FIELD OF THE INVENTION

The invention concerns a light emitting device which comprises a central core element and a plurality of LED light sources where the central core element is a cylindrical element comprising a longitudinal direction and a cross-section in a direction perpendicular to the longitudinal direction, the cross section being shaped like a polygon with N sides. Such light emitting devices are typically made in the form of light bulbs, and in particular for retrofitting incandescent light bulbs.

As used herein the term "added LED light source" is intended to refer to a LED light source added to a central core element with N sides and already being provided with N LED light sources, i.e. any one or more LED light source from number N+l and upwards of the plurality of LED light sources.

BACKGROUND OF THE INVENTION

Light bulbs with LED light sources placed upon a central core element in the form of a regular octagon (i.e. a polygon having 8 sides) extended like a cylinder. The octagon was chosen for the first light bulbs with this architecture and needed 8 LED light sources to have a satisfactory lumen output. Also the diameter of the octagon (diameter of the smallest circle that can fully encircle the octagon) needed to be below 26 mm to assure that it did not touch the glass of the outer bulb while inserted vertically into the bulb. On one hand, to ensure good cooling the octagon needs to have as large a surface as possible, i.e. as large a radius as possible. On the other hand, to ensure good optical performance, i.e. the smallest possible illuminance variation of the glass bulb, the octagon should preferably be small. CN 202001900 U describes an example of such a light bulb, and furthermore teaches that to ensure a high brightness and a homogeneous illuminance of such a light bulb the LED light sources are preferably to be equally distributed over the surfaces of the octagon with the same number of LED light sources present on all surfaces.

However, as the quality of the LED light sources available improves, less LED light sources are needed to achieve a satisfactory high output intensity. Likewise, when smaller and cheaper LED light sources become available, more LED light sources are needed to achieve a satisfactory high output intensity. Also, lamps with different light levels or different white colors (different efficiencies of the LED light sources) will need different numbers of LED light sources. Furthermore, for the currently known architecture, changing the octagon to a hexagon (6 sides), heptagon (7 sides) or nonagon (9 sides) is costly.

Therefore, a central core element having a shape that is, at least to a large extent, LED light source count indifferent will have a big benefit.

Still considering a bulb with a central core element in the form of an octagon with 8 LED light sources, it is known that the direct (i.e. from the LED light sources) illuminance variation inside the bulb (in the plane of the LED light sources) is small and that the resulting luminance variation on the outside of the bulb will, for a fairly scattering bulb, be nearly invisible to the human eye. Also part of the direct light on the inside of the bulb will not be transmitted but will scattered back into the bulb. Most of this scattered light will have a second chance to escape the bulb at another position. As a result, the luminance variation on the outside is smaller than the illuminance variation on the inside. The thicker the scattering coating that the bulb may comprise, the smaller the luminance variation on the outer surface of the bulb, but the lower the optical efficiency.

However, if a ninth LED is to be added to the octagon, then the result is that the direct illuminance variation will increase considerably. This large variation will result in such a luminance variation on the outside of the bulb that it will be noticed by humans and not be appreciated. A thicker coating in the bulb can reduce the outside variation, but this will have a cost penalty as the optical efficiency will decrease.

Another way of decreasing the luminance variation without changing the coating (and optical efficiency) and without changing the octagon, that has been attempted, is to place the ninth LED as far from the LED already present on the relevant side of the central core element as possible. It can be shown by calculations that the direct illumination variation expressed as (Emax-Emin)/(Emax+Emin), Emax being the maximum illuminance and Emin the minimum illuminance, thereby will decrease from +/- 0.29 to about +/- 0.18. A further decrease may be obtained by increasing the length of the side of the octagon on which the ninth LED is to be placed. In case of 8 LED light sources the thus adjusted octagon will give slightly worse homogeneity while in case of 9 LED light sources the homogeneity will improve. However, the homogeneity is still too low, and the luminance variation will thus still be noticeable by the human eye.

