Patent Description:
The present inventive subject matter relates to optical waveguides, and more particularly to optical waveguides for general lighting.

An optical waveguide mixes and directs light emitted by one or more light sources, such as one or more light emitting diodes (LEDs). A typical optical waveguide includes three main components: one or more coupling elements, one or more distribution elements, and one or more extraction elements. The coupling component(s) direct light into the distribution element(s), and condition the light to interact with the subsequent components. The one or more distribution elements control how light flows through the waveguide and is dependent on the waveguide geometry and material. The extraction element(s) determine how light is removed by controlling where and in what direction the light exits the waveguide.

When designing a coupling optic, the primary considerations are: maximizing the efficiency of light transfer from the source into the waveguide; controlling the location of light injected into the waveguide; and controlling the angular distribution of the light in the coupling optic. One way of controlling the spatial and angular spread of injected light is by fitting each source with a dedicated lens. These lenses can be disposed with an air gap between the lens and the coupling optic, or may be manufactured from the same piece of material that defines the waveguide's distribution element(s). Discrete coupling optics allow numerous advantages such as higher efficiency coupling, controlled overlap of light flux from the sources, and angular control of how the injected light interacts with the remaining elements of the waveguide. Discrete coupling optics use refraction, total internal reflection, and surface or volume scattering to control the distribution of light injected into the waveguide.

After light has been coupled into the waveguide, it must be guided and conditioned to the locations of extraction. The simplest example is a fiber-optic cable, which is designed to transport light from one end of the cable to another with minimal loss in between. To achieve this, fiber optic cables are only gradually curved and sharp bends in the waveguide are avoided. In accordance with well-known principles of total internal reflectance light traveling through a waveguide is reflected back into the waveguide from an outer surface thereof, provided that the incident light does not exceed a critical angle with respect to the surface.

In order for an extraction element to remove light from the waveguide, the light must first contact the feature comprising the element. By appropriately shaping the waveguide surfaces, one can control the flow of light across the extraction feature(s). Specifically, selecting the spacing, shape, and other characteristic(s) of the extraction features affects the appearance of the waveguide, its resulting distribution, and efficiency.

<CIT> discloses a waveguide bend element configured to change a direction of travel of light from a first direction to a second direction. The waveguide bend element includes a collector element that collects light emitted from a light source and directs the light into an input face of the waveguide bend element. Light entering the bend element is reflected internally along an outer surface and exits the element at an output face. The outer surface comprises beveled angular surfaces or a curved surface oriented such that most of the light entering the bend element is internally reflected until the light reaches the output face.

<CIT> discloses a light emitting panel assembly that comprises a transparent light emitting panel having a light input surface, a light transition area, and one or more light sources. Light sources are preferably embedded or bonded in the light transition area to eliminate any air gaps, thus reducing light loss and maximizing the emitted light. The light transition area may include reflective and/or refractive surfaces around and behind each light source to reflect and/or refract and focus the light more efficiently through the light transition area into the light input surface of the light emitting panel. A pattern of light extracting deformities, or any change in the shape or geometry of the panel surface, and/or a coating that causes a portion of the light to be emitted, may be provided on one or both sides of the panel members. A variable pattern of deformities may break up the light rays such that the internal angle of reflection of a portion of the light rays will be great enough to cause the light rays either to be emitted out of the panel or reflected back through the panel and emitted out of the other side.

Lighting Components, Inc. of Niles, Illinois, manufactures a waveguide having a wedge shape with a thick end, a narrow end, and two main faces therebetween. Pyramid-shaped extraction features are formed on both main faces. The wedge waveguide is used as an exit sign such that the thick end of the sign is positioned adjacent a ceiling and the narrow end extends downwardly. Light enters the waveguide at the thick end and is directed down and away from the waveguide by the pyramid-shaped extraction features. <CIT>) discloses a back light device comprising a light guide plate including a light guide body having a surface for receiving light and a low refractive index layer having a lower refractive index than the light guide body. Light is reflected by the low refractive index layer and is emitted from a light emitting surface of the light guide body. <CIT>) discloses a lighting assembly that utilizes the combination of a wave-guide and a Fresnel prism to advantageously direct light emitted from a light source. Light emitted from the light source passes either from the light source into the wave-guide or through a light pipe into the wave-guide. The wave-guide will select the exiting angle of the light rays so that the light will emit from the wave-guide at approximately the same angle and pass into the Fresnel prism. The Fresnel prism will cause the light to be emitted from the lighting assembly at nearly a normal angle.

