GB2520248A

GB2520248A - Lighting unit

Info

Publication number: GB2520248A
Application number: GB1319900.5A
Authority: GB
Inventors: Charles Justin Maeers
Original assignee: Collingwood Lighting Ltd
Current assignee: Collingwood Lighting Ltd
Priority date: 2013-11-12
Filing date: 2013-11-12
Publication date: 2015-05-20
Anticipated expiration: 2033-11-12
Also published as: GB201319900D0; GB2520248B

Abstract

A lighting unit 1 includes at least one solid state lighting device 3, a reflector 5, and a phase change heat transfer device 7. The solid state lighting device 3 is mounted on the phase change heat transfer device 7 so that light emitted from the solid state lighting device 3 is oriented towards the reflector 5 which reflects the light incident upon it outwardly from the lighting unit 1. Heat generated by the solid state lighting device 3 is conducted away from the solid state lighting device 3 via the phase change heat transfer device 7.

Description

LIGHTING UNIT

The present invention relates to a lighting unit, which is typically adapted to be mounted in an aperture in a partition, for example a wall or ceiling panel. In particular, the invention relates to a lighting unit including at least one solid state lighting element, such as a Light Emitting Diode (LED) type devices.

LED technology is prevalent within the lighting industry. LEDs convert electrical energy into light and heat. LEDs are very efficient, generally resulting in less energy consumption when a lamp utilising incandescence or fluorescence is replaced with one designed to utilise LEDs.

One of the challenges with LED devices is thermal management. This is because the location at which the conversion from electrical energy to light and heat within the LED structure is physically very small. This small part of the LED gets very hot during use and can dramatically reduce the light output and lifetime of the LED if the heat is not quickly and efficiently removed. Heat is typically removed from an LED using an aluminium heat sink, which is connected directly to the LED, or to an aluminium heat spreader plate, on which the LEDs are mounted. Heat is then transferred to the environment, typically by air flowing over the heat sink.

Thus for efficacious solid state lighting units, there is a need for effective thennal management.

Reflector lighting units are a type of lighting unit that includes a reflector and a lamp (bulb), such as a conventional AR1 11, which is positioned within the lighting unit such the light emitting part of the lamp is oriented inwards towards the reflector rather than oriented in the more conventional outward facing direction.

AR1 11 lamps were originally of the halogen typc, however in recent years Ceramic Metal Halide lamp (CMH) versions have become most prevalent. A typical example of a CMH AR1 11 reflector lighting unit is shown in Figure 1. The CMH lamp 1 is mounted axially within the centre of a reflector 5 and is supported by arms 7 that extend across the lighting unit opening 9. The light emitting part of the lamp 1 is oriented inwards towards the rcflector 5, and the rear of the lamp (the outward facing part) is covered by an end cap 11 to block direct view of the light source, thereby reducing glare.

CMH AR1 11 lamps offer high precision and low glare light distributions. AR lamps are important in applications where a reduced glare and a crisp beam edge is required, such as highlighting products in a display or illuminating a table surface in a restaurant.

Recently, LED replacement lighting units for the AR1 11 have been introduced to the market. There are three main types of LED AR1 II replacement designs.

The first design type has LEDs mounted to an aluminium heat sink which directs the light forwards (outwards -i.e. is different from the way in which the reflector unit of Figure 1 operates) via a Total Internal Reflection (TIR) lens. This method results in a very high glare effect which can cause discomfort, and totally disregards the low glare effect achieved by the conventional CMH version AR 111. It typically does not feature an aluminised reflector or a light source mounted along the centnl axis of the lighting unit.

The second design type attempts to recreate the low glare effect of the conventional CMH in that it utilises LEDs that are mounted facing towards the reflector to assist in producing a low glare effect. In this design type, a Printed Circuit Board (PCB) having one or more LEDs is mounted similar to the CMH lamp in Figure 1, with the light emitting part of the LEDs being oriented towards the reflector. The PCB is supported by arms 7 in a similar fashion to that shown in Figure 1. These arms are also used in the thermal management system to remove heat from the LEDs by conducting the heat through the anns to a dissipation area of the reflector and heat sink. A drawback to this arrangement is that the support arms block a significant proportion of the light emitted and can create shadows.

