The present invention relates to a light-emitting device comprising a light source for emitting light of at least a first wavelength range; a light guide; a plurality of outcoupling elements for outcoupling light from the light guide; a reflective member arranged to reflect light that is outcoupled from the light guide; and a wavelength converting member comprising a wavelength converting material.
Light-emitting diode (LED) based lighting devices are increasingly used for a wide variety of lighting applications. LEDs offer advantages over traditional light sources such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage, high purity of spectral colors and fast modulation of lumen output. However, one issue with LED lighting is the provision of “warm” white light. LEDs with high lumen efficacy (˜75 1 m/watt) available today produce light with a high color temperature (˜6000 K) and are thus perceived as “cold” white. For most general illumination applications a color temperature of 3000 K or less is preferred. In addition, the light should have a good color rendering index.
Low color temperature with a good color rendering index can be accomplished by means of phosphor in combination with illumination of a LED. Conventionally, the phosphor is embedded in glue that is directly attached to the LED chip. However, in such a solution the phosphor is exposed to the heat generated by the LED and to the light flux at the same time. As a result, very often this type of LED and phosphor solution does not meet the lifetime requirements necessary.
US 2007/0086184 A1 discloses an illumination system that includes one or more light sources that produce primary light, a light-mixing zone that homogenizes the primary light, a wavelength converting layer that converts the primary light to a secondary light, and a light-transmitting zone that receives the secondary light and transmits the secondary light. However, the wavelength converting layer of this system risks being overheated due to the generation of heat by the wavelength conversion event, resulting in reduced wavelength conversion efficiency (a phenomenon known as thermal quenching). Thus, there exists a need in the art for improved light-emitting devices.
It is an object of the present invention to provide an improved light-emitting device. A particular object is to provide a light-emitting device that is particularly suitable for use in a LED based lighting arrangement, which is efficient and which allows for efficiency and tuning of the color, color temperature and/or color rendering index (CRI) of the emitted light.
In one aspect, the invention relates to a light-emitting device comprising:
a light source for emitting light of a first wavelength range;
a light guide having a light receiving surface for receiving at least part of said light emitted by the light source, a front surface and a rear surface, for guiding light of said first wavelength range by total internal reflection at said front surface and said rear surface;
a plurality of outcoupling elements for outcoupling light from the light guide such that at least part of the light that is outcoupled by the outcoupling elements exits the light guide through said rear surface;
a reflective member arranged in rear of said light guide to reflect light that is outcoupled from the light guide; and
a wavelength converting member comprising a wavelength converting material arranged outside the light guide to convert light of said first wavelength range to light of a second wavelength range.
The light-emitting device according to the invention benefits from the advantages of having the wavelength converting material arranged at a distance from the light source; for example, when using a plurality of LEDs for a light source, the light from several LEDs may be mixed before reaching the wavelength converting material, so that differences in emission characteristics between individual LEDs are averaged out, leading to no visible artifacts. Futhermore, the light-emitting device according to the invention has high lumen efficacy, since there is little chance a ray of light will be lost by being backscattered towards the LED die, and it also enables high light recycling efficiency, since wavelength converted light emitted in the “wrong” direction may be reflected in the direction of an observer.
Furthermore, arranging the wavelength converting material outside the light guide allows efficient cooling of the wavelength converting material, thus avoiding thermal quenching of the wavelength converting material.
Advantageously, in the light-emitting device according to the invention, the color, color temperature and/or CRI may be tuned by modifying the wavelength converting member (e.g. relative coverage of wavelength converting material). As a result, “warm” or “cold” white light may be obtained as desired. In most general lighting applications, a “warm” white light (that is, white light having a low color temperature) is desirable. Furthermore, by adapting the coverage of the outcoupling elements, a desired distribution of light from the light guide may be obtained.
In order to further improve the light recycling efficiency and mixing and/or distribution of light, the reflective member of the light-emitting device may be diffusive.
