Patent Description:
In this context, the IR light has a wavelength belonging to the range from <NUM>,<NUM> to <NUM> and visible light has a wavelength belonging to the range <NUM>,<NUM> to <NUM>,<NUM>.

Thermal emitters for such diverse applications as for example (but not limited to) infrared spectroscopy, illumination for gas sensing, hyperspectral imaging, machine vision, are known.

Examples of known thermal emitters are described in the patent applications <CIT>, <CIT> or <CIT> filed by the applicant.

Thermal emitters emit light according to the blackbody theory of radiation. This gives detailed information about how the emission intensity varies with temperature and wavelength.

However, no real materials are truly black, so the blackbody emissivity has to be scaled by a parameter called the emissivity, ε, which is a function of wavelength and temperature.

Most thermal emitters are based on materials which are as black as possible, i.e., which have an ε at the wavelength range of interest close to <NUM>. The drawback is that there are very few materials with high emissivities and in general they can only survive relatively low temperatures, i.e., to temperature below <NUM>.

Some IR thermal emitters are made from a refractory material. A refractory material is a material with a melting point above <NUM>. Examples of refractory materials are Tungsten, Titanium, Hafnium, Zirconium, Tantalum, Molybdenum and their Nitrides, Oxides and Carbides. Refractory metals are metals with a melting point above <NUM>.

At IR wavelength refractory metals are quite reflective (Reflectivity ranging from <NUM>% to more than <NUM>%) and the corresponding emissivity belongs in general to the range of <NUM> to <NUM>. The advantage of refractory metals is that they are stable at high temperature, the disadvantage is their low intrinsic emissivity.

Flat thermal emitter devices, i.e., thermal emitter devices comprising a substantially flat emitting membrane, are Lambertian emitters. Wire thermal emitter devices are Lambertian on one axis (in general, the axis of the filament) and uniform on a second axis. For a Lambertian emitter, the majority of the power is being emitted in a cone at <NUM>°.

A thermal light emitting device comprises in general a housing, mainly to protect the emitter from oxidation. This housing can include elements to enhance the performance of the thermal emitter in an optical system. A common issue is how to get light from the thermal emitter into the optical system. In order to use the most power possible, light at very high angles (i.e. to angles higher than <NUM>°or lower than -<NUM>°) should be collected. It is also desirable to make the thermal emitter device as compact as possible.

A common solution, used notably for wire thermal emitter devices, is to place the thermal emitter device into a parabolic reflector. However, if the size of the parabolic reflector (or mirror) is similar to the thermal emitter device size, then shadowing occurs, i.e., the thermal emitter device itself blocks the light reflected from the parabolic reflector. Moreover, this solution is not suitable for flat thermal emitter devices. Finally, the parabolic reflector has low efficiency for collecting light from the top side of the thermal emitter device.

Another approach is just to use a lens, comprising a first lens surface and a second lens surface (opposite to the first lens surface), at least one lens surface facing one of the surfaces of the thermal emitting membrane. The lens should be very large to maximize the collected light. However, in this case, light at high angles is lost due to reflection.

By assume a Lambertian thermal membrane emitting with a random polarization, then some light is lost at the first lens surface due to reflection. Another fraction is lost at the second lens surface.

The normal way to overcome these losses is to reduce the reflections using an anti-reflective coating. Some documents disclose the use of a reflective layer placed under the membrane to improve the emissivity of the emitter (<CIT> or <CIT>) or on side walls of the emitter device (<CIT>).

However, the anti-reflective coatings are expensive, they are complicated to fabricate for wide wavelength ranges, and also have a limited range of angles over which they work. Finally, anti-reflective coatings are clearly not ideal when dealing with thermal sources, as the wavelength range is large e.g., <NUM> -<NUM>, and the range of emitting angles is also very large (Lambertian source).

Other broadband techniques involve subwavelength structures such as the so-called "moth eye structures". Moth-eye structures are also expensive to fabricate and are generally not available in standard commercial processes.

<CIT> and <CIT> disclose a thermal emitter device according to the preamble of claim <NUM>.