The third design type features two LED sources assembled onto a thermally conductive partition, one on each side of the partition, which divides the entire reflector surface into two parts. Typically the two LED devices are mounted back to back on the partition. The light emitted from each LED source is directed sideways (i.e. substantially perpendicularly to the central axis of the lighting unit) towards its respective part of the reflector. An end cap is also used to reduce direct glare. A drawback to this arrangement is that no light is reflected from the reflector where the partition bisects the reflector.

It is an object of the present invention to provide a lighting unit that mitigates at least one of the aforesaid disadvantages, or at least provide an alternative lighting unit.

According to the present invention there is provided a lighting unit including at least one solid state lighting device, a reflector, and a phase change heat transfer device.

The solid state lighting device is mounted on the phase change heat transfer device. The solid state lighting device is oriented on the phase change heat transfer device such that light emitted from the solid state lighting device, in use, is oriented towards the reflector, said reflector being arranged to reflect the light incident upon it outwardly from the lighting unit, and wherein heat generated, in use, by the solid state lighting device is conducted away from the solid state lighting device via the phase change heat transfer device.

The invention provides a thermally efficient solid state lighting unit with an improved lighting effect. Since the solid state lighting device is mounted on the phase change heat transfer device there are no arms, or a central partition, to obstruct the light that is emitted from the lighting unit, thus providing a more even beam of light than prior art LED lighting units that are arranged according to the second and third design types from the prior art. Furthermore, since the lighting unit is arranged as a reflector lighting unit, it does not suffer from the harsh glare generated by prior art units arranged according to the first

design type from the prior art.

Advantageously the reflector has a central axis, the phase change heat transfer device has a longitudinal axis, and the longitudinal axis of the phase change heat transfer device is preferably substantially co-axial with the central axis of the reflector.

Advantageously the reflector can include a substantially dish-shaped reflective surface.

Thus the reflector provides a substantially concave reflective surface. Advantageously at least a part of the reflective surface can be substantially parabolic.

Advantageously the reflector can include an aperture, and the phase change heat transfer device protrudes through the aperture. Advantageously the aperture is arranged substantially co-axially with the central axis of the reflector. In preferred embodiments the reflector is in the form of a dish-shaped annulus, with the phase change heat transfer device protruding therethrough.

Advantageously the phase change heat transfer device is elongate, for example can be in the form of a heat pipe. Advantageously the solid state lighting device is mounted on the phase change heat transfer device towards one end thereof. Thus the elongate heat transfer device acts as a columnar support for the solid state lighting device. This helps to improve the quality of the light beam emitted from the unit since there are no radial support arms to obstruct the beam. Preferably the phase change heat transfer device includes first and second ends, and the solid state lighting device is mounted on the heat transfer device towards the first end. Advantageously the first end of the heat transfer device protrudes through the aperture in the reflector.

The solid state lighting device can be mounted on the phase changc heat transfer device either directly or indirectly. By indirectly it is meant that there is at least one intermediate component, or substance, such as a thermal pad or a thermal paste between the solid state lighting device and the phase change heat transfer device. In preferred embodiments, the number of thermal interfaces between the solid state lighting device and the phase change heat transfer device is minimised, however it will be appreciated by the skilled person that the use of thermal pads and/or thermal pastes is generally advantageous since they substantially eliminate air pockets between connecting portions of the solid state lighting device and the phase change heat transfer device.

Advantageously the reflector provides a substantially continuous reflective surface around the phase change heat transfer device. Since the phase change heat transfer device is elongate it only penetrates the reflector through the central aperture, and therefore the remaining part of the reflective surface is unbroken / uninterrupted. Thus the phase change heat transfer device provides a column-like (columnar) support for the solid state lighting device. This provides more even lighting.

Advantageously the reflector can comprise a thin walled body, typically having a thickness in the range 0.5 to 3mm. The reflector can include a metalized plastics body. Alternatively, the reflector can include a metallic body, such as aluminium body.

The reflector can include a flange at its open end to increase its rigidity.

