WO2014191251A1

WO2014191251A1 - Lighting device and method of manufacturing the same

Info

Publication number: WO2014191251A1
Application number: PCT/EP2014/060281
Authority: WO
Inventors: Susanne Lina GOLDBACH; Paul Dijkstra; Lilianne Elisabeth Maria VAN LIEBERGEN; Boudewijn Ruben DE JONG; Martin Hillebrand Blees
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-05-28
Filing date: 2014-05-20
Publication date: 2014-12-04

Abstract

A lighting device (100, 200, 500, 501, 700, 705) is provided comprising a plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) arranged on top surfaces of a plurality of substrates (550, 650, 750). The substrates are at least partly transparent for visible light and form a major part of a housing (110, 210) which defines a cavity (114, 214, 514). The top surfaces of the plurality of substrates face the cavity and each of the plurality of solid state light sources is positioned such that, in operation, each of the plurality of solid state light sources emits light in a direction away from the top surfaces of the plurality of substrates.

Description

Lighting device and method of manufacturing the same

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting devices comprising solid state light sources. The present invention also relates to methods of manufacturing such a lighting device.

BACKGROUND OF THE INVENTION

Lighting devices comprising solid state light sources, such as light emitting diodes (LEDs), OLEDs, and laser diodes such as a vertical-cavity surface-emitting laser (VCSEL), are known in the art. Lighting devices, such as light bulbs, that are equipped with LEDs should provide a light distribution and lumen output that is at least comparable to that of the traditional lighting devices and the cost level should not be too high compared to the traditional lighting devices.

US2012/0314409A1 discloses a semiconductor-based tube-type lighting apparatus which comprises an elongated light-transmitting tube, a linear slit formed on the tube in a longitudinal direction and at least one bar-shaped optical semiconductor module secured to the light-transmitting tube with edges of the slit fitted into side surfaces of the bar- shaped optical semiconductor module. The bar-shaped optical semiconductor module includes a heat sink, a Printed Circuit Board (PCB) attached to the heat sink and an array of semiconductor optical devices arranged on the PCB.

SUMMARY OF THE INVENTION

The present invention has been made with respect to the above considerations. It is an object of the present invention to provide a lighting device comprising solid state light emitting devices and a method of manufacturing such a lighting device, which is cost- effective and which provides for an optimum light distribution.

This and other objects are achieved by providing a lighting device and a method having the features defined in the independent claims. Preferred embodiments are defined in the dependent claims. Hence, according to a first aspect of the invention, there is provided a lighting device comprising a plurality of solid state light sources arranged on top surfaces of a plurality of substrates. The substrates are at least partly transparent for visible light and form a major part of a housing which defines a cavity. The top surfaces of the plurality of substrates face the cavity and each of the plurality of solid state light sources is positioned such that, in operation, each of the plurality of solid state light sources emits light in a direction away from the top surfaces of the plurality of substrates.

Thus according to the invention a lighting device is provided in which the solid state light sources emit light away from the top surface of the substrates and, hence, towards the cavity, or the inside of the housing, in a direction substantially towards a top surface that is opposite to the light emitting solid state light source. Light emitting surfaces of the solid state light sources are facing away from the top surfaces of the substrates. This provides for a greater flexibility in providing an optimum light distribution because the light emitted by the solid state light sources travels a relatively large distance before it exits the housing which will lead to a less spotty appearance in which the light emitting surfaces of the individual solid state light sources are less clearly distinguished. The emitted light does not exit the housing via the top surface on which it is arranged, but travels away from the top surface and hence will exit the housing at a further side of the housing, for example at a surface of the housing which is substantially opposite to the surface where the light is emitted. In this way the light spreads out before it exits the housing which results in a less spotty appearance and hence an improved light distribution.

