THERMAL LIGHT EMITTING DEVICE WITH INTEGRATED FILTER

Abstract

A light emitter module includes a refractory membrane arranged to be heated to a thermal emission temperature such that an emitting surface of the membrane emits radiation in the IR and/or visible spectrum. The radiation is collimated by transmissive optical element adjacent to the emitting surface with a curved exit surface on which an optical filter is deposited. The transmissive optical element may be a planoconvex lens. The disclosure relates also to compound sources with several thermal sources facing an array of micro-lenses with a common plane entry surface on the backside and a plurality of convex surfaces on the forward side, each covered by an optical filter.

Claims

1. A light emitter module comprising: a refractory membrane arranged to be heated to a thermal emission temperature such that an emitting surface of the membrane emits radiation in the IR and/or visible spectrum; a transmissive optical element adjacent to the emitting surface comprising a curved surface configured such that at least a part of the radiation from the emitting surface enters the transmissive optical element and crosses the curved surface; and an optical filter on the curved surface, wherein the transmissive optical element has a reflectivity normal to the curved surface comprised in the range 4% to 40%, and wherein a distance between the transmissive optical element and the emitting surface is equal or lower than L/4, where L denotes a largest transversal dimension of the refractory membrane.

2. The light emitter module of claim 1, wherein the optical filter is an interferential filter.

3. The light emitter module of claim 1, wherein the curved surface is a convex surface, and/or the transmissive optical element is a planoconvex lens.

4. The light emitter module of claim 1, wherein a distance between the transmissive optical element and the emitting surface is equal or lower than L/8.

5. The thermal emitter module of claim 1, wherein the transmissive optical element is made of glass, silicon, sapphire, quartz, germanium, and/or a MID-Far thermal material, such as CaF2, MgF2, ZnSe, ZnS, NaCl.

6. The thermal emitter module of claim 1, wherein the refractory membrane is made by or comprises a refractory material, e.g., a refractory metal and/or an alloy of refractory metals and/or a refractory ceramic, or wherein the refractory membrane is made of tungsten.

7. The thermal emitter module of claim 1, comprising a blocking aperture around the curved surface.

8. A compound emitter device including a plurality of the thermal emitting modules of claim 1.

9. The compound emitter device of claim 8, in which the optical filters of the thermal emitting modules have different central wavelengths and/or pass bandwidth, configured such that subsets of the thermal emitting modules can be selected and activated.

10. The compound emitter device of claim 8, wherein the radiation emitted by the thermal emitting modules is concentrated in a target spot.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention are disclosed in the description and illustrated by the drawings in which:

[0046] FIGS. 1 to 3 illustrate a cut view of thermal emitter devices.

[0047] FIG. 4 illustrates a cut view of a thermal emitter device comprising a thin lens, while

[0048] FIG. 5 illustrates a cut view of a thick lens of a thermal emitter device.

[0049] FIG. 6 shows a cut view of the thick lens of a thermal emitter device of FIG. 5 as well as the light propagation beyond the lens exit surface.

[0050] FIG. 7 illustrates a cut view of a thick lens of a thermal emitter device and a mirror on a portion of the lens exit surface.

[0051] FIG. 8 illustrates schematically a thermal emitter system comprising a cold mirror, i.e., with a mirror that does not emit at the wavelength of interest.

[0052] FIGS. 9 to 11 show a cut view of a thermal emitter device with an off-axis mirror on the lens exit surface.

[0053] FIG. 12 shows a perspective view of part of a thermal emitter device according to another embodiment of the invention.

[0054] FIG. 13 illustrates a cut view of a thermal emitter device with a filter.

[0055] FIGS. 14 and 15 relate to a compound thermal emitter with four emitting modules and four corresponding lenses.

[0056] 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 are present, some reference signs may have been omitted to avoid overcrowding the figures.

