Abstract:
A highly efficient, lightweight solid state lighting panel is disclosed that has multiple LED sources, and emits a substantially uniform light intensity between said sources. The light from these sources is directed towards a highly reflective, diffusive, backing. These LED sources are placed between the reflector and a partially transmissive, partially reflective output coupler to form a cavity. The LEDs, which are mounted to printed circuit boards to form strips, can be either attached to the inner surface of the diffuser with adhesive, or suspended on a thermally dissipative structure within the cavity. By optimizing the reflector to have as high of a reflectance value as possible (&gt;95%) along with a output diffuser with about 50% transmission and 50% reflection, one can obtain cavity transmissions higher than 90%. The output from this disclosed design is more pleasant to look at than those with LEDs that directly illuminate the diffuser, causing hotspots.

Description:
RELATED APPLICATION 
     This application is the U.S. National Stage of International Application No. PCT/US2011/021102, filed 13 Jan. 2011, which designates the U.S., published in English, and claims the benefit of U.S. Provisional Application No. 61/336,148, filed Jan. 15, 2010. The entire teachings of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     Popular conventional lighting panels use either incandescent or fluorescent light sources. Conventional light fixtures are fragile, heavy in weight, difficult to manufacture, and have many replacement components such as ballasts, which are potential failures in addition to the fluorescent bulb. 
     In contrast, contemporary light sources, such as light emitting diodes, are substantially lighter in weight, more energy efficient and gradually age, rather than catastrophically fail. When the thermal and electronic drive characteristics are properly managed, light-emitting diode (LED) products are predicted to still deliver an average of 70% of initial intensity after 50,000 hours of operation, which at 12 hours per day, 365 days per year, amounts to a lifetime of 11 years. The direct nature of LEDs can result in efficiencies approaching 200 lumens per watt of electric power. 
     LEDs have many other desirable features. They are fully dimmable without color variation. They instantly turn on, have full color, and emit more light and less heat than other sources. LEDs can be sorted for photometric luminous, flux (in lumens), color, wavelength, and forward voltage. They are also extremely durable, and contain no harmful mercury vapors, such as those used in fluorescent light sources. 
     LED benefits are based on good thermal system design to achieve the best efficiency and reliability. The LED absolute maximum thermal ratings should be maintained for LED junction and printed circuit board temperature. The temperature of an LED should be managed in order to achieve the LED&#39;s maximum rated life. Thermal resistance causes a temperature difference between the source of the heat and the exit surface for heat. The less heat retained by the LED, the better its performance and the longer its lifetime. Despite the advantages of LEDs over other light sources, current designs have several problems. 
     Contemporary LED lighting panels are of two designs: One is waveguide-edge illumination with the light directed into a transparent or translucent waveguide and the LEDs placed along the perimeter of a panel. These panels are quite heavy (a 2′×2′ waveguide weighs 6-12 lbs.) and require the LEDs with the incumbent thermal loading on the perimeter. The need to use high power LEDs, due to the limited perimeter available, further exacerbates the thermal problem. The second is direct illumination with the light output from the LED die pointed towards a diffusive panel some distance away. In its simplest form, the LEDs deliver an intensity profile to the observer with many hot spots, which is unpleasant to look at. To eliminate this unpleasant look, diffuser panels are often used to soften the glare. The highest amount of softening is seen with the lowest value of transmission. These diffusers lower the efficiency of the panels&#39; output due to this low transmission. In order to achieve a reasonably uniform illumination, complex optics and a substantial depth must be provided. This configuration—with the LEDs mounted on a top panel complicates the LEDs and the thermal management of the environment by concentrating the waste heat in the closed space between the fixture and the ceiling. 
     SUMMARY 
     Embodiments of the present invention include LED lighting panels (luminaires), methods of making LED lighting panels, and methods of illuminating environments with LED lighting panels. In one example, the inventive lighting panel includes a reflective surface disposed opposite to and spaced apart from a partially transmitting, partially reflecting diffusive coupler. Together, the reflective surface and the diffusive coupler define a cavity that contains LEDs whose positions within the cavity are selected to provide substantially uniform illumination (e.g., illumination whose intensity varies by about ±15% or less) between adjacent LEDs. Turning on the LEDs causes the LEDs to emit light towards the reflective surface and away from the diffusive coupler. The reflective surface reflects the light towards the diffusive coupler, which reflects a portion of the incident light back towards the reflective surface and transmits another portion of the incident light into the illuminated environment. 