An optical waveguide includes a waveguide body that mixes and directs light out one or more surfaces. A typical waveguide body requires multiple layers of optical materials to control and extract light from the waveguide body. While each additional layer affords increased optical control, such a design can be expensive owing to the number of layers that are used and optical losses are typically encountered at each interface between layers.

In accordance with one aspect, the claimed invention is directed to the distribution and extraction of light from an optical waveguide with a high degree of control and a minimal number of layers. In a particular embodiment, a waveguide body achieves a high degree of optical control, optical efficiency, and aesthetic appearance with only two layers. The first layer is a block or other shaped body of optically transmissive material. The first layer is tapered in that the layer has opposed major surfaces, a relatively thick input surface disposed at first ends of the major surfaces and into which light developed by a light source is coupled and further having a relatively thin end surface disposed at second ends of the major surfaces. The second layer is also an optically transmissive material, and is bonded to one of the major surfaces of the first layer. The outside surface of the second layer may be formed with an array of surface features. The second layer has an index of refraction lower than the first layer and higher than the surrounding environment. Thus, for example, if the first layer has an index of refraction of <NUM> and the surrounding environment is air having an index of refraction of <NUM>, the second layer has an index of refraction between <NUM> and <NUM>.

In this particular embodiment, because the first layer is tapered, the light inside bounces at increasingly steep angles against its surfaces through total internal reflection (TIR) and eventually escapes the first layer. This extraction occurs when the light reaches the layer's critical angle, which is defined by the difference in refractive index between the material of the first layer and its surrounding environment. Because the difference in refractive indices between the first layer and the second layer is less than the difference in refractive indices between the first layer and air, the critical angle at the bonded interface is reduced. As a result, light escapes through the bonded interface before light can escape through any of the non-bonded surfaces. In addition, in a particular embodiment, the surface texture of the second layer is selected to control the angular distribution of the extracted light. In this way, both the spatial location of extraction and angular direction of extraction are controlled.

It has been found that by selecting the degree of taper of the first layer and the index of refraction of the second layer, light can be extracted solely through the bonded interface, with negligible light escaping from other surfaces. This may eliminate or at least reduce the need for a reflector behind the waveguide body to collect any stray light. Inasmuch as the second layer serves the purposes of both extraction and distribution control, such element eliminates the need for a still further layer that controls light after extraction.

The interface between the first layer and the second layer, particularly, the surface finish of the interface and the method of bonding, are important considerations. The second layer could be created by one of many methods: a solid layer of controlled index material deposited on the first layer, a thin film micropatterned on the exterior surface of the first layer, a curable coating applied to the surface of the first layer, etc. are all possible. The two-layer construction could even be created by bonding two similar layers together and then subsequently changing the index of refraction of one or both of the layers.

In the example described above, the second layer may cover an entire surface of the first layer and only extracts light from a single side. However, it should be evident that the present invention applies to a wide range of waveguide geometries where extraction must be controlled with as few layers as possible. Additionally, raising the refractive index of the second layer above the index of refraction of the surrounding environment can serve a similar purpose as lowering the index of the extraction layer. Such an arrangement may even afford additional control, as the critical angle between the waveguide body and surrounding environment (such as air) becomes harder to break. The relative refractive index values between the adjacent layers is an important consideration.

The claimed invention controls stray light, provides high efficiency extraction and highly uniform extraction, and can be used to create a fully luminous output. The present invention can be used in any type of lamp or luminaire, such as a troffer.

Still further, the materials used herein may include an acrylic material, a silicone, a polycarbonate, or other suitable material(s) to achieve a desired effect and/or appearance.

