A WAVELENGTH CONVERTING ELEMENT, A LIGHT EMITTING MODULE AND A LUMINAIRE

Abstract

A wavelength converting element (100), a light emitting module and a luminaire are provided. The wavelength converting element comprises a luminescent element (104) and a light transmitting cooling support (112). The luminescent element comprises a luminescent material (102) and a light transmitting sealing envelope (108) for protecting the luminescent material against environmental influences. The sealing envelope has a first thermal conductivity. The cooling support has a second thermal conductivity that is at least two times the first thermal conductivity. The cooling support comprises a first surface (113) and the sealing envelope comprises a second surface (105). The first surface and the second surface face towards each other. The first surface is thermally coupled to the second surface for allowing through the second surface a conduction of heat towards the cooling support to enable a redistribution of the heat generated in the luminescent element.

Claims

1. A wavelength converting element comprising: a luminescent element comprising a luminescent material and a sealing envelope, the luminescent material being configured to absorb a portion of impinging light and to convert a portion of the absorbed light towards light of another color, the luminescent material being provided in the sealing envelope, the sealing envelope comprising two layers of glass in between which the luminescent material is provided, the sealing envelope being light transmitting, having a first thermal conductivity and being configured to protect the luminescent material against environmental influences, a cooling support made of a light transmitting material having a second thermal conductivity that is larger than two times the first thermal conductivity, wherein the cooling support comprises a first surface, the sealing envelope comprises a second surface, the first surface faces towards the second surface, and the first surface is thermally coupled to the second surface for allowing through the second surface a conduction of heat towards the cooling support to enable a redistribution of the heat generated in the luminescent element, wherein a first ratio of the thermal conductivity of the layers of glass and a thickness of the one of the layers of glass which is arranged between the luminescent material and the cooling support is larger than 200 W/m.sup.2K, wherein the cooling support is thermally coupled to the luminescent element via a layer of light transmitting glue, wherein a second ratio of a thermal conductivity of the light transmitting glue and a thickness of the layer of light transmitting glue is larger than 100 W/m.sup.2K.

2. A wavelength converting element according to claim 1, wherein the first thermal conductivity is smaller than 5 W/mK, or the second thermal conductivity is larger than 10 W/mK, or the first thermal conductivity is smaller than 5 W/mK and the second thermal conductivity is larger than 10 W/mK.

3. A wavelength converting element according to claim 1, wherein the sealing envelope provides a barrier for moisture and/or air that has a penetration rate that is smaller than 10.sup.6 mbar l/s.

4. A wavelength converting element according to claim 1, wherein the sealing envelope comprises sealing material provided in between the two layers of glass and arranged around the luminescent material, the sealing material being configured to provide a barrier for moisture and/or air.

5. A wavelength converting element according to claim 1, wherein the first ratio is larger than 3500 W/m.sup.2K.

6. (canceled)

7. (canceled)

8. A wavelength converting element according to claim 1, wherein the cooling support comprises one of the materials alumina, sapphire, spinel, AlON, SiC or MgO.

9. A wavelength converting element according to claim 1, wherein the cooling support is a layer and a thickness of the layer is larger than 0.1 mm and optionally smaller than 2.0 mm.

10. A wavelength converting element according to claim 1 comprising a layer of a further luminescent material being configured to absorb a portion of impinging light and to convert the absorbed portion towards light of a further color, the further luminescent material being less sensitive to environmental influences than the luminescent material.

11. A wavelength converting element according to claim 1, wherein the luminescent material is configured to emit the another color of light in a narrow light emission distribution having a spectral width of not more than 75 nm expressed as a Full Width Half Maximum Value.

12. A light emitting module comprising: a light emitter for emitting light, a wavelength converting element according to claim 1, the wavelength converting element being arranged to receive light from the light emitter.

13. A light emitting module according to claim 12, wherein the light emitting module also comprises a thermal conductive housing and the cooling support of the wavelength converting element is thermally coupled to the thermal conductive housing.

14. A light emitting module according to claim 13, wherein the thermal conductive housing comprises a light exit window, the light emitter is arranged to emit light towards the light exit window, the wavelength converting element forms the light exit window and an edge of the cooling support being thermally coupled to the thermal conductive housing.

