WO2011144597A2

WO2011144597A2 - A lighting device

Info

Publication number: WO2011144597A2
Application number: PCT/EP2011/057948
Authority: WO
Inventors: Oliver Shakespeare
Original assignee: Oliver Shakespeare
Priority date: 2010-05-21
Filing date: 2011-05-17
Publication date: 2011-11-24
Also published as: WO2011144597A3; GB2473311B; GB2473311A; GB201008459D0

Abstract

A lighting device uses a plurality of LEDs (16). The LEDs are positioned to form a plurality of sets (18,20,22) of LEDs such that the LEDs of each set are arranged around a respective circle and the circles of the respective sets are mutually concentric. The device also comprises circuitry adapted to address together the LEDs of each set, separately from the LEDs of the or each other set, for powering together the LEDs of said each set.

Description

A LIGHTING DEVICE

The invention relates to a lighting device having a plurality of LEDs (light emitting diodes).

It is known to provide a lighting device having a plurality of LEDs. The use of a plurality of LEDs allows greater illumination as compared to a single LED. The known device can be provided with an optical array to direct the light emitted by the LEDs into a beam of desired shape and size. It is possible to dim the known lighting device using pulse width modulation. In this way, the amount of light emitted by each LED is reduced and the intensity of the light beam is reduced.

However, dimming of the light emitted by the known lighting device in this way is not always satisfactory. In many situations, it is desirable to use the same lighting device to produce a relatively homogenous lighting of relatively high intensity, for example when it is desired to undertake tasks using the lighting, and also for the lighting device to be dimmable to produce increased ambience or mood. When the known lighting device described above is dimmed using pulse wave modulation, the shape and size of the light beam remains the same although the intensity is reduced. This often results in a flat rather unappealing lighting rather than providing the desired ambience.

In accordance with the invention, there is provided a lighting device comprising: a plurality of LEDs, wherein the LEDs are positioned to fonn a plurality of sets of LEDs such that the LEDs of each set are arranged around a respective circle and the circles of the respective sets are mutually concentric; circuitry adapted to address together the LEDs of each set, separately from the LEDs of the or each other set, for powering together the LEDs of said each set; and optical means; the lighting device being operable so that when the LEDs of any set, other than the set which is positioned radially innermost, are powered, the LEDs of the or each set which lies radially inwardly of said any set will also be powered, and wherein, when the lighting device is operated in this way, the optical means directs light from the powered LEDs into a diverging conical beam such that the cone angle of the beam increases as the number of powered sets increases. Hence, it is possible to operate the lighting device so that dimming of the device is accompanied by narrowing of the illumination beam.

As used herein, the term LED (light emitting diode) refers to any semi-conductor diode which radiates in the visible region. It is known to incorporate multiple LEDs onto a single chip. Such a chip is often referred to as a multi-die LED with each LED on the chip being referred to as a die. The term LED as used herein applied to both a separate LED and also to an individual die on a multi-die LED.

The following is a more detailed description, by way of a example, of embodiments of the invention, reference being made to the appended drawings in which:

Figure 1 is a plan view of a light generating unit having a plurality of LEDs;

Figure 2 is a circuit diagram showing the electrical connections between the LEDs of the light generating unit of Figure 1 ;

Figure 3 is a plan view of the light generating unit of Figure 1 showing additional components of the unit;

Figure 4 is a cross-sectional view of part of the light generating unit of Figure 3;

Figure 5 is an exploded view showing assembly of the light generating unit of Figure 3 together with the components of an optical array;

Figure 6 is a cross- sectional view showing the light generating unit of Figure 3 when assembled with the optical array of Figure 5;

Figures 7a,8a and 9a show three different modes of operation of the light generating unit of Figure 3;

Figures 7b,8b and 9b show the beams of light that are produced by the light generating unit of Figure 3 when assembled with the optical array as shown in Figure 6, the conical beam of Figure 7b corresponding to the mode of operation shown in Figure 7a, the conical beam shown in Figure 8b corresponding to the mode of operation shown in Figure 8a, and the conical beam shown in Figure 9b corresponding to the mode of operation shown in Figure 9a;

Figure 10 is an Illuminating Engineering Society (IES) polar distribution plot representing the beam of light shown in Figure 7b;

Figure 1 1 is an IES polar distribution plot representing the beam of light shown in Figure 8b;

Figure 12 is an IES polar distribution plot representing the beam of light shown in Figure 9b;