The substrates are at least partly transparent for visible light. In operation the light emitted by solid state light sources will be less obstructed by the substrates which form the major part of the housing. In order to provide an optimum light distribution at least a part of the light is transferred through the housing, and therefore at least a part of the housing should be transparent for light. By choosing the appropriate material for the substrates, that form the major part of the housing, the light distribution of the lighting device can even be improved further, for example by selecting a material with a suitable scattering properties while keeping the light transmission capability of the housing close to 100%. For example alumina is a suitable material with a light transmission capability between 95% and 98%. In general there will be a trade-off defining the level of transparency of the substrates, and thus a major part of the housing, because on the one hand a more diffuse, i.e. less clear, substrate contributes to a more uniform light distribution with a reduced spottiness, but on the other hand a less diffuse, i.e. more clear or transparent substrate enhances the efficiency of the lighting device.

Furthermore, depending on certain properties of the solid state light sources, a certain degree of mixing of light inside the cavity of the housing can be achieved before the light exits the housing. In an embodiment the substrates are provided with a layer of a wavelength converting material, such as phosphor, in which case for example blue LEDs may be used wherein the wavelength converting material converts the blue light to white light.

A major, or main, part of the housing is formed by the plurality of substrates, which housing defines a cavity. In other words, the substrates define a main part of the housing and also define the cavity. The top surfaces of the plurality of substrates face the cavity and thus the substrates surround the cavity. The major, or main, part should be understood as being the largest part, for example more than 50%, of the housing up to the entire housing that is formed by the substrates. This advantageously combines two functions for the substrates as being a major part of the housing of the lighting device and the carrier of the solid state light sources. In embodiments the major, or main, part of the housing that is formed by the plurality of substrates are side walls, or side surfaces, of the housing. For example, three rigid and flat substrates define side walls of a triangular shaped housing, and six rigid and flat substrates define side walls of a hexagonal shaped housing.

The solid state light source may, for example, be light emitting diodes (LEDs), but also other solid state light sources may be used, such as laser diodes or OLEDs.

The plurality of solid state light sources are arranged on the top surfaces of the substrates that form the major part of the housing. This provides for a simple and cheap solution because less components are required to position the solid state light sources at the right location than according to the prior art lighting devices. No separate substrate or PCB (Printed Circuit Board) is required and the solid state light sources are simply mounted on the substrates or PCB, for example through a printed electronics technology, wherein the substrates and PCB also function as the housing. This enables a low cost technology to be used for manufacturing such a lighting device, because the printed electronics technology is a relatively cheap mass manufacturing method. Furthermore, in this way the invention provides for a reduction of material required for a lighting device equipped with solid state light sources. Advantageously standard components may be used in which a standard sized substrate is used on which one or more solid state light source(s) is/are placed and which can be used for several types of lighting devices, for example by varying the number of solid state light sources or the type of solid state light sources on the substrate. Preferably the size of the substrate and/or the number of substrates used is made suitable for a certain type, and size, of lighting device, for example a candle or a bulb type of lighting device, and other examples are a tube (TLED) and CFL retrofit (PLED). Furthermore, the use of a substrate, or PCB, allows for the use of a relatively cheap and standard manufacturing method and standard equipment to manufacture the lighting devices.

According to an embodiment the substrates are made of a thermally conductive material. A substrate or housing material that is thermally conductive provides for a relatively efficient transport of heat generated by the solid state light sources away via the housing, formed for a major part by the substrates, on which the solid state light sources are positioned. In this embodiment the housing thus serves as a heat sink or heat spreader that transports the heat generated by the solid state light source away from the solid state light source. This avoids the use of a separate heat sink. In embodiments a connection between the solid state light source and the top surface of the substrates is also thermally conductive.

In an embodiment of the lighting device according to the invention the light device further comprises a thermally conductive pattern on the top surfaces of the plurality of substrates which is thermally connected to at least one of the plurality the solid state light sources. This provides for a spreading of the heat that is generated by the solid state light sources. The thermally conductive pattern for example comprises relatively short thermally conductive tracks in a star pattern around each solid state light source.

