Patent Publication Number: US-2021173141-A1

Title: Light guide apparatus and fabrication method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application No. 62/506,222, entitled LIGHT GUIDE APPARATUS AND FABRICATION METHOD THEREOF, filed on May 15, 2017 and is continuation in part of co-pending U.S. patent application Ser. No. 15/260,877, entitled LIGHT GUIDE APPARATUS AND FABRICATION METHOD THEREOF, filed on Sep. 9, 2016, which claims priority of U.S. Provisional Patent Application No. 62/216,503, entitled LIGHT GUIDE APPARATUS AND FABRICATION METHOD THEREOF, filed on Sep. 10, 2015, and is a continuation in part of U.S. patent application Ser. No. 15/203,384, entitled LIGHT GUIDE APPARATUS AND FABRICATION METHOD THEREOF, filed on Jul. 6, 2016, which is a continuation of International Application No. PCT/US2015/10296 filed on Jan. 6, 2015 and entitled LIGHT GUIDE APPARATUS AND FABRICATION METHOD THEREOF, which in turn claims priority to U.S. patent application Ser. No. 14/148,388 filed on Jan. 6, 2014 and to U.S. Provisional Patent Application No. 62/099,864 filed on Jan. 5, 2015, all of which are incorporated by reference herein in their entirety for all purposes. 
    
    
     BACKGROUND 
     The present teachings relate to a light guide apparatus and a fabrication method thereof. More particularly, the present teachings relate to a light guide apparatus for collecting light and delivering the collected light, and a fabrication method thereof. 
     Light guide apparatuses, such as light pipes, optical fibers and planar waveguides, have been used to direct the propagation of light beams using the principle of total internal reflection (TIR) at the interface between an optically dense medium and an optically rare medium. Traditionally, light pipes (or optical fibers) need to have the light pumped into them from the end of the pipe (or optical fiber) with the condition that angle of light falls within the acceptance cone of the pipe to be guided therein. Other than light propagation, light guide apparatuses have also been used as solar concentrators to focus light in a small area. However, owing to the constant motion of the sun relative to earth, solar concentrators require a tracking system, because they only work at certain fixed small angular range of the sun relative to the solar concentrator. Moreover, many of these light guide apparatuses are rather bulky. 
     Accordingly, there is a need to develop a new light guide apparatus that can concentrate light without using a tracking system. There is also a need for a new light guide apparatus that can redirect light impinging on the apparatus over a wide range of incident angles. 
     SUMMARY 
     A light guide apparatus that can redirect light impinging on the apparatus over a wide range of incident angles from the side and can concentrate light without using a tracking system (low concentration) or limited use of tracking system (high concentration) and methods for fabrication are disclosed herein below. 
     One novelty presented in this disclosure, also relies on a peculiar behavior in Snell&#39;s law that has not been explicitly been harnessed in prior art. When light is incident from a dense medium to a rare medium at an angle slightly less (1-3 degrees) than the critical angle for total internal reflection, the variation in angle of refraction is highly sensitive to the variations in angle of incidence. E.g. For Glass/air interface, a 1 degree reduction in angle of incidence below critical angle, changes the angle of refraction by 12 degrees. A further 1 degree reduction changes the angle of refraction by 5 degrees. In this disclosure, this anomalous light bending effect near total internal reflection conditions (herein called as near-TIR) is harnessed in combination with the well-known effects of refraction and total internal reflection at optical interfaces. 
     In one aspect, the present disclosure provides a light guide apparatus having a core that defines a longitudinal axis. The core includes a first optically transparent section comprising a first optical medium having a first index of refraction; and a second optically transparent section comprising a second optical medium having a second index of refraction, an interface between the first optically transparent section and the second optically transparent section defining a shape. The shape, the first index of refraction, and the second index of refraction are configured such that light entering the core is deflected at the interface at an angle such that, when the light impinges on a core-cladding interface, the light impinges on the core cladding interface at an angle at least equal to a critical angle for total internal reflection. 
     In one embodiment, the shape comprises a first frustum of a first half angle, wherein a central axis of the first conical frustum substantially coincides with the longitudinal axis of the core. 
     In one embodiment, the core further comprises a third optically transparent section comprising a third optical medium, the third optically transparent section interfacing with the second optically transparent section to define a central cylinder. The third optical medium may be air or vacuum. The second optical medium may be same as the third optical medium. 
     In one embodiment, the shape further comprises a second conical frustum of a second half angle, a central axis of the second conical frustum substantially coinciding with the longitudinal axis of the core. The second half angle is greater than the first half angle. A bottom base circumference of the first conical frustum coincides with a bottom base circumference of the second conical frustum. A top base circumference of the first and second conical frustums substantially coincides with a curvilinear side surface of the central cylinder. A bottom base circumference of the first conical frustum substantially coincides with a bottom base circumference of the second conical frustum. 
     In one embodiment, the first half angle ranges from about 0.05 degrees to about 75 degrees, and the second half angle ranges from about 2 degrees to about 85 degrees. 
     In one embodiment, the second frustum comprises a semicircular conical frustum, and the first conical frustum comprises a semicircular conical frustum. 
     In one embodiment, the first refractive index of the first optical medium ranges from about 1.4 to about 2.4, and the second refractive index of the second optical medium ranges from about 1.3 to about 2.2. 
     In one embodiment, the shape comprises a plurality of first conical frustums of the first half angle and a plurality of second conical frustums of the second half angle, wherein central axes of the first and second conical frustums substantially coincide with the longitudinal axis of the core. One of the first conical frustums comprises a top base circumference that coincides with a top base circumference of a neighboring one of the second conical frustums. 