15. A luminaire comprising a wavelength converting element according to claim 1.

16. A luminaire comprising a light emitting module according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the drawings:

[0033] FIG. 1 schematically shows three embodiments of a wavelength converting element according to an aspect of the invention,

[0034] FIGS. 2a and 2b schematically show embodiments of a light emitting module according to another aspect of the invention,

[0035] FIG. 3 schematically shows three other embodiments of wavelength converting elements in which a layer of further luminescent material is provided,

[0036] FIG. 4 schematically shows an embodiment of a wavelength converting element wherein the further luminescent material is provided in the luminescent element,

[0037] FIG. 5a schematically shows an embodiment of a lamp, and

[0038] FIG. 5b schematically shows an embodiment of a luminaire.

[0039] It should be noted that items denoted by the same reference numerals in different Figures have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item have been explained, there is no necessity for repeated explanation thereof in the detailed description.

[0040] The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly.

DETAILED DESCRIPTION

[0041] FIG. 1 schematically shows three embodiments of a wavelength converting element 100, 130, 160 according to an aspect of the invention. A first embodiment of a wavelength converting element 100 is presented at the top end of FIG. 1. The wavelength converting element 100 comprises a luminescent element 104 which comprises luminescent material 102 provided in a sealing envelope 108 made of glass. The sealing envelope 108 is thermally coupled to a cooling support 112. The thermal coupling between the sealing envelope 108 and the cooling support may be provided by, for example, a layer of glue 110. The cooling support 112 comprises a first surface 113 that faces towards the luminescent element 104. The luminescent element 104 has a second surface 105 that faces towards the cooling support 112. The second surface is formed by a surface of the sealing envelope 108. The second surface 105 is (optionally along its whole surface) thermally coupled to the first surface 113.

[0042] The luminescent material 102 is configured to absorb a portion of impinging light according to an absorption spectral distribution and convert the absorbed light towards light of another color according to a light emission spectral distribution. The luminescent material 102 is sensitive to environmental conditions, such as, air and/or moisture. Typically, luminescent materials that emit light in a relatively narrow light emission spectral distribution (meaning that the full width half maximum of that distribution is smaller than 75 nanometer) are sensitive to moisture and/or air. Examples of such luminescent materials are particles that show quantum confinement and have at least in one dimension a size in the nanometer range. Showing quantum confinement means that the particles have optical properties that depend on the size of the particles. Examples of such materials are quantum dots, quantum rods and quantum tetrapods. Other typical narrow band luminescent materials that are sensitive to air and/or moisture are some inorganic phosphors like Thiogallates, such as, for example, Strontium Thiogallates. Other materials may be CaSSe and SSON:Eu. As shown in FIG. 1, the luminescent material 102 may be provided as a layer. The layer has a certain thickness as indicated in FIG. 1 by th1. The thickness of the layer is such that a required amount of luminescent material 102 may be provided to obtain a required light conversion. The thickness is typically in a range from 0.05 mm to 1 mm. The luminescent material 102 may comprise one specific type of a luminescent material, but may also comprises a mix of different types of luminescent materials that have, for example, different light emission spectra. It might be that the luminescent material 102 is the only material present in the sealing envelope 108, but, in other embodiment, the luminescent material may be provided in a matrix, such as a matrix polymer, or, for example, in a liquid inside the sealing envelope 108.

[0043] The sealing envelope 108 is configured to and arranged for protecting the luminescent material 102 against air and/or moisture. Thus, the material of the sealing envelope 108 provides a barrier for air and/or moisture. In an embodiment, the thermal conductivity of the material of the sealing envelope 108 is lower than 5 W/mK. In another embodiment, the thermal conductivity of the material of the sealing envelope 108 is lower than 3 W/mK. In a further embodiment, the thermal conductivity of the material of the sealing envelope 108 is lower than 2 W/mK. The sealing envelope is light transmitting to allow light to be transmitted towards the luminescent material 102 and to allow the light that is generated in the luminescent material 102 to be emitted in a direction away from the luminescent material 102. A thickness of the sealing envelope 108 is made relatively small because the sealing envelope 108 would otherwise form a too large thermal barrier for heat that is generated in the luminescent material 102. A typical thickness of the sealing envelope is in a range from 200 micrometer to 1 mm. The thickness is measured in a direction from the luminescent material 102 towards the cooling support. In FIG. 1 the thickness of the sealing envelope is indicated with th2. A first ratio of the thermal conductivity of the material of the sealing envelope and a thickness th2 of the sealing envelope is larger than 200 W/m.sup.2K to prevent that the sealing envelope is a too large barrier for heat. Optionally, the first ratio is larger than 3500 W/m.sup.2K.

