Patent Application
20140085924
The present invention relates to lighting, and more specifically, to an edge-lit light panel.
Edge-lit light panels are particularly handy for the purposes of office lighting, retail lighting, signage lighting, and so forth. In an edge-lit light panel, one or more light sources direct light into a light guide through its lateral edge. The light propagates generally laterally within the light guide, reflecting off its front and back surfaces through total internal reflection. Typically, the light guide includes a diffuser within its volume or on its front or back surfaces, which redirects the internal light out of the light guide into a desired pattern.
Conventional edge-lit light panels suffer from a variety of deficiencies. For instance, there may be excessive reflection losses incurred as light is coupled into the light guide at its lateral edge. In addition, because the light guides are typically molded from a non-bendable material, it is difficult to change the geometry of the light guide once a part has been manufactured. Finally, if the properties of a light source are varied, it generally requires a redesign of the light guide, which can be time-consuming and expensive. Accordingly, there exists a need for a light panel that can reduce reflection losses at the lateral edge of the light guide, and that can accommodate different light sources without a redesign of the light guide.
Embodiments disclosed herein provide a light panel including a transparent light guide having a front face, a rear face and a lateral perimeter. The light guide is capable of receiving light through its lateral perimeter and laterally transmitting the light via total internal reflection. The light panel also includes a diffuser attached to the front face or the rear face of the light guide. The diffuser has a non-uniform diffusivity that varies over a lateral extent of the diffuser. The diffuser is capable of coupling light out of the light guide and transmitting said light into a predetermined distribution generally perpendicular to the light panel. In some embodiments, the light panel also includes at least one concave feature disposed on the lateral perimeter. Light produced from a light source disposed proximate a center of the concave feature enters through the concave feature and forms internal light having a range of propagation angles. The range includes central propagation angles and peripheral propagation angles. In some embodiments, the light panel also includes at least one inclined surface disposed on the lateral perimeter. Internal light at the peripheral propagation angles is received by the at least one inclined surface and is totally internally reflected by the at least one inclined surface. Internal light at the central propagation angles misses the at least one inclined surface.
In an embodiment, there is provided a light panel. The light panel includes: a transparent light guide having a front face, a rear face and a lateral perimeter, the transparent light guide being capable of receiving light through its lateral perimeter and laterally transmitting the light via total internal reflection; and a diffuser attached to the front face or the rear face of the transparent light guide, the diffuser having a non-uniform diffusivity that varies over a lateral extent of the diffuser, the diffuser being capable of coupling light out of the transparent light guide and transmitting the light into a predetermined distribution generally perpendicular to the light panel.
In a related embodiment, the diffusivity may increase away from a lateral perimeter of the diffuser. In another related embodiment, the diffuser may have a smooth side adhered to the front face or the rear face of the light guide. In yet another related embodiment, the diffuser may be laminated onto the light guide.
In still another related embodiment, the diffuser may be a volume diffuser. In a further related embodiment, the volume diffuser may include a plurality of particles having a refractive index differing from that of a background material, and the density of the plurality of particles may increase away from the lateral perimeter of the diffuser. In a further related embodiment, the density of the plurality of particles may be a maximum at a central region of the light panel. In another further related embodiment, the light panel, the light guide, and the diffuser may each have a generally rectangular perimeter, the diffuser may have a plurality of concentric bands, the density of particles may be generally uniform within each concentric band in the plurality of concentric bands, and a central band in the plurality of concentric bands may have a peak particle density.
In yet still another related embodiment, the diffuser may be a surface diffuser having one or more diffusing features on at least one of the front and rear surfaces of the diffuser.
