Abstract:
In accordance with various embodiments, planar light sheets are conformed to interior volumes of housings to produce predetermined spatial optical characteristics such as luminous intensity distributions.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/993,455, filed May 15, 2014. This application is also a continuation-in-part of U.S. patent application Ser. No. 14/303,197, filed Jun. 12, 2014, which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/834,183, filed Jun. 12, 2013. This application is also a continuation-in-part of U.S. patent application Ser. No. 14/301,859, filed Jun. 11, 2014, which (i) claims the benefit of and priority to U.S. Provisional Patent Application No. 61/834,183, filed Jun. 12, 2013, and (ii) is a continuation-in-part of U.S. patent application Ser. No. 14/195,175, filed on Mar. 3, 2014, which is a continuation of U.S. patent application Ser. No. 13/970,027, filed Aug. 19, 2013, which is a continuation-in-part of U.S. patent application Ser. No. 13/799,807, filed Mar. 13, 2013, which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/697,411, filed Sep. 6, 2012. The entire disclosure of each of these references is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     In various embodiments, the present invention generally relates to electronic devices, and more specifically to array-based lighting devices. 
     BACKGROUND 
     Solid-state lighting is an attractive alternative to incandescent and fluorescent lighting systems for a wide range of applications because of its relatively higher efficiency, robustness, and long life. In many cases, the long operational life of LEDs permits installation into systems such as fixtures, housings, or extrusions without provision for changing the LED source(s), because the LED lifetime exceeds the expected system lifetime. In many applications it is desired to minimize the size and/or volume of such systems. However, conventional LED systems typically require some form of heat-sinking or thermal management that is difficult to incorporate, particularly in relatively small and enclosed spaces. This is a particular issue for systems formed within a housing or extrusion. In such cases, the housing or extrusion typically must be large enough to accommodate not only the LEDs, circuit boards, and any heat sinks, but also must be engineered to not overheat from heat generated by the LEDs during operation. Such constraints may require additional ventilation, for example, passive ventilation or active ventilation such as fans, to prevent deleterious heat buildup. These issues typically lead to undesirably large, thick, and potentially complicated lighting systems. 
     In some applications it may be desirable for the housing of an LED-based lighting system to provide additional functionality, for example to aid in light distribution control, e.g., by using optics to achieve a specific light distribution pattern. For example, cove lighting or wall wash lighting systems often include a housing incorporating optics to achieve the desired light distribution patterns. The size and complexity of the optical elements are related to the size of the light source and the desired light distribution pattern, in many cases leading again to undesirably large systems. 
     In some applications the housing may provide protection for the LEDs, for example mechanical protection (e.g., protection against an object hitting or crushing the lighting system) or protection from environmental factors such as water, rain, snow, or the like. Outdoor applications typically require a housing for physical and environmental protection of the LEDs and associated circuitry. Such housings are often undesirably large because of the need to enclose the LEDs, circuit board, heat sinks, and associated control and drive circuitry. 
     Finally, many applications for backlighting and illuminated panels and signs require custom sizing to fit in a particular location. Systems featuring relatively few high-brightness LEDs on rigid circuit boards may be difficult to use cost-effectively in a wide range of installations while achieving high illumination uniformity and high efficiency. 
     Accordingly there is a need for solutions that provide LED-based lighting systems having a small form factor, provide the desired light distribution pattern, and which are simple to install. 
     SUMMARY 
     Embodiments of the present invention relate to illumination systems based on flexible light sheets and that incorporate additional functionality that enables various different mechanical mounting and electrical and/or mechanical joining techniques. For example, illumination systems in accordance with embodiments of the invention incorporate rigid, semi-rigid, flexible, and/or positionable housings that at least partially enclose flexible light sheets and that may also provide electrical connectivity. 
     In various embodiments of the present invention, flexible planar light sheets incorporating light-emitting elements are deformed to fit within three-dimensional, non-planar interior volumes of housings. As used herein, a light sheet may be deformed by being bent, curved, folded, and/or creased, either temporarily or permanently. That is, a deformed light sheet may recover to its undeformed, planar configuration in the absence of a deforming force and/or when not disposed within the shaped housing. In their undeformed states, light sheets in accordance with embodiments of the invention typically emit light having a particular spatial optical characteristic (e.g., a luminous intensity distribution, a correlated color temperature distribution, a color rendering index distribution, and/or a spectral power distribution), and, when deformed within a housing, the light sheets emit light having a different spatial optical characteristic. In various embodiments of the invention, light sheets are deformed via bending or curving, and the resulting radius of curvature of the light sheet is less than a maximum bending tolerance of the light sheet (i.e., the maximum amount of bending before the light sheet undergoes partial or complete mechanical or electrical failure). 
     Additional details of lighting systems in accordance with embodiments of the present invention appear within U.S. patent application Ser. No. 13/799,807, filed Mar. 13, 2013 (the &#39;807 application), and U.S. patent application Ser. No. 13/748,864, filed Jan. 24, 2013 (the &#39;864 application), the entire disclosure of each of which is incorporated by reference herein. 
     In an aspect, embodiments of the invention feature a lighting system that includes or consists essentially of a flexible light sheet and a housing having a three-dimensional, non-planar interior volume. The light sheet includes or consists essentially of (i) a flexible substrate having first and second opposed surfaces, (ii) a plurality of light-emitting elements disposed over at least one of the first or second surfaces of the substrate, and (iii) a plurality of conductive traces disposed on the substrate and electrically interconnecting the plurality of light-emitting elements. When the light sheet is in an undeformed configuration, (a) the light sheet is substantially planar and (b) the light-emitting elements of the light sheet collectively emit light having a first spatial optical characteristic. The light sheet is disposed within the interior volume of the housing. The light sheet is deformed to conform to the interior volume of the housing. The light-emitting elements of the light sheet collectively emit light through the housing with a second spatial optical characteristic different from the first spatial optical characteristic. 
     Embodiments of the invention may include one or more of the following in any of a variety of combinations. The first and/or second spatial optical characteristics may include or consist essentially of luminous intensity distributions. The first and/or second spatial optical characteristics may each include or consist essentially of a luminous intensity distribution, a correlated color temperature distribution, a color rendering index distribution, and/or a spectral power distribution. The light-emitting elements may or may not be in physical contact with at least a portion of the interior surface of the housing. The deformed light sheet may include or consist essentially of at least two portions. Two or more of the portions (even each portion) may have a different curvature. At least one portion may be substantially planar. Two or more of the portions (even each portion) may be non-coplanar. The deformed light sheet may include one or more folds (i.e., regions along which the light sheet is at least partially folded over on itself). The interior volume of the housing may be defined by and disposed between interior and exterior surfaces of the housing. The light sheet may be disposed between the interior and exterior surfaces of the housing. The light-emitting elements may be disposed over both the first and second surfaces of the substrate (e.g., both first and second surfaces of the substrate may have light-emitting elements disposed thereon). The housing may define an opening, at least at one of two opposed ends, for accessing the interior volume. The housing may be a unitary (i.e., one-piece) structure, e.g., an extrusion. The housing may include or consist essentially of at least two joined sections. The sections may be removable from each other. At least one end cap may cover at least a portion of the opening. The end cap may be removable. The housing may include mounting hardware for mounting the lighting system. The housing may define an opening for accessing the interior volume. At least one end cap may cover the opening. The at least one end cap may be removable. The housing may include or consist essentially of at least two joined sections. The sections may be removable from each other. The housing may include mounting hardware for mounting the lighting system. 
     The light sheet may include first and second spaced-apart power conductors for supplying power to the light-emitting elements. One or more electrical connectors may be disposed on the housing (e.g., on an interior and/or on an exterior surface of the housing). The one or more electrical connectors may include or consist essentially of (i) a first electrical connector electrically coupled to the first power conductor and (ii) a second electrical connector electrically coupled to the second power conductor. One or more electrical connectors may extend outside of the housing, and the one or more electrical connectors may provide electrical coupling to the first and/or second power conductors from outside of the housing. Portions of the housing may be electrically conductive and may provide electrical coupling to the first and second power conductors from outside of the housing. 
     The plurality of light-emitting elements may be spaced apart and interconnected, via the plurality of conductive traces, into one or more light-emitting strings. Each light-emitting string may have (i) a first end electrically connected to the first power conductor and (ii) a second end electrically connected to the second power conductor. The light sheet may include one or more control elements each configured to control current to one or more of the light-emitting strings. The one or more control elements may be disposed on the first and/or second surfaces of the substrate. The one or more control elements may each be electrically coupled to at least one (or only one) light-emitting string. The substrate may be separable, via a cut spanning the first and second power conductors and not crossing a first light-emitting string, into two partial substrates each including or consisting essentially of (i) one or more light-emitting strings and (ii) portions of the first and second power conductors configured to supply power to and thereby illuminate the one or more light-emitting strings of the partial substrate. Along each light-emitting string, a first pitch (or spacing) at which the first light-emitting elements are spaced may be substantially constant. The one or more light-emitting strings may include or consist essentially of a plurality of light-emitting strings. Over the substrate, the light-emitting elements may be spaced apart at the first pitch, and the first pitch may be maintained between light-emitting elements of different ones of the light-emitting strings. 
     The lighting system may include a power supply for energizing the plurality of light-emitting elements. The power supply may be disposed within the housing. The lighting system may include at least one battery for energizing the plurality of light-emitting elements. The at least one battery may be disposed within the housing. At least one sensing element may be disposed within and/or on the housing. The at least one sensing element may include or consist essentially of a light sensor, an occupancy sensor, a temperature sensor, a smoke detector, and/or a fire detector. The housing may be in thermal contact with the light sheet and/or the light-emitting elements, whereby the housing is a heat sink for the light sheet and/or the light-emitting elements. 
