FIELD OF THE INVENTION

The invention relates to a light emitting device comprising at least one first LED adapted for, in operation, emitting light with a first spectral distribution and a least one second LED adapted for, in operation, emitting light with a second spectral distribution.

BACKGROUND OF THE INVENTION

Within the lighting business, a goal is to optimize the visible light generated by LED modules, characterized by the lumen equivalent, as well as the electrical

performance, i.e. efficacy, while still keeping the cost low. In recent years the most optimum way of generating a high lumen equivalent and efficacy have been investigated - cf.

"Research challenges to ultra-efficient inorganic solid-state lighting" by J.M. Phillips et al. Laser & Photon Rev. 1, No. 4, 307 - 333 (2007). At the moment (partial) phosphor conversion of blue LEDs with narrow-band phosphors is the most beneficial route, as the performance of direct red and especially green LEDs still lagged behind the quantum efficiencies of the narrow-band phosphors.

With the performance of direct red and green LEDs steadily increasing the use of LEDs in the combination RGB (red - green - blue) or RGYB (red - green - yellow - blue) and also addition of violet (400nm) component to obtain crispy white is becoming more and more relevant. LEDs have a narrow emission spectrum and the wavelength can be shifted depending on the manufacturing conditions allowing for a good control over the final emission spectrum. Most importantly a multi-LED configuration also offers the possibility of electrical control of LED module emission in terms of color and white light color temperature as well as color rendering. This can be an added feature for the end-customer or be used as a late-stage configuration in the factory to tune between the different color temperatures of the portfolio. An important consideration in connection with white light LED modules is to achieve a homogeneous far- field light distribution.

In order to achieve a homogeneous far- field light distribution for white light LED modules based on multi-color LEDs it is necessary to distribute the LEDs

homogeneously over the LED board or LED strip. For low output power applications where only few LEDs are needed, e.g. one per color, this is however difficult as there are not enough LEDs available to achieve a homogenous distribution. For a larger number of LEDs this is considerably easier. However, it also has to be taken into account the LEDs of one color should be connected in a single string (combining connections in parallel and/or series) to facilitate the electrical connection of the LED board. For some configurations this might however not be possible for small sized modules, e.g. spot applications.

By way of example, considering a white light module based on red, green and blue LEDs, the green LEDs are connected first, at the outside of the board, to leave as much flexibility for the connections of the other LEDs as possible. In a second step a possible connection for the red LEDs is considered. However, the connection of the red LEDs will necessarily split the blue LEDs into at least two groups, such that the blue LEDs within a group can be connected, but blue LEDs of different groups cannot be connected. In these cases, multi-layer boards can be used to connect the LEDs of one kind or color in separated layers. These multi-layer boards are however very expensive. Alternatively, additional channels could be added, here it would be two blue channels, which adds complexity to the driving scheme. Moreover, already the red circuit cannot be accessed from the side of the board, so the electrical connection for the red as well as the two blue channels would need to cross the green circuit, which is not preferred as it poses reliability risks (wires can break or cause short circuits).

Furthermore, even with optimized LED positions, the far- field light pattern is still not satisfactory. In most cases a metal reflector is used to mix the colors to achieve a homogeneous far-field, which adds to the size of the final lamp/luminaire. The less the colors are mixed, the more complex the design of the reflector has to be, which also adds to the cost of the final lamp/luminaire. For example, for a reference LED module with four red and blue LEDs, a simple, smooth reflector does not yield a well-mixed far- field. A structured reflector is needed to get a good result and this structure has to be tuned to the LED layout. Also, the LED layout has been optimized for a module with five blue and red LEDs. This does lead to an improvement for all reflectors, but further optimization is still needed.

WO-2007/067513 discloses a LED package having a plurality of LEDs of differing wavelength. For collecting light emitted by the LEDs into a unitized beam, and for distributing the collected light into a common aperture, the LED package has an optic comprising lobes connected to an integrating rod, wherein the lobes are azimuthally equally spaced about the aperture of the LED package. An alternative attempt at mixing different colors is mentioned in US

2004/0120647 Al describing an illumination device with a plurality of LEDs and an optical element in the form of a prism complex comprising a first trapezoid waveguide and a second and a third triangular waveguide mutually separated by means of suitable dichroic filters. The second and a third triangular waveguide are arranged in such a way that one side forms part of an incidence plane arranged adjacent the LEDs, another side provided with a dichroic filter is joined to the first waveguide and the third surface forms an outer side of the optical element extending between the incidence plane and a light emergence plane. Hence, the second and third triangular waveguides are closed towards the light emergence plane.

With this solution, however, the far- field light pattern is still not satisfactory.

