Patent Publication Number: US-8542964-B2

Title: Waveguide sheet containing in-coupling, propagation, and out-coupling regions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/041,698, filed on Mar. 7, 2011, which is a continuation of U.S. patent application Ser. No. 12/324,535, filed on Nov. 26, 2008, now issued as U.S. Pat. No. 7,929,816, which claims priority to and the benefit of U.S. Provisional Patent Application No. 61/006,110, filed on Dec. 19, 2007; U.S. Provisional Patent Application No. 61/064,384, filed on Mar. 3, 2008; U.S. Provisional Patent Application No. 61/127,095, filed on May 9, 2008; U.S. Provisional Patent Application No. 61/076,427, filed on Jun. 27, 2008; and U.S. Provisional Patent Application No. 61/135,098, filed on Jul. 16, 2008. The entire disclosure of each of these applications is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     In various embodiments, the present invention relates to optics, and in particular to optical waveguides. 
     BACKGROUND 
     The technology to transmit and guide light through optical systems exploits a physical phenomenon in which light is confined within a material surrounded by other materials with lower refractive index. Such optical systems are generally referred to as optical waveguides, and are employed to direct, diffuse, and/or polarize light in many applications, e.g., optical communication and illumination. 
     When a ray of light moves within a transparent substrate and strikes one of its internal surfaces at a certain angle, the ray of light is either reflected from the surface or refracted into the open air in contact with the substrate. The condition according to which the light is reflected or refracted is determined by Snell&#39;s law, which relates the impinging angle, the refracting angle (in the case of refraction) and the refractive indices of both the substrate and the air. Broadly speaking, depending on the wavelength of the light, for a sufficiently large impinging angle (above the “critical angle”) no refraction occurs, and the energy of the light is trapped within the substrate. In other words, the light is reflected from the internal surface as if from a mirror. Under these conditions, total internal reflection is said to take place. 
     Many optical systems operate according to the principle of total internal reflection. Optical fiber represents one such system. Optical fibers are transparent, flexible rods of glass or plastic, basically composed of a core and cladding. The core is the inner part of the fiber, through which light is guided, while the cladding surrounds it completely. The refractive index of the core is higher than that of the cladding, so that light in the core impinging the boundary with the cladding at an angle equal to or exceeding the critical angle is confined in the core by total internal reflection. Thus, geometric optics may be used to derive the largest angle at which total internal reflection occurs. An important parameter of every optical fiber (or any other light-transmitting optical system) is known as the “numerical aperture,” which is defined as the sine of the largest incident light ray angle that is successfully transmitted through the optical fiber, multiplied by the index of refraction of the medium from which the light ray enters the optical fiber. 
     Another optical system designed for guiding light is the graded-index optical fiber, in which the light ray is guided by refraction rather than by total internal reflection. In this optical fiber, the refractive index decreases gradually from the center outwards along the radial direction, and finally drops to the same value as the cladding at the edge of the core. As the refractive index does not change abruptly at the boundary between the core and the cladding, there is no total internal reflection. However, the refraction nonetheless bends the guided light rays back into the center of the core while the light passes through layers with lower refractive indices. 
     Another type of optical system is based on photonic materials, where light is confined within a bandgap material surrounding the light. In this type of optical system, also known as a photonic material waveguide, the light is confined in the vicinity of a low-index region. One example of a photonic material waveguide is a silica fiber having an array of small air holes throughout its length. 
     International Patent Application Publication No. WO2004/053531, the entire contents of which are hereby incorporated by reference, discloses a waveguide for propagating and emitting light. The waveguide is made of a flexible, multilayer waveguide material in which the refractive index of one layer is larger than the refractive index of the other layers to allow propagation of light via total internal reflection. One layer of the waveguide material comprises one or more impurities which scatter the light to thereby emit a portion thereof through the surface of the waveguide material. 
     Impurities for light scattering are also employed in light diffusers (also known as light-scattering films or diffusing films), which diffuse light from a source in order to attain a uniform luminance. For example, in a liquid crystal display device a light diffuser is placed between the light source or light reflector and the liquid crystal panel so as to diffuse the illuminating light, allowing the device to be used as a plane or flat light source as well as enhancing the luminance on the front side of the device. 
     Conventional illumination apparatuses capable of emitting diffused light with uniform luminance are complicated to manufacture and too large for many applications. They tend to be unitary and large rather than small and scalable. Additionally, such apparatuses often exhibit insufficient color mixing and diffusion to emit light with a high degree of color and luminance uniformity. 
     SUMMARY 
     The foregoing limitations of conventional illumination apparatuses are herein addressed by utilizing a waveguide that incorporates spatially distinct in-coupling, propagation, and out-coupling regions and/or that is easily manufactured as a group of aligned core structures. Generally, embodiments of the invention propagate and diffuse light until it exits though a surface of the waveguide device or a portion thereof; for example, light entering the in-coupling region is substantially retained within the waveguide until it is emitted from the out-coupling region. Embodiments of the invention successfully provide an optical waveguide device that may be tiled or overlapped. As further detailed herein, the optical properties of the waveguide may be tailored to the requirements of particular applications. 
     In one aspect, embodiments of the invention feature an illumination structure including a substantially non-fiber waveguide. The waveguide may include or consist essentially of a discrete in-coupling region for receiving light, a discrete propagation region for propagating light substantially without emission, and a discrete out-coupling region for emitting light. The illumination structure may include a first cladding layer that is disposed over (and may be in direct physical contact with) the top surface of the waveguide, as well as a second cladding layer that is disposed over (and may be in direct physical contact with) the bottom surface of the waveguide. The in-coupling region and/or the out-coupling region may include a plurality of scattering particles, and the propagation region may be substantially free of scattering particles. The scattering particles may have a concentration that varies across at least one dimension of the out-coupling region, and the concentration of scattering particles may increase with distance from the in-coupling region. 
     Embodiments of the invention may feature one or more of the following. The out-coupling region may include or consist essentially of a plurality of core structures, at least some of which include a plurality of scattering particles. At least one of the size, the concentration, or the type of the scattering particles may vary among at least two of the core structures. There may be substantially no overlap between individual core structures in a direction perpendicular to a general direction of light propagation therethrough. Each of the core structures may have a substantially quadrilateral cross-sectional area. The cross-sectional area may be rectangular. The illumination structure may further include means for emitting light disposed proximate the in-coupling region and/or disposed within the waveguide. A reflector may be disposed proximate a surface of the in-coupling region. The waveguide may be substantially planar. The optical mean free path of light in the propagation region may be substantially constant, and the optical mean free path of light in the out-coupling region may vary substantially monotonically. 
     The illumination structure may include a second substantially non-fiber waveguide disposed above and/or in direct contact with the substantially non-fiber waveguide. The second substantially non-fiber waveguide may include or consist essentially of a discrete in-coupling region for receiving light, a discrete propagation region for propagating light, and a discrete out-coupling region for emitting light. The out-coupling region of the substantially non-fiber waveguide may be substantially vertically aligned with the out-coupling region of the second substantially non-fiber waveguide. 
     In another aspect, embodiments of the invention feature a method of producing light. The method includes providing a substantially non-fiber waveguide including or consisting essentially of a discrete in-coupling region for receiving light, a discrete propagation region for propagating light without emission, and a discrete out-coupling region for emitting light. The method also includes emitting light proximate the in-coupling region, whereby the light propagates through the propagation region with substantially no emission therefrom into the out-coupling region. The light is emitted from the out-coupling region. 
     These and other objects, along with advantages and features of the present invention herein disclosed, 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 may exist in various combinations and permutations. 