     The LEDs may be mounted on strips which are separated from each other by a distance x. In these examples, the reflective surface and the diffusive coupler are spaced apart by a distance t=(x/2)/tan α, where α is an angle within a range of about 30° and about 60°, or, more preferably, within a range of between about 50 and about 55°. The LEDs and LED strips can also be mounted on heat-dissipating elements that dissipate heat generated by the LEDs into the environment illuminated by the lighting panel. In some cases, the heat-dissipating elements are secured with mechanical standoffs at positions spaced apart from the diffusive coupler. 
     Typically, each inventive lighting panel includes a reflective surface whose reflectivity is at least about 95% (e.g., about 97%) across the visible spectrum. Each inventive lighting panel can also include a diffusive coupler that is about 50% reflective and about 50% transmissive across the visible spectrum. Inventive lighting panels may also include a second partially transmitting, partially reflecting diffusive coupler that is disposed opposite to and spaced apart from the first diffusive coupler to define a second cavity between the first diffusive coupler and the environment. Some of the light transmitted by the first diffusive coupler reflects off the second diffusive coupler back towards the reflective coupler. The rest of the light transmitted by the first diffusive coupler propagates through the second diffusive coupler to provide substantially uniform illumination to the environment. 
     An inventive lighting panel can be made by forming a reflective surface, disposing more than one LED opposite the reflective surface for emitting light towards the reflective surface, and disposing a partially transmitting, partially reflecting diffusive coupler opposite to and spaced apart from the reflective surface to form a cavity containing the LEDs. The cavity can be formed before or after the LEDs are disposed opposite the reflective surface, which can be formed by coating a substrate with a reflective coating having a reflectivity of at least about 95% (or, more preferably, about 97%) across the visible spectrum. 
     The LEDs are spaced apart from each other by a distance x, and the distance between the diffusive coupler and the reflective surface is t=(x/2)/tan α, where α is an angle within a range of about 30° and about 60°, e.g., within a range of between about 50° and about 55°. If desired, a second diffusive coupler can be disposed opposite to and spaced apart from the diffusive coupler and the reflective surface to define a second cavity. The first and second cavities can be sealed to minimize cleaning and maintenance costs. 
     Embodiments of the present invention overcome issues associated with other LED lighting panels while retaining the advantages of LED illumination. By directing the LED output away from the observer to the upper interior diffuse reflective optical surface, the inventive LED lighting panels provide a diffuse illumination pattern that is much less harsh than the illumination patterns provided by LEDs oriented towards the illuminated environment, and appears substantially uniform between sources. Multiple reflections within the cavity formed by the reflective surface and the diffusive coupler distribute light more uniformly across the output of the panel, creating a more pleasant illumination at efficiencies of over 90%. Inventive LED panels with a second diffusive coupler can provide a substantially uniform illumination pattern, with less visualization of the strip shadow. When mounted on a ceiling or wall, the inventive lighting panels direct heat generated by the LEDs away from the ceiling or wall and towards the illuminated space, reducing the thermal load on the ceiling or wall, or dissipates it within the unit by a thermal frame. As a result, ceilings and walls with inventive lighting panels can be insulated more thoroughly to reduce heating costs. In addition, the same air conditioning that cools the illuminated room can be used to cool the lighting panel during the hot season. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, 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 embodiments of the present invention. 
         FIG. 1  is a side view of a preferred embodiment of the present invention having an intermediate light source element. 
         FIG. 2  is a schematic side view of another embodiment of the present invention having a bottom, composite light source element. 
         FIG. 3  is a schematic side view of another embodiment of the present invention having a bottom, composite light source element with a top integrated wall-diffuser element. 
         FIGS. 4A-4D  are graphic representations of the spatial light distribution when varying the relative spacing of the elements of a preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A description of example embodiments of the invention follows. The teachings of all patents, published applications, and references cited herein are incorporated by reference in their entirety. 
     Embodiments of the present invention relate to a lightweight LED fixture/panel which may be specifically adapted to drop ceiling grids or other structures of any size, geometric shape, flat, formed or combination thereof. The fixtures/panels may be drop panels. More particularly, inventive embodiments include an LED lighting system having a novel construction enabling high efficiency, light weight, a directed thermal signature into a climate-controlled environment, and substantially uniform spatial light emission between the sources. 
