FIELD OF THE INVENTION

The present invention relates to lighting elements and lighting devices comprising light emitting diodes. BACKGROUND OF THE INVENTION

Using light emitting diodes, LEDs, as light sources provides several advantages compared to using traditional incandescent lamps and halogen lamps, such as higher energy efficiency and faster response to power switching. However, spectral properties and spatial distribution of the light output from LEDs differ quite dramatically from that of traditional light sources, which causes difficulties when designing retrofit LED- based lamps to mimic the light distribution of traditional lamps.

A particular challenge is to find arrangements of LEDs providing uniform intensity, i.e. providing illumination having similar intensity in all directions, such as the isotropic intensity distribution provided by traditional incandescent lamps. Moreover, LEDs are easily overheated and this needs to be taken into account when designing LED based lighting devices. The need of protecting the LEDs from overheating often results in fairly complicated lamp designs which are adapted for a specific type of LED or for a specific lumen output.

Although several designs of LED based lighting devices have been proposed in the prior art, there is a need for further development of LED based lighting elements and/or lighting devices in order to solve or alleviate at least some of the problems described above.

SUMMARY OF THE INVENTION

An object of at least some of the embodiments of the present invention is to provide a lighting device and/or a lighting element solving or alleviating one or more of the above described problems. This and further objects may for example be achieved by means of a lighting element having the features defined in the independent claim. Preferable embodiments of the invention are characterized by the dependent claims. According to an aspect of the present invention, there is provided a lighting element comprising a first housing and at least one light emitting diode. The first housing defines a hollow interior volume. The at least one light emitting diode is arranged along the first housing for illuminating the interior volume. The first housing is adapted to scatter light received from the at least one light emitting diode and to output the scattered light from the lighting element.

The intensity of light emitted by a light emitting diode, LED, is usually strongest in a forward direction, e.g. in a direction orthogonal to a light emitting surface of the LED (i.e. the normal to the light emitting surface). In other directions, the intensity is lower. The intensity at a given direction, deviating by an angle from the forward direction, decreases as the angle increases. The inventors have realized that this non-uniform intensity distribution (i.e. with respect to angle/direction) may be compensated for by arranging at least one LED along a first housing such that the at least one LED illuminates an interior volume defined by the first housing. The first housing may receive light from the at least one LED at different distances and angles so as to compensate for the non-uniform intensity of the light emitted by the at least one LED. The first housing may be adapted to compensate for the non-uniform intensity of the light emitted by the at least one LED, whereby the illuminance received (from the at least one LED) at the first housing is more uniform than the intensity of the light emitted by the at least one LED. Further, by having the first housing scatter the light received from the at least one LED by the first housing and output the scattered light as output of the lighting element, a light output of the lighting element may be provided which is more uniform than the light output of the at least one LED.

For example, a spherical (or sphere-like) shape of the first housing causes light emitted in the forward direction by the at least one LED, i.e. the light having the highest intensity, to propagate a relatively long distance within the interior volume before being received by the first housing, and causes light emitted at large angles (e.g. between 45 and 90 degrees) from the forward direction, i.e. light having a lower intensity, to propagate a relatively short distance before being received by the first housing.

Despite the fact that a LED emits light with a rather non-uniform distribution, the present invention is advantageous in that a more uniform light output may be provided using at least one LED as light source. Moreover, while LEDs usually emit light in directions forming a hemisphere or less, lighting elements of at least some embodiments of the present invention may provide illumination in just about all directions, i.e. directions forming a sphere (except possibly for directions obscured by components, such as the at least one LED itself, heat sink elements or parts for mounting of the lighting element or of the at least one LED).

The first housing may for example be a sphere (or have a spherical shape). If the at least one LED emits light according to a Lambertian intensity distribution (i.e.

satisfying Lambert's cosine law), this shape may be particularly suitable for compensating for the non-uniformity of the light emitted by the at least one LED. In other words, the light emitted by the at least one LED may be received by the spherical housing at uniform illuminance.

In another example, the first housing may be an ellipsoid. The three axes of symmetry of the ellipsoid may have different lengths. In some examples, the lengths of the axes may be such that the longest is for example at most 4, 5 or 10 times the length of the shortest.

