Abstract:
A bidirectional light sheet including at least a first and second array of bare LED chips with top and bottom electrodes, where the arrays of LEDs are sandwiched between at least two transparent substrates having conductors bonded to the electrodes without wires to form the light sheet. The LEDs of the array are arranged within the light sheets to emit light from opposite surfaces of the light sheet to create a bidirectional light sheet. The bidirectional light sheet is suspended from a ceiling to be non-parallel to the ceiling.

Description:
FIELD OF INVENTION 
     This invention relates to solid state illumination and, in particular, to a substantially flat bidirectional light sheet containing light emitting dies, such as light emitting diodes (LEDs), where the light sheet is orientated at a non-parallel angle, such as vertically, with respect to a ceiling. 
     BACKGROUND 
     High power LEDs are the conventional choice for general solid state lighting applications. Such high power white LEDs are extremely bright and can have luminous efficacies between 100 and 200 lumens/watt. The input power of a single high-power LED is typically greater than 0.5 watt and may be greater than 10 watts. Such LEDs generate considerable heat since they are only about 1 mm 2  in area, so the required packaging is fairly complex and expensive. Although a bare high-power LED chip typically costs well under $1.00 (e.g., $0.10), the packaged LED typically costs around $1.50-$3.00. This makes a high output (e.g., 3000+lumens) solid state luminaire relatively expensive and not a commercially feasible alternative for a fluorescent light fixture, commonly used for general illumination. Further, the optics required to convert the high brightness point sources into a substantially homogeneous, broad angle αmission for an office environment (where glare control is important) is extremely challenging. 
     To greatly reduce the cost of a large area, high lumen output light source, it is known to sandwich an array of bare LED dice between a reflective bottom sheet having conductors and a top transparent sheet having conductors. The LEDs have top and bottom electrodes that contact a set of conductors. When the conductors on the sheets are energized, the LEDs emit light through only the transparent sheet. The light sheet may be flexible. 
     Such prior art light sheets are not bidirectional. 
     It is also well known to provide a light emitting panel as a luminaire for general illumination, where the panel is oriented so that its light emitting surface is parallel to a floor. 
     It may be desirable in certain environments to provide a cost-effective luminaire that generates lighting effects other than those of the above-described prior art luminaires. 
     SUMMARY 
     Bidirectional light sheets and novel orientations of the light sheets are described. The light sheets can be formed to have any dimensions, including narrow strips. 
     In one embodiment, an array of bare light emitting diode (LED) chips, having top electrodes and bottom electrodes, are sandwiched between two or more substrates having conductors formed on their surfaces. LEDs with top and bottom electrodes are typically referred to as vertical LEDs. The bottom electrode of commercially available vertical LEDs is reflective and covers the entire bottom surface of the LED. Therefore, the typical vertical LED emits light only from its top surface and sides. The top electrode is intended by the LED manufacturer to be bonded to a thin wire using ultrasonic bonding or other bonding technique. 
     The light sheets used in embodiments of the present invention employ conductors on the substrates that electrically contact the LED electrodes without using wires. The conductors may connect any number of LEDs in series and are ultimately connected to a power source. In another embodiment, wires may be used for the connections, adding considerable cost and complexity to the light sheet. 
     In one embodiment, the orientations of the vertical LEDs are alternated so that the conductors on the substrates connect an anode of one LED to the cathode of the adjacent LED for a series connection. In this way, the LEDs having one orientation emit light in one general direction, and LEDs having the opposite orientation emit light in the opposite direction. Therefore, the light sheet emits bidirectional light. Reflectors (e.g., prisms) in the substrates may be used to direct any side light toward the desired light output surface of the sheet. 
     In other embodiments, two light sheets are effectively affixed back-to-back, where the light sheets emit light in opposite directions to form a bidirectional light sheet. A reflective sheet may be used as an intermediate layer between the opposing light sheets. 
     In one embodiment, control electronics may be located on or in an intermediate layer between the light sheets. 
     In an application of a bidirectional light sheet, the sheet (e.g., a strip) may be suspended from a ceiling so that it is orientated vertically (i.e., perpendicular to the ceiling and floor). Optics may be molded into the light emitting surfaces to angle the peak light intensity downward (e.g., at 55 degrees relative to vertical) to avoid glare and to merge the light of one fixture with light from adjacent fixtures. Other ways of directing the light may also be used, such as locating the LED chips in reflective cups or deep wells that emit a collimated beam of light at any selected angle. Portions of the light sheet, or another light sheet in the same fixture, may also be designed to direct light upward to reflect off the ceiling to achieve broad illumination. Any combinations of peak intensity angles may be achieved. 