Advantageously the phase change heat transfer device includes a thermally conductive casing, which typically includes a thermally conductive metal such as copper. The casing is closed at each end, and is typically in the form of a pipe.

The phase change heat transfer device includes a phase change material (PCM), such as water or ethanol, which is located inside the casing. In preferred embodiments, the interior of the casing is low pressure, i.e. there is a partial vacuum. At the hot end of the casing (the end adjacent to the solid state lighting device) the PCM is in liquid form. Heat generated by the solid state lighting device, in use, vaporises the liquid PCM, which then travels along the interior of the casing towards the colder end where it condenses back into a liquid thereby giving up heat and cooling the solid state lighting device. The liquid PCM returns to the hot end of the casing, and the cycle is repeated while heat is generated by the solid state lighting device. Thus the phase change heat transfer device transfers heat away from the solid state lighting unit by means of conduction via the thermally conductive casing and by operation of the PCM within the casing.

Advantageously the lighting unit can include a plurality of cooling fins connected to the phase change heat transfer device. The cooling fins are arranged to transfer heat from the phase change heat transfer device to the environment. The fins are thermally conductive, and preferably include a thermally conductive metal such as aluminium and/or copper. The fins are located along a significant proportion of the length of the phase change heat transfer device.

At least one of the fins, and preferably a plurality of the fins, can include a substantially annular disc. Advantageously each fin includes a substantially annular disc. Each disc is arranged substantially perpendicularly to the longitudinal axis of the phase change heat transfer device.

At least one of the fins, and preferably a plurality of the fins, can include at least one finlet to increase the surface area of the fin. Advantageously each finlet can comprise an upstanding formation that projects out the plane of the annular disc.

Advantageously the lighting unit can include a light guide for mounting over at least a portion of the phase change heat transfer device. The light guide includes a reflective surface for redirecting some of the light emitted from the solid state lighting device towards the reflector. The light guide directs light emitted by the solid state lighting device over a large area of the reflector. Additionally or alternatively, the phase change heat transfer device can include a reflective surface for the same purpose.

Advantageously solid state lighting unit faces towards at least one of the reflector and the light guide.

Advantageously the light guide can have a substantially circular cross-section. The light guide can be arranged to have a substantially circular cross-section at any given location along the length of its reflective surface. Advantageously the diameter of a first end of the light guide adjacent to the solid state lighting device is smaller than the diameter of a second end of the light guide adjacent to the reflector.

Advantageously the light guide can include a tapered portion. This presents an inclined reflective surface to the solid state lighting device. For example, the light guide can include at least one substantially frusto-conical portion. Preferably the light guide includes first and second frusto-conical portions. The base of the first frusto-conical portion can be connected to the top of the second frusto conical portion.

Advantageously the light guide can include a curved outer surface. Preferably the light guide includes a convex outer surface to direct light over a large area of the reflector.

Advantageously the light guide can include a locking fonnation, such as at least one locking foot, to lock the light guide to the reflector.

Advantageously the light guide can include a thin walled body. The light guide is preferably tubular.

Advantageously the solid state lighting device can include a PCB. The PCB is arranged substantially perpendicularly to the longitudinal axis of the phase change heat transfer device. Advantageously the solid state lighting unit is attached to the phase change heat transfer device via the PCB. The PCB is arranged substantially perpendicularly to the longitudinal axis of the phase change heat transfer device. The PCB includes an aperture and the PCB is mounted onto the phase change heat transfer device via the aperture. One end of the phase change heat transfer device protrudes through the aperture fonned in the PCB. Advantageously the PCB is mounted on the phase change heat transfer device towards one end thereof Advantageously the solid state lighting device includes at least one LED. The or each LED is mounted on the PCB. In preferred embodiments a plurality of LEDs are distributed on a planar surface of the PCB. For example, the LEDs are oriented inwards towards the reflector and/or light guide. The LEDs are distributed about the central axis of the reflector.

The LEDs are oriented towards a central portion of the reflector. In embodiments including the light guide, the LEDs are preferably aligned with an inclined reflective surface of the light guide. The diameter of the light guide somewhere along its length is preferably greater than the distance between the LEDs.