A further problem arises in case of an octagon with further added LED light sources, i.e. with 10, 11, 12, 13, 14 or 15 LED light sources. The tenth LED light source will be placed on the opposite side of that on which the ninth LED light source is placed. If an eleventh LED light source is to be placed then this can be done on a side of the octagon being equally far from the sides on which the ninth and tenth LED light source, respectively, is placed. However between the ninth and tenth LED light source there will then be three empty sides, while between the tenth and eleventh LED light source as well as between the eleventh and the ninth LED light source there will be only one empty side. Therefore, in the case of eleven LED light sources, a different setup is preferred, namely one in which one of the ninth and the tenth LED light source is moved such that the ninth and the tenth LED light source are placed one side closer to one another. Therefore, when one wants to optimize the shape of the octagon in order to decrease the luminance variation, the case with 10 LED light sources will require one particular side to be increased in length, while the case with 11 LED light sources will require a different particular side to be increased in length. These two requirements are in conflict and hence it is impossible to achieve a sufficiently low and thus satisfactory luminance variation in all cases. This same conflict has been shown to come up also for a hexagon shape or a nonagon shape.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome these problems, and to provide a light emitting device with a structure and an arrangement of a plurality of LED light sources that allows for achieving a homogenous illuminance for all numbers of added LED light sources, and which is thus simpler and less costly to produce and acquire.

According to a first aspect of the invention, this and other objects are achieved by means of a light emitting device comprising a central core element and a plurality of LED light sources, the central core element being a cylindrical element comprising a longitudinal direction and a cross-section in a direction perpendicular to the longitudinal direction, the cross section being shaped like a polygon with N sides, N being equal to any one of 4 and an odd integer equal to or larger than 3, and the plurality of LED light sources being arranged on the N sides of the central core element in such a way that

on at least one of the N sides the number of LED light sources equals n+land on the remaining of the N sides the number of LED light sources equals n, where n is an integer being equal to or larger than 1 , and that

for all numbers of the N sides with n LED light sources the difference between the smallest number of adjacent sides with n LED light sources and the largest number of adjacent sides with n LED light sources is always maximally 1. Particularly, by providing a light emitting device with a central core element with a cross section being shaped like a polygon with N sides, N being equal to any one of 4 and an odd integer equal to or larger than 3, the central core element is provided with such a shape and structure that the illuminance of the light emitting device is largely or even fully unaffected by the specific total number of LED light sources arranged on the sides of the central core element.

Providing a light emitting device with a plurality of LED light sources arranged on the N sides of the central core element in such a way that the two above- mentioned points are fulfilled provides for a light emitting device having a highly

homogenous illuminance irrespective of the number of LED light sources added to the central core element of the light emitting device and thus irrespective of the specific total number of LED light sources arranged on the sides of the central core element.

Furthermore, when the plurality of LED light sources are arranged on the central core element in such a way that the two above-mentioned points are fulfilled, it turns out to apply in case of any number of added LEDs that no LED light sources need to be moved to other surfaces of the central core element in order to fulfill the two above- mentioned points. Thereby, a highly homogenous illuminance is obtained in a very simple manner. Thus a significantly simplified light emitting device being considerably more simple and cost effective to produce is provided.

Hence, a light emitting device is thereby provided which comprises a structure and an arrangement of a plurality of LED light sources that allows for achieving a homogenous illuminance for all numbers of added LED light sources, and which is thus simpler and less costly to produce and acquire.

In an embodiment at least one side of the N sides has a length that is different from the length of the remaining sides of the N sides.

Thereby a light emitting device with a further improved direct illumination variation is provided.

In an embodiment the N sides comprise mutually different lengths.

Thereby a light emitting device with an even further improved direct illumination variation is provided.

In an embodiment the plurality of LED light sources comprises kN+1 LED light sources, where k is an integer being equal to or larger than 1 , and wherein the side of the N sides on which LED light source number kN+1 of the plurality of LED light source is arranged is longer than at least some of the remaining sides on the N sides. In an embodiment the plurality of LED light sources comprises kN+1 LED light sources, where k is an integer being equal to or larger than 1 , and wherein the side of the N sides on which LED light source number kN+1 of the plurality of LED light source is arranged is the longest of the N sides.

By any of the two above embodiments, a light emitting device with kN+1

LED light sources and a particularly homogenous illuminance as well as a particularly low direct illumination variance is obtained.