    
    
     
       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: 
         FIG. 1   a  is a schematic illustration showing a perspective view of an optical waveguide device which comprises a plurality of core structures joined in a side-by-side configuration, according to some embodiments of the present invention; 
         FIG. 1   b  is a schematic illustration showing a cross-sectional view along the line A-A of  FIG. 1   a , according to some embodiments of the present invention; 
         FIG. 1   c  is a schematic illustration showing a perspective view of the device optical waveguide device in embodiments in which the device comprises one or more cladding layers; 
         FIGS. 1   d  and  1   e  are schematic illustrations showing cross sectional views along line B-B of  FIG. 1   c;    
         FIGS. 1   f  and  1   g  are schematic illustrations showing perspective views of the optical waveguide device without ( FIG. 1   f ) and with ( FIG. 1   g ) claddings, in an embodiment in which the core layer of the device is formed of core structures in the shape of plaques; 
         FIG. 1   h  is a schematic illustration showing a perspective view of an optical waveguide device which comprises a plurality of core structures joined in a nested configuration, according to some embodiments of the present invention; 
         FIGS. 2   a  and  2   b  show representative examples of an optical mean free path as a function of a lateral direction, according to some embodiments of the present invention; 
         FIGS. 3   a - 3   d  are schematic illustrations showing fragmentary cross-sectional views of the device, according to some embodiments of the present invention; 
         FIGS. 4   a - 4   d  are schematic illustrations showing cross-sectional views of an optical funnel, according to some embodiments of the present invention; 
         FIGS. 4   e - 4   g  are cross-sectional views of an optical waveguide device having substantially no line-of-sight between a photoluminescent material and a light-emitting element in an optical funnel, according to some embodiments of the present invention; 
         FIGS. 4   h - 4   j  are cross-sectional views of an optical waveguide device having substantially no line-of-sight between a photoluminescent material and a light-emitting element embedded within the waveguide device, according to some embodiments of the present invention; 
         FIG. 5  is a schematic illustration of a coextrusion apparatus for forming a core layer, according to some embodiments of the present invention; 
         FIG. 6  is a schematic illustration of a coextrusion apparatus for forming a core layer and one or more cladding layers, according to some embodiments of the present invention; 
         FIGS. 7   a  and  7   b  are schematic illustrations of a process for forming a core layer and optionally one or more cladding layers using extrusion coating technique, according to various exemplary embodiments of the present invention; 
         FIGS. 8   a - 8   c  are schematic illustrations of a process for forming a core layer and optionally one or more cladding layers using lamination technique, according to various exemplary embodiments of the present invention; 
         FIGS. 9   a - 9   c  are schematic illustrations of a process for forming a core layer and optionally one or more cladding layers using tiling technique, according to various exemplary embodiments of the present invention; 
         FIGS. 10   a - 10   c  are schematic illustrations of a process for manufacturing a core layer by co-injection technique, according to various exemplary embodiments of the present invention; 
         FIG. 11  is a schematic illustration showing a perspective view of a multilayer optical waveguide device, according to some embodiments of the present invention; 
         FIG. 12   a  is a plan view of an illumination panel incorporating multiple optical waveguide devices, according to some embodiments of the present invention; 
         FIG. 12   b  is an exploded view of a display device incorporating the illumination panel depicted in  FIG. 12   a ; and 
         FIG. 13  is a schematic illustration of a light-emitting element that includes a phosphor layer. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1   a - 1   h  illustrate an optical waveguide device  10  according to various exemplary embodiments of the present invention. Device  10  generally has an open shape (i.e., non-tubular) such as the shape of a sheet, e.g., a planar sheet. Typically, device  10  is a non-fiber device, i.e., not a substantially cylindrical waveguide in which a light-conducting core is surrounded by a layer of cladding, and is solid (i.e., not hollow). In various exemplary embodiments of the invention the sheet is flexible and hence may be also assume a non-planar shape. For example, when the sheet is placed on a curved surface, the sheet may acquire the curvature of the surface. Device  10  may also have a certain degree of elasticity. Thus, one or more of the layers of device  10  may be made, for example, from an elastomer. In some embodiments, device  10  is substantially rigid. 
     Device  10  includes a core layer  16  formed of a plurality of core structures  18  joined in, e.g., a side-by-side or nested configuration. Core structures  18  (designated in  FIGS. 1   a - 1   h  by reference numerals  18 - 1 ,  18 - 2 , . . . ,  18 - n ) may take the form of elongated bands arranged side-by-side (see, FIG.  1 . a ), or may instead have a nested configuration (see, e.g.,  FIG. 1   h ) with their common ends joined. The width of structures  18  (along the x direction in  FIGS. 1   a - 1   h ) may vary. 
     For clarity of presentation device  10  is shown as planar, with layer  16  being parallel to the x-y plane, and each elongated core structure  18  extending along the y direction. The ordinarily skilled person would know how to tailor the following description for non-planar devices. For example, each section of a non-planar sheet may be described using a Cartesian x-y-z coordinate system which is rotated such that the section is tangential to the x-y plane and normal to the z direction. The x, y and z directions are referred to herein as the “lateral,” “longitudinal” and “normal” directions, respectively. 
     Although core structures  18  are shown in  FIGS. 1   a - 1   g  as having a rectangular cross-section, this need not necessarily be the case, depending on the application. 
     With more specific reference to  FIGS. 1   a  and  1   b ,  FIG. 1   a  is a perspective view of device  10  and  FIG. 1   b  is a cross-sectional view along the line A-A of  FIG. 1   a  in embodiments in which core layer  16  is at least partially surrounded by air.  FIG. 1   b  is a cross-sectional view along the line A-A of a device similar to that illustrated  FIGS. 1   a  and  1   h  in embodiments in which core layer  16  is at least partially surrounded by air. 
     In embodiments in which core layer  16  is at least partially surrounded by air, structures  18  are characterized by a refractive index which is larger than the refractive index of the surrounding air. In such configuration, when light strikes the internal surface of layer  16  at an angle larger than the critical angle, θ c ≡sin −1 (n 1 /n 2 ), where n 1  and n 2  are the refractive indices of the air and the core layer, respectively, the light energy is trapped within core layer  16  and propagates therethrough via total internal reflection. Light may also propagate through device  10  when the impinging angle is smaller than the critical angle, in which case one portion of the light is emitted and the other portion thereof continues to propagate. The difference between the indices of refraction the core layer and surrounding air may be selected in accordance with the desired propagation angle of the light. 
     Typically, the refractive index of air is about 1; hence, core structures  18  typically include or consist essentially of a waveguide material having a refractive index greater than 1. Representative examples of waveguide materials suitable for the core structures include, without limitation, a thermoplastic such as a polycarbonate, methacrylate (PMMA), and/or polyurethane (TPU) (aliphatic) with a refractive index of about 1.50, TPU (aromatic) with a refractive index of from about 1.58 to about 1.60, amorphous nylon such as GRILAMID supplied by EMS Glivory (e.g., GRILAMID TR90 with refractive index of about 1.54), polymethylpentene, TPX: supplied by Mitsui with a refractive index of about 1.46, polyvinylidene fluoride (PVDF) with a refractive index of about 1.34, or other thermoplastic fluorocarbon polymers, and/or STYROLUX (UV stabilized) supplied by BASF with refractive index of about 1.58. 
       FIGS. 1   c - 1   e  are perspective ( FIG. 1   c ) and cross-sectional ( FIGS. 1   d  and  1   e ) views along line B-B of device  10  in embodiments in which device  10  further comprises one or more cladding layers. Although the core layer of the device shown in  FIG. 1   c  is illustrated as having the core structures in a side-by-side configuration, this need not necessarily be the case, since, for some applications, it may be desired to have the core layer arranged in a nested configuration (e.g., the configuration schematically illustrated in  FIG. 1   h ). 
     As shown in  FIGS. 1   c - 1   e , device  10  includes a first cladding layer  12 , a second cladding layer  14 , and core layer  16  interposed between cladding layers  12 ,  14 . Typically, the elongated structures of core  16  extend along the length of the cladding layers. 
     The refractive index of the cladding layers is typically smaller than the refractive index of the core layer. As a result, when light strikes the internal surface of the cladding layers at an impinging angle larger than the critical angle (θ c ≡sin −1 (n 1 /n 2 ), where n 1  and n 2  are the refractive indices of the cladding and core layers, respectively), the light energy is trapped within core layer  16 , and the fight propagates therethrough. The light may also propagate through device  10  when the impinging angle is smaller than the critical angle, in which case one portion of the light is emitted and the other portion continues to propagate. The difference between the indices of refraction of the layers is preferably selected in accordance with the desired propagation angle of the light. 
     In the embodiments in which the cladding layers are employed, core structures  18  include or consist essentially of a waveguide material such as those identified above for the embodiment lacking cladding layers, and preferably have relatively high refractive indices. 
     In accordance with embodiments of the present invention, the indices of refraction are selected such that propagation angle is from about 2° to about 55°. For example, core layer  16  may be made of GRILAMID TR90 with a refractive index of about 1.54, and cladding layers  12 ,  14  may be made of TPX with refractive index of about 1.46, so that Δn≡n 2 −n 2 ≈0.08 and n 1 /n 2 ≈0.948, corresponding to a propagation angle of 90°−sin −1 (0.948), or approximately ±19°. In another example, a core layer  16  made of TPU (aromatic) with a refractive index of about 1.60 without cladding has a corresponding propagation angle of 90°−sin −1 (1/1.6), or approximately ±51°. 
     In some embodiments of the invention, core structures  18  do not have elongated shapes.  FIGS. 1   f  and  1   g  are perspective views of device  10  without ( FIG. 1   f ) and with ( FIG. 1   g ) claddings, in an embodiment in which core structures  18  are in the shape of plaques (e.g., polygonal plaques such as squares or rectangles). The ordinarily skilled person will know how to construct a cross-sectional view of these illustrations, which may be similar to  FIGS. 1   b ,  1   d  and  1   e.    
     The partitioning of core layer  16  into core structures  18  (elongated or shaped as plaques) may be accomplished by any process known in the art, such as, but not limited to, coextrusion, extrusion, coating, coinjection molding, lamination, tiling, and the like. For example, two adjacent structures may be welded at their joined ends, bonded by an adhesive material disposed along their length and/or width, etc. A process for forming core layer  16  according to some embodiments of the present invention is provided below. 