     The lighting system may include a second flexible light sheet and a second housing having a three-dimensional, non-planar interior volume. The second light sheet may include or consist essentially of (i) a second flexible substrate having first and second opposed surfaces, (ii) a plurality of second light-emitting elements disposed over at least one of the first or second surfaces of the second substrate, and (iii) a plurality of conductive traces disposed on the second substrate and electrically interconnecting the plurality of second light-emitting elements. When the second light sheet is in an undeformed configuration, (a) the second light sheet may be substantially planar and (b) the second light-emitting elements of the second light sheet may collectively emit light having a third spatial optical characteristic. The second light sheet may be disposed within the interior volume of the second housing. The second light sheet may be deformed to conform to the interior volume of the second housing. The second light-emitting elements of the second light sheet may collectively emit light through the second housing with a fourth spatial optical characteristic different from the third spatial optical characteristic. The first spatial optical characteristic may be the same as the third spatial optical characteristic. The second spatial optical characteristic may be the same as the fourth spatial optical characteristic. The first spatial optical characteristic may be different from the third spatial optical characteristic. The second spatial optical characteristic may be different from the fourth spatial optical characteristic. The light sheet may be electrically coupled to the second light sheet. The housing may be mechanically coupled to the second housing. 
     The lighting system may include a second flexible light sheet that includes or consists essentially of (i) a second flexible substrate having first and second opposed surfaces, (ii) a plurality of second light-emitting elements disposed over at least one of the first or second surfaces of the second substrate, and (iii) a plurality of conductive traces disposed on the second substrate and electrically interconnecting the plurality of second light-emitting elements. When the second light sheet is in an undeformed configuration, (a) the second light sheet may be substantially planar and (b) the second light-emitting elements of the second light sheet may collectively emit light having a third spatial optical characteristic. The second light sheet may be disposed within the interior volume of the housing. The second light sheet may be deformed to conform to the interior volume of the housing. The second light-emitting elements of the second light sheet may collectively emit light through the housing with a fourth spatial optical characteristic different from the third spatial optical characteristic. The first spatial optical characteristic may be the same as the third spatial optical characteristic. The second spatial optical characteristic may be the same as the fourth spatial optical characteristic. The first spatial optical characteristic may be different from the third spatial optical characteristic. The second spatial optical characteristic may be different from the fourth spatial optical characteristic. The light sheet may be electrically coupled to the second light sheet. 
     The lighting system may have an ingress protection rating of at least IP 65, as specified by International Protection Marking in International Electrotechnical Commission (IEC) standard 60529. The light sheet may have a thickness in the range of 0.25 mm to 25 mm (inclusive). The light-emitting elements may emit substantially white light. The light-emitting elements may include or consist essentially of light-emitting diodes (e.g., bare-die light-emitting diodes and/or packaged light-emitting diodes). At least a portion of the housing may be transparent to a wavelength of light emitted by the light-emitting elements. At least a portion of the housing may have a transmittance greater than 60% to a wavelength of light emitted by the light-emitting elements. The housing may include or consist essentially of one or more diffusers or diffusing elements. The housing may define one or more optical elements. Each light-emitting element may be associated with, and substantially aligned to, one optical element. The one or more optical elements may include or consist essentially of a refractive optic, a reflecting optic, a total internal reflectance optic, and/or a Fresnel optic. The lighting system may include one or more support structures (e.g., solid support structures) to which the housing is mechanically coupled. The lighting system may be flexible. The lighting system may be positionable, whereby the lighting system maintains a deformed configuration in the absence of a deforming force. A curvature of the light sheet within the housing may be less than a maximum bending tolerance of the light sheet. The substrate may include or consist essentially of polyethylene naphthalate, polyethylene terephthalate, polycarbonate, polyethersulfone, polyester, polyimide, polyethylene, fiberglass, FR4, and/or paper. The conductive traces may include or consist essentially of copper and/or aluminum. The interior volume of the housing may include or consist essentially of at least two portions each having a different curvature. The interior volume of the housing may include or consist essentially of at least two portions each having a different width or diameter. 
     In another aspect, embodiments of the invention feature a lighting system that includes or consists essentially of a flexible light sheet and a housing having a three-dimensional, non-planar interior volume. The light sheet includes or consists essentially of (i) a flexible substrate having first and second opposed surfaces, (ii) a plurality of light-emitting elements disposed over at least one of the first or second surfaces of the substrate, and (iii) a plurality of conductive traces disposed on the substrate and electrically interconnecting the plurality of light-emitting elements. When the light sheet is in an uncurved configuration, (a) the light sheet is substantially planar and (b) the light-emitting elements of the light sheet collectively emit light having a first luminous intensity distribution. The light sheet is disposed within the interior volume of the housing. The light sheet is curved to conform to the interior volume of the housing, the curvature of the light sheet within the housing being less than a maximum bending tolerance of the light sheet. The light-emitting elements of the light sheet collectively emit light through the housing with a second luminous intensity distribution different from the first luminous intensity distribution. 
     These and other objects, along with advantages and features of the invention, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. As used herein, the terms “about,” “approximately,” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. 
     Herein, two components such as light-emitting elements and/or optical elements being “aligned” or “associated” with each other may refer to such components being mechanically and/or optically aligned. By “mechanically aligned” is meant coaxial or situated along a parallel axis. By “optically aligned” is meant that at least some light (or other electromagnetic signal) emitted by or passing through one component passes through and/or is emitted by the other. As used herein, the terms “phosphor,” “wavelength-conversion material,” and “light-conversion material” refer to any material that shifts the wavelength of light striking it and/or that is luminescent, fluorescent, and/or phosphorescent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIGS. 1A and 1B  are side views of illumination systems in accordance with various embodiments of the invention; 
         FIG. 1C  is a perspective view of the illumination system of  FIG. 1A ; 
         FIG. 1D  is a side view of an illumination system in accordance with various embodiments of the invention; 
         FIGS. 2A-2C  are side views of illumination systems in accordance with various embodiments of the invention; 
         FIG. 3  is a perspective view of an illumination system in accordance with various embodiments of the invention; 
         FIG. 4A  is a side view of an illumination system in accordance with various embodiments of the invention; 
         FIGS. 4B and 4C  are side views of the illumination system of  FIG. 4A  at earlier stages of manufacture; 
         FIGS. 5 and 6  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIGS. 7A and 7B  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIGS. 8A-8E  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIG. 9A  is a circuit diagram of a light sheet in accordance with various embodiments of the invention; 
         FIGS. 9B and 9C  are schematics of a portion of a light sheet in accordance with various embodiments of the invention; 
         FIG. 10  is a schematic view of an illumination system in accordance with various embodiments of the invention; 
         FIG. 11A  is a partial circuit diagram of a portion of an illumination system in accordance with various embodiments of the invention; 
         FIG. 11B  is a plan-view schematic of a portion of a light sheet in accordance with various embodiments of the invention; 
         FIG. 11C  is a cross-sectional schematic of the light-sheet portion of  FIG. 11B ; 
         FIG. 11D  is a cross-sectional schematic of the interior of a housing element in accordance with various embodiments of the invention; 
         FIGS. 12A and 12B  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIGS. 13A and 13B  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIGS. 14A and 14B  are schematic views of illumination systems in accordance with various embodiments of the invention; 
         FIG. 15A  is a schematic plan view of a light-emitting element in accordance with various embodiments of the invention; 
         FIGS. 15B and 15C  are light distribution patterns in accordance with various embodiments of the invention; and 
         FIGS. 15D, 15E, and 15F  are schematic views of illumination systems in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  depicts an exemplary lighting device  100  in accordance with embodiments of the present invention, although alternative devices or systems with similar functionality are also within the scope of the invention. As depicted, lighting device  100  includes or consists essentially of one or more flexible light sheets  110  and one or more housing elements  120  having one or more interior regions  130 . In some embodiments, light sheets  110  may include an array of light-emitting elements (LEEs)  140 , for example as shown in  FIG. 1B .  FIG. 1C  shows an isometric view of the structure of  FIG. 1A . In the embodiment shown in  FIG. 1C , housing element  120  partially encloses light sheet  110 , covering substantially all of the flat face of light sheet  110 , but not covering portions of the edges of light sheet  110  (both the visible edge and the back edge not visible in the view of  FIG. 1C ; see also  FIG. 3 ). In some embodiments, one or more end caps may be included to completely or substantially enclose light sheet  110  in housing element  120 . As will be discussed herein, in some embodiments lighting device  100  may include additional elements, for example end caps, connectors, mounting hardware, additional support or decorative structures, power supplies or drivers, sensing elements, control elements or the like. 
     In various embodiments, housing element  120 , also referred to herein as an extrusion, holds or positions flexible light sheet  110  in a specific, pre-determined position or shape in an interior region  130  to achieve one or more specific characteristics or attributes. For example, in some embodiments housing element  120  may hold light sheet  110  in a specific position to achieve a specific light distribution pattern. For example, as shown in  FIG. 1A , housing element  120  retains light sheet  110  in a U-shape, resulting in a different light distribution pattern from lighting device  100  than would be achieved using a flat light sheet  110 . 
     In various embodiments, housing element  120  or portions of housing element  120  may be manufactured using an extrusion process (and thus, housing element  120  or portions of housing element  120  may be an “extrusion”). In various embodiments, the extrusion process permits a relatively cost-effective manufacturing method for housing element  120 . In various embodiments, an extruded housing element  120  may be transparent to a wavelength of light emitted by LEEs  140  or a portion of housing element  120  may be transparent to a wavelength of light emitted by LEEs  140 . In various embodiments, housing element  120  may include one or more optical elements or components (e.g., lenses) thereon, and/or one or more optical elements or components may be defined by the housing element  120  itself. In various embodiments, one or more optical elements may be incorporated into housing element  120  during the extrusion process. For example, one or more linear optical elements may be formed in or as part of housing element  120  in the extrusion direction. 
