WO2010052633A1

WO2010052633A1 - Light guide with outcoupling elements

Info

Publication number: WO2010052633A1
Application number: PCT/IB2009/054858
Authority: WO
Inventors: Marcellinus P. C. M. Krijn; Gabriel-Eugen Onac
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2008-11-05
Filing date: 2009-11-02
Publication date: 2010-05-14
Anticipated expiration: 2011-05-05

Abstract

The invention relates to a light guide (3) comprising a light receiving surface (4) for receiving light, a front surface (6) and a rear surface (7). The light guide is adapted to guide light received by said light receiving surface by means of total internal reflection at said front surface and said rear surface, and further comprises a plurality of outcoupling elements (5) arranged on said rear surface 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 front surface. At least part of said outcoupling elements comprises a wavelength converting material. The light guide according to the invention allows tuning of the color temperature of white light emitted by a light-emitting device comprising the light guide by modifying the composition and/or distribution of the outcoupling elements.

Description

Light guide with outcoupling elements

FIELD OF THE INVENTION

The present invention relates to a light guide comprising a light receiving surface for receiving light, a front surface and a rear surface, said light guide being adapted to guide light by means of total internal reflection at said front surface and said rear surface, and further comprising a plurality of outcoupling elements arranged on said rear surface 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 via said front surface.

BACKGROUND OF THE INVENTION 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 lm/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 a wavelength converting material, also known as a phosphor, illuminated by an 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 Al 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. Hence, the wavelength converting material risks being overheated, 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.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved light-emitting device.

In one aspect, the invention relates to a light guide comprising a light receiving surface for receiving light, a front surface and a rear surface, said light guide being adapted to guide light received by said light receiving surface by means of total internal reflection at said front surface and said rear surface, and further comprising a plurality of outcoupling elements arranged on said rear surface 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 front surface, wherein at least part of said outcoupling elements comprises a wavelength converting material.

The light guide according to the invention is easily produced and also easily employed in a light-emitting device. Having the wavelength converting material arranged on the outside of the light guide allows efficient cooling of the wavelength converting material, thus reducing or avoiding any thermal quenching. Furthermore, heat may also be dissipated from the wavelength converting material using a heat sink arranged in thermal contact with the wavelength converting material, without the heat sink blocking the path of light to an observer. Also, arranging the wavelength converting material directly on the light guide saves space. Advantageously, using the light guide according to the invention, the color, color temperature and/or color rendering index of the light emitted by a light-emitting device comprising said light guide may be tuned by modifying the composition and/or the distribution of the outcoupling elements (e.g. to alter the relative coverage of a 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 relative coverage of the outcoupling elements, a desired distribution of light may also be obtained. For example, the relative coverage of the rear surface of the light guide by the outcoupling elements may increase with the distance from the light receiving surface along the light guide, resulting in a more uniform distribution of light from the light guide.

In embodiments of the invention, a first outcoupling element of the light guide may comprise a first wavelength converting material. Furthermore, a second outcoupling element may comprise a second wavelength converting material. By including more than one wavelength converting material in the outcoupling elements, the wavelength range(s) of the light emitted by a light-emitting device comprising the light guide may be adapted, allowing tuning of the color, color temperature and/or the color rendering index of the light. Also, when the first and the second wavelength converting materials are contained in different outcoupling elements, the relative coverage of the light guide by one wavelength converting material may be easily adapted, independently of the relative coverage by the other wavelength converting material. Furthermore, at least part of the outcoupling elements may comprise a scattering material.

In another aspect, the invention relates to a light-emitting device comprising a light source for emitting light of a first wavelength range and a light guide as described above. The light source may comprise at least one LED. 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 may be averaged out, leading to no visible artifacts. Furthermore, 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 light source. Also, arranging the wavelength converting material on the light guide 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.

The light-emitting device may further comprise a reflective member arranged to reflect light that is outcoupled from said light guide. The use of a reflective member may increase the portion of light emitted in the desired direction. Also, the use of a reflective member may increase the amount of light of said first wavelength range that is converted by the wavelength converting material, and hence may improve the efficacy of the device. Furthermore, the reflective member may improve the mixing of light, e.g. mixing of unconverted and converted light. Typically, the wavelength converting material is adapted to convert light of said first wavelength to light of a second wavelength range. Said first wavelength range may be from 380 to 520 nm, for example from 420 to 480 nm.

