Patent Application
20110032729
Embodiments described herein relate to optical systems. More particularly, embodiments described herein relate to optical systems using phosphors to down convert light.
LEDs are used to generate light for a variety of applications. In some cases, phosphors are used in conjunction with the LEDs to produce a desired color of light. In traditional systems of using phosphors with LEDs, phosphors are coated on a dome that surrounds the LED. These systems, however, suffer from heat related inefficiencies.
An LED inherently heats when transforming electrical energy to light. The addition of phosphors to an LED package causes additional heating through absorption of light by the LED and transference of heat from phosphors to the LED. Heat causes the LED efficiency and phosphor quantum efficiencies to drop, thereby reducing the overall LED package efficiency.
To address the issue of absorption, the LED must be highly reflective of the down-converted light generated by the phosphors, adding complication to the LED device. To address heat transfer from the phosphors to the LED, the phosphors can be disposed in a layer removed from the LED chip. In such an arrangement, the LED is typically surrounded by a cup with the LED at the bottom of the cup on a phosphor layer disposed at the other end. The LED provides light to the phosphor layer which down converts the light. Some portion of the down-converted is emitted out of the cup (i.e., away from the LED), while another portion is emitted back into the cup (i.e., toward the LED). In such an arrangement, the LED still absorbs a large amount of back-scattered light. Moreover, it is difficult to cool the phosphors without placing a cooling mechanism between the phosphor layer and the intended target for the light.
Additional problems arise when using multiple colors of phosphors to attain a specific color point or to match the color filters of LCD panels. Namely, phosphors can self-absorb. For instance a red-emitting phosphor may absorb down-converted light from a green-emitting phosphor instead of the pump wavelength. Such absorption introduces losses into the system making it difficult to minimize absorption and maximize package efficiency in the system. Additionally, when multiple phosphors are used in proximity to each other, it is difficult to achieve pump light uniformity to the phosphors.
Embodiments described herein provide optical systems in which phosphors are used to down-convert light. In general, optical systems can include a light guide configured to propagate light from an entrance face to a distal end along a propagation axis using total internal reflection. A phosphor layer can be disposed orthogonal to the entrance surface of the light guide.
The orthogonal arrangement can help reduce heating of the LED and phosphors. Depending on the length scales, the pump source only occupies a small angular subtense as viewed by the phosphor. Consequently, the amount of light backscattered by the phosphors that will reach the light source may be relatively small, thereby reducing absorptive heating at the light source. Furthermore, while the pump source may have a relatively high exitance, the phosphor may have a relatively low irradiance. This implies that per unit area, the flux density of pump energy on the phosphor is relatively small, thus leading to low thermal rise due to Stoke Shifts. To further reduce heating, the phosphors can be independently cooled without placing the cooling mechanisms between the phosphors and the intended target.
The phosphor layer can comprise multiple colors of phosphors with areas of each color spatially separated from other colors by a gap. It is believed that such an arrangement can reduce re-absorption in the phosphor layer, thereby increasing overall package efficiency. Color blending from the various colors of phosphors can occur in the light guide or external to the light guide. For example, according to one embodiment, the exit surface of the light guide can be a selected distance from the phosphor layer so that color blending primarily occurs in the light guide and the light guide emits a substantially uniform color from the exit surface. In another embodiment, the light guide can be configured so that color blending primarily occurs external to the light guide.
The optical system can include a reflector to reflect light emitted by phosphors or escaping from sidewalls of the light guide. The use of reflector can increase overall efficiency of the optical system to redirect down-converted light that might otherwise be lost.
Embodiments of optical systems described herein provide advantages over traditional systems of using phosphors in conjunction with light sources by reducing heating at the light source due to absorption of down-converted light.
Embodiments described herein provide another advantage by potentially leading to lower thermal rise due to Stoke's shift.
Embodiments described herein provide yet another advantage because a light source's temperature no longer has a significant influence on the phosphor temperature and vice versa.
Embodiments described herein provide yet another advantage by allowing for independent cooling of phosphors over a much larger surface area.
Embodiments described herein provide yet another advantage by reducing phosphor self-absorption.