Therefore, there is a need of a thermal emitter device where the emitted light is used as efficiently as possible and that overcomes the shortcomings and limitations of the state of the art.

An aim of the present invention is the provision of a thermal emitter device that overcomes the shortcomings and limitations of the state of the art.

Another aim of the invention is the provision of a thermal emitter device with improved efficiency and/or easy to fabricate.

Another aim of the invention is the provision of a thermal emitter device where the emitted light is used as efficiently as possible.

According to the invention, these aims are attained by the object of the attached claims, and especially by a thermal emitter device according to claim <NUM>.

The thermal emitter device according to the invention comprises a thermal emitting membrane comprising a surface, wherein the thermal emitting membrane is arranged to be heated to a thermal emission temperature so that the surface radiates IR or visible light.

In this context, the term "membrane" designates an element whose thickness is lower than its other two dimensions. In this context, the term "membrane" is a synonymous of the term "(hot)-plate". In this context, a membrane is arranged to keep its own shape independently on the temperatures and it is held at several points. In other words, in this context, a membrane does not buckle nor break at high temperatures. In one preferred embodiment, the membrane is substantially planar. In one preferred embodiment, the membrane can support itself, i.e., it is structurally independent. In another embodiment, the membrane cannot support itself, unless attached on all sides.

According to the invention, the emissivity of the surface is lower than <NUM>. In fact, the invention is useful for low-emissivity materials, i.e., for materials having an emissivity lower than <NUM>. In other words, there is not so much interest for enhancing the emissivity of good emitter materials, i.e., of materials having an emissivity equal or higher than <NUM>.

The thermal emitter device according to the invention comprises a lens, the lens comprising a lens surface, the lens surface facing the surface of the thermal emitting membrane and having a reflectivity normal to the lens surface comprised in the range <NUM>% to <NUM>%, so as to partially reflect the radiated IR or visible light.

In this context, the term "lens" is not necessarily a synonymous of a flat lens. In other words, a lens could be flat, or it could be not-flat, e.g. it can have a convex shape.

According to the invention, the distance between the lens surface and said one of the first or second surfaces is equal or lower than L/<NUM>, where L is a major length of the thermal emitting membrane. In other words, according to the invention the lens is placed really "close" to the thermal emitter device. In this way, a part of the IR or visible light reflected by the lens is reabsorbed by the thermal emitting membrane, and another part of the light reflected by the lens is reflected by the thermal emitting membrane toward the lens, having therefore another chance to go through the lens, thereby increasing the efficiency the thermal emitter device.

Since the efficiency is increased, then for a fixed radiance it is possible to lower the temperature. Thermal emitters operating at lower temperatures will typically have a longer operating lifetime. In other words, a user who requires a specific spectral radiance will lower the operating temperature and hence improve the lifetime.

The thermal emitter devices according to the invention are not perfect blackbodies. They have an emissivity lower than <NUM> depending on wavelength and material. This means they have a reflectivity of <NUM>% or higher. According to the invention, the thermal emitter device is placed "close" to a partially reflective lens: therefore, a part of the emitted light goes through the lens, and another part of the emitted light will be reflected by the lens, will hit an emitter surface, and either will be reabsorbed by the thermal emitter device or will reflected by the thermal emitter device towards the lens, having then a second chance to go through the lens.

Thanks to the reflection of the thermal emitter device, there is then an improvement in transmission. Moreover, there is an additional gain, since the remaining power is not truly lost as it is absorbed by the thermal emitter device and therefore increases the efficiency of the emitter and/or its lifetime.

According to one embodiment, the thermal emitting membrane is made by or comprises a refractory material, e.g., a refractory metal, a refractory ceramic (such as carbides or nitrides) and/or an alloy of refractory metals.

According to one embodiment, the distance between the lens and the surface of the emitting membrane is equal or lower than U8. In this embodiment, the lens is closer to the emitting membrane, thereby increasing more the efficiency and/or the lifetime of the thermal emitter device.