       FIG. 1  is a simplified schematic side view of a light-emitting diode (LED) light fixture/panel  100  provided in accordance with the present invention shown in a configuration for a drop ceiling installation having a bottom transmissive diffusing panel  110 , also known as a diffusive coupler  110 , an intermediately positioned LED light source elements  120  arranged in adjacent linear arrays  122 ,  122 ′ and affixed to a supporting frame  130  and a top diffuse reflective surface  140  which may be a surface of the top panel  150 . (The supporting frame  130  can itself be made of the same diffusive, partially transmitting, partially reflecting material used to form the diffusive  110 . That is, the supporting frame  130  can act as a second diffusive coupler.) A supporting perimeter frame  152  supports and dictates the spacing of the diffusive coupler  110 , the reflective surface  140 , and the supporting frame  130 . The supporting perimeter frame  152  may also be sealed and impervious to dust and water, reducing maintenance costs and increasing fixture life. 
     In operation, light  160  from the light source elements  120  is directed towards the top reflective diffuse surface  140  where it is diffusively reflected towards the bottom transmissive diffusing panel  110 . The bottom transmissive panel  110  diffusively transmits a percentage of the light  160  into an illuminated environment  170 , and reflects a percentage of light  162  back towards the top reflective surface  140 . The first reflected light  162  is again reflected from the top reflective surface  140  and a percentage of this light  162  is transmitted into the environment  170 . A percentage of first reflected light  162  is again reflected by the bottom panel  110  towards the top reflective surface  140  and returned to the cycle as second reflected light  164 . If absorption in the cavity is kept low, this cycle of transmission and reflection may continue until a substantial percentage of the light is transmitted into the environment  170 . The efficiency of inventive LED panels may be further improved by affixing another reflective surface to the bottom of the panels and other non-emissive surfaces of the supporting frame  130  and perimeter  152  frames. 
     By manufacturing a bottom transmissive panel  110  that has a low coefficient of light absorption, preferably less than 1%, and a top reflective diffuse surface  140  with a high reflectivity, preferably greater than 97%, overall efficiencies of greater than 90% may be achieved. The percent transmission of the bottom panel  110  matters, and 50% transmission appears to near optimum. Bottom panels with reflectivities of 70% and transmissivities of about 30% are also possible. 
     In a preferred embodiment, the bottom transmissive panel (diffusive coupler)  110  is formed of 2447 white colored acrylic and the top reflective surface  140  is formed of aluminum, steel, or polyvinyl chloride (PVC) coated with WhiteOptics™ Reflector (e.g., White97™ Film) or another suitable reflective coating. For more details of suitable reflective coatings, see, e.g., U.S. application Ser. No. 12/728,160 to Eric Teather, which is incorporated herein by reference in its entirety. 
     In an exemplary description of the manufacturing of the disclosed panel, one first obtains high efficiency LED strips from an LED manufacturer, and adheres the LED strips to the inside of the diffusive coupler, with the designed spacing. The electrical input to the multiple strips is soldered to a parallel bus circuit board that distribute the electrical energy to the strips. A reflective backing is prepared by sticking a White Optics reflector sheet to a foamed PVC backing, such as Sintra, formed to the desired cavity depth. The backing and the diffusive coupler are brought together with adhesive, after a power cord is attached to the parallel bus. The unit is then energized and the output is tested. If it passes testing, an edging is applied around the unit to further seal it and to protect the edges. 
     The proper spacing of the light source elements  120 , the bottom panel  110  and the top surface  140  provides optimum performance and the most pleasant looking output to the observer. In an embodiment of light source elements  120  as shown in  FIG. 1 , the spacing of the linear light arrays  122 ,  122 ′ is dependent on their angular spatial distribution. 
       FIG. 2  shows an inventive panel  300  that includes linear strips  122 ,  122 ′ that are rigid enough to support the LEDs  120  in the cavity between the diffusive coupler  110  and the reflective surface  140  without an additional supporting frame. Sufficiently rigid linear strips  122 ,  122 ′, such as those made of or supported with aluminum or printed circuit board (PCB) substrate, are mechanically connected to the panel&#39;s peripheral frame, e.g., with standoffs or other suitable connections. Preferred panels include rigid strips  122 ,  122 ′ that are suspended within the cavity between the diffusive coupler  110  and reflective surface  140  at a height sufficient to prevent shadows from appearing in the illumination pattern as described below. 