     In one embodiment, a luminaire is created with a plurality of pivotable bidirectional light sheets so the user can customize the light emission pattern. In another embodiment, the flexible light sheet may be formed as a cylinder and suspended from the ceiling to provide uniform illumination of the floor and ceiling. In another embodiment, the light sheet may be formed as a truncated pyramid and suspended from the ceiling. 
     Light emitting dies other than LEDs may also be used. 
     Other variations are described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The below described drawings are presented to illustrate some possible examples of the invention. 
         FIG. 1  is a simplified perspective view of a portion of a bidirectional light sheet, in accordance with one embodiment of the invention, showing some light emitting areas. 
         FIG. 2  illustrates a series connection of LEDs in the light sheet of  FIG. 1 . 
         FIG. 3  is a cross-sectional view along line  3 - 3  in  FIG. 1  showing LEDs in opposite orientations being connected in series. 
         FIG. 4  is a cross-sectional view along line  3 - 3  in  FIG. 1  showing back-to-back light sheets containing flip-chip LEDs connected in series. 
         FIG. 5  is a cross-sectional view along line  3 - 3  in  FIG. 1  showing back-to-back light sheets containing vertical LEDs connected in series. 
         FIG. 6  illustrates a conductor connection in the light sheet of  FIG. 5  showing the series connection between adjacent LEDs. 
         FIG. 7  is a perspective view of any of the bidirectional light sheets being orientated approximately perpendicular to a ceiling and emitting light at a variety of peak intensity angles. 
         FIG. 8  is a bottom up view of a luminaire containing a plurality of the bidirectional light sheets and adjustable to emit light at a variety of peak intensity angles. 
         FIG. 9  is a side view of the luminaire of  FIG. 8 . 
         FIG. 10  is a perspective view of a bidirectional light sheet that is bent to form a cylinder, where the cylinder is suspended from a ceiling. 
         FIG. 11  illustrates two curved bidirectional light sheets that may be angled in any direction and curved to have any radius. The light sheets may be suspended from a ceiling. 
         FIG. 12A  is a side view of bidirectional light sheets forming a truncated pyramid, shown suspended from a ceiling. 
         FIG. 12B  is a bottom up view of the luminaire of  FIG. 12A . 
     
    
    
     Elements that are the same or similar are labeled with the same numerals. 
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a portion of a light sheet  10 , showing a simplified pseudo-random pattern of LED areas  12 . The LED areas  12  may instead be in an ordered pattern. There may be 500 or more low power LEDs in the light sheet  10  to generate the approximately 3700 lumens (per the DOE CALiPER benchmark test) needed to replace a standard fluorescent fixture typically found in offices. 
     The pseudo-random pattern may repeat around the light sheet  10  (only the portion within the dashed outline is shown). A pseudo-random pattern is preferred over an ordered pattern since, if one or more LEDs fail or have a poor electrical connection, its absence will be significantly harder to notice. 
     In one embodiment, the light sheet  10  is generally formed of three main layers: a transparent bottom substrate  14  having an electrode and conductor pattern; an intermediate sheet  16  acting as a spacer and optional reflector; and a transparent top substrate  18  having an electrode and conductor pattern. In one embodiment, the LED chips are electrically connected between electrodes on the bottom substrate  14  and electrodes on the top substrate  18 . The light sheet  10  is very thin, such as a few millimeters, and is flexible. 
       FIG. 2  illustrates a sample pattern of conductors  19  on the top substrate  18  and/or bottom substrate  14  that connect any number of LED chips in series. In the example of  FIG. 2 , two groups of series-connected LEDs in the LED areas  12  are connected in parallel. The parallel connection may be selected by a customizable interconnector  22  external to the light sheet. The customizable interconnection of the LED chips allows the drive voltage and current to be selected by the customer or be customized for a particular size of light sheet. There may be many strings of LED chips in the light sheet that are connected together in series, parallel, or connected to different power supplies. 
     A DC or AC power supply  23  is shown connected to the connector  22 . An input of the power supply  23  may be connected to the mains voltage. If the voltage drop of an LED series string is sufficiently high, the series string of LEDs may be driven by a rectified mains voltage (e.g., 120 VAC). 
