FIELD OF THE INVENTION

The present invention relates to a light emitting arrangement, as well as a lamp and a luminaire, respectively, comprising said light emitting arrangement. BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are typically mounted on a submount wafer that later can be diced to separate out the individual LEDs/submounts. Each submount portion of the wafer has top electrodes that are bonded to electrodes on the LED. In a flip-chip mounting configuration, an LED with a light-transmissive substrate and front-side electrodes is bonded "face down" to bonding bumps of a mount. The flip-chip arrangement is, for example, advantageous with regard to thermal heat sinking due to the proximity of the light active layers to the submounts.

Wavelength converting materials, also called luminescent materials or phosphors, may be used with LEDs, such as group Ill-nitride LEDs, to produce light emitting devices emitting light of various wavelengths. The light emitted by the LED is firstly absorbed by the wavelength converting material, and then the wavelength converting material converts the light into light of a different wavelength than that of the light absorbed. As a consequence of the light conversion not being complete, energy is dissipated in the wavelength converting material, typically a wavelength converting layer, which leads to an increase in temperature in both the wavelength converting layer and in the adjacent LED. The temperature increase results in an overall efficiency decrease as well as in degradation of both the wavelength converting material and the LED. There is a need in the art to improve the thermal management in such light emitting arrangements.

US 2010/0244662 Al has proposed a luminescent light source including a substrate; a light-emitting element mounted on the substrate; and a phosphor layer that covers the light-emitting element and is filled in an interstice between a principal surface of the substrate and the light-emitting element, in order to improve the heat dissipation properties of the arrangement. Still, there is a need to increase the efficiency of light conversion in light emitting arrangements comprising so called phosphor converted light emitting diodes, and to further decrease the energy dissipated in the wavelength converting layer and the light emitting diode of such an arrangement.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partly overcome this problem, and to provide a light emitting arrangement having a configuration of its wavelength converting layers improving the overall efficiency of the arrangement.

According to a first aspect of the invention, this and other objects are achieved by a light emitting arrangement comprising a light emitting diode, which is arranged on a thermally conductive support, as well as a first wavelength converting layer and a second wavelength converting layer. The light emitting diode is arranged to emit first primary light towards the thermally conductive support and to emit second primary light away from the thermally conductive support. The first wavelength converting layer comprises a first wavelength converting material for converting first primary light into first converted light. The second wavelength converting layer comprises a second wavelength converting material for converting second primary light into second converted light. The first wavelength converting layer and the second wavelength converting layer are arranged on opposite sides of the light emitting diode. The first wavelength converting layer is in thermal contact with the thermally conductive support. Optionally, the light emitting arrangement may further comprise a reflective layer arranged to reflect first converted light. The first converted light may be reflected towards the light emitting diode and/or away from the thermally conductive support. The reflective layer may be thermally conductive.

The light emitting diode may be transmissive to first converted light emitted by the first wavelength converting layer.

When highly energetic blue light is converted to light of a longer wavelength with less energy, e.g. yellow light or red light, energy, typically in the form of heat, is dissipated in the wavelength converting material. The energy dissipation is even larger if the conversion of a blue photon does not take place with 100 % efficiency. The energy dissipation leads to a temperature increase in the material in which the light conversion is taking place and in the material adjacent to said material, typically in an LED. A temperature increase in components, like wavelength converting layers and LEDs, results in decreased overall efficiency as well as faster degradation of the same components. The light emitting arrangement according to embodiments of the present invention at least partly overcomes this problem. The proposed light emitting arrangements present improved overall efficiency, at least partly due to a shortened thermal path by arranging a first wavelength converting layer between the support and the LED, and, in some embodiments, in part also due to reduced thickness of the second wavelength converting layer arranged on the side of the LED opposite to the side on which the first wavelength converting layer is arranged, compared to conventional devices.

Advantageously, the light emitting arrangement according to embodiments of the present invention may also comprise a reflective layer comprising a reflective material, preferably a thermally conductive reflective layer.

According to an embodiment, the light emitting arrangement may further comprise a reflective layer, which may be positioned relative to the light emitting diode such that it is arranged to reflect first converted light, for example in a direction away from the support.