Furthermore, an optical element of this type will have to be provided with relatively long waveguides and thus be made rather bulky in order to achieve a satisfactory mixing of the colors to achieve a homogeneous far- field. Thus, the problems related to the bulky size of the final lamp/luminaire still remain unsolved.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem, and to provide a light emitting device which is compact both as regards the circuit board or base on which the LEDs are mounted and the optical element in front of the LEDs, which provides for a simple electrical connection of the LEDs, and which with a plurality of LEDs yields a homogeneous far- field light pattern.

According to a first aspect of the invention, this and other objects are achieved by means of a light emitting device comprising at least one first LED adapted for, in operation, emitting light with a first spectral distribution, a least one second LED adapted for, in operation, emitting light with a second spectral distribution, and an optical element comprising a first light input surface, a second light input surface, a light exit surface, at least one first through light guiding element extending from the first light input surface towards the light exit surface, and at least one second through light guiding element extending from the second light input surface towards the light exit surface, the at least one first through light guiding element comprising a first end surface at the first light input surface and a second end surface facing the light exit surface, the at least one second through light guiding element comprising a first end surface at the second light input surface and a second end surface facing the light exit surface, the optical element being adapted for: receiving the light with the first spectral distribution from the at least one first LED at the first light input surface,

receiving the light with the second spectral distribution from the at least one second LED at the second light input surface,

- redistributing at least a part of the light with the first spectral distribution by guiding it through the at least one first through light guiding element,

redistributing at least a part of the light with the second spectral distribution by guiding it through the at least one second through light guiding element,

mixing the redistributed light to obtain mixed light with a third spectral distribution, and

coupling the mixed light with the third spectral distribution out of the light exit surface.

Further to the above, the first and second through light guiding elements are channels provided in the optical element, wherein the channels comprise walls, and wherein the walls are any one or more of twisted and turning.

Optical elements with channels as through light guiding elements for redistributing and mixing light of different spectral distributions are known in the art.

However, in these known examples the channels are either straight or have a tapered (or tapered-like) shape. The inventors of the present invention have realized that providing the channels with twisted and/or turning walls has the advantage of enabling customized and/or optimized redistribution of the light adapted to the particular architecture of the light emitting device as well as to the particular number and arrangement of the LEDs of the light emitting device.

Also, twisted and/or turned walls and thus channels enable the provision of longer channels and thus paths of propagation for the light, which have the further advantage of ensuring proper and sufficient mixing of the light during propagation of the light through the through light guiding elements of the optical element.

Overall, an even higher degree of freedom in the design of the optical element is thus achieved.

Previously, optical elements having through light guiding elements in the form of channels with twisted and/or turning walls were very difficult if not impossible to manufacture due to their complicated construction architecture and/or design, but recent developments in the field of additive manufacturing (also known as 3D printing) have enabled a relatively simple and straightforward method of manufacturing such optical elements. The optical element may therefore be a 3D-printed optical element.

By providing both the first and second light guiding elements as through light guiding elements each having a first and a second end surface a light emitting device with a particularly simple and cost efficient structure is obtained while still enabling obtaining a high degree of mixing of the light emitted by the LEDs.

By providing a light emitting device with an optical element in which at least a part of the light with the first spectral distribution is redistributed by guiding it through the at least one first through light guiding element, at least a part of the light with the second spectral distribution is redistributed by guiding it through the at least one second through light guiding element, and the redistributed light is mixed to obtain mixed light with a third spectral distribution, particularly to obtain white light, a light emitting device is obtained with which a highly homogeneous far- field light pattern may be achieved. Furthermore, the use of a more compact optical element becomes possible as both the redistribution and the mixing of the light may take place in the light guiding elements.

Furthermore, the provision of an optical element as described above enables mounting the first and second LEDs, as well as where applicable even further LEDs emitting light with other spectral distributions than the first and second spectral distributions, on a single layer LED board. This provides for a significant simplification of the light emitting device, both in terms of further compactness and in term of simpler connections of the LEDs. This in turn provides a light emitting device which is considerably cheaper to manufacture as compared to the prior art types.

Overall, a light emitting device is thus obtained which is very compact - in practice a total height of less than 5 - 7 cm may be obtained - and which provides for a simple electrical connection of the LEDs, and which with a plurality of LED modules yields a homogeneous far- field light pattern.

The first and second through light guiding elements are channels provided in the optical element.

Thereby, the optical element may be provided with particularly simple light guiding elements, which in principle eliminates the need for further elements, such as coatings, filters or the like, aiding in guiding the light.

Providing the through light guiding elements as channels, specifically, provides for a light emitting device which is particularly simple in structure and cheap in manufacture, which has a large degree of freedom as regards the design of the optical element and with which a completely homogenous far- field light pattern may be obtained.