In another example, the first housing may be sphere-like, i.e. it may have a shape similar to that of a sphere. For example, the (sphere-like) first housing may be an ellipsoid with axes of similar lengths, e.g. the longest being at most 1.5 or 2 times longer than the shortest, it may be convex, i.e. it may be the boundary of a convex set, and/or it may have similar smoothness as a sphere, i.e. no edges or corners, but may deviate slightly from a spherical shape, e.g. by being deformed as if compressed from one or more directions.

It is to be noted that the first housing (e.g. sphere, ellipsoid, sphere-like housing or convex housing, as in the examples described above) may have one or more holes through which the at least one LED (or a heat sink element or mounting parts of the lighting element or of the at least one LED) may be arranged. The diameter of these one or more holes is smaller than the diameter of the first housing. The one or more holes may for example be small in comparison with the first housing, i.e. the diameter of the one or more holes may for example be at most half, a quarter, an eight or a sixteenth of the diameter of the first housing.

The first housing defines a hollow interior volume, i.e. the first housing encloses a volume (the hollow interior volume) that fills the first housing from inside and has the same shape as the inside or inner surface of the first housing. The outside shape of the housing may differ from the shape of the interior volume if the thickness of the first housing is not constant. The first housing may have holes, e.g. through which the at least one LED (or a heat sink element or mounting parts of the lighting element or of the at least one LED) may be arranged. In at least some embodiments, the interior volume may be defined by theoretically extrapolating the first housing (e.g. using differentials up to any suitable order) to cover such holes.

The at least one light emitting diode, LED, is arranged along the first housing, i.e. it may be arranged in contact with the first housing, it may be arranged close to the first housing relative to the diameter of the first housing, e.g. at a distance less than a quarter, an eight, a tenth, a twentieth or a hundredth of the diameter of the first housing,; and/or it may be arranged at one or more holes/gaps of the first housing (the holes/gaps for example being small in comparison with the first housing, i.e. the diameter of the one or more holes for example being at most half, a quarter, an eight or a sixteenth of the diameter of the first housing). The at least one light emitting diode, LED, may e.g. be arranged at the inside of the first housing, or may be arranged such that at least part of the at least one LED is located outside the first housing. The at least one LED may e.g. be arranged facing the center of the interior volume of the first housing. For example, the at least one LED may be arranged at a hole of the first housing, a light emitting surface of the at least one LED facing the interior volume.

The at least one LED may be a single LED or a plurality of LEDs. The at least one LED may be of any type, depending on the application. For example, the at least one LED may be of a single color, or may comprise two or more differently colored LEDs (e.g. blue, green, red) for providing a white light mix. The at least one LED may be adapted to emit light according to a Lambertian luminous intensity distribution, i.e. it may be adapted to emit light (at least approximately) satisfying Lambert's cosine law.

As the at least one LED is arranged along the first housing, cooling of the at least one LED is facilitated as compared to lighting devices in which LEDs are arranged closer to the center. For example, one or more heat sink elements may be arranged along the first housing and in contact with the at least one LED to protect it from overheating, which obscures less light than if the one or more heat sink elements were arranged in contact with a LED located in the center of the lighting element.

A lighting element according to the present invention is advantageous in that it requires less components than many other LED based lighting devices.

Another advantage is that different desired light output levels and/or colors of the emitted light can be easily realized.

In an example embodiment, the first housing may be so diffuse that its scattering of the light received from the at least one LED prevents individual components located inside the lighting element from being perceived from the outside, so that the lighting element may appear as (or similar to) one single light source.

In an example embodiment, any portion of the first housing may be at most ten times as thick as any other portion of the first housing, i.e. if the thickness of the first housing is different at different portions of the first housing, then the thickest portion of the first housing may be at most ten times as thick as the thinnest portion of the first housing. Herein, "thickness" refers to a distance measured in a direction (e.g. perpendicular to the first housing) from the interior of the first housing to the outside of the first housing (i.e. from an inner surface of the first housing to an outer surface of the first housing). It will be appreciated that holes or perforations of the first housing for arranging components such as the LED are not considered while comparing thicknesses of different parts of the first housing. Moreover, "portions" refers to parts of the first housing located in different directions from the center of the first housing. In case the first housing comprises several layers arranged on top of each other, two such layers are not considered as different

"portions" of the first housing.