Each LED can include a lens, for example a domed lens. Advantageously each lens has a central axis. The central axis of at least one of the lenses, preferably a plurality of lenses, and more preferably still each of the lenses, is arranged substantially parallel to the central axis of the lighting unit.

Advantageously the PCB can comprise a thermally conductive plate. The thermally conductive plate comprises a thermally conductive material, or materials, such as aluminium and/or copper. The PCB provides a thermally conductive bridge between the LEDs and the phase change heat transfer device.

Advantageously the lighting unit can include an end cap mounted over the rear of the solid state lighting device. The end cap prevents light from being accidently emitted directly from the solid state lighting device in an outwardly direction, thereby reducing glare. Thus the majority, if not all, of the light emitted from the solid state lighting device exits the lighting unit after having been reflected by the reflector. Preferably the end cap sits over the end of the phase change heat transfer device and the PCB.

The lighting unit according to the invention is mountable in an aperture formed in a partition, such as a wall or ceiling, and can include mounting means for holding the lighting unit within the aperture. For example, the lighting unit can be in the form of a downlight. In this arrangement the heat transfer device cycle is moderately aided by gravity to return the condensed liquid to the hot end of the heat transfer device. In this arrangement, when the lighting unit is located in a substantially horizontal ceiling, the LEDs are oriented in an upwards direction and the reflective surface of the reflector directs light in a downwardly direction.

According to another aspect of the invention there is provided a lighting unit including at least one solid state lighting device, a reflector, and a thermally conductive columnar support member.

The solid state lighting device is mounted in thermal contact with the columnar support member. The solid state lighting device is oriented on the columnar support member such that light emitted from the solid state lighting device, in use, is oriented towards the reflector, and the reflector is arranged to reflect the light incident upon it outwardly from the lighting unit. Heat generated, in use, by the solid state lighting device is conducted away from the solid state lighting device via the columnar support member.

The invention provides a thermally efficient solid state lighting unit with an improved lighting effect. Since the solid state lighting device is mounted on a columnar support there are no arms, or a central partition, to obstruct the light that is emitted from the lighting unit, thus providing a more even beam of light than prior art LED lighting units that are arranged according to the second and third design types from the prior art. Furthermore, since the lighting unit is arrangcd as a reflector lighting unit, it does not suffer from the harsh glare generated by prior art units arranged according to the first design type from the

prior art.

The columnar support member can include a phase change heat transfer device. The phase change hcat transfcr device is elongate and the solid state lighting device is mounted thereon. This reduces the optical impact of the heat transfer device on light emitted from the reflector.

Advantageously the solid state lighting device can include a PCB. The solid state lighting device is attached to the columnar support member via the PCB, and said PCB is arranged substantially perpendicularly to the longitudinal axis of the columnar support member.

The solid state lighting device can be mounted on the columnar support member either directly or indirectly. By indirectly it is meant that there is at least one intermediate component, or substance, such as a thermal pad or a thermal paste between the solid state lighting device and the columnar support member. In preferred embodiments, the number of thermal interfaces between the solid state lighting device and the colunmar support is minimised, however it will be appreciated by the skilled person that the use of thermal pads and/or thermal pastes is generally advantageous since they substantially eliminate air pockets between connecting portions of the solid state lighting device and the columnar support member.

Advantageously the second aspect of the invention can include features from the first aspect of the invention.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic view of a prior art lighting unit; Figure 2 is a cross-sectional side view through a lighting unit according to an embodiment of the invention; Figure 3 is a cross-sectional side view of part of the lighting unit of Figure 2, illustrating diagranunatically the direction that light is emitted from LEDs and the direction that light is reflected out of the lighting unit; and Figure 4 is the cross-sectional side view of Figure 2, illustrating diagrammatically a main thermally conductive pathway from the LEDs to cooling fms via a heat pipe.

A lighting unit 1 according to one embodiment of the invention is shown in Figures 2 to 4.