In an embodiment the first and second through light guiding elements are channels provided in the optical element, the channels comprise walls, and the walls comprises one or more of a material and a coating being any one or more of opaque and scattering, scattering, forward scattering, translucent and reflective material.

By providing the walls with an opaque scattering or a scattering or a forward scattering material or coating, a more efficient mixing of the redistributed light during propagation of the light through the through light guiding elements of the optical element is obtained. Thereby a further improved far- field light pattern is achieved. The use of a scattering material or coating furthermore being opaque has the additional effect of obtaining a collimating effect on the light as well as of lowering the loss of light through the walls of the channels. The use of a forward scattering material or coating has the additional effect of lowering the loss of light even further.

By providing the walls with a translucent material or coating cross-talk between certain channels is allowed without limiting the performance of the device. Thereby an even better redistribution of the received light may be obtained.

By providing the walls with a reflective material or coating the loss of light through the walls of the channels during the redistribution of the received light may be considerably lowered or even eliminated altogether, which results in an increased efficiency of the light emitting device.

In an embodiment the first and second through light guiding elements are channels provided in the optical element, the channels comprise walls, different parts of the walls comprise different optical properties, and the optical properties are chosen as one or more property from the group of properties comprising reflective, opaque, scattering, forward scattering and translucent.

In addition to the above-mentioned advantages related to scattering, opaque and/or reflective walls, this has the effect of enabling redistribution of light in one part of the through light guiding elements and mixing of light in another, particularly subsequent, part of the light guiding elements. Thereby full redistribution of the light may be achieved before mixing the light, which in turn provides for an even further improved far- field light pattern.

Translucent properties have the further advantage of allowing cross-talk between certain channels without limiting the performance of the device. Thereby an even better redistribution of the received light may be obtained. It is furthermore noted that the material used for the optical element and/or the walls of the through light guiding elements should preferably not be absorbing or have a very low absorption, so as to ensure that as little light as possible is lost in the structure.

In an embodiment the first and second through light guiding elements are channels provided in the optical element and the channels comprise a shaped feature adjacent to any one or more of the first light input surface, the second light input surface and the light exit surface.

Such a shaped feature has the advantage of improving the in-coupling of light if provided at the light input surface and/or the out-coupling of light, where provided at the light exit surface, thereby lowering the amount of light lost in in-coupling and/or out- coupling of the light, thus in turn improving the efficiency of the light emitting device.

Alternatively, the first and second through light guiding elements may be at least one waveguide provided in the optical element and wherein the at least one waveguide comprises at least one through hole provided at a position above an LED of the at least one first LED and the at least one second LED.

Thereby a light emitting device with a particularly simple structure with which light emitted by the LEDs may be redistributed, mixed and coupled out in a particularly simple manner is obtained.

The at least one through hole may comprise any one of straight walls, tapered walls and a shaped feature adjacent to any one or more of the first light input surface, the second light input surface and the light exit surface.

Straight walls have the advantage of being particularly simple and thus cheap to manufacture.

Tapered walls have the additional advantage that light, which would otherwise be transmitted through the waveguide will be incident on the wall of the hole in an angle large enough to undergo be coupled into the waveguide. Thus the in-coupling of light into the waveguide, and consequently both the redistribution and mixing of light is improved, while the loss of light is lowered.

Shaped features have the advantage of improving the in-coupling of light if provided at the light input surface and/or the out-coupling of light, where provided at the light exit surface, thereby lowering the amount of light lost in in-coupling and/or out- coupling of the light, thus in turn improving the efficiency of the light emitting device.

In an embodiment at least one of the first and second through light guiding elements comprises a scattering material. Thereby a more efficient mixing of the redistributed light during propagation of the light through the through light guiding elements of the optical element is obtained. Thereby a further improved far- field light pattern is achieved. If the scattering material is furthermore opaque the additional effect of obtaining a collimating effect on the light as well as of lowering the loss of light through the walls of the channels is obtained.

In an embodiment the light emitting device further comprises any one or more of a diffusing element, a reflective coating or element, a colored element, such as an element with wavelength dependent absorption or reflection features in the visible spectral range, a light conversion element (including phosphors) and at least one light out-coupling element arranged on the light exit surface of the optical element.

Diffusing elements provide a further light mixing effect and thus have the advantage of further improving the mixing of the light, which in turn improved the far- field light pattern.

Reflective coatings or elements have the advantage that light otherwise lost will be reflected back into the light guiding elements. In case of through light guiding elements in the form of waveguides, light, which would otherwise be transmitted through the waveguide will be reflected at the light output surface and thus be coupled into the waveguide. Thus both the redistribution and mixing of light is improved, while the loss of light is lowered.

Colored elements and light conversion elements have the advantage of enabling a change of optical properties of the emitted light within the visible wavelength range.

