivity of the phase-change material is well- characterised, it may be deposited also on the emitting surface in addition, or in alternative, to the second surface. This may be advantageous to prevent spills of molten material on the substrate.

[0027] The phase-change material may be also applied totally or in part to the heaters. In this variant, the material is spaced apart from the emitting surface and closer to the region where the heat is produced, which may result in a faster control time. The amount of phase-change material may be limited by the available area, however.

[0028] In embodiments, the inventive device may include, within the monitoring unit or beside it, a control circuit, that maintains the emitter at a desired temperature, using the knowledge that, at the plateau, the temperature of the emitter plate is equal to the phase change temperatureThe control circuit is configured to alter the drive power to achieve this goal, for example by using a feedback loop.

[0029] The control circuit can operate as close as possible to the phase change temperature, in which case it may use the fact that the derivative of the temperature at the plateau is close to zero. It may also, as it will be disclosed further, use the plateau as a reference and drive the emitter at a fixed intensity that is in a predetermined ratio with the intensity at the plateau, where "intensity" here indicates the value of the heating current, or the resistance of the heaters, or the luminous output, or another suitable temperature-related quantity.

[0030] As mentioned above, the power drive may be pulsed to allow the observation of the change of phases. A feedback loop would be implemented such that the resistance does not change with the power modulation, (slow drive can occur).

[0031] In a possible mode of operation, the emission intensity and the resistance of the heaters are measured at the temperature of the phase change, then the device is operated at a different power, below or above the transition temperature, controlling the resistance such that it is a given fraction of the resistance at the transition temperature. The ratio between the resistance at the operation point and the resistance at the transition point may be, for example a predetermined value between 90% and 110%, or any other suitable value. In this manner of operating the device, the temperature curve of the heaters is not explicitly calibrated; nevertheless, the operating temperature is linked to the known temperature of the phase change. The measure of the resistance at the temperature of the phase change can be repeated as often as needed, for example hourly, to suppress aging, drift, and other undesirable disturbances.

[0032] This mode of operation is beneficial to the operating life of the device, since, by choosing an operating temperature below the melting point of the phase-change substance, the latter is in a solid state most of the time. Moreover, the control system may be simpler to implement.

[0033] The number of phase transitions observable in the device of the invention is not limited to one. Variants of the invention may exhibit a second phase transition at a different transition temperature than the first one, or any number of phase transitions at different temperatures. Having more than one phase transitions provides multiple reference temperatures and allows choosing a precise operating point in an extended range of temperature. This may be achieved by determining the resistance of the device at the known temperatures where the phase changes occur and interpolating or extrapolating the resistance to determine the resistance value corresponding to a desired operating temperature.

[0034] According to the needs , the phase-change material or materials may be patterned or deposited in a continuous layer on the plate and / or on the heater rather than covering the whole available surface.

[0035] In the present disclosure the wordings "infrared radiation", "infrared light" and "IR light" are treated as equivalent or synonyms and denote an electromagnetic radiation with a wavelength belonging to the range between 0.8 pm to 20 pm. Similarly, "visible light" and "visible radiation" refer to emissions with a wavelength between 0.4 pm and 0.8 pm. The sources treated in this disclosure are grey- or blackbody broadband and emit at the same time both in the infrared and in the visible spectrum but may be denoted by any one of the above expressions according to where the dominant emission or the emission of interest lies.

[0036] In the same context, the terms "plate", "membrane" will be used to denote elements whose thickness is lower than its other two dimensions. In embodiments, the thickness of the membrane may be less than 1 / 10thor even less than 1 / 100thof the other two dimensions, "hot-plate" and "micro hot-plate" may be used as synonyms. Preferably, the membranes are self- supporting and can be held in positions by a limited number of anchor points at their periphery, and are essentially planar. Importantly, the membranes of the invention are configured and supported such that theydo not break or distort severely at the intended operating temperature, although a certain degree of deformation is unavoidable and acceptable.