In an example embodiment, the first housing may comprise a base material and a plurality of impurities distributed within the base material. The impurities may be of a material having a refractive index which is different than a refractive index of the base material. With the present example embodiment, the first housing is more diffuse and the received light is more efficiently scattered. The impurities may e.g. have spherical shape, or other similar shapes. The impurities may be granules or other small (compared to the thickness of the first housing, e.g. the diameter of the impurities may be less than, or in the order of, 1/100, 1/1000 or 1/10000 of this thickness) pieces of material. For example, the impurities may be of a material (e.g. Ti0 ₂ ) having a refractive index (e.g. 1.7) which is higher than the refractive index (e.g. 1.5) of the base material (e.g. glass or acrylic).

Alternatively, the impurities may be of a material (e.g. silicone) having a refractive index (e.g. 1.4) which is lower than that the refractive index (e.g. 1.5) of the base material (e.g. glass or acrylic).

Additionally, or alternatively, the inside and/or outside (i.e. the inner and/or outer surface) of the first housing may have a surface structure contributing to the scattering of light.

In an example embodiment, the first housing may be adapted to receive light, from the at least one light emitting diode, at a frequency (or wavelength), or according to a spectrum centered at a frequency (or wavelength). The first housing may be adapted to preserve the frequency (or wavelength) of the received light. The light emitting diode, LED, may emit light at a single frequency or may emit light according to a spectrum centered at a frequency. The first housing may receive light from the LED having this frequency or spectrum and may preserve this frequency or spectrum. Alternatively, the light from the LED may be absorbed on its way to the first housing, e.g. by a wavelength converting element or material, such that light with one or more different frequencies (or wavelengths) are received at the first housing. The first housing may be adapted to preserve these one or more different frequencies.

According to an embodiment, there is provided a lighting device comprising a lighting element according to any one of the preceding embodiments (and/or examples), and a second housing arranged to enclose the lighting element. The second housing may be adapted to scatter light received from the lighting element, and may optionally be adapted to emit this scattered light as output of the lighting device. Scattering the light outputted by the lighting element at the second housing may provide an even more uniform light output than the light output by the lighting element only.

The second housing may for example be a sphere (or have a spherical shape). In another example, the second housing may be sphere-like, i.e. it may have a shape similar to that of a sphere (as described above in relation to the first housing). For example, the second housing may be convex (i.e. it may be the boundary of a convex set) and/or may have similar smoothness as a sphere (i.e. no edges or corners) but may deviate slightly from a spherical shape, e.g. by being deformed as if compressed from one or more directions. In another example, the second housing may be an ellipsoid. The three axes of symmetry of the ellipsoid may have different lengths. In some examples, the lengths of the axes may be such that the longest is for example at most 4, 5 or 10 times the length of the shortest.

According to an embodiment, the first housing and the second housing may be

(at least almost/approximately) concentric, i.e. the first housing may have a center (located in the interior volume defined by the first housing) which coincides with the center of the second housing. In at least some embodiments, this enables a more uniform illumination of the second housing by the first housing. The center of a housing may for example be defined as a geometric mean of the points of the housing, or as the center of gravity of the housing. The first and second housings may for example be concentric spheres.

Additionally, or alternatively, the second housing may be arranged at a (positive) distance from the lighting element (or from the first housing) i.e. the second housing may be substantially larger than the first housing and may be arranged such that it encloses the first housing without touching it. The distance between the first and second housings allows for the light emitted from one point of the first housing to be mixed with light emitted from other points of the first housing, before reaching the second housing. As a result, a more uniform illumination of the second housing is provided, in particular in embodiments where the first housing does not emit light uniformly. The (positive) distance may also allow for multiple reflections of light between the first and second housings, which may contribute in providing a more uniform illumination of the second housing and/or in providing a more uniform light output from the lighting device.

According to an embodiment, the lighting device may further comprise a heat sink element arranged in thermal contact with the at least one LED. The second housing may be adapted to (at least partially) enclose the heat sink element. The heat sink element may be adapted to prevent/protect the at least one LED from overheating. The second housing (at least partially) enclosing the heat sink element allows for the lighting device to provide a more uniform light output. Indeed, light from the at least one LED or first housing, which may have been blocked/shadowed by the heat sink element, may be compensated for by light which has not been blocked/shadowed and which is scattered by the second housing. In this way, the heat sink element (and its blocking effect on the emitted light) may be at least partially concealed from the outside of the lighting device by the second housing. The lighting device may optionally comprise other components, such as components for mounting of the lighting device or components for electrical control of the at least one LED.