In an embodiment, the reflective layer may be a thermally conductive support comprising reflective material. In some embodiments, the reflective layer may be a first wavelength converting layer comprising reflective material. In some embodiments, the reflective layer may be a separate (preferably thermally conductive) layer comprising reflective material.

The reflective layer may be arranged between the support and the first wavelength converting layer. The reflective layer may be arranged in thermal contact with the support. The reflective layer may be arranged in direct physical contact with the support. The first wavelength converting layer may be arranged in direct physical contact with the reflective layer. Direct physical contact may favor thermal contact and/or optical contact.

According to an embodiment, the light emitting arrangement may further comprise an additional thermally conductive layer being arranged in thermal contact with the support.

According to an embodiment, a Stokes shift of the wavelength converting material of the first wavelength converting layer may preferably be larger than the Stokes shift of the wavelength converting material of the second wavelength converting layer.

Hence, the wavelength converting layer having the larger Stokes shift and in which more energy is thus dissipated, is arranged closer to and in closer thermal contact with the support, e.g. a heat sink, than the wavelength converting layer having the smaller Stokes shift. According to an embodiment, the wavelength converting material of at least one of the first wavelength converting layer and the second wavelength converting layer may comprise inorganic phosphor particles. Alternatively or additionally, the wavelength converting material of at least one of the first wavelength converting layer and the second wavelength converting layer may comprise quantum dots.

According to an embodiment, at least one of the first wavelength converting layer and the second wavelength converting layer may comprise an at least partially transparent matrix. Typically, the wavelength converting material of at least one of the first wavelength converting layer and the second wavelength converting layer may be contained in an at least partially transparent matrix. The at least partially transparent matrix may further comprise light scattering elements.

According to an embodiment, at least one of the first wavelength converting layer and the second wavelength converting layer further may comprise at least one thermally conductive element. Preferably, the first wavelength converting layer comprises at least one thermally conductive element.

According to an embodiment, the first wavelength converting layer and the second wavelength converting layer may be arranged on the light emitting diode. For example the wavelength converting layers may be in direct optical contact with the light emitting diode. According to other embodiments, the light emitting diode and at least one wavelength converting member, typically the second wavelength converting layer, may be arranged mutually spaced apart.

According to an embodiment, the support may be a heat sink.

The improvement in overall efficiency when using a "direct configuration" (e.g. as illustrated in Fig. 1) of a light emitting arrangement according to embodiments of the invention compared to a conventional light emitting arrangement (which has its wavelength converting layer(s) on the same side of the light emitting diode, more precisely on the side facing away from the support) has, in experiments performed by the inventors, been seen to be approximately 5-8 % when using a wavelength converting material, in particular quantum dots, emitting red light in the first wavelength converting layer. In such embodiments, the light emitting arrangement may lack a reflective layer. The improvement in overall efficiency, based on the reduction in temperature in the wavelength converting layer emitting red light, was measured by a temperature map.

The overall efficiency may be further boosted up to approximately 15 % by using a wavelength converting material emitting red light contained in a matrix as the first wavelength layer, wherein the matrix further comprises reflective material. Preferably, the wavelength converting material is contained in the matrix in the form of quantum dots. In such embodiments, a reflective layer may be present in the form of a first wavelength converting layer comprising reflective material.

In another aspect, the invention provides a luminaire or a lamp comprising a light emitting arrangement as described herein.

The light emitting arrangement of embodiments of the present invention differs from the above-mentioned US 2010/0244662 for example by presenting an at least partially transparent light emitting diode emitting first primary light in a direction towards the support as well as second primary light in a direction away from the support. Further, the light emitting arrangement according to embodiments of the invention does not have to be provided with the same material on both sides of the light emitting diode, e.g. the first wavelength converting layer may be substantially different from the second wavelength converting layer. Further, the light emitting arrangement according to embodiments of the present invention may comprise a reflective layer.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 illustrates a light emitting arrangement according to an embodiment of the invention, where the light emitting arrangement has a direct configuration.

Fig. 2 illustrates a light emitting arrangement according to an embodiment of the invention, where the light emitting arrangement has a remote configuration or a vicinity configuration.