In an embodiment the plurality of substrates comprises fins protruding from the housing. This further enhances the thermal performance of the housing and the lighting device, because the fins provide for additional cooling. The fins may for example be formed by edges of the substrates that protrude from the housing.

In a further embodiment the plurality of substrates comprises at least three substrates. The substrates are easy to handle and the use of at least three substrates provides for a greater freedom in providing for an optimum lighting distribution in several types of lighting devices. Preferably the at least three substrates are flat. Furthermore, the at least three substrates are preferably mechanically connected, and preferably also thermally connected for an efficient transport of heat generated by the solid state light sources.

In another embodiment, at least one substrate is flexible. This allows for shaping a part of the housing with the flexible substrate, which in this embodiment is made from a flexible material, and for more design freedom in providing for an optimum lighting distribution. In an embodiment a plurality of flexible substrates forms the housing and thus defines the major, or main, part or the whole shape of the housing.

In an embodiment, the lighting device further comprises an optical element positioned in the cavity of the housing. The optical element for example provides for a redistribution of the light in the cavity and thereby may improve the light distribution of the light emitted from the housing. For example a reflector may be placed in the cavity, for example at the bottom of the housing. In a further example the reflector is cone shaped pointing in a direction towards the cavity, or pointing in a direction away from the cavity.

In an embodiment the solid state light sources are positioned such that, in operation, the light emitted by the plurality of solid state light sources provides for a predefined light distribution. The light distribution of the lighting device depends, amongst others, on the number of solid state light sources, but also on the positioning of the solid state light sources in the housing and the optical properties of the material of the housing through which the light exits. The positioning of a solid state light source for example comprises the physical location of the solid state light source in the housing and/or the mutual distance between neighboring solid state light sources and/or the angle of the main direction of the emitted light with respect to the top surface of the substrate. A predefined light distribution is a light distribution that is required depending on the application of the lighting device. For example, in case the lighting device represents a bulb, the lighting distribution that is required should fulfill a certain standard, for example energy star, such that it resembles the light distribution of a standard incandescent light bulb. This predefined lighting distribution is then achieved by an appropriate positioning of the solid state light sources in the housing which can be calculated by, for example, ray trace simulation software.

In an embodiment a light emitting surface of at least one of the plurality of solid state light sources is not positioned in parallel with respect to the top surface of the substrates. In this way the light distribution may be optimized further by providing for an inclination of the light emitting surface of the solid state light sources with respect to the top surface of the substrate on which the solid state light source is arranged. Thus, the light emitting surface of the solid state light sources, or of at least one or more of the solid state light sources, is not placed in parallel with a, for example side, surface of the housing that is formed by a substrate but will have a specific angle with this surface of the housing such that the light is emitted in a specific direction different from a direction perpendicular to the local surface of the housing. In an embodiment of the lighting device according to the invention electrical tracks are provided on the top surface of the substrates. The electrical tracks may provide for an electrical connection from the solid state light sources to, for example, a driver. In a further embodiment an electrical connection is provided between the electrical tracks of the plurality of substrates thereby providing an electrical series connection between the plurality of solid state light sources.

According to another aspect of the present invention a lamp is provided comprising a lighting device according to the invention and a driver.

According to another aspect of the present invention, there is provided a method of manufacturing a lighting device comprising the steps of:

positioning a plurality of solid state light sources on top surfaces of a plurality of substrates which are at least partly transparent for visible light, such that, in operation, each of the plurality of solid state light sources emits light in a direction away from the top surfaces of the plurality of substrates;

- arranging the plurality of substrates such that a major part of a housing is formed which defines a cavity.

Advantages of the lighting device according to the invention are similarly applicable to the method according to the invention. An additional advantage is that a relatively simple and cheap method is provided in which the solid state light sources may be arranged with a mass manufacturing method, such as, for example, a printed electronics technology.