Light out-coupling elements have the advantage of enabling localized out- coupling of mixed light at a desired position, and may in this way also increase the intensity of the light emitted by the light emitting device.

By way of examples such light out-coupling elements may be a roughness provided to the surface of a waveguide or a hole with a suitable shape or a local adjustment of properties such as an addition of extra scattering elements at a desired position.

In an embodiment the optical element further comprises a first part comprising a reflective material and a second part comprising a scattering material, the first part being arranged between the plurality of LEDs and the second part.

Thereby redistribution of light in one part of the through light guiding elements and mixing of light in another, particularly subsequent, part of the through light guiding elements is enabled. Thereby full redistribution of the light may be achieved before mixing the light, which in turn provides for an even further improved far- field light pattern.

In an embodiment the light emitting device further comprises an element for electrically or manually rotating and/or tilting the optical element.

Thereby a far- field light pattern shaped and/or colored according to specific needs, applications and/or desires may be achieved.

In an embodiment the light emitting device further comprises at least one base on which at least one of the at least one first LED and the at least one second LED is arranged.

Non-limiting examples of such a base is one or more separate LED strips, a flat single-layer or multi-layer LED board, a flat single-layer or multi-layer LED board with a LED strip mounted on more than one surface thereof, and bases with other shapes than flat.

Thereby, a light emitting device is provided which provides for improved flexibility in architecture and/or design and which may be made even more compact.

The invention further relates to a lamp, a luminaire, or a lighting system comprising a light emitting device according to the invention.

Such a lamp, a luminaire, or a lighting system may be used in one or more of the following applications: digital projection, automotive lighting, stage lighting, shop lighting, home lighting, accent lighting, spot lighting, theater lighting, fiber optic lighting, display systems, warning lighting systems, medical lighting applications, decorative lighting applications.

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 A shows a schematic cross sectional view of a first embodiment of a light emitting device according to the invention comprising an optical element with three light guiding elements, each in the form of a plurality of channels.

Figs. IB, 1C and ID shows three different schematic cross sectional views of the light emitting device according to Fig. 1, featuring the three through light guiding elements shown separately. Fig. 2 shows a schematic cross sectional view of an embodiment of an optical element with through light guiding elements in the form of a plurality of channels.

Fig. 3 shows a schematic cross sectional view of a second embodiment of a light emitting device according to the invention comprising an optical element with two light guiding elements, each in the form of a plurality of channels.

Fig. 4 shows a schematic cross sectional view of a third embodiment of a light emitting device according to the invention comprising an optical element with a through light guiding element in the form of a waveguide.

Fig. 5 shows a schematic cross sectional view of a fourth embodiment of a light emitting device according to the invention comprising an optical element with a through light guiding element in the form of a waveguide with a hole provided at a position corresponding to one of a plurality of LEDs.

Fig. 6 shows a schematic cross sectional view of a light emitting device according to the invention illustrating further embodiments of a through light guiding element according to Fig. 5.

Fig. 7 shows a schematic top view of a fifth embodiment of a light emitting device according to the invention comprising an optical element with a through light guiding element in the form of a plate shaped waveguide.

Fig. 8 shows a schematic perspective view of a sixth embodiment of a light emitting device according to the invention comprising an optical element with a plurality of through light guiding elements in the form of waveguides arranged in a grid.

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

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. Referring now to Fig. 1 A, a light emitting device 1 according to a first embodiment of the invention is shown.

The light emitting device 1 comprises a base 8, such as a single-layer printed circuit board, as well as a first LED 21, a second LED 22 and a third LED 23 arranged on the base 8.

Irrespective of the embodiment the base 8 is an optional element. Non-limiting examples of a base 8 is one or more separate LED strips, a flat single-layer or multi-layer LED board, a flat single-layer or multi-layer LED board with a LED strip mounted on more than one surface thereof, and bases with other shapes than flat. Where a base 8 is provided configurations with LEDs mounted in different orientations is enabled.

The LEDs 21, 22, 23 are adapted for, in operation, emitting light comprising three different spectral distributions. More particularly, in the embodiment shown, the first LED 21 is adapted for emitting light with a first spectral distribution, in this example green light, the second LED 22 is adapted for emitting light with a second spectral distribution, in this example red light, and the third LED 23 is adapted for emitting light with a fourth spectral distribution, in this example blue light.

Irrespective of the embodiment, in an alternative more than one LED emitting light with the same spectral distribution may be provided. Irrespective of the embodiment, in another alternative one or more LEDs emitting light with two different spectral distributions, e.g. red and blue LEDs, or one or more LEDs emitting light with four or more different spectral distributions, e.g. red, blue, green and yellow LEDs, may be provided.

The light emitting device 1 further comprises an optical element 3 comprising a first light input surface 3 la, a second light input surface 3 lb, a third light input surface 31c and a light exit surface 32. In some embodiments the LEDS 21, 22, 23 may be arranged directly on the optical element 3 rather than on a base 8.