     Whether or not device  10  includes cladding layers, and irrespectively of the shape and arrangement of the core structures forming layer  16 , some of the core structures include additives selected to provide the individual core structures with a predetermined effective refractive index. The effective refractive index depends on the type and concentration of the additive. Typically, higher additive concentrations provide higher effective refractive indices. The additives may take the form of light-scattering particles  20  embedded in one or more of the core structures. In various exemplary embodiments of the invention, the size, concentration, refractive index, and/or type of light-scattering particles  20  varies among at least two of the core structures. 
     Particles  20  are dispersed within core structures  18  and facilitate emission of the light from a surface  23  of core layer  16  and/or a surface  24  of cladding layer  14  (in the embodiments in which cladding layer  14  is employed). Particles  20  serve as scatterers and typically scatter optical radiation in more than one direction. When light is scattered by a particle  20  such that the impinging angle is below the critical angle, no total internal reflection occurs and the scattered light is emitted through surface  23  and/or surface  24 . 
     The light-scattering particles may be beads, e.g., glass beads, or other ceramic particles, rubber particles, silica particles, particles including or consisting essentially of inorganic materials such as BaSO 4  or TiO 2 , particles including or consisting essentially of a phosphor material (as further described below), and the like. In an embodiment, the light-scattering particles are substantially or even completely non-phosphorescent. Such non-phosphorescent particles merely scatter light without converting the wavelength of any of the light striking the particles. The term “light-scattering particles” may also refer to non-solid objects embedded in the waveguide material from which core structure are made, provided that such objects are capable of scattering the light. Representative example of suitable non-solid objects include, without limitation, closed voids within the core structures, e.g., air bubbles, and/or droplets of liquid embedded within the core structures. The light-scattering particles may also be organic or biological particles, such as, but not limited to, liposomes. In some embodiments, optical elements such as microlenses are utilized in conjunction with, or even instead of, light-scattering particles. In other embodiments, optical elements include or consist essentially of structures such as hemispheres or diffusive dots. In such embodiments, the optical elements function to out-couple light propagating through device  10 . As utilized herein, “optical elements” may generically refer to elements such as microlenses as well as light-scattering particles, e.g., non-photoluminescent particles. 
     In accordance with various embodiments of the present invention, the concentration, size and/or type of particles is selected such as to provide illumination at a predetermined profile (e.g., intensity profile) from predetermined regions of surface  23  or  24 . For example, in regions of device  10  where a larger portion of the propagated light is to be emitted through the surface, the concentration of particles  20  may be large and/or the particles may have a type and/or size which provides them with high scattering properties; in regions where a smaller portion of the light is to be emitted the concentration of particles  20  may be smaller and/or the particles may have a type and/or size which provides them with lower scattering properties; and in surface regions from which no light is to be emitted, substantially no particles are embedded in core structures  18 . 
     As will be appreciated by one ordinarily skilled in the art, the energy trapped in waveguide device  10  decreases each time a light ray is emitted through surface  23  or  24 . On the other hand, it may be desired to use device  10  to provide a uniform surface illumination. Thus, as the overall amount of energy decreases with each emission, a uniform surface illumination may be achieved by gradually increasing the ratio between the emitted light and the propagated light. According to some embodiments of the present invention, the increasing ratio of emitted light to propagated light is achieved by an appropriate selection of the distribution, type, refractive index, and/or size of particles  20  in the core layer  16 . For example, at regions in which it is desired to have uniform surface illumination, the concentration of particles  20  may be an increasing function of the optical distance traversed by the propagated light. 
     Generally, the optical output at specific and predetermined regions may be controlled by arranging the core structures  18  such that different core structures have different concentrations, sizes, refractive indices, and/or types of particles  20 . 
     In various exemplary embodiments of the invention, the core structures  18  are arranged to define a first zone  26  and a second zone  28 . First and second zones  26 ,  28  may include portions of core layer  16  such that a profile of an optical mean free path characterizing core layer  16  is generally flat across the first zone  26  and monotonically varying across the second zone  28 . 
     The optical mean free path may be measured directly by positioning a bulk material in front of a light-emitting element and measuring the optical output through the bulk at a given direction as a function of the thickness of the bulk. Typically, when a bulk material, t mm in thickness, reduces the optical output of a light source at the forward direction by 50%, the material is said to have a mean free path of t mm. 
       FIG. 2   a  shows a representative example of an optical mean free path as a function of the lateral direction x. As shown, the optical mean free path is substantially constant in zone  26 , and is a decreasing function of x in zone  28 . The decrement of the optical mean free path in region  28  facilitates an increasing ratio between the emitted portion and propagated portions of the light. 
     Zone  26  may include one or more core structures and is typically devoid of light-scattering particles  20 . In this embodiment, zone  26  propagates light with minimal or no emissions from surfaces  23  or  24 , i.e., zone  26  is a propagation region. Zone  28  may include a plurality, e.g., three or more, of core structures  18  each having particles  20  embedded therein. In such an embodiment, zone  28  provides illumination by out-coupling light from core  16  (i.e., zone  28  is an out-coupling region for light propagated through zone  26 ). The brightness of the illumination from zone  28  may be substantially uniform. 
     Brightness uniformity may be calculated by considering the luminance deviation across the range of azimuthal angles as a fraction of the average luminance across that range. A more simple definition of the brightness uniformity BU is BU=1−(L MAX −L MIN )/(L MAX +L MIN ), where L MAX  and L MIN  are, respectively, the maximal and minimal luminance values across the predetermined range of azimuthal angles. 
     The term “substantially uniform brightness” refers to a BU value which is at least 0.8 when calculated according to the above formula. In some embodiments of the invention the value of BU is at least 0.85, more preferably at least 0.9, and still more preferably at least 0.95. 
     To achieve a decreasing optical mean free path, the concentration of particles  20  in the core structures  18  of zone  28  may be an increasing function of the distance from zone  26 . Alternatively or additionally, the type and/or size of the particles in the individual core structures  18  of zone  28  may vary to achieve the desired profile. As shown in  FIG. 1   b , the concentration, type, size, and/or refractive index of panicles  20  in zone  28  may change in a direction of light propagation through device  10  (denoted as the x direction in  FIG. 1   b ). However, for any cross-section through zone  28 , the concentration, type, size, and/or refractive index of particles  20  may be substantially constant in at least one of the directions perpendicular to the light-propagation direction (e.g., the v and z directions in  FIG. 1   b ). For example, each core structure  18  in zone  28  may have a substantially constant concentration, type, size, and/or refractive index of particles  20  therewithin, but this value may change in at least one (or every) other core structure  18  in zone  28 . 
     Some embodiments of the present invention include a third zone  30 . As shown in  FIG. 1   a , third zone  30  may be proximate or in direct contact with first zone  26  and away from second zone  28 . Third zone  30  may comprise or consist essentially of one or more core structures  18  having light-scattering particles  20  embedded therein. A representative example of an optical mean free path in the embodiment in which three zones are defined is illustrated in  FIG. 2   b.    
     Zone  30  may be an in-coupling region for facilitating the entry of light into device  10 . Light enters device  10  at zone  30 , propagates through zone  26  and exits (i.e., is out-coupled) at zone  28 . One or more of the core structures  18 , typically the first and last structures (i.e., structures  18 - 1  and  18 - n  in the illustration of  FIG. 1   d ) may be made light-reflective so as to prevent or reduce optical losses through the side(s) of device  10 . The characteristic refractive index of such light-reflective core structures  18  is preferably above 2. A representative example of a material having a sufficiently high refractive index suitable for the present embodiment is TiO 2 , which has a refractive index of about 2.5. Alternatively, light-reflective structures  22  may be disposed proximate the entire height of device  10  as shown in  FIG. 1   e.    
     Coupling of light into device  10  may be facilitated using an optical funnel  32  positioned adjacent to layer  12  or layer  16  at zone  30 . Funnel  32  is preferably configured to receive light from one or more light-emitting elements and to transmit the light into layer  12  or layer  16 . The principle of operation of funnel  30  according to some embodiments of the present invention is further detailed herein under with reference to  FIG. 4 . 
     To prevent or reduce optical losses through the portion of cladding layer  14  which overlaps zone  30 , device  10  may further include one or more light reflectors  36  adjacent to cladding layer  14  at the region of cladding layer  14  which overlaps zone  30 . Reflector(s)  36  reduce illumination in any direction other than a circumferential direction. 
     In various exemplary embodiments of the invention, zone  30  of device  10  includes one or more components that cause the light exiting zone  30  (into zone  26 ) to have a predetermined optical profile, such as, but not limited to, a substantially uniform color profile or substantially uniform white light. This embodiment may be implemented by color mixing, optical means, or may be implemented via luminescence, a phenomenon in which energy is absorbed by a substance, commonly called a luminescent, and is emitted in the form of light. The wavelength of the emitted light differs from the characteristic wavelength of the absorbed energy (the characteristic wavelength equals hc/E, where h is the Plank&#39;s constant, c is the speed of light and E is the energy absorbed by the luminescent). Luminescence is a widely occurring phenomenon which may be classified according to the excitation mechanism as well as according to the emission mechanism. Examples of such classifications include photoluminescence and electroluminescence. Photoluminescence is sub-classified to fluorescence and phosphorescence. 