Optionally, these parts may be at least partially enclosed by the second housing, similarly to the heat sink element.

According to an embodiment, the heat sink element may comprise a stem extending at least partway between the first housing and the second housing, and at least one fin arranged (in a direction) along the stem (e.g. the at least one fin may be arranged in contact with the stem and may extend along at least a part of the stem in a direction between the first and second housings). The stem facilitates transportation of heat away from the lighting element and in particular from the at least one light emitting diode, towards the second housing (and/or to the outside of the enclosure defined by the second housing). The at least one fin increases the surface area of the heat sink element and improves heat transfer to the surroundings of the heat sink element. The stem may for example be arranged partly or entirely between the first and second housing, i.e. there may be parts of the stem extending outside the enclosure defined by the second housing and/or inside the enclosure defined by the first housing. The stem may be in direct contact with one or more of the first and second housings, and/or it may be in thermal contact with one or more of these housings via other parts of the heat sink element. The at least one fin may optionally be at least two fins. The at least one fin may optionally extend at least partly around the outside of the first housing.

According to an embodiment, the first housing has a center and the stem may extend in a direction which is at least approximately radial relative to the center, i.e. in a direction leading towards or from the center. In the present embodiment, the at least one fin may extend at least approximately perpendicularly out from the stem, i.e. the at least one fin may extend from the stem in a direction at least approximately perpendicular to the direction of the stem which is radial relative to the center, as described above. The present embodiment is advantageous in that the amount of light shadowed by the stem and the at least one fin is reduced. The at least one fin may optionally be flat to further reduce the amount of light shadowed.

According to an embodiment, the at least one fin may be arranged separate, or at a distance, from the second housing. This separation provides room, or a gap, between the at least one fin and the second housing for air, or any gases present in the lighting device, to flow between regions/volumes separated by the at least one fin, which may further improve the efficiency of the heat sink element. The distance between the at least one fin and the second housing also provides room for mechanical fastening means, such as a clamp, for fastening the heat sink element to the second housing. For example, the heat sink element may comprise a base plate which is attached to the stem and which may be attached to the second housing via a clamp mechanism.

The heat sink element (and/or parts thereof) may optionally be optically reflecting to reduce absorption of light and to increase uniformity of the illumination of the second housing. The heat sink element may be at least partially covered by a reflective, preferably diffusely reflecting, material such as white anodized material or matt white paint. For example, the heat sink element may comprise a reflective surface.

According to an embodiment, the second housing may have a shape deviating from a spherical shape and the first housing may be adapted to compensate for this deviation so as to output scattered light providing a uniform illumination of the second housing. In the present embodiment, the scattering of light provided by the second housing contributes to the uniformity of the light output of the lighting device. This allows for using a first housing deviating more from a spherical shape than in embodiments without the second housing. Such deviations may be particularly suitable for certain shapes of the second housing (e.g. the shape of an ellipsoid with axes of different lengths). When uniformly illuminating a spherical second housing from the inside, the lighting element may for example be located at the center of the interior of the second housing and may be adapted to output light uniformly in substantially all directions. In case the second housing is not spherical (e.g. still sphere-like, but deviating from a perfectly spherical shape), the lighting element may preferably output light at (e.g. slightly) different intensities in different directions, so as to illuminate the second housing uniformly. Hence, the first housing may be adapted to compensate for the shape of the second housing by providing such light output. The light output of the first housing may e.g. be controlled by the geometry of the first housing. For example, the first housing may have a geometry (such as shape or thickness) compensating for a deviation in shape of the second housing, from a sphere. Additionally, or alternatively, the second housing may have a non-uniform thickness and the first housing may be adapted to compensate for this deviation (via its thickness and/or shape) so as to output scattered light providing a uniform illumination of the second housing.

In one example, a distance from a center of the interior volume, defined by the first housing, to the second housing along a first ray from the center (i.e. in a first direction), is shorter than a distance from the center to the second housing, along a second ray from the center (i.e. in a second direction, different from the first direction). In the present example, a distance from the center to the first housing, along the first ray, is longer (or, in an alternative example, shorter) than a distance from the center to the first housing, along the second ray. The shape of the first housing may then be used to adapt the light output of the first housing to match (or fit) the shape of the second housing.