According to one embodiment, the lens is made of glass, silicon, sapphire, quartz, germanium, and/or a MID-Far thermal material, such as CaF2, MgF2, ZnSe, ZnS, NaCl.

According to one embodiment, the thermal emitter device comprises a lid and the lens is placed in or on the lid. In one embodiment, the lens is the lid.

According to one embodiment, the lens surface is a lens entry surface, the lens comprising a lens exit surface.

According to one embodiment, the lens is "thin". In this embodiment, the thickness of the lens is such that the lens exit surface is also be deemed as being "close" to the emitter surface. In this embodiment, when calculating the thickness of "thin" lens, the refraction of light in the lens material should be taken into account and the lens apparent thickness should be used.

The lens apparent thickness formula for a lens material with a given refractive index n is well known and depends on the incident angle on the lens. For an angle equal to <NUM>°, the lens apparent thickness formula is the real thickness of the lens, multiplied by a scale factor equal to <NUM>/sqrt(2n^<NUM>-<NUM>).

According to this embodiment, a lens is "thin" if its lens apparent thickness is less than U4 (or U8). In this embodiment, the distance between the lens entry surface and a (first) surface of the thermal emitting membrane, is less than U4 (or U8).

According to one embodiment, the lens is a flat lens. In this context, a lens is flat if both of the lens surfaces are flat and sensibly parallel.

According to one embodiment, the lens is "thick". According to this embodiment, a lens is "thick" if its lens apparent thickness is higher than U4 (or U8). In this embodiment, the distance between the lens entry surface and a (first) surface of the thermal emitting membrane, is less than U4 (or U8).

According to one embodiment, the lens exit surface is curved so as to refocus the light back to the emitter and/or for making the emission more directional.

According to one embodiment, the thermal emitter device comprises a mirror on at least a portion of the lens exit surface.

According to one embodiment, the mirror is off-axis. The lens has in general a symmetry axis. In this context, the expression "the mirror is off-axis" indicates that the symmetry axis of the lens does not pass thought the mirror.

According to one embodiment, the mirror is a cold mirror, i.e., a mirror whose reflectivity normal to the mirror surface is higher than <NUM>% (i.e., it is a highly reflecting mirror).

According to one embodiment, the mirror comprises an opening.

According to one embodiment, the portion of the lens facing the opening has a shape different from the shape of the lens which does not face the opening, in order to control the emitted light further.

According to one embodiment, the thermal emitter device comprises a plurality of resistive arms connected to the thermal emitting membrane, wherein the thermal emitting membrane is suspended by the resistive arms, wherein the thermal emitting membrane is heated to a thermal emission temperature via said resistive arms.

According to one embodiment, the surface is a first surface, the device comprising a second surface being opposite to the first surface,.

<FIG> illustrates a cut section of a portion of a thermal emitter device <NUM> according to one embodiment of the invention. In this embodiment, the thermal emitter device <NUM> comprises a thermal emitting membrane <NUM> comprising a first surface <NUM> and a second surface <NUM>, the second surface <NUM> being opposite to the first surface <NUM>, wherein the thermal emitting membrane <NUM> is arranged to be heated to a thermal emission temperature so that the first and second surfaces <NUM>, <NUM> radiate light <NUM> at the thermal emission temperature. The size and the proportion of the different elements illustrated in <FIG> are just indicative and do not necessarily correspond to the real size respectively proportion.

According to the invention, the emissivity ε of a surface, for example of the first surface <NUM>, is lower than <NUM>. In one embodiment, the membrane <NUM> is monolithic. If the membrane <NUM> is monolithic, then the second surface <NUM> will have the same emissivity ε of the first surface <NUM>. In one embodiment, the first and second surfaces <NUM>, <NUM> are made by the same material. In another embodiment, the first and second surfaces <NUM>, <NUM> are made by different materials, but having both an emissivity lower than <NUM>. Non limitative examples of material having an emissivity lower than <NUM> in the IR and visible spectrum comprises a refractory material, e.g., a refractory metal and their alloys. Examples of refractory metals are Tungsten, Titanium, Hafnium, Zirconium, Tantalum, Molybdenum and their Nitrides, Oxides and Carbides.