Fig. 3 illustrates a light emitting arrangement according to an embodiment of the invention, where the light emitting arrangement is arranged in a reflective container.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of

embodiments of the present invention. Like reference numerals refer to like elements throughout. DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

As used herein, "first primary light" refers to light emitted by a light emitting diode in a direction towards a thermally conductive support. Typically, the first primary light is emitted through a first surface of the light emitting diode.

As used herein, "second primary light" refers to light emitted by a light emitting diode in a direction away from a thermally conductive support. Typically, the second primary light is emitted through a second surface of the light emitting diode.

As used herein, "first converted light" refers to converted light emitted by a wavelength converting material comprised within a first wavelength converting layer.

Typically, the wavelength converting material of the first wavelength layer absorbs first primary light and emits first converted light.

As used herein, "second converted light" refers to converted light emitted by a wavelength converting material comprised within a second wavelength converting layer. Typically, the wavelength converting material of the second wavelength layer absorbs second primary light and emits second converted light.

As used herein, "reflective layer" refers to a layer comprising reflective material. A thermally conductive reflective layer may be a thermally conductive support comprising reflective material. Alternatively, the reflective layer may be a first wavelength converting layer comprising reflective material. Alternatively, the reflective layer may be a separate (preferably thermally conductive) layer comprising reflective material, typically called a separate reflective layer.

By "thermal contact" is meant that components being in thermal contact have the ability to exchange heat with each other.

By "Stokes shift" is meant the difference (in wavelength or frequency units) between spectral positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) arising from the same electronic transition.

By "arranged on the opposite side" is meant that an object is positioned somewhere on the opposite side of an element compared to the side of reference. A component being arranged on the opposite side of an element may be arranged distant from, adjacent to or in direct contact with said element. Thus, "arranged on the opposite side" should not be construed as arranged directly onto an opposite surface of the element.

By "remote configuration", in the context of the present invention, is meant that at least one wavelength converting layer is arranged at a certain distance from the light emitting diode. In other words, at least one wavelength converting layer, preferably the second wavelength converting layer, and the light emitting diode are mutually spaced apart. By "vicinity configuration" is herein meant that at least one wavelength converting layer is arranged at a distance away from the light emitting diode. In other words, at least one wavelength converting layer, preferably the second wavelength converting layer, and the light emitting diode are mutually spaced apart. In the "remote configuration", the at least one wavelength converting layer is typically arranged at a larger distance from the light emitting diode than in the "vicinity configuration".

By "direct configuration" in the context of the present invention, is meant that both wavelength converting layers are arranged on the light emitting diode, thus forming part of a layer stack.

By "overall efficiency" is meant the total efficiency with which the system converts electrical power to optical power. The overall efficiency is for example dependent on the efficiency of the conversion of light from the light emitting diode by wavelength converting layers to longer wavelengths.

Figs. 1 to 3 illustrate light emitting arrangements 10 comprising a thermally conductive support 20, a separate reflective layer 30, a first wavelength converting layer 40 comprising a wavelength converting material, a light emitting diode 50, and a second wavelength converting layer 60 comprising a wavelength converting material.

The reflective layer 30 is typically arranged in thermal contact with the support 20. Preferably, the reflective layer 30 is arranged in direct thermal and physical contact with the support 20. The reflective layer 30 may cover the support 20 partially or completely, preferably completely.

In the embodiments illustrated in Figs. 1 to 3, the first wavelength converting layer 40 is arranged in thermal contact with the reflective layer 30. The first wavelength converting layer 40 may also be arranged in direct physical contact with the reflective layer 30. The support 20 may function as a heat sink of the light emitting arrangement, and, for example, heat in the first wavelength converting member 40 and the light emitting diode 50 may be carried off via the reflective layer 30 to the support 20. The first wavelength converting layer 40 is arranged between the reflective layer 30 and the light emitting diode 50. The second wavelength converting layer 60 is arranged on the opposite side of the light emitting diode 50 compared to the first wavelength converting layer 40.

The light emitting diode 50 is transmissive at least to the first converted light emitted by the first wavelength converting layer 40. Further, the light emitting diode 50 is arranged to at least partly emit first primary light towards the reflective layer 30 and the support 20.

The light emitting diode 50 has a first surface 51, on which the first wavelength converting layer 40 may be arranged, and a second surface 52, on which the second wavelength converting layer 60 may be arranged. The second surface 52 faces away from the support 20 and the first surface 51 faces towards the support 20. The second surface 52 is arranged opposite to the first surface 51.

