fig1 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 . fig2 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 fig2 , 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 fig3 , 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 . fig4 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 . fig5 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 . fig6 is a top down view of the light sheet portion of fig5 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 . fig7 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 fig7 , 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 . fig8 is a bottom up view , and fig9 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 . fig1 - 12b illustrate additional luminaires that reflect light off the ceiling . fig1 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 . fig1 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 fig1 . 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 . fig1 a 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 . fig1 b is a bottom up view of the luminaire of fig1 a . 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 .