WO2008087572A1

WO2008087572A1 - Light emitting device with improved heat transport

Info

Publication number: WO2008087572A1
Application number: PCT/IB2008/050091
Authority: WO
Inventors: Stefan M. Verbrugh; Leendert Van Der Tempel
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-01-19
Filing date: 2008-01-11
Publication date: 2008-07-24
Anticipated expiration: 2009-07-19
Also published as: TW200901507A; JP2010517213A

Abstract

A light emitting device is provided, comprising at least one light emitting diode (2), at least one heat conductive substrate (3) to transport heat away from said at least one light emitting diode (2), and a light guide (4) being arranged to receive light emitted by said at least one light emitting diode (2). The side (5) of the light guide (4) that faces the heat conductive substrate (3) comprises at least one area portion (7), of which a major area- percentage is separated from said substrate (3) by a gap (8) having a width of from 2 to 200 µm. By the provision of such a well-defined air gap, high thermal transport is achievable from the heat conductive substrate to the light guide, while allowing total internal reflection in the backside of the light guide.

Description

Light emitting device with improved heat transport

FIELD OF THE INVENTION

The present invention relates to a light emitting device comprising at least one light emitting diode, at least one heat conductive substrate and a light guide plate being arranged to receive light emitted by said at least one light emitting diode, and to a luminary comprising at least one such light emitting device.

BACKGROUND OF THE INVENTION

Semiconductor light-emitting devices comprising light emitting diodes (LEDs) are among the most efficient and robust light sources currently available.

Due to their small size, potential energy savings and long life, LEDs are rapidly evolving to become a viable light source for general lighting applications.

In order to produce general ambient lighting, LEDs may be combined with additional optics, such as a light guide which is able to convert and mix the light emitted from the LED into several beam distributions, thereby lighting up an interior space, for example an office. A light guide may be arranged on one or a plurality of LEDs.

However, in operation, high-power LEDs generate energy, both in terms of heat and in terms of light intensity. In order to operate the LED well, heat must be conducted away in an efficient manner. If the LED is not cooled effectively, the brightness, efficiency and lumen output of the LED diminish.

In conventional light emitting devices, the LEDs are mounted on a substrates which spreads the dissipating heat over a large area in order to overcome the problem of heat dissipation.

Such light emitting devices comprising one or a plurality of LEDs arranged on a substrate may be connected with a light guide for general lighting applications. The light is distributed in the light guide to obtain an desirably homogenous illumination over the whole surface of the light guide.

US 2005/0265029 Al, Epstein et al, describes an LED-array system comprising an LED-array and an optical sheet adapted to spread the light emitted by the LEDs over the essentially entire surface of the optical sheet. The cooling of the heat spreaders by convection and thermal radiation to ambient is however somewhat uncertain, since the surface that it is arranged on, such as a ceiling or wall, may be thermally insulated. To make the device more robust and generally applicable regardless the ceiling quality, it would be is advantageous to cool of the heat spreader also through the light guide, since the light guide is in contact with the surrounding atmosphere.

In US 2005/0265029 Al, an intermediate layer is however typically arranged between the LED-array and the optical sheet. This intermediate layer may hamper the heat transport from the LED-array to the optical sheet.

If the light guide is arranged in direct contact with the conductive substrate, heat conduction is possible. However, the optical efficiency of the system will then be reduced since the possibility to total internal reflection on the back surface of the light guide is removed.

A strong mechanical bonding between the light guide and the thermally conductive substrate unavoidably leads to absorption of light and/or disturbance of the quality of the light beam.

Thus, there is a need in the art to overcome this contradiction and to provide a light emitting device where the heat dissipated by the LEDs is efficiently transported away from the LEDs, and where a high light utilization efficiency is possible in the light guide.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partly overcome these problems, and to provide a light emitting device comprising at least one light emitting diode, a heat conductive substrate and light guide plate, where the light guide plate allows total internal reflection in the surfaces thereof while the device exhibits good heat transfer from the heat conductive substrate to the light guide plate.

Thus, in a first aspect, the present invention provides a light emitting device comprising at least one light emitting diode, at least one heat conductive substrate to transport heat away from said at least one light emitting diode, and a light guide being arranged to receive at least part of the light emitted by said at least one light emitting diode.

The light guide has a side that faces said at least one heat conductive substrate, which side comprises at least one area portion, of which a major area percentage is separated from said substrate by an gap having a width of from about 2 to about 200 μm. In a light emitting device of the present invention, light emitted by the LEDs enters into the light guide and propagates therein. During operation, the light emitting diodes dissipate heat, and this heat is to a large extent transported away from the LED by the heat conductive substrate.