The light emitting diode emits first primary light from its side facing the support 20 and second primary light from its side facing the second wavelength layer 60. The light emitting diode 50 may emit light through both the first surface 51 and the second surface 52.

The first wavelength converting layer 40 may be arranged in direct physical contact with the first surface 51. Alternatively, the first wavelength converting layer 40 and the first surface 51 may be arranged mutually spaced apart. The first wavelength converting layer 40 may cover the first surface 51 partially or completely, preferably completely.

The second wavelength converting layer 60 may be arranged in direct physical contact with the second surface 52. Alternatively, the wavelength converting layer 60 and the second surface 52 may be arranged mutually spaced apart. The second wavelength converting layer 60 may cover the second surface 52 partially or completely, preferably completely.

A transparent material, such as sapphire, glass or a thin layer of GaN, may be present on the second surface 52 of the light emitting diode 50, i.e. between the light emitting diode 50 and the second wavelength converting layer 60.

The light emitting diode 50 may be electrically connected to the support 20 as well as to the environment outside the light emitting arrangement herein described. The first surface 51 of the light emitting diode 50 is preferably equipped with electrodes. However, for simplicity, the electrodes are not shown in the drawings. The embodiment shown in Fig. 1 represents an example of a light emitting arrangement 10 having a direct configuration, wherein the first wavelength converting layer 40 and the second wavelength converting layer 60, respectively, are arranged in direct optical contact with the light emitting diode 50.

The light emitting arrangement according to an embodiment of the invention may for instance be a GaN-based flip chip light emitting diode with direct phosphor configuration. A flip chip configuration is well-known to the person skilled in the art.

The light emitting arrangement 10, shown in Fig. 1, comprises a support 20 being completely covered by a separate reflective layer 30. The reflective layer 30 is in both thermal and physical direct contact with the support 20. The reflective layer 30 is, in turn, completely covered by a first wavelength converting layer 40. The first wavelength converting layer 40 is in both thermal and physical direct contact with the reflective layer 30. The first wavelength converting layer 40 may comprise a wavelength converting material emitting red light as first converted light. On the opposite side of the first wavelength converting layer 40 compared to the reflective layer 30, a light emitting diode 50 is arranged. The light emitting diode 50 has a first surface 51 covering the first wavelength converting layer 40 completely. The light emitting diode 50 has a second surface 52 opposite to the first surface 51. The second surface 52 is completely covered by a second wavelength converting layer 60. The second wavelength converting layer 60 may comprise a wavelength converting material emitting yellow light as second converted light.

The light emitting arrangement may have either a direct configuration, such as illustrated in Fig. l, or a remote configuration or a vicinity configuration, such as illustrated in Fig. 2 or in Fig. 3.

In Fig. 2, a light emitting arrangement 10 according to an embodiment of the invention having a remote configuration or a vicinity configuration is illustrated. In this light emitting arrangement 10, the second wavelength converting layer 60 is arranged at a distance from the light emitting diode 50. The second wavelength converting layer 60 and the light emitting diode 50 are arranged mutually spaced apart.

A light emitting arrangement having a remote configuration or a vicinity configuration may be arranged within or on a reflective container 70, shown in Fig. 3, or within or on a heat sink. The support 20 may be arranged inside the reflective container or the heat sink or may be an integrated part of the reflective container or the heat sink. As shown in Fig. 3, the second wavelength converting layer 60 may have a surface area that is larger than the surface area of the light emitting diode 50. Any or all of the support 20, the reflective layer 30 and the first wavelength converting layer 40 may also have a surface area larger than the surface area of the light emitting diode 50.

The thermally conductive support may be formed of a ceramic material, for instance comprising A1 ₂ 0 ₃ and/or A1N, a silicone material, a composite material or a laminate material. The substrate may further be formed of a conductive layer, such as of aluminum or silver, covered by an insulating layer.

According to embodiments of the invention, the light emitting arrangement may comprise a separate reflective layer. Alternatively or additionally, the support and/or the first wavelength converting layer may comprise a refiective material.