[0044] The sealing envelope may be manufactured for the largest part of glass. Techniques to obtain such a glass sealing envelope are, for example, glass blowing, glass welding, glass-glass frit bonding by using a laser to heat the frit, or, for example, glass-glass sealing by glue (and, optionally, using a getter within the sealed space to absorb air and/or moisturethis technology is known in the field of sealing Organic Light Emitting Diodes). Glass has a typical thermal conductivity of about 0.7 to 1.4 W/mK. Fused silica and quartz have a thermal conductivity up to 1.4 W/mK. Different types of borosilicate (including AF45 and eagle glass) have a thermal conductivity in the range from 0.9 to 1.2 W/mK. Different types of soda lime glass have a thermal conductivity in the range from 0.7 to 1.3 W/mK.

[0045] The layer of glue 110 may be used to fasten the luminescent element 104 to the cooling support 112 and to provide the thermal coupling between the luminescent element 104 and the cooling support 112. The layer of glue 110 has a thickness which is indicated in FIG. 1 with th3. The thickness of the layer of glue 110 may be relatively thin, for example, in the order of one hundred or a few hundred micrometers. The thermal conductivity of the glue is larger than 0.1 W/mK, but, in an embodiment, larger than 0.2 W/mK. Optionally, second ratio of a thermal conductivity of the light transmitting glue and a thickness th3 of the layer of light transmitting glue is larger than 100 W/m.sup.2K to prevent that the layer of glue 110 has a too large thermal resistance. Optionally, the second ratio is larger than 2000 W/m.sup.2K. The layer of glue 110 is also light transmitting to allow a transmission of light from the cooling support 112 to the luminescent element 104 and vice versa. Because the layer of glue 110 may become relatively warm, the glue should be stable, for example, the glue may be LED grade material, which means that it is stable at elevated temperatures and high fluxes of incident light, for example, high fluxes of incident blue light. Stable at least means that no optical degradation occurs and that there is about no delamination of the two components that are glued to each other. For example, Silicone KJR9222 and KJR9224 (Shin-Etsu) or Lumisil 400 (Wacker) have been tested as glues that have such characteristics. Other adhesives that could be used are suitable acrylates or epoxies, such as, for example, the Delo-family (Katiobond).

[0046] It is schematically drawn by means of arrow 106 that heat that is generated by the luminescent material 102 may be well conducted towards the cooling support 112 as long as a thermal resistance of a thermal path through the sealing envelope 108 and the layer of glue 110 is relatively low. By choosing appropriate materials the glue, and choosing appropriate layer thicknesses for the glass sealing envelope 108 and the layer of glue 110, a relatively large amount of heat generated in the luminescent material 102 may be conducted towards the cooling support 112. The cooling support 112 redistributes the heat such that a more uniform temperature distribution is obtained through the wavelength converting element 100.

[0047] The cooling support 112 is made of a light transmitting material and has a relatively high thermal conductivity. In an embodiment, the thermal conductivity of the material of the cooling support 112 is larger than 10 W/mK, or, in another embodiment, larger than 15 W/mK, or, in a further embodiment, larger than 20 W/mK. The thickness of the cooling support is indicated in FIG. 1 by th4. The thickness th4 is sufficient large such that a large amount of heat may be transported by the cooling support 112, but not too large so that it does not introduce a too big thermal resistance in the heat path from the luminescent material to a potential heat sink. The thickness th4 is, for example, larger than 0.1 mm, or, in another embodiment, larger than 0.5 mm, or in a further embodiment, larger than 0.8 mm. The thickness th4 of the cooling support is, for example, smaller than 2 mm. Thereby the cooling support strongly contributes to the redistribution of heat within the whole wavelength conversion element 100 such that no relatively warm hotspots are present while other parts of the wavelength conversion element 100 are relatively cool.

[0048] Other adhesives that could be used are suitable acrylates or epoxies, such as, for example, the Delo-family (Katiobond). via the glue 112 also contributes to the fact that heat is better conducted towards an environment of the wavelength conversion element 100. Useful materials for the cooling support are ceramic Alumina, sapphire, spinel, AlON, SiC, MgO.