In embodiments of the invention, the light-emitting device may further comprise a heat sink arranged in thermal contact with the wavelength converting material.

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 currently preferred embodiments of the invention, in which:

Fig. 1 is a schematic cross-sectional view of a light guide according to an embodiment of the invention;

Fig. 2 is a schematic cross-sectional view of a light-emitting device according to an embodiment of the invention; Fig. 3 is a schematic cross-sectional view of a light-emitting device according to another embodiment of the invention;

Fig. 4 is a perspective top view of a light guide according to another embodiment of the invention; and

Fig. 5 is a graph showing the color coordinates as measured for a light guide according to an embodiment of the invention and the black body curve.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in the Figures, the sizes of layers and domains may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention.

Fig. 1 shows a currently preferred embodiment of the invention. Light emitted by a light source (not shown) is received into a light guide 3 via a light receiving surface 4 thereof. The light received into the light guide 3 via the light receiving surface 4 may be light of the wavelength range of from 380 to 520 nm. Examples of light sources (e.g., LEDs) suitable for use with the light guide 3 are described below with reference to Fig. 2. 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. For example, each light receiving surface 4 may receive light emitted by a separate light source, e.g. 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 also has a front surface 6 and a rear surface 7. Light received into the light guide via the light receiving surface 4 is subsequently guided by means of total internal reflection at at least the front surface 6 and the rear surface 7.

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.

The light guide may have any suitable shape, for example the shape of a rod, a plate, a disc or the like. For example, the light guide may have a flat disc-like or toroidal-like shape and may at least partially encircle a light source, the light receiving surface 4 forming an inner surface facing a light source.

In embodiments of the invention, the light guide 3 may have the shape of a plate and may comprise at least one cavity or hole, in which the light source is arranged, said cavity or hole thus forming an optical chamber and defining at least one light receiving surface. Such a cavity or hole may for example have the shape of a diamond. Each such diamond-shaped cavity or hole typically defines at least two light receiving surfaces through which light from a single light source, e.g. an LED, may be coupled into the light guide 3. In yet other embodiments of the invention, the light guide 3 may comprise a plurality of cavities or holes, optionally arranged in at least one array, each cavity or hole being adapted to receive a light source. A light source, for example an LED, may be arranged in each cavity or hole to emit light which is coupled into the light guide 3 via each light receiving surface. For example, a very thin plate-shaped light guide may comprise two arrays of said cavities or holes, located along the respective long sides of the plate-shaped light guide, and an LED may be arranged in each cavity or hole.

The light guide may be made of any suitable material conventionally for light guides or other optical structures. Such materials are known to those skilled in the art.

Furthermore, the front surface 6 and the rear surface 7 define part of the outer surface of the light guide 3, and typically form interfaces between the light guide 3 and at least one medium or material outside the light guide 3. The medium or material outside the light guide 3 may be air, or it may be a liquid or solid material. For example, the light guide 3 may be at least partly embedded in a transparent material having an index of refraction that is less than the index of refraction of the light guide. Such a material may form a cladding layer functioning e.g. as a scratch resistant layer. Furthermore, for the purpose of mechanical support, the light guide may be in contact with a solid material. In case the index of refraction of a mechanical support material is higher than the index of refraction of the light guide, the contact area of the light guide with said material should be small in order not to result in extensive outcoupling of light by the mechanical support material, which is generally undesired.

The light guide 3 further comprises a plurality of outcoupling elements 5 arranged on the rear surface 7. 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 front surface 6. Hence, at least part of the light reflected by an outcoupling element 5 exits the light guide 3 via the front surface 6. 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 of light at the front surface 6. In the embodiment shown in Fig. 1, each outcoupling element 5 is formed by a discrete domain of material deposited onto said surface.

Furthermore, at least some of the outcoupling elements 5 comprise a wavelength converting material. The wavelength converting material is adapted to absorb at least part of the incident light and re-emit light of different (typically longer) wavelength. For example, an outcoupling element may comprise a first wavelength converting material adapted to absorb light of said first wavelength range and to emit light of a second wavelength range.