Figure 13 is a visual representation of the illumination produced by the light beam of Figure 7b;

Figure 14 is a visual representation of the illumination produced by the light beam of Figure 8b;

Figure 15 is a visual representation of the illumination produced by the light beam of Figure 9b; Figure 16a shows a first electronic control system for controlling operation of the light generating unit shown in Figure 3; Figure 16b represents the operation of the light generating unit of Figure 3 when controlled by the first electronic control system of Figure 16a;

Figure 17a shows a second electronic control system for controlling the operation of the light generating unit of Figure 3;

Figure 17b represents the operation of the light generating unit of Figure 3 when under the control of the second electronic control system shown in Figure 17 a; Figure 18a shows a third electronic control system for controlling the light generating unit of Figure 3;

Figure 18b represents the operation of the light generating unit of Figure 3 when under the control of the third electronic control system shown in Figure 18a;

Figure 19 shows a luminair having the general configuration of a GUlO MR16 lamp, and incorporating the light generating unit of Figure 3 and the optical array of Figure 6;

Figure 20 is an exploded view of the luminair of Figure 1 ;

Figure 21 is an end view of the luminair of Figures 19 and 20; and Figure 22 is a cross-sectional view of the luminair of Figures 19 to 21. Figure 1 shows a light generating unit (10) which may be used in the construction of an LED based luminair. The light generating unit 10 includes a support 12 which is formed from aluminium so as to facilitate heat dissipation. The support 12 has a generally planar side 14 on which are mounted a plurality of LEDs 16.

As shown in Figure 1, the LEDs 16 are positioned in three separate sets 18,20,22. The LEDs 16 of the first set 18 are arranged around a first circle; the LEDs 16 of the second set 20 are arranged around a second circle; and the LEDs 16 of the third set 22 are arranged around a third circle. The three circles of the three sets 18,20,22 are concentric, with the circle of the first set 18 being positioned radially innermost, the circle of the third set 22 being positioned radially outermost, and the circle of the second set 20 being positioned intermediate the first set 18 and the third set 22.

In the specific embodiment shown in Figure 1, the LEDs 16 are formed on a single chip (not shown) which is mounted on the aluminium support 12. It will be appreciated, however, that this need not be the case and that the LEDs 16 may be separate from one another and mounted individually on the support 12.

Figure 1 also shows gold bond wires 24, first, second and third conductive tracks 26,28,30, and an earth conductive track 32. The conductive tracks 26,28,30,32 and the gold bond wires 24 are connected to the LEDs 16 so that the first inner set 18 of LEDs 16 can be operated independently of the second and third sets 20,22; so that the second intermediate set 20 of LEDs 18 can be operated independently of the first and third sets 18,22; and the third outer set 22 of LEDs 16 can be operated independently of the first and second sets 18,20. The connections between the LEDs 16, formed by the gold bond wires 24 and the conductive tracks 26,28,30,32 are shown schematically in the form of a circuit diagram in Figure 2. As shown in Figures 1 and 2, the first inner set 18 has six LEDs 16. The six LEDs 16 of the first inner set 18 are connected together in series by the gold bond wires 24. One end of the inner set 18 is connected to the first conductive track 26 which, in turn, leads to a first electrical terminal 34. The other end of the first inner set 18 is connected to the fourth conductive track 32, which serves as an earth, and which in turn leads to a fourth earth terminal 40. Hence, the LEDs 16 of the first inner set 18 can be powered by applying a voltage between the first terminal 34 and the fourth earth terminal 40.

As shown in Figures 1 and 2, the second intermediate set 20 consists of twelve LEDs 16. The twelve LEDs 16 are connected together in two branches arranged in parallel, with each branch having six LEDs 16 connected in series. Each such branch of the second set 20 has a respective first end connected to the second conductive track 28 and a respective second end connected to the fourth earth conductive track 32. The second conductive track 28 leads to a second teraiinal 36. Hence, the LEDs 16 of the second intermediate set 20 can be powered, independently of the first and third sets 18,22, by applying a voltage between the second terminal 36 and the fourth earth terminal 40.

The third outer set 22 has twenty four LEDs 16. The twenty four LEDs 16 are connected together in four branches in parallel, and with each such branch having six of the LEDs 16 connected together in series. Each such branch has a respective first end connected to the third conductive track 30 and a respective second end connected to the fourth earth conductive track 32. The third conductive track 30 is connected to a third terminal 38. Hence, the third outer set 22 of LEDs 16 can be powered, independently of the first and second sets 18,20, by applying a voltage between the third terminal 38 and the fourth terminal 40.