In an embodiment the plurality of LED light sources comprises kN+2 LED light sources, where k is an integer being equal to or larger than 1 , the side of the N sides on which a first of LED light sources number kN+1 and kN+2 of the plurality of LED light source is arranged is the longest of the N sides, and the side of the N sides on which a second of LED light sources number kN+1 and kN+2 of the plurality of LED light source is arranged is the second longest of the N sides.

Thereby a light emitting device with kN+2 LED light sources and a particularly homogenous illuminance as well as a particularly low direct illumination variance is obtained.

In an embodiment the plurality of LED light sources comprises kN+3 LED light sources, where k is an integer being equal to or larger than 1 , the side of the N sides on which a first of LED light sources number kN+1, kN+2 and kN+3 of the plurality of LED light source is arranged is the longest of the N sides, the side of the N sides on which a second of the LED light sources number kN+1, kN+2 and kN+3 of the plurality of LED light source is arranged is the second longest of the N sides, and the side of the N sides on which a third of the LED light sources number kN+1, kN+2 and kN+3 of the plurality of LED light source is arranged is the third longest of the N sides.

Thereby a light emitting device with a particularly homogenous illuminance as well as a particularly low direct illumination variance is obtained, not only for the case of a light emitting device with kN+3 LED light sources, but for any light emitting device with any number of light sources equal to or larger than N+3.

In an embodiment the N sides are of equal length, i.e. the central core element has the cross sectional shape of a regular polygon with N sides.

Thereby a central core element with a particularly simple shape is provided for, which in turn provides for a light emitting device being particularly simple and cost effective to produce. In an embodiment the plurality of LED light sources comprises kN+2 LED light sources, where k is an integer being equal to or larger than 1 , and wherein light sources number kN+1 and kN+2, respectively, of the plurality of LED light sources is arranged on two sides of the N sides being farthest away one another.

Thereby a light emitting device with kN+2 LED light sources, a particularly simple construction and a particularly homogenous illuminance as well as a particularly low direct illumination variance is obtained.

In an embodiment the plurality of LED light sources comprises kN+3 LED light sources, where k is an integer being equal to or larger than 1, two of LED light sources number kN+1, kN+2 and kN+3, respectively, of the plurality of LED light sources are arranged on two sides of the N sides being farthest away one another, and the third of LED light sources number kN+1, kN+2 and kN+3, respectively, of the plurality of LED light sources is arranged on a side of the N sides being equally far away from the sides on which the remaining two of LED light sources number kN+1, kN+2 and kN+3, respectively, are arranged.

Thereby a light emitting device with kN+3 LED light sources, a particularly simple construction and a particularly homogenous illuminance as well as a particularly low direct illumination variance is obtained.

In an embodiment at least one of the plurality of LED light sources is arranged on a side of the N sides of the central core element in such a way that its position on the side is variable in at least one direction.

Thereby it becomes possible to further optimize the homogenous illuminance as well as the direct illumination variance even further by optimizing the position of the respective LED light sources on the respective surfaces of the central core element.

In an embodiment this is obtained in that least one of the plurality of LED light sources is arranged on the side in such a way as to be displaceable, continuously and/or stepwise, in the at least one direction.

In an embodiment the central core element is a prismatic element. Thereby a further optimization the homogenous illuminance as well as the direct illumination variance is obtained.

The light emitting device according to the invention may furthermore comprise any one or more of a homogenous outer bulb, a scattering coating and a socket element. In an embodiment the longest distance between two points on a periphery of a cross-section of the central core element is at least 26 mm.

The light emitting device according to the invention may be a light bulb, such as an incandescent light bulb or any other type of light bulb.

The invention thus furthermore concerns a light bulb comprising a light emitting device according to the invention.

According to a second aspect of the invention the above and other objects are likewise achieved by means of a method for arranging a plurality of LED light sources on a central core element of a light emitting device, the method comprising the steps of

providing a plurality of LED light sources,

providing a central core element being a cylindrical element comprising a longitudinal direction and a cross-section in a direction perpendicular to the longitudinal direction, the cross section being shaped like a polygon with N sides, wherein N is equal to any one of 4 and an odd integer equal to or larger than 3, and

arranging the plurality of LED light sources on the N sides of the central core element in such a way that

on at least one of the N sides the number of LED light sources equals n+1 and on the remaining of the N sides the number of LED light sources equals n, where n is an integer being equal to or larger than 1 , and that

- for any number of the N sides with n LED light sources the difference between the smallest number of adjacent sides with n LED light sources and the largest number of adjacent sides with n LED light sources is always maximally 1.