Although the first and second surfaces <NUM>, <NUM> have been represented as parallel, this is not essential for the invention. Although the first and second surfaces <NUM>, <NUM> 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 <NUM>.

In the illustrated embodiment, the thermal emitting membrane <NUM> is a single piece membrane. In another embodiment (not illustrated), the thermal emitting membrane <NUM> is a multi-layer membrane, i.e., it comprises at least one layer (of a different material) between the first and second surfaces <NUM>, <NUM>.

In the embodiment of <FIG>, the thermal emitter device <NUM> comprises a plurality of resistive arms <NUM> connected to the thermal emitting membrane <NUM>. In the embodiment of <FIG>, the resistive arms <NUM> connect the thermal emitting membrane <NUM> to a support <NUM>. The thermal emitting membrane <NUM> is suspended by the resistive arms <NUM>, and it is heated to a thermal emission temperature via those resistive arms <NUM>.

According to the invention, the thermal emitter device <NUM> comprises also a lens <NUM>. The lens <NUM> comprises a lens entry surface <NUM>, which faces the first surface <NUM> of the thermal emitting membrane <NUM> in <FIG>. The lens <NUM> comprises a lens exit surface <NUM>, opposite to the lens entry surface <NUM>.

In the embodiment of <FIG>, the lens entry surface <NUM> is substantially flat and the lens exit surface <NUM> comprises a curved portion <NUM>, in particular a convex portion <NUM>.

In the embodiment of <FIG>, 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.

In the embodiment of <FIG>, the thermal emitting membrane <NUM> is placed in a housing <NUM> defined by the lens <NUM> and the support <NUM>. In one embodiment, this housing <NUM> comprises vacuum or a controlled atmosphere e.g., without oxygen or other gases which would react with the emitting material at high temperature.

According to the invention, the lens <NUM> has a reflectivity normal to a lens surface, e.g., the lens entry surface <NUM>, comprised in the range <NUM>% to <NUM>%, so as to partially reflect the radiated light.

According to one embodiment, the lens <NUM> is made of glass, silicon, sapphire, quartz, germanium, and/or a MID-Far thermal material, such as CaF2, MgF2, ZnSe, ZnS, NaCl.

According to the invention, the distance d between the lens entry surface <NUM> and the first surface <NUM> of the thermal emitting membrane <NUM> is equal or lower than L/<NUM>, where L is a major dimension of the thermal emitting membrane <NUM>.

If the thermal emitting membrane <NUM> has a rectangular section, its major dimension L is the longer side of the rectangular section. If the thermal emitting membrane <NUM> has a circular section, its major dimension L is the diameter of the circular section.

In other words, according to the invention the lens <NUM> is placed really "close" to the thermal emitter device <NUM>. In this way, a part of the light reflected by the lens <NUM> is reabsorbed by the thermal emitting membrane <NUM>, and another part of the light reflected by the lens <NUM> is reflected by the thermal emitting membrane <NUM> toward the lens <NUM>, having therefore another chance to go through the lens: this allows to increase the efficiency and/or the lifetime of the thermal emitter device.

According to one embodiment, the distance d between the lens entry surface <NUM> and the first surface <NUM> of the thermal emitting membrane <NUM> is equal or lower than U8. In this embodiment, the lens <NUM> is closer to the thermal emitting membrane <NUM>, thereby increasing more the efficiency and/or the lifetime of the thermal emitter device.

In one embodiment, the thermal emitter device <NUM> comprises a lid and the lens <NUM> is placed in or on the lid.

Using a lens <NUM> close to the thermal emitting membrane <NUM> changes the angle dispersion of the thermal emitted light. The refraction at the interface between the housing <NUM> and the lens entry surface <NUM> 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 <NUM> due to the material of the lens <NUM> a. For example, if the lens is made of glass, the maximum angle is about <NUM>°; if the lens <NUM> is made of in silicon, the maximum angle is about <NUM>°.