       FIG. 2  also illustrates the spacing of adjacent LEDs  120  according to the formula, t=(x/2)/tan α, where t is the separation distance between the reflective surface  140  and the LEDs  120 , x is the distance between adjacent strips  122 ,  122 ′, and α is an angle within a range between about 30 and about 60°. The angle α is normally the point in the LEDs distribution where the light intensity has dropped to about 60% of the on axis intensity. In other words, when viewed from an angle α, the LED elements  120  appear to emit about 60% as much light as they appear to emit when viewed head on. As an example, a 2′×2′ panel suitable for use in a suspended ceiling may include ten evenly separated strips  122 ,  122 ′ with approximately two inches between adjacent strips  122 ,  122 ′. Applying the above formula yields separation between the sources  120  and the top diffuse reflector  140  of about 
             t   =         (       2   ″     /   2     )       tan   ⁢           ⁢   50   ⁢   °       =       0.84   ″     .             
Moving the LEDs  120  closer together does not affect the uniformity of the illumination pattern, but decreasing the distance from the LEDs  120  to the reflector  140  causes a visible darkening between sources  120 , which is unpleasant to the viewer. Economically, one would like to have the fewest number of strips, hence the largest separation between strips for a given cavity depth.
 
       FIG. 3  is a simplified schematic side view of a thermal-dissipating embodiment of the present invention having a bottom, composite light source element shown in a configuration for a drop ceiling installation having a bottom transmissive diffusing panel  110 , a bottom-positioned LED light source elements  120  affixed to a heat-dissipating element  132 , and a top diffuse reflective surface  140  which may be a surface of the top panel  150 . Each composite light source comprises LEDs  120  affixed to a particular heat-dissipating element  132 . In operation, the heat-dissipating element  132  distributes the heat generated by the light source elements  120  throughout its volume and more evenly transfers the heat to the interior space and bottom panel  110 . In preferred embodiments, the heat-dissipating element  132  is made of PCB substrate, aluminum, or another suitable thermally conductive material. Sufficiently rigid heat-dissipating elements  132  may be used to support the LEDs  120  above the diffusive coupler as described above with respect to  FIG. 2 . 
     Inventive lighting panels are lighter, thinner, and provide higher light intensity at substantially lower fabrication and maintenance costs than prior art luminaires. They also dissipate heat into the illuminated environment instead of into the ceiling space. In winter when heating the room, the LEDs  120  help modestly heat the room. In the summer when the room is cooled, the LEDs  120  are subject to a lower ambient temperature than that in the space above the ceiling, which increases both the efficiency and lifetime of the LEDs  120 . Dissipating the heat into the room also allows for thermal insulation to be placed on top of the panel in the ceiling space to further insulate the room. 
       FIG. 3  also shows how construction of inventive lighting panels may be further simplified by manufacturing an integrated top panel  150  and perimeter frame  152 ,  152 ′. In operation, the perimeter frame  152 ,  152 ′ incorporates supports and attachment elements for the LED strips  122 ,  122 ′ and bottom panel  110 . The full interior surface of the lighting panel may be diffusely reflective  140 . The diffusive coupler  110 , top panel  150 , and perimeter frame  152 ,  152 ′, may be sealed to provide a dust- and waterproof cavity for the LEDs  120 . The perimeter frame  152 ,  152 ′ may also dissipate heat that is transmitted from the LEDs  120  via the strips  122 ,  122 ′. 
       FIGS. 4A-4D  are graphical representations of spatial light distributions of preferred embodiments where the spacing of the light source arrays  122 ,  122 ′, the bottom panel  110 , and the reflective surface  140  are selected to produce illumination patterns that are substantially uniform between adjacent arrays  122 ,  122 ′. That is, the light arrays  122 ,  122 ′ are positioned to emit patterns that vary in intensity by about ±15% or less between adjacent arrays  122 ,  122 ′. Variations in intensity of ±15% or less are hard for the human eye to discern. Varying the relative and absolute distances of the linear light arrays  122 ,  122 ′ from the bottom panel  110  and the reflective surface  140  affects the visibility of shadows cast by the arrays  122 ,  122 ′. The generalized effect is shown in the corresponding graph of intensity versus position where  FIG. 4A  shows the shadow when the arrays  122 ,  122 ′ are affixed to the bottom panel  110  occluding the bottom panel  110 , and  FIG. 4D  shows a uniform intensity where the spacing is optimized to eliminate the visible shadow due to the LED strips  122 ,  122 ′. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.