The light input surfaces 31a, 31b, 31c, respectively, are each adapted for receiving light emitted from one of the three LEDs 21, 22, 23. More generally, and irrespective of the embodiment, each light input surface is adapted for receiving light having a specific spectral distribution.

The light exit surface 32 is adapted for emitting light received at the light input surfaces 31a, 31b, 31c and guided to the at least one light exit surface 32. As will be apparent from the following, the light emitted from the light exit surface 32 comprises a third spectral distribution. The optical element 3 comprises three through light guiding elements 41, 42 and 43. Figs. IB, 1C and ID each show one of the three respective through light guiding elements 41, 42 and 43 separately.

Each of the three through light guiding elements 41, 42 and 43 comprise a first end surface 414, 424 and 434, respectively, at the first, second and third light input surface 31a, 31b and 31c, respectively, as well as a second end surface 415, 425, 435, respectively facing the light exit surface 32. Furthermore, in the embodiment shown on Fig. 1 A - ID, the respective light guiding elements 41, 42 and 43 extend all of the way from the respective light input surface 3 la, 3 lb, 3 lc to the light exit surface 32. In other words, the second end surfaces 415, 425, 435 are arranged at the light exit surface 32.

Alternatively, the respective light guiding elements 41, 42 and 43 may extend only a part of the way from the respective light input surface 3 la, 3 lb, 3 lc to the light exit surface 32, such that the second end surfaces 415, 425, 435 are arranged at a distance from and facing the light exit surface 32.

Generally, the optical element 3 functions as follows. Light with the first spectral distribution emitted from the first LED 21 is received at the first light input surface 31a, light with the second spectral distribution emitted from the second LED 22 is received at the second light input surface 31b and light with the fourth spectral distribution emitted from the third LED 23 is received at the third light input surface 3 lc. At least a part of the light with the first spectral distribution is redistributed by guiding it through the at least one first through light guiding element 41, at least a part of the light with the second spectral distribution is redistributed by guiding it through the at least one second through light guiding element 42 and at least a part of the light with the fourth spectral distribution is redistributed by guiding it through the at least one third through light guiding element 43. The

redistributed light is, during and/or after redistribution, mixed to obtain mixed light with a third spectral distribution, particularly white light, and finally the mixed light with the third spectral distribution is coupled out of the light exit surface 32.

In the embodiment of Fig. 1A, the through light guiding element 41 (cf. Fig. 1C) is adapted for collecting and redistributing the light emitted from the first, in this case green, LED 21 and received at the first light input surface 3 la of the optical element 3, while the through light guiding element 42 (cf. Fig. ID) is adapted for collecting and redistributing the light emitted from the second, in this case red, LED 22 and received at the second light input surface 31b of the optical element 3, and the through light guiding element 43 (cf. Fig. IB) is adapted for collecting and redistributing the light emitted from the third, in this case blue, LED 23 and received at the third light input surface 3 lc of the optical element 3.

Referring to Figs. IB, 1C and ID, each of the three through light guiding elements 41, 42 and 43 in this embodiment is provided as a plurality of through channels. More particularly, the through light guiding element 41 comprises three channels 411, 412 and 413, cf. Fig. 1C. Likewise, the through light guiding element 42 comprises three channels 421, 422 and 423, cf. Fig. ID, and the through light guiding element 43 comprises three channels 431, 432 and 433, cf. Fig. IB.

Irrespective of the embodiment the channels extend from the light input surface 31 of the light exit surface 32 of the optical element 3. Also the channels are irrespective of the embodiment through channels in the sense that they are open at both the respective light input surface and at the light output surface 32.

Furthermore, in the embodiment shown on Figs. 1 A to ID, each of the three through light guiding elements 41, 42 and 43 are provided with a tapered shape in such a way that the width of each of the channels of the respective through light guiding element 41, 42, 43 at the light input surface 31 is smaller than the width of each of the channels of the respective through light guiding element 41, 42, 43 at the light exit surface 32. In other words each of the three through light guiding elements 41, 42 and 43 is provided with a tapered shape in such a way that the width of each of the three through light guiding elements 41, 42 and 43 increases in a direction from the light input surface 31 to the light exit surface 32. Thereby, the light emitted by the respective LEDs 21, 22, 23 may be distributed over the complete light exit surface 32.

In an alternative, the channels may be provided with a straight shape.

Irrespective of the embodiment the channels comprise walls. The walls, which may be partially or fully circumferential walls, delimit the channels within the optical element 3.