     In some embodiments, housing element  120  may hold light sheet  110  in a specific position to achieve a specific thermal profile. For example, in various embodiments, all or a portion of housing element  120  may act as a heat sink for conducting heat away from light sheet  110  and/or LEEs  140 . In various embodiments, housing element  120  may be in thermal contact with a portion of light sheet  110  and/or a portion of LEEs  140  and may act as a heat sink for heat generated in light sheet  110  and/or LEEs  140 . In various embodiments, heat generated in portions of light sheet  110  and/or LEEs  140  may be distributed over the relatively larger area of housing element  120 , thus resulting in a reduced temperature of LEEs  140  and/or light sheet  110 . In various embodiments, because housing element  120  has a relatively large area, the temperature of housing element  120  may be relatively lower than the temperature of light sheet  110  and/or LEEs  140  without the use of housing element  120  as a heat sink. In some embodiments, light having different characteristics, for example correlated color temperature (CCT), color rendering index (CRI), angular light distribution pattern, or the like may be emitted from different portions of light sheet  110 . In some embodiments, housing element  120  may shape or hold different portions of light sheet  110  in specific positions to achieve specific but different characteristics, e.g., optical characteristics such as light intensity, CCT, CRI, spectral power distribution or the like, in different regions of the light distribution pattern emitted by lighting device  100 . As shown in  FIG. 1D , portions of light sheet  110  retained within different portions of housing element  120  may have different numbers or areal densities of LEEs  140  thereon in order to achieve different illumination characteristics for light emitted from different portions of the housing element  120 . 
     In some embodiments, housing element  120  may form or hold different portions of light sheet  110  in specific positions to achieve specific but different characteristics, for example thermal characteristics such as temperature or thermal resistance or the like in different regions of housing element  120  and/or lighting device  100 . 
     While in  FIGS. 1A-1D , housing element  120  has a U-shape, this is not a limitation of the present invention, and in other embodiments housing element  120  may have a different curved shape, or may include one or more straight portions, or a combination of straight and curved portions, or may have any arbitrary shape.  FIG. 2A  depicts an embodiment having a curved housing element  120  enclosing light sheet  110 . In some embodiments, light from light sheet  110  may be emitted from one side of light sheet  110 , for example the side identified as side  230 , generally in a direction  210 , while in other embodiments, light from light sheet  110  may be emitted from a different or opposite side of light sheet  110 , for example the side identified as side  240 , generally in a direction  220  (i.e., generally opposite direction  210 ).  FIG. 2B  depicts an embodiment in which housing element  120  includes substantially straight or planar portions, for example having an L shape. The shape of housing  110  shown in  FIG. 2B  is not a limitation of the present invention, and in other embodiments housing  110  may have other shapes including or consisting essentially of two or more planar sections, or one or more planar sections and one or more curved sections.  FIG. 2C  depicts an embodiment in which housing element  120  includes both substantially curved and substantially straight or planar portions. 
     In some embodiments, housing element  120  is constructed such that interior region  130  is only accessible through one or both of the ends of housing element  120 . For example, as depicted in  FIG. 3 , one or more light sheets  110  must be inserted into interior region  130  of housing element  120  via one or both of the ends of housing element  120 . In other words, in some embodiments the portions of housing element  120  that are adjacent to or in contact with LEEs  140 , the surface of light sheet  110  on which LEEs  140  are disposed, or the side of light sheet  110  opposite that on which LEEs  140  are disposed do not move or open to permit placement of light sheet  110  in housing element  120 . In some embodiments, portions of interior region  130  may be formed from one non-separable portion of housing element  120 . 
     In various embodiments of the present invention, light sheet  110  may curved into shape to fit within housing element  120 , while in other embodiments light sheet  110  may include one or more folds or creases. For example, in the lighting systems of  FIG. 2B  and  FIG. 3 , light sheet  110  may be folded or creased to fit into the L-shaped housing element  120 . 
     In some embodiments, one or more portions of housing element  120  that are adjacent to or in contact with the surfaces of LEEs  140 , light sheet  110  on which LEEs  140  are disposed, or the side of light sheet  110  opposite that on which LEEs  140  are disposed may move or open to permit placement of light sheet  110  in housing element  120 , or placement of light sheet  110  in interior region  130  in housing element  120 . For example, in one embodiment portions of interior region  130  may be formed by two or more separable portions of housing element  120 .  FIG. 4A  shows one example of an embodiment of the present invention showing a separable clam-shell-type housing element  120  that features housing element portions  120 ′ and  120 ″. In one embodiment, light sheet  110  may be placed in between housing element portions  120 ; and  120 ′; as shown in  FIG. 4B , during the assembly process. Housing element portions  120 ; and  120 ′; are then brought together and mated together to form a housing element  120 . During this process, light sheet  110  deforms to substantially conform to the interior shape of interior region  130 , as shown in  FIG. 4C , resulting in the structure of  FIG. 4A . In some embodiments, light sheet  110  may already be shaped to substantially conform to the interior shape of interior region  130 , and thus may not deform substantially during assembly of housing element portions  120 ′ and  120 ″. 
       FIGS. 1A-4C  show various geometries for lighting devices  100  and housing elements  120 ; however, these are not limitations of the present invention, and in other embodiments lighting device  100  and housing element  120  may have any shape or geometry. While  FIGS. 4A-4C  show housing  120  composed of two housing element portions, this is not a limitation of the present invention, and in other embodiments housing element  120  may be composed of more than two housing element portions. 
     In some embodiments, lighting device  100  may be configured to protect light sheet  110 , for example to provide mechanical protection, protection from dust, water, etc. One method for rating different levels of environmental protection is an IP rating as specified by International Protection Marking in IEC standard 60529, providing classification of degrees of protection provided by enclosures for electrical equipment, the entirety of which is hereby incorporated by reference herein. In some embodiments, lighting device  100  may have any IP rating, for example from IP00 to IP 69k, or any other IP rating. In some embodiments, lighting device  100  has an IP 65 rating or an IP 66 rating or an IP 67 rating or an IP 68 rating. In general for an IP XY rating, “X” indicates the level of protection for access to electrical parts and ingress to solid foreign objects, while “Y” indicates the level of protection for ingress of harmful water. For example, an IP44 rating provides access and ingress protection for objects greater than about 1 mm and protection from water splashing on the system. In another example, an IP66 rating provides a dust-tight enclosure and protection from water jets incident on the system. Specific details of the requirements and test method are detailed within the IP specification. 
     In some embodiments, housing element  120  may have an end cap or closure on one or both ends of housing element  120 .  FIG. 5  shows one example of an embodiment of an end cap  510 . In some embodiments, end cap  510  may be removable, for example it may be affixed to housing element  120  or lighting device  100  using a reversible closure, e.g., a press-fit end cap, a threaded end cap, screws, bolts, removable or reversible adhesive, or the like. In some embodiments, end cap  510  may be substantially permanently affixed to housing element  110  or lighting device  100 ; that is, it may be affixed in a manner not meant to be removed during normal operation or maintenance. In some embodiments, end cap  510  may be sealed to housing element  110  or lighting device  100  using an o-ring seal. In some embodiments, an o-ring seal may be removable during normal operation and maintenance, while in other embodiments an o-ring seal may be designed not to be removed during normal operation and maintenance. In some embodiments, end cap  510  may be affixed to housing element  110  or lighting device  100  using, for example, glue, adhesive, tape, room temperature vulcanization (RTV) silicone, rivets, UV-curable adhesive, or the like. 
     In some embodiments, two housing elements  110  may be joined or coupled together, for example to make a longer lighting device  600 , using a connector  610 , as shown in  FIG. 6 . As discussed herein, in some embodiments connector  610  may be removable, for example, a press-fit end cap, a threaded end cap, screws, bolts, or the like, while in other embodiments connector  610  may be designed to be permanently installed, i.e., to not be removed during normal operation and maintenance. As discussed herein, connector  610  may be coupled to housing elements  120  using a variety of permanent means, for example glue, adhesive, tape, RTV silicone, screws, rivets, or the like. While  FIG. 6  shows two housing elements  120  coupled together, this is not a limitation of the present invention, and in other embodiments more than two housing elements  120  may be joined or coupled together via, e.g., the use of multiple connectors  610 . 
     In some embodiments, connector  610  may be designed to connect two portions of lighting device  100  such that the distance between adjacent LEEs  140  across the joint between the two portions of lighting device  100 , or two portions of light sheet  110 , is the same as or substantially the same as that between adjacent LEEs  140  on each light sheet  110 . The ability to position or join multiple lighting devices  100  provides a system that may be utilized to make arbitrarily large assemblies having uniform luminance with no relatively darker areas in the joint regions between adjacent lighting devices  100 . 
     In various embodiments, connector  610  may include or consist essentially of a mechanical connector joining two housing elements  120  together; however, this is not a limitation of the present invention, and in other embodiments connector  610  may include other or additional functionality, for example, electrical coupling between portions of light sheet  110 , between portions of lighting devices  100 , or the like. In various embodiments, electrical coupling may be utilized to provide energization or power to light sheets  110  and/or to provide control and/or communication to and between light sheets  110 . 
     In some embodiments, lighting device  100  incorporates a driver or power supply to provide power to light sheet  110 . In some embodiments, lighting device  100  is configured to receive one or more control or communication signals, for example to energize or de-energize light sheet  110 , to dim or increase the intensity of light emitted by light sheet  110 , and/or to change one or more characteristics of light emitted by lighting device  100 , for example the CCT, CRI, and/or the light distribution pattern of the light emitted by light sheet  110 . In some embodiments, lighting device  100  may include more than one light sheet  110 , and in some such embodiments control or communication signals may provide different information or signals to different light sheets  110 . In some embodiments, different information or signals may be provided to different portions of one light sheet  110 . 