In order to operate LEDs at temperatures at which they perform long times and efficiently, it is preferred that the heat energy is transported away from the device. One option is to use the light guide as a heat sink, being cooled down by the surrounding atmosphere. The light guide typically has a relatively large surface area towards the atmosphere, and is thus suitable as a means for transporting heat away from the device.

In order to transport heat from the heat conductive substrate to the light guide, good thermal transport properties there between are desired.

Meanwhile, and in order to obtain good light propagation, it is preferred that total internal reflection can occur in the surfaces of the light guide, including the back surface.

By arranging at least one portion of said side of the light guide such that a major area percentage of said at least one portion is located at a certain, well-defined distance from the heat conductive substrate, good heat transfer is obtained from the heat conductive substrate to the light guide, while enabling total internal reflection (TIR) in said portion(s).

The distance between the light guide and the substrate must be substantially above the wavelength of the light in order to allow total internal reflection, and is typically about 2 μm or more. If the distance is smaller, total internal reflection will not occur, leading to a drastically impaired light propagation in the light guide due to absorption of light. At the same time, the distance should be small in order to maintain a good heat transfer from the light guide to the heat conductive substrate, and should typically not exceed about 200 μm.

In embodiments of the present invention, said at least one area portion, which is separated from said heat conductive substrate by said gap, is essentially parallel to said heat conductive substrate. Hence, a spatially extending area-portion of said side of the light guide plate is arranged at the prescribed distance from the heat conductive substrate, allowing efficient heat transfer from the substrate while also allowing total internal reflection at in said at least one area portion.

In embodiments of the present invention, at least 90 area-%, for example more than 95, such as more than 99 area-% of said at least one area portion is separated from said substrate by said gap. In order to utilize as much as possible of the back surface for total internal reflection and simultaneous good heat transfer, a high area-percentage of the area portions parallel to the substrate should be located at the above defined distance from the substrate.

For increasing the heat transfer from the light guide plate to the heat conductive substrate, the width of the gap should be as small as possible, while being relatively simple to obtain, such as from 5 to 25 μm.

In embodiments of the present invention, the area portions being parallel to said heat conductive substrate may constitute a major area-percentage of said back side.

The gap typically comprises a gas to enable conduction of heat from the heat conductive substrate to the light guide plate over the gap. The gas is typically that of the ambient atmosphere, e.g. air, or may be another gas or gas mixture especially selected for the circumstances.

In order to utilize as much as possible of the back surface for total internal reflection and simultaneous good heat transfer, the area portions of the back side of the light guide that fulfills the above requirements regarding the width of the air gap should be large, such as constituting a major portion of the back side, such as at least 50 area-%, more than 75 area-% or more than 90 area%.

In embodiments of the present invention, at least one spacer element is arranged between said substrate and said area portions.

In order to stabilize the width of the air gap between the light guide and the substrate, it may be advantageous to arrange spacer elements there between. The surface of the spacer element is small and is included in the area-percentage above that does not fulfill the above requirements regarding the gap width.

In embodiments of the present invention, the thermal conductivity of the heat conductive substrate is preferably higher than that for the light guide.

In embodiments of the present invention, the heat conductive substrate may comprise a porous surface arranged against said area portions of said light guide plate. When the light guide plate is arranged against a porous substrate surface, the above- mentioned area portions of the backside are in contact with the substrate in a plurality of contact locations. However, each such contact location represents a very small area, such that the major area-percentage of the area portions of are distanced from the substrate material by an air-gap of the above prescribed width. The use of a porous heat conductive substrate is advantageous in that the prescribed distance between the light guide plate and the substrate may be formed by merely placing the light guide plate against the porous substrate surface, which facilitates the production of such devices.

Metal foam and metal wool are preferred porous materials for use in the heat conductive material due to the desired combination of high porosity and high thermal conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention.

Figure 1 illustrates one embodiment of a light emitting device of the present invention.

Figure 2 illustrates another embodiment of a light emitting device of the present invention.

Figure 3 illustrates the thermal conductivity of an air gap at 5O⁰C as a function of the thickness of the air gap.

DETAILED DESCRIPTION

The present invention relates to a light emitting device comprising at least one light emitting diode, at least one heat conductive substrate and a light guide plate being arranged to receive light emitted by said at least one light emitting diode.

One embodiment of a light emitting device according to the present invention is illustrated in fig 1.