In an embodiment of the method according to the invention, the housing comprises at least three substrates further comprising the step of creating a single housing that defines the cavity by arranging the at least three substrates adjacent to each other after the step of positioning the plurality of solid state light sources. The method is further improved by providing the housing in three or more separate parts and positioning the solid state light sources on the top surfaces of the substrates which further simplifies the positioning of the solid state light sources, after which the three or more separate substrates are assembled resulting in a single housing that defines the cavity.

In an embodiment electrical tracks are for example printed and sintered/cured to fixate the tracks onto the top surface of the substrates. A solder paste, such as SAC (silver aluminum copper) material, is applied on which the solid state light sources are subsequently electrically bonded. Optionally electrically conducting glue, for example silver filled, may be used, or any other suitable way to connect the solid state light source to a surface. It is noted that the invention relates to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the lighting device are all combinable with the various embodiments described for the method of manufacturing a lighting device, as defined in accordance with the first aspect of the present invention.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

This 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.

Figures 1 , 2a and 2b are schematic cross-sectional views of lighting devices exemplifying general embodiments;

Figures 3 and 4 are schematic cross-sectional views of lighting devices according to embodiments of the invention;

Figure 5 shows an embodiment of a substrate lay-out according to an embodiment of the invention; and

Figures 6 and 7 show three-dimensional exploded views of lighting devices according to embodiments of the invention.

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

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

Figure 1 is a schematic cross-sectional view of a lighting device 100 according to a general embodiment comprising a housing 110, which has an inner surface 112 and which defines a cavity 114. The housing 110 is at least partly transparent for visible light and thus has a sufficiently high light transmission capability. The transparency of the housing 110 may, for example, range between 10% and 100% and should have a value such that the light that is generated by solid state light sources is visible from outside the housing and such that the lighting device does not lose too much light because of the transparency of the housing 110, i.e. the lighting device should be efficient. The solid state light sources, such as light emitting diodes (LEDs) 120, are attached on the inner surface of the housing 110. Each LED 120 has a light emitting window or surface 122 where the light that is generated by the LED 120 is emitted by the LED. The dotted arrows in Figure 1 indicate the light paths of the light that is emitted from the light emitting surface 122 of the LEDs 120. The light is emitted in a direction away from the closest, or adjacent, inner surface 112 of the housing 110 into the cavity 114 and spreads out before it exits the housing 110. This spreading out of the light inside the cavity 114 of the housing 110 reduces the spotty appearance as compared to lighting devices in which the LEDs are located such that the light emitting surfaces of the LEDs face and are adjacent to the inner surface of the housing. The light emitted by these LEDs will not traverse the cavity but will travel a shortest distance to the closest inner surface of the housing and will exit the housing at a location which is closest to the LED. The light emitted by the LEDs 120 travels through, and crosses, the cavity to the inner surface of the housing 110 that is approximately opposite in relation to the position of the LED 120. Thus, an improved light distribution is achieved with a reduced spotty appearance and resembling the light distribution of an incandescent bulb as much as possible. In

embodiments the LEDs 120 are distributed on the inner surface of the housing. In this way, for example, light emitted by a first LED may exit the housing close to a second LED that is substantially opposite to the position of the first LED, and light emitted by the second LED may exit the housing close to the first LED.

The light distribution can be optimized by positioning the LEDs 120 on the inner surface of the housing according to a specific pattern. The positioning of the LED for example comprises the physical location of the LED in the housing, the mutual distance between neighboring LEDs and/or the angle of the main direction of the emitted light with respect to the adjacent inner surface of the housing. A predefined light distribution is a light distribution generated by a lighting device that is required depending on the application of the lighting device. For example, in case the lighting device represents a bulb, the lighting distribution that is required should fulfill a certain standard, for example energy star, such that it resembles the light distribution of a standard incandescent light bulb. This predefined lighting distribution is then achieved by an appropriate positioning of the LEDs in the housing which can be calculated by, for example, ray trace simulation software.