     In some embodiments, a power supply and/or driver  710  may be separate from lighting device  100 , as depicted schematically in  FIG. 7A .  FIG. 7A  shows power supply/driver  710  providing power to lighting device  100 . Power is supplied to power supply/driver  710  through a connection  720  (e.g., one or more wires or other electrical conductors). Communication and control signals may be provided to power supply driver  710 , for example by connection  730 , or to lighting device  100 , for example by connection  740 . In some embodiments, connections  720 ,  730 , and  740  are wired, that is are formed using a physical connection; however, this is not a limitation of the present invention, and in other embodiments connections  720 ,  730 , and/or  740  may be provided by other means, for example wirelessly, for example using wireless power transfer, light-based communication, radio-based communication, Wi-Fi, Bluetooth, infrared light, ultraviolet light, or the like. 
     In some embodiments, driver  710  may be physically incorporated into lighting device  100 . For example, in some embodiments driver  710  may be incorporated into lighting device  100  as depicted in the cross-section schematic shown in  FIG. 7B . While  FIG. 7B  shows driver  710  in a support structure  750 , this is not a limitation of the present invention, and in other embodiments driver  710  may be inside interior region  130 . 
     In some embodiments, where driver  710  is separate from lighting device  100  or where driver  710  is not inside interior region  130 , electrical connection to light sheet  110  may be made through a portion of housing element  120  or through end cap  510  or through connector  610 . In some embodiments where driver  710  is inside interior region  130 , electrical connection between driver  710  and light sheet  110  may be completely contained within interior region  130 , while electrical connection to driver  710  may be made through a portion of housing element  120  or through end cap  610  or through connector  710 . 
     In some embodiments, a lighting device  100  may be battery powered. In some aspects of such embodiments, one or more batteries may be contained within interior region  130  or inside lighting device  100  but not inside interior region  130 , for example similar to the device shown in  FIG. 7B , but with driver  710  replaced by a battery and optional driver. Alternatively, one or more batteries may be external to lighting device  100 . In some embodiments, a battery-operated lighting device may have an IP rating, for example an IP 65 rating, an IP 66 rating, an IP 67 rating, or an IP 68 rating. In some embodiments, a battery-operated lighting device may incorporate a non-rechargeable or a rechargeable battery source to power light sheet  110 . In some embodiments, a battery-operated lighting device may incorporate a rechargeable battery capable of being charged wirelessly, for example inductively. In various embodiments, a battery-operated lighting device may incorporate or be connectable to one or more photovoltaic cells, batteries, generators, energy harvesting devices, or the like. 
     In some embodiments of the present invention, two or more lighting devices  100  may be electrically coupled together, or one or more lighting devices  100  may be electrically coupled to one or more drivers or power supplies, such as driver  710 . In some embodiments, control or communication signals may be required to be supplied to lighting device  100 , driver  710 , and/or light sheet  110 . In some embodiments of the present invention, control or communication signals may be supplied through direct coupling (i.e., wired or physical electrical coupling), through an electrical connector, optically, wirelessly, by induction, or by other means, as discussed herein. 
     In some embodiments, end cap  510  and/or connector  610  may include a means for electrically coupling two or more lighting devices  100 , or coupling one or more lighting devices  100  to one or more drivers  710 , or to provide control and/or communications signals to lighting device  100 , for example to light sheet  110  and/or driver  710 . For example, in one embodiment end cap  510  and/or connector  610  may include one or more electrical connectors, such as a pin-and-jack system, whereby adjacent light sheets  110  are electrically coupled through the electrical connector. For example, in one embodiment connector  610  may feature a connector electrically coupled to one or more power conductors on light sheet  110 , and the connector may be electrically coupled to a corresponding connector on an adjacent light sheet  110  through connector  610 . In one embodiment, connector  610  permits physical close coupling of adjacent lighting devices  100 , while in other embodiments a jumper wire may be used to electrically couple two or more spaced-apart lighting devices  100 . In some embodiments, wires may be soldered or otherwise electrically coupled to power conductors  910 ,  920  (as discussed with reference to  FIGS. 9A-9C ) and multiple lighting devices  100  may be electrically coupled through standard wiring techniques, for example using connectors, wire nuts, soldering, or the like. In one embodiment, magnets of the appropriate polarity may be mounted or disposed within or at the ends of housing elements  120  or within end cap  510  and/or connector  610 , such that each frame may be mechanically and electrically connected via the magnets. In other embodiments of the present invention, other means may be used for electrically coupling two or more lighting devices  100  or coupling one or more lighting devices  100  to one or more drivers  710 , or to provide control and/or communications signals to lighting device  100 , for example using radio-based signals or light-based signals. 
       FIG. 7B  depicts another aspect of embodiments of the present invention. In  FIGS. 1A-6  housing element  120  has been depicted as being symmetric or substantially symmetric. However, this is not a limitation of the present invention, and in other embodiments housing element  120  may be asymmetric in one or more dimensions 
     While various ones of  FIGS. 1A-6  show lighting device  100  as straight, this is not a limitation of the present invention, and in other embodiments the lighting device may be curved or have multiple straight segments or have any shape.  FIGS. 8A-8E  depict various non-limiting geometries of lighting devices of the present invention.  FIG. 8A  depicts a lighting system having a curved housing element  120 . In some embodiments, the light distribution pattern of lighting device  100  may be controlled by both a curved or shaped light sheet  110  (identified by the dashed line in  FIG. 8A ) and a curved or shaped housing element  120 .  FIG. 8B  depicts another embodiment of a lighting device of the present invention. In one embodiment, the lighting device of  FIG. 8B  may be mounted at the intersection of two planar surfaces, for example a ceiling and wall. While  FIG. 8B  shows a structure that is about 25% of a full circle, this is not a limitation of the present invention, and in other embodiments the lighting device may be any portion of a full circle. 
       FIG. 8C  depicts an embodiment of the present invention in which light sheet  110  (identified by the dashed line in  FIG. 8C ) has a varying width along the length of housing element  120 . Housing element  120  and interior space  130  vary in a similar fashion as the width of light sheet  110 , such that light sheet  110  is held in place along housing element  120 . In some embodiments of this lighting device, this may permit varying of the light intensity and/or light distribution pattern in different directions. For example, in one embodiment of the lighting device of  FIG. 8C , the light intensity may be greater in direction  810  than in direction  820 . In one embodiment of the lighting device of  FIG. 8C , the light distribution pattern may be wider in direction  810  than in direction  820 .  FIGS. 8D and 8E  show additional illustrative, non-limiting embodiments. 
     In some embodiments, lighting device  100  may be flexible in the sense of being pliant in response to a force and resilient, i.e., tending to elastically resume an original configuration upon removal of the force. Such lighting devices may have a radius of curvature of about 50 cm or less, or about 20 cm or less, or about 5 cm or less, or about 1 cm or less. In some embodiments, housing elements  120  have a Young&#39;s Modulus less than about 50×10 9  N/m 2 , less than about 10×10 9  N/m 2 , or even less than about 5×10 9  N/m 2 . In some embodiments, housing elements  120  have a Shore A hardness value less than about 100; a Shore D hardness less than about 100; and/or a Rockwell hardness less than about 150. 
     In some embodiments, lighting device  100  is positionable, in the sense that it is pliant in response to a force, as with a flexible element, but that the element substantially retains the deformed shape upon removal of the force. Such lighting devices may have essentially any radius of curvature, but in particular may have a radius of curvature of about 50 cm or less, or about 20 cm or less, or about 5 cm or less, or about 1 cm or less. 
     In some embodiments, lighting device  100  and/or housing element  120  may be rigid or substantially rigid, in the sense that they are not pliant in response to a force, i.e., tending to plastically deform or break in response to a force. 
     In some embodiments, lighting device  100  and/or housing element  120  are semi-rigid, having a deformation characteristic between that of a flexible element and a rigid or substantially rigid element. Such elements may have a radius of curvature greater than about 1 cm. 
     In some embodiments, light sheet  110  typically includes or consists essentially of an array of LEEs  140  electrically coupled by conductive elements formed on a flexible substrate, for example as described in U.S. patent application Ser. No. 13/799,807, filed Mar. 13, 2013 (the &#39;807 application), or U.S. patent application Ser. No. 13/970,027, filed Aug. 19, 2013 (the &#39;027 application), the entire disclosure of each of which is herein hereby incorporated by reference. 
       FIG. 9A  depicts an exemplary circuit topology, in accordance with embodiments of the present invention, which features conductive elements  960 , at least two power conductors  910 ,  920 , multiple LEEs  140 , and optional control elements (CEs)  940 . In some embodiments, LEEs  140  may be configured in a regular periodic array, for example a substantially square or rectangular array, where LEEs  140  are separated by pitch (or “spacing”)  923  in the one direction (for example vertical direction) by pitch  925  in a substantially orthogonal direction. In some embodiments, pitch  925  is the same as or substantially the same as pitch  923 . While the geometrical layout and pitches  923  and  925  are described in connection with the circuit schematic shown in  FIG. 9A , such geometry, layout, and pitches are not limitations of the present invention, and in other embodiments the physical layout of the circuit elements may be different than the circuit topology shown in  FIG. 9A . Additionally, other embodiments may have different circuit topologies, for example LEEs  140  may be electrically coupled in parallel, in a combination of series and parallel, or any other arrangement. In some embodiments, more than one group of electrically connected LEEs  140  may be electrically coupled to one CE  940 , while other embodiments may not require any CEs  140 . The specific circuit topology is not a limitation of the present invention. 