Embodiments described herein provide another advantage by allowing the use of nano phosphor particles or quantum dots. Because the nanoparticles/quantum dots can be positioned away from the source and can be independently cooled, the temperature of the nanoparticles/quantum dots can be controlled to prevent heat degradation of the binder material used with the nanoparticles/quantum dots.
A more complete understanding of the embodiments and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:
The disclosure and various features and advantageous details thereof are explained more fully with reference to the exemplary, and therefore non-limiting, embodiments illustrated in the accompanying drawings and detailed in the following description. Descriptions of known starting materials and processes may be omitted so as not to unnecessarily obscure the disclosure in detail. It should be understood, however, that the detailed description and the specific examples, while indicating the preferred embodiments, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.
As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, product, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, process, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).
Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of, any term or terms with which they are utilized. Instead these examples or illustrations are to be regarded as being described with respect to one particular embodiment and as illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized encompass other embodiments as well as implementations and adaptations thereof which may or may not be given therewith or elsewhere in the specification and all such embodiments are intended to be included within the scope of that term or terms. Language designating such non-limiting examples and illustrations includes, but is not limited to: “for example,” “for instance,” “e.g.,” “in one embodiment,” and the like.
Embodiments described herein provide optical systems in which phosphors are used to down-convert light. The phosphors are disposed on a light guide orthogonal to an entrance surface to the light guide. This orthogonal separation can reduce the amount of light from the phosphors that re-enters the pump source and prevent heat from the phosphors from heating the pump source.
Light guide 110 is an optical wave guide that propagates light from entrance face 120 to a distal end 140 along a primary propagation axis 117. Light guide 110 is formed of a material to promote total internal reflection of light from light source 105. Example materials include, but are not limited to, glass, extruded plastic, polyacrylate, polycarbonate or other material. Light guide 110 can be square, rectangular, tubular or otherwise shaped.
Phosphor layer 115 is disposed on one or more surfaces that are orthogonal to entrance surface 120. The phosphors can be applied according to any technique known or developed in the art. By way of example, but not limitation, phosphor layer 115 can include phosphor particles mixed with an adhesive, such a silicone. The particles in phosphor layer 115 can include quantum dots, phosphor nano-particles or other sizes of phosphor particles. The size, concentration, density, thickness, pattern, emission wavelength or other property of the particles can vary along the length of the light guide to control the uniformity or color and to direct the appropriate amount of energy out of the system. Phosphor layer 115 can be disposed along the entire length of light guide 110, a substantial portion of light guide 110 or along any desired portion of light guide 110.
Phosphor layer 115 can include various colors of phosphors. Light guide 110 can be configured so that color blending occurs in light guide 110. For example, according to one embodiment, surface 125 and exit surface 130 can be a selected distance “h” apart such that color from the various phosphors is primarily blended in light guide 110. Consequently, light guide 110 will emit light of a desired color from surface 130, though there may be some edge effects. In another embodiment, light guide 110 may emit light that has noticeably different colors in near field, but that become blended external to light guide 110 to become a desired color at far field (e.g., as seen by human, electronic observer or other target 197).
In general, the further away a particular phosphor particle is from entrance face 120, the less likely light emitted by that particle will reenter the pump source. In the example of
While particles further away from entrance face 120 are less likely to emit light that will be absorbed by light source 105, such particles are also less likely to receive light from light source 105 in the first place. If phosphor layer 115 is uniform over a relatively long light guide 110, the area of light guide 110 closer to light source 105 may emit noticeably brighter light. To account for this, the phosphor particle density distribution can increase along the length of light guide 110 to produce a more uniform emission pattern from light guide 110.
As light propagates along light guide 110, some light that will be incident on surface 125 and will be down converted by phosphors in phosphor layer 115. The phosphor will emit some portion of the down-converted light back into light guide 110. The down-converted light can exit light guide 110 through exit surface 130.
In general, light down-converted by the phosphors will exit light guide 110 from exit surface 130. However, because phosphors are lambertian emitters, the phosphors will emit some portion of light away from light guide 110. Additionally, even if the down-converted light is emitted into the light guide 110, some portion of the light may exit sidewalls 157. According to one embodiment, a reflector can be used to direct light in a desired direction.