Further embodiments of a method according to the invention appear from the detailed description and the dependent method claims.

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 shows a schematic illustration of a first embodiment of a light emitting device according to the invention and comprising a central core element with a cross sectional shape of a polygon with N = 7 sides, i.e. a heptagon, and a plurality of LED light sources. Figs. 2A to 2D shows cross sectional views of a first embodiment of a central core element of a light emitting device according to Fig. 1 seen along the line X-X, the central core element having the cross sectional shape of a regular heptagon, and illustrating four different orders of arrangement of seven additional LED light sources on the seven sides of the central core element.

Figs. 3A to 3D shows cross sectional views of a second embodiment of a central core element of a light emitting device according to Fig. 1, the central core element having the cross sectional shape of an irregular heptagon, and illustrating four different orders of arrangement of seven additional LED light sources on the seven sides of the central core element.

Fig. 4 shows a graph illustrating the direct illumination variation on an outer bulb of a light emitting device according to the invention as a function of the number of LED light sources for a regular heptagon and for an irregular heptagon, respectively.

Fig. 5 shows a graph illustrating the direct illumination variation on an outer bulb of a light emitting device according to the invention as a function of the number of LED light sources for a regular heptagon and for an irregular heptagon, respectively, in a case where the position of the LED light sources on a side of the central core element may be varied and where the position of the LED light sources therefore has been optimized.

Figs. 6A to 6D shows the optimized LED light source positions for the irregular optimized heptagon for the respective cases of 7 LED light sources (Fig. 6A), 8 LED light sources (Fig. 6B), 9 LED light sources (Fig. 6C) and 10 LED light sources (Fig. 6D).

Fig. 7 shows a graph illustrating the far field luminous intensity as a function of the number of LED light sources for a light emitting device according to the invention comprising a central core element being a regular heptagon and an irregular heptagon, respectively, and further comprising an outer bulb which is non-scattering.

Fig. 8 shows a schematic illustration of a second embodiment of a light emitting device according to the invention and comprising a central core element with a cross sectional shape of a polygon with N = 5 sides, i.e. a pentagon, and a plurality of LED light sources.

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.

Fig. 1 shows a first embodiment of a light emitting device 10 according to the invention. The light emitting device 10 comprises a central core element 11 and a plurality of LED light sources 12. The central core element is a cylindrical element comprising a longitudinal direction L and a cross-section in a direction perpendicular to the longitudinal direction, the cross section being shaped like a polygon with N = seven sides, i.e. a heptagon. The plurality of LED light sources 12 are arranged on the seven sides of the central core element in a way that will be described further below.

In the embodiment shown, the light emitting device 10 further comprises a homogenous outer bulb 13 as well as a socket element 14 with electrical connectors 15, which are all optional. The homogenous outer bulb 13 may comprise a scattering coating which is also optional.

Hence, the light emitting device 10 is in the embodiment shown a light bulb, such as an incandescent light bulb. Other types of light bulbs are, however, also feasible.

Figs. 2A to 2D shows cross sectional views of a first embodiment of a central core element 11 of the light emitting device according to Fig. 1. In this embodiment the central core element is provided with the cross sectional shape of a regular heptagon. In other words all seven sides have the same length.

For the central core element 11 shaped as a regular heptagon and initially comprising seven LED light sources, one on each side, four possible orders of LED light source addition are possible when taking one specific side as the starting point. These four possibilities are illustrated in Figs. 2A to 2D, respectively, in which reference numerals 1 to 7 as adhered to each of the seven sides of the central core element 11 illustrate the order in which LED light sources number eight and upwards are added to the central core element 11. In other words LED light source number eight is added to side 1 , LED light source number nine to side 2, LED light source number ten to side 3 and so forth.

The four possibilities illustrated in Figs. 2A to 2D have all been shown to provide for a very homogenous illuminance for all numbers of LEDs when LED light sources number eight and upwards are added to the central core element 11 in the order illustrated, although with small mutual differences. It has furthermore been shown that the possibility illustrated in Fig. 2C provides for the most homogenous illuminance of the four possibilities. For all four options, and for any numbers of added LED light sources, the smallest and largest number of adjacent sides with no additional LED light source differs by only 1. Also an LED light source may be added or removed without having to move already placed LED light source(s) to a different side of the central core element 11.