       FIG. 9A  shows two power conductors  910  and  920 , which may be used to provide power to strings  950  of LEEs  140 . Each string  950  may include two or more electrically coupled LEEs  140 . LEEs  140  in string  950  may be electrically coupled in series, as shown in  FIG. 9A ; however, this is not a limitation of the present invention, and in other embodiments other examples of electrical coupling may be utilized, for example LEEs in parallel or in any combination of series and parallel connections.  FIG. 9A  shows CE  940  connected in series with LEEs  140  of string  950 ; however, this is not a limitation of the present invention, and in other embodiments CE  940  may have different electrical coupling between power conductors  910 ,  920 , or may be absent altogether. For example, in some embodiments CE  940  may be separately electrically coupled to power conductors  910 ,  920  and to the LEE string  950 , while in other embodiments each CE  940  may be electrically coupled to two or more strings. The number of strings electrically coupled to each CE  940  is not a limitation of the present invention. Combinations of structures described herein, as well as other electrical connections, all fall within the scope of the present invention. Power conductors  910 ,  920  may be used to provide power to strings  950 , for example AC power, DC power, or power modulated in any other fashion. 
     Referring to  FIGS. 9B and 9C  that depict schematics of exemplary light sheets  110 , a light sheet  110  may feature an array of LEEs  140  each electrically coupled between conductive traces  960 , and power conductors  910  and  920  providing power to conductive traces  960  and CEs  940 , all of which are disposed over a substrate  965 . As utilized herein, a “wiring board” refers to a substrate for LEEs with or without additional elements such as conductive traces or CEs. A wiring board may also be referred to as a light sheet or a circuit board.  FIG. 9B  shows a portion of light sheet  110 . In the exemplary embodiment depicted in  FIG. 9B , power conductors  910 ,  920  are spaced apart from each other and light-emitting strings (or simply “strings”)  950  are connected in parallel across power conductors  910 ,  920 . In some embodiments, for example as shown in  FIG. 9B , strings  950  do not cross (i.e., intersect) each other. In other words, power conductors  910 ,  920  are oriented in one direction and strings  950  are oriented such that they span power conductors  910 ,  920  in a different direction. As shown in  FIG. 9B , strings  950  are substantially perpendicular to power conductors  910 ,  920 . However, this is not a limitation of the present invention, and in other embodiments at least some segments (i.e., portions connecting two or more LEEs  140 ), or even the entire strings  950 , may define a line (not necessarily a straight line) that is not perpendicular to power conductors  910 ,  920  yet is (at least for an entire string  950 ) not parallel to power conductors  910 ,  920 . In other embodiments strings  950  may intersect, for example one string  950  splitting into two or more strings  950 , or two or more strings  950  joining to form a reduced number of strings  950 . In some embodiments, conductive traces  960  may cross over each other without being electrically coupled, and in some embodiments strings  950  may cross over or under each other without being electrically coupled (e.g., they may be electrically insulated from each other). In some embodiments all or a portion of one or more strings  950  may be outside of the area spanned by power conductors  910 ,  920 . Various examples of string geometries and conformations utilized in embodiments of the present invention are detailed in the &#39;807 and &#39;027 applications. 
     As shown, LEEs  140  are positioned across substrate  965  in a regular periodic array, although this is not a limitation of the present invention, and in other embodiments LEEs  140  may occupy any positions on light sheet  110 . Power conductors  910  and  920  provide power to each LEE string, for example the string  950  encircled by the dashed line in  FIG. 9B . Each LEE string  950  typically includes multiple conductive traces  960  that interconnect multiple LEEs  140 , as well as one or more CEs  940 , which in  FIG. 9B  is in series with LEEs  140 . String  950  shown in  FIG. 9B  is a folded string, i.e., a string that has three segments electrically coupled in series but positioned as three adjacent segments. A string segment is a portion of a string spanning all or a portion of the region between power conductors  910  and  920  in  FIG. 9B . In light sheet  110 , some string segments may include LEEs  140  while others do not. However, in other embodiments the distribution and position of LEEs  140  along conductive elements  960  and string segments may be different. In some embodiments, a string  950  may be a straight string, i.e., a string with no folds, as shown in  FIG. 9C . (For simplicity, the example shown in  FIG. 9C  does not depict CEs  940 ). One end of string  950  is electrically coupled to power conductor  910 , while the other end of string  950  is electrically coupled to power conductor  920 . As will be discussed, the number of segments in a string  950  is not a limitation of the present invention. Various examples of straight and folded strings utilized in embodiments of the present invention are detailed in the &#39;807 and &#39;027 applications. 
       FIGS. 9A and 9B  illustrate three aspects in accordance with various embodiments of the present invention. The first is the multiple strings  950  that are powered by the set of power conductors  910 ,  920 . The second is the positional relationship between the locations of LEEs  140  and CE  940 , which is disposed between the conductive traces  960  and between power conductors  910 , 920 . The third is the inclusion of a CE  940  in each string of series-connected LEEs  140 . Combinations of these three aspects enable light sheet  110  to be economically manufactured in very long lengths, for example in a roll-to-roll process, and cut to specified lengths, forming light sheets, while maintaining the ability to tile, or place light sheets adjacent to each other (e.g., in the length direction), with no or substantially no change in pitch between LEEs  140  or in the optical characteristics across the joint between two adjacent light sheets, as discussed in more detail in the &#39;807 and &#39;027 applications. 
     In an exemplary embodiment, CE  940  is configured to maintain a constant or substantially constant current through LEEs  140  of string  950 . For example, in some embodiments, a constant voltage may be applied to power conductors  910 ,  920 , which may, under certain circumstances may have some variation, or the sum of the forward voltages of LEEs  140  in different strings may be somewhat different, for example as a result of manufacturing tolerances, or the component and/or operational values of the element(s) within CE  940  may vary, for example as a result of manufacturing tolerances or changes in operating temperature, and CE  940  acts to maintain the current through LEEs  140  substantially constant in the face of these variations. In other words, in some embodiments the input to the light sheet is a constant voltage that is applied to power conductors  910 ,  920 , and CEs  940  regulate the current in each string to which it is connected, e.g., regulate the current to a constant or substantially constant current through LEEs  140 . The design of CE  940  may be varied to provide different levels of control or variation of the current through LEEs  140 . In some embodiments, CEs  940  may control the current through LEEs  140  to be substantially constant with a variation of less than about ±25%. In some embodiments, CEs  940  may control the current through LEEs  140  to be substantially constant with a variation of less than about ±15%. In some embodiments, CEs  940  may control the current through LEEs  140  to be substantially constant with a variation of less than about ±10%. In some embodiments, CEs  940  may control the current through LEEs  140  to be substantially constant with a variation of less than about ±5%. 
     In some embodiments, CEs  940  may, in response to a control signal, act to maintain a constant or substantially constant current through LEEs  140  until instructed to change to a different constant or substantially constant current, for example by an external control signal. In some embodiments, as detailed herein, all CEs  940  on a sheet may act in concert, that is maintain or change the current through all associated LEEs  140 ; however, this is not a limitation of the present invention, and in other embodiments one or more CEs  940  may be individually instructed and/or energized. 
     In some embodiments LEEs  140  may include or consist essentially of light-emitting diodes (LEDs) or lasers. In some embodiments, light emitted from light sheet  110  is in the form of an array of bright spots, or light-emission points, resulting in a pixelated pattern. However, this is not a limitation of the present invention, and in other embodiments light sheet  110  includes different types of light emitters, for example organic LEDs (OLEDs). In some embodiments, light sheet  110  may emit light homogeneously or substantially homogeneously, for example light sheet  110  may include an array of LEEs  140  behind an optic or diffuser that spreads the light from LEEs  140  homogeneously or substantially homogeneously. In some embodiments, light sheet  110  may include one or more OLEDs emitting homogeneously or substantially homogeneously over light sheet  110 . 
     In the embodiment depicted in  FIG. 9B , LEEs  140  are distributed substantially uniformly over light sheet  110 ; however, this is not a limitation of the present invention, and in other embodiments, LEEs  140  may have a non-uniform distribution, for example as shown in  FIG. 1D , where the structure has a higher density of LEEs  140  on the vertical portion of light sheet  120 , for example to increase the light intensity in the lateral direction. As will be understood, the distributions of LEE  140  on light sheet  110  shown in  FIG. 9B  or  FIG. 1D  are not limitations of the present invention, and other embodiments may have other distributions of LEEs  140 . In some embodiments, one or more portions of light sheet  110  may be unpopulated with LEEs  140 . In some embodiments, the distribution of LEEs  140  on light sheet  110  is specifically chosen to achieve one or more characteristics, for example optical, electrical, thermal or the like, as described herein. In some embodiments, the distribution of LEEs  140  on light sheet  110  may be chosen to create a certain desired decorative look. 
     As described herein, lighting device  100  may be designed to be cut to length. In some embodiments, lighting device  100  is cut to length with light sheet  110  installed; however, in other embodiments the housing element  120  may be cut to length before incorporation or installation of light sheet  110 . In some embodiments, light sheet  110  may also be cut to length, as discussed in more detail in the &#39;807 and &#39;027 applications. For example, in some embodiments of the present invention light sheet  110  may be cut between strings  950 . 