     In one aspect, the present disclosure provides a light guide apparatus comprising a core defining a longitudinal axis; and a cladding layer on the core. The cladding layer comprises a first optical medium having a first index of refraction and an inclusion structure embedded in the optical medium. The inclusion structure comprises a second optical medium having a second index of refraction. The inclusion structure defines an interface between the first optical medium and the second optical medium. The inclusion structure, the first index of refraction, and the second index of refraction are configured such that light incident on the interface is totally internally reflected and propagates at a predetermined grazing angle with respect to the longitudinal axis. The light is incident on the cladding layer in a predetermined range of angles from a normal direction substantially perpendicular to the longitudinal axis. 
     In one embodiment, the interface has one of a conic shape, a semi-conic shape, a parabolic conic shape, and an ellipsoidal shape. Surfaces of the inclusion structure may be textured, and the second optical medium may be air. The inclusion structure may have a semi-conic shape. The core may have a semi-cylindrical shape, and the core may be tapered. 
     In one embodiment, a cross section of the cladding layer has an outer circumference of a shape selected from the group consisting of a circle, an N-sided polygon, an ellipse, a semicircle, and a bounded shape of two circular arcs, wherein N is a natural number ranging from 3 to 100. In one embodiment, the first refractive index of the first optical medium of the cladding layer ranges from about 1.3 to about 1.8. 
     In one embodiment, the core comprises at least an optically transparent medium, and a refractive index of the optically transparent medium of the cylindrical core is greater than that of the first optical medium of the cladding layer. 
     In one embodiment, the core comprises a first optically transparent section comprising a third optical medium having a third index of refraction; and a second optically transparent section comprising a fourth optical medium having a fourth index of refraction; an interface between the third optically transparent section and the optically fourth transparent section defining a shape; the shape, the third index of refraction and the fourth index of refraction being configured such that light entering the core is deflected at the interface at an angle such that, when the light impinges on a core-cladding interface, the light impinges on the core cladding interface at an angle at least equal to a critical angle for total internal reflection. 
     In one embodiment, the shape comprises a first conical frustum of a first half angle, wherein a central axis of the first conical frustum substantially coincides with the longitudinal axis of the cylindrical core. The cylindrical core further comprises a third transparent section comprising a third optical medium, the third transparent section interfacing with the second transparent section to define a central cylinder. The shape further comprises a second conical frustum of a second half angle, a central axis of the second conical frustum substantially coinciding with the longitudinal axis of the cylindrical core. 
     In one aspect, the present disclosure provides a light guide apparatus comprising a core defining a longitudinal axis, a cladding layer on the core, and a super-cladding layer on the cladding layer. The cladding layer is configured to deflect light incident on the cladding layer, the light being incident on the cladding layer in a predetermined range of angles from a normal direction substantially perpendicular to the longitudinal axis. The light is deflected to a direction that forms a grazing angle with respect to the longitudinal axis. The super-cladding layer comprises a first optically transparent medium that receives incident light. A second optically transparent medium interfaces with the first optically transparent medium to define a heterogeneous interface. The heterogeneous interface comprises a plurality of bi-conic shapes. 
     In one aspect, the present disclosure provides a light guide apparatus comprising a super-cladding layer, wherein the super-cladding layer comprises a first optically transparent medium that receives incident light; and a second optically transparent medium interfacing with the first optically transparent medium to define a heterogeneous interface, wherein the heterogeneous interface comprises a plurality of shapes; each shape from the plurality of shapes configured to deflect a light beam, incident on the super-cladding layer in a first predetermined range of angles from a normal direction substantially perpendicular to a longitudinal axis, into a second predetermined range of angles. 
     In one embodiment, the light guide apparatus further comprises a core defining the longitudinal axis; a cladding layer disposed on the core, wherein the cladding layer is configured to deflect light incident on the cladding layer, the light being incident in said second predetermined range of angles from a normal direction substantially perpendicular to the longitudinal axis; the light being deflected to a direction that forms a grazing angle with respect to the longitudinal axis. The plurality of shapes comprises a plurality of bi-conic shapes; and wherein an angle subtended by surfaces of two neighboring bi-conic shapes ranges from about 2 degrees to about 30 degrees. 
     In one embodiment, a refractive index of the first optically transparent medium ranges from about 1.3 to about 2.4, and a refractive index of the second optically transparent medium ranges from about 1.3 to about 2.4. A refractive index difference of the first and second optically transparent media may range from about 0.01 to about 0.30. The super-cladding layer is configured to convert a light beam having an incident angle within about ±30 degrees with respect to the normal direction to a light beam having an angle within about ±5 degrees or less with respect to the normal direction. 
     In one embodiment, the core comprises a third optically transparent section comprising a first optical medium with a first index of refraction; and a fourth optically transparent section comprising a second optical medium with a second index of refraction; an interface between the third optically transparent section and the fourth optically transparent section defining a shape; the shape, the first index of refraction and the second index of refraction being configured such that light entering the core is deflected at the interface at an angle such that, when the light impinges on a core-cladding interface, the light impinges on the core cladding interface at an angle at least equal to a critical angle for total internal reflection. 
     In one embodiment, the shape comprises a first conical frustum of a first half angle, wherein a central axis of the first conical frustum substantially coincides with the longitudinal axis of the core. 
     In one embodiment, the cladding layer comprises a third optically transparent section having a first index of refraction and an inclusion structure embedded in the third optical medium; the inclusion structure comprising a fourth optical medium having a second index of refraction; the inclusion structure defining an interface between the third optical medium and the fourth optical medium; the inclusion structure, the first index of refraction and the second index of refraction configured such that light incident on the interface is totally internally reflected and propagates at a predetermined grazing angle with respect to the longitudinal axis; the light being incident in a predetermined range of angles from a normal direction substantially perpendicular to the longitudinal axis. The grazing angle may range from about 0.1 degrees to about 40 degrees. 