Figure 2a shows a schematic cross-sectional view of a lighting device 200 according to an embodiment in which the invention is exemplified with a retrofit light bulb. As is shown in Figure 2a, the lighting device 200 comprises a housing 210 in the shape of a retrofit light bulb, which has an inner surface 212 and which defines a cavity 214. Similar to the embodiment exemplified in Figure 1, also this housing 210 is at least partly transparent for visible light. On the inner surface 212 a plurality of LEDs 220A, 220B are positioned such that the light distribution generated by the lighting device 200 is optimized and fulfills, for example, the energy star standard as much as possible. Each of the LEDs 220A, 220B have a light emitting surface that faces the cavity 214 and thus light is emitted by each LED 220A, 220B in a direction away from the nearest inner surface of the housing 210.

Figure 2b shows a projected cross-sectional view of planes according to A-A' and B-B', as exemplified in Figure 2a, and the largest cross-section of the housing 210. The circles represent different cross-sectional planes of the housing 210. The outer circle represents the cross-section of the housing 210 with the largest diameter and the inner, dotted, circle represents the cross-sections along A-A' and B-B', as shown in Figure 2a, on which the LEDs 220A, 220B are positioned. As can be seen from Figure 2b, the, in this case three, LEDs 220A are positioned equidistant on a circle on the inner surface of the housing at the cross-sectional plane A-A' and the, in this case three, LEDs 220B are positioned equidistant on a circle on the inner surface of the housing 210 at the cross-sectional plane B-B'.

Furthermore, when projected onto a common plane in Figure 2b, each LED 220A on the plane A-A' is positioned equidistant on a circle from a closest LED 220B on the plane B-B'. For example, the LEDs 220A are positioned on a circle at 0° degrees, 120° and 240°, and the LEDs 220B are positioned on a circle at 60°, 180° and 300°. In this example the angular distance on a circle of the projected planes A-A' and B-B' between each LED 220A, 220B is 60°. Also other angular distances and number of LEDs are envisaged to produce a required lighting distribution.

Thus in this example, half of the LEDs 220 A, 220B are positioned equidistant on the inner surface of the housing in the plane A-A', and the other half of the LEDs 220A, 220B are positioned equidistant on the inner surface of the housing in the second plane B-B', the first plane A-A' being parallel to but at a different location than the second plane B-B' and the LEDs 220A positioned on the first plane A-A' are positioned at locations different from the LEDs 220B positioned on the second plane B-B'. In general, a predefined lighting distribution can further be achieved by an inclination of a LED with respect to the inner surface of the housing where the LED is located (not shown), such as on the top surface of a substrate which forms a major part of the housing. In that case the light emitting surface of the LED is not positioned in parallel with the inner surface of the housing, but is tilted or inclined, thereby defining an angle with respect to the (local) inner surface of the housing. In that case the light is emitted by the LED in a direction which is not perpendicular to the inner surface of the housing but at an angle not equal to zero with respect to the perpendicular direction of the inner surface of the housing. In this way the light distribution can be optimized and tailored independent of the shape of the inner surface of the housing.

Other examples of retrofit type lamps in which the invention may be provided are retrofit candle type bulbs and retrofit TL tubes.

Figure 3 shows a schematic cross-sectional view of a lighting device 500 according to an embodiment of the invention in which LEDs 520 are arranged on, in this case, six substrates 550. Another amount of substrates may also be used, for example depending on the shape of the housing and/or the required lighting distribution and/or the desired light level and/or any other desired light property of the lighting device. The substrates may be flexible or rigid substrates and form side walls or surfaces of a housing of the lighting device 500 which housing defines a cavity 514. In other words, the substrates 550 surround and define the cavity 514. Furthermore, the substrates are at least partially transparent for visible light having a suitable light transmission capability. The LEDs 520 have a light emitting surface 522 facing the cavity 514. The housing in this example represents that of a retrofit light bulb, but it may also be a candle light bulb, a tube (TLED), CFL retrofit (PLED) or any other shape or form.