The lighting unit 1 is in the form of a downlight. The lighting unit 1 includes a solid state lighting device 3, a reflector 5, and a phase change heat transfer device, which is typically in the form of a heat pipe 7. The lighting unit 1 further includes a light guide 9, an end cap 11 and cooling fins 13.

The solid state lighting device 3 includes a Printed Circuit Board (PCB) 15 and a plurality of Light Emitting Diodes (LEDs) 17, typically 2 to 10 LEDs 17. The PCB 15 includes a thermally conductive plate, such as a copper plate 16. The PCB 15 is substantially planar and the LEDs 17 are mounted on one of the substantially planar faces I Oa, lob of the copper plate 16. The LEDs 17 are soldered onto the plate 16. As well as providing a mounting surface for the LEDs 17, the copper plate 16 is arranged to transfer heat, which is generated in use, away from the LEDs 17. The PCB 15 includes an aperture 16c which is formed through the copper plate 16. The aperture 16c is centrally located on the substantially planar faces 16a,l6b. The LEDs 17 are distributed about the aperture lóc, preferably in a regular fashion. For example, are evenly angularly distributed. The maximum separation of the LEDs 17 on the PCB 15 is preferably less than the maximum diameter of the light guide 9.

Each LED 17 includes a lens 17a. Each lens 17a has a central axis Y-Y (see Figure 4).

The PCB 15 is mounted over one end 33 of the heat pipe via the aperture 16c. The LEDs 17 face towards the reflector 5. The PCB 15 can be attached to the heat pipe directly, or alternatively there can be at least one intermediate thermally conductive component or substance, for example a thermally conductive pad or paste for removing air pockets between the PCB 15 and heat pipe 7.

The reflector 5 is an optical device that is arranged to reflect light emitted by the LEDs outwards from the lighting unit 1. The reflector 5 comprises a thin walled construction, and is typically made from a metalized plastic, or a metallic substance, such as aluminium. The thickness of the walls is typically 0.5 to 3mm. The reflector 5 includes a curved reflective surface 19, which gives the reflector a substantially dish-shaped or concave configuration.

A first part 21 of the reflector has a substantially parabolic form. A second part 23 includes a flange 25 and a lip 27.

An aperture 29 is formed through the reflector 5. The aperture 29 is located centrally within the dish-shaped reflector structure, and is centred about a central axis Z-Z of the reflector 5.

The reflector 5 is attached to the heat pipe 7. Thc heat pipe 7 protrudes through the aperture 29 and supports the solid state lighting device 3, without any lateral structural elements such as support arms. Thus the heat pipe 7 is in the form of a columnar support.

This is significant since the reflector 5 surrounds the heat pipe 7 with an unbroken reflective surface 19, which provides a more even beam of light from the lighting unit 1, and there are no radial support arms to obscure the beam of light.

The longitudinal axis of the heat pipe 7 is arranged substantially co-axially with the central axis Z-Z of the reflector. The solid state lighting device 3 is mounted on the heat pipe 7 such that the PCB 15 is arranged substantially perpendicularly with respect to the longitudinal axis of the heat pipe 7 (and hence central axis of the reflector Z-Z), with the LEDs 17 being oriented inwards towards a central portion of the reflector and the light guide 9 such that light emitted from the LEDs is directed towards the reflector 5 and the light guide 9. The axes Y-Y of the LED lenses 1 7a are arranged substantially parallel with the central axis of the reflector Z-Z. When the downlight is located in a ceiling, the LEDs are oriented substantially vertically upwards, and hence are located on an upper side 1 6a of the PCB, and the reflector 5 is oriented downwards with the central axis Z-Z being arranged substantially vertically.

The heat pipe 7 is elongate, and includes a thermally conductive casing 31, which is typically made from copper. The casing is closed at each end. The heat pipe 7 includes a phase change material (PCM), such as water or ethanol, which is located inside the casing.

The interior of the casing is low pressure, i.e. there is a partial vacuum. At the hot end 33 of the casing (the end adjacent to the solid state lighting device 3) the PCM is in liquid form. Heat generated by the solid state lighting device 3, in use, vaporises the liquid PCM, which then travels along the interior of the casing towards a cold end 35 where it condenses back into a liquid thereby give up heat. The liquid PCM returns to the hot end 33 of the casing, and the cycle is repeated while heat is generated by the solid state lighting device 3.