EXAMPLES OF EMBODIMENTS OF THE PRESENT INVENTION

[0057] FIG. 1 illustrates a cut section of a portion of a thermal emitter device 1 that may be part of embodiments of the invention. In this embodiment, the thermal emitter device 1 comprises a thermal emitting membrane 10 comprising a first surface 11 and a second surface 12, the second surface 12 being opposite to the first surface 11, wherein the thermal emitting membrane 1 is arranged to be heated to a thermal emission temperature so that the first and second surfaces 11, 12 radiate light 100 at the thermal emission temperature. The size and the proportion of the different elements illustrated in FIG. 1 are just indicative and do not necessarily correspond to the real size respectively proportion.

[0058] The emissivity of a surface, for example of the first surface 11, will vary according to the material chose, the surface state and the wavelength, and is lower than 0.7 in most cases. In embodiments, the membrane 10 may be monolithic or the first and second surfaces may be made by the same material in which case the second surface 12 will have the same emissivity as the first surface 11. In other embodiments, the first and second surfaces 11, 12 are made by different materials with different emissivity, both lower than 0.7. Non limitative examples of material having an emissivity lower than 0.7 in the IR and visible spectrum comprises refractory metals such as Tungsten, Titanium, Hafnium, Zirconium, Tantalum, Molybdenum, their alloys, their Nitrides, Oxides and Carbides.

[0059] Although the first and second surfaces 11, 12 have been represented as parallel, this is not essential for the invention. Although the first and second surfaces 11, 12 have been represented as substantially plate-like, again this is not essential for the invention. However, the invention is particularly adapted for a flat thermal emitting membrane 10.

[0060] In the illustrated device, the thermal emitting membrane 10 is a single piece membrane. In other (not illustrated) embodiments, the thermal emitting membrane 10 may have a multi-layer structure comprising at least one layer (of a different material) between the first and second surfaces 11, 12.

[0061] In FIG. 1, the thermal emitter device 1 comprises a plurality of resistive arms 4 connected to the thermal emitting membrane 10 and connecting the thermal emitting membrane 10 to a support 13. The thermal emitting membrane 10 is suspended by the resistive arms 4, and it is heated to a thermal emission temperature via those resistive arms 4.

[0062] Importantly, the thermal emitter device 1 comprises also a lens 2 that comprises a lens entry surface 21, which faces the first surface 11 of the thermal emitting membrane 10 in FIG. 1. The lens 2 comprises a lens exit surface 22, opposite to the lens entry surface 21.

[0063] In the embodiment of FIG. 1, the lens entry surface 21 is substantially flat and the lens exit surface 22 comprises a curved portion 24, in this example, the curved portion 24 is convex.

[0064] In the embodiment of FIG. 1, the lens is monobloc and made by the same material. In other embodiments, the lens could comprise two or more pieces and/or could be made of different materials. In one embodiment, a (plano-convex) lens is placed on the lid, e. g. with glue or any other adapted fixation means.

[0065] In the embodiment of FIG. 1, the thermal emitting membrane 10 is placed in a housing 8 defined by the lens 2 and the support 13. In one embodiment, this housing 8 comprises vacuum or a controlled atmosphere e.g., without oxygen or other gases which would react with the emitting material at high temperature.

[0066] In embodiments, the lens 2 has a reflectivity normal to a lens surface, e. g., the lens entry surface 21, comprised in the range 4% to 40%, to partially reflect the radiated light. It may be made of glass, silicon, sapphire, quartz, germanium, and/or a MID-Far thermal material, such as CaF.sub.2, MgF.sub.2, ZnSe, ZnS, NaCl.

[0067] According to the invention, the distance d between the lens entry surface 21 and the first surface 11 of the thermal emitting membrane 10 is equal or lower than L/4, where L is a major dimension of the thermal emitting membrane 10.

[0068] If the thermal emitting membrane 10 has a rectangular section, its major dimension L is the longer side of the rectangular section. If the thermal emitting membrane 10 has a circular section, its major dimension L is the diameter of the circular section.

[0069] In other words, it is preferred that the lens 2 be placed close to the thermal emitter device 10. In this way, a part of the light reflected by the lens 2 is reabsorbed by the thermal emitting membrane 10, and another part of the light reflected by the lens 2 is reflected by the thermal emitting membrane 10 toward the lens 2, having therefore another chance to go through the lens: this allows to increase the efficiency and/or the lifetime of the thermal emitter device.