The wavelength converting member and the reflective member may be provided on different sides of the light guide, so as to provide good mixing and distribution of light. For example, the wavelength converting member may be provided in front of the light guide. Alternatively, the wavelength converting material may be arranged in the path of light from the light guide to the reflective member, typically between the light guide and the reflective member. In embodiments of the invention the wavelength converting material may be arranged on the reflective member; thus, the wavelength converting material can be efficiently cooled using a heat sink arranged in thermal contact with the wavelength converting material via the reflective member without the heat sink blocking the path of light to an observer. For example, a heat sink may be arranged on the rear side of the reflective member. Also, arranging the wavelength converting material on the reflective member saves space, and avoids any unwanted Fresnel reflections caused by a transparent substrate for supporting a wavelength converting material through which light is to be transmitted.
Furthermore, the wavelength converting member may comprise a plurality of discrete domains comprising wavelength converting material. Advantageously, the relative coverage (%) by the wavelength converting material may then be easily adapted during production by adapting the density of the domains and/or their size(s). Thus, a desired color and/or color temperature and/or color rendering index may be obtained. Also, domains comprising different types of wavelength converting materials may be easily produced.
Alternatively or additionally to said discrete domains comprising wavelength converting material, said wavelength converting member may comprise a continuous layer comprising a wavelength converting material. A continuous layer may provide improved uniformity of coverage of the wavelength converting material.
Furthermore, said plurality of outcoupling elements may be provided on an outer surface of the light guide. Typically, the outcoupling elements may be provided on said front surface of the light guide, or, alternatively, on said rear surface of the light guide. The outcoupling elements may comprise a scattering material. Using a scattering material for the outcoupling elements is cheap and, since no structural elements have to be produced in the light guide, production of the light guide is simplified.
In embodiments of the invention, the relative coverage of the front surface by the outcoupling elements may increase with the distance from the light receiving surface along the light guide. Hence, the outcoupling of light of uniform intensity all over the length of the light guide may be achieved.
Typically, the light source of the light-emitting device according to the invention comprises at least one light-emitting diode (LED).
In another aspect, the invention relates to the light guide as such of any embodiment of the light-emitting device described herein.
These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, in which:
As illustrated in the Figures, the sizes of layers and domains are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention.
Optionally, the light source may comprise a plurality of LEDs having different emission characteristics. For example, of a plurality of LEDs, at least one LED may emit light predominantly at 470 nm, whereas at least one other LED may emit light predominantly at 450 nm. By adapting the relative emission of different wavelengths from the light source, the color temperature of the light emitted by the light-emitting device may be tuned. As a result, “warm” or “cold” white light may be obtained as desired.
Light of said first wavelength range emitted by the light source 2, and optionally light of other wavelength ranges also emitted by the light source 2, may be coupled into a light guide 3 via a light receiving surface 4 of the light guide 3. Typically, the light source 2 is arranged adjacent to the light guide 3 and, in operation, emitting light generally in the direction of the light receiving surface 4. However, the light source may also emit light in other directions, in which case the light may be redirected by a reflective material before reaching the light receiving surface 4. In embodiments of the invention, the light guide 3 may comprise a plurality of light receiving surfaces, each light receiving surface 4 receiving light emitted by at least one light source 2. For example, each light receiving surface 4 may receive light emitted by a separate LED. Alternatively, a plurality of light receiving surfaces may receive light emitted by the same light source, e.g. the same LED.
The light guide 3 moreover has a front surface 31 and a rear surface 32. Having been coupled into the light guide 3, light of said first wavelength range is propagated by total internal reflection at at least the front surface 31 and the rear surface 32. The light guide 3 may be made of any material conventionally used for light guides.
As used herein, the term “light guide” refers to an optical element adapted to receive light emitted by a light source and in which at least part of said light is subject to total internal reflection at at least one surface of the light guide. Typically, light is subject to total internal reflection at at least two surfaces, such as a front surface and a rear surface. In the case of a cylindrical or tubular light guide, however, light may be subject to total internal reflection at a continuous envelope surface of the light guide.