     In some embodiments, light sheet  110  does not require any additional thermal management or heat-sinking, i.e., the heat generated by LEEs  140  is at least partially accommodated by the structure of light sheet  110  itself, for example substrate  965  and/or conductive elements  960  and/or power conductors  910 ,  920 . In some embodiments, light sheet  110  may be incorporated into housing element  120  without the need for additional passive or active cooling of light sheet  110 . In some embodiments, housing element  120  may include or consist essentially of a wide range of materials without the need for a relatively high thermal conductivity to aid in heat extraction from light sheet  110 . For example, in some embodiments all or portions of housing element  120  may include or consist essentially of one or more of the following materials: plastic, acrylic, polycarbonate, glass, silicone, polyurethane, wood, fabric, or the like. In some embodiments, housing element  120  may have a thermal conductivity less than about 5 W/·K, less than about 2 W/m·K, less than about 1 W/m·K, or even less than 0.5 W/m·K. In some embodiments, portions of housing element  120  in contact with LEEs  140  may have a thermal conductivity less than about 5 W/m·K, less than about 2 W/m·K, less than about 1 W/m·K, or even less than 0.5 W/m·K. In some embodiments, portions of housing element  120  adjacent to or in contact with light sheet  110  may have a thermal conductivity less than about 5 W/m·K, less than about 2 W/m·K, less than about 1 W/m·K, or even less than 0.5 W/m·K. In some embodiments, all or portions of housing element  120  may include or consist essentially of a metal, for example aluminum, copper, steel, brass, or the like. 
     In various embodiments, lighting device  100  may be mounted (e.g., to a mounting surface such as a wall, a ceiling, or a fixture), for example using screws or nails (or other fasteners) that may be inserted through mounting holes defined by and/or extending at least partially through housing  120 ; however, this is not a limitation of the present invention, and in other embodiments lighting device  100  may be mounted by other means, for example staples, tape, double-sided tape, magnets, a hook-and-loop fastener such as Velcro, or the like. In various embodiments, lighting device  100  may include or incorporate mounting elements, for example double-sided tape or barbed pins that may be used to mount lighting device  100  to a mounting surface. In some embodiments, lighting device  100  may be suspended from a surface, for example a roof or ceiling. In various embodiments, housing elements  120  may include mounting hardware, for example, wall anchors, hooks, pre-installed screws, eyelets, magnets, tape, adhesive, hook-and-loop fasteners such as Velcro, and/or holes for mechanical fasteners such as nails or screws, and the like, to aid in installation of lighting device  100 . 
     In some embodiments, one or more electrical conductors may be incorporated into housing element  120 , for example to provide power and/or communication and/or control signals to light sheet  110 . For example, in one embodiment housing element  120  features a conductive element  1010  that is disposed on or formed within or embedded or partially embedded into housing element  120 .  FIG. 10  shows conductive elements  1010  disposed on exterior housing element  120  or between exterior housing element  120  and light sheet  110  and electrically coupled to power conductors  910 ,  920  on light sheet  110 . As shown in  FIG. 10 , power conductors  910 ,  920  are electrically coupled to conductive elements  960 , which are electrically coupled to LEEs  140 . Light sheet  110  is clamped between interior housing element portion  120 ′ and exterior housing element portion  120 , forming an electrical and mechanical connection between conductive elements  1010  and power conductors  910 ,  920 . In some embodiments, conductive element  1010  may be mounted on a surface (e.g., an external surface) of housing element  120 . In various embodiments, conductive element  1010  includes or consists essentially of one or more electrically conductive materials such as metals such as aluminum, copper, silver, gold, steel, or the like. In various embodiments, conductive element  1010  includes an electrically conductive tape, for example one that is conductive in both the lateral and z (i.e., through-thickness) directions, such that a low-resistance pathway forms between power conductor  910  and conductive element  1010 , and conductive element  1010  forms a low-resistance pathway in parallel with power conductor  910 . In some embodiments, conductive element  1010  includes or consists essentially of a combination of materials, for example a metal layer over which is formed a conductive adhesive or a conductive tape. In various embodiments, a pin, a rivet, or a barbed pin electrically coupled to conductive element  1010  and disposed on or within housing element  120  may pierce and electrically couple to a conductive trace on light sheet  110 . In various embodiments, a pin or a barbed pin electrically coupled to a conductive trace on light sheet  110  may pierce and electrically couple to a conductive element  1010 . 
     In some embodiments, housing element  120  may include more than one conductive element  1010 . For example, conductive elements in housing element  120  may be used, in addition to powering lighting device  100 , to provide communication and control signals to and from lighting device  100 . In some embodiments, conductive elements in housing  120  may be used to provide electrical crossovers, to permit additional circuitry complexity while still using only one layer of conductive elements  960  on substrate  965 . For example,  FIG. 11A  shows an electrical schematic of a system having two different types of LEEs  140 ,  140 ′. In some embodiments, LEE  140  may have a different color than LEE  140 ′, or a different intensity, or a different light distribution pattern, or a difference in any other electrical and/or optical properties. In one embodiment, LEE  140  and  140 ′ may both emit white light, but with different color temperatures, and the color temperature of the light panel may be adjusted by changing the light intensity emitted by strings with different color-temperature LEEs. For example in various embodiments LEEs  140  may have a correlated color temperature (CCT) of about 2000K and LEEs  140 ′ may have a CCT of about 10,000K and the CCT of the ensemble may be varied between about 2000K and about 10,000K by varying the power delivered to strings having LEEs  140  and  140 ′. In various embodiments, LEEs  140  may have a CCT of about 2700K, and LEEs  140 ′ may have a CCT of about 5000K, and the CCT of the ensemble may be varied between about 2700K and about 5000K by varying the power delivered to strings having LEEs  140  and  140 ′. 
     In various embodiments, the circuit is driven by a substantially constant voltage supply that is pulse-width modulated, that is, the voltage is kept substantially the same during the “on” phase, and the light intensity is varied by changing the duty cycle, or the ratio of “on” to “off” time of the power signal. The circuit of  FIG. 11A  typically requires the power to the two different types of strings to be modulated separately, and thus typically requires three, or perhaps four (if separate returns are required) conductors. As understood by those skilled in the art, this requires an electrical cross-over or a similar electrical routing scheme. While light sheets with multiple conductive layers may be manufactured, these are relatively more expensive than light sheets or circuit boards with one layer of conductive elements. In various embodiments of the present invention, conductive elements within housing element  120  may form one or more electrical cross-overs, permitting circuits such as that shown in  FIG. 11A  to be realized with a light sheet or circuit board having only one conductive layer. 
       FIG. 11B  shows one example of a pattern of power conductor traces for power conductors  920  and  920 ′ on light sheet  110 , that, combined with the housing element  120 , permit realization of circuits requiring crossovers with a light sheet having a single conductive layer.  FIG. 11B  shows a portion of a light sheet, including substrate  965  on which power conductors  920  and  920 ′ as well as conductive elements  960  have been formed. Conductive elements  960  electrically couple LEEs  140 , such that LEEs  140 ′ are electrically coupled to power conductor  920 ′ and LEEs  140  are electrically coupled to power conductor  920 . However, as shown in  FIG. 11B , power conductor  920  is discontinuous and requires a crossover in a region  1120  to form a complete circuit.  FIG. 11C  shows a cross-section of the structure of  FIG. 11B  through cut-line A-A′. As shown in  FIG. 11C , conductive element  1010  associated with power conductor  920  in top housing element  120 T is formed such that it does not electrically couple with power conductor  920 ′. In some embodiments, this may be achieved by spacing conductive element  1010  apart from power conductor  920 ′, while in other embodiments an insulating layer, for example plastic or insulating tape or paper or other insulator may be positioned between power conductor  920 ′ and conductive element  1010 . Not shown in  FIG. 11C  is conductive element  1010 ′, which is associated with power conductor  920 ′, in top housing element  120 T.  FIG. 11D  shows a plan view of the inside of top housing element  120 T, showing both conductive elements  1010  and  1010 ′, where conductive element  1010  has region  1120  that is designed to prevent electrical coupling to the underlying portion of power conductor  920 ′. 
     While  FIGS. 11A-11C  show a system having one level of electrical cross-over, this is not a limitation of the present invention, and in other embodiments more than one level of cross-over may be utilized. In some embodiments, two levels may be utilized, with a lighting system having two housing elements, with each housing element having one level of cross-over. In other embodiments, more than one level of cross-over may be utilized in a single housing element  120 . While  FIG. 11C  shows one form of cross-over, this is not a limitation of the present invention, and in other embodiments other types of cross-overs may be formed. For example, cross-overs may be formed using any of the approaches described herein for electrically coupling multiple housing elements together. 
     While conductive element  1010  has been described as disposed on or part of housing  120 , this is not a limitation of the present invention, and in other embodiments conductive element may be separate from housing  120 . In various embodiments, conductive element  1010  may be separate from housing  120  but disposed within housing  120 . 
     In preferred embodiments, at least a portion of housing element  120  is transparent to a wavelength of light emitted by LEEs  140 . In some embodiments, substantially all of housing element  120  is transparent to a wavelength of light emitted by LEEs  140 . In some embodiments, housing element  120  may include or consist essentially of glass or plastic. In some embodiments, housing element  120  may include or consist essentially of at least one of silicone, polycarbonate, acrylic, polystyrene, plexiglass, silicone, polyurethane, or the like. In some embodiments, housing element  120  may include a portion that is transparent to a wavelength of light emitted by LEEs  140  and a portion substantially not transparent to (e.g., translucent, opaque, and/or reflective to) a wavelength of light emitted by LEE  140 . In some embodiments, the transparent portion of housing element  120  has a transmittance greater than 80% for a wavelength of light emitted by LEEs  140 , or greater than 90% for a wavelength of light emitted by LEEs  140 , or greater than 95% for a wavelength of light emitted by LEEs  140 . In some embodiments, housing element  120  may include a diffuser or diffusing elements. For example, a portion or substantially all of housing element  120  may be manufactured from an optically diffusing material. 
     In some embodiments, housing elements  120  may include one or more optical elements, for example to modify the light distribution pattern or to change the appearance of light sheet  110 , in either or both the illuminated or non-illuminated state. For example, in some embodiments the optical elements may be used to produce a specific light distribution pattern, for example a collimated light distribution pattern, a batwing light distribution pattern, an offset collimated light distribution pattern or the like, as described in U.S. patent application Ser. No. 13/693,632, filed on Dec. 4, 2012, the entirety of which is hereby incorporated by reference herein. 