While reflector 165 is shown on the three sides of the light guide, the reflector may be on one or two sides of the light guide. In other embodiments, the reflector may also be disposed to reflect light from the end of the light guide opposite of the pump source. If the light guide is shaped for angular control, an orthogonally separable diffuser can be used to divert light toward the phosphor.
According to one embodiment, reflector 165 touches, but is not in intimate contact with light guide 110. In other words, reflector 165 can be lightly set without an optical interface leaving inherently small air gaps. In this case, the reflector 165 may contact the light guide 110 in limited places, but gaps still exist between a majority of reflector 165 and light guide 165. In other embodiments, reflector 165 does not make contact with light guide 110. A gap, which is potentially very thin, can be maintained between reflector 165 and the light guide 110 to preserve total internal reflection. While gaps between light guide 110 and reflector 165 may simply filled with the surrounding medium (e.g., air), they may also be filled with a material having an index of refraction that preserves total internal reflection in light guide 110. In other embodiments, reflector 165 may be in intimate contact with light guide 110. That is, reflector 165 may be pressed against light guide 110 or coupled to light guide 110 with a liquid, adhesive, compliant material or other material.
According to one embodiment, the optical system can be configured so that scattered pump light or down-converted light will strike the reflector. Pump light that remains inside light guide 110 may not make it out the light guide on the first pass, but upon subsequent passes and scattering, the optical system will allow the majority of the energy to escape.
To minimize light loss through gaps 190, gaps 190 can include features 195 to scatter light, such as surface roughening, micro-facets or other features that cause light incident on features 195 to scatter. In other embodiments, the optical system can include reflectors (e.g., reflector 165) to reflect light that may otherwise escape gaps 190.
In the embodiments of
In some cases, the pump source is not directly in line with the light guide but can be optically coupled to the light guide using fiber optics, reflectors or other optical coupling mechanisms.
Orthogonally separated phosphors can be used with light guides having a variety of shapes.
One advantage of light bulb 450 is that the light source 405 can be securely mounted near the socket, rather than near the center of glass bulb 455. Because the light is guided by light guide 410 to the phosphors, light will appear to an observer to be generated at a more traditional location (e.g., near the center of glass bulb 455). Because the phosphors are remote from the light source 405, overheating of the light source 405 is reduced or avoided.
Embodiments described herein provide optical systems in which a phosphor layer is disposed orthogonal to an entrance surface of a light guide. The phosphor layer can be disposed on the light guide by being disposed directly on the surface of the light guide or disposed on the light guide with other layers in between. The phosphor layer can include phosphor particles mixed in silicone or other adhesive, phosphors embedded in a clear plastic or acrylic sheet that is optically coupled the surface of the light guide, phosphors sandwiched between sheets of material or phosphors otherwise disposed so that light from the light guide can be incident on the phosphors. The phosphor layer can include a continuous layer of phosphors or spatially separated sections. The size, concentration, density, thickness, pattern, emission wavelength or other property of the particles can vary along the length of the light guide to control the uniformity or color along the light guide and to direct the appropriate amount of energy out of the system.
According to one embodiment, phosphors can be located remote from an LED pump source. That is, the distance of the phosphors from the LED is at least 2:1 of the LED die width. In other embodiments the phosphors may be located closer to the LED (e.g., to be proximate to the exit surface of the LED) or may be located at much farther distances (e.g., greater 10:1).
Additionally, embodiments described herein can include features to cool the phosphors including heat sinks, heat pipes, convective air cooling, fluid cooling or other cooling mechanisms. According to one embodiment, the optical systems can be arranged so that the temperature of the phosphors will not degrade a binding material.
While this disclosure describes particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the disclosure.
This application claims the benefit of priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 61/229,642 by inventors Dung T. Duong and Hyunchul Ko, entitled “Orthogonally Separable Light Bar”, filed Jul. 29, 2009, the entire contents of which are hereby expressly incorporated by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61229642 | Jul 2009 | US |