In the embodiment shown in
Furthermore, the front surface 31 of the light guide 3 of
Light outcoupling elements 5 are provided on the light guide 3 for outcoupling light therefrom. The outcoupling elements are adapted to reflect and/or scatter at least part the incident light at an angle which does not result in total internal reflection when the reflected and/or scattered light subsequently meets the rear surface 32. Hence, at least part of the light reflected by an outcoupling element 5 exits the light guide 3 via the rear surface 32. Another part of the light reflected or scattered by an outcoupling element 5 may be so at an angle which results in total internal reflection at the rear surface 32.
Thus, having been reflected and/or scattered by an outcoupling element, a ray of light may exit the light guide upon its very next incidence on an interface between the light guide and a medium, such as air, outside the light guide. However, part of the light incident on an outcoupling element will be reflected at an angle which results in continued total internal reflection within the light guide 3. Typically, the outcoupling elements 5 achieve outcoupling of light of said first wavelength range from the light guide 3.
The light outcoupling elements 5 of the embodiment shown in
In embodiments of the invention, the light outcoupling elements transmit little or no light of said first wavelength range. Since light of the first wavelength range (e.g. blue light) that is transmitted might not be received by a wavelength converting material for conversion to the second wavelength range (e.g. yellow light), the performance of the white light-emitting device may be affected by the amount of light of said first wavelength range that is lost by being transmitted by the outcoupling elements. Typically, the light outcoupling elements may transmit 30% or less of the incident light of said first wavelength range. In order to further improve the efficacy of the light-emitting device, 20% or less, for example 10% or less, of the incident light of the first wavelength range may be transmitted by the outcoupling elements.
The distribution of light outcoupling elements 5 may be adapted to obtain the desired distribution of light emitted from the light-emitting device. For example, the relative coverage of the outcoupling elements may increase along the length of the light guide, so that the outcoupling elements are more densely arranged in a region of the light guide 3 far away from the light receiving surface 4 than in a region of the light guide 3 close to the light receiving surface 4. Such a distribution of the outcoupling elements enables outcoupling of light of uniform intensity all over the length of the light guide. The light outcoupling elements 5 may be arranged in any suitable pattern to obtain a desired light outcoupling distribution from the light guide. A possible distribution of the outcoupling elements is illustrated in
Furthermore, a reflective member 6 is arranged to reflect light that has been outcoupled through the rear surface 32 back towards the light guide 3, through which the reflected light may then be transmitted without being subject to total internal reflection. The reflective member 6 is typically provided in rear of the light guide 3. The reflective member may be a diffuse reflective plate or layer made of any conventional reflective material used in the art, for example a metal or a reflective polymer such as MCPET.
Furthermore, a plurality of domains 7 comprising a wavelength converting material 7 are arranged on the reflective member 6. Thus, the reflective member 6 is a combined reflective and wavelength converting member. The wavelength converting material is adapted to convert light of a first wavelength range to light of a second wavelength range, i.e., to absorb light of said first wavelength range and emit light of said second wavelength range. Thus, light that is outcoupled from the light guide 3 by the light outcoupling elements 5 provided on the front surface 31 of the light guide 3 may exit the light guide 3 through the rear surface 32 and may then either be directly reflected back towards the light guide by the reflective member 6, or, when incident on a domain 7 comprising wavelength converting material, be converted and/or scattered by the wavelength converting material. A part of the light that is emitted or scattered by the wavelength converting material may also be reflected towards the light guide 3 by the reflective member 6. Light of said first wavelength that is reflected and/or scattered by the reflective member 6 and/or the wavelength converting material, and light of said second wavelength emitted by the wavelength converting material, may be transmitted through the light guide 3 to exit the light guide via the front surface 31. Thus the light emitting device 1 provides good mixing of unconverted and converted light.
Since light emitted by the wavelength converting material is scattered by the wavelength converting material, and also may be diffusively reflected by the reflective member 6, part of the converted light may be subject to total internal reflection after entering the light guide 3. However, a major part of the light that is subject to total internal reflection within the light guide 3 is light of said first wavelength range which has not (yet) been outcoupled from the light guide 3.