     In one aspect, the present disclosure provides a solar panel comprising a light guide apparatus for receiving sun light, the light guide apparatus comprising a plurality of parallel light pipes, and a photovoltaic cell optically coupled to an end of the parallel light pipes. Each light pipe comprises a core defining a longitudinal axis and a normal direction substantially perpendicular to the longitudinal axis; a cladding layer on the core, wherein the cladding layer is configured to convert sun light from a first predetermined range of angles (about ±5 degrees in an exemplary embodiment) with respect to the normal direction to a direction that forms a grazing angle with respect to the longitudinal axis; and a collimating layer on the cladding layer, wherein the collimating layer is configured to convert sun light from an incident angle within a second predetermined range of angles (in an exemplary embodiment about ±30 degrees) with respect to the normal direction to an angle within the first predetermined range of angles (in an exemplary embodiment, about ±5 degrees) with respect to the normal direction. 
     In one embodiment, the solar panel further comprises a reflector under the light guide apparatus for reflecting escaped light back to the light guide apparatus. 
     In one embodiment, the light guide apparatus consists of a multi-layer stack of multiple optical media such that the interface between each media consists of an array of scalene prisms with predetermined angles of the prism faces for each interface. The angle of the faces of scalene prisms that constitute a particular interface are chosen such that light entering the light guide apparatus at a fixed range of angles is impinging either under conditions of near-TIR or near normal incidence or TIR or refraction so as to achieve efficient light coupling in the light guide for a wide range of angles of input light. In one aspect, the scalene prism array interface that defines the core is designed in such a way that the light impinges on one of the faces of the prism under near-TIR conditions to achieve anomalous light bending and breaks the angular symmetry of the light to achieve the light trapping. 
     In one aspect, the present disclosure provides a method of fabricating a light guide assembly. The method comprises forming a number of protrusions on a surface of a first optically transparent material; forming a number of indentations on a surface of a second optically transparent material; each indentation from the number of indentations being configured so that a shape of said each indentation is similar to a shape of each protrusion from the number of protrusions and dimensions of said each indentation are bigger than dimensions of said each protrusion; and assembling said surface of the first optically transparent material over said surface of the second optically transparent material such a space is disposed between surfaces and a location of each protrusion corresponds to a location of each indentation, forming a number of inclusions. In one embodiment, air is disposed in the space between the surfaces. In one embodiment, the method further comprises depositing a layer of a third optically transparent material of a substantially constant thickness over said surface of the first optically transparent material; said substantially constant thickness configured such that a shape of said surface of the first optically transparent material, after deposition is substantially congruent with a shape of said surface of the second optically transparent material; wherein after assembling the third optically transparent material is disposed in the space between protrusions and indentations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is to be read in conjunction with the accompanying drawings, in which: 
         FIGS. 1( a ) through 1( d )  illustrate a light guide apparatus having a core and a cladding layer, in accordance with various embodiments of the present disclosure; 
         FIGS. 2( a ) through 2( d )  illustrate a light guide apparatus having a core, in accordance with various embodiments of the present disclosure; 
         FIG. 3  illustrates a light guide apparatus having a core, a cladding layer, and a super-cladding layer, in accordance with one embodiment of the present disclosure; 
         FIGS. 4( a ) and 4( b )  illustrate ray tracing diagrams for a light guide apparatus having a core and a cladding layer, in accordance with one embodiment of the present disclosure; 
         FIG. 5  illustrates a ray tracing diagram for a light guide apparatus having a core, a cladding layer, and a super-cladding layer, in accordance with one embodiment of the present disclosure; 
         FIGS. 6( a ) through 6( c )  illustrate ray tracing diagrams for a light guide apparatus having a core, in accordance with various embodiments of the present disclosure; 
         FIG. 7  illustrates a perspective view of a solar panel including an array of light guide apparatuses, in accordance with one embodiment of the present disclosure; 
         FIGS. 8( a ) through 8( g )  illustrate various views of systems including an array of light guide apparatuses, in accordance with various embodiments of the present disclosure; 
         FIGS. 8 ( e   1 ) through  8 ( e   4 ) illustrate design variations of the planar core layer with prismatic hetero-interface in accordance with various embodiments in the present disclosure. 
         FIGS. 9( a ) through 9( f )  illustrate a planar light guide apparatuses, in accordance with various embodiments of the present disclosure; 
         FIGS. 9 ( a   1 ) through  9 ( a   3 ) illustrate an alternate construction of the planar light concentrator in accordance with various embodiments of the present disclosure. 
         FIG. 9 ( b   1 ) illustrates an exemplary design of a planar light concentrator consisting of multiple layers of polymers in a specific interfacial geometry with respect to each other. 
         FIG. 9 ( b   2 ) through  9 ( b   5 ) are exemplary ray tracing simulations of design shown in  FIG. 9 ( b   1 ). 