[0037] The invention uses, in embodiments, an evacuated envelope to protect the source from the atmosphere and reduce heat losses. The expression "vacuum" is used in this disclosure to designate any pressure below standard conditions, preferably not above 1 mbar, better 10-3mbar or less.Short description of the drawings

[0038] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:Figures 1a and 1 b illustrate schematically the structure of the incandescent membrane of the invention, in two variants. The first one has a circular general shape and is supported by acuate arms. The latter is square and the short arm that hold it in place include each an optional compliant structure.Figure 2 shows a device according to the invention, in section.Figure 3 plots the time-evolution of the electric resistance of a device according to the invention.Figure 4 is a phase diagram of a binary system that, when used in the invention, provides more than one reference points.Figure 5 shows, schematically, a possible structure of a control circuit used in the invention.Figure 6 illustrates some step of a process of the invention, as a flowchart.Figure 7 plots the time evolution of the electric resistance and of the luminosity in a device according to the invention.

[0039] In the figures, remarkable elements are identified by reference signs that are repeated in the text. The same reference sign may be used to identify distinct elements that are identical, similar, or technically equivalent. When many identical, similar or equivalent elements occur on a drawing, some reference signs may have been omitted to avoid cluttering the figure.Examples of embodiments of the present invention

[0040] With reference to figures 1a and 1b, the source of the invention has a 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 source such that the plate, and especially the support arms 342, 344 carry a suitable 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.

[0041] Preferably the plate 30 and the arms 342, 344 are fabricated out of 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, graphite, and many other.

[0042] The source of the invention comprises in general a housing, not represented, 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 a gas composition based on an inert gas, such as Argon or Xenon.

[0043] In the depicted embodiment, the plate is suspended above a reflector 20 by the arms 342, 344, such that the back surface of the plate is facing the reflector 20. The reflector may comprise a layer of a substance 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 at lower temperatures, or when the radiation of interest is at longer wavelengths for example at wavelengths greater than three microns.

[0044] The inventors found that a certain degree of elasticity and / or compliance in the arms 342, 344 provides an elastic buffer against the expansion and contraction over the large temperature changes that the device must withstand and increases the useful life of the device. In figure 1a, the flexibility is provided by slender arms 342 that have an arcuate form and meet the plate 30 at an angle, rather than radially. When heated to the operating temperature, thermal expansion is absorbed by a deformation of the arms 342 and a rotation of the whole plate 30 around the central axis.

[0045] Figure 1 b presents a different structure that allows to achieve the same goal. Here compliant structures on the arms 344 take care of the thermal 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.

[0046] Figure 2 is a simplified representation of a device according to the invention, in section. The device is built on a substrate 10, which may be silicon, or another suitable material, preferably with a high thermal conductivity. The plate 30 is suspended by the arms 34 above a recess in the substrate, and the reflector 20 is realised by a suitable IR-reflecting layer, for example a thin film of gold. The window 50 above the plate 30 is a of a suitable material transparent to the desired radiation, such as glass, sapphire, germanium, diamond, silicon, or other. The distance between the window 50 and the plate 30 is defined by the spacing layer 95 whose thickness may be of five millimetres or lower, preferably less than one millimetre. In a variant, the window 50 is essentially parallel to the plate 30. The window 50 transmits the radiation emitted by the the plate 30 to an outer region where the radiation can be used, providing separation between the source and the outer region. If separation is not needed or desired, window 50 may be replaced by a simple aperture.

[0047] If required, the source may include a filter layer 51 that transmits a part of the spectrum generated by the emitter 30 and reflects part of the radiation back to the emitter or absorbs it. An anti-reflective coating may also be added.

[0048] As mentioned above, the emitter is protected from the atmospheric gases by a suitable envelope, not represented, and lies in an evacuated space, or in a protective atmosphere. A getter 92 may be used to maintain the vacuum or the purity of the protective atmosphere.

[0049] The heat generated in the heating arms 30 is dissipated radiatively by the plate 30 and by thermal conduction towards the substrate 20. In a situation where the heat power is constant, the plate will find an operating temperature at which the power losses equal the input power.

[0050] An advantage of the invention lies in the high compactness of the emitter devices that can be integrated in electronic circuits withconventional fabrication techniques. Preferably, the device is enclosed in a package, not represented in figure 1, that is suitable for soldering on a circuit board, for example a SMD package. The dimensions of the emitter plate and of the packaged device may vary according to the intended application but, in general, they do not exceed 20 mm in any dimension, and are preferably less than 10 mm in any dimension.