     In some embodiments, at least one LEE is associated with at least one optical element, where the optical element is positioned to receive light from the LEE. In some embodiments, each LEE is associated with one optical element. In some embodiments, some LEEs are associated with optical elements while others are not. In some embodiments, all optical elements are the same or substantially the same, while other embodiments may include more than one type of optical element. In some embodiments, optical elements may be used to produce asymmetric or non-symmetric or arbitrary light distribution patterns. 
     In some embodiments, the optical elements are formed as part of housing element  120 , while in other embodiments the optical elements are disposed on a surface of housing element  120 , for example the interior or exterior surface of housing element  120 . In some embodiments, the optical elements are disposed on light sheet  110 . In some embodiments, an optical element may be a refractive optic, a reflecting optic, a total internal reflectance (TIR) optic, a Fresnel optic, or the like. 
       FIG. 12A  shows one embodiment of the present invention in which exterior housing element  120  includes optical elements  1210 . As shown in  FIG. 12A , the center of each LEE  140  is aligned with or substantially aligned with the center of an optical element  1210 ; however, this is not a limitation of the present invention, and other embodiments may include an offset between the centers of LEEs  140  and optical elements  1210 .  FIG. 12B  shows an embodiment of the present invention having two different types of optical elements,  1210  and  1210 ′. For example, in one embodiment the device of  FIG. 12B  may be mounted on the ceiling and optical elements  1210  may be engineered to produce a bat-wing light distribution pattern to provide uniform or substantially uniform illumination on an underlying work surface while optical elements  1210 ′ may be engineered to produce a collimated light distribution pattern to illuminate the ceiling and walls, thus providing ambient or indirect illumination to the space. In some embodiments of the present invention, optical elements, for example optical elements  1210  and/or  1210 ′ may be engineered to provide low glare within the space. 
     In some embodiments of the present invention, housing element  120  has sub-elements or divisions that correspond to the pitch between LEEs, or that correspond to multiples of the pitch between LEEs, for example integer multiples of the pitch between LEEs. As depicted in  FIG. 13A , housing element  120  encloses light sheet  110  having three LEEs  140  or three rows of LEEs  140 , and housing element  120  has three sections  1310 ,  1320 , and  1330 , each of which corresponds to one LEE or one row of LEEs, where each LEE or each row of LEEs is separated by a pitch  1350 . In some embodiments, all LEEs  140  may be directed (i.e., pointing and/or positioned to emit light) in the same or substantially the same direction; however, this is not a limitation of the present invention, and in other embodiments different LEEs  140  may be directed in different directions, for example as depicted in  FIG. 13B . 
       FIG. 14A  shows another embodiment of the present invention including interior and exterior housing elements  120 ′,  120 , each having four sides  1410 ,  1420 ,  1430 , and  1440 . In the depicted embodiment, sides  1410  and  1420  include two LEEs  140  or rows of LEEs  140 , side  1430  includes three LEEs  140  or rows of LEEs  140 , and side  1440  includes one LEE  140  or row of LEEs  140 . In one embodiment, the structure of  FIG. 14A  may be mounted vertically (as shown) and produce a light distribution pattern such that the light intensity in a direction generally perpendicular to sides  1410  and  1420  are about the same, and are less than the light intensity in a direction generally perpendicular to side  1430  and more than the light intensity in a direction generally perpendicular to side  1440 . In some embodiments, the relative light intensity emitted from different sides is proportional or substantially proportional to the number of LEEs  140  on that side. In some embodiments, all LEEs  140  are driven at the same or substantially the same current; however, this is not a limitation of the present invention, and in other embodiments different LEEs may be driven at different currents. 
       FIG. 14B  shows another embodiment of the present invention including interior and exterior housing elements  120 ′,  120 , each having three sides identified as  1450 ,  1460 , and  1470 . In this embodiment, sides  1450  and  1470  have the same number of LEEs  140 , while side  1460  has no LEEs  140 . In some embodiments, all LEEs  140  are driven at the same current; however, this is not a limitation of the present invention, and in other embodiments different LEEs  140  may be driven at different currents. For example, in some embodiments LEEs  140  on side  1450  may be driven at a higher current than LEEs  140  on side  1470 , and the light intensity in a direction generally perpendicular to side  1450  may then be higher than a light intensity in a direction generally perpendicular to side  1470 . In some embodiments, light may be emitted from side  1460 , for example light resulting from light scattering and/or from light emitted through the back of light sheet  110 . In some embodiments, light sheet  110  and/or substrate  965  may be transparent; however, this is not a limitation of the present invention, and in other embodiments light sheet  110  and/or substrate  965  may be white or any color. In some embodiments, light sheet  110  may be translucent, while in other embodiments it may be opaque. 
     In various embodiments, the substrate or light sheet may be formed to position the LEEs in a specific orientation, for example to achieve one or more optical characteristics. For example, in various embodiments the emitting area of a LEE  140  may have a non-symmetric shape and/or may have a non-symmetric light distribution pattern with respect to the normal to the emitting plane or surface. In such embodiments, LEEs  140  may be disposed on light sheet or substrate  110  in more than one direction. For example,  FIG. 15A  shows a plan view (looking down on the emitting surface) of LEE  140  having a rectangular emitting surface with a long side  1520  and a short side  1510 .  FIG. 15B  shows a schematic of one possible light distribution pattern  1530  looking perpendicular to cut line A-A′, while  FIG. 15C  shows a possible light distribution pattern  1540  looking perpendicular to cut line B-B′. In  FIGS. 15B and 15D , “x” represents a lateral dimension, “y” represents the direction orthogonal to the emitting surface, and curves  1530  and  1540  represent the intensity at a specific points in the plane defined by cut lines A-A′ perpendicular to the emitting surface and in the plane defined by cut lines B-B′ perpendicular to the emitting surface, respectively. 
     In various embodiments, LEEs  140  may be oriented in more than one direction to achieve a specific light distribution pattern. For example,  FIG. 15D  shows a system in which LEEs  140  are oriented with short face  1510  parallel to the plane of the drawing, resulting in light distribution pattern  1530  (shown in a dashed line). In various embodiments, LEEs  140  may be oriented with long  1520  parallel to the plane of the drawing, resulting in light distribution pattern  1540  (shown in a dashed line). In various embodiments, light sheet  110  may be oriented to direct light in various directions. For example,  FIG. 15F  shows an embodiment of the present invention in which light sheet  110  is shaped such that the light emission  1550  (shown in a dashed line) is substantially perpendicular to the light emission direction shown in  FIGS. 15D and 15E  (housing  120  is not shown in  FIG. 15F  for clarity). In various embodiments, light sheet  110  may be oriented such that the light emission is in any direction. In various embodiments, all LEEs  140  may be oriented in the same or substantially the same direction, while in other embodiments LEEs  140  or groups of LEEs  140  may be oriented in differing directions. 
     Housing element  120  may be manufactured using a variety of techniques, for example extrusion, molding, three-dimensional (3D) printing, machining, or the like. The method of manufacture of housing element  120  is not a limitation of the present invention. As utilized herein, the term “light-emitting element” (LEE) refers to any device that emits electromagnetic radiation within a wavelength regime of interest, for example, visible, infrared or ultraviolet regime, when activated, by applying a potential difference across the device or passing a current through the device. Examples of light-emitting elements include solid-state, organic, polymer, phosphor-coated or high-flux LEDs, laser diodes or other similar devices as would be readily understood. The emitted radiation of an LEE may be visible, such as red, blue or green, or invisible, such as infrared or ultraviolet. An LEE may produce radiation of a continuous or discontinuous spread of wavelengths. An LEE may feature a phosphorescent or fluorescent material, also known as a light-conversion material, for converting a portion of its emissions from one set of wavelengths to another. In some embodiments, the light from an LEE includes or consists essentially of a combination of light directly emitted by the LEE and light emitted by an adjacent or surrounding light-conversion material. An LEE may include multiple LEEs, each emitting essentially the same or different wavelengths. In some embodiments, a LEE is an LED that may feature a reflector over all or a portion of its surface upon which electrical contacts are positioned. The reflector may also be formed over all or a portion of the contacts themselves. In some embodiments, the contacts are themselves reflective. Herein “reflective” is defined as having a reflectivity greater than 65% for a wavelength of light emitted by the LEE on which the contacts are disposed. In some embodiments, an LEE may include or consist essentially of an electronic device or circuit or a passive device or circuit. In some embodiments, an LEE includes or consists essentially of multiple devices, for example an LED and a Zener diode for static-electricity protection. In some embodiments, an LEE may include or consist essentially of a packaged LED, i.e., a bare LED die encased or partially encased in a package. In some embodiments, the packaged LED may also include a light-conversion material. In some embodiments, the light from the LEE may include or consist essentially of light emitted only by the light-conversion material, while in other embodiments the light from the LEE may include or consist essentially of a combination of light emitted from an LED and from the light-conversion material. In some embodiments, the light from the LEE may include or consist essentially of light emitted only by an LED. 