         FIGS. 10( a ) through 10( c )  illustrate strategies for materials selection in accordance with various embodiments of the present disclosure; 
         FIGS. 11( a )  through  11   d ) illustrate a method for fabricating a light guide apparatus, in accordance with one embodiment of the present disclosure; and 
         FIG. 12( a )  illustrates a smart window using a light guide apparatus, in accordance with one embodiment of the present disclosure; 
         FIG. 12( b )  illustrates a lighting device using a light guide apparatus in accordance with one embodiment of the present disclosure; 
         FIG. 12 ( b   1 ) illustrates an illumination device according to one embodiment of the present disclosure; 
         FIG. 12  ( b   2 ) illustrates operation of the illumination device of  FIG. 12 ( b   1 ); 
         FIG. 12 b     2 - a  shows one embodiment using asymmetric and symmetric V groove interfaces; 
         FIG. 12 ( b   3 ) illustrates an illumination device according to another embodiment of the present disclosure; 
         FIG. 12  ( b   4 ) illustrates an illumination device according to yet another embodiment of the present disclosure; 
         FIG. 12 ( b   5 ) illustrates the use of an illumination device according to embodiments of the present disclosure in illuminating a display; 
         FIG. 12 ( b   6 ) illustrates another use of an illumination device according to embodiments of the present disclosure in illuminating a display; 
         FIG. 12 ( b   7 ) shows another embodiment using the edge-lit light guide collimator as described in this patent disclosure; 
         FIG. 12 ( b   8 ) shows the top view of an embodiment of a rectangular light guide with beveled corners; 
         FIG. 12 ( b   9 ) shows the top view of another embodiment of a rectangular light guide with beveled corners; 
         FIG. 12 ( b   10 ) shows the top view of an embodiment for collimating light on both axes; 
         FIG. 12 ( b   11 )) shows the top view of a circular edge-lit light guide as described in this patent disclosure; 
         FIG. 12 ( b   12 ) shows the top view of a circular light guide which a variation of the embodiment shown in  FIG. 12  ( b   11 ); 
         FIG. 12 ( b   13 ) shows the top view of an inner edge-lit light guide as described in this patent disclosure; 
         FIG. 12( c )  illustrates optical collectors for indoor lighting using a light guide apparatus in accordance with one embodiment of the present disclosure 
         FIG. 12( d )  illustrates solar thermal device using a light guide apparatus in accordance with one embodiment of the present disclosure; 
         FIG. 12( e )  illustrates optical laminates on photovoltaic devices and modules using a light guide apparatus in accordance with one embodiment of the present disclosure; 
         FIG. 12( f )  illustrates light trapping optics for luminescent concentrators using a light guide apparatus in accordance with one embodiment of the present disclosure; and 
         FIG. 12( g )  illustrates a device for optical pumping of lasers using a light guide apparatus in accordance with one embodiment of the present disclosure; 
         FIG. 13( a )  shows a method to successively bend light using combination of flat and textured interfaces; 
         FIG. 13 ( b )  a method to trap and recycle light in a dense medium using successive light bending to shallow angles; 
         FIG. 13( c )  show the zemax simulation of this optics with zoomed-in version of light trapping at the interface; 
         FIG. 13( d )  shows use of light trapping to concentrate light for concentrated photovoltaics by placing the solar cell at edge; 
         FIG. 13( e )  shows use of light trapping to concentrate light for concentrated photovoltaics by placing the solar cell at bottom surface; 
         FIG. 13( f )  shows use of light trapping optics to fabricate a hybrid panel that utilizes both multi-junction solar cells as well as single junction solar cells; 
         FIG. 13( g )  shows use of light bending and light trapping optics to recycle light in a solar panel; 
         FIG. 14 ( a )  shows an achromatic Flat lens using the successive light bending method; 
         FIG. 14( b )  shows use of a flat lens array to manage emission cone of output light from an array of light emitting diodes; 
         FIG. 15 ( a )  shows an angle selective reflector consisting of two dense optical media with textured interface between them; 
         FIG. 15( b )  shows an angle selective reflector consisting of multiple layers of optical media of different refractive index with a combination of textured or flat interfaces between them; 
         FIG. 15( c )  shows an angle selective reflector consisting of multiple layers of optical media of different refractive index with interfaces being either textured or flat or a combination of textured and flat construction; 
         FIG. 15 ( d )  shows use of angle selective reflector as a window film for energy efficiency and privacy such that sunlight is blocked; 
         FIG. 16  shows alternative Method to trap light in air medium using successive light bending method; 
         FIG. 17 ( a )  shows a device to collimate, homogenize and achieve glare-reduction from a light source using a multilayer stack of optical media as shown. 
         FIG. 17( b )  shows the visual depictions of the “onward face” and “leeward face” of the prism as described in the mechanism of light collimation for  FIG. 17( a ) ; also shown is the interaction of the light with the onward and leeward faces of the prism array interface: 
         FIG. 17( c )  shoes ray tracing simulation of device shown in  FIG. 17( a )  showing the collimated light output; and 
         FIG. 17( d )  shows the beam profile of the output light for the method shown in  FIG. 17( a )  achieving light collimation, glare reduction and light homogenization; X-axis shows angle values and Y-axis shows radiance in angle space. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and systems for collecting light into a light guide apparatus of these teachings as the light impinges on the apparatus over a wide range of angles are disclosed herein below. 
     In one instance, described herein includes an apparatus and a method to pump light into the light pipe from sides of the light pipe along the length of the pipe so that, for a uniform illumination over the pipe, light ends up accumulating inside the core of the pipe while traveling along the length of the pipe. This can be used to concentrate the light into core. 
     The following detailed description is of the best currently contemplated modes of carrying out these teachings. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of these teachings, since the scope of these teachings is best defined by the appended claims. Although the teachings have been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims. 
     As used herein, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. 
     Except where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Further, any quantity modified by the term “about” or the like should be understood as encompassing a range of ±10% of that quantity. 
     “Light,” as used herein refers to electromagnetic radiation and is not limited to only the visible range of wavelengths. 