[0051] Importantly, the source of the invention comprises a phasechange material exhibiting a change of phase at a transition temperature that is in an operating temperature range of the light source. Figure 3 show the resistance of an emitter according to the invention that includes a layer of phase-change material, as a function of time. The resistance of the emitter is directly related to its temperature.

[0052] The plot represents the resistance of the device (line 120) rising from the cold state to an equilibrium value where the electric power equals the thermal losses. Three segments can be identified in the time axis. In the first, marked "A", the temperature — and the resistance — rise from the initial values, corresponding to the cold state. The temperature is below the phase change temperature of the phase-change materials 40. The radiative losses can be approximated by the Stefan-Boltzmann's law:P(T) = Ae(T)oT4where A denotes the area of the emitter, e the temperature-dependent emissivity of the plate, and o is the Stefan-Boltzmann constant. One observes an increase of the resistance with a progressively diminishing slope.

[0053] In the segment "B" the resistance does not rise any more. The heaters and / or the plate have attained the phase-change temperature. The power difference between the Joule heating and the heat dissipation is entirely consumed by the latent heat of the material that is undergoing thephase transition. In this regime, the temperature is sensibly constant, and equal to the temperature Tmof the phase change.

[0054] Eventually, all the available material changes phase, and the resistance, with the temperature, start to rise again (segment "C") until the whole plate thermalizes at the temperature TI.

[0055] Starting from segment "D", the heating power is reduced, and the temperature of the plate drops until, in segment "E" it reaches the temperature Tm again where the phase change material starts freezing and the temperature remains essentially constant. Only in segment "F", when the phase change material is completely solidified, the temperature starts to drop towards the new equilibrium value T2.

[0056] lines 121 represent the notional temperature profile of an emitter with the same thermal capacity and emissivity, but without any phase transition. The temperature rises to Ti and falls to T2 continuously and smoothly, without the horizontal plateaux that characterize the phase transition. It can be shown that the hatched areas 123 between the plots 120 and 121 are proportional to the latent heat consumed in the melting, respectively freezing of the phase change material. It can be seen also that, since T2 is considerably closer than Ti to the melting point Tm, the horizontal plateau in "E" is longer than that in "B".

[0057] The plot of figure 3 represents the temperature of the emitter through the resistance of the heaters. While the temperature and the resistance are directly related, the resistance can be measured only to the limit of the instrumental errors, and the temperature-resistance relationship cannot be known with absolute certainty, because it depends on the exact geometry of the device, on its age, and other parameters. The temperature of the change of phase, in contrast, is very precisely known for any given substance.

[0058] The resistance of the heaters is not the only temperature-linked quantity that can be used in the invention. Heater current, heater voltage drop, and luminous intensity, for example, may also show plateaux at the values corresponding to the phase transition in a similar fashion as the resistance.

[0059] The device of the invention can then determine the occurrence of the temperature plateaux in "B" and "E" control the power to maintain the temperature at the phase change point. Also, the device can record the value of R(Tm), the resistance at the melting point (or equivalently, freezing point), and use it to obtain a calibration function R(T) linking the observed temperature-dependent quantity (in this example the resistance of the heaters) and the plate temperature, or control the heating power such that the resistance remains constant at a different value, in a predetermined relationship with R(Tm), for example such that R = a R(Tm), where a denotes a predetermined ratio. The device may also evaluate the areas 123 and the corresponding values of the latent heat, which offer a measure of the residual amount of phase transition material.

[0060] In the same way, the device of the invention may record the value of another suitable temperature-dependent quantity, such as the heating current, the heating voltage, or the luminous intensity at the melting point or at the freezing point, and use it to link this quantity to the true temperature of the emitter. One may also have variants of the invention that control the power to maintain the temperature at the phase change point by observing a plateau in the heating current, or the heating voltage drop or the luminous intensity.

[0061] Preferably, the invention uses materials that are solid at ambient temperature and melt, or sublimate, at a known transition temperature. There are many suitable materials that have a phase change at a transition temperature between 500 K and the melting point of tungsten, for example, preferably higher 800 K or higher than 1900 K. the following materials exhibit a melting / freezing point in a broad temperature range.