In another example, a distance from a center of the interior volume, defined by the first housing, to the second housing along a first ray from the center (i.e. in a first direction), is shorter than a distance from the center to the second housing along a second ray from the center (i.e. in a second direction, different from the first direction). In the present example, the first housing is thicker (or, in an alternative embodiment, thinner) at an intersection with the first ray than at an intersection with the second ray. The thickness of the first housing may then be used to adapt the light output of the first housing to fit the shape of the second housing.

According to an embodiment, the first housing may be adapted to provide a first scattering distribution and the second housing may be adapted to provide a second scattering distribution. The scattering distribution of the first housing may be broader/wider than the scattering distribution of the second housing, e.g. the first housing may be more diffuse than the second housing so as to provide a more efficient scattering. The light received by the second housing may already be quite uniform and the second housing may only need to provide relatively little scattering in order for the output of the lighting device to be uniform (e.g. for shadowing effects caused by heating elements and other components not to be visible from outside the lighting device). Hence, the second housing may be made less diffuse than the first housing, e.g. allowing for an increased light output of the lighting device and/or increased energy efficiency.

Additionally, or alternatively, the second housing may comprise a base material and a plurality of impurities distributed within the base material and/or the inside and/or outside of the first housing may have a surface structure contributing to the scattering of light. The impurities may be of a material having a different refractive index than a refractive index of the base material. The impurities may e.g. have a spherical shape, or other similar shapes. The impurities may be granules or other small pieces of material (compared to the thickness of the second housing, e.g. the diameters of the impurities may be less than, or in the order of, 1/100, 1/1000 or 1/10000 of the thickness).

In an example embodiment, the second housing may comprise a material which has a relatively higher thermal conductivity than plastic, such as glass. The present example embodiment is advantageous in that it provides lighting devices without need for active cooling.

It will be appreciated that any of the features in the embodiments described above for a lighting element/device according the present invention may be combined with other embodiments of lighting elements/devices according to the present invention. Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art will realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, in which:

Fig. 1 is a schematic side-view of a lighting element according to an embodiment of the present invention; Fig. 2 is a schematic side-view of a lighting device according to an

embodiment of the present invention; and

Fig. 3 is a perspective view of a lighting element and a heat sink element according to an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

A lighting element according to an embodiment of the present invention will now be described with reference to figure 1.

A lighting element 10 is depicted, comprising a first spherical housing 11 (or sphere) which defines a hollow spherical interior volume 12. A light emitting diode 13, LED, is arranged at the bottom of the first sphere 11 and configured to emit light into the interior volume 12. The emitted light illuminates the interior volume 12, and thereby also the first sphere 11 from inside. The first sphere 11 is configured to scatter the light received from the light emitting diode 13 and to output the scattered light as output of the lighting element 10.

The LED 13 may be configured to emit light according to a Lambertian intensity distribution, i.e. the intensity I of the emitted light is maximal in a forward direction orthogonal to a light emitting surface of the LED, and decreases according to Ι(θ) = I ₀ x cos(6) where Θ is the angle formed between the forward direction and the current direction, and where I ₀ is the maximum intensity. In the present embodiment, the forward direction of the LED 13 is towards the center 14 of the first sphere 11.

The propagation distance R of a ray of light emitted at a first angle θι from the LED 13 to the first sphere 11 may be expressed as to R(0 _X ) = D x 005(0 _! ) where D is the inner diameter of the first sphere 11. The ray of light emitted by the LED 13 at the first angle θι may be received by the first sphere 11 at a first point 15. As the normal direction of the sphere 11 at the first point 15 is directed towards the center 14 of the sphere 11, there is an angle θι between this normal direction and the direction of the light received at the first point 15, introducing a factor cos(0i) in the received illuminance at the point 15. Similarly, the propagation distance R of a ray of light emitted at a second angle θ ₂ may be expressed as R(0 ₂ ) = D x cos(6 ₂ ). This ray of light may be received by the first sphere 11 at a second point 16, and the direction of the received light introduces a factor cos(0 ₂ ) in the received illuminance at the point 16. Because of the shape of the first sphere 11, the illuminance at the inside or inner surface of the first sphere 11 is proportional to ^cos ^*j ⁽⁹⁾ ₌ iiL [ _e ¾ j _s constant with respect to the angle Θ. Indeed, the factors cos(0) caused by the Lambertian intensity distribution, the propagation distance of the light, and the angle under which the light is received, compensate each other. In case the LED does not emit light according to a

Lambertian intensity distribution, the shape of the first housing (i.e. the sphere 11) may be designed differently such as to compensate for the non-Lambertian intensity distribution.