In another embodiment, there is provided a light panel. The light panel includes: a transparent light guide having a lateral perimeter, the transparent light guide being capable of receiving light through its lateral perimeter and laterally transmitting the light via total internal reflection; at least one concave feature disposed on the lateral perimeter, wherein light produced from a light source disposed proximate a center of the concave feature enters through the concave feature and forms internal light having a range of propagation angles, the range including central propagation angles and peripheral propagation angles; and at least one inclined surface disposed on the lateral perimeter, wherein internal light at the peripheral propagation angles is received by the at least one inclined surface and is totally internally reflected by the at least one inclined surface, and wherein internal light at the central propagation angles misses the at least one inclined surface.
In a related embodiment, the at least one concave feature may have a circular cross-section. In another related embodiment, the light source may be a plurality of solid state light sources, and the light panel may further include the plurality of solid state light sources.
In a further related embodiment, each solid state light source in the plurality of solid state light sources may have a corresponding concave feature and may be disposed proximate the center of the corresponding concave feature. In a further related embodiment, each solid state light source in the plurality of solid state light sources may have a corresponding dome disposed between the solids state light source and the corresponding concave feature.
In another further related embodiment, at each point of the inclined surface, a light ray originating at a solid state light source, passing through the concave feature and striking the inclined surface may strike the inclined surface at an incident angle greater than the critical angle, thereby leading to total internal reflection of the light ray.
In still another related embodiment, each inclined surface may be a flat bevel extending along an edge of the light panel. In yet another related embodiment, each inclined surface may be a rounded, convex bevel extending inward from a front or back surface of the light guide to a lateral edge of the light guide. In still yet another related embodiment, the at least one concave feature and the inclined surface may be integral with the light guide. In yet still another related embodiment, the at least one concave feature and the inclined surface may be included on a lateral feature element, the lateral feature element being separate from and attached to the light guide. In still another related embodiment, at least one side of the lateral perimeter may include: a lateral edge being generally flat and perpendicular to front and rear faces of the light guide; a plurality of concave features disposed in a line along the lateral edge, each concave feature being generally spherical and extending into the light panel from the lateral edge; a pair of inclined surfaces extending from the front and rear faces of the light guide to the lateral edge, respectively, the pair of inclined surfaces being symmetrical from the front to the rear and from the rear to the front, respectively; wherein for light entering the light panel through a concave feature and striking one of the inclined surfaces, the light may be totally internally reflected.
The foregoing and other objects, features and advantages disclosed herein will be apparent from the following description of particular embodiments disclosed herein, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles disclosed herein.
Throughout, the directional terms “up”, “down”, “top”, “bottom”, “side”, “lateral”, “longitudinal” and the like are used to describe the absolute and relative orientations of particular elements. For these descriptions, it is assumed that light exits through a “front” of the light panel, with a spatial distribution centered around a longitudinal axis that is generally perpendicular to the front of the light panel. It will be understood that while such descriptions provide orientations that occur in typical use, other orientations are certainly possible. The noted descriptive terms, as used herein, still apply if the light panel is pointed upward, downward, horizontally, or in any other suitable orientation.
An edge-lit light panel is disclosed, which includes a transparent light guide. Light, such as from one or more LED arrays, is coupled into the light guide through its lateral edge, and propagates generally laterally within the light guide via total internal reflection. A diffuser is attached to the front or rear face of the light guide, such as by lamination. The diffuser may have a refractive index matched to that of the light guide. The light guide may have one or more concave features on its lateral edge to reduce reflection losses at high angles of incidence. The concave features may include a single, one-dimensional groove that includes all the LEDs along a particular straight edge of the light panel, or may include a series of concave dimples, with one dimple for each LED.
The above paragraph is merely a generalization of several of the elements and features described in detail below, and should not be construed as limiting in any way.
In general, the shape of a footprint of the light panel 1 is largely aesthetic, and depends on the particular design requirements for the light panel 1. For instance, if the light panel 1 is to be used in office overhead lighting, it may be desirable to have the light panel 1 be square to fit into a two foot by two foot ceiling grid panel, or be rectangular to fit into two adjacent two foot by two foot ceiling grid panels. The footprint of the light guide 2 may be generally rectangular, as shown in
The light panel 1, as drawn in
The above description includes light propagating within the light guide 2, which is from total internal reflection from the front and back sides of the light guide 2. Below is described a mechanism for coupling light out of the light guide, which involves a diffuser attached to the front or back surfaces of the light guide 2, and a mechanism for coupling light into the light guide 2, which can reduce reflection losses at the lateral perimeter of the light guide 2.