In more generalized terms, and applying to all embodiments described herein, the central core element may be described as having a cross section being shaped like a polygon with N sides. In the above described case, N equals seven. In other embodiments described further below N may equal another odd integer being equal to or larger than three, such as five or nine, or even four.

The plurality of LED light sources may then be arranged on the N sides of the central core element in such a way that at least one of the N sides comprises n+1 LED light sources and the remaining of the N sides comprise n LED light sources, where n is an integer being equal to or larger than 1 , and that for all numbers or counts of the N sides comprising n LED light sources the difference between the smallest number of adjacent sides comprising n LED light sources and the largest number of adjacent sides comprising n LED light sources is always maximally 1.

The four possible orders of addition of LED light sources illustrated in Figs. 2A to 2D, respectively, may also be described in terms of steps of a method according to the second aspect of the invention.

In this case, the step of arranging the plurality of LED light sources on the N =

7 sides of the central core element comprises, in addition to the steps recited in the introductory description, at least the two first of the following steps carried out in the order mentioned:

a) arranging light source number kN+1 , where k is an integer being equal to or larger than 1, of the plurality of LED light sources is arranged on a randomly chosen side of the N sides,

b) arranging light source number kN+2 of the plurality of LED light sources on one of the two sides of the N sides being farthest away from the side on which light source number kN+1 is arranged,

c) arranging light source number kN+3 of the plurality of LED light sources on a side of the N sides being equally far away from the sides on which LED light sources number kN+1 and kN+2, respectively, are arranged, d) arranging light source number kN+4 of the plurality of LED light sources on one of the two sides of the N sides being farthest away from the side on which light source number kN+3 is arranged, and

e) arranging light source number kN+5 of the plurality of LED light sources on one of the two sides of the N sides being next to the side on which light source number kN+3 is arranged.

LED light sources number kN+6 and kN+7 may then be placed on the two remaining sides of the N sides of the central core element in any order desired.

It is noted that the steps a) to d) described above will also apply in the case of the number of sides N being any other odd integer equal to or larger than 5. Furthermore, in the case of the number of sides N being any odd integer equal to or larger than 9, step e) will also be applicable.

Turning now to Figs. 3A to 3D cross sectional views of a second embodiment of a central core element 111 of the light emitting device according to Fig. 1 is shown. In this embodiment the central core element 111 is provided with the cross sectional shape of an irregular heptagon. In the embodiment shown on Figs. 3A to 3D all of the seven sides have different lengths.

Furthermore, the central core element 111 shown in Fig. 3A has been obtained with the central core element 11 shown in Fig. 2 A as the starting point, upon which the length of each of the respective seven sides has been optimized until arriving at the central core element 111 shown in Fig. 3A with the aim of obtaining an even more homogenous illuminance. The central core elements 111 shown in Figs. 3B, 3C and 3D, respectively, have been obtained in an analogous manner with the central core element 11 shown in Figs. 2B, 2C and 2D, respectively, as the starting point.

In other embodiments in which an irregular heptagon is provided, at least one of the seven sides has a length differing from, and in particular being longer than, the remaining sides.

Still referring to Figs. 3A to 3D, these figures illustrate the optimized relation between the lengths of the respective seven sides of the central core element 111. As in the above case four possible orders of LED light source addition are possible. These four possibilities are illustrated in Figs. 3A to 3D, respectively, in which reference numerals 1 to 7 as adhered to each of the seven sides of the central core element 111 illustrate the order in which LED light sources number eight and upwards are added to the central core element 111. The four possibilities illustrated in Figs. 3A to 3D have all been shown to provide for a very homogenous illuminance for all numbers of LEDs when LED light sources number eight and upwards are added to the central core element 111 in the order illustrated, although with small mutual differences. It has furthermore been shown that the possibility illustrated in Fig. 3C provides for the most homogenous illuminance of the four possibilities.