Referring to <FIG>, which represent the state of art, a waveguide body <NUM> includes a first portion or layer <NUM> having an input surface <NUM> and an end surface <NUM> opposite the input surface <NUM>. A second layer <NUM> is bonded or otherwise secured to a first major surface <NUM> of the first layer. The layers <NUM>, <NUM> are optically transmissive. Light developed by one or more light sources, such as an LED <NUM> (<FIG>), is directed into the input surface <NUM> and may be emitted out an outer surface <NUM> of the second layer <NUM>.

Preferably, the first major surface <NUM> and an opposite major surface <NUM> of the first layer <NUM> are both planar. It should be noted, however, that either or both of these surfaces <NUM>, <NUM> may be curved, if desired. Further, in the preferred embodiment, the surfaces <NUM> and <NUM> are disposed at an angle relative to one another so that the first layer <NUM> is tapered in overall shape. Still further in the preferred embodiment, the outer surface <NUM> of the second layer <NUM> is textured. Preferably, the textured surface <NUM> is stepped, so that light exiting the first layer <NUM> strikes the surface <NUM> in perpendicular fashion. This minimizes the amount of light reflected back into the body. If desired, an inner surface <NUM> of the second layer <NUM> and/or the surface <NUM> of the first layer may be smooth or textured. Also in the illustrated embodiment, the layer <NUM> is fabricated of an acrylic or other suitable optical material and has a first index of refraction. The second layer <NUM> is made of a different material than the first layer <NUM>, such as silicone or a doped acrylic material, and has a second index of refraction. In the embodiment of <FIG>, the first index of refraction is preferably greater than the second index of refraction. Still further, in accordance with the illustrated embodiment when used in air having an index of refraction of about <NUM>, the first index of refraction is about equal to <NUM> and the second index of refraction is between about <NUM> and about <NUM>, and is preferably about <NUM>. If desired, the waveguide body <NUM> may be used in a different environment having a different index of refraction less than the index of refraction of the second layer <NUM>.

Still further in accordance with the illustrated embodiment, the textured surface <NUM> comprises alternating ridges and troughs <NUM>, <NUM> and/or any other structures. The textured surface <NUM> may be formed by any suitable forming process, such as injection molding, embossing, stamping, milling, calendering, laser etching, or the like.

The light source <NUM> may develop light that is directly coupled into the waveguide body <NUM> via an air gap <NUM> and/or a coupling optic (not shown). The light source <NUM> may be a white LED or may comprise multiple LEDs including a phosphor-coated LED either alone or in combination with a color LED, such as a green LED, etc. In those cases where a soft white illumination is to be produced, the light source <NUM> typically includes a blue shifted yellow LED and a red LED. Different color temperatures and appearances could be produced using other LED combinations, as is known in the art. In one embodiment, the light source <NUM> comprises any LED, for example, an MT-G LED incorporating TrueWhite® LED technology as developed and manufactured by Cree, Inc. , the assignee of the present application.

Light emitted from the light source <NUM> enters the waveguide body <NUM> through the input surface <NUM>. The light is reflected by total internal reflection within the first layer <NUM> until the critical angle of the first layer <NUM> is exceeded, whereupon the light passes through the second layer <NUM> and exits the outer surface <NUM>, shown by the light ray <NUM>. The waveguide body <NUM> is designed to emit all or substantially all of the light from the outer surface <NUM> as the light travels through the waveguide body <NUM>. The light is directed outwardly in a controlled manner by the textured surface, for example, as seen by the light ray <NUM> of <FIG>.

Any remaining light that does not exit the outer surface <NUM> may exit the waveguide <NUM> at the end surface <NUM>. Alternatively, the end surface <NUM> may be coated with a reflective material, such as a white or silvered material to reflect any remaining light back into the waveguide body <NUM>, if desired.

As shown in <FIG> and as described above, the taper of the waveguide body <NUM> is linear between the input surface <NUM> and the end surface <NUM>. According to one embodiment, a first thickness at the input surface <NUM> is about <NUM> to about <NUM> or more mm, and more preferably is between about <NUM> and about <NUM>, and most preferably is equal to about <NUM>. Further, a second thickness of the end surface is <NUM> or less.