     The terms “light pipe,” “optical fiber,” and “optical pipe” are used herein below to describe the light guide apparatus of the present disclosure and are used interchangeably herein. The term optical pipe should be taken to limit the embodiment to a particular geometry. 
     The term “near-TIR” is used here in to describe the situation in which light is incident from a dense medium to a rare medium at an angle at most 7 degrees less than the critical angle for total internal reflection. 
     A “scalene triangle,” as used herein, is a triangle where all sides have a different length. A “scalene prism,” as used here in, is a prism whose cross-section is a scalene triangle. A “scalene trapezoid,” as used herein, is a trapezoid form by truncating a scalene triangle. 
     In one embodiment of the system of these teachings, the light is concentrated into a compact design of the light pipe by accumulating light into the cores of an array of light pipes at wide angles of incidence. This eliminates the need for solar tracking systems because the optics approach of these teachings can address the variation of angles of incidence during the day as well as during the year. Such side pumped optical pipes and planar concentrators that use the design elements of these teachings can be used for many applications such as solar panels, power producing smart windows, indoor lighting, solar thermal, side pumped lasers etc., as described herein below. 
     In one or more embodiments, the light guide apparatus of these teachings includes a core defining a longitudinal axis, a cladding layer on the core, the cladding layer having a first optical medium having a first index of refraction and an inclusion structure embedded in the optical medium, the inclusion structure including a second optical medium having a second index of refraction, the inclusion structure defining an interface between the first optical medium and the second optical medium, the inclusion structure, the first index of refraction and the second index of refraction configured such that light incident on the interface is totally internally reflected and propagates at a predetermined grazing angle with respect to the longitudinal axis; the light being incident in a predetermined range of angles from a normal direction substantially perpendicular to the longitudinal axis. 
     In one instance, the light guide apparatus of these teachings includes a core defining a longitudinal axis, a cladding layer on the core, the cladding layer having a first optical medium having a first index of refraction and an inclusion structure embedded in the optical medium, the inclusion structure including a second optical medium having a second index of refraction, the inclusion structure defining an interface between the first optical medium and the second optical medium, the inclusion structure, the first index of refraction and the second index of refraction configured such that light incident on the interface under near-TIR conditions or TIR conditions and experiences significant deflection. This leads to light trapping in the core due to disruption in the angular symmetry of light. It should be noted here that even though TIR conditions break the symmetry of light and help in initial trapping the light in the core, the long distance propagation of light is not optimal when this mechanism of light trapping is used. When near-TIR is used as a light trapping mechanism, both initial light trapping and propagation of trapped light are optimal. 
     In one instance, the shape of the interface is one of a conic shape, a semi-conic shape, a parabolic conic shape, and an ellipsoidal shape. 
     In another instance, surfaces of the inclusion structure are textured. 
     In yet another instance, the second optical medium is air, these teachings not being limited to only that instance. 
     Although in the embodiments shown below the cladding and the core are shown as cylinders, a number of other geometries are within the scope of these teachings. For instance, the cladding and the core can be one of a circle, an N-sided polygon, an ellipse, a semicircle, or a bounded shape of two circular arcs. In one embodiment, N is a natural number ranging from 3 to 100. 
     In still another instance, the inclusion structure has a semi-conic shape, and the core has a semi-cylindrical shape. In one embodiment, the first refractive index of the first optical medium of the cladding layer ranges from about 1.3 to about 2.2. 
     Although in the embodiments shown below, a cone (or an array of cones or semi-cones) is shown to have a circular base. However, it is implicit that the base can be an ellipse or polygon or parabola. 
     Unless otherwise stated, the dimensions of the optical features and thickness of optical layers can be in range of 0.5 microns to 10 meters. The length of the light guide can be 0.5 microns and above and can have portions along the length where the optical features as absent. 
       FIGS. 1( a ) through 1( d )  illustrate a light guide apparatus having a core  108  and a cladding layer  104 , in accordance with various embodiments of the present disclosure. As shown in  FIGS. 1( a ) through 1( d ) , the light guide apparatus  102  (or optical pipe  102  or fiber  102 ) comprises a cladding layer  104  and a core  108 . The general approach is to take light incident on the outer surface of the optical pipe  102  and convert the light to a grazing incidence with respect to the longitudinal axis  110  of the optical pipe  102 . This grazing incident light then enter the core  108  of the fiber  102  which includes optical elements that trap and propagate the light along the length of the fiber  102  inside the core  108 . 
     As shown in  FIGS. 1( a ) through 1( d ) , the cladding layer  104  includes a monolithic optically transparent medium with air inclusion  106  in the shape of a cone. The refractive index of this medium is between 1.3 and 2.4. The half angle of the cone is chosen to be around the critical angle for total internal reflection at the dense medium/air interface. That is, when light is incident at an angle normal to the axis  110  of the fiber  102 , the light gets reflected and become grazing incident with respect to the axis  110  of optical pipe  102 . A higher half-angle of the cone helps to achieve light in a wider angular range, because light at angles less than normal with respect to the axis can also be totally-internally-reflected.  FIGS. 2( a ) through 2( d )  illustrate a light guide apparatus having a core  108 , in accordance with various embodiments of the present disclosure. This core layer optics traps the light inside the core  108  and allows the light to propagate substantially without any loss. 
     Referring again to  FIGS. 1( a ) through 1( d ) , various elements as shown therein are described in further detail as follows. 
     In the embodiment shown in  FIG. 1( a ) , a cylinder represents an optical fiber or light pipe or optical pipe  102  in which light is pumped and propagates. This structure is herein referred to as an optical pipe  102 . It is to be understood that the cross-section of the cylinder A 1  is not necessarily circle. Instead, the cross-section of cylinder A 1  can be, for example, an n-sided polygon (where n can be between 3 and 100), or an ellipse, or a semicircle, or bounded by two arcs of a circle. 