[0070] According to one embodiment, the distance d between the lens entry surface 21 and the first surface 11 of the thermal emitting membrane 10 is equal or lower than L/8. In this embodiment, the lens 2 is closer to the thermal emitting membrane 10, thereby increasing more the efficiency and/or the lifetime of the thermal emitter device.

[0071] In one embodiment, the thermal emitter device 1 comprises a lid and the lens 2 is placed in or on the lid.

[0072] Using a lens 2 close to the thermal emitting membrane 10 changes the angle dispersion of the thermal emitted light. The refraction at the interface between the housing 8 and the lens entry surface 21 allows to convert all angles, so that all light propagates at angles less than a maximum angle related to the angle of total internal reflection at surface 21 due to the material of the lens 2 a. For example, if the lens is made of glass, the maximum angle is about 40; if the lens 2 is made of in silicon, the maximum angle is about 16.

[0073] FIG. 2 illustrates a cut view of a lens 2 of a thermal emitter device according to another embodiment of the invention. In this embodiment, the lens 2 is made by silicon and comprises an entry surface lens 21 and an exit surface lens 22 substantially parallel to the entry surface lens 21, both the entry surface lens 21 and the exit surface lens 22 being substantially flat.

[0074] By assuming a Lambertian source S emitting at, for example, a wavelength of 1.5 microns with a random polarization (and schematically representing a thermal emitting membrane 10), then about 32% of the light is lost at the lens entry surface 21 due to reflection. A slightly smaller fraction 27% is lost at the lens exit surface 22. The total transmission of the lens therefore 50%:

[00001]$\begin{array}{cc}0.5=\left(1.000-0.315\right)\left(1.00-0.27\right)& \left(1\right)\end{array}$

[0075] Instead of considering this loss as a drawback to be improved, e.g., by using anti-reflective coating, the thermal emitter device 1 according to the invention exploits those reflections, by using a thermal emitting membrane 10 which is not a perfect blackbody.

[0076] The thermal emitting membrane 10 has emissivity of lower than 0.7, depending on wavelength and material. This means it has a reflectivity of 30% or higher. According to the invention, the thermal emitting membrane 10 is placed close to the lens; therefore, the light reflected from the lens 2 will hit the first surface 11 of the thermal emitting membrane 10, and either be reabsorbed by the thermal emitting membrane 10 or reflected by the thermal emitting membrane 10 towards the lens, which then has a second chance to go through the lens 2.

[0077] Let T.sub.lens being the transmission of the first surface of the lens 21, then the light transmitted at the first pass is simply T.sub.lens. Let R.sub.lens being the light reflected by the lens. After reflection R.sub.lens from the thermal emitting membrane 10 with reflectivity R.sub.emitter then after one round trip and additional R.sub.lens R.sub.emitter of light will impinge on the lens 2. Therefore, the total light transmitted after first pass and a single round trip is

[00002]$\begin{array}{cc}{T}_{\mathrm{lens}}\left(1+R_{\mathrm{lens}}{R}_{\mathrm{emitter}}\right),& \left(2\right)\end{array}$

and after n round trips it becomes:

[00003]$\begin{array}{cc}{T}_{\mathrm{lens}}\left(1+R_{\mathrm{lens}}{R}_{\mathrm{emitter}}+{\left(R_{\mathrm{lens}}{R}_{\mathrm{emitter}}\right)}^2+.Math.+{\left(R_{\mathrm{lens}}{R}_{\mathrm{emitter}}\right)}^n\right)& \left(3\right)\end{array}$

[0078] Table 1 indicates the total light transmitted after a certain number of round trips, for a thermal emitter device having an emissivity equal to 0.4 and a reflectivity R.sub.emitter equal to 0.6, and Table 2 indicates the total light transmitted after a certain number of round trips, for a thermal emitter device having an emissivity equal to 0.2 and a reflectivity R.sub.emitter equal to 0.8:

TABLE-US-00001 TABLE 1 No of round trips Transmission 0 68.49% 1 81.43% 2 83.88% 3 84.35% N 84.45%

TABLE-US-00002 TABLE 2 No of round trips Transmission 1 85.75% 2 90.10% 3 91.20% N 91.57%

[0079] These two examples show that most a considerable improvement in transmission occurs via reflection from the thermal emitting membrane 10. As discussed, there is also an additional gain in that the remaining power is not truly lost as it is absorbed by the thermal emitting membrane 10 and therefore increases its efficiency.