     As shown in  FIG. 3 , to achieve a series connection of LED chips using top and bottom conductors, some LEDs chips  26  are mounted on the bottom substrate  14  with their anodes  27  connected to the bottom substrate electrodes  28  and other LED chips  26  are mounted with their cathodes  30  connected to the bottom substrate electrodes  28 . Ideally, adjacent LED chips are reversely mounted to simplify the series connection pattern. The conductors  19  between the electrodes then connects the LED chips in series. A similar pattern of conductors  32  on the top substrate  18  connects the cathodes of LED chips  26  to the anodes of adjacent LED chips  26 . 
     In another embodiment, it is also possible to connect the LED chips in two anti-parallel series branches, or derivatives thereof, that will enable the LED chips to be driven directly from AC, such as directly from the mains voltage. 
     Since the cathodes  30  of the LED chips  26  are typically large reflectors that cover the entire bottom surface of the LED chips, the light emitted from the oppositely orientated LED chips  26  will be in opposite directions. Reflectors  36  molded into the substrates  14 / 18  or intermediate sheet  16  may be used to reflect side light toward the output surfaces of the light sheet. 
     If the LED chips  26  emit blue light, phosphor  38  may be deposited over the light path to convert the blue light to white light, as shown by the light rays  40 . Phosphor may also be incorporated into an encapsulant that fills the holes in the intermediate sheet  16  surrounding the LED chips  26 . 
     Additional details of the various bidirectional light sheets shown herein may be found in U.S. application Ser. No. 12/870,760, filed on Aug. 27, 2010, entitled, Solid State Light Sheet for General Illumination, by Louis Lerman et al., incorporated herein by reference. 
       FIG. 4  illustrates another bidirectional light sheet, where the LED chips  44  are flip chips, with anode and cathode electrodes  46  on the bottom surface of the LED chips  44 . One set of LED chips  44  are sandwiched between a top substrate  18  and a bottom substrate  14 , and another set of LED chips is sandwiched between the same bottom substrate  18  and another substrate  48 . Alternately, two light sheets may be separately manufactured and laminated together back-to-back. A reflector layer may be positioned between the two sets of LED chips. The LED chips in each set may be connected in any combination of series and parallel. 
       FIG. 5  illustrates another embodiment of a bidirectional light sheet, where the top substrate  18  and bottom substrate  14  have conductors  50  and  52  that overlap when the substrates are laminated together to form a series connection between LED chips  26 . Two light sheets are laminated together with a reflective layer  53  between them to cause light to be emitted bidirectionally from the back-to-back light sheets. 
       FIG. 6  is a top down view of the light sheet portion of  FIG. 5  showing the overlapping conductors  50  and  52  connecting the LED chips  26  in series. 
     The substrate electrodes over the LED chip anodes may by transparent conductors, such as ITO (indium-doped tin oxide) or ATO (antimony-doped tin oxide) layers, to avoid blocking light. 
     The intermediate layer between the sets of LED chips may include control electronics and/or cross-over conductors for interconnecting the LED chips and controlling brightness. 
       FIG. 7  illustrates any of the bidirectional light sheets being suspended from a ceiling  60  by wires  61  and orientated approximately perpendicular to the ceiling  60 . The wires  61  may conduct a low DC voltage (e.g., 24 volts DC) to the LED chips or may supply a mains voltage to a power converter in the luminaire. The light sheets are shown emitting light  62  at a variety of peak intensity angles. Lenses  63  may be molded in the transparent surfaces of any of the light sheets to direct the peak intensity at any angle. The lenses may be Fresnel lenses, elongated grooved lenses, or other lens shapes to achieve the desired light emission angles. Other ways of directing the light may also be used in any of the embodiments, such as locating the LED chips in reflective cups or deep wells that emit a collimated beam of light at any selected angle. This can be done by angling the cups or shaping the cups. 
     In  FIG. 7 , two, bidirectional light sheets  64  and  66  are mounted together in the same luminaire, where the light sheet  64  has lenses that generally direct light downward, and the light sheet  66  has lenses that generally direct light upward to reflect off the ceiling  60 . Light from adjacent, identical luminaires merge across the floor and ceiling to create an overall smooth lighting effect. The luminaires may replace standard fluorescent lamp troffers, yet not require any space above the ceiling. This enables the luminaires to be used where the ceiling is not a drop down ceiling. 