<FIG> illustrates a cut view of a lens <NUM> of a thermal emitter device according to another embodiment of the invention. In this embodiment, the lens <NUM> is made by silicon and comprises an entry surface lens <NUM> and an exit surface lens <NUM> substantially parallel to the entry surface lens <NUM>, both the entry surface lens <NUM> and the exit surface lens <NUM> being substantially flat.

By assuming a Lambertian source S emitting at, for example, a wavelength of <NUM> microns with a random polarization (and schematically representing a thermal emitting membrane <NUM>), then about <NUM>% of the light is lost at the lens entry surface <NUM> due to reflection. A slightly smaller fraction <NUM>% is lost at the lens exit surface <NUM>. The total transmission of the lens therefore <NUM>%: <MAT>.

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

The thermal emitting membrane <NUM> has emissivity of lower than <NUM>, depending on wavelength and material. This means it has a reflectivity of <NUM>% or higher. According to the invention, the thermal emitting membrane <NUM> is placed close to the lens: therefore, the light reflected from the lens <NUM> will hit the first surface <NUM> of the thermal emitting membrane <NUM>, and either be reabsorbed by the thermal emitting membrane <NUM> or reflected by the thermal emitting membrane <NUM> towards the lens, which then has a second chance to go through the lens <NUM>.

Let Tlens being the transmission of the first surface of the lens <NUM>, then the light transmitted at the first pass is simply Tlens. Let Rlens being the light reflected by the lens. After reflection Rlens from the thermal emitting membrane <NUM> with reflectivity Remitter then after one round trip and additional RlensRemitter of light will impinge on the lens <NUM>. Therefore, the total light transmitted after first pass and a single round trip is <MAT> and after n round trips it becomes <MAT>.

Table <NUM> indicates the total light transmitted after a certain number of round trips, for a thermal emitter device having an emissivity equal to <NUM> and a reflectivity Remitter equal to <NUM>, and Table <NUM> indicates the total light transmitted after a certain number of round trips, for a thermal emitter device having an emissivity equal to <NUM> and a reflectivity Remitter equal to <NUM>:.

These two examples show that most a considerable improvement in transmission occurs via reflection from the thermal emitting membrane <NUM>. 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 <NUM> and therefore increases its efficiency.

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

<FIG> illustrates a cut view of a thermal emitter device <NUM> according to another embodiment of the invention.

Complete numerical simulations with ray-tracing software performed by the applicant with a thermal emitter device according to the invention, a lens <NUM> having an index of refraction of <NUM> and a thermal emitting membrane <NUM> having an emissivity of <NUM> show that up to <NUM> % of thermal emitted light can be transmitted thought the lens <NUM>, and the other <NUM>% is absorbed by the thermal emitting membrane <NUM>.

A similar advantage can be obtained by exploiting the lens exit surface <NUM>, as long as the lens <NUM> is "thin". In other words, the thickness of the lens <NUM> is such that lens exit surface <NUM> can also be deemed as being close to lens entrance surface <NUM> as defined above.

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 <NUM> itself is "thin".

In this context, a lens <NUM> is "thin" if the lens apparent thickness is less than U4 (or U8). In this embodiment, the distance between the lens entry surface <NUM> and the surface <NUM> of the thermal emitting membrane, is less than U4 (or U8).

<FIG> illustrates a cut view of a thermal emitter device <NUM> comprising a "thin" lens <NUM>, according to another embodiment of the invention. The refractive index of the lens <NUM> is taken to be around <NUM> in <FIG>.

For example, with a lens material with an index of refraction of <NUM>, light at <NUM>° is refracted to <NUM>°. The tan of <NUM>° is <NUM>. More specifically, the thickness is the apparent thickness of the lens when viewed at <NUM>°. For example, if the refractive index, n, is <NUM> then the scale factor is <NUM>, so the window appears <NUM> times closer than in reality. For n=<NUM>, the scale factor is <NUM>.

Complete numerical simulations with ray-tracing software performed by the applicant with a thermal emitting membrane <NUM> of <NUM> in diameter show that the lens entry surface <NUM> should be <NUM> away from its first surface <NUM> for it to be close. The scaled thickness of the thin lens should be likewise <NUM>. For an index of <NUM> this would mean that the real thickness of the lens could be <NUM>/<NUM> =<NUM>.