[0049] In FIG. 1 the presented embodiments are drawn in cross-sectional view. The presented cross-sectional view of wavelength converting element 100 may be a cross section of a disk shaped wavelength converting element 100, or a square or rectangular box shaped wavelength converting element 100. As such, the three dimensional shape of the luminescent element 104 and/or of the cooling support 112 may also be one of disk shaped or square or rectangular box shaped. In practical embodiments, the cooling support 112 forms a layer and the luminescent material 102 is also provided in a layer between two layers of glass.

[0050] Wavelength converting element 130 has a different cross-sectional view. Except for the shape of the wavelength converting element 130 and the embodiment of the sealing envelope of the wavelength converting element 130, wavelength converting element 130 is similar to the above discussed wavelength converting element 100. The presented cross-sectional shape has the shape of half an ellipse (or, in another embodiment, half a circle). This means that the three dimensional shape of the wavelength converting element 130 may be a shape of a dome or a shape of a tunnel. This implies that the luminescent element and the cooling support 142 have also such a shape. The embodiment of the luminescent element of wavelength conversion element 130 comprises two dome shaped or tunnel shaped layers of glass 138, 139 in between which a layer of the luminescent material 132 is provided. At an edge of the layer of luminescent material 132 an opening between the two layers of glass 138, 139 is sealed by a sealing material 137. The sealing material 137 may be a dedicated type of glue which forms a relatively good barrier for moisture and/or air. The sealing material 137 may also be based on glass and may be welded to the two layers of glass 138, 139 by locally heating the material and the neighboring glass. Such local heating may be obtained by impinging a relatively small, but powerful, laser bundle to the location where the sealing material 137 must be welded to the two layers of glass 138, 139. In between one of the layer of glass 138, 139 and the cooling support 142 a layer of light transmitting glue 140 is provided. Embodiments of the glue, the luminescent material 132 and further characteristics of the elements of the wavelength conversion element 130 are discussed in the context of wavelength conversion element 100.

[0051] At the bottom end of FIG. 1 another embodiment of a wavelength conversion element 160 has been presented. Except for the shape of the cooling support 172 and the embodiment of the sealing envelope, the wavelength conversion element 160 is similar to wavelength conversion element 100. In line with wavelength conversion element 130, the sealing envelope comprises two layers of glass 168, 169. The two layers of glass 168, 169 have a relatively flat shape and may be disk shaped, square or rectangular shaped, or have any other appropriate flat shape. The luminescent material 102 is provided in between the two layers of glass 168, 169 and at an edge of the luminescent material 102 (close to the edges of the two layers of glass 168, 169) the space in between the two layer of glass 168, 169 is sealed by means of sealing material 167 (of which embodiments have already been discussed above). The wavelength conversion element 160 further comprises a (circular or rectangular) tray shaped cooling support 172. The luminescent element that is formed by the two layer of glass 168, 169, the luminescent material 102 and the sealing material 167 is provided inside the tray shaped cooling support 172. The luminescent element is glued by means of a layer of light transmitting glue 170 to the cooling support 172. In this embodiment, a better thermal coupling is obtained between the luminescent element and the cooling support because a larger portion of the luminescent element is via the blue in contact with the cooling support. Embodiments of the glue, the luminescent material 102 and further characteristics of the elements of the wavelength conversion element 160 are discussed in the context of wavelength conversion element 100.