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 by a light outcoupling element might not have another chance of being converted by the wavelength converting material 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.

Optionally, an outcoupling element may comprise a second wavelength converting material different from said first wavelength converting material. It may desirable to use a second type of 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. When present, 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 the same wavelength different from that emitted by the first wavelength converting material. Advantageously, by extending the wavelength range of the wavelength converted light, the color rendering index of a light- emitting device using a light guide as described herein may be improved and/or, in the case of white light, the color temperature may be decreased. 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.

The wavelength converting material(s) 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 Ce and Pr; 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 (Yi__xLu_x)₃B₅θi₂: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 (Yi__xLu_x)3Al5θi2: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 (Ba₅Sr₅Ca)₂SIsNs :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 first wavelength converting material typically absorbs 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 than 380 nm or higher than 520 nm. The wavelength converting material may emit light in the wavelength range of from 450 to 750 nm. By using two or more types of wavelength converting material, 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 exiting the light guide 3 may be tuned by adapting the relative coverage of each wavelength converting material. The preferred coverage depends on (1) the thickness of the light guide, and (2) the distance the light must travel in the light guide before meeting a wavelength converting material. Generally, the thinner the light guide, the lower should be the wavelength converting material coverage. Also generally, the larger the distance the light must travel, the lower should be the wavelength converting material coverage. Often a large light guide means large distances for the light to travel. For example, for a 1 mm thick plate-shaped light guide having a diagonal size of 32", an aspect ratio of 16:9 and LEDs located in both the long sides of the light guide, the relative coverage of wavelength converting material may typically vary from 2 % close to each long side of the light guide to 10 % in the middle of the light guide (the position farthest from the LEDs). However, the desired relative coverage may also be affected by the type of outcoupling elements used. For example, the relative coverage of wavelength converting material may be in the range of from 1 to 80 %.

In embodiments of the invention, at least some of the outcoupling elements 5 may further comprise a scattering material, such as titanium dioxide. Other scattering materials suitable for use in the outcoupling elements are known to a person skilled in the art. For example, some of the outcoupling elements may comprise a scattering material and no wavelength converting material, whereas other outcoupling elements may comprise both a scattering material and at least one wavelength converting material. The use of a scattering material, in addition to the wavelength converting material, in the outcoupling elements 5 enables further tuning of the color temperature of the light exiting the light guide 3. The distribution of outcoupling elements 5 on the rear surface 7 of the light guide 3 may be adapted to obtain the desired distribution of light emitted from the light- emitting device. For example, the number of outcoupling elements 5 per unit area or their size(s) may increase along the length of the light guide, so that the outcoupling elements are more densely arranged far away from the light receiving surface 4 than close to the same. Such a distribution of the outcoupling elements provides a more uniform outcoupling of light over the length of the light guide 3. The outcoupling elements 5 may be arranged in any suitable pattern to obtain a desired light outcoupling distribution from the light guide 3. A possible distribution of outcoupling elements is illustrated in Fig. 4, which is a perspective view of a light guide 3 comprising a light receiving surface 4 and having a plurality of outcoupling elements 5 arranged on its rear side 7.

Advantageously, the outcoupling elements 5 of the light guide according to embodiments of the invention may be thermally connected to a heat sink for dissipation of heat generated by the wavelength converting material comprised in the outcoupling elements 5. Typically, heat generated by the wavelength converting material of the outcoupling elements 5 may be transferred along a heat transfer path extending from the outcoupling elements 5 to a heat sink arranged in thermal contact with the outcoupling elements 5. The heat sink may thus be arranged on a rear side of the light guide 3. As a result, 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 the 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 outcoupling elements, either directly via mechanical pressure, or via an adhesive material, The heat sink is typically not in optical contact with the light guide 3.

Fig. 2 shows a light-emitting device according to a currently preferred embodiment of the invention. The light emitting device 1 comprises a light source 2 which is adapted to emit light of a first wavelength range which is typically the range of from 380 to 520 nm, preferably from 440 to 480 nm and more preferably from 450 to 470 nm. The light source 2 may comprise at least one LED. 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. LEDs having emission wavelength ranges as described above, as well as LEDs having other emission wavelengths, are known to persons skilled in the art. By adapting the relative emission of different wavelengths from the light source and/or of wavelength converted light from the wavelength converting material, 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. In most general lighting applications, a "warm" white light (that is, white light having a low color temperature) is desirable.