[0080] The applicant has found that two round trips are enough to give most of the gain from light being reflected from thermal emitting membrane 10. A close distance between the lens 2 and the thermal emitting membrane 10 have been defined based on those considerations.

[0081] FIG. 3 illustrates a cut view of a thermal emitter device 1 according to another embodiment of the invention.

[0082] Complete numerical simulations with ray-tracing software performed by the applicant with a thermal emitter device according to the invention, a lens 2 having an index of refraction of 3.5 and a thermal emitting membrane 10 having an emissivity of 0.4 show that up to 84.4% of thermal emitted light can be transmitted thought the lens 2, and the other 15.4% is absorbed by the thermal emitting membrane 10.

[0083] A similar advantage can be obtained by exploiting the lens exit surface 22, if the lens 2 is thin. In other words, the thickness of the lens 2 is such that lens exit surface 22 can also be deemed as being close to lens entrance surface 21 as defined above.

[0084] Complete numerical simulations with ray-tracing software performed by the applicant show that the transmission through the thermal light emitting device according to the invention is enhanced if the lens 2 itself is thin.

[0085] In this context, a lens 2 is thin if the lens apparent thickness is less than L/4 (or L/8). In this embodiment, the distance between the lens entry surface 21 and the surface 11 of the thermal emitting membrane, is less than L/4 (or L/8).

[0086] FIG. 4 illustrates a cut view of a thermal emitter device 1 comprising a thin lens 2, according to another embodiment of the invention. The refractive index of the lens 2 is taken to be around 3.5 in FIG. 4.

[0087] For example, with a lens material with an index of refraction of 3.5, light at 45 is refracted to 11.6. The tan of 11.6 is 0.2. More specifically, the thickness is the apparent thickness of the lens when viewed at 45. For example, if the refractive index, n, is 3.5 then the scale factor is 0.21, so the window appears 0.21 times closer than in reality. For n=1.5, the scale factor is 0.53.

[0088] Complete numerical simulations with ray-tracing software performed by the applicant with a thermal emitting membrane 10 of 100 m in diameter show that the lens entry surface 21 should be 20 m away from its first surface 11 for it to be close. The scaled thickness of the thin lens should be likewise 20 m. For an index of 3.5 this would mean that the real thickness of the lens could be 20/0.21=95 m.

[0089] In one embodiment, the entry and the lens exit surfaces 21, 22 of a thin lens 2 are substantially flat.

[0090] FIG. 5 illustrates a cut view of a thick lens 2 of a thermal emitter device according to another embodiment of the invention. The showed map scale gives just an indication of a possible size of the thick lens 2 and should not be considered as limitative.

[0091] In the embodiment of FIG. 5, the lens exit surface 22 is (at least partially) curved, to refocus via reflection at least part of the light back onto the thermal emitting membrane 10. In one embodiment, at least a portion of the lens exit surface 22 is convex. In the embodiment of FIG. 5, all the lenses exit surface 22 is convex.

[0092] Tests performed by the applicant show that the net transmission with a thick lens 2 can be estimated to be about 71% with the remaining 29% being reabsorbed by the thermal emitting membrane 10.

[0093] There is an additional advantage to use a lens 2 comprising an exit curved lens exit surface 22. Not only does it enhance the efficiency of the thermal emitter device 1, but it also makes the emission more directional.

[0094] Tests performed by the applicant show that for a lens having an index of refraction of 3.5, the angular spread of the light beams is +/11.6 simply by refraction at the lens entry surface. The numerical aperture NA of the thermal emitting membrane 10 has been changed from 0.95 to about 0.2, which has a huge advantage in many applications as no other external optical elements are needed.

[0095] In one embodiment, the thermal emitter device 1 comprises an external optics to collimate further the emitted light.