     A photoluminescent is generally a material which absorbs energy is in the form of light. A fluorescent material is a material which emits light upon return to the base state from a singlet excitation, and a phosphorescent materials is a material which emits light upon return to the base state from a triplet excitation. In fluorescent materials, or fluorophores, the electron de-excitation occurs almost spontaneously, and the emission ceases when the source of the energy exciting the fluorophore is removed. In phosphor materials, or phosphors, the excitation state involves a change of spin state, which decays only slowly. In phosphorescence, light emitted by an atom or molecule persists after the excitation source is removed. 
     Photoluminescent materials are used according to various embodiments of the present invention for altering the color of light. Since blue light has a short wavelength (compared, e.g., to green or red light), and since the light emitted by a photoluminescent material has a longer wavelength than the absorbed light, blue light generated by a blue light-emitting element such as a light-emitting diode (LED) may be readily converted to visible light having a longer wavelength. Accordingly, in various exemplary embodiments of the invention a specific light profile on the exit of light into zone  26  is provided using one or more photoluminescent layers disposed on or embedded in device  10 . 
     The term “photoluminescent layer” is commonly used herein to describe one photoluminescent layer or a plurality of photoluminescent layers. Additionally, a photoluminescent layer may include one or more types of photoluminescent species. In any event, a photoluminescent layer is characterized by an absorption spectrum (i.e., a range of wavelengths of light absorbed by the photoluminescent molecules to effect quantum transition to a higher energy level) and an emission spectrum (i.e., a range of wavelengths of light emitted by the photoluminescent molecules as a result of quantum transition to a lower energy level). The emission spectrum of the photoluminescent layer is typically wider and shifted relative to its absorption spectrum. The difference in wavelength between the apex of the absorption and emission spectra of the photoluminescent layer is referred to as the Stokes shift of the photoluminescent layer. 
     The absorption spectrum of the photoluminescent layer preferably overlaps, at least partially, the emission spectrum of the light source which feeds device  10 . More preferably, for each characteristic emission spectrum of the light source, there is at least one photoluminescent layer having an absorption spectrum overlapping the characteristic emission spectrum. According to some embodiments of the present invention, the apex of the source&#39;s emission spectrum lies in the spectrum of the photoluminescent layer, and/or the apex of the photoluminescent layer&#39;s absorption spectrum lies in the spectrum of the light source. 
     The photoluminescent layer may “convert” the wavelength of a portion of the light emitted by the light source. More specifically, for each photon which is successfully absorbed by the layer, a new photon is emitted. Depending on the type of photoluminescent, the emitted photon may have a wavelength which is longer or shorter than the wavelength of the absorbed photon. Photons which do not interact with the photoluminescent layer propagate therethrough. The combination of converted light and non-converted light forms the profile of light entering zone  26 . This “mixed” light is preferably spectrally different from each of the converted light and the non-converted light. Since the mixed light is formed by the superposition of the converted light and the non-converted light, the spectrum of the mixed light generally contains all of the wavelengths of the converted light and the non-converted light. 
     In preferred embodiments, the photoluminescent material is disposed neither on an outer surface of device  10  nor directly on a light-emitting element  34 . Rather, as described further below, the photoluminescent material (e.g., in the form of particles and/or a layer or layers is disposed within device  10  some distance away from light-emitting element  34 . 
       FIGS. 3   a - d  are fragmentary schematic illustrations of device  10  showing a cross-section of zone  30  parallel to the z-x plane. Several components of device  10  are omitted from  FIGS. 3   a - d  for clarity of presentation.  FIG. 3   a  illustrates an embodiment in which the elongated structures at the ends of zone  30  (structures  18 - 1  and  18 - 3 , in the present example) include or consist essentially of photoluminescent material, e.g., a phosphor or a fluorophore.  FIG. 3   b  illustrates an embodiment in which one or more of the inner elongated structures of zone  30  (structure  18 - 2 , in the present example) include or consist essentially of photoluminescent material.  FIG. 3   c  is a schematic illustration of an embodiment in which a photoluminescent layer  38 , which may include or consist essentially of a photoluminescent material such as a phosphor or a fluorophore, is disposed on the surface of layer  12  and/or layer  14 . In this embodiment, the wavelength of the light is changed via the multiple impingements of the light on surface of layer  12  and/or  14 . In an embodiment, only one of the surfaces is coated by the photoluminescent layer  38 . For example, the surface of layer  14  may be coated by the photoluminescent layer  38  and the surface of layer  12  may be left exposed for better light coupling between layer  12  and the light-emitting element or funnel  32 . 
     Photoluminescent material may also be incorporated in the form of particles, as illustrated in  FIG. 3   d . A plurality of photoluminescent particles  128  may be distributed within one or more of the core structures  18  in accordance with the desired light output profile. For example, in one embodiment, the particles  128  are uniformly distributed in all the core structures  18 . In another embodiment, the particles are distributed such that there are core structures  18  with a higher population of the particles  128  and core structures  18  with a lower population of the particles  128 , depending on the desired profile in or near each core structure. 
     A cross-sectional view of an exemplary embodiment of optical funnel  32  is illustrated in  FIG. 4   a . Optical funnel  32  receives the light from one or more light-emitting elements  34  and distributes it prior to entry of the light into layer  12  (not shown in  FIG. 4 , see  FIGS. 1   d  and  1   e ) so as to establish a plurality of entry locations within zone  30  (hence improving the uniformity of light distribution within zone  30 ). Light-emitting elements  34  may be arranged near funnel  32  or they may be embedded in funnel  32 . Efficient optical transmission between funnel  32  and layer  12  is preferably ensured by impedance matching therebetween. Each light-emitting element  34  may be a discrete light source, e.g., an LED. In various embodiments, each light-emitting element  34  is a substantially unpackaged (or “bare”) LED die. In such embodiments, funnel  32  or other portions of device  10  (such as zone  30 , as described further below) function as the “package” for light-emitting element  34 . In preferred embodiments of the invention, bare LED dies do not include a phosphor or other photoluminescent material as a portion thereof (e.g., on a common substrate therewith or incorporated into or onto the LED semiconductor layer structure). Where a single light-emitting element  34  is described herein, more than one light-emitting element  34  could generally also be utilized, and vice versa. Generally, light is emitted from light-emitting element  34  upon supply of electrical current thereto. 
     Funnel  32  may be made as a surface-emitting waveguide or surface-emitting optical cavity which receives the light generated by light-emitting elements  34  through an entry surface  142 , distributes it within an internal volume  148 , and emits it through an exit surface  144 , which is typically opposite to the entry surface  142 . 
     In some embodiments of the present invention, funnel  32  comprises one or more light reflectors  146 , which are typically arranged peripherally about volume  148  so as to form an optical cavity or an optical resonator within volume  148 . One or more light reflectors  146  may also be formed on or attached to the entry surface  142  of funnel  32 . In this embodiment, one or more openings  150  are formed on the reflectors  146  at the entry surface, thus allowing light to enter volume  148 . Openings  150  may be substantially aligned, e.g., in the x-y plane, with light-emitting elements  34 . 
     Funnel  32  may include or consist essentially of a waveguide material, or it may be filled with a medium having a small absorption coefficient to the spectrum or spectra emitted by the light-emitting elements  34 . For, example, funnel  32  may be filled with air, or be made of a waveguide material which is similar or identical to the material of the cladding layers  12  and/or  14 . The advantage of using air is its low absorption coefficient, and the advantage of a waveguide material identical to material of the cladding layers  12 ,  14  is impedance matching therewith. 
     When funnel  32  is filled with medium having a small absorption coefficient (e.g., air), there may be no impedance matching at exit surface  144  of funnel  32 . Thus, some reflections and refraction events may occur upon the impingement of light on the interface between funnel  32  and the cladding layer  12 . Neither refraction nor reflection events cause significant optical losses; refraction events contribute to the distribution of light within zone  30 , and reflection events contribute to the distribution of light within volume  148 . 
     In various exemplary embodiments of the invention, funnel  32  is supplemented by photoluminescent material for controlling the output profile of the light, as schematically illustrated in  FIGS. 4   b - 4   d . For clarity of presentation, the reflectors  146  are not shown in  FIGS. 4   b - 4   d . In any of the embodiments, funnel  32  may include one or more light reflectors  146  as detailed above. In the embodiment illustrated in  FIG. 4   b , a photoluminescent layer  38  is interposed between layer  12  and funnel  32 ; in the embodiment illustrated in  FIG. 4   c , photoluminescent layer  38  is embedded in funnel  32 ; and in the embodiment illustrated in  FIG. 4   d  a plurality of photoluminescent particles  128  is distributed within funnel  32 . 