In one embodiment, the entry and the lens exit surfaces <NUM>, <NUM> of a "thin" lenses <NUM> are substantially flat.

<FIG> illustrates a cut view of a "thick" lens <NUM> 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 <NUM> and should not be considered as limitative.

In the embodiment of <FIG>, the lens exit surface <NUM> is (at least partially) curved, so as to refocus via reflection at least part of the light back onto the thermal emitting membrane <NUM>. In one embodiment, at least a portion of the lens exit surface <NUM> is convex. In the embodiment of <FIG>, all the lenses exit surface <NUM> is convex.

Tests performed by the applicant show that the net transmission with a "thick" lens <NUM> can be estimated to be about <NUM>% with the remaining <NUM>% being reabsorbed by the thermal emitting membrane <NUM>.

There is an additional advantage to use a lens <NUM> comprising an exit curved lens exit surface <NUM>. Not only does it enhance the efficiency of the thermal emitter device <NUM>, but it also makes the emission more directional.

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

In one embodiment, the thermal emitter device <NUM> comprises an external optics to collimate further the emitted light.

<FIG> illustrates a cut view of the "thick" lens <NUM> of a thermal emitter device <NUM> of <FIG>, 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 <NUM>. In another (not illustrated) embodiment, the beams could be deviated at the lens exit surface <NUM>. The showed map scale gives just an indication of a possible size of the "thick" lens <NUM> and should not be considered as limitative.

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

<FIG> illustrates a cut view of a "thick" lens <NUM> of a thermal emitter device, comprising a mirror on a portion <NUM> of the lens exit surface <NUM>, according to another embodiment of the invention. The showed map scale gives just an indication of a possible size of the "thick" lens <NUM> and should not be considered as limitative.

In the embodiment of <FIG>, the lens exit surface <NUM> comprised a curved portion <NUM>. The mirror <NUM> is placed at the two ends of the curved portion <NUM>, by restricting therefore the exit angle of the light beam, thereby improving its directionality.

In one preferred embodiment, the thermal emitting membrane <NUM> (not visible in <FIG>) is curved. This allows to increase more the number of trips on the emitted light in the lens <NUM>.

In order for the light reflected from the mirror <NUM> on the lens <NUM> and for the light reflected from the thermal emitting membrane to escape, in one embodiment, the mirror portion <NUM> 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 <NUM> does not retract exactly the original path. The bowing should be small on the scale of the scale of the emitter dimension.

Tests performed by the applicant show that a bowed mirror <NUM> couples the light reflected from the mirror <NUM> into the escape cone, thereby directly improving the efficiency of the thermal emitter device.

In one embodiment, the mirrored portion <NUM> comprises an off-axis aperture on the exit lens surface. This allows to improve the device emissivity.

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

<FIG> illustrates schematically a thermal emitter system <NUM> comprising a cold mirror <NUM>, i.e., with a mirror that does not emit at the wavelength of interest. Although in <FIG> 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> are just indicative and do not necessarily correspond to the actual size respectively proportion. The same applies to the inclination of the depicted arrows.

For an absorbing material ε = <NUM> - Rm, where Rm 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.

This embodiment is based on the reflection by the cold mirror <NUM> of some of the light emitted from the first thermal emitter device <NUM> back off the same surface, in order to increase the effective emissivity or the first thermal emitter device <NUM>.

Let P<NUM> being the power emitted by the first thermal emitter device <NUM> towards the optic <NUM> and towards the mirror <NUM>. Then: <MAT>.

The power reflected back by the cold mirror <NUM> having a reflectivity R towards the first thermal emitter device <NUM> is then equal to: <MAT>.

The power P<NUM> reflected back by the cold mirror <NUM> is then reflected by the emitter as P<NUM>: <MAT> where Rm is the reflectivity of the material of the first thermal emitter device <NUM>.