[0096] FIG. 6 illustrates a cut view of the thick lens 2 of a thermal emitter device 1 of FIG. 5, with an embodiment of the light propagation beyond the lens exit surface. In the illustrated embodiment, the beams are not deviated at the lens exit surface 22. In another (not illustrated) embodiment, the beams could be deviated at the lens exit surface 22. The showed map scale gives just an indication of a possible size of the thick lens 2 and should not be considered as limitative.

[0097] In order to restrict the opening of the lens 2, it is possible to either change the shape of the lens 2 or put a mirror on a portion part of the exit surface 22 of the lens. This mirror will block light and reflect it back onto the thermal emitting membrane 10, with the double advantage that the light can be reflected from the thermal emitting membrane 10 or reabsorbed in the thermal emitting membrane 10.

[0098] FIG. 7 illustrates a cut view of a thick lens 2 of a thermal emitter device, comprising a mirror on a portion 23 of the lens exit surface 22, according to another embodiment of the invention. The showed map scale gives just an indication of a possible size of the thick lens 2 and should not be considered as limitative.

[0099] In the embodiment of FIG. 7, the lens exit surface 22 comprised a curved portion 24. The mirror 23 is placed at the two ends of the curved portion 24, by restricting therefore the exit angle of the light beam, thereby improving its directionality.

[0100] In one preferred embodiment, the thermal emitting membrane 10 (not visible in FIG. 7) is curved. This allows to increase more the number of trips on the emitted light in the lens 2.

[0101] In order for the light reflected from the mirror 23 on the lens 2 and for the light reflected from the thermal emitting membrane to escape, in one embodiment, the mirror portion 23 is slightly defocused, i.e., the emitter is not placed at the exact focal point, the blur should remain small on a scale of the emitter dimension; in another embodiment, the thermal emitting membrane is slightly curved (bowed upwards towards the lens), so that the light reflected from the mirror 23 does not retract exactly the original path. The bowing should be small on the scale of the scale of the emitter dimension.

[0102] Tests performed by the applicant show that a bowed mirror 23 couples the light reflected from the mirror 23 into the escape cone, thereby directly improving the efficiency of the thermal emitter device.

[0103] In one embodiment, the mirrored portion 23 comprises an off-axis aperture on the exit lens surface. This allows to improve the device emissivity.

[0104] In one embodiment, the device emissivity is improved by using a using a (cold) mirror.

[0105] FIG. 8 illustrates schematically a thermal emitter system 1000 comprising a cold mirror 200, i.e., with a mirror that does not emit at the wavelength of interest. Although in FIG. 8 the mirror is illustrated as a curved one, the invention is not limited to a curved mirror, but include any shape of mirrors, comprising e.g., flat mirrors. The size and the proportion of the different elements of FIG. 8 are just indicative and do not necessarily correspond to the actual size respectively proportion. The same applies to the inclination of the depicted arrows.

[0106] For an absorbing material =1R.sub.m, where R.sub.m is the reflectivity of the material. By reflecting some of the light emitted from the material back off the same surface, then it is possible to increase the effective emissivity.

[0107] This embodiment is based on the reflection by the cold mirror 200 of some of the light emitted from the first thermal emitter device 100 back off the same surface to increase the effective emissivity or the first thermal emitter device 100.

[0108] Let P.sub.1 being the power emitted by the first thermal emitter device 100 towards the optic 300 and towards the mirror 200. Then:

[00004]$\begin{array}{cc}{P}_1={\mathrm{dA}}_1{}_1& \left(4\right)\end{array}$

[0109] The power reflected back by the cold mirror 200 having a reflectivity R towards the first thermal emitter device 100 is then equal to:

[00005]$\begin{array}{cc}{P}_2=R{\mathrm{dA}}_1{}_1& \left(5\right)\end{array}$

[0110] The power P.sub.2 reflected by the cold mirror 200 is then reflected by the emitter as P.sub.3:

[00006]$\begin{array}{cc}{P}_3=R_{m}R{\mathrm{dA}}_1{}_1& \left(6\right)\end{array}$

where R.sub.m is the reflectivity of the material of the first thermal emitter device 100.