The light emitting device 1 according to this embodiment comprises a plurality of mutually spaced apart LEDs 2 emitting light into an optically transparent/translucent light guide plate 4. The light guide plate 4 has a backside 5 and an opposing front side 6, through which light, received into the light guide plate 4, is coupled out of the plate.

On the back side 5 of the light guide plate 4 is located a plurality of heat conductive substrates 3, which are arranged to conduct heat away from the light emitting diodes 2.

The backside 5 of the light guide plate 4 comprises area portions 7 which are essentially parallel to the substrate 3.

The area portions 7 of the backside 5 are separated from the substrate 3 by an air gap 8. Typically, the light emitting device 1 is arranged on or at some distance from a surface, such as a ceiling or wall 10.

The term "light guide", as used herein, refers to an element that receives light through a light-receiving surface and within which the light propagates towards a light output surface without significant transmission losses. In general, light guides operate on the principle of total internal reflection, whereby light traveling through the light guide is reflected at the surfaces of the light guide based on differences in the indices of refraction of the material of the light guide and the material immediately surrounding it, e.g. air, cladding, etc.

The material of the light guide is at least partly transmissive, e.g. translucent or even transparent, at least for light emitted by the light emitting diodes of the light emitting device.

Examples of suitable light guide material include, but are not limited to, glass, transparent ceramics and transparent polymers, such as poly(methylmethacrylate) (PMMA) or polycarbonate. When the light guide is made of a polymeric material, it is preferably formed by means of injection molding.

The light guide material should preferably have a refractive index being is substantially higher than the material, typically air, immediately surrounding it. Hence, a refractive index of above about 1,4 is generally suitable.

The light guide 4 is typically in the general shape of a plate with a front and a backside, where it is intended that the light should exit the light emitting device through the front side. The light may for example be coupled into the light guide plate through the backside of the light guide plate or through lateral surfaces of the light guide plate.

The shape of the light guide may vary, and many shapes are possible and known to those skilled in the art. The light guide typically comprises some kind of light extraction structures such that light propagating in the light guide eventually can be coupled out of the light guide, preferably at the front side thereof.

For example, the backside of the light guide may have a repetitive saw tooth shape whereas the front side is essentially flat. Light propagating within the light guide is alternately reflected on the front side and on the backside. Due to the saw tooth shape of the back side, the angle of incidence on the front side will vary for each reflection, and will eventually reach below the critical angle for total internal reflection, so that light will be coupled out of the light guide. In another example, the light guide comprises internal air slots formed at a certain angle to the front surface.

Several examples of such light extraction structures are known in the art. The exact realization of the light extraction structures in the light guide is however not essential for the present invention, and essentially all light guide designs known to those skilled in the art can be used in a light emitting device of the present invention.

The term "light emitting diode" (herein also abbreviated LED) as used herein refers to all types of light emitting diodes, including, but not limited to inorganic based LEDs, organic based LEDs (OLED) and polymeric based LEDs (polyLED).

Typically, the LEDs are adapted to emit light in the visible or near- visible wavelength range, from UV to IR-light.

Several approaches are possible for coupling light into the light guide plate, as is known in the art. For example, the light emitting diode may be merely placed at a light receiving surface of the light guide plate, and be allowed to emit light towards this light receiving surface. The light receiving surface may for example be the back side of the light guide or portions thereof, may be a lateral edge side connecting the back side and the front side, or may be the side walls of a recess in the light guide, for example in a opening through the substrate.

The light emitting diodes may be arranged at a distance from the light guide or may be at least partly located within the light guide, such as molded into the light guide.

Depending on the light guide design, it may be suitable to use top emissive or side emissive light emitting diodes.

The precise design or coupling light from the light emitting diodes into the light guide plate is however not essential for the present invention, and essentially all light guide designs known to those skilled in the art can be used in a light emitting device of the present invention.

The light emitting diodes are connected to a driving circuitry (not shown) for providing the LEDs with driving electricity.

During operation, the LEDs dissipate heat. Much of the heat is taken up by the heat conductive substrates (heat spreaders) 3 arranged in thermal contact with the LEDs. Typically, portions of the LED are in physical contact with the substrate, either directly or via solder bumps or the like.

At too high temperatures, the LEDs are damaged and emit less light. Hence, it is desired to transport the heat away from the LEDs. By arranging the light guide 4 in such that a good thermal conductivity is obtained between it and heat conductive substrate 3, the light guide 4 may act as a means for transporting heat away from the device while being cooled down by the surrounding atmosphere.