Therefore, the total power towards the optics <NUM> is P<NUM> + P<NUM> and is equal to: <MAT>.

The total emission power is conserved, less possible loss in the mirror <NUM>. The power towards optic can never exceed dA, · Ω<NUM>, so that the second law of thermodynamics is satisfied.

The thermal emitter device according to one embodiment of the invention is an implementation of the idea depicted in <FIG>.

<FIG> illustrates a cut view of a thermal emitter device <NUM> according to one embodiment of the invention, comprising an off-axis mirror <NUM>. The mirror comprises an opening <NUM>. The thermal emitter device <NUM> of <FIG> comprises also a (not-illustrated) lens, according to the disclosure.

In the embodiment of <FIG>, the emission in a cone Ω<NUM> towards the mirror <NUM> is reflected back on to the thermal emitting membrane <NUM>. Part of the power is reabsorbed in the thermal emitting membrane <NUM> and part of the power is reflected out through the opening <NUM>, which sums with the original power emitted towards the opening <NUM>, thus enhancing the power out.

<FIG> illustrates a cut view of a thermal emitter device <NUM> according to another embodiment of the invention, comprising an off-axis mirror on the lens exit surface <NUM>.

In this embodiment, the opening <NUM> is on the lens exit surface <NUM><NUM> so the light is more directional. This embodiment combines the advantage of a (close) lens (to collect angles) along with the mirror <NUM> to reflect light off the sample.

In this configuration, the opening could have a different shape to the rest of the lens <NUM>, in order to control the light further.

<FIG> shows an example of a thermal emitter device <NUM> according to the invention, wherein the thermal emitting membrane <NUM> comprises a plurality of resistive arms <NUM> connected to the thermal emitting membrane <NUM>, wherein the thermal emitting membrane <NUM> is suspended by the resistive arms, wherein the thermal emitting membrane <NUM> is heated to a thermal emission temperature via those resistive arms <NUM>. Each of the arms <NUM> in the illustrated example of <FIG> has a length <NUM>, a width <NUM> and a thickness <NUM>, and a cross-sectional area which is much smaller than that of the membrane <NUM>. The connection pads <NUM> are designed to provide mechanical connection to a substrate such that the membrane <NUM> is only supported relative to the substrate by the arms <NUM> and pads <NUM>. The connection pads <NUM> provide electrical connection to the arms <NUM>, and thereby to the membrane <NUM>. The membrane <NUM>, pads <NUM> and arms <NUM> are preferably made of a single contiguous piece of material. Other features and other embodiments of this thermal emitter device <NUM> and/or of this emitting membrane <NUM> are described in the documents <CIT>, <CIT> or <CIT> filed by the applicant.

In the embodiment of <FIG> the membrane <NUM> comprises different holes, as described in the patent application having the application number <CIT> filed by the applicant.

Claim 1:
A thermal emitter device (<NUM>), comprising:
- a thermal emitting membrane (<NUM>) comprising a surface (<NUM>; <NUM>), wherein the thermal emitting membrane (<NUM>) is arranged to be heated to a thermal emission temperature so that the surface radiates IR or visible light,
- a lens (<NUM>), comprising a lens surface (<NUM>), the lens surface facing the surface (<NUM>; <NUM>) of the thermal emitting membrane (<NUM>), characterized in that the emissivity of the surface (<NUM>; <NUM>) is lower than <NUM>,
that the lens surface has a reflectivity normal to the lens surface comprised in the range <NUM>% to <NUM>%, so as to partially reflect the radiated IR or visible light,
that the distance (d) between the lens surface (<NUM>) and the surface (<NUM>; <NUM>) is equal or lower than L/<NUM>, where L is a major length of the thermal emitting membrane (<NUM>),
so that a part of the IR or visible light reflected by the lens (<NUM>) is reabsorbed by the thermal emitting membrane (<NUM>), and another part of the IR or visible light reflected by the lens (<NUM>) is reflected by the thermal emitting membrane (<NUM>) toward the lens (<NUM>), having therefore another chance to go through the lens, thereby increasing the efficiency of the thermal emitter device (<NUM>).