The substrates 550 may be separate substrates, thus not connected to each other mechanically, before the substrates are arranged to form a major or main part, in this embodiment the side walls or surfaces, of the housing. The arrangement of the substrates 550 to form the major part, in this case the side walls, of the housing may be done by any suitable mechanical connection, for example the substrates may be mechanically connected to each other and/or the substrates may be mechanically connected via a common mechanical fixation. The substrates 550 may, additionally or alternatively, be mechanically connected to each other via, for example, hinges or any kind of flexible connection before the substrates are arranged to form a major part of the housing, which allows a simple configuration of the six substrates to form the, in this case, side walls of the housing. In case the substrate 550 is flexible, then the shape of the flexible substrate defines at least a part of the shape of a surface of the housing so as to provide an improved mechanical, and optionally also thermal, fixation to the housing (not shown in this Figure).

Figure 4 shows a schematic cross-section view of a lighting device 501 according to an embodiment of the invention in which LEDs 520 are placed on one or more substrates 555 that are preferably at least partially transparent for visible light and which exemplifies another type of arrangement of the substrates to form a major part of the housing as compared to the embodiment shown in Figure 3. The substrates 555 in this embodiment are arranged in the lighting device in a star-shape form in which one edge of each substrate is a protruding edge of the housing. This substrate configuration results in that part of each substrate 555, i.e. the protruding edge, represents a fin 557 which functions as a heat sink thereby improving the thermal behavior of the lighting device. Also in this embodiment, just as in the embodiment shown in Figure 3, the housing represents that of a retrofit light bulb, but it may also be a candle light bulb, a tubular bulb (TLED), a CFL (PLED) bulb or any other shape or form.

Figure 5 shows an embodiment of a substrate layout with preferably transparent and rigid substrates 650 with a top surface on which LEDs 620 are provided. In this example one LED per substrate 650 is provided, but embodiments with more than one LED per substrate are also envisaged. The substrates 650 have rectangular shapes with long edges 655 and Figure 5 shows the arrangement of substrates before the, in this case six, substrates 650 are arranged to form a major part of the housing of a lighting device, for example the light bulb as is shown in Figure 3. Electrical wiring 660, for example metal tracks combined with wiring between the substrates 650, provides for a, in this case, series connection between the LEDs 620. The individual substrates 650 are subsequently assembled in a lamp, in this case by placing them upwards in a direction perpendicular to the plane of the substrate arrangement shown in Figure 5 resulting in a substrate arrangement in which the, in this case six, substrates are placed with the long edges 655 in parallel to each other and the light emitting surfaces of the LEDs 620 face the enclosure formed by the six substrates 650, and the substrates 650 function as side surfaces or walls of a housing.

The substrates as disclosed above may be made from glass, plastic or ceramic material. In embodiments the substrates as discussed above have good thermal properties such that they are able to function as an efficient heat conductor and heat spreader for spreading and conducting the heat produced by the LEDs. For example a substrate made of a ceramic material and with a size of 22mm x 62mm can be used to provide for a lumen output of 130 lumen, and applying six of these substrates in a retrofit lamp results in a total lumen output of 800 lumen.

Figure 6 shows a three-dimensional exploded view of a lighting device 700 according to an embodiment of the invention with a lamp foot 790 and a holder 780 for accommodating electronic circuits, such as a driver on a PCB. The lighting device 700 has in this case a hexagonal outer shape and comprises six rigid substrates 750 that are at least partly transparent for visible light. In this example the side walls of the housing are formed by the six substrates 750 that define the hexagonal shape of the housing. Other outer shapes, such as a square or triangular shape, are also envisaged with the appropriate amount and/or shape of substrates, for example four substrates for a square shaped housing and three substrates for a triangular shaped housing. On a top surface of each of the substrates a plurality of LEDs 720 is provided that are electrically interconnected through metal tracks 760. The LEDs 720 are positioned and orientated such that each LED 720 is arranged to direct light into a direction away from the part of the surface of the substrate 750 on which the LED 720 is positioned. In other words, the light emitting surface of each LED 720 faces the enclosure that is formed when the substrates 750 are arranged in the holder 780. The substrates 750 are mechanically fixated into the holder 780 via a suitable mechanical connection, for example a snap-fit. Additionally an electrical connection to the driver may be established by the mechanical assembly of the substrates into the holder 780. On top of the lighting device 700 a cap 775 is provided which is preferably made of a diffusive material, for example plastics, glass or ceramic. The housing is in this embodiment formed by the six substrates 750, the cap 775 and the holder 780 after the substrates are arranged in the holder 780. The top surfaces of the substrates 750 define the inner surfaces of the housing.