The heat conductive substrate typically comprises a material having a high thermal conductivity, such as, but not limited to, metallic materials, for example, copper, aluminum, steel etc, and alloys thereof, and other materials, such as plastics or ceramic materials having a high thermal conductivity.

Typically, the thermal conductivity of the heat spreader material should be significantly higher than the conductivity of the light guide material (and of the air). Transparent polymers usually have thermal conductivities around 0.2 W/mK and glass around 1 W/mK. Thus, the thermal conductivity of the heat spreader material should typically be above about 2, such as from about 2 to about 2500 W/mK, which applies to almost all metals and many ceramics.

The heat conductive substrate may be a separate element or may be a layer of a heat conductive material arranged on another carrier material.

As mentioned above, it is preferred that the light guide allows total internal reflection on both the front and the backside thereof. Hence, the backside 5 should not be in contact with the heat conductive substrate 3. On the other hand, the backside 5 should be close to the heat conductive substrate to allow good heat transport.

Hence, the backside 5 of the light guide 4 should comprise area portions 7 that are essentially parallel to the underlying heat conductive substrate, and where at least a major area-percentage of the area portions 7 is separated from the heat conductive substrate by an air gap. Where this gap is formed, total internal reflection in the light guide 4 is allowed while a good heat transport to the heat conductive substrate 3 is obtained. It is preferred that this area-percentage, in which the gap formed, is as high as possible. Hence, preferably at least 50, such as at least 90, for example at least 95 or at least 99 area-% of the parallel portions of the light guide should form the prescribed air gap towards the heat conductive substrate.

For maximum efficiency in terms of combined total internal reflection and heat transport, the area portions 7 that are essentially parallel to the heat conductive substrate should represent a major area-percentage of the backside 5 of the light guide.

Typically, the area portions 7 that are essentially parallel to the heat conductive substrates 3 represent at least about 50, such as at least 75, for example at least 90, area-% of the back side of the light guide. The area portions 7 are typically distributed over the back side 5 of the light guide 4. For example, such area portions 7 are located between two adjacent spaced apart light emitting diodes 2, 2' such that the heat transport to the light guide is distributed over the area of the back side 5.

In order to allow total internal reflection on the backside of the light guide, the air gap between the area portions 7 and the conductive substrate 3 should substantially exceed the wavelength of the light emitted by the light emitting diodes 2. Hence, the width of the air gap should exceed about 2 μm. In order to ensure the existence of the gap when taking into account manufacturing tolerances and the like, the width should preferably exceed about 5 μm.

In order to keep the heat transport from the heat conductive substrate to the light guide as high as possible, the width of the air gap should however not be too high, and should typically not exceed about 200 μm. With a lower width comes a better heat transport, and thus the width may be lower than about 200 μm, such as lower than about 100 μm, for example lower than about 25 μm.

Those skilled in the art should realize that the above used term "air gap" does not limit the application to the fact that the gap is filled with air. As will be appreciated, the gap between the light guide and the substrate is an open gap, which is filled with the gas of the actual atmosphere (such as air, but alternatively other gases/gas mixtures, such as nitrogen, helium, etc), or with any other gas or gas mixture which has the desired properties. Especially, the gas should be able to conduct heat from the heat conductive substrate to the light guide. For example, the conductivity of nitrogen and helium is in the order of five times higher than that of air.

The shape of the heat conductive substrate 3 is adapted to the shape of the backside 5 of the light guide 4 (or vice versa) such that the above-defined features regarding the air gap are fulfilled.

The shape of the backside 5 may for example be flat, concave, convex, or may have an irregular or regular repetitive structure, such as a saw tooth structure.

The heat conductive substrate may cover essentially the full backside 5 of the light guide or may alternatively cover parts of the conductive substrate. Especially, the heat conductive substrate may contain openings, for example such that the conductive lines to the LEDs can be guided through those openings.

Spacer elements 9 may be arranged between the heat conductive substrate 3 and the light guide 4 in order to stabilize the assembly and to maintain the prescribed air gap. The area footprint of the spacer elements 9 should be small in respect to the area portions 7, such that they essentially do not interfere with the total internal reflection. In general, the footprint of the spacer elements 9 is included in the minor area-percentage of the area portions 7 that does not keep within the prescribed distance from the heat conductive substrate.

The spacer elements 9 may be formed as protrusions from the light guide 4 and/or from the heat conductive substrate 3, or may be separate elements, such as separate small elements, such as spheres or wires.

The light guide 4 and the substrate 3 may be held together by a holding means, such as a spring (not shown), for example with the spacer elements 9 maintaining the air gap 8 between the light guide 4 and the substrates 3.