Fins 1 3 are attached to the heat pipe 7 and are arranged to transfer heat from the heat pipe to the environment by convection and radiation. Each fin 13 comprises a substantially annular disk like member that is arranged substantially perpendicularly to the longitudinal axis of the heat pipe 7. Each fin 13 includes a plurality of raised protrusions 37, which are sometimes referred to as finlets, to increase the surface area thereof, thereby increasing the hcat transfer efficiency. The finlets 37 also promote air circulation between the fins 13, thereby improving heat transfer to the environment by convection.

The light guide 9 has an outer reflective surface that is arranged to redirect light incident upon it towards the reflector 5, thereby ensuring that light emitted from the LEDs 17 is incident over a large part (substantially all) of the reflective surface 19. The light guide 9 has first and second substantially frusto-conical parts 39,41, wherein the taper angle of the first part 39 with respect to the axis Z-Z is greater than the taper angle of the second part. It can be seen from Figure 2, that at least some of the LEDs 17 are aligned with the inclined surface of the first frusto-conical part 39.

The light guide 9 has a tubular structure, and is mounted over the heat pipe 7. The light guide 9 is typically made from a metalized plastic, or a metal such as aluminium. The structure is thin walled, typically having a wall thickness of around 0.5 to 3mm. The light guide 9 includes feet 43 that are arranged to pass through the aperture 29, and to interlock with the first fin 13a, which abuts the rear of the reflector, thereby fixing the light guide 9 to the reflector 5. Typically, the light guide 9 has a circular cross-section along its reflective length (cross-section taken substantially perpendicularly to the axis Z-Z), thereby providing the columnar support with a smooth reflective surface.

The cap 11 is mounted over the hot end 33 of the heat pipe and at least part of the solid state lighting device 3. The cap II is provided to prevent light being emitted directly from the LEDs out of the lighting unit. This helps to reduce glare.

In use, light (indicated diagrammatically by arrows having reference number 45 in Figure 3) emitted from the LEDs 17 is incident upon the reflective surface 19 and the light guide 9 since the LEDs 17 face towards both the reflector 5 and light guide 9. The light guide 9 reflects the light 45 incident upon it to the reflective surface 19 of the reflector 5. The reflective surface 19 redirects the light 45 out of the lighting unit. The arrangement of the LEDs 17, light guide 9 and reflector 5 helps to provide a more even beam of light emitted from the lighting unit 1, said beam having reduced glare and a crisp edge.

The arrows in Figure 4 indicate how heat is conducted away from the LEDs 17, in use, to the environment. Heat is conducted from the LEDs 17 to the PCB 15, from the PCB 15 to the heat pipe 7, and from the heat pipe 7 to the cooling fins 13 and reflector 5. Heat is transferred from the cooling fins 13 and reflector 5 to the environment mainly by convection and radiation. The thermal management provided by the heat pipe 7 helps to keep the LEDs 17 cool in operation thereby increasing the useful lifetime of the LEDs and improving the performance of the lighting unit 1. The heat pipe 7 provides a very effective and efficient thermal management system.

It will be appreciated by the skilled person that modifications can be made to the above-mentioned embodiment that fall within the scope of the invention, for example the light guide 9 may take any suitable form guiding light from the LEDs 17 to the reflector 5. For example, the light guide can have a curved outer surface and/or a single substantially frusto-conical structure.

The heat pipe can include a reflective surface to enable it to act as a light guide. This can be as an alternative to, or in addition to, the current light guide.

The lighting unit can be arranged into a type of light other than a downlight, for example a lighting unit that is arranged for mounting in or on a wall, or a free standing lighting unit.

It is envisaged that is some applications, it may be possible to use a different type of elongate (columnar) support member from a heat pipe, for example in low power LED lighting applications where thermal management issues are less of a concern.

The embodiment above is arranged in the form of a downlight, where light exits the lighting unit is a downwards orientation. It is possible to have the light in a different orientation, for example such that light is emitted in an upwards direction. It is to be noted that the orientation of the lighting unit does not prevent the heat pipe from operating.