As the illuminance is constant along the inner surface of the first sphere 11, the first sphere 11 may provide a uniform luminance as output of the lighting element 10 by scattering the light received from the at least one LED 13. Scattering may for example be achieved with a first sphere 11 comprising a base material in which a plurality of impurities has been distributed. The base material may have a relatively lower refractive index, such as glass or acrylic having a refractive index of about 1.5, while the impurities may be small spheres of a material having a relatively higher refractive index, such as Ti0 ₂ having a refractive index of about 1.7. Alternatively, the impurities may be of a material having an ever lower refractive index, such as silicone having a refractive index of about 1.4.

Scattering may also be achieved in other ways, such as by giving the inside and/or the outside of the first sphere 11 a surface structure. A combination of volume scattering (using impurities) and surface scattering is also possible.

It is to be noted that in some embodiments, the first sphere 11 may be replaced by another housing, such as e.g. a sphere-like housing, an ellipsoid or a convex housing. Although the illuminance at the inside (or inner surface) of such a housing may not be constant (in contrast to the case of a sphere 11, in which the illuminance may be at least approximately constant), factors caused by the intensity distribution of the LED, the propagation distance of the light, and the angle under which the light is received, may still compensate each other at least partially. Hence, the lighting element may still provide a quite uniform light output. Similarly, the LED may not necessarily be configured to emit light exactly according to a Lambertian intensity distribution. As long as the intensity distribution is sufficiently similar to Lambertian, a (partial) compensation/cancellation between the intensity distribution of the LED, the propagation distance of the light, and the angle under which the light is received, may still be achieved.

A lighting device according to an embodiment of the present invention will now be described with reference to figure 2. Figure 2 shows a lighting device 20 comprising a lighting element 21 identical to the lighting element 10 described with reference to figure 1 except that first housing 22 of the lighting element 21 in figure 2 is not a sphere, but an ellipsoid having a vertical axis which is longer than its two horizontal axes. The lighting element 21 is enclosed by a second housing 23, having the shape of an ellipsoid with a vertical axis shorter than its two horizontal axes. The second housing 23 is concentric with the first housing 22 of the lighting element 21, and arranged at a (positive) distance from the lighting element 21 so as to leave an open space 24 between the two housings. Light emitted from different points of the first housing 22 may be mixed in this open space 24 and light may be reflected multiple times between the two housings and within the first housing 22.

A heat sink element 25 may be arranged in thermal contact with the LED (not shown in figure 2) of the lighting element 21. The heat sink element 25 is adapted to prevent the LED from overheating and is also, at least partially, enclosed by the second housing 23.

As the second housing 23 is an ellipsoid, the distance from the center 26 of the housings, to the second housing 23 depends on along which direction the distance is measured. Referring to the example configuration shown in figure 2, the distance from the center 26 to the second housing 23, along a vertical ray 27, is shorter than the distance from the center 26 to the second housing 23, along a horizontal ray 28. The lighting element 21 is adapted to compensate for the deviation of the second housing 23 from a perfectly spherical shape. The distance from the center 26 to the first housing 22 is longer along the vertical ray 27 than along the horizontal ray 28. Moreover, the first housing 22 may be thicker at an intersection with the vertical ray 27 than at an intersection with the horizontal ray 28. This causes the light output of the lighting element 21 to be stronger in the horizontal direction than in the vertical direction, allowing for a more uniform illumination of the second housing 23.

It will be appreciated that the shapes of the first housing 22 and the second housing 23 depicted in figure 2 are only examples presented for illustrative purposes.