[0111] Therefore, the total power towards the optics 300 is P.sub.1+P.sub.3 and is equal to:

[00007]$\begin{array}{cc}{P}_1+P_3={\mathrm{dA}}_1{}_1\left(1+R_{m}R\right)& \left(7\right)\end{array}$

[0112] The total emission power is conserved, less possible loss in the mirror 200. The power towards optic can never exceed dA.sub.1.Math..sub.1, so that the second law of thermodynamics is satisfied.

[0113] The thermal emitter device according to one embodiment of the invention is an implementation of the idea depicted in FIG. 8.

[0114] FIG. 9 illustrates a cut view of a thermal emitter device 1 according to one embodiment of the invention, comprising an off-axis mirror 23. The mirror comprises an opening 26. The thermal emitter device 1 of FIG. 9 comprises also a (not-illustrated) lens, according to the disclosure.

[0115] In the embodiment of FIG. 9, the emission in a cone 21 towards the mirror 23 is reflected back on to the thermal emitting membrane 10. Part of the power is reabsorbed in the thermal emitting membrane 10 and part of the power is reflected out through the opening 26, which sums with the original power emitted towards the opening 26, thus enhancing the power out.

[0116] FIG. 10 illustrates a cut view of a thermal emitter device 1 according to another embodiment of the invention, comprising an off-axis mirror on the lens exit surface 22.

[0117] In this embodiment, the opening 26 is on the lens exit surface 22 22 so the light is more directional. This embodiment combines the advantage of a (close) lens (to collect angles) along with the mirror 23 to reflect light off the sample.

[0118] FIG. 11 illustrates a cut view of a thermal emitter device 1 according to another embodiment of the invention, comprising an off-axis mirror on the lens exit surface 22.

[0119] In this configuration, the opening could have a different shape to the rest of the lens 2, to control the light further.

[0120] FIG. 12 shows an example of a thermal emitter device 1 according to the invention, wherein the thermal emitting membrane 10 comprises a plurality of resistive arms 4 connected to the thermal emitting membrane 10, wherein the thermal emitting membrane 10 is suspended by the resistive arms, wherein the thermal emitting membrane 10 is heated to a thermal emission temperature via those resistive arms 4. Each of the arms 4 in the illustrated example of FIG. 12 has a length 5, a width 6 and a thickness 7, and a cross-sectional area which is much smaller than that of the membrane 10. The connection pads 3 are designed to provide mechanical connection to a substrate such that the membrane 10 is only supported relative to the substrate by the arms 4 and pads 3. The connection pads 3 provide electrical connection to the arms 4, and thereby to the membrane 10. The membrane 10, pads 3 and arms 4 are preferably made of a single contiguous piece of material. Other features and other embodiments of this thermal emitter device 1 and/or of this emitting membrane 10 are described in the documents WO2020012042, WO2021144463 or WO2021144464 filed by the applicant and enclosed here by reference.

[0121] Advantageously, the thermal emitter device may be manufactured at the micrometer scale on a wafer substrate.

[0122] In the embodiment of FIG. 12 the membrane 10 comprises different holes, as described in the patent application having the application number EP20220155542 filed by the applicant, and here enclosed by reference.

[0123] The presence of the holes on the membrane 10 as described in the patent application having the application number EP20220155542 is not limited to the embodiment of FIG. 12, but it applies also to the other embodiment of the present invention which comprise a mirror.

[0124] FIG. 13 shows an emitter device with a thermal source 10 and a transmissive element 2 configured as a thick lens 2 with a curved exit surface 22 covered by a thin filter layer 120. Remarkably, the curvature is chosen in a way that reduces the variations in the angle between the light rays crossing the exit surface and the surface itself, which is beneficial, because it mitigates the problems related to the angular dependence of the filter. By choosing the geometry carefully, light rays can be made essentially normal to the surface and to the filter layer, at least for a small thermal source; however, the invention does not require a perfect normal incidence everywhere.