Claims

CLAIMS1. A lighting unit including at least one solid state lighting device, a reflector, and a phase change heat transfer device, wherein the solid state lighting device is mounted in thermal contact with the phase change heat transfer device in a manner such that light emitted from the solid state lighting device, in use, is oriented towards the reflector, said reflector being arranged to reflect the light incident upon it outwardly from the lighting unit, and wherein heat generated, in use, by the solid state lighting device is transferred from the solid state lighting device to the phase change heat transfer device by conduction.
2. A lighting unit according to claim 1, wherein the reflector has a central axis, the phase change heat transfer device has a longitudinal axis, and the longitudinal axis of the phase change heat transfer device is substantially co-axial with the central axis of the reflector.
3. A lighting unit according to claim I or 2, wherein the reflector includes a substantially dish-shaped reflective surface.
4. A lighting unit according to any one of the preceding claims, wherein the reflector includes an aperture, and the phase change heat transfer device protrudes through the aperture.
5. A lighting unit according to any one of the preceding claims, wherein the reflector includes a substantially continuous reflective surface around the phase change heat transfer device.
6. A lighting unit according to any one of the preceding claims, wherein the reflector includes a flange at its open end to increase its rigidity
7. A lighting unit according to any one of the preceding claims, wherein phase change heat transfer device is elongate, for example is in the form of a heat pipe.
8. A lighting unit according to any one of the preceding claims, wherein the heat pipe includes first and second ends, and the solid state lighting device is mounted on the heat pipe towards the first end.
9. A lighting unit according to any one of the preceding claims, including a plurality of cooling fins connected to the heat transfer device, said cooling fins being arranged to transfer heat from the phase change heat transfer device to the environment.
10. A lighting unit according to claim 9, wherein at least one of the fins, and preferably a plurality of the fins, includes a substantially annular disc.
11. A lighting unit according to claim 10, wherein at least one of the fins, and preferably a plurality of the fins, includes at least one finlet to increase the surface area of the fin.
12. A lighting unit according to any one of the preceding claims, including a light guide for mounting over at least a portion of the phase change heat transfer device, said light guide being arranged to reflect light emitted from the solid state light device, which is incident upon it, towards the reflector.
13. A lighting unit according to claim 12, wherein the light guide includes a substantially circular cross-section.
14. A lighting unit according to claim 13, wherein the light guide has a first end located adjacent the solid state lighting device and a second end located towards the reflector, wherein the diameter of the light guide at the first end is smaller than the diameter of the light guide towards the second end.
15. A lighting unit according to claim 12 or 13, wherein the light guide includes a tapered portion.
16. A lighting unit according to any one of the preceding claims, wherein the solid state lighting device includes a PCB.
17. A lighting unit according to claim 16, wherein the PCB is arranged substantially perpendicularly to the longitudinal axis of the phase change heat transfer device.
18. A lighting unit according to claim 16 or 17, wherein the solid state lighting device is attached to the phase change heat transfer device via the PCB.
19. A lighting unit according to claim 18, wherein the PCB is arranged substantially perpendicularly to the longitudinal axis of the phase change heat transfer device.
20. A lighting unit according to any one of the preceding claims, wherein the solid state lighting device includes at least one LED, wherein the or each LED faces towards at least one of the reflector and the light guide.
21. A lighting unit according to claim 20, wherein at least one LED includes a lens, said lens having a central axis, wherein the central axis of the lens is arranged substantially parallel to the central axis of the reflector.
22. A lighting unit according to claim 20 or 21, when dependent on claim 16, wherein the PCB includes a thermally conductive plate for conducting heat from thc LEDs to the phase change heat transfer device.
23. A lighting unit according to any one of the preceding claims, including an end cap mounted over the rear of the solid state lighting device.
24. A lighting unit according to any one of the preceding claims, wherein the lighting unit is mountable in an aperture formed in a partition, such as a wall or ceiling, and includes mounting means for holding the lighting unit within the aperture.
25. A lighting unit according to claim 24, wherein the lighting unit is in the form of a downlight.