[0062] Table 1 lists the latent heat of fusion, the melting point ant the vapour pressure for some elements that may be used as phase-change materials to provide a reliable reference point according to the invention. The list is not exhaustive: many substances, including compounds and mixtures, with a melting point lower than the melting point of the plate, which could be made of tungsten, could be used as phase-change materials. They are too numerous to be listed.Temperature forHfus m.p Vapor pressure = 1[J / g] [K] Pa) [K]Al 396 933 1482 Be 1356 1558 1462 C 356 3825 2700 Cr 394 2133 1656 Co 275 1768 1790 Cu 206 1357 1509 Au 63 1336 1646 Ir 135 2739 2713 Fe 247 1811 1728 Mg 358 923 701 Mn 240 1517 1228 Mo 375 2893 2742 Ni 293 1726 1783 Nb 288 2743 2942 Os 163 3306 3160 Pt 103 2041 2330 Rh 211 2238 2288 Se 68 493 500 Si 1787 1684 1908 Ag 105 1235 1283 Ta 199 3253 3297 Th 69 2023 2633 Sn 59 505Ti 390 1943 1982 W 190 3673 3477 V 448 2173 2101 Zn 112 693 610Table 1

[0063] Preferably, the material of the hot plate will exhibit a low heat capacity (heat capacity of pure tungsten is 134 J / (Kg K) ). Phase change materials with a higher latent heat are advantageous. Ideally, the phasechange material should wet the substrate on which it is deposited, to avoid balling up or dripping of molten material. Al, Cu, Au, Ag, Ni, Pd, Pt, Nb, Mo wet tungsten and, should provide an even film of liquid metal on the surface.

[0064] Sublimating materials, such as Cr, Zn, Cd, As, Sb avoid the problem of the liquid phase entirely, but may be difficult to use in a sealed evacuated envelope. Vapour pressure defines the maximum operational life and should be as low as possible to minimise evaporation.

[0065] Several combinations may be used in the invention, according to the desired used case. Molybdenum, for example, wets tungsten and has a reasonable latent heat. It may be used to provide a thermal radiation source that has a stable working point at its melting point of 2894 K. Niobium, Gold, Silicon, iridium and other suitable substances could also be used, at their respective melting point.

[0066] The invention is not limited to having a single phase transition at one temperature, and may include two or more phase-change materials with distinct melting points, for example, and obtain in this way two reference temperature points. These temperature points can be used to calibrate more precisely the resistance of the heaters, or another suitable quantity, as a function of the temperature, or control the power at a desired operating temperature, relative to the melting points.

[0067] Alloys can be used in lieu of pure materials, particular those mixture system that present a eutectic point at a given composition. In theory any phase changes including latent heat can be used in the frame of the invention, but the transitions with a favourably high latent heat are between solid and liquid, mostly. It may be possible to use multiple phase transitions in an alloy to define multiple transition temperatures. Figure 5,for example, shows a simplified phase diagram of the Ag+Cu system, and may be applied to several binary systems as well. The vertical line 167 is the locus of the states of a given selected composition of an alloy of Silver and Copper. When the temperature reaches the eutectic temperature (779 K, the eutectic point is labelled as 160), the system exhibits a first change of state (point 165a) at the solidus line between the solid solution Ag+Cu and a mixture of solid silver and molten alloy (L+Ag). This transition involves a latent heat and can be used as a reference point in the invention.

[0068] As the temperature rises, the same composition meets, in point 165b, the liquidus line where the L+Ag phase is in equilibrium with the liquid alloy (L) This provides a second reference point that can be used in the invention.

[0069] The phase change material may be provided on any heated surface of the source, either of the plate or of the heaters. In may be deposited in a film on the emitting surface of the plate (position 40a in figure 2), provided its emissivity is well characterised and sufficiently high. It may also be on the lower side of the plate, facing the substrate (position 40b). In alternative, or in addition, the phase change material may be on the upper side of the heaters (40c) or on their lower side (40d). In all these locations, the phase change materials may be deposited as a continuous film, or patterned.

[0070] Figure 5 is an idealized structure of a control circuit that may be used, in the invention, to stabilise the operating point on a fixed point of temperature marked by a change of phase. The power source 200 is a controlled source that supplies the heaters 35, here represented as a simple resistive load. The power source 200 may approximate an ideal voltage source, a real source with a suitable internal resistance or an active power source with a l / V characteristics designed to provide fast heating and stable operation at the desired operating point of the light source. A monitor and control circuit 250 is configured to monitor the heater's resistance 7? (t), which is a measure of their instantaneous temperature. This can beachieved by measuring the voltage drop 7(t) and the current / (t) of the heaters 50, or in any other way. The current may be read by a sensor 258 (for example a shunt resistor, a current transformer, or a Hall sensor), or it may be obtained in any other way.