In this embodiment the lighting device is further equipped with a reflector 770 which redirects light emitted towards the holder 780 thereby providing for a tailored light distribution and furthermore reducing the loss of light and improving the efficiency of the lighting device 700. The reflector is for example cone shaped with the top of the cone pointing in a direction towards the holder 780 as shown in Figure 6. In another embodiment (not shown) the top of the cone is pointing into the direction of the cap 775 thereby providing for another light distribution, in combination with a pre-selected positioning of the LEDs 720 on the substrates 750.

Figure 7 shows a three-dimensional exploded view of a lighting device 705 according to an embodiment of the invention which is largely similar to the lighting device 700 shown in Figure 6 and which provides for an improved spreading of heat generated by the LEDs 720. The difference with the lighting device 700 of Figure 6 is that the lighting device 705 also comprises a thermally conductive pattern in the form of heat spreading tracks 725 provided on the substrate and thermally connected to the LEDs 720 and the substrates 750. The heat spreading tracks have, in this case, a star shape pattern and may be made of a material with a relatively high heat conductivity, for example metal. The heat spreading tracks provide for an improved spreading of the heat generated by the LEDs 720 on the substrate and therefore an improved thermal behavior and longer lifetime of the LEDs 720.

In a first stage of an example of a method of manufacturing a lighting device according to an embodiment of the invention, electrical tracks and pads are provided on a top surface of a plurality of substrates. The electrical tracks for interconnecting the LEDs can be provided by for example printing technologies such as screen printing, dispensing, ink jet printing of materials such as Ag or Cu on the top surface of the substrates, such as for example a glass or plastic substrate. Other suitable techniques or materials may also be used to provide for electrical tracks and pads for electrically interconnecting the LEDs and/or electrically connecting the LEDs to a power supply. Additional manufacturing steps such as sintering and curing may be applied if needed.

Thereafter the LEDs are placed on suitable locations on the top surface of the substrates. For example a well-known pick & place technique may be used for this purpose. Additional electrical components can also be placed on the top surface of the substrates, for example the driver can at least partly be integrated on the top surface of the substrates. At this stage, optionally electrical connectors, if required, can also be placed using the pick & place technique. If needed, reflow soldering can be applied to provide for a good electrical connection between the LEDs and other electrical components and the electrical tracks / pads.

A housing is manufactured by forming a major part of the housing from the plurality of substrates, for example by arranging the plurality of substrates adjacent to each other. The housing defines a cavity and the top surfaces of the substrates face the cavity. The substrates are mechanically fixated, for example via gluing, at this stage of the manufacturing process. If needed, the housing is then electrically connected to the driver, for example with pins of the connectors, and, if needed, the lamp-driver assembly is then connected to an end cap, for example by overmolding via injection molding or by any other suitable technique.

In an embodiment the substrates are for example PCBs or flexible substrates on which the LEDs are mounted. For example, three flat and rigid substrates may define a triangular shaped housing. In another example, a flexible substrate is formed such that it defines a cylindrical or other required shaped housing. In general the substrates used for the invention may be made for example of glass, which is a relatively cheap material with relatively good thermal properties and which can also be a flexible substrate with a thickness of approximately 100 micrometers, or a plastic material. Printing electronics on the substrates may for example comprise the printing of silver tracks printed on a glass substrate which function as electrical connections and optionally also as thermally conductive tracks for spreading out the heat generated by the solid state light source.