If there were no diffusing element on, in or attached to the light guide 2, then light entering from a lateral edge would propagate along the full lateral extent of the light guide 2 via multiple total internal reflections from its front 3 and back 4 surfaces, and would fail to exit the light guide in a desired manner. In other words, the light panel 1 would not be emitting light across its lateral surface with the desired angular profile. In order to couple light out of the light panel 1, an element in, on, or attached to the light guide 2 has to disrupt the conditions that lead to total internal reflection. Such an element is a diffuser 6, and in
Note that if the particles, all of a known size distribution, were uniformly distributed throughout the diffuser 6, then the output from the diffuser would be brightest near its lateral edges, and would decrease toward the center of the diffuser 6. In other words, if the diffuser were uniform, the diffuser would pick off a fraction of the available light for each unit of area; such a “picking off” would lead to dimmed light at the center because less light is available to “pick off”. As a result, the diffuser 6 should be non-uniform, with more diffusivity at its center than at its edges. The non-uniformity may be achieved by changing the size distribution of particles, the refractive index of particles, or the particle density within the diffuser 6. It is assumed that the design/simulation phase of the light panel 1 may produce sufficient specifications for the diffuser 6, so that output of the light panel 1, through the diffuser 6, may achieve the desired specifications. For instance, if the light panel 1 is used for lighting, the panel 1 may have a specification for brightness of a maximum deviation from uniformity, as a function of area and/or angle. Other suitable specifications may be used as well.
In general, the diffuser 6, whether a surface diffuser and/or a volume diffuser, is manufactured separately from the light guide 2, and is then attached to the light guide 2. The attachment may be via lamination, adhesive, welding, optical contacting or any suitable method. Note that in some embodiments, the light extraction properties of the diffuser 6 are used to produce different lit appearances for the light panel 1, depending on viewing angle or viewing location. Note that in some embodiments, the light extraction properties of the diffuser 6 are used to control the beam angle of light exiting the light panel 1. Note that in some embodiments, the light extraction properties of the diffuser 6 are used to account for light guides 2 having various footprints, such as rectangular, non-rectangular, round, curved, out-of-plane, and so forth. Note that in some embodiments, the light extraction properties of the diffuser 6 are used to control the color of the light panel output (assuming that some of the solid state light sources in the arrays 7 have differing colors). Thus, in some embodiments, at least two of the solid state light sources in the arrays 7 emit light having different colors. In some embodiments, the relative amounts of differently colored light are controlled by a closed-loop color mixing feedback system. In some embodiments, the color mixing feedback system includes a thermal sensor. In some embodiments, the color mixing feedback system includes an optical sensor. Any and/or all of these sensors may be mounted proximate the light panel 1, such as but not limited to adjacent to any of the lateral edges, adjacent to the back face, and/or adjacent to the front face of the panel.
A mechanism for coupling light into the light guide 2 is now described. Light is produced by one or more arrays of solid state light sources 7. In
For the light guide 2 of
The concave structure 51 is shown in cross-section in
As noted above, the concave structure 51 may be a partial sphere, or other two-dimensional concave structure, where the lateral edge of the light guide 2 has one concave structure for each solid state light source. As an alternative, the lateral edge may have a concave structure 51 that includes more than one solid state light source. For instance, the lateral edge may include a groove that subtends all the solid state light sources along the edge, or a subset of all the solid state light sources. Similarly, there may be multiple concave features along the lateral edge, each of which subtends one or more solid state light sources.