     Various embodiments of the present invention feature one or more light-emitting elements  34  embedded within zone  30  of device  10  and/or photoluminescent material (e.g., photoluminescent layer  38  and/or particles  128 ) disposed within device  10  outside of the direct “line-of-sight” from light-emitting elements  34 . That is, in such embodiments, there is no direct, straight-line optical path between the light-emitting elements  34  and the photoluminescent material; rather, light emitted from light-emitting elements  34  reflects from a reflector, a surface, or an interface within device  10  before reaching the photoluminescent material. Thus, any light striking and being back-reflected from the photoluminescent material will not propagate directly back into light-emitting element  34  (where it could be absorbed, thus reducing overall light output and efficiency of device  10 ). Rather, light reflecting from the photoluminescent material will tend to remain within device  10  and eventually reflected back toward zone  28  to be out-coupled. In some embodiments, there is substantially no direct line-of-sight between light-emitting element  34  and the photoluminescent material, i.e., less than approximately 5% of the light from light-emitting element  34  has a direct line-of-sight to the photoluminescent material; any losses thereof are therefore negligible. 
     Whether or not the photoluminescent material is within a direct line-of-sight of light-emitting element  34 , the photoluminescent material may advantageously be located remotely in relation to light-emitting element  34 , i.e., it may be present in zone  26  and/or zone  28  rather than proximate light-emitting element  34  (in zone  30  or in funnel  32 , for example). The quantum efficiency (or other performance metric) of the photoluminescent material may degrade when the material is exposed to elevated temperatures, e.g., temperatures greater than approximately 50° C. Remote placement of the photoluminescent material prevents the temperature of the material from rising during operation due to, e.g., heat given off by light-emitting element  34 . Instead, the temperature of remotely placed luminescent material will generally remain at the ambient temperature of the surroundings of device  10 . Generally, the temperature of the luminescent material may remain at least approximately 30° C., or even at least 100° C. less than the temperature of light-emitting element  34  during operation. 
     During assembly of device  10 , elevated temperatures capable of damaging (e.g., degrading the quantum efficiency of) the photoluminescent material are often required when affixing or embedding light-emitting element  34  into device  10 . Remote placement of the photoluminescent material enables the photoluminescent material to be provided within device  10  prior to the addition of light-emitting element  34 —the distance therebetween prevents the elevated temperatures from damaging the photoluminescent material. 
     A remotely placed photoluminescent material may be located in any one or more of a variety of locations, as depicted in  FIGS. 4   e - 4   j .  FIG. 4   e  depicts a photoluminescent layer  38  within zone  26  and outside the direct line-of-sight of light-emitting element(s) in funnel  32  (e.g., as illustrated in  FIG. 4   a ). At least a portion of the light propagating through zone  26  is converted by photoluminescent layer  38  to light of a different wavelength, and then the converted and unconverted light components enter zone  28  where they are out-coupled together to form, e.g., substantially white light. In this and similar configurations, the propagating light converted by the photoluminescent material travels in a direction substantially perpendicular to the direction of the eventual out-coupled light. Such configurations may enable superior uniformity, brightness, and color of the out-coupled light. 
       FIG. 4   f  depicts potential locations in zone  28  for the photoluminescent material, which are also outside the direct line-of-sight of light-emitting element(s) in funnel  32 . First, photoluminescent particles  128  may be utilized in conjunction with (or instead of) particles  20 : at least a portion of light striking particles  128  is converted to light of a different wavelength, and the light out-coupled from zone  28  is, e.g., substantially white. Additionally (or instead), photoluminescent layer  38  may be disposed within zone  28 , e.g., proximate a top edge thereof. In this configuration, at least a portion of the light already being out-coupled (i.e., on its way out of device  10 ) is converted to light of a different wavelength. The exiting converted and unconverted light mix to form, e.g., substantially white light. In configurations featuring particles  20  (or other optical element(s)) disposed between light-emitting element  34  and a photoluminescent material (e.g., photoluminescent layer  38  disposed along the top edge of zone  28 ), the uniformity of the light striking the photoluminescent material may be greater than the uniformity of the light striking particles  20 . That is, the scattering by particles  20  increases the uniformity of the light, which then strikes the photoluminescent material and is out-coupled from device  10  with a high level of uniformity. The line of sight between light-emitting element  34  and the photoluminescent material may not be eliminated by placement of particles  20  therebetween, as some light may propagate through the region populated with particles  20  without being scattered thereby. 
       FIG. 4   g  depicts possible locations for a photoluminescent material described with reference to  FIGS. 4   e  and  4   f , any of which (or any combination of which) may be utilized in conjunction with a device  10  shaped to eliminate the direct line-of-sight between the light-emitting element(s) in funnel  32  and photoluminescent layer  38  and/or particles  128 . As shown in  FIG. 4   g , device  10  may include a bend, curve, or other geometry in zone  26  (or even in zone  28 ) which facilitates the elimination of a direct line-of-sight between the light-emitting element(s) and the photoluminescent material. This geometry may also facilitate subsequent “tiling” of multiple devices  10  to form an illumination panel, e.g., a panel in which the zones  28  of devices  10  overlie zones  26  and/or  30  of adjacent devices  10  (as further described below with reference to  FIGS. 12   a  and  12   b ). The shape depicted in  FIG. 4   g  is exemplary, and many other configurations are possible. 
       FIGS. 4   h - 4   j  are analogous to  FIGS. 4   e - 4   g , respectively, but depict one or more light-emitting elements  34  embedded within device  10  (here shown embedded within a core structure  18  of zone  30 ) rather than coupled to device  10  via funnel  32 . As shown by the schematic break within zone  26  in  FIGS. 4   h - 4   j , zone  26  may be elongated and/or be sized and shaped so as to substantially or completely eliminate the direct line-of-sight between light-emitting element(s)  34  and photoluminescent layer  38  and/or particles  128 . Each device  10  depicted in  FIGS. 4   e - 4   j  may also incorporate cladding layers  12 , 14 , e.g., as illustrated in  FIGS. 1   c - 1   e.    
     In a preferred embodiment, light from light-emitting element  34  (whether embedded within device  10  or operated in conjunction with funnel  32 ) generally enters zone  30  in an “in-coupling direction,” i.e., along the z axis indicated in  FIG. 1   b . Once in-coupled into device  10  by scattering from particles  20  and/or reflector  36 , the light generally propagates through device  10  (e.g., through zone  26 ) in a “propagation direction” that is substantially perpendicular to the in-coupling direction. As illustrated in  FIG. 1   b , the propagation direction is generally along the x axis. After the light enters zone  28 , it is generally out-coupled from device  10  (i.e., emitted from surface  23  and/or  24 ) in an “out-coupling direction” that is substantially perpendicular to the propagation direction (e.g., along the z axis indicated in  FIG. 1   b ). Thus, the in-coupling direction and the out-coupling direction may be substantially parallel. In some embodiments in which photoluminescent layer  38  and/or particles  128  are present, at least a portion of the light propagating in device  10  in the propagation direction is stimulated by photoluminescent layer  38  and/or particles  128 , giving rise to the mixed light that is out-coupled from device  10  in an out-coupling direction substantially perpendicular to the propagation direction. This configuration may enable better brightness and/or color uniformity than devices in which stimulated light (i.e., light before or as it strikes a photoluminescent material) propagates in a direction that is not substantially perpendicular (e.g., a substantially parallel direction) to an out-coupling direction of the mixed light resulting from stimulation by the photoluminescent material. 
     Phosphors are widely used for coating individual LEDs, typically to obtain White light therefrom. However, photoluminescent layers incorporated in waveguide devices as described herein have not been employed. The advantage of providing photoluminescent layer  38  and/or particles  128  (in layer  16  and/or funnel  32 ) as opposed to on each individual light-emitting element, is that waveguide device  10  diffuses the light before emitting it. Thus, instead of collecting light from a point light source (e.g., a LED), photoluminescent layer  38  and/or particles  128  collects light having a predetermined extent. This configuration allows a better control on the light profile provided by device  10 . 
     Many types of phosphorescent and fluorescent substance are contemplated. Representative examples include, without limitation, the phosphors disclosed in U.S. Pat. Nos. 5,813,752, 5,813,753, 5,847,507, 5,959,316, 6,155,699, 6,351,069, 6,501,100, 6,501,102, 6,522,065, 6,614,179, 6,621,211, 6,635,363, 6,635,987, 6,680,004, 6,765,237, 6,853,131, 6,890,234, 6,917,057, 6,939,481, 6,982,522, 7,015,510, 7,026,756, 7,045,826, and 7,005,086, the entire disclosures of which are hereby incorporated by reference. In an embodiment, the quantum efficiency of photoluminescent layer  38  and/or particles  128  is only stable up to a temperature of approximately 50° C. However, in many configurations the temperature of such materials remains lower than this level due to spatial separation of photoluminescent layer  38  and/or particles  128  from the light-emitting element(s). In various embodiments, layer  38  and/or particles  128  include or consist essentially of one or more electroluminescent materials rather than (or in addition to) photoluminescent materials. Such electroluminescent materials may include or consist essentially of quantum dot materials and/or organic LED (OLED) materials. Suitable quantum dots may include or consist essentially of cadmium selenide. 