[0071] The monitor and control circuit 250 is configured to detect, for example by observing when the first derivative dR / dt is close to zero, the temperature plateau that indicates that the phase change material is melting or freezing. The circuit 250 could then generate a signal Q indicating that the emitter is at the desired temperature and, as soon as the resistance drifts away from the stable value, correct the setting of the programmable source 200 to stabilise the system on the transition temperature of the phase change.

[0072] The monitor and control circuit may also include a sensor of the radiated intensity 256. Since the phase transition of the material is also accompanied by a change in the emissivity e of the material, the phase transition may also be visible as a discontinuity in the emitted luminous power if, for example, the phase transition material is on the emitting surface of the plate (position 40a in figure 2). In these cases, the monitor and control circuit is configured to use this information in place of or together with the resistance / ?(t) to determine when the phase transitions occur.

[0073] Figure 7 plots The resistance (curve 120) and the luminous emission (curve 130) of a source according to the invention during a heating transient in which the drive voltage across the heater is suddenly increased (curve 110). The additional power results in the melting of the transition material and, as in the example of figure 3, a temperature plateau that manifests in a corresponding plateau 128 of the resistance as well as in a plateau 138 of the luminous intensity 130 that is detectable using the sensor 256. In this plot, the luminosity plateau 130 appears more easily distinguishable than the resistance one. A corresponding plateauwould appear also in the cooling transient at the freezing point of the transition material, but this is not shown in the plot.

[0074] The inventive source has inherently a broad spectrum, close to that of a black body at the same temperature. The luminosity plateau 130, however, appears at all wavelengths. Advantageously, the sensor 256 may be sensitive only to a part, possibly narrow, of the emission spectrum. In a preferred embodiment, the sensor 256 is a silicon photodiode, sensitive only to the visible or near-infrared part of the emission spectrum. The luminosity plateau 130 is readily apparent in the signal recorded by a silicon photodiode, even if the silicon detector is sensitive only to a small window in the emission spectrum of the source.

[0075] The power source 200 is preferably a pulsed one, delivering short pulses. The control loop should be faster than the thermalisation time of the emitting plate to better detect the delay in the heating — or cooling — caused by the phase change. The monitor and control circuit 250 may be configured to change the amplitude and / or the width and / or the repetition frequency of the pulses to stabilise the temperature of the system.

[0076] Figure 6 represents, in a flowchart, a possible process to determine the occurrence of the phase transitions. In this example, starting from an initial state below the melting point, the heating voltage, which may start from an initial value V = 0 is increased stepwise by a determined amount AV0(step 181) then, in step 182 the instantaneous values of current and resistance are measured. In step 183 the control system decides whether there is a change of phase, for example by detecting a plateau where dR / dt « 0.

[0077] If no melting is observed, the process returns to step 181 where the heating voltage is further increased by AVo and the measure is repeated. Otherwise, in step 184 the heating voltage is reduced by a halfstep -1 / 2 A0, optionally after a delay, and the resistance is evaluated again (step 185). In step 186 the control system decides whether the systemis at the phase change temperature or, if required, reduces the voltage further by branching to the step 184.

[0078] In successive iteration (not illustrated in the flowchart), the process applies progressively decreasing voltage steps of amplitude ±A0 / ( / V + 1) to the heating voltage. N denotes the number of observed phase transition and the sign of the step is positive when the temperature is below the transition point, negative when it is above.

[0079] This method converges to a temperature equal to the melting temperature, T = Tm, provided the initial step A0is chosen small enough such that the first phase transition is not missed. Eventually, the steps become so small that the transition is no longer observable. This may be remedied by applying a minimum and a maximum value to the step amplitude to avoid a runaway.