If desired, the first and/or second layers may be of other shape(s). For example, <FIG> illustrates an embodiment where the first layer <NUM> and the second layer <NUM> are both tapered in opposite directions and are secured together in overlapping relationship to form a waveguide body <NUM> that is of substantially constant thickness throughout. Preferably, the body has a thickness of about <NUM> to about <NUM> or more mm, and more preferably is between about <NUM> and about <NUM>, and most preferably is equal to about <NUM>.

Further, one or more discontinuous features may be used in place of or alternatively to the continuous second layer <NUM>. <FIG> illustrates an embodiment according to the claimed invention wherein a discontinuous layer comprising equally spaced linear facets <NUM> are secured or formed atop the first layer <NUM>. <FIG> shows an embodiment in which a discontinuous layer comprising discrete facets <NUM> are secured or formed in a regular array atop the first layer <NUM>. If desired, the facets <NUM> may be unequally spaced or the discrete facets <NUM> may be disposed irregularly across the layer <NUM>. In both <FIG> the index of refraction of the first layer <NUM> is greater than the index of refraction of the facets <NUM> or <NUM> and the index of refraction of the surroundings is less than the index of refraction of the facets <NUM> or <NUM>. Preferably, the facets <NUM>, <NUM> are smooth. In general, the number, geometry, and spatial array of such features across the first layer <NUM> affects the uniformity and distribution of emitted light.

<FIG> illustrates an embodiment according to the claimed invention, in which a body of material <NUM> is disposed in the first layer <NUM>. The body of material may be bonded to the layer <NUM>. Contrary to the previous embodiments, the first layer <NUM> has an index of refraction less than an index of refraction of the body of material <NUM>. The body of material <NUM> may, for example, be a polycarbonate plastic material having an index of refraction of about <NUM>. The body of material <NUM> is shown as being v-shaped, although other shapes could alternatively be used. Other similar or identical bodies of material could be disposed at spaced locations in the layer <NUM>. The layer <NUM> with the bodies of material <NUM> therein, and, optionally, the layer <NUM>, direct light away from the surface <NUM> so that substantially all light is directed out the surface <NUM>.

Other embodiments of the disclosure including all of the possible different and various combinations of the individual features of each of the foregoing embodiments and examples are specifically included herein. For example, the material used as the send layer <NUM> may have a varying index of refraction over the extent thereof. The waveguide and the components thereof may have different shapes. In addition, the various layers need not be of the shapes described, and one layer could extend into or away from the adjacent layer, if desired. Such an embodiment, which is part of the claimed invention, is shown in <FIG>, in which the layers have a non-planar interface <NUM>. Also as seen in <FIG>, a reflective layer or coating is disposed on a bottom surface of the first layer <NUM>. The reflective layer <NUM> may be a specular material, a white diffuse material, or the like.

Claim 1:
An optical waveguide body (<NUM>), comprising:
a first layer (<NUM>) of optically transmissive material comprising a thickness defined by a first major surface (<NUM>) and an opposite second major surface (<NUM>);
a second layer (<NUM>) of optically transmissive material defined by a third inner surface (<NUM>) and a fourth outer surface (<NUM>) opposite the third surface (<NUM>) ; said third surface (<NUM>) being in contact with the second surface (<NUM>) of the first layer (<NUM>);
wherein the second layer (<NUM>) comprises a plurality of ridges (<NUM>) and troughs (<NUM>) disposed on the fourth surface (<NUM>) thereof; and
wherein the first layer (<NUM>) has a first index of refraction and the second layer (<NUM>) has a second index of refraction less than the first index of refraction and wherein the first and second indices of refraction are both greater than about <NUM>;
wherein the first surface and fourth surface are tapered toward one another and wherein the light is directed out of the fourth surface (<NUM>), characterized in that the first and second layers (<NUM>, <NUM>) have a non-planar interface; a first part of the second layer (<NUM>) extends into the first layer (<NUM>) and a second part of the second layer (<NUM>) extends away from the first layer (<NUM>).