     There is more than one configuration in which photoluminescent layer  38  may be used. In one embodiment, photoluminescent layer  38  complements the light emitted by light-emitting elements  34  to create a white light, e.g., using dichromatic, trichromatic, tetrachromatic or multichromatic approach. For example, a blue-yellow dichromatic approach may be employed, in which case blue light-emitting elements (e.g., InGaN LEDs with a peak emission wavelength at about 460 nm) are used, and photoluminescent layer  38  may include or consist essentially of phosphor molecules with an absorption spectrum in the blue range and an emission spectrum extending to the yellow range (e.g., cerium-activated yttrium aluminum garnet, or strontium silicate europium). Since the scattering angle of light sharply depends on the frequency of the light (fourth-power dependence for Rayleigh scattering, or second-power dependence for Mie scattering), the blue light generated by the blue light-emitting elements  34  is efficiently diffused in the waveguide material before interacting with photoluminescent layer  38  and/or particles  128 . Layer  38  and/or particles  128  emit light in its emission spectrum and complement the blue light which is not absorbed by photoluminescent layer  38  and/or particles  128  to white light. 
     In another dichromatic configuration, ultraviolet light-emitting elements (e.g., LEDs of GaN, AlGaN, and/or InGaN with a peak emission wavelength between 360 nm and 420 nm) are used. Light of such ultraviolet light-emitting elements is efficiently diffused in the waveguide material. To provide substantially white light, two photoluminescent layers  38  and/or two types of photoluminescent particles  128  are preferably employed. One such photoluminescent layer and/or type of particles may be characterized by an absorption spectrum in the ultraviolet range and emission spectrum in the orange range (with peak emission wavelength from about 570 nm to about 620 nm), and another photoluminescent layer and/or type of particles may be characterized by an absorption spectrum in the ultraviolet range and emission spectrum in the blue-green range (with peak emission wavelength from about 480 nm to about 500 nm). The orange light and blue-green light emitted by the two photoluminescent layers  38  and/or two types of photoluminescent particles  128  blend to appear as white light to an observer. Since the light emitted by the ultraviolet light-emitting elements is above or dose to the end of the visual range, it is not discerned by the observer. When two photoluminescent layers  38  are employed, they may be deposited one on top of the other so as to improve the uniformity. Alternatively, a single photoluminescent layer  38  having two types of photoluminescent material with the above emission spectra may be utilized. 
     In another embodiment a trichromatic approach is employed. For example, blue light-emitting elements may be employed as described above, with two photoluminescent layers  38  and/or two types of photoluminescent particles  128 . A first photoluminescent layer  38  and/or type of photoluminescent particles  128  may include or consist essentially of phosphor molecules with an absorption spectrum in the blue range and an emission spectrum extending to the yellow range as described above, and a second photoluminescent layer  38  and/or type of photoluminescent particles  128  may include or consist essentially of phosphor molecules with an absorption spectrum in the blue range and an emission spectrum extending to the red range (e.g., cerium-activated yttrium aluminum garnet doped with a trivalent ion of praseodymium, or europium-activated strontium sulphide). The unabsorbed blue light, the yellow light, and the red light blend to appear as white light to an observer. 
     Also contemplated is a configuration is which light-emitting elements  34  with different emission spectra are employed and several photoluminescent layers  38  are deposited and/or several types of photoluminescent particles  128  are distributed, such that the absorption spectrum of each photoluminescent layer  38  and/or type of photoluminescent particles  128  overlaps one of the emission spectra of the light-emitting elements  34 , and all the emitted colors (of the light-emitting elements  34  and the photoluminescent layers  38  and/or particles  128 ) blend to appear as white light. The advantage of such a multi-chromatic configuration is that it provides a high-quality white balance because it allows better control of the various spectral components of the light in a localized manner, e.g., along an edge or surface of device  10 . 
     The color composite of the white output light depends on the intensities and spectral distributions of the emanating light emissions. These depend on the spectral characteristics and spatial distribution of the light-emitting elements  34 , and, in the embodiments in which one or more photoluminescent components (layers  38  and/or particles  128 ) are employed, on the spectral characteristics of the photoluminescent components and on the amount of unabsorbed light. The amount of light unabsorbed by the photoluminescent components is, in turn, a function of the characteristics of the components, e.g., thickness of the photoluminescent layer(s)  38 , density of photoluminescent material(s), and the like. By judiciously selecting the emission spectra of light-emitting element  34  and optionally the thickness, density, and spectral characteristics (absorption and emission spectra) of photoluminescent layer  38  and/or particles  128 , device  10  may provide substantially uniform white light. 
     In any of the above embodiments, the “whiteness” of the light may be tailored according to the specific application for which device  10  is intended. For example, when device  10  is incorporated as backlight of an LCD device, the spectral components of the light provided by device  10  may be selected in accordance with the spectral characteristics of the color filters of the liquid crystal panel. In other words, since a typical liquid crystal panel includes an arrangement of color filters operating at a plurality of distinct colors, the white light provided by device  10  includes at least at the distinct colors of such filters. This configuration significantly improves the optical efficiency as well as the image quality provided by the LCD device, because the optical losses due to mismatch between the spectral components of the backlight unit and the color filters of the liquid crystal panel are reduced or eliminated. 
     Thus, in the embodiment in which the white light is achieved by light-emitting elements  34  emitting different colors of light (e.g., red light, green light and blue light), the emission spectra of the light-emitting elements  34  are preferably selected to substantially overlap the characteristic spectra of the color filters of an LCD panel. In the embodiment in which device  10  is supplemented by one or more photoluminescent components (layers  38  and/or particles  128 ), the emission spectra of the photoluminescent components and optionally the emission spectrum (or spectra) of the light-emitting elements are preferably selected to overlap the characteristic spectra of the color filters of an LCD panel. Typically, the overlap between a characteristic emission spectrum and a characteristic filter spectrum is about 70% spectral overlap, more preferably about 80% spectral overlap, and even more preferably about 90%. 
     The following is a description of a production process for the core layer  16  and the optical waveguide device  10  according to various exemplary embodiments of the present invention. 
     In some embodiments, the core layer is formed by coextrusion. As used herein, the term “coextrusion” refers to the process of simultaneous extrusion of several die outputs which are welded together before chilling to form an extrudate having an open shape, e.g., a non-tubular sheet. An extrudate formed by a coextrusion process according to some embodiments of the present invention may be a single-layer structure or a laminate structure having two or more layers. In some embodiments of the present invention the coextrusion process is employed in an extrusion coating process in which an extrudate formed by the coextrusion process is applied so as to coat one or more existing layers. 
     Thus, a plurality of light-transmissive compositions in a molten or plastic state may be coextruded to form the elongated core structures of core layer  16 . Each light-transmissive composition may be extruded to form a single core structure  18 , and may be a polymeric material mixed with light-scattering particles of type, size and concentration selected to provide the core structure  18  with the desired optical properties (e.g., mean free path). 
     A coextrusion apparatus  50  which may be used according to some embodiments of the present invention is schematically illustrated in  FIG. 5 . As shown therein, several melt or plasticized streams  52  (three in the illustration) are individually extruded from a plurality of extruders  54 . The melt streams comprise light-transmissive compositions in accordance with the respective core structures  18  to be formed. Extruders  54  discharge the compositions, which are conveyed by conventional conduits (not shown) to a coextrusion die or feedblock  56 . Die  56  combines and arranges the compositions and issues a composite flat stream  58  in which the various compositions flow side-by-side. A chill roller system  60  quenches stream  58  to form core layer  16  which includes or consists essentially of a plurality of core structures  18  as described above. The formed core structures  18  may have any shape or cross-section, e.g., rectangular or triangular. 
     One or more of the extruded core structures  18  (e.g., the sidemost elongated structure  18 - 1  and  18 - n , see  FIG. 1   a ) may be made reflective. This may be achieved by judicious selection of the composition from which these core structures are formed. For example, a composition characterized by high refractive index (e.g., 2 or more) may be fed to the respective extruder  54 . A representative example of a material having a sufficiently high refractive index for reflectivity is TiO 2 , which has a refractive index of about 2.5. Also contemplated, is the use or incorporation of a substantially opaque composition or the incorporation of reflective particles at a sufficiently high density to make core structure  18  reflective. 
     Coextrusion apparatus  50  may be adapted to simultaneously form the core layer  16  as well as the cladding layers  12 ,  14 . This approach is illustrated in  FIG. 6 , which shows apparatus  50  with a die  56  configured to combine and arrange the compositions into a laminated flat stream  58  in which the intermediate layer of stream  58  is composed of side-by-side flow of the various compositions of the core layer  16  and the outer layers of the stream are composed of the compositions of the cladding layers  12 ,  14 . In an embodiment, additional layers are formed above and below the cladding layers  12 ,  14 , e.g., for the purpose of protecting or reenforcing the cladding layers  12 ,  14 . 
     An alternative embodiment is illustrated in  FIGS. 7   a  and  7   b . In this embodiment, apparatus  50  performs an extrusion coating process, whereby the core structures  18  of core layer  16  are coextruded on a cladding layer  12  which is already in a dimensionally stable (i.e., rigid) state. Once core layer  16  is dimensionally stable (e.g., following cooling, or treatment with roller system  60 ), cladding layer  14  may optionally be laminated on core layer  16  to form a three-layer structure. 