     One or more non-LEE devices such as Zener diodes, transient voltage suppressors (TVSs), varistors, etc., may be placed on each light sheet to protect the LEEs  140  from damage that may be caused by high-voltage events, such as electrostatic discharge (ESD) or lightning strikes. In one embodiment, conductive trace segments shown in  FIG. 9B or 9C  between the LEE strings  950  may be used for placement of a single protection device per light sheet, where the device spans the positive and negative power traces, for example power conductors  910 ,  920 . These trace segments also serve to provide a uniform visual pattern of lines in the web direction, which may be more aesthetically pleasing than a light sheet with noticeable gaps between LEE strings  950 . In a more general sense, in addition to conductive traces  960  that are part of string  950 , additional conductive traces  960  that may or may not be electrically coupled to other strings  950  and/or power conductors  910 ,  920  may be formed on substrate  965 , for example to provide additional power conduction pathways or to achieve a decorative or aesthetically pleasing look to the pattern on the light sheet or to provide a communication pathway to one or more CEs  940 , for example to provide a control signal to the one or more CEs  940 . These trace segments also serve to provide a uniform visual pattern of lines in the web direction, which may be more aesthetically pleasing than a light sheet with noticeable gaps between LEE strings  950 . 
     In one embodiment, an LEE  140  includes or consists essentially of a bare semiconductor die, while in other embodiments LEE  140  includes or consists essentially of a packaged LED. 
     In some embodiments, LEE  140  may include or consist essentially of a “white die” that includes an LED that is integrated with a light-conversion material (e.g., a phosphor) before being attached to the light sheet, as described in U.S. patent application Ser. No. 13/748,864, filed Jan. 24, 2013, or U.S. patent application Ser. No. 13/949,543, filed Jul. 24, 2013, the entire disclosure of each of which is incorporated by reference herein. 
     In some embodiments, LEEs  140  may emit light in a relatively small wavelength range, for example having a full width at half maximum in the range of about 20 nm to about 200 nm. In some embodiments, all LEEs  140  may emit light of the same or substantially the same wavelength, while in other embodiments different LEEs  140  may emit light of different wavelengths. In some embodiments LEEs  140  may emit white light, for example that is perceived as white light by the eye. In some embodiments, the white light may be visible light with a spectral power distribution the chromaticity of which is close to the blackbody locus in the CIE 1931 xy or similar color space. In some embodiments, white light has a color temperature in the range of about 2000 K to about 10,000 K. The emission wavelength, full width at half maximum (FWHM) of the emitted light or radiation or other optical characteristics of LEEs  140  may not be all the same and are not a limitation of the present invention. 
     Substrate  965  may include or consist essentially of a semicrystalline or amorphous material, e.g., polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polycarbonate, polyethersulfone, polyester, polyimide, polyethylene, fiberglass, FR4, metal core printed circuit board, (MCPCB), and/or paper. Substrate  965  may include multiple layers, for example, a semicrystalline or amorphous material, e.g., PEN, PET, polycarbonate, polyethersulfone, polyester, polyimide, polyethylene, and/or paper formed over a second substrate for example comprising, acrylic, aluminum, steel and the like. Depending upon the desired application for which embodiments of the invention are utilized, substrate  965  may be substantially optically transparent, translucent, or opaque. For example, substrate  965  may exhibit a transmittance or a reflectivity greater than 70% for optical wavelengths ranging between approximately 400 nm and approximately 700 nm. In some embodiments substrate  965  may exhibit a transmittance or a reflectivity of greater than 70% for one or more wavelengths emitted by LEE  140 . Substrate  965  may also be substantially insulating, and may have an electrical resistivity greater than approximately 100 ohm-cm, greater than approximately 1×10 6  ohm-cm, or even greater than approximately 1×10 10  ohm-cm. In some embodiments substrate  965  may have a thickness in the range of about 10 μm to about 500 μm. 
     Conductive elements, e.g., power conductors  910 ,  920  and conductive traces  960 , may be formed via conventional deposition, photolithography, and etching processes, plating processes, lamination, lamination and patterning, evaporation sputtering or the like or may be formed using a variety of different printing processes. For example, power conductors  910 ,  920  and conductive traces  960  may be formed via screen printing, flexographic printing, ink-jet printing, and/or gravure printing. Power conductors  910 ,  920  and conductive traces  960  may include or consist essentially of a conductive material (e.g., an ink or a metal, metal film or other conductive materials or the like), which may include one or more elements such as silver, gold, aluminum, chromium, copper, and/or carbon. Power conductors  910 ,  920  and conductive traces  960  may have a thickness in the range of about 50 nm to about 1000 μm. In some embodiments, the thickness of power conductors  910 ,  920  and conductive traces  960  may be determined by the current to be carried thereby. While the thickness of one or more of power conductors  910 ,  920  and conductive traces  960  may vary, the thickness is generally substantially uniform along the length of the trace to simplify processing. However, this is not a limitation of the present invention, and in other embodiments the thickness and/or material of power conductors  910 ,  920  and conductive traces  960  may vary. In some embodiments, all or a portion of power conductors  910 ,  920  and conductive traces  960  may be covered or encapsulated. In some embodiments, a layer of material, for example insulating material, may be formed over all or a portion of power conductors  910 ,  920  and conductive traces  960 . Such a material may include, e.g., a sheet of material such as used for substrate  265 , a printed layer, for example using screen, ink jet, stencil or other printing means, a laminated layer, or the like. Such a printed layer may include, for example, an ink, a plastic and oxide, or the like. The covering material and/or the method by which it is applied is not a limitation of the present invention. 
     In one embodiment, conductive traces  960  are formed with a gap between adjacent conductive traces  960 , and LEEs  140  and CEs  940  are electrically coupled to conductive traces  960  using conductive adhesive, e.g., an isotropically conductive adhesive and/or an ACA, as described in U.S. Pat. No. 8,384,121, filed on Jun. 29, 2011, the entire disclosure of which is incorporated herein by reference. ACAs may be utilized with or without stud bumps and embodiments of the present invention are not limited by the particular mode of operation of the ACA. For example, the ACA may utilize a magnetic field rather than pressure (e.g., the ZTACH ACA available from SunRay Scientific of Mt. Laurel, N.J., for which a magnetic field is applied during curing in order to align magnetic conductive particles to form electrically conductive “columns” in the desired conduction direction). Furthermore, various embodiments utilize one or more other electrically conductive adhesives, e.g., isotropically conductive adhesives, non-conductive adhesives, in addition to or instead of one or more ACAs. In other embodiments, LEEs  140  and CEs  940  may be attached to and/or electrically coupled to conductive traces  960  by other means, for example solder, reflow solder, wave solder, wire bonding, or the like. The method by which LEEs  140  and CEs  940  are attached to conductive traces  960  is not a limitation of the present invention. 
     CE  940  may be one component or multiple active and/or passive components. In one embodiment, power conductors  910 ,  920  provide a DC voltage or substantially DC voltage and CE  940  includes or consists essentially of a resistor, e.g. a current-limiting resistor. The choice of the resistance value may be a trade-off between a number of parameters and characteristics that may include, e.g., efficiency and current stability. In general, a larger resistance will result in reduced efficiency but greater current stability, while a smaller resistance will result in increased efficiency but reduced current stability. Variations in the current may result from variations in the input voltage (for example across power conductors  910 ,  920 ), variations in forward voltage of the LEEs  140  within the string, variations in the value of the current-limiting resistor, variations in current that may occur if one or more LEEs  140  in the string become short-circuited or the like. In the case of CE  940  including or consisting essentially of a resistor, in some embodiments CE  940  is a discrete resistor formed within or on conductive traces  960 , such as a chip resistor, a bare-die resistor or surface mount device (SMD) resistor. 
     As discussed above, in embodiments where CE  940  includes or consists essentially of a resistor, there may be trade-offs between efficiency and current stability. While such trade-offs may be acceptable in certain products, other products may require relatively better current stability at higher efficiencies, and in these cases CE  940  may include or consist essentially of multiple components or a circuit element, as discussed above. In some embodiments CE  940  includes or consists essentially of a field-effect transistor (FET) and a resistor. In another embodiment CE  940  includes or consists essentially of two bipolar junction transistors (BJTs) and two resistors. 
     In some embodiments, the efficiency and current stability increase with the number of components, as does the cost. In some embodiments where CE  940  includes or consists essentially of multiple components, the components may be in discrete form (i.e., each component individually electrically coupled to conductive traces  960 ) or in hybrid form (where multiple separate components are mounted on a submount, which is then electrically coupled to conductive traces  960 ), or in monolithic form (where multiple components are integrated on a semiconductor chip, for example a silicon-based or other semiconductor-based integrated circuit). In some embodiments, CE  940  may be in bare-die form, while in other embodiments CE  940  may be packaged or potted or the like. In some embodiments, CE  940  may include or consist essentially of a bare-die integrated circuit. In some embodiments, the integrated circuit includes or consists essentially of multiple active and/or passive devices that are fabricated on a common semiconductor substrate. 
     In other embodiments, power conductors  910 ,  920  may provide AC power, or power modulated at different frequencies and in these embodiments CEs  940  may be selected accordingly or may be omitted. In one embodiment, power conductors  910 ,  920  may provide a standard line voltage, for example about 120 VAC or about 940 VAC or about 277 VAC, for example at about 50 Hz or about 60 Hz. In some embodiments, CEs  940  may accommodate a plurality of input types, and thus be so-called “universal” CEs  940 , while in other embodiments different CEs  940  may be required for different input types. The actual component or components of CEs  940  are not limiting to this invention; however, in preferred embodiments of this invention, the positioning of CEs  940  does not disrupt the LEE pitch. In another embodiment of this invention, the positioning of CEs  940  is independent of LEE pitch. As discussed herein, CEs  940  and LEEs  230  may be electrically coupled to conductive traces  960  using a variety of means, for example solder, conductive adhesive or anisotropic conductive adhesive (ACA); however, the method of electrical coupling of CEs  140  and LEEs  230  is not a limitation of the present invention. 
     In some embodiments driver  710  is a substantially constant voltage supply, the output of which is pulse-width modulated to permit dimming of LEEs  140  on light sheet  110 . In some embodiments, driver  710  is a UL class 2 system having a voltage output not exceeding 60 V. 
     The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.