The wavelength converting material may be any suitable wavelength converting material, also known as a phosphor, known in the art. However, preferred wavelength converting materials may be selected from garnets and nitrides, especially doped with trivalent cerium or divalent europium, respectively. Embodiments of garnets especially include A3B5O12 garnets, wherein A comprises at least yttrium (Y) or lutetium (Lu) and wherein B comprises at least aluminum (Al). Such garnet may be doped with cerium (Ce), with praseodymium (Pr) or a combination of cerium and praseodymium; especially however with Ce. Typically, B comprises aluminum; however, B may also partly comprise gallium (Ga) and/or scandium (Sc) and/or indium (In). In another variant, B and O may at least partly be replaced by Si and N. The element A may especially be selected from the group consisting of yttrium (Y), gadolinium (Gd), terbium (Tb) and lutetium (Lu). Typically, Gd and/or Tb are only present up to an amount of about 20% of A. In a specific embodiment, the wavelength converting material comprises (Y1−xLux)3B5O12:Ce, wherein x is equal to or larger than 0 and equal to or smaller than 1. The term “:Ce”, indicates that part of the metal ions (i.e. in the garnets: part of the “A” ions) in the wavelength converting material is replaced by Ce. For instance, assuming (Y1−xLux)3Al5O12:Ce, part of Y and/or Lu is replaced by Ce. This notation is known to the person skilled in the art. Ce will replace A in general for not more than 10%.
In other embodiments, the wavelength converting material may comprise one or more materials selected from the group consisting of (Ba,Sr,Ca)S:Eu, (Ba,Sr,Ca)AlSiN3:Eu and (Ba,Sr,Ca)2Si5N8:Eu. In these compounds, europium (Eu) is substantially or only divalent, and replaces one or more of the indicated divalent cations. In general, Eu will not be present in amounts larger than 10% of the cation. The term “:Eu” indicates that part of the metal ions is replaced by Eu. Divalent europium will in general replace divalent cations, such as the above divalent alkaline earth cations, especially Ca, Sr or Ba.
The wavelength converting material is adapted to absorb light in said first wavelength range emitted by the light source, which is typically light in the range of from 380 to 520 nm, preferably from 440 to 480 nm and more preferably from 450 to 470 nm; however, if the light source emits light in a wavelength range other than 380-520 nm, the wavelength converting material may be adapted to absorb light of a wavelength range having at least one endpoint lower and/or higher than 380-520 nm. The wavelength converting material may emit light in the wavelength range of from 450 to 750 nm.
When the wavelength converting member comprises discrete domains comprising a wavelength converting material, the color temperature of the light emitted from the light-emitting device may be tuned by adapting the relative coverage of the wavelength converting material. For example, the relative coverage of domains comprising wavelength converting material at a concentration of 20% may be in the range of 40-80%.
It may desirable to use more than one wavelength converting material in order to provide conversion from and/or to a wider range of wavelengths than can be achieved using a single wavelength converting material. Therefore, in embodiments of the invention, the light-emitting device may comprise a second wavelength converting material. Typically, the wavelength converting member comprises discrete domains comprising said second wavelength converting material in addition to the domains comprising the first wavelength converting material described above.
The second wavelength converting material typically a) absorbs light of the same wavelength range as said first wavelength converting material and emits light of a wavelength range different from that emitted by the first wavelength converting material, or b) absorbs light of a wavelength range different from that absorbed by the first wavelength converting material and emits light of a wavelength range different from that emitted by the first wavelength converting material. However, it is also possible that the second wavelength converting material absorbs and emits light of substantially the same wavelength ranges as the first wavelength converting material.
In embodiments of the invention, both wavelength converting materials may absorb light of different subranges of said first wavelength range emitted by the light source.
Advantageously, by extending the wavelength range of the wavelength converted light, the color rendering index may be improved and/or, in the case of white light, the color temperature may be decreased.
The wavelength converting member may also comprise a further wavelength converting material adapted to absorb and emit light of desired wavelength ranges.
By using two or more types of wavelength converting materials, light emitted by the light source may be efficiently converted and the color and/or the color temperature and/or the color rendering index of the light emitted by the light-emitting device may be tuned by adapting the relative coverage and concentration of each wavelength converting material.