Still referring to figs. IB, 1C and ID, the channels 411, 412, 413; 421, 422, 423; 431, 432, 433 of the respective through light guiding element 41, 42, 43 comprise walls. The walls of the channels are in figs. IB, 1C and ID exemplified by walls 531, 532; 511, 512; 521, 522, respectively, of the central channels 423, 412 and 422, respectively.

Furthermore, the 411, 412, 413; 421, 422, 423; 431, 432, 433 of the respective through light guiding element 41, 42, 43 are merging close to the exit surface in such a way that e.g. channel 413, 423 and 433 are merged, to enable mixing of the light with the first, second and fourth spectral distributions to obtain the mixed light with the third spectral distribution at the light exit surface 32.

The walls of the channels may comprise one or more of an opaque scattering material or coating, a scattering material or coating, a forward scattering material or coating, a translucent material or coating and a reflective material or coating.

The walls may also comprise one or more different optical properties, such optical properties for instance being reflective, opaque, scattering, forward scattering and translucent.

Referring to the embodiment of an optical element 3 shown in Fig. 2, this particular optical element 3 comprises through light guiding elements in the form of through channels 41 and 42. The channels denoted 42 and shown in dotted lines are arranged behind the channels 41 shown in full lines. Thus, only the light input surface 31a corresponding to the channels 41 is visible in Fig. 2. In this case, the walls of the channels 41 and 42 are at least partially scattering, in the optimum case at least partially forward scattering.

Alternatively or additionally, parts of the walls of the channels can also be translucent as cross-talk between certain channels does not limit the performance of the device.

Furthermore, and irrespective of the embodiment, the material used for the optical element 3 and its through light guiding elements should not be absorbing or have a very low absorption, such that as little light as possible is lost in the structure.

It is noted that some particular designs, such as that shown in Fig. 2, are advantageously made not by traditional fabrication methods, but by 3D printing, as not all openings of the channels 41, 42 on the light exit surface 32 can be seen simultaneously. Therefore (injection) molding is excluded as a possible manufacturing method and machining would take a lot of time and might also not be appropriate for small sized channels and thin walls between these channels. Furthermore, as shown on Fig. 2, the channels are drawn in a single plane for simplicity, even though they will actually be arranged in a three dimensional space, optionally including twists and turns.

Referring now to Fig. 3 a light emitting device 101 according to a second embodiment of the invention is shown. The light emitting device 101 comprises a single- layer base 8, such as a single layer printed circuit board, as well as two LEDs 22 and 23 arranged on the single-layer base 8.

More particularly, in the embodiment shown, the LED 22 is adapted for emitting light with a first spectral distribution and the LED 23 is adapted for emitting light with a second spectral distribution. More particularly, in the embodiment shown, the LED 22 is adapted for emitting red light Rl, R2 and the LED 23 is adapted for emitting blue light Bl, B2.

The light emitting device 101 further comprises an optical element 3 comprising a first light input surface 3 la, a second light input surface 31b and a light exit surface 32. The first light input surface 3 la is adapted for receiving light emitted from the LED 22 and the second light input surface 3 lb is adapted for receiving light emitted from the LED 23. The light exit surface 32 is adapted for emitting light received at the light input surfaces 31a, 31b and guided to the at least one light exit surface 32.

The optical element 3 comprises two through light guiding elements, each comprising two channels 421, 422 and 431, 432, respectively. The optical element 3 further comprises two parts, namely a reflective part 33 and a scattering part 34.

Each of the channels 421, 422 and 431, 432, respectively, of the two through light guiding elements comprises a first end surface 4241, 4242 and 4341, 4342, respectively, at the first and second light input surface 31a and 31b respectively. Each of the channels 421 , 422 and 431, 432, respectively, of the two through light guiding elements further comprises a second end surface 4251, 4252 and 4351, 4352, respectively facing the light exit surface 32. In the embodiments shown in Fig. 3, each of the channels 421, 422 and 431, 432,

respectively, of the two through light guiding elements extends only a part of the way from the respective light input surface 3 la, 3 lb, 3 lc to the light exit surface 32. The second end surfaces 4251, 4252 and 4351, 4352 are thus arranged at a distance from and facing the light exit surface 32, and more particularly at the transition between the reflective part 33 and the scattering part 34 of the optical element 3.

In an alternative, the channels 421, 422 and 431, 432, respectively, and thus the through light guiding elements, may comprise a reflective part formed in the reflective part 33 of the optical element 3 and a scattering part formed in the scattering part 34 of the optical element 3. More particularly, the walls of the channels 421, 422 and 431, 432, respectively, may in this case comprise a reflective part and a scattering part.