In embodiments of the invention, the light emitting arrangement may lack a separate reflective layer, and the reflective material may instead be comprised in at least one of the support and the first wavelength converting layer. In embodiments where the first wavelength converting layer comprises the reflective material, the first wavelength converting layer may be in thermal contact, and optionally also in direct physical contact, with the support. The reflective material and the wavelength converting material may be contained in a common matrix.

The reflective layer may be a specular refiective layer. A specular refiective layer may be a multilayer dielectric reflector (DR) formed of organic or inorganic layers. The specular reflective layer may also be a layer of silver, aluminum or other reflective metal, or a combination of a dielectric reflector and metal. Alternatively, the refiective layer may be a diffuse reflective layer. A diffuse reflective layer may be a metallic layer with a rough surface. A diffuse reflective layer may be a layer comprising particles of e.g. A10 _x or TiO _x .

In an embodiment of the invention, the thermally conductive support may at least be transparent to first converted light, and the reflective layer may be arranged on the opposite side of the support in relation to the light emitting diode, to reflect at least first converted light and optionally also first primary light back through the support in the direction of the light emitting diode. In such embodiments the refiective layer is not necessarily thermally conductive.

The light emitting diode may be a group III nitride-based light emitting diode. The group III nitrides, i.e. A1N, GaN and InN, represent an important trio of semiconductors because of their direct band gaps which span the range 1.95 eV to 6.2 eV, including the whole of the visible region and extending well out into the ultraviolet range. GaN-based light emitting diodes typically emit light from green to ultraviolet. An example of a light emitting diode according to an embodiment of the invention is a blue light emitting diode, e.g. a blue GaN-based light emitting diode. A blue light emitting diode typically emits a blue light having a wavelength in the range of from 380 nm to 470 nm.

The first wavelength converting layer may include a wavelength converting material emitting red light as first converted light, a wavelength converting material emitting yellow light as first converted light, or a combination thereof. Also a wavelength converting material emitting green light as first converted light may be considered, either alone or in a combination with other wavelength converting materials emitting converted light of other color(s). Preferably, the first wavelength converting layer comprises a wavelength converting material emitting red light as first converted light.

For a wavelength converting material emitting red light, the Stokes shift is larger than for wavelength converting materials emitting light of shorter wavelengths, thus the wavelength converting material emitting red light tends to get hotter than the wavelength converting material emitting e.g. green light or yellow light. It is therefore advantageous to arrange the material having the larger Stokes shift closer to the heat sink than the material having the smaller Stokes shift. A light emitting arrangement in which the wavelength converting layer having the relatively large Stokes shift is arranged closer to the support/heat sink than the wavelength converting layer having a relatively small Stokes shift may reduce or prevent thermal quenching.

The first wavelength converting layer may be made thin and have a uniform thickness. Preferably, the thickness of the first phosphor layer is in the range of from 5 micrometer to 500 micrometer.

The first wavelength converting layer may be provided by an underfill process, which is known to the person skilled in the art. In an underfill process, a liquid material is typically injected between the light emitting diode and the support, followed by polymerization. The underfill is used to provide mechanical support and to enforce electrical contact between the light emitting diode and the support. Frequently used underfill materials include silicone, resin, e.g. an epoxy resin, and the like. Typically, in embodiments of the present invention, the material being underfilled further comprises a wavelength converting material. The material being underfilled may further comprise a reflective material.

The second wavelength converting layer may include a wavelength converting material emitting red light as second converted light, a wavelength converting material emitting yellow light as second converted light, or a combination thereof. Also a wavelength converting material emitting green light as second converted light may be considered, either alone or in a combination with other wavelength converting materials emitting converted light of different color(s). For example, the second wavelength converting layer includes a wavelength converting material emitting yellow light as second converted light.

The second wavelength converting layer may be made thin and have a uniform thickness. Preferably, the thickness of the second wavelength converting layer is in the range of from 2 micrometer to 5 mm.

The second wavelength converting layer may be pre-formed as a ceramic sheet and thereafter be affixed to the second surface of the light emitting diode. Alternatively, particles of a wavelength converting material in the second wavelength converting layer may be thin-film deposited on the second surface, such as by electrophoresis.