[0080] The process represented in figure 6 can be used to maintain the device of the invention at the temperature of the phase change for an extended interval, by varying the heating voltage up and down by progressively diminishing steps. Other processes to achieve this goal are possible, however.Reference symbols in the figures

[0081] 10 substrate20 mirror30 plate32 contact pad34 arm35 electrical resistance of the heaters40a phase change material, on a top side of the plate40 b phase change material, on a bottom side of the plate 40c phase change material, on a top side of an arm 40d phase change material, on a bottom side of an arm 50 window51 filter92 getter95 spacer, support120 plot of the temperature121 plot of the temperature without phase transitions123 area proportional to the latent heat 128 temperature plateau130 plot of the luminosity138 luminosity plateauA initial rise B interval of constant temperature — melting or sublimation C thermalisation D initial dropE interval of constant temperature — freezing F thermalisation 160 eutectic point165 phase transition, transition temperature167 selected composition 181 voltage step up182 measure of resistance 183 search for a plateau184 voltage step down185 measure of resistance186 search for a plateau200 power source 250 monitor and control unit256 radiantion sensor258 current sensor 342 arcuate arm344 arm with compliant structure

Claims

Claims1. A visible / IR light source comprising a plate and one heater or a plurality of heaters arranged to heat the plate to an incandescent state such that an emitting surface of the plate emits radiation in the infrared and / or visible spectrum and a phase-change material exhibiting a change of phase at a transition temperature that is in an operating temperature range of the light source, the phase-change material lying on a surface of the plate or on a surface of the heater or heaters, characterized by a monitoring unit configured to determine that a temperature of the plate is at the transition temperature by detecting a plateau of temperature in a heating or cooling phase of the plate.

2. The light source of the preceding claims, wherein the phase-change material exhibits melting, freezing, or sublimation at the transition temperature.

3. The light source of any one of the preceding claims, wherein the phasechange material is any one of Al, Ti, Be, Rh, V, Cu, Au, Ag, Ni, Pd, Pt, Mo, Nb, Ir, Pt, Zn, Cr, Sb, Cd, As, Si, or an alloy thereof.

4. The light source of any one of the preceding claims, wherein the phasechange material melts at the transition temperature and, in the liquid state, wets a material of the surface on which it lies.

5. The light source of any one of the preceding claims, wherein the transition temperature is between 800 K and the melting point of tungsten.

6. The light source of any one of the preceding claims wherein the phasechange material is an alloy exhibiting at least a second phase change at a second transition temperature different from the transition temperature, or the light source comprises at least a second phase-change materialexhibiting a second phase change at a second transition temperature different from the transition temperature.

7. The light source of any one of the preceding claims, wherein the monitoring unit is configured to measure a temperature of the plate through a temperature-dependent quantity, for example an electric resistance or an electric current or a voltage drop of said heater or heaters or a radiation emitted by the plate.

8. The light source of the preceding claims, configured to stabilise the temperature of the plate to a desired value equal, below, or above the transition temperature, based on the knowledge that the temperature of the plate is equal to the transition temperature when the plateau occurs.

9. The light source any one of the preceding claims, wherein the heater or heaters are conductive arms holding the plate.

10. The light source any one of the preceding claims including a reflector parallel to and separate from the substrate a second surface of the plate.11.The light source of any one of the preceding claims, including a window or an aperture through which the radiation emitted by the plate is transmitted to an outer region.

12. The light source of any one of the preceding claims, the plate being essentially flat and having a surface of 0.3 square mm or less.

13. The light source of any one of the preceding claims, wherein the phasechange material or the phase-change materials are patterned on said surface of the plate or of the heater or heaters.

14. The light source of any one of the preceding claims, wherein the phasechange material forms a continuous layer on said surface of the plate or of the heater or heaters.

15. The light source of any one of the preceding claims, wherein the phasechange material is on the emitting surface and / or on the second surface, and / or on a surface of the heater or heaters.

16. A method of using the light source of any one of the preceding claims, the method including the steps of heating the plate to incandescence, detecting the plateau of temperature by measuring a temperaturedependent quantity.

17. The method of the preceding claim, wherein the heating is obtained by passing a current through one resistive heater, or a plurality of resistive heaters, and the temperature-dependent quantity is a resistance or a current or a voltage drop of the heater or of the heaters.

18. The method of claim 15, wherein the temperature-dependent quantity is luminous emission.

19. The method of any one of claims 15-17, including determining a calibration function linking the temperature-dependent quantity to the temperature or inversely and using the calibration function to stabilise a temperature of the plate to a desired value by controlling the temperaturedependent quantity.

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