In both of the above-mentioned cases, the light Bl, B2, Rl, Rl, as the case may be, is redistributed when propagating through the reflective part 33 of the optical element 3 and subsequently mixed when propagating through the scattering part 34 of the optical element 3, thus forming mixed light M. In this way all light is efficiently guided away from the LEDs 22, 23 by means of the channels 421 , 422, 431, 432 and efficient color mixing is obtained in the scattering part 34 of the optical element 3. For further improvement of the color mixing, diffusers 6, such as translucent diffusers, are provided in or on the light exit surface 32 of the optical element 3. In this way a further improvement of the mixing of the light is achieved as a final output consisting of scattered diffuse light S is obtained.

Turning now to Fig. 4, a schematic cross sectional view of a fourth embodiment of a light emitting device 102 according to the invention is shown. The light emitting device 102 comprises an optical element with a through light guiding element 3 in the form of a waveguide 10.

The light emitting device 102 comprises a single-layer base 8, such as a single layer printed circuit board, as well as a first LED 21, a second LED 22 and a third LED 23 arranged on the single-layer base 8.

The LEDs 21, 22, 23 are adapted for, in operation, emitting light comprising three different spectral distributions. More particularly, in the embodiment shown, the first LED 21 is adapted for emitting light with a first spectral distribution, in this example green light, the second LED 22 is adapted for emitting light with a second spectral distribution, in this example red light, and the third LED 23 is adapted for emitting light with a fourth spectral distribution, in this example blue light.

The light emitting device 102 further comprises an optical element 3 comprising a first light input surface 31a, a second light input surface 31b, a third light input surface 31c and a light exit surface 32.

The light input surfaces 31a, 31b, 31c, respectively, are each adapted for receiving light emitted from one of the three LEDs 21, 22, 23. More generally, and irrespective of the embodiment, each light input surface is adapted for receiving light having a specific spectral distribution. The light exit surface 32 is adapted for emitting light received at the light input surfaces 3 la, 3 lb, 3 lc and guided to the at least one light exit surface 32. As will be apparent from the following, the light emitted from the light exit surface 32 comprises a third spectral distribution.

The optical element 3 of the light emitting device 102 comprises a through light guiding element in the form of a waveguide 10. The waveguide 10 may, irrespective of the embodiment, be shaped as a rod or a plate and may have any feasible cross sectional shape.

The waveguide 10 comprises a first end surface 414, 424 and 434, respectively, at the first, second and third light input surface 31a, 31b and 31c, respectively, as well as a second end surface 415, 425, 435, respectively facing the light exit surface 32. In the embodiment shown on Fig. 4, the second end surfaces 415, 425, 435 are arranged at the light exit surface 32.

The waveguide 10 may, irrespective of the embodiment, comprise one or more of an opaque scattering material or coating, a scattering material or coating, a forward scattering material or coating, a translucent material or coating and a reflective material or coating.

The waveguide 10 may, irrespective of the embodiment, also comprise one or more different optical properties such optical properties for instance being reflective, opaque, scattering, forward scattering and translucent.

The optical element 3 further comprises a light out-coupling structure or element 7 arranged on or at the light exit surface. One or more light out-coupling structure or elements 7 may be provided. As may be seen the second end surfaces 415, 425, 435 are in the embodiment shown in Fig. 4 as coinciding with each other and furthermore with the position of the light out-coupling structure 7.

Generally, the light emitting device 102 functions in a way that is analogous to that described in relation to Fig. 1 above. Additionally, however, in the waveguide 10 the light is guided to the light out-coupling structure or element 7 at which it is emitted from the optical element 3 and thus from the light emitting device 102. On the way through the waveguide 10, the light from the plurality of LEDs is mixed such that the light emitted from the optical element 3, and thus from the light emitting device 102, is mixed light with a third spectral distribution, particularly white light.

Turning now to Fig. 5, a schematic cross sectional view of a fifth embodiment of a light emitting device 103 according to the invention is shown. The light emitting device 103 of Fig. 5 is very similar to that shown in Fig. 4 and described above in both structure and function. Thus, only the features of the light emitting device 103 of Fig. 5 differing from that shown in Fig. 4 and described above will be described in the following.

The waveguide 10 of the optical element 3 of the light emitting device 103 is in this embodiment provided with a hole, or cavity, 11. The optical element 3 is arranged such that the hole 11 of the waveguide 10 is arranged directly above one of the LEDs, here the LED 21.

The waveguide 10 comprises a first end surface 424 and 434, respectively, at the first and second light input surface 31a and 31b, respectively, as well as a second end surface 415, 425, respectively, facing the light exit surface 32a and 32b, respectively. In the embodiment shown on Fig. 4, the second end surfaces 415, 425 are arranged at the respective light exit surface 32a, 32b.

Thus, in this case a first part of the light emitted by each of the LEDs 21, 22 and 23 is transmitted and emitted directly from the light emitting device 103. This is indicated by solid arrows in Fig. 5.