Another embodiment of a light emitting device according to the present invention is illustrated in figure 2.

In this embodiment, the at least one heat conductive substrate 23 is porous or at least has a porous surface facing the backside 5 of the light guide 4, and the conductive substrate 23 is arranged such that area portions 7 of the backside 5 of the light guide 4 are in direct contact to the conductive substrate 23.

Due to the porosity of the conductive substrate 23, a major area-percentage of the area portions 7 form an air gap 8 to the substrate 23, which air gap has a width of between 2 and 200 μm. By choosing a substrate material with a suitable porosity, at least 50, such as at least 90, for example at least 95 area-% of the area portions 27 will form an air gap 8 with the prescribed width to the heat conductive substrate, i.e. 2-200 μm, such as 5-25 μm.

Non- limiting examples of porous substrate material include metal foam, such as aluminum foam or metal wool. Metal foam is a cellular structure consisting of a solid metal - frequently aluminum - containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open- cell foam). The defining characteristic of metal foams is a very high porosity: typically, well over 80% of the volume consists of void spaces.

The graph in figure 3 describes the thermal conductance (heat transfer coefficient) of an air gap in W/m²K as a function of the width of the air gap in meters, calculated for a temperature of 50°C, which may represent a typical temperature in the air gap of a light emitting diode of the present invention, since this temperature is above normal ambient temperature, but well below the maximum use temperature of about 70⁰C for polymethylmethacrylate), one of the preferred light guide materials. As can be seen, the conductivity drastically decreases with increasing air gap width. As a comparison, a typical LED ("Luxeon LED", Lumileds) has a conductance of 100 W/m²K, which here corresponds to an air gap with a width of 200 μm.

Hence, when the air gap between the light guide and the heat conductive substrate is below 200 μm, the conductance of in this air gap exceeds the conductance through the LED (for example when the LED forms a contact from the light guide to the substrate.)

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. For example, luminary based on a the light emitting device of the present invention may comprise one or more, i.e. a plurality, of set(s) of light emitting devices of the present invention, i.e. comprising a plurality of separate light guides. For example, the separate light guides may be tiled such as to collectively cover a larger area than the area of a single light guide. Partitioning the device into a plurality of light guide may further prove useful, for example in order to maintain flatness and prevent warpage of the device due to expansion, swell and shrinkage differences between metal and polymer.

Light emitting devices of the present application may be used as luminaries for general lighting, such as in office spaces, homes and vehicles. Further, they may be used in or as back lighting units for transmissive or transflective display devices, such as LCD display devices.

Claims

CLAIMS:

1. A light emitting device (1) comprising at least one light emitting diode (2), at least one heat conductive substrate (3) to transport heat away from said at least one light emitting diode (2), and a light guide (4) being arranged to receive light emitted by said at least one light emitting diode (2) , wherein said light guide (4) has a side (5) facing said at least one substrate (3), which side (5) comprises at least one area portion (7) of which a major area percentage is separated from said heat conductive substrate (3) by a gap (8) having a width of from 2 to 200 μm.

2. A light emitting device according to claim 1, wherein said at least one area portion (7) is parallel to said at least one heat conductive substrate (3).

3. A light emitting device according to any of the preceding claims, wherein at least 90 area-% of said at least one area portion (7) is separated from said heat conductive substrate (3) by said gap (8).

4. A light emitting device according to any of the preceding claims, wherein the width of said gap (8) is from 5 to 25 μm.

5. A light emitting device according to any of the preceding claims, wherein said gap (8) comprises a gas.

6. A light emitting device according to any of the preceding claims, wherein said area portions (7) being parallel to said heat conductive substrate (3) constitutes a major area- percentage of said side (5).

7. A light emitting device according to any of the preceding claims, wherein at least one spacer element (9) is arranged between said heat conductive substrate (3) and said area portions (7).

8. A light emitting device according to any of the preceding claims wherein said heat conductive substrate (3) comprises a material having a thermal conductivity higher than the thermal conductivity of said light guide.

9. A light emitting device according to any of the preceding claims, wherein said heat conductive substrate (23) comprises a porous surface arranged against said area portions (7) of said light guide plate.

10. A light emitting device according to claim 7, wherein said substrate (23) comprises metal foam or metal wool.

11. A light emitting device according to any of the preceding claims, comprising at least two spaced apart light emitting diodes (2), and wherein one of said at least one area portion (7) is located between two adjacent such light emitting diodes.

12. A luminary comprising at least one light emitting device according to any of the preceding claims.