Embodiments may be envisaged in which the second housing 23 is of a different shape (e.g. a sphere, a sphere-like housing, a convex housing etc.), and in which the first housing 22 has a shape and/or thickness adapted to compensate for the particular shape of the second housing 23. In some embodiments, the second housing 23 may be a sphere, and the lighting element 21 may be the lighting element 10 depicted in figure 1, i.e. the first housing 22 may also be a sphere. It is also to be noted that the lighting device 20 may comprise further elements, such as components for mounting the lighting device, and/or components for electric control of the LED. These additional components may be at least partially enclosed by the second housing 23 in order to provide a more uniform light output.

An example of a heat sink element according to an embodiment of the present invention will now be described with reference to figure 3.

Figure 3 is a perspective view of a lighting element 31 and a heat sink element 32 arranged in thermal contact with the at least one light emitting diode (not shown in figure 3) of the lighting element 31. The lighting element 31 depicted in figure 3 is spherical, but other shapes are also envisaged, such as ellipsoids, and sphere-like shapes, as described above. The heat sink element 32 comprises a cylindrical stem 33 arranged below the lighting element 31 and extending downwards, i.e. in a radial direction from the center of the lighting element 31. A plurality of fins 34 is distributed around the stem 33 to increase the surface area of the heat sink element 32. The fins 34 are preferably flat. The fins 34 extend downwards along the stem 33 and perpendicularly out from the stem 33 so as to reduce the amount of light obscured by the fins 34. The fins 34 may be thin (e.g. 0.1 mm) and may extend a bit up around the lighting element 31 (e.g. to increase the surface area of the fins 34). The stem 33 may for example be arranged on a (e.g. cylindrical) base plate 35. The heat sink element 32 (or at least some of its parts) may be white anodized or painted matt white thereby allowing heat transfer by thermal radiation and reducing absorption of light by the heat sink element 32, which may further improve the cooling performance of the heat sink element 32.

Similar to the heat sink 25 depicted in figure 2, the lighting element 31 and the heat sink 32 depicted in figure 3 may be enclosed by a second housing (not shown in figure 3) for scattering the light received from the lighting element 31. The heat sink element 32 may be fixed to the second housing via a mechanical fixation such as a clamp mechanism holding the base plate 35. The lighting element 31 and the heat sink element 32 may be arranged through an opening in the second housing (not shown in figure 3). In order to increase the surface area of the heat sink element 32, the outline of the heat sink element 32 may be adapted to fit the opening in the second housing.

The fins 34 may be arranged at a distance from the second housing (not shown in figure 3) and/or from the base plate 35, thereby providing room for air to flow between compartments/volumes defined by the fins 34 along the stem 33. Such flow of air may be caused by relatively warmer air rising in the lighting device in which the heat sink element 32 is arranged. The distance between the fins 34 and the second housing and/or from the base plate 35 may also facilitate mechanical fixation of the heat sink element 32 to the second housing.

The stem 33 may for example be hollow so as to allow feeding of cables from outside the second housing through the stem 33 to the lighting element 31.

The increased efficiency of the heat sink element 32 depicted in figure 3 may reduce the need for active cooling, and may thereby reduce noise and/or power consumption and/or may increase reliability of lighting devices according to embodiments of the present invention. Nevertheless, embodiments of lighting devices according to the present invention are envisaged in which active cooling (such as by an electric fan) is used in addition to (or as an alternative to) heat sink elements such as the heat sink element 32 depicted in figure 3, or the heat sink element 25 depicted in figure 2. For example, an electric fan may be arranged between the first housing and the second housing.

It is to be noted that the heat sink element depicted in figure 3 serves as an example of a heat sink element according to at least some embodiments of the present invention, and that other heat sink elements are also envisaged. For example, a heat sink element according to an example embodiment of the present invention may have different types of fins and/or other parts for increasing the surface area of the heat sink element. The fins may for example be oriented differently, may have different shapes, and/or may be arranged in other locations than in figure 3. Moreover, a lighting device according to embodiments of the present invention may for example comprise more than one heat sink element.

While embodiments of the invention have been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. For example, it is possible to operate the invention in an

embodiment wherein more than one light emitting diode is used to illuminate the interior volume. For example, a red, a green and a blue light emitting diode may emit light which is mixed via scattering and reflections to provide a white light mix as output of the lighting element and/or lighting device. Moreover, one or more wavelength converting elements may be arranged inside the first housing, between the first and second housings, and/or outside the second housing to convert one or more wavelengths of the light emitted by the at least one LED. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art 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. Any reference signs in the claims should not be construed as limiting the scope.