     Once the core layer  16  and optionally the cladding layers  12 ,  14  are coextruded, the layer(s) may be further treated while the compositions are in molten or plastic state. One example of such treatment is application of heat and/or pressure so as to at least partially mix respective compositions at common edges of adjacent core structures  18 . When the adjacent core structures  18  have different particle concentrations (including the case of two adjacent structures in which one has a zero concentration and the other has a non-zero concentration), the heat and/or pressure treatment may result in a concentration gradient across the lateral direction of the core structures  18 . This embodiment is particularly useful when it is desired to have a smooth profile along the optical mean free path. Post-extrusion treatment of the formed core structures  18  may be performed by roller system  60  (prior to the cooling of the extruded structures), or it may be done using another roller system. When the compositions comprise thermoplastic materials, the post-extrusion treatment may be performed after the structures are cooled. In this embodiment, the post-extrusion treatment may include reheating of the core structures. 
     The optical waveguide device featured in embodiments of the present invention may also be manufactured by a lamination process. Suitable lamination processes may be employed on both thermoset and thermoplastic materials. Any lamination technique suitable for the materials from which core layer  16  and cladding layers  12 ,  14  are formed may be employed. The lamination process may be executed with or without a solid support. When a solid support e.g., a metal support or other rigid support) is employed, it is preferably designed and constructed to allow lamination of individual core structures  18  in a side-by-side fashion. Thus, the solid support preferably fixes each individual core structure  18  to its place sidewise with a previously laminated elongated structure. 
     A lamination technique according to various embodiments is schematically illustrated in  FIGS. 8   a - 8   c . The process starts with a substrate  62  ( FIG. 8   a ), on which the lamination process is executed. The process continues with the lamination of a plurality of core structures  18  (e.g., elongated core structures) in a side-by-side configuration on a substrate  62  to form core layer  16  ( FIG. 8   b ). The lamination may be performed by heat-and-press, with or without adhesives. Optionally, but not obligatorily, substrate  62  may serve as a cladding layer (e.g., layer  12  of  FIGS. 1   c - 1   e  or  1   g ). In this embodiment, substrate  62  preferably includes or consists essentially of a flexible cladding material and is preferably laid on a support substrate (not shown), which is desirably planar. 
     One or more light-reflective structures may be laminated sidewise relative to core layer  16 . This may be done in a similar manner to the lamination of the other core structures  18 . 
     Once laminated side-by-side, the core structures  18  may be joined at their common ends using any technique known in the art, including, without limitation, adhesive bonding, solvent bonding, or welding (also known as fusion bonding). The lamination of core  16  on substrate  62  may be preceded by a step in which an adhesive optical material is applied on substrate  62 . If desired, substrate  62  may be removed following the lamination of the core structures  18 . It this embodiment, the air serves as the “cladding” layer as detailed above. 
     In various exemplary embodiments of the invention the process continues by laminating cladding layer  14  on core layer  16  ( FIG. 8   c ). Optionally, an optical adhesive may be applied on core layer  16  prior to the lamination of cladding layer  14  thereon. 
     An additional technique for fabricating device  10  is illustrated in  FIGS. 9   a - 9   c . The process starts with substrate  62  ( FIG. 9   a ). A plurality of core structures  18  having the shape of plaques are tiled in a side-by-side configuration on a substrate  62  to form core layer  16  ( FIG. 9   b ). The tiling may be performed by lamination techniques such as heat-and-press, with or without adhesives. Optionally, but not obligatorily, substrate  62  may serve as a cladding layer (e.g., layer  12  of  FIGS. 1   c - e  or  1   g ). In this embodiment, substrate  62  is made of a flexible cladding material and is preferably laid on a support substrate (not shown), which is preferably planar. 
     One or more light-reflective structures may be laminated sidewise relative to core layer  16 . This may be done in a similar manner to the lamination of the other core structures  18 . 
     Once laminated side-by-side, the core structures  18  may be joined at their common ends using any technique known in the art, including, without limitation, adhesive bonding, solvent bonding, or welding. The lamination of core  16  on substrate  62  may be preceded by a step in which an adhesive optical material is applied on substrate  62 . If desired, substrate  62  may be removed following the lamination of the core structures  18 . In this embodiment, the air serves as the “cladding” layer as detailed above. In various exemplary embodiments of the invention the process continues by laminating cladding layer  14  on core layer  16  ( FIG. 9   c ). Optionally, an optical adhesive may be applied on core layer  16  prior to the lamination of cladding layer  14  thereon. 
     Following the lamination process of any of the above embodiments, one or more additional layers (not shown) may be attached to cladding layers  12  and/or  14 . This may be achieved using any procedure known in the art, including, without limitation, printing, embossing, lamination, and the like. The attachment of the additional layers may be performed using any technique, including, without limitation, adhesive bonding, solvent bonding, welding, mechanical fastening, co-consolidation, and the like. The additional layer may cover the entire surface area of the cladding or a portion thereof. For example, a reflective foil  36  (see, e.g.,  FIG. 1   a ) may be attached to cladding layer  14 . Also contemplated are jacket layers for protecting the cladding layers  12 ,  14 . 
     An additional technique for fabricating device  10  according to some embodiments of the present invention is illustrated in  FIGS. 10   a - 10   c . In these embodiments co-injection molding is employed. Co-injection molding is a variant of a process known as injection molding. In injection molding thermoplastic polymers or the like are fed from a hopper into a barrel, melted by a reciprocating screw and/or electric heat, and are propelled forward by a ram (piston, plunger) or the screw used as a plunger) a mold cavity, which is cooled to below the heat-distortion temperature of the resin. 
     Co-injection molding takes advantage of a characteristic of injection molding called fountain flow. As the cavity is filled, the material at the melt front moves from the center line of the stream to the cavity walls. The walls are typically kept below the transition temperature of the melt such that the material that touches the walls cools rapidly and freezes in place. This provides insulating layers through which new melt makes its way to the melt front. 
     In some embodiments of the present invention, the co-injection technique is employed for forming a core layer  16  having a plurality of core structures  18  in a nested configuration. A co-injection molding system suitable for the present embodiments is illustrated in  FIG. 10   a . The system typically includes a co-injection manifold  230  mounted relative to a mold cavity  220 , and shaped according to the desired shape of the device. In various exemplary embodiments of the invention, mold cavity  220  has a substantially planar shape. 
     Manifold  230  includes a nozzle housing  234  having forward and rearward ends. The illustrated nozzle housing  234  is generally V-shaped, but any other shape suitable for co-injection may be utilized. Nozzle housing  234  includes a plurality of arms  254 , each having a rearward end  262 , and includes an outwardly extending mounting portion  266 . Arms  254  are supported by mounting columns  236 , which are typically fixedly mounted on a horizontal surface of a machine base sled (not shown). 
     Housing  234  has an outlet  270  in its forward end, as well as a plurality of inlets  274  in the rearward end of each arm. Outlet  270  communicates with an inlet  226  of cavity  220 . Inlets  274  of housing  234  respectively communicate with a plurality of injection nozzles  284  of respective injection units (not shown). Each injection nozzle is typically fed by a different light-transmissive composition as described above. 
     Manifold  230  also includes a valve  258  movable between a plurality of positions. In each position, valve  258  open a fluid communication channel between one of inlets  274  and outlet  270 . Also contemplated is a position in which valve  258  closes all communication channels. Valve  258  may be moved relative to housing  234  by a hydraulic cylinder  278  mounted on the manifold  230 . 
     The co-injection system may operate as follows. The nozzle housing is oriented such that each injection nozzle provides one type of light-transmissive composition. The co-injection process begins with the valve  258  in a position selected such that a first light-transmissive composition (e.g., a composition with low concentration of light-scattering particles), in a molten or plastic state, flows through the outlet  270 . The selected composition is injected into the mold cavity  220 . The valve  258  is then moved to another position to allow flow of a second light-transmissive composition (e.g., a composition with a higher concentration of light-scattering particles), in a molten or plastic state, through the outlet  270 . By the effect of fountain flow described above, the second composition is nested into the first composition. The process is optionally continued by repositioning the valve  258  so as to inject into the mold a third composition in a molten or plastic state. The third composition is nested into the previously injected second composition. The third composition may have a concentration of light-scattering particles higher than that of the second composition. 
     Any number of light-transmissive compositions may be serially injected into the mold so as to form a core layer  16  with a plurality of core structures  18  (which may be flexible) joined in a nested configuration. The melt fronts of the different light-transmissive compositions are designated in  FIG. 10   a  by reference numerals  222 - 1 , . . . ,  222 - n . The propagation of each melt front nesting into previously injected light-transmissive compositions is shown by arrows. An advantage of using a co-injection manifold for manufacturing the core layer  16  is that it allows more flexibility in selecting the characteristics of the different core structures  18 . A continuous or semi-continuous control on the operation of the co-injection manifold may facilitate formation of core structures  18  in a manner such that the characteristic mean free path varies substantially smoothly from one core structure  18  to the other. Since the effective refractive index varies with the characteristic mean free path, various embodiments of the present invention allow production of an optical waveguide device  10  having a graded effective refractive index along the lateral direction. 