It will be noted that although the first, second and third sets 18,20 and 22 have different numbers of LEDs 16, the LEDs 16 are connected, in each case, with groups of six LEDs 16 in series. In this way, the same electrical current can be applied to every single LED 16 on the light generating unit 10. The relevance of this is that the wavelength of light emitted by an LED is dependent on the current applied to the LED. It is preferable for each LED 16 on the light generating unit 10 to emit light at the same wavelength, and this is clearly facilitated by the electrical connections described above.

As shown in Figure 4, the light generating unit 10 is also provided with first, second and third annular walls 42,44,46. The first annular wall 42 surrounds the first inner set 18 of LEDs 16 and separates the first inner set 18 from the second intermediate set 20. The second annular wall 44 separates the second intermediate set 20 of LEDs 16 from the third outer set 22. The third annular wall 46 lies radially outwardly of the third outer set 22 at LEDs 16. As shown in Figure 4, each annular wall 42,44,46 has a triangular cross-section with a flat base adhered to the aluminium support 12.

In addition, as seen in Figures 3 and 4, the light generating unit 10 is provided with first, second and third diffusers 48,50,52. The first diffuser 48 has a disklike shape and is received within the first annular wall 42, the first diffuser 48 lies above the LEDs 16 of the first inner set 18. The second diffuser 50 has an annular shape and is received between the first annular wall 42 and the second annular wall 44. The second diffuser 50 lies above the LEDs 16 of the second intermediate set 20. The third diffuser 52 also has an annular shape and is received between the second annular wall 44 and the third annular wall 46. The third diffuser 52 lies above the LEDs 16 of the third outer set 22.

Each one of the first, second and third diffusers 48,50,52 is formed from a diffusing plastics material impregnated with phosphor. The LEDs 1 emit blue light which is converted by the phosphor to yellow light. In addition, a proportion of the blue light from the LEDs 16 leaks through the diffusers 48,50,52. This leaked blue light combines with the yellow light emitted by the phosphor to form white light. Each diffuser 48,50,52 acts to ensure that the light emitted by the corresponding set 18,20,22 of LEDs 16 is substantially homogenous around the respective circle of that set 18,20,22. The first, second and third annular walls 42,44,46 are formed from an opaque white plastics material. The annular walls 42,44,46 serve to ensure that light emitted from the LEDs 16 of any one of the sets 18,20,22 does not impinge on a diffuser 48,50,52 overlying a different one of the sets 18,20,22. In addition, the triangular cross-section of the annular walls 42,44,46 helps to direct light from the LEDs 16 to the corresponding overlying diffuser 48,50,52 and, by reflecting the light, helps to further homogenise the light.

The light generating unit 10 will generally be used in combination with an optical array, the optical array being configured to direct emitted light into a beam of desired shape. In the current embodiment, a co ical beam is preferred.

The light generating unit 10 is shown in combination with an optical array in Figures 5 and 6. As seen in these Figures, the optical array comprises an inner lens 54 and an outer lens 56 which are held together by a lens spacer 58. The inner and outer lenses 54,56 may have any configuration such that the optical array directs the light emitted by the LEDs 16 in a beam having the desired shape. As shown in Figures 5 and 6, the lens spacer 58 mounts the inner and outer lenses 54,56 on the planar side 14 of the support 12 so that the inner and outer lenses 54,56 overlie the first, second and third diffusers 48,50,52.

As seen in Figures 7b,8b and 9b, light from the LEDs 16 after passage through the diffusers 48,50,52 enters the inner lens 54 and subsequently the outer lens 56. The inner and outer lenses 54,56 in combination act to produce a conical beam of light. Figures 7b,8b and 9b, together with the associated Figures 7a,8a and 9a, demonstrate three alternative modes of operation of the light generating unit 10. In Figures 7a and 7b, all three sets 18,20,22 of the LEDs 16 are powered so that all three sets emit light. This light, after passing through the three diffusers 48,50,52 is directed by the inner and outer lenses 54,56 into a conical beam of light 60 which is relatively wide. For example, the beam of light 60 may have a cone angle (the angle from the central axis 59 to the outer perimeter) of about 30°.