The more diffuse, i.e. less clear, the substrates are, the more the housing (formed by the substrates) contributes to a uniform light distribution with a reduced spottiness. However, more clear or highly transparent substrates enhance the efficiency of the lighting device. In general there will be a trade-off defining the level of transparency of the substrates, and thus a major part of the housing.

In an embodiment the substrates are provided with a layer of a wavelength converting material, such as phosphor, in which case for example blue LEDs may be used wherein the wavelength converting material converts the blue light to white light.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, the shape of the housing may be any shape suitable for the intended application, for example candle-like, tubular, etc. and the material of the housing may have any suitable optical properties, such as scattering, light transmission, etc. For example, the solid state light sources may be any type that is suitable for the intended application, for example light emitting diodes, laser diodes, OLEDs, etc. For example, any optical element may be combined with the lighting device to obtain a specific lighting distribution, such as a reflector, a diffusive element, a lens, etc.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A lighting device (100, 200, 500, 501, 700, 705) comprising a plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) arranged on top surfaces of a plurality of substrates (550, 650, 750), the substrates being at least partly transparent for visible light, wherein the substrates (550, 650, 750) form a major part of a housing (110, 210) which defines a cavity (114, 214, 514) and the top surfaces of the plurality of substrates facing the cavity (514), and wherein each of the plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) is positioned such that, in operation, each of the plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) emits light in a direction away from the top surfaces of the plurality of substrates (550, 650, 750).

2. The lighting device as claimed in claim 1, wherein the substrates (550, 650, 750) form side walls of the housing.

3. The lighting device as claimed in claim 1 or 2, wherein the substrates (550, 650, 750) are made of a thermally conductive material.

4. The lighting device as claimed in claim 1, 2 or 3, further comprising a thermally conductive pattern (725) on the top surfaces of the plurality of substrates (550, 650, 750) which is thermally connected to at least one of the plurality of solid state light sources (520, 620, 720).

5. The lighting device as claimed in claim 1, wherein the plurality of substrates (550, 650, 750) comprises fins protruding from the housing.

6. The lighting device as claimed in any of the previous claims, wherein the substrates (550, 650, 750) are rigid.

7. The lighting device as claimed in any of the previous claims, wherein at least one substrate of the plurality of substrates is flexible.

8. The lighting device as claimed in any of the previous claims, further comprising an optical element (770) positioned in the cavity (114, 214, 514).

9. The lighting device as claimed in any of the previous claims, wherein a light emitting surface of at least one of the plurality of LEDs (120, 220A, 220B, 520, 620, 720) is not positioned in parallel with respect to the top surface of the plurality of substrates (550, 650, 750).

10. The lighting device as claimed in any of the previous claims, wherein electrical tracks (760) are provided on the top surfaces of the plurality of substrates (550, 650, 750).

11. The lighting device as claimed in claim 10, further comprising an electrical connection between the electrical tracks (760) of the plurality of substrates (550, 650, 750) thereby providing an electrical series connection between the plurality of solid state light sources (520, 620, 720).

12. The lighting device as claimed in any of the previous claims, wherein the substrates (550, 650, 750) are mechanically interconnected via a flexible connection.

13. A lamp comprising a lighting device according to any of the previous claims and a driver.

14. A method of manufacturing a lighting device comprising the steps of:

positioning a plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) on top surfaces of a plurality of substrates (550, 650, 750) which are at least partly transparent for visible light, such that, in operation, each of the plurality of solid state light sources (120, 220A, 220B, 520, 620, 720) emits light in a direction away from the top surfaces of the plurality of substrates (550, 650, 750);

arranging the plurality of substrates (550, 650, 750) such that a major part of a housing (110, 210) is formed which housing defines a cavity (114, 214, 514).

15. The method as claimed in claim 14, wherein the housing comprises at least three substrates further comprising the step of creating a single housing that defines the cavity by arranging the at least three substrates adjacent to each other after the step of positioning the plurality of solid state light sources.