     Once the core layer  16  is formed, it is typically released from the mold. A top view of the core layer  16  once released from the mold is illustrated in  FIG. 10   b . The procedure optionally and preferably continues by cutting the core layer  16  along the lateral direction so as to remove one or more marginal regions  228  therefrom, thereby providing a core layer  16  in which the core structures  18  are joined in a side-by-side configuration. Shown in  FIG. 10   b  are two cut lines  224  parallel to the lateral direction along which the core layer may be cut. A top view of the core layer  16  once cut along cut lines  224  is illustrated in  FIG. 10   c . The procedure may continue to form additional layers such as cladding layers  12 ,  14 , and/or photoluminescent layers  38  on the core structure  16  as detailed above. In some embodiments of the present invention, the co-injection system is configured to inject also the cladding layers  12 ,  14 . 
     Referring to  FIG. 11 , in various embodiments of the invention, multilayer  110  optical waveguide device  100  comprises multiple waveguide devices  10 , each of which may be fabricated as described above. The devices  10  in multilayer device  100  may be disposed in a vertically “stacked” configuration as depicted in  FIG. 11 . A layer  1100  of low-refractive-index material may be disposed between each “layer”  10  in order to prevent undesired light propagation from one layer  10  to another. Light may be coupled in to zone  30  of each layer  10  from a different light source, or the same light source may be utilized for each layer  10 . In a particular embodiment, multilayer device  100  provides controllable RUB illumination by including different types of photoluminescent particles  128  in each layer  10 . For example, a bottom layer  10  may include photoluminescent particles  128  that emit red light, a middle layer  10  may include photoluminescent particles  128  that emit green light, and a top layer  10  may include photoluminescent particles  128  that emit blue light. Such a multicolor multilayer device  100  may be suitable for LCD backlight applications. As shown in  FIG. 11 , the zones  28  of the layers  10  may be substantially vertically aligned such that light emitted from the bottom layer  10  travels through the other layers  10  before finally being emitted from multilayer device  100 . In other words, each zone  28  in multilayer device  100  may have substantially no vertical overlap with zones  26 ,  30  of the other layers  10 . 
     Referring to  FIGS. 12   a  and  12   b , in various embodiments of the invention, multiple optical waveguide devices  10  are utilized together to provide enhanced functionality. Illumination panel  1200  includes or consists essentially of a plurality of optical waveguide devices  10  attached together at their edges (or overlapped) in a “tiled” fashion. In order to provide substantially uniform illumination across the entire surface of illumination panel  1200 , the waveguide devices  10  may be tiled together such that only out-coupling region  28  of each device  10  is visible. In-coupling region  30  and propagation region  26  of each device  10  may therefore be disposed beneath adjoining devices  10  and not visible. While illumination panel  1200  is illustrated as substantially planar, the flexibility of each waveguide device  10  enables illumination panel  1200  to be configured in a variety of shapes, including curved sheets and even spheres. 
     Illumination panel  1200  may be utilized to provide substantially uniform illumination in a variety of applications. For example, illumination panel  1200  may itself be utilized as a luminaire for lighting applications. In another embodiment, illumination panel  1200  is utilized as a backlight unit for a display device  1210 , e.g., a liquid crystal display (LCD). Display device  1210  may additionally include an LCD panel  1220  defining a plurality of pixels, and may be actuated by signals received from control circuitry  1230 . 
     Referring to  FIG. 13 , a phosphor layer  1302  may be added to a light-emitting element  1300 . The phosphor layer  1302  converts light emitted from the in-coupling region  1304  from the light source  1306 , such as an LED, into a different color (i.e., changes the spectrum). For example, part of the light from a blue LED may be converted to yellow light, which mixes with the remaining blue light to provide white output illumination. In other embodiments, phosphor material is placed at any location in the optical path, including locations without any direct line of sight from any light source. 
     The waveguide materials from which the waveguide device  10  is made may include or consist essentially of one or more polymeric materials. The polymeric material may optionally include a rubbery or rubber-like material. The material may be formed by dip-molding in a dipping medium, for example, a hydrocarbon solvent in which a rubbery material is dissolved or dispersed. The polymeric material optionally and preferably has a predetermined level of cross-linking, which is preferably between particular limits. The cross-linking may optionally be physical cross-linking, chemical cross-linking, or a combination thereof. A non-limiting illustrative example of a chemically cross-linked polymer is cross-linked polyisoprene rubber. Non-limiting illustrative examples of physically cross-linked polymers include cross-linked block co-polymers and segmented co-polymers, which may be cross-linked due to, e.g., micro-phase separation. The material is optionally cross-linked through application of radiation, such as, but not limited to, electron beam radiation and/or electromagnetic (e.g., ultraviolet) radiation. 
     Although not limited to rubber itself, the material optionally and preferably has the physical characteristics (e.g., parameters relating to tensile strength and elasticity) of rubber. For example, the waveguide material may be characterized by a tensile set value which is below 5%. The tensile set value generally depends on the degree of cross-linking and is a measure of the ability of a flexible material, after having been stretched either by inflation or by an externally applied force, to return to its original dimensions upon deflation or removal of the applied force. 
     The tensile set value may be determined by, for example, placing two reference marks on a strip of the waveguide material and noting the distance between them, stretching the strip to a certain degree, for example, by increasing its elongation to 90% of its expected ultimate elongation, holding the stretch for a certain period of time, e.g., one minute, then releasing the strip and allowing it to return to its relaxed length, and re-measuring the distance between the two reference marks. The tensile set value is then determined by comparing the measurements before and after the stretch, subtracting one from the other, and dividing the difference by the measurement taken before the stretch. In a preferred embodiment, using a stretch of 90% of an expected ultimate elongation and a holding time of one minute, the preferred tensile set value is less than 5%. Also contemplated are materials having about 30% plastic elongation and less than 5% elastic elongation. 
     Other exemplary materials, which may optionally be used alone or in combination with each other, or with one or more of the above rubber materials, include but are not limited to, crosslinked polymers such as: polyolefins, including but not limited to, polyisoprene, polybutadiene, ethylene-propylene copolymers, chlorinated olefins such as polychloroprene (neoprene) block copolymers, including diblock-, triblock-, multiblock- or star-block-, such as: styrene-butadiene-styrene copolymers, or styrene-isoprene-styrene copolymers (preferably with styrene content from about 1% to about 37%), segmented copolymers such as polyurethanes, polyether-urethanes, segmented polyether copolymers, silicone polymers, including copolymers, and fluorinated polymers and copolymers. In some embodiments of the present invention, the waveguide material may include or consist essentially of IOTEK. 
     The embedded particles may be glass beads, BaSO 4  particles, and/or similar particles. The volume density of the particles may be from about 0.1% to about 5%. 
     The number of extruders used to fabricate the core layer  16  may number from three to approximately 10. When the cladding layers  12 ,  14  are formed simultaneously with the core layer  16  the number of extruders may number from three to approximately 15. The total width of the coextrusion die may be about 400 mm to about 1200 mm, and it may be constructed and designed to provide from about 20 to about 100 side-by-side core structures  18 . 
     The thickness of the cladding layers  12 ,  14  may be about 10 μm to about 100 μm. The thickness of the core layer  16  may be about 400 μm to about 1300 μm. The number of core structures  18  in the core layer may be approximately 20 structures to approximately 100 structures. The width of a single core structure  18  may be about 5 mm to about 30 mm. 
     Examples 
     The core structures  18  of an optical waveguide device  10  were fabricated from polyurethane. Two outer in-coupling zones  30  were each approximately 22 mm wide and included 0.5% volume density of VELVOLUX M synthetic BaSO 4  particles (available from Sachtleben Chemie GmbH of Duisburg, Germany) having approximate diameters of 5 μm. Propagation zones  26  were each approximately 29 mm wide and were substantially particle-free. The center out-coupling zone  28  was approximately 77 mm wide, and was composed of three core structures  18 . The outer core structures  18  were approximately 26 mm wide and included 0.35% volume density of VELVOLUX M synthetic BaSO 4  particles. The middle core structure  18  of out-coupling zone  28  was approximately 25 mm wide and contained 0.2% volume density of BLANC FIXE F synthetic BaSO 4  particles (also available from Sachtleben Chemie GmbH of Duisburg, Germany) having approximate diameters of 1 μm. Out-coupling region  28  exhibited a nine-point average brightness of approximately 9078 Nits, with a uniformity of approximately 10%. 
     Another optical waveguide device  10  was fabricated from IOTEK, and included a propagation zone  26  that was substantially particle-free and had a width of approximately 11 mm. Out-coupling zone  28  was composed of three core structures  18 . In increasing distance from propagation zone  26 , these core structures  18  were 1) a 17 mm-wide region having 0.75% volume density of 5 μm-diameter BaSO 4  particles, 2) a 10 mm-wide region having 1.5% volume density of 5 μm-diameter BaSO 4  particles, and 3) a 10 mm-wider region having 3% volume density of 5 μm-diameter BaSO 4  particles. Illumination from this out-coupling zone  28  was approximately uniform across its width. 
     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.