     In the embodiment shown in  FIG. 1( a ) , the optical pipe  102  has a longitudinal axis  110 . 
     In the embodiment shown in  FIG. 1( a ) , a line perpendicular to the central axis  110  of the optical pipe  102  provides another axis. An orthogonal coordinate system is used in this embodiment such that the x-axis is always oriented along the longitudinal axis  110  and y-axis is denoted as radial axis  112 . The radial plane is defined as the cross-section of the optical pipe  102  and is aligned with the y-z plane. (It should be noted that this notation is not a limitation of these teachings.) 
     In the embodiment shown in  FIG. 1( a ) , a cladding layer  104  of the optical pipe  102  is made of a material (optically transparent medium). In one instance, the refractive index of the optically transparent material in the cladding layer ranges from about 1.3 to about 2.2. 
     In the embodiment shown in  FIG. 1( a ) , there is an inclusion  106  in the cladding layer  104  which may have a shape of a conical frustum. In the embodiment shown, the inclusion is an air inclusion. (It should be noted that other optical materials can be used for the inclusion.) It is appreciated that variations on the shape of this inclusion  106  are possible. The purpose of this cladding layer  106  is that, if any light hits the dense-material/air interface between the air inclusion  106  and the optically transparent medium of the cladding layer  104 , the light undergoing total-internal-reflection becomes a grazing angle (in one embodiment, 40-0.1 degrees) with respect to the axis  110  of the optical pipe  102 . 
     In the embodiment shown in  FIG. 1( a ) , there is a core  108  of the optical pipe  102 , which is further described below. This core layer  108  includes one or more optically transparent media arranged in a particular geometry where at least one of the media in the core  108  has an index of refraction higher than that of the cladding material. At least one of the materials in the core  108  may be a lasing medium or composed of a luminescent material with light absorbing properties. 
     In the embodiment shown in  FIGS. 1( a )-1( b ) , a light ray that is incident on a side of the optical pipe  102  is incident on the optical medium-air inclusion interface at an angle higher than the critical angle for total internal reflection. 
     In the embodiment shown in  FIGS. 1( a )-1( b ) , a light ray that is reflected from the optical medium-air inclusion interface is incident on the core at grazing angles (in one instance, ranging from about 35 degrees to about zero degrees). 
     In the embodiment shown in  FIG. 1( b ) , the air inclusion is similar to that of  FIG. 1( a ) . The shape of the embodiment shown in  FIG. 1( b )  is a parabolic conical frustum. That is, the longitudinal cross-section of the cone looks like a section of a parabola. This structure appears to be the most optimal results in terms of guiding the light with wide angular aperture into the core  102 . 
     In the embodiment shown in  FIG. 1( c ) , an air inclusion is shown that has a shape similar to that of  FIG. 1( b ) . The profile of the cone of  FIG. 1( c ) , as seen in the longitudinal cross-section, is a convex parabola. It is appreciated that various other shapes of  FIG. 1( c )  are possible, such as, for example, ellipsoidal. 
     In the embodiment shown in  FIG. 1( d ) , an air inclusion  118  of conical shape with a textures surface  122  includes one or more textured features. The shape of the textured features may be, for example, a semi-circle, quarter circle, ⅛ th  arc of a circle, etc. The textured surface  122  may also be a section of a parabolic or ellipse. Also, the top and bottom surfaces  122  and  124  can be either both concave, both convex, top convex/bottom concave, or top concave/bottom convex. These textured surfaces  122  and  124  may also include straight lines with slopes different from each other and/or different than that of the cone itself. There may be 2-10 of these textured features on the surface of one individual conical air inclusion  118 . The texture of the upper surface can be different from the texture of the lower surface. In one embodiment, the textured features on surfaces  122  and  124 , as a whole, may constitute an air inclusion of a spiral shape. 
     Further, although not shown, it is appreciated that the bottom and top surfaces of the conical air inclusion  106 ,  114 ,  116 , or  118  are not parallel, such that the top surface  122  is a cone of a higher angle, whereas the bottom surface  124  is a cone of a lower angle. The profile of the each of the cone surfaces can be any of the shapes described herein above. 
     In addition, although not shown, it is appreciated that the angle of the bottom surface with respect to the central axis may be 90 degrees. Thus, the air inclusion  106 ,  114 ,  116 , and  118  is bound by a circular base (whose plane is perpendicular to the surface of the central axis) and a conical surface. The conical surface can be of any shape as described herein above. 
     Moreover, it is appreciated that the air inclusions need not be air or vacuum. For example, the air inclusions may be filled with optically transparent medium of refractive index lower than that of the material of the cladding  104 . 
     The ratio of the outer radius of the cladding layer  104  to the outer radius of the core  108  can be in the range of the refractive index of the cladding material. Thus, the ratio should be in the range of about 1.3 to about 2.0. 
     In one or more other embodiments, the core includes a first optically transparent section comprising a first optical medium having a first index of refraction, a second optically transparent section comprising a second optical medium having a second index of refraction, an interface between the first optically transparent section and the second optically transparent section defining a shape, the shape, the first index of refraction and the second index of refraction being configured such that light entering the core is deflected at the interface at an angle such that, when the light impinges on a core-cladding interface, the light impinges on the core cladding interface at an angle at least equal to a critical angle for total internal reflection. 