A second mode of operation is shown in Figures 8a and 8b. In this mode of operation, the first inner set 18 and the second intermediate set 20 of LEDs 16 are powered, whereas the third outer set 22 of LEDs 16 are not powered. Light from the first inner set 18 and the second intermediate set 20 passes through, respectively, the first diffuser 48 and the second diffuser 50. After passage through the diffusers 48,50, the light is directed by the inner and outer lenses 54,56 into a conical beam 62 having a smaller cone angle (for example about 17½°).

Figures 9a and 9b show a third mode of operation in which the first inner set 18 of LEDs is powered, but the second and third sets 20,22 of LEDs 16 are not powered. In this mode of operation, light from the first inner set 18 passes through the first diffuser 48 and after passage through the inner and outer lenses 54,56 forms a cone 64 having a narrow cone angle (for example about 5°).

It will be noted that in the three modes of operation described above with respect to Figures 7a,7b,8a,8b,9a and 9b, when the second intermediate set 20 is powered, then the first inner set 18 will also be powered, and when the third outer set 22 is powered, then both the inner set and the intermediate set 18,20 are also powered. In this way, in each one of the modes of operation, the resultant beam of light 60,62,64 tends to have a constant intensity along a radius extending from the cone axis 59 to the periphery (although it will be noted that complete homogeneity of intensity is not required). Another preferred feature of the current embodiment is that the illuminance of the beam remains generally the same (although not necessarily precisely the same) regardless of the mode of operation. As used herein, the term illuminance refers to the intensity of the light of the conical beam 60,62,64. More specifically, the term illuminance refers to the entire luminous flux, derived from the conical beam 60,62,64, which impinges on an imaginary plane, and the amount of the entire luminous flux being normalised to the area of the beam impinging on the plane. The imaginary plane is located at a predetermined distance from the light source and lies normal to the conical axis 59 of the beam 60,62,64. Hence, the conical beam 60,62,64 intersects with the imaginary plane at a circle and the entire luminous flux falling within this circle is divided by the area of the circle in order to obtain the illuminance. As discussed above, it is preferred that the illuminance of the conical beam remains generally the same between the first mode of operation with all three sets 18,20,22 powered, the second mode of operation with the first inner set 18 and the second intermediate set 20 powered and the third mode of operation with only the first inner set 18 powered. The respective numbers of LEDs 16 in the first, second and third sets 18,20 and 22 are chosen with a view to achieving this. As described above and shown in Figures 1 and 2, the third outer set 22 has more LEDs 16 than the second intermediate set 20 and the second intermediate set 20 has more LEDs than the first inner set 18. In practice, larger radially outwardly positioned circles of LEDs contribute light to a greater volume of the conical beam as compared to smaller radially inner circles of LEDs. In view of this, the provision of a greater number of LEDs 16 in a radially outer set of LEDs as compared to a radially inner set of LEDs helps to produce a generally similar illuminance between the three modes of operation described above. In order to achieve generally similar illuminance, it is also possible to provide radially outer sets of LEDs with a greater density of LEDs per unit area, or with brighter LEDs. Any measure which allows a radially outer set of LEDs to produce a greater flux of light may be used to help achieve the preferred constancy of illuminance.

Figures 10,1 1 and 12 are IES polar distribution plots representing the cones of light shown, respectively, in Figures 7b,8b and 9b. Hence, Figure 10 shows the mode of operation in which the first, second and third sets 18,20 and 22 are all powered, Figure 11 shows the second mode of operation in which the first and second sets 18,20 are powered and the third set 22 is not powered, and Figure 12 shows the third mode of operation in which only the first inner set 18 is powered. Each one of the IES polar distribution plots (Figures 10 to 12) has an origin which is located at the centre of the top of the plot. This corresponds to the source of light. The angle of the shaded region, adjacent the origin, represents the angle of divergence of the light beam 60,62,64. The vertical line extending down from the origin corresponds to the axis of the conical beam. The IES polar distribution plots of Figures 10 to 12 represent the light intensity of the conical beams 60,62,64 at any angle from the beam axis. The intensity is proportional to the length of a line extending from the origin to the base of the shaded region at the appropriate angle from the axis. As will be seen in Figures 10,1 1 and 12, the light intensity remains generally constant between the first mode of operation, the second mode of operation and the third mode of operation.