Preferably, the Stokes shift of the first wavelength converting layer is larger than the Stokes shift of the second wavelength converting layer. The Stokes shift of the first wavelength converting layer is preferably in the range of from 100 nm to 400 nm. The Stokes shift of the second wavelength converting layer is preferably in the range of from 10 nm to 300 nm.

The first wavelength converting layer may be substantially different from the second wavelength converting layer. By the term "substantially different", it is herein meant that the first wavelength converting layer may include a wavelength converting material, or a combination of wavelength converting materials, being different from the wavelength converting material(s) in the second wavelength converting layer with regard to the kind, the concentration, the ratio or the mixture of wavelength converting material(s). Such different wavelength converting layers typically have different emission spectra.

The wavelength converting material in the first wavelength converting layer and second wavelength converting layer, respectively, may be present in the form of inorganic wavelength converting material, such as inorganic phosphors.

Examples of inorganic wavelength converting materials may include, but are not limited to, cerium (Ce) doped YAG (Y ₃ Al ₅ 0i ₂ ) or LuAG (Lu ₃ Al ₅ 0i ₂ ). Ce doped YAG emits yellowish light, whereas Ce doped LuAG emits yellow-greenish light. Examples of other inorganic phosphors materials which emit red light may include, but are not limited to ECAS (ECAS, which is Cai_ _x AlSiN ₃ :Eu _x wherein 0<x<l; preferably 0<x<0.2) and BSSN (BSSNE, which is Ba ₂ _ _x _ _z M _x Si ₅ -yAl _y N8-yO _y :Eu _z wherein M represents Sr or Ca, 0<x<l and preferably 0<x<0.2, 0<y<4, and 0.0005<z<0.05).

The wavelength converting material in the form of inorganic wavelength converting material may be contained in an at least partially transparent matrix. The matrix may be completely transparent. Alternatively, the matrix may be at least partially transparent, such as translucent or only transparent in some area(s) of the matrix. The at least partially transparent matrix may be a silicon-based polymer.

The wavelength converting material in the form of inorganic wavelength converting material may also be sintered particles.

The wavelength converting material of the first wavelength converting layer and/or of the second wavelength converting layer, respectively, may be an organic wavelength converting material, such as organic phosphor. In particular, one or more organic wavelength converting material may be used in embodiments using the remote configuration or the vicinity configuration, e.g. as illustrated in Fig. 2 and Fig. 3.

Examples of suitable organic wavelength converting materials are organic luminescent materials based on perylene derivatives, for example compounds sold under the name Lumogen ^® by BASF. Examples of suitable compounds include, but are not limited to, Lumogen® Red F305, Lumogen ^® Orange F240, Lumogen ^® Yellow F083, and

Lumogen ^® F170. Advantageously, a layer comprising organic luminescent material may be transparent and non-scattering.

The wavelength converting material in the form of organic wavelength converting material may be contained in an at least partially transparent matrix. The matrix may be completely transparent. Alternatively, the matrix may be at least partially transparent, such as translucent or only transparent in some area(s) of the matrix. The at least partially transparent matrix may comprise for example a polymer, epoxy or silicon-based polymers such as a silicone. Examples of suitable polymer materials are polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and

polycarbonate (PC).

Non-limiting examples of wavelength converting materials emitting red light are eCas, BSSN, nitride silicate-based Sr ₂ Si ₅ N8:Eu ²⁺ , nitride alumino silicate-based

CaAlSiN ₃ :Eu ²⁺ , oxonitride aluminosilicate-based Sr ₂ Si ₄ A10Nv:Eu ²⁺ and LOS-based

La ₂ 0 ₂ S:Eu ³⁺ . Non- limiting examples of wavelength converting materials emitting yellow light are (Sr, Ba) ₂ Si0 ₄ :Eu ²⁺ and (Y, Gd) ₃ Al ₅ 0i ₂ :Ce ³⁺ , such as YAG. Non-limiting examples of wavelength converting materials emitting green light are SSON, BaMgAlioOi ₇ :Eu ²⁺ , BaMgAlioOi ₇ :Mn ²⁺ , SrAl ₂ 0 ₄ :Eu ²⁺ and silicate-based (Ba, Sr) ₂ Si0 ₄ :Eu ²⁺ .