[0052] FIGS. 2a and 2b schematically show embodiments of a light emitting module 200, 250 according to another aspect of the invention. FIG. 2a shows light emitting module 200 which comprises a wavelength converting element 201 which may be similar to wavelength converting element 100 or 160 of FIG. 1. Light emitting module 200 further comprises a thermally conductive housing 204 and comprises one or more light emitters 208. The thermally conductive housing 204 encloses a space 202 which is, for example, filled with air. The inner walls 210 of the thermally conductive housing 204 that are facing towards the space 202 may be provided with a light reflective coating or layer (not shown) such that light that impinges on the inner walls 210 is reflected instead of absorbed. Within the space 202 are provided the one or more light emitters 208. Optionally the light emitters 208 are provided with a dome shaped optical element 209 which, for example, contributes to a good light extraction from the light emitters 208 and/or which may refract the light emitted by the light emitters 208 such that a wider light beam is emitted by the light emitters 208. At one side of the thermally conductive housing a light exit window 212 is provided. At the light exit window 212 is provided the wavelength converting element 201. At least an edge of the cooling support 112 is thermally coupled to the thermally conductive housing 204. This thermal coupling may be obtained by, for example, a thin layer of glue (which has a sufficient high thermal conductivity, but in practical embodiments, the thermal conductivity of the glue is not really high). The cooling support 112 may also be arranged in direct contact with the thermally conductive housing 204. As shown in FIG. 2a, edges of the sealing envelope 108 may also be directly in contact with the thermally conductive housing 204 or the edges of the sealing envelope 108 are also thermally coupled to the thermally conductive housing 204 by means of a thin layer of glue. By means of arrow 106 it is schematically indicated how heat may be conducted from the luminescent material 102, via the sealing envelope 208, the layer of glue 110 and the cooling support 112 towards the thermally conductive housing 204.

[0053] In an embodiment, walls of the thermally conductive housing 204 may also have a lower part that is relatively thick and may have an upper part that be relatively thin (the upper part is a portion that is close to the light exit window 212) such that the walls of the thermally conductive housing have a profile in which the wavelength converting element 201 fits (which means, in which the wavelength converting element 201 may be laid/glued). Thereby a portion of a surface of the cooling support 112, which faces towards the space 202, is also in contact with an upper part of the thermally conductive wall to obtain a better thermal coupling.

[0054] As shown in FIG. 2a, the light emitting module 200 may optionally have a heat sink 206. The heat sink 206 may be thermally coupled to a surface of the thermally conductive housing 204 that is facing away from the space 202 (and, in particular, in FIG. 2a a surface that is opposite a surface on which the light emitters 208 are provided). The thermally conductive housing 204 may conduct heat that it received from the wavelength converting element towards the heat sink 206.

[0055] In FIG. 2a it has been drawn that the cooling support 112 faces the space 202 in which the light emitters 208 are provided. In another embodiment, the wavelength converting element 201 may also arranged up-side-down in the thermally conductive housing 204 such that the cooling support layer faces the ambient and a portion of the sealing envelope faces the space 202.

[0056] In FIG. 2a three light emitters 208 have been drawn. Embodiments of the light emitting module may comprise one, two, three or more light emitters 208. In an embodiment the light emitters are solid state light emitters. For example, the light emitters 208 are Light Emitting Diodes (LEDs). The light emitters 208 may emit blue light and the luminescent material(s) 102 of the wavelength converting element may be configured to convert a portion of the received blue light towards yellow light such that a combination of yellow light and blue light may result in a white light emission. The luminescent material(s) 102 may also be configured to convert a portion of the blue light towards red light such that the light emitted by the light emitting module 200 comprises a more smooth light emission distribution and may have a higher Color Rendering Index (CRI). It is to be noted that embodiments of the luminescent materials 102 are not limited to yellow or red emitting luminescent materials.

[0057] In FIG. 2b another embodiment of a light emitting module 250 has been presented. The light emitting module 250 comprises a thermally conductive housing 254 which encloses a cavity and inside this cavity are provided light emitters 208. The walls of the cavity may be provided with a light reflective coating or layer. The light emitting module 250 also comprises wavelength converting element 251 which is similar to wavelength converting elements 100, 160 of FIG. 1 except that the cooling support 262 is relatively thick and fills for the largest part the cavity that is enclosed by the thermally conductive housing 254. In an embodiment, the cooling support 262 may be in direct contact with the light emitters 208 such that light emitted by the light emitters 208 is well coupled into the cooling support 262. In another practical embodiment, a light transmitting medium 264, for example, Silicone, is provided in between the light emitters 208 and the cooling support 262. The light transmitting medium 264 assist in the outcoupling of light from the light emitters 208 and allows the transmission of the light towards and into the cooling support 262. The cooling support 262 is along a relatively large surface in thermal contact with the thermally conductive housing 254 such that a relatively large portion of the heat that is received from the luminescent material 102 may be conducted towards the thermally conductive housing 254. As shown in FIG. 2b, it is not necessary that the luminescent element with sealing envelope 108 and luminescent material 102 is arranged in between walls of the thermally conductive housing 254the luminescent element may protrude out of the thermally conductive housing 254.