Figure 13 provides a visual representation of the illumination that would be provided in the first mode of operation in which the first, second and third sets 18,20 and 22 are all powered. Figure 14 shows a visual representation of the illumination that would be provided in the second mode of operation when the first inner set 18 and the second intermediate set 20 are powered, but when the third outer set 22 are not powered. Figure 15 shows a visual representation of the illumination that would be provided when only the inner set 18 of LEDs 1 is powered. Figures 16a, 17a and 18a show three alternative electronic control systems controlling the powering of the three sets 18,20,22 of LEDs 16. Each of the three electronic control systems can be used to produce the first, second and third modes of operation described above. The first electronic control system shown in Figure 16a makes use of a standard on/off household light switch 66. The light switch 66 is connected to a constant current PSU 68 which, in turn, is connected to an LED driver 70. The LED driver 70 is connected to a shift register 72 with hold up memory which, in turn, is connected to each of the first, second and third sets 18,20,22 of LEDs 16. In operation, on initial powering of the main switch 66, all three of the first, second and third sets 18,20 and 22 are powered and produce light. Hence, the light generating unit 10 is in the first mode of operation described above. On a brief power interrupt (for example turning the mains switch 66 off for two seconds and then turning it back on) the third outer set 22 of LEDs 16 is extinguished. Hence, the light generating unit 10 is now in the second mode of operation described above. On a second power interrupt, the second intermediate set 20 of LEDs 16 is extinguished. Hence, the light generating unit 10 is now in the third mode of operation described above. Finally, on a third power interrupt (or, alternatively, on an extended power interrupt) the first inner set 18 of LEDs 16 is extinguished. The operation of the first, second and third sets 18,20,22 of LEDs 16, under the control of the first electronic control system is shown in Figure 16b. As seen in Figure 16b, each one of the first, second and third sets 18,20,22 is either 100% on or off.

Figure 17a shows the second electronic control system. The second electronic control system includes a triac mains dimmer switch 74 which is connected to a triac dimmable PSU/LED driver 76. In turn, the driver 76 is connected to a shift register 78. In operation, on turning the dimmer switch 74 towards full power, the first, second and third sets 18,20,22 of LEDs 16 are powered in turn. The operation of the light generating unit 10, under the control of the second electronic control system, is shown graphically in Figure 17b. Between dimmer switch positions 1 and 4, the first inner set 18 of LEDs 16 is powered 100% while the second and third sets 20,22 are turned off. Between dimmer switch positions 5 and 7, the first and second sets 18,20 of LEDs 16 are turned on while the third outer set 22 is turned off. Between switch positions 7 and 10, all three of the first, second and third sets 18,20,22 of the LEDs 16 are powered.

Figure 18a shows the third electronic control system. In this system, a triac mains dimmable switch 80 is connected to first, second and third triac dimmable LED drivers 82,84,86. Each dimmable LED driver 82,84,86 is connected to a respective one of the first, second and third sets 18,20,22 of the light generating unit 10. In this embodiment pulse width modulation enables the luminous flux of the first, second and third sets 18,20,22 to be varied. As the dimmer switch is turned from position 1 to position 4, the first inner set 18 is operated to provide a gradually increasing intensity of light. From switch positions 4 to 10, the first inner set 18 is 100% illuminated. From dimmer switch positions 1 to 4, the second intermediate set 20 is turned off. As the dimmer switch position is moved from 4 to 7, the second intermediate set 20 is operated so that the emitted light gradually increases from zero to 100%. From dimmer switch positions 7 to 10, the second intermediate set 20 is fully illuminated. From dimmer switch positions 1 to 7, the third outer set 22 is unpowered. As the dimmer switch position is moved from 7 to 10, the third outer set 22 is operated so as to gradually increase the intensity of illumination from zero to 100%. This operation is shown in Figure 18b.

Figures 19 to 22 show a luminair 90 utilising the light generating unit 10 and the optical array shown in Figures 5 and 6. The construction of the luminair 90 is best shown in Figure 20. In Figure 20, numeral 92 identifies a unit consisting of the light generating unit 10 mounted together with the optical array shown in Figures 5 and 6. In addition, the luminair 90 includes a moulded lamp cap 94, an inductor 96, a combined heat sync and housing 98, a printed circuit board 100 and a clear plastic Fresnel lens 102. The luminair 90 is similar in configuration to an LED based MR16 lamp.