The wavelength converting material in the first wavelength converting layer and second wavelength converting layer, respectively, may be present in the form of quantum dots. An advantage of quantum dots is that they cause no scattering of light. Quantum dots are small crystals of semiconducting material generally having a width or diameter of only a few nanometers, which exhibit size-dependent electronic and optical properties which are different from those of bulk solids. When excited by incident light, a quantum dot emits light of a color determined by the size and material of the crystal. Light of a particular color can therefore be produced by adapting the size of the dots. Most known quantum dots with emission in the visible range are based on cadmium selenide (CdSe) with shell such as cadmium sulfide (CdS) and zinc sulfide (ZnS). Cadmium free quantum dots such as indium phosphide (InP), and copper indium sulfide (CuInS ₂ ) and/or silver indium sulfide (AgInS ₂ ) can also be used. Quantum dots show very narrow emission band and thus they show saturated colors. Furthermore, the emission color can be tuned by adapting the size of the quantum dots. Any type of quantum dot known in the art may be used in the present invention, provided that it has the appropriate wavelength conversion characteristics. However, it may be preferred for reasons of environmental safety and concern to use cadmium- free quantum dots or at least quantum dots having very low cadmium content.

The matrix containing the wavelength converting material may further comprise light scattering elements. Light scattering elements may be light scattering particles, e.g. Al ₂ 0 ₃ particles and/or Ti0 ₂ particles. Light scattering elements may also be light scattering pores. Particularly, wavelength converting material in the form of quantum dots may be contained in a reflective matrix. The reflective matrix typically comprises a light scattering material, e.g. titanium oxide and/or boron nitride.

The first wavelength converting layer may further comprise at least one thermally conductive element. The at least one thermally conductive element may be thermally conductive particles dispersed within the layer, preferably blended with the matrix of the layer. The at least one thermally conductive element may improve the dissipation of heat from the first wavelength converting layer and the light emitting diode, via the reflective layer if present, to the support. Typically, the support may serve as a heat sink.

Advantageously, in addition to high reflectivity, titanium oxide and boron nitride have good heat conductivity and may also be used as said thermally conductive element. Alternatively or additionally, the at least one thermally conductive element may be a transparent graphene, e.g. monolayer graphene or multilayer graphene.

In embodiments of the present invention, the light emitting arrangement may include an additional layer. Such an additional layer may be a third wavelength converting layer, another optically functional layer, or an optically transmissive and/or transparent layer, etc.

The additional layer may be arranged between the light emitting diode and the second wavelength converting layer. Alternatively, the additional layer may be arranged in between the reflective layer and the first wavelength converting layer, or between the first wavelength converting layer and the light emitting diode. If a third wavelength converting layer is arranged between the light emitting diode and the support, the light emitting diode may be transmissive also to the light emitted by the third wavelength converting layer, i.e. third converted light.

The additional layer may be an additional thermally conductive layer, e.g. a layer comprising graphene, having a relatively good thermal conductivity. The additional thermally conductive layer may be arranged in thermal contact with the thermally conductive support. The additional thermally conductive layer may be arranged between the reflective layer and the first wavelength converting layer. Alternatively or additionally, the additional thermally conductive layer may be arranged between the support and the reflective layer. A further possibility is to have the additional thermally conductive layer arranged between the support and the first wavelength converting layer.

The light emitting arrangement according to an embodiment of the invention may also comprise more than one light emitting diode. For instance, a light emitting arrangement may comprise a common support, a common reflective layer, and a multiple number of light emitting diodes each having their first surface covered by a first wavelength converting layer. A second wavelength converting layer may cover the second surface of each light emitting diode, or may be a common second wavelength converting layer for the entire light emitting arrangement.

The light emitting arrangement according to embodiments of the present invention may be useful in various types of lighting applications, such as in functional lighting and general illumination. For example, the light emitting arrangement may be used in a lamp or luminaire intended for illumination. The light emitting arrangement may for example be used in backlights, automotive headlights and/or flashlights.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, although the drawings illustrate embodiments using a single light emitting diode and associated layers (substrate, reflective layers, first and second wavelength converting layers), it is envisaged that a plurality of light emitting diodes could be used, optionally using the corresponding number of associated layers, or sharing one of more of said associated layers with at least one other light emitting diode.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.