     In one or more other embodiments, the core includes a first optically transparent section comprising a first optical medium having a first index of refraction, a second optically transparent section comprising a second optical medium having a second index of refraction, an interface between the first optically transparent section and the second optically transparent section defining a shape, the shape, the first index of refraction and the second index of refraction being configured such that light entering the core impinges on the interface between the first optically transparent section and the second optically transparent section at an angle at least equal to an angle for Near-TIR and is deflected such that, it is eventually trapped in the core. 
     Referring to  FIGS. 2( a ) through 2( d ) , various embodiments of the core in the system of these teachings are shown therein are described in further detail as follows. 
     One strategy behind the optical design of the core  108  of these teachings is to modify the angle of the light in the core  108  by creating asymmetric interfaces (with respect the cylindrical surfaces) between two or more heterogeneous materials. Light either gets reflected (through total-internal-reflection, TIR) or refracted at these asymmetric hetero-interfaces, such that when the light hits the cladding-core cylindrical interface again, the incident angle is more than the critical angle for total internal reflection at the cladding-core interface. The various optical elements described below (as shown in  FIGS. 2( a ) through 2( d ) ) illustrate examples of this approach. One feature of this design is that the same optics has to trap the light that enters the core  108  for the first time and then keep interacting with the trapped light multiple times during its propagation along the length of the optical pipe  102  without forcing it out of the core  108 . Another strategy to trap light in the core is to choose the angle of the hetero-interfaces in the core such that the light is incident on that interface at an angle slightly less (0.1 to 7 degrees) than the critical angle for total internal reflection (near-TIR condition). As described previously, there is a large bending of light when light is close to total internal reflection. On its way back, the light beam does not encounter the same conditions of near-TIR passing through the interface and hence the angle does not return to its original angle. Thus the angular symmetry of light is broken in the core leading to light trapping in the core. 
     In the embodiment shown in  FIGS. 2( a ) through 2( d ) , the core  108  that has a cylindrical shape similar to the one described in  FIGS. 1( a )-1( d ) . (It should be noted that this is not a limitation of these teachings. Embodiments of the cladding on the core in which the cladding of core are not cylindrical are also within the scope of these teachings.) The core  108  includes at least one optically transparent medium which has a refractive index (in the range of about 1.4 to about 2.4) that is always higher than that of the cladding medium and forms the boundary with the cladding which can be a cylindrical shape. 
     In one instance, the first optical medium  202  is a high index material which can be in the shape of a cone. The second optical medium  208  is an air inclusion or a material of index different than that of the first optical medium  202 , which can be in the shape of a cone. The refractive index of the second optical medium  208  is preferably less than that of the first optical medium  202 , such that there is a possibility of total internal reflection at the interface between the first optical medium  202  and the second optical medium  208 . The half angle of the interface is in the range of about 0.05 degrees to about 20 degrees. The half angle is chosen to be lower if the light incident on the core  108  from cladding  104  is at a shallow angle. A higher angle of the interface can guide light into the core  108  even when the incidence angle of the light is shallower. Therefore, somewhat higher angle in the range of 5-20 degree is preferable. 
     When the angle of the conical interface is steep, the heterogeneous interface boundary  206  is reduced to a point too quickly. Accordingly, the interface boundary can include repetition of the conical interface  206  between the two heterogeneous materials along the length of the optical pipe so that the light can keep propagating, as shown in  FIG. 2( c ) . 
     Referring to  FIGS. 2( c ) and 2( d ) , a second optical medium  208  interfaces with the first optical medium  202 . While the heterogeneous interface  206  traps light, it still may lead to some of the light leaking out of the core  108  and subsequently the optical pipe  102 . The design shown in  FIGS. 2( c ) and 2( d )  can achieve better performance and a substantially lossless propagation of trapped light. As can be seen from the cross-sectional profile, the heterogeneous interface  206  between the two optical media  202  and  208  of the core  108  has a “nested conical shape” such that one cone has a higher angle than the other. This can achieve almost completely lossless trapping and propagation of light in the core  108  over lengths of optical pipe  102  greater than 1 meter. 
     In one exemplary embodiment, when the angle of the light incident on the core  108  is about 20 degrees, the refractive indices of materials in the core  108  are about 1.6 and about 1.5 (ratio=1.067), and half-angles of the cones, related to angle  1  and angle  2  in  FIGS. 2( c ) and 2( d ) , at the heterogeneous interface is between about 14 degree and about 26 degrees. (It should be noted that this disclosure is not limited only to the exemplary embodiment.) When the incident light angles are increased up to about 30 degree, the half-angles of the conical heterogeneous interfaces  206  also increase proportionally while maintaining their difference in the range of about 5-15 degrees. In order for this design to work best, the refractive index difference between the two materials must be small (in the range of about 0.01 to about 0.2). At higher refractive index difference, the light may start to leak out of the core  108 . 
     In one embodiment, the core has a third optical medium, which can be, in one instance, an air cavity,  210 . In this design, there is a cylindrical (hollow, in one instance) space inside the core  108 . The shape of the heterogeneous interface between two materials is the same as described and shown in  FIGS. 2( c ) and 2( d ) . The ratio of radius of core  108  to the radius of cross-section of the air cavity  210  is in the range of about 1.05 to about 2.0. The optimal value of this ratio is around the value of refractive index of the core medium. 