The electronics provided in the luminair 90 allow operation of the light generating unit 10 to provide the first, second and third modes of operation described above. As an alternative the electronics may be separate from the luminair 90.

It will be appreciated that the embodiments described above in detail are examples of the invention and are not limiting on the scope of protection. Many different possible adaptations may be made while remaining within the scope of the invention as defined in the claims. Some potential adaptations are discussed below. The light generating unit 10 described above has three sets 18,20,22 of LEDs 16. However, it is possible to use different plural numbers of sets of LEDs. For example, the light generating unit 10 may have two sets of LEDs, or any plural number of sets of LEDs that can be accommodated within the confines of a suitably sized light generating unit. Regardless of the number of sets of LEDs, the LEDs 16 of each set will be arranged around a respective circle and the circles of the respective sets will be mutually concentric.

As described above, it is a preferred feature of the invention that when any set of LEDs is powered (other than the innermost set) the or each set of LEDs which lie radially inwardly will also be powered.

In the example described above, the LEDs 16 produce blue light and some of this blue light is converted into yellow light by phosphor incorporated into the first, second and third diffusers 48,50,52. However, different arrangements may also be used. For example, the LEDs 1 may be provided with a coating comprising phosphor which will, in a similar manner, serve to convert a proportion of the emitted blue light into yellow light. In this case, the first, second and third diffusers 48,50,52 need not contain phosphor as they only then serve to diffuse the light. The optical array need not be as shown in Figures 5 and 6. Any suitable optical array, comprising any suitable number of lenses, may be used. Other components may be included in the optical array.

Claims

1. A lighting device comprising: a plurality of LEDs, wherein the LEDs are positioned to form a plurality of sets of LEDs such that the LEDs of each set are arranged around a respective circle and the circles of the respective sets are mutually concentric; circuitry adapted to address together the LEDs of each set, separately from the LEDs of the or each other set, for powering together the LEDs of said each set; and optical means; the lighting device being operable so that when the LEDs of any set, other than the set which is positioned radially innermost, are powered, the LEDs of the or each set which lies radially inwardly of said any set will also be powered, and wherein, when the lighting device is operated in this way, the optical means directs light from the powered LEDs into a diverging conical beam such that the cone angle of the beam increases as the number of powered sets increases.

2. A lighting device according to claim 1, wherein the optical means includes at least one lens located for directing light from each powered LED.

3. A lighting device according to claim 1 or claim 2, wherein a radially outer one of said sets of LEDs produced a greater light output as compared to a radially inner one of said sets of LEDs, so that, when the device is operated in a way such that when the LEDs of any set, other than the set which is positioned radially innermost, are powered, the LEDs of the or each set which lies radially inwardly of said any set will also be powered, then said greater light output tends to reduce differences in illuminance of the conical beam as the number of powered sets of LEDs is changed.

4. A lighting device according to claim 3, wherein said greater output of light is produced by providing said radially outer set of LEDs with a greater number of LEDs, or with a greater density per unit area of LEDs, or with brighter LEDs, as compared to said radially inner set of LEDs.

5. A lighting device according to any preceding claim, and including a controller, wherein the controller is operative so that when the LEDs of any set, other than the set which is positioned radially innermost, are powered, the LEDs of the or each set which lies radially inwardly of said any set will also be powered.

6. A lighting device according to any preceding claim, wherein each set of LEDs is provided with a respective diffuser to increase the homogeneity of the light output around the circle of said each set.

7. A lighting device according to claim 6, including means preventing or reducing the impingement of light from the LEDs of any one set on the diffuser of any other set.

8. A lighting device according to claim 7, wherein the means comprises one or more circular walls, the or each wall inteiposed between the LEDs of one set and the LEDs of an adjacent set.

9. A lighting device according to claim 8, wherein the or each wall is white.

10. A lighting device according to any preceding claim, wherein the circuitry includes a plurality of temrinals, the LEDs of each set being electrically connected to a respective one of the terminals.

1 1. A lighting device according to any preceding claim, wherein the LEDs are individual die on a multi-die LED chip.

12. A lighting device according to claim 3, wherein the radially inner set of LEDs has a first number of LEDs connected together in series and the radially outer set has a second number of LEDs that is an integer multiple of the first number, the LEDs of the outer set being connected together in series in branches with the branches being connected together in parallel and with each branch having said first number of LEDs connected together in series.

13. A lighting device substantially as hereinbefore described with reference to the drawings.