Since the side-view cross-sections in
In
It is instructive to examine in more detail the lateral perimeter 5 of the light guide 2. The lateral perimeter 5 may, and in some embodiments does, include two specific features that help reduce losses due to surface reflections incurred when light enters the light guide 2. First, the lateral perimeter 5 may, and in some embodiments does, include a concave structure 51 adjacent to one or more solid state light sources 7, which may reduce the angle of incidence at which the incident rays strike the light guide 2, and may thereby reduce or eliminate the instances of high angles of incidence that may have high power reflectivities. As noted above, the concave structure 51 may be spherical or cylindrical in shape, which may subtend a single solid state light source or several solid state light sources. In general, the cross-sectional shape of the concave structure 51 need not be truly circular to realize the benefits of reducing the angle of incidence, which in turn can reduce the amount of power lost in reflection. For instance, the concave structure 51 may include an elongated dimple, an irregularly shaped dimple, a concave dimple having corners, a concave dimple lacking corners, or any other suitable shape. As a generality, any shape that reduces the angle of incidence entering the light guide 2, for high angles of incidence, may help reduce the amount of light reflected at those high angles of incidence, and may therefore increase the fractional amount of light that enters the light guide 2.
Note that in addition to being beneficial for use with a bare solid state light source emission surface, as is shown in
Second, the lateral perimeter 5 may have an inclined portion 52, which can help enforce a condition of total internal reflection inside the light guide 2 for light that has entered through the concave structure 51. Note that if the light guide 2 lacked an inclined portion 52 and were strictly rectangular in all dimensions, then there would be a region near the top of the light guide 2 at which light inside the light guide would fail to satisfy the condition for total internal reflection. In this region, light would undesirably exit the light guide 2. The inclined portion 52 is intended to keep that otherwise-exiting light inside the light guide, by introducing a reflection that directs it downward. Many geometries are possible for the inclined portion 52. Two example geometries are shown in
It is assumed that the light guide 2 has a refractive index denoted by n, and that angles of incidence are drawn with respect to the local surface normal. For a ray propagating at almost 90 degrees (horizontally toward the front surface 53 in
For a ray propagating toward the bottom of the inclined portion 52, a similar condition holds. We want to ensure that a ray that strikes the front and back surface 53 directly would do so under the condition of total internal reflection, even at the topmost portion of the front and back surface 53. Such a ray forms an angle of incidence (with respect to the local surface normal) of θ2. The angle θ2 is the critical angle, so that a ray exiting into air would do so at grazing exitance, generally parallel to the surface and downward in
Note that these rules-of-thumb are simple approximations, and that actual geometry may be specified easily during the simulations performed in the design phase of the device. Such simulations need not strictly adhere to any or all of the three assumptions noted above, since the simulation software is significantly more sophisticated than the simple equations outlined above.
A second example is shown in
Although the inclined surfaces 52 shown in
Thus far, it has been assumed that the features on the lateral perimeter 5 of the light guide 2 are made integral with the light guide 2. In some embodiments, it is desirable to form these features on a separate lateral feature element 9, then attach the lateral feature element 9 to the light guide.
Unless otherwise stated, use of the word “substantially” may be construed to include a precise relationship, condition, arrangement, orientation, and/or other characteristic, and deviations thereof as understood by one of ordinary skill in the art, to the extent that such deviations do not materially affect the disclosed methods and systems.
Throughout the entirety of the present disclosure, use of the articles “a” and/or “an” and/or “the” to modify a noun may be understood to be used for convenience and to include one, or more than one, of the modified noun, unless otherwise specifically stated. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
Elements, components, modules, and/or parts thereof that are described and/or otherwise portrayed through the figures to communicate with, be associated with, and/or be based on, something else, may be understood to so communicate, be associated with, and or be based on in a direct and/or indirect manner, unless otherwise stipulated herein.
Although the methods and systems have been described relative to a specific embodiment thereof, they are not so limited. Obviously many modifications and variations may become apparent in light of the above teachings. Many additional changes in the details, materials, and arrangement of parts, herein described and illustrated, may be made by those skilled in the art.