[0058] FIG. 3 schematically shows three other embodiments of a wavelength converting elements 300, 330, 360 in which a layer of further luminescent material is provided. Basically, the arrangement of the wavelength converting element 300, 330, 360 is similar to the arrangement of wavelength converting elements 100, 160 of FIG. 1 except that an additional layer 302 of further luminescent material is provided. The further luminescent material is to a lesser extent sensitive to air and/or moisture than the luminescent material 102 is and, as such, the further luminescent material is not sealed and protected against air and/or moisture. The further luminescent material is configured to absorb a portion of impinging light and convert the absorbed portion towards light of a further color. For example, the further luminescent material may be a yellow/orange emitting inorganic phosphor (e.g. YAG:Ce (for example, NYAG) or LuAG:Ce). Often these further luminescent materials have a relatively broad light emission spectrum. In the different embodiments of the wavelength converting element 300, 330, 360 the additional layer 302 of the further luminescent material is arranged at different positions. In wavelength converting element 300, the additional layer 302 of the further luminescent material is arranged at a surface of the cooling support 112 that is opposite a surface of the cooling support 112 that is thermally coupled to the luminescent converter 104. In wavelength converting element 330, the additional layer 302 of the further luminescent material is arranged at a surface of the luminescent converter 104 that is opposite a surface of the luminescent converter 104 that is thermally coupled to the cooling support 112. In wavelength converting element 360, the additional layer 302 of the further luminescent material is arranged in between the cooling support 112 and the luminescent converter 104.

[0059] In another embodiment, layer 302 is an optical layer with specific optical properties (that are different from being luminescent). The optical layer may comprise scattering material, may be a filter or may comprise specific optical structures for redirecting or refracting light like outcoupling structures or micro-lenses. It is to be noted that such an optical layer may also be combined with the additional layer of further luminescent material.

[0060] FIG. 4 schematically shows an embodiment of a wavelength converting element 400 wherein the further luminescent material 402 is provided in the luminescent element 404. Except the addition of the further luminescent material 402, the wavelength converting element 400 is similar to wavelength converting elements 100, 160 of FIG. 1. Although it is not required to seal the further luminescent material (as discussed in the context of FIG. 3), this further luminescent material 402 may be provided within the sealing envelope 108 together with the luminescent material 102 that is sensitive to air and/or moisture. In FIG. 4 two distinct layers, each one with one of the luminescent materials 102, 402, are drawn inside the sealing envelope 108, but, in other embodiment, the different luminescent materials 102, 402 may be provided as a mix inside the sealing envelope 108.

[0061] FIG. 5a schematically shows an embodiment of a lamp 500. The lamp 500 has, for example, a shape of a traditional incandescent lamp and is, as such, a retro-fit incandescent lamp. The lamp 500 may comprise, for example, one or more light emitting modules (not shown) according to previously discussed embodiments of the light emitting modules or the lamp 500 may comprise one or more wavelength conversion elements (not shown) according to previously discussed embodiments of the wavelength converting elements.

[0062] FIG. 5b schematically shows an embodiment of a luminaire 550. The luminaire 550 comprises, for example, one or more light emitting modules (not shown) according to previously discussed embodiments of the light emitting modules. In another embodiment, the luminaire 550 comprises one or more lamps (not shown) according to the embodiment of FIG. 5a. In yet a further embodiment, the luminaire 550 comprises one or more wavelength conversion elements (not shown) according to previously discussed embodiments of the wavelength converting elements.

[0063] In summary, a wavelength converting element, a light emitting module and a luminaire are provided. The wavelength converting element comprises a luminescent element and a light transmitting cooling support. The luminescent element comprises a luminescent material and a light transmitting sealing envelope for protecting the luminescent material against environmental influences. The sealing envelope has a first thermal conductivity. The cooling support has a second thermal conductivity that is at least two times the first thermal conductivity. The cooling support comprises a first surface and the sealing envelope comprises a second surface. The first surface and the second surface face towards each other. The first surface is thermally coupled to the second surface for allowing through the second surface a conduction of heat towards the cooling support to enable a redistribution of the heat generated in the luminescent element.

[0064] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

[0065] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article a or an preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.