A second part of the light emitted by each of the LEDs 21, 22 and 23 is coupled into the optical element 3 and thus the waveguide 10, guided through the waveguide 10 and emitted from or coupled out of the waveguide at a position of a different LED than that from which the light was originally emitted. This is indicated by dashed arrows in Fig. 5.

Referring now to Fig. 6, further embodiments of an optical element 3 with a through light guiding element in the form of a waveguide 10 with holes 11 is described.

The performance and properties of the optical element 3 may be adjusted by adjusting the construction of a hole, or cavity, 11 at or near the light input surface 31, e.g. the size and shape of the part of the hole 11 closest to the LEDs 21. For example, changing the angle of the sides of the hole 11 will allow more or less light to be coupled into the optical element 3.

In the case of an optical element 3 with a straight hole 11 , the light emitted by the LED 21 under small angles with respect to the axis of the hole 11 will either be reflected and leave the hole 11 or be transmitted through the optical element 3. It will not be coupled into the optical element 3 as the angle with respect to the light input surface 31 of the waveguide 3 is too large for total internal reflection.

If the side walls 5 of the hole 11 are instead tapered or partially tapered, at least a part of the light that would otherwise have left the hole 11 after reflection is now reflected off one side of the wall 5 of the hole 11 under a larger angle and can be coupled into the waveguide 10 on the opposite side of the wall 5.

To improve the in-coupling of the previously transmitted light, the wall 5 of the hole 11 may be modified at the light exit surface 32 of the optical element 3 to show a slope 52 as shown on the left-hand-side of Fig. 6, and/or a localized, reflective coating 9 may be provided at the light exit surface 32 of the waveguide 3 and/or the wall 5 of the hole 11 may be provided with a modified shape 51 at the light input surface 31 of the optical element 3 as shown on the right-hand-side of Fig. 6.

It is to be noted that it is also feasible to provide one or more of the through light guiding elements in the form of channels of the embodiments described in relation to Figs. 1 A to 3 above with one or more of the features described with reference to Fig. 6 in relation to the holes 11 provided in the waveguide 10.

Turning now to Fig. 7, a schematic top view of a fifth embodiment of a light emitting device 104 according to the invention is shown. The light emitting device 104 comprises an optical element 3 with a through light guiding element in the form of a plate shaped waveguide 10. The light emitting device 104 has the same general construction and function as that described above in relation to Fig. 4.

The light emitting device 104 comprises nine LEDs adapted for, in operation, emitting light with three different spectral distributions. More particularly, in the embodiment shown, the three LEDs denoted 21 are adapted for emitting green light, the three LEDs denoted 22 are adapted for emitting red light and the three LEDs denoted 23 are adapted for emitting blue light.

The light emitting device 104 further comprises nine light out-coupling structures or elements 7 at which mixed light may be emitted or extracted from the optical element 3 and thus from the light emitting device 104.

Turning now to Fig. 8, a schematic top view of a sixth embodiment of a light emitting device 105 according to the invention is shown. The light emitting device 105 comprises an optical element 3 with through light guiding elements in the form of six waveguides 10a, 10b, 10c, lOd, lOe and lOf arranged in a grid. The light emitting device 105 has the same general construction and function as that described above in relation to Fig. 4.

The light emitting device 105 comprises nine LEDs adapted for, in operation, emitting light with three different spectral distributions. More particularly, in the embodiment shown, the three LEDs denoted 21 are adapted for emitting green light, the three LEDs denoted 22 are adapted for emitting red light and the three LEDs denoted 23 are adapted for emitting blue light.

The light emitting device 105 further comprises nine light out-coupling structures or elements 7, one arranged at each of the nine intersections between two of the six waveguides 10a, 10b, 10c, lOd, lOe and lOf, at which mixed light may be emitted or extracted from the optical element 3 and thus from the light emitting device 105.

By means of each of the above-described embodiments a light output with a very homogeneous far field may be obtained.

However, it is also possible to modify a light emitting device according to the invention in such a way as to obtain a light output with an inhomogeneous far field pattern. By way of example a third spectral distribution, e.g. representing white light, can be obtained in the center of the light exit surface 32 and a spectral distribution, e.g. corresponding to a specific color, can be obtained at the rim, or vice versa. This is achieved by having the through light guiding elements for all input spectral distributions ending in the center of the exit surface and only a subset of the through light guiding elements having openings also at the rim. Alternatively or in addition thereto the light exit surface 32 may be provided with a shaped element adapted for shaping the output light beam emitted from the light exit surface 32.

Additionally or alternatively, the different specific far- field patterns may be achieved by providing a light emitting device according to the invention with an element for electrically or manually rotating or tilting the optical element 3 with respect to the LEDs 21, 22, 23 and the base 8.

The person skilled in the art realizes that the present invention is by no means limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

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.