Furthermore, the wavelength converting body may be thermally connected to a heat sink for dissipation of heat generated by the wavelength converting material. For example, heat generated by the wavelength converting material may transferred along a heat transfer path extending from the wavelength converting material via the reflective member 6 to a heat sink arranged in thermal contact with the reflective member 6. Typically, the heat sink is arranged on a rear side of the reflective member 6. Thus, advantageously, heat may be efficiently transported away from the wavelength converting material, so that thermal quenching of the wavelength converting material is avoided, without the path of light being interrupted by a heat sink. The heat sink may be of any material conventionally used in the art for heat dissipation structures, for example a metal, e.g. aluminum or copper. For example, the heat sink may be a patterned heat conductive plate that is in contact with the reflective member or another substrate on the wavelength converting material is arranged, either directly via mechanical pressure, or via an adhesive material, The heat sink is typically not in optical contact with the light guide 3.
In another embodiment of the invention illustrated in
The wavelength converting member 8 may comprise a second wavelength converting material as described above. Furthermore, as shown in
In embodiments of the invention, as an alternative to or in addition to the discrete domains comprising a wavelength converting material, the wavelength converting member 8 may comprise a continuous layer comprising at least one wavelength converting material. Optionally, such a layer may also comprise a scattering material, for example titanium dioxide. In such embodiments, the color temperature of the light emitted by the light-emitting device may be tuned by adapting the concentration of wavelength converting material in the continuous layer, the thickness of the continuous layer and/or the wavelength converting material composition of the continuous layer.
For example, the wavelength converting member 8 may comprise a continuous layer comprising said first wavelength converting material and, arranged on said continuous layer, discrete domains comprising said second wavelength converting material. Alternatively, the continuous layer may comprise said second wavelength converting material and the discrete domains arranged thereon may comprise said first wavelength converting layer. Alternatively, a continuous layer may comprise both said first wavelength converting material and said second wavelength converting material. The coverage, concentration and pattern of the discrete domains and/or the continuous layer comprising wavelength converting material, respectively, may be as described above.
The wavelength converting body 8 may be thermally connected to a heat sink for the dissipation of heat generated by the wavelength converting material.
In a further embodiment of the invention illustrated in
In a further embodiment of the invention the light source 2 comprises a plurality of LEDs. The LEDs may be adapted to emit light of said first wavelength range, and optionally they may emit light of different subranges of said first wavelength range. For example one LED may emit light predominantly at 470 nm, whereas another LED may emit light predominantly at 450 nm. By adapting the relative emission of different wavelengths from the light source, the color temperature of the light emitted by the light-emitting device may be tuned. Furthermore, since light from different LEDs may be mixed before entering the light guide 3 via the light receiving surface 4, the emission characteristics of an individual LED may have a less pronounced effect on the light reaching the wavelength converting member 8, compared to the case when only one LED is used in the light source 2. Alternatively, the plurality of LEDs may comprise at least one LED emitting light of said first wavelength and at least one LED emitting light of a wavelength range different from said first wavelength range. For example, in addition to one or more LED(s) emitting light in the blue light wavelength range, at least one LED emitting light in the green light wavelength range could be used. In embodiments in which the light source comprises LEDs emitting light of different wavelength ranges, typically first and second wavelength converting materials having different absorption and optionally also different emission wavelength ranges are used.
In order to test the wavelength conversion and reflection parts according to embodiments of the present invention, different sets of dots of a Ce-doped yttrium aluminum garnet (also referred to as Ce:YAG) phosphor material embedded in a transparent resin were deposited each onto a white diffuser (MCPET, Furukawa Electric). The dots were deposited in a fine regular rectangular pattern. The estimated concentration of Ce:YAG material in the dots was 20%. The sets of dots represented a phosphor coverage of 25%, 44% and 100%, respectively. The sets of dots, and the white diffuser without any phosphor dots (representing 0% phosphor coverage), were illuminated perpendicularly with light from LEDs emitting light of a wavelength of 460 nm. The resulting color varied from blue (0% coverage) to yellow (100% coverage). The color coordinates of the light emanating from the respective sets were measured. The results are presented in
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Number | Date | Country | Kind |
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08168388.0 | Nov 2008 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB09/54720 | 10/26/2009 | WO | 00 | 4/26/2011 |