[0125] The shift of the filter's central wavelength depends on the refractive index of the central layer of the filter. For each material, an angle range can be determined in which the shift of the central wavelength is not significative and can be neglected. If the shape of the exit surface is chosen such that the angle between the light rays and the normal remains in this angle range, the performance of the filter will be essentially the same as for a collimated source. The angle range is about 8 for glass and glass-like windows. For silicon or similar materials, it could be as high as 30.

[0126] Preferably, the filter layer 120 is a thin film interference filter comprising a stack of dielectric layers deposited on the curved surface. The example depicted has a flat entry surface and a convex exit surface and is advantageous, because the curvature of the exit surface does not need to be extreme. Highly curved surface pose technical issues for depositing thin film filters. Other configurations are possible.

[0127] The example shown combines the filter on a curved surface with a lens close to the emitter membrane disclosed previously. This combination is particularly advantageous because it provides enhanced coupling and excellent light collection in a small package.

[0128] As the light leaving the emitter device is filtered and has a narrow bandwidth, it can be focused more effectively on smaller detectors. Small detectors can be cheaper and provide better performances and especially less noise, than large ones. Preferably, as disclosed in previous embodiments, the device may also include a blocking aperture around the curved exit surface of the lens, to prevent light from leaving the system at unwanted angles, for example a metallised reflective layer 23.

[0129] The emitter can be fabricated on a wafer, as it has been disclosed above. In this case, the transmissive optical element can also be fabricated in the same way. The filter layer 120 can be deposited on the lens. The fabrication process can be parallelised, to realize an integrated array of emitters, micro-lenses and filters that can be fabricated at wafer scale.

[0130] FIGS. 14 and 15 show a compound emitter with an array of several thermal sources 10, each facing the flat backside of an array of micro-lenses with a common plane entry surface on the backside and a plurality of convex surfaces 24 on the forward side, each covered by an optical filter.

[0131] FIG. 14 shows the array from the exit face, and FIG. 15 is a perspective view. FIG. 15 is not to scale: the distance between the source and the target has been artificially reduced for better legibility. The collimation introduced by the micro-lens array concentrates the light in the target spot 140.

[0132] The geometry can be optimised further by displacing slightly the lenses to enhance the overlap of the individual emission spots, as visible in FIG. 14, where the individual sources 10 are slightly offset outwards relative to the axis of the respective axis of the micro-lenses 24. The figures show four emitters, but the invention is not so limited. This variant is especially advantageous in spectrometers, for example, when it is desired to concentrate the radiation on a small-area detector.

[0133] Optionally, the emitter device could include a plurality of modules, each with an emitting refractory membrane, a transmissive element with a curved surface and a filter, as disclosed above, where the filters have different transmission functions, characterised by different central wavelengths and, possibly, bandwidths. In this variant, the wavelength emitted can be changed by selecting a subset of the modules, for example for on-band and off-band detection in a spectrometer system.

[0134] The arrangement described herein is particularly advantageous when the emitting surface is flat, and the back side of the micro-lenses is close to the emitter, as in the micro-emitters of the invention, otherwise the angle range would be too large. It would be much harder to obtain the same results with other sources with an irregular distribution of emission such as LEDs.

REFERENCE SIGNS USED IN THE DRAWINGS

[0135] 1 Thermal emitter device [0136] 2 Lens [0137] 3 Connection pad [0138] 4 Arm [0139] 5 Length of the arm [0140] 6 Width of the arm [0141] 7 Thickness of the arm [0142] 8 Housing [0143] 10 Thermal emitting membrane [0144] 11 First surface [0145] 12 Second surface [0146] 13 Support [0147] 21 Entry lens surface [0148] 22 Exit lens surface [0149] 23 Mirrored portion [0150] 24 Curved portion [0151] 26 Opening [0152] 20 Cold mirror [0153] 100 Emitted light [0154] 120 filter layer [0155] 140 target spot [0156] 200 Cold mirror [0157] 300 Optics [0158] 400 Second thermal emitter device [0159] 1000 Thermal emitter system [0160] d Distance [0161] P1, . . . Pj Powers [0162] t Thickness of the lens [0163] S Lambertian source [0164] 1, 2 Solid angles