The order in which LED light sources number eight and upwards are added to the central core element 111 is fixed in the sense that the first additional LED light source, i.e. LED light source number eight, is added on the longest side (the side denoted 1 in Figs. 3A to 3D), the second additional LED light source, i.e. LED light source number nine, is added on the second longest side (the side denoted 2 in Figs. 3 A to 3D) and the third additional LED light source, i.e. LED light source number ten, is added on the third longest side (the side denoted 3 in Figs. 3A to 3D). The remaining order of addition may be as illustrated on any one of Figs. 3 A to 3D, respectively. Alternatively, the order og addition may be continued such that the seventh additional LED light source, i.e. LED light source number fourteen, is added to the shortest side.

In more generalized terms, and applying to all embodiments described herein, the central core element may be described as having a cross section being shaped like a polygon with N sides, where at least one side of the N sides has a length that is different from the length of the remaining sides of the N sides.

The plurality of LED light sources may then be arranged on the N sides of the central core element in the same generally described way as above following the description of Figs. 2A to 2D, and furthermore in such a way that the side of the N sides on which LED light source number kN+1, where k is an integer being equal to or larger than 1, of the plurality of LED light source is arranged is longer than at least some of the remaining sides on the N sides.

Likewise, the case illustrated in Figs. 3A to 3D may be generalized such as to say that the N sides comprise mutually different lengths.

The plurality of LED light sources may then be arranged on the N sides of the central core element in the same generally described way as above following the description of Figs. 2 A to 2D, and furthermore in such a way that the side of the N sides on which LED light source number kN+1 of the plurality of LED light source is arranged is the longest of the N sides. The side of the N sides on which LED light source number kN+2 of the plurality of LED light source is arranged is then the second longest of the N sides. Likewise, the side of the N sides on which LED light source number kN+3 of the plurality of LED light source is arranged is then the third longest of the N sides.

Even more generalized, the plurality of LED light sources may in addition to the above be arranged on the N sides of the central core element in the same generally described way as above following the description of Figs. 2A to 2D, and furthermore in such a way that the respective LED light sources number kN+1 up to kN+7 of the plurality of LED light sources are arranged one on each side of the N sides in an order starting with the longest of the N sides and continuing with sides of the N sides having successively smaller lengths.

The four possible orders of addition of LED light sources illustrated in Figs. 3A to 3D, respectively, may also be described in terms of steps of a method according to the second aspect of the invention.

In this case the step of arranging the plurality of LED light sources on the N sides of the central core element comprises, in addition to the steps recited in the introductory description, at least one of the following three steps carried out in the order mentioned:

a) arranging LED light source number kN+1, where k is an integer being equal to or larger than 1, of the plurality of LED light sources on a side of the N sides being the longest of the N sides,

b) arranging LED light source number kN+2 of the plurality of LED light sources on a side of the N sides being the second longest of the N sides, and

c) arranging LED light source number kN+3 of the plurality of LED light sources on a side of the N sides being the third longest of the N sides.

Furthermore, the step of arranging the plurality of LED light sources on the N sides of the central core element may comprise arranging LED light sources number kN+1 up to kN+7, where k is an integer being equal to or larger than 1, of the plurality of LED light sources one on each side of the N sides in an order starting with the longest of the N sides and continuing with sides of the N sides having successively smaller lengths.

Referring again to Figs. 3A to 3D, to assure the required cooling surface area, the diameter of the heptagon was set to 26 mm, this however being optional as it is no necessity for optical reasons. For optimization simplicity the LED light sources were placed in the centre of the sides of the central core element 111. It was found that all four possibilities perform approximately equally well. However, and as mentioned above, the possibility shown in Fig. 3C turned out to perform slightly better than the other three possibilities (Fig. 3A, 3B and 3D) and therefore this setup was used for the measurements to be described in the following. Furthermore, it is in some embodiments feasible that at least one of the plurality of LED light sources is arranged on a side of the N sides of the central core element in such a way that the position of the LED light source on the side is variable in at least one direction, i.e. by being continuously or stepwise displaceable.