     The light angles coming from both sides of the light sheet may be mirror images for symmetry or may be asymmetrical. 
     Instead of a flat light sheet, the light sheet may be bent to form an arc or other shape, depending on the desired emission pattern. 
     The light sheet may be affixed to the ceiling at non-parallel angles other than a vertical orientation, depending on the particular light effect desired. However, a symmetrical light emission for room illumination will typically be desired. 
     In another embodiment, there are a variety of lenses in a single light sheet to direct the light at two or more different angles. This may be used to create a very compact luminaire formed of one or more light sheets. 
     Many other aesthetic light patterns may be generated from the vertical orientation of the bidirectional light sheets and the types of lenses formed in the light sheets. 
       FIG. 8  is a bottom up view, and  FIG. 9  is a side view, of a luminaire containing four bidirectional light sheets  70 , which are adjustable to emit light at a variety of peak intensity angles. Each light sheet may output light at a certain downward peak intensity angle, such as 55 degrees relative to the nadir, or each light sheet may emit at a different peak intensity angle. The angles of the physical light sheets  70  may be adjusted by pivoting  71  the light sheet around an axis. For example, one edge of each light sheet may be connected to a pivoting support on the luminaire base  72 . The peak intensity light rays  74  from the four light sheets  70  are shown being at different angles. Any number of light sheets  70  at any orientation (e.g., diagonal, parallel, perpendicular) may be used in the luminaire. 
     The bidirectionality of the flexible light sheet is very useful in hanging luminaires where it is desired to illuminate the ceiling as well as the floor. Illuminating a ceiling creates a pleasant aesthetic effect and provides more uniform lighting throughout the room.  FIGS. 10-12B  illustrate additional luminaires that reflect light off the ceiling. 
       FIG. 10  is a perspective view of a bidirectional light sheet  78  that is bent to form a cylinder, where the cylinder is suspended from a ceiling  80 . The flexible light sheet  78  may be supported along its edges by a plastic cylindrical frame that is suspended from the ceiling  80  by wires  82 . The curvature of the light sheet  78  causes light to be evenly emitted 360 degrees around a central axis. The peak intensity of light may be directed downward to avoid glare by lenses or other optical means. In one embodiment, the peak intensity is at 55 degrees relative to the nadir. The light emitted from the inside surface of the cylinder is both directed upward to reflect off the ceiling  80  and downward to avoid any dark spot under the luminaire. Angled light rays  84  are shown being emitted from the outer surface of the light sheet  78 . The outer surface may also emit a percentage of the light toward the ceiling  80  for more uniform illumination of the ceiling  80 . An angled light ray  86  is shown being emitted from the inside surface of the light sheet  78  and reflected off the ceiling  80  to avoid a dark spot above the luminaire, and downward light rays  88  are shown being emitted from the inside surface of the light sheet  78  to avoid a dark spot under the luminaire. 
       FIG. 11  illustrates two curved bidirectional light sheets  90  and  92  that may be angled in any direction and curved to have any radius. The light sheets  90 / 92  may be suspended from a ceiling as in  FIG. 10 . The light sheets  90 / 92  may each be supported by a frame to allow each to be independently tilted and pivoted around a central axis. Since the light  94  emitted by each bidirectional light sheet  90 / 92  is asymmetrical, virtually any light pattern may be created by changing the angles and directions of the light sheets  90 / 92 . 
       FIG. 12A  is a side view of bidirectional light sheets  96  forming a truncated pyramid, shown suspended from a ceiling  80 . The light sheets  96  are directed at a downward angle, such as at a 55 degree angle, to direct light  98  downward. This provides 360 degree coverage of the floor. To avoid any dark spot above the luminaire and to illuminate the ceiling well beyond the area of the light sheets  96 , the inside surfaces of the light sheets  96  direct light  100  toward the ceiling  80 . A light sheet  96  may form the flat bottom surface of the luminaire, or the bottom may be open for increased air circulation. 
       FIG. 12B  is a bottom up view of the luminaire of  FIG. 12A . The light sheets  96  may be at any angle, such as to minimize glare. Lenses in the light sheet surfaces may be used to direct the light emission. 
     Other uses of a non-parallel oriented bidirectional light sheet are also envisioned. 
     The various features of all embodiments may be combined in any combination. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all changes and modifications that fall within the true spirit and scope of the invention.