The light emitting device 1 further comprises a light guide 3, the features of which may be as described above. The light guide 3 extends longitudinally from the light source 2, the light receiving surface 4 of the light guide facing the light source 2. At least part of the light emitted by the light source 2 is received into the light guide 3 via the light receiving surface 4, where it is subject to total internal reflection at the front surface 6 and the rear surface 7 until it is outcoupled by an outcoupling element 5 arranged on the rear surface 7.

The light-emitting device 1 may optionally comprise a reflective member 8 arranged in rear of the light guide 3 to reflect light that is scattered away from the light guide 3 by an outcoupling element, as illustrated in Fig. 2. In this case, the light-emitting device typically emits light in the direction of an observer located in front of the light guide 3. The use of a rear reflective member 8 may increase the portion of light emitted in the desired direction. Also, the use of a rear reflective member may increase the amount of light of said first wavelength range that is converted by the wavelength converting material, and hence may improve the efficacy of the device. Alternatively, as is shown in Fig. 3, the reflective member 8 may be arranged in front of the light guide 3 in order to reflect light that is outcoupled from the light guide via the front surface 6 so that it is transmitted again through the light guide, thus improving the mixing of light. When a reflective member is arranged in front of the light guide, the light-emitting device 1 may emit light in the direction of an observer located in rear of the light guide 3.

Advantageously, as described above with reference to the light guide per se, in the light-emitting device 1, the outcoupling elements 5 of the light guide 3 may be thermally connected to a heat sink for dissipation of heat generated by the wavelength converting material comprised in the outcoupling elements 5.

Example

A regular pattern of phosphor dots (3 mm pitch) was deposited on a light guide having a thickness of 1 mm. The light guide was illuminated with blue light from LEDs by coupling the light into the light guide. The color coordinates of the outcoupled light was measured and the result is presented in Fig. 5, in which the dashed line represents the radiation of a black body at temperatures ranging from about 1500 K to infinity. The result implies that it is possible to find a phosphor coverage percentage that yields white light. By altering the composition of the wavelength converting material, light of any desired color temperature could be obtained.

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.

Claims

CLAIMS:

1. Light guide (3) comprising a light receiving surface (4) for receiving light, a front surface (6) and a rear surface (7), said light guide (3) being adapted to guide light received by said light receiving surface (4) by means of total internal reflection at said front surface (6) and said rear surface (7), and further comprising a plurality of outcoupling elements (5) arranged on said rear surface (7) for outcoupling light from the light guide (3) such that at least part of the light that is outcoupled by the outcoupling elements (5) exits the light guide (3) through said front surface (6), wherein at least part of said outcoupling elements (5) comprises a wavelength converting material.

2. Light guide according to claim 1, wherein the coverage of the rear surface (7) by the outcoupling elements (5) increases with the distance from the light receiving surface (4) along the light guide (3).

3. Light guide according to claim 1 or 2, wherein a first outcoupling element (5) comprises a first wavelength converting material.

4. Light guide according to any one of the preceding claims, wherein a second outcoupling element (5) comprises a second wavelength converting material.

5. Light guide according to any one of the preceding claims, wherein at least part of said outcoupling elements (5) comprises a scattering material.

6. Light-emitting device (1) comprising: a light source (2) for emitting light of a first wavelength range; and - a light guide (3) according to any one of the claims 1 to 5.

7. Light-emitting device according to claim 6, further comprising a reflective member (8) arranged to reflect light that is outcoupled from said light guide (3).

8. Light-emitting device according to claim 6 or 7, wherein said light source (2) comprises at least one LED.

9. Light-emitting device according to any one of the claims 6 to 8, further comprising a heat sink arranged in thermal contact with the wavelength converting material.

10. Light-emitting device according to any one of the claims 6 to 9, wherein the wavelength converting material is adapted to convert light of said first wavelength to light of a second wavelength range.

11. Light-emitting device according to any one of the claims 6 to 10, wherein said first wavelength range is from 380 to 520 nm.