Fig. 4 shows the direct illumination variation on an outer bulb 13 of a light emitting device according to the invention as a function of the number of LED light sources for a central core element 11 shaped as a regular heptagon (any one of Figs. 2 A to 2D) and for the optimized heptagon (Fig. 3C, i.e. a central core element 111 shaped as an irregular heptagon with optimized side lengths), respectively. For reference, the direct illumination variation on an outer bulb of a light emitting device according to the invention as a function of the number of LED light sources for a central core element shaped as a regular polygon with N sides is also shown. Indeed, as may be seen, the optimized heptagon has a lower worst case direct illumination variation with the number of LED light sources as compared to the regular heptagon.

The improvement may seem small, but when the position of the LED light sources on the respective sides of the central core element is variable, the improvement is even better. This is illustrated in Fig. 5. For the optimized heptagon the worst case variation in direct illumination on the outer bulb is below 15% and this is for the number of LED light sources being seven. The illumination at the outside of the bulb will even be lower and this will be difficult to see by a human eye. Therefore this irregular heptagon shape is

advantageous compared to regular shapes.

Figs. 6A to 6D illustrates the optimized positions of the LED light sources 12 on the respective sides of the central core element 111 with the cross section of an irregular optimized heptagon for the cases of seven, eight, nine and ten LED light sources,

respectively. Indeed the longer sides allow two LED light sources placed on the same side of the central core element to be placed further apart. The circles on each of the graphs, being denoted 16 on Fig. 6A, illustrate the location of the outer bulb 13, where the illumination variation is calculated as shown in Figs. 4 and 5.

Fig. 7 illustrates the far field luminous intensity variation as a function of the number of LED light sources for a central core element 11 shaped as a regular heptagon (any one of Figs. 2A to 2D) and for the optimized heptagon (Fig. 3C, i.e. a central core element 111 shaped as an irregular heptagon with optimized side lengths), respectively. For reference, the far field luminous intensity variation as a function of the number of LED light sources for a central core element shaped as a regular polygon with N sides is also shown. All three graphs shown further apply to the case of a light emitting device further comprising an outer bulb which is non-scattering. Again a satisfactory performance is shown. Also, making the heptagon irregular hardly affects the far field homogeneity.

Fig. 8 shows a second embodiment of a light emitting device 100 according to the invention. The light emitting device 100 comprises a central core element 110 and a plurality of LED light sources 12. The central core element 110 is a cylindrical element comprising a longitudinal direction L and a cross-section in a direction perpendicular to the longitudinal direction, the cross section being shaped like a polygon with five sides, i.e. a pentagon.

The plurality of LED light sources 12 are arranged on the five sides of the central core element 110 following the same principle as described above. That is, the plurality of LED light sources may then be arranged on the N sides of the central core element in such a way that at least one of the N sides comprises n+1 LED light sources and the remaining of the N sides comprise n LED light sources, where n is an integer being equal to or larger than 1 , and that for all numbers or counts of the N sides comprising n LED light sources the difference between the smallest number of adjacent sides comprising n LED light sources and the largest number of adjacent sides comprising n LED light sources is always maximally 1.

Likewise, the five sides of the central core element 110 may be modified in length following the same principles as described above and exemplified by means of the central core element 111 of the light emitting device 10 according to the first embodiment of the invention.

The light emitting device 100 further comprises a homogenous outer bulb 13 as well as a socket element 14 with electrical connectors 15. The homogenous outer bulb 13 may comprise a scattering coating.

Hence, the light emitting device 100 is in this case a light bulb, such as an incandescent light bulb.

In even further embodiments, the central core element of a light emitting device according to the embodiment may also be a central core element having a cross section being shaped like a polygon with two, three or four sides.

In such embodiments, the plurality of LED light sources may also be arranged on the sides of the central core element having a cross section being shaped like a polygon with two, three or four sides following the same principle as described above. That is, the plurality of LED light sources may then be arranged on the N sides of the central core element in such a way that at least one of the N sides comprises n+1 LED light sources and the remaining of the N sides comprise n LED light sources, where n is an integer being equal to or larger than 1 , and that for all numbers or counts of the N sides comprising n LED light sources the difference between the smallest number of adjacent sides comprising n LED light sources and the largest number of adjacent sides comprising n LED light sources is always maximally 1.

Likewise, the sides of a central core element having a cross section being shaped like a polygon with two, three or four sides may also be modified in length following the same principles as described above and exemplified by means of the central core element 111 of the light emitting device 10 according to the first embodiment of the invention.

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

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.