       FIGS. 2( c ) and 2( d )  show another design variation having a first optical medium  212  and a second optical medium  214 . The high index medium in the core  108  can have the shape of an annular cylinder with the cladding layer outside it. On the inner side of this annular cylinder, there can be two optical media in the shape as shown in  FIGS. 2( c ) and 2( d )  with the option of having the possibility of a third optical medium  222 . The two heterogeneous optical media  212  and  214  have a difference of refractive indices in the range of about 0.02 to about 0.2. The medium with higher index is in contact with the high index annular cylindrical core materials. In that embodiment, the core includes three optically transparent optical media with refractive index of about 1.5 to about 2.2 for outermost layer, refractive index of about 1.4 to about 2.0 for intermediate layer, which has a conical interface with the innermost layer whose refractive index is in the range of about 1.3 to about 1.9. In any circumstance, the outermost core layer has a refractive index greater than that of the cladding layer  104  in its vicinity by 0.1. 
     It is important to point out here that the profile of the hetero-interface in the longitudinal cross-section of core layer in  FIG. 2( a ) through ( c )  can consist of curved lines instead of straight lines. These curved lines could be sections of a parabola, ellipse or circle or a free form curve y=Ax n  where A is a constant and n is a real number between 0.1 and 10. 
     In one or more embodiments, the light guide apparatus of these teachings also includes a super-cladding layer disposed on the cladding layer, wherein the super-cladding layer includes a first optically transparent medium that receives incident light and a second optically transparent medium interfacing with the first optically transparent medium to define a heterogeneous interface, wherein the heterogeneous interface, the first optical medium and the second optical medium are configured to convert a light beam incident on the super cladding at a first range of angles into a light beam exiting the super cladding in a second range of angles, the first range being wider than the second range. In one instance, the heterogeneous interface includes a number of bi-conic shapes. 
     For certain embodiments, the light guide apparatuses as shown in  FIGS. 1( a ) through 1( d ) and 2( a ) through 2( d )  work well without additional layers, to guide light with an angular variation of about ±10 degrees with respect to the normal to the central axis  110 . However, many light sources emit light with an angular spread of about ±30 degrees or more. For example, the sunlight is parallel but the sun&#39;s position has a seasonal variation of ±22.5 degrees. In such a situation, the best alternative is to guide light prior to entry of the cladding layer and the core. In order to pump light with an angular variation (of about ±30 degrees) from the normal to the central axis  110 , an additional cladding layer (or a super-cladding layer) can be used as a light collimator. 
       FIG. 3  illustrates a light guide apparatus having a core, a cladding layer, and a super-cladding layer, in accordance with one embodiment of the present disclosure. 
     A super-cladding layer  302 , which includes a heterogeneous interface  310  between two optically transparent media  304  and  306  in the shape as shown in the  FIG. 3 . 
     In one exemplary embodiment, the first optically transparent medium  304  has refractive index in the range of about 1.3 to about 2.2. 
     In the exemplary embodiment, the second optically transparent medium  306  has refractive index in the range of about 1.35 to about 2.4. The value of refractive index of the first optical medium  304  is matched to the refractive index of cladding layer  104 . For optimal operation, the refractive index difference between the first optical medium  304  and the second optical medium  306  is in the range of about 0.02 to about 0.25. 
     As shown in  FIG. 3 , in the exemplary embodiment shown therein, the shape of the interface  310  is described as a repetitive unit whose primary unit consists of two steep cones attached back-to-back. (it should be noted that these teachings are not limited only to the exemplary embodiment.) The half-angle of these cones are in the range of about 75-89 degrees. As can be seen from the cross-sectional view, the angle  308  subtended by the surfaces of these two cones on each other at the point of contact is in the range of about 2-30 degrees. 
     The effect of the super-cladding layer  302  is to act as a “collimator” and convert light beam in a first predetermined range (in one exemplary embodiment, about ±30 degree) into light of angle of a second predetermined range (in the exemplary embodiment, about 5 degrees or less) around the normal to the axis  110  of the optical pipe  102 . It should be noted that this method is beneficial to most of the rays but not all rays. Therefore the optical efficiency of the supercladding layer is high but not 100%. 
       FIGS. 4( a ) and 4( b )  illustrate ray tracing diagrams for a light guide apparatus having a semi-conical core and a semi-conical cladding layer, in accordance with one embodiment of the present disclosure.  FIG. 5  illustrates a ray tracing diagram for a light guide apparatus having a semi-conical core, a semi-conical cladding layer, and a semi-conical super-cladding layer, in accordance with one embodiment of the present disclosure.  FIGS. 4 and 5  show the ray tracing diagrams to illustrate the path of light pumped into the optical pipe from the sides. 
     In  FIG. 4( a ) , a parallel beam of light rays  408 , which are perpendicular to the longitudinal axis  110 , enter the cladding layer  104  of the optical pipe  102 . The light rays  408  hit the parabolic conical air inclusions  406  at an angle above the critical angle for total internal reflection at the dense medium/air interface resulting in a reflected light rays at grazing incidence with respect to the longitudinal axis  110  on the core  108 . The rays hit the semi-conical heterogeneous interface  414  and an angle greater that the critical angle for total internal reflection between optically dense medium  412  and optically rare medium  416 , where the light rays  408  get deflected such that when the light rays  408  are incident on the core-cladding interface again, the angle of the incident ray is more than the critical angle at that interface and hence contained within the core. The trapped beam is shown as  410 . In this particular ray tracing simulation, air is used for the optically rare medium  416  (refractive index=1). 
       FIG. 4( b )  shows the ray tracing simulation of the same system as shown in  FIG. 4( a ) , but with various parallel beams of light rays. Each of these light beams lies within the same plane. This plane is at an angle of 10 degrees from the radial axis  112  which itself is normal to the longitudinal axis  110  of the optical pipe  102 . As can be seen from  FIG. 4( b ) , a majority of the light rays ( 410 ) are contained within the dense medium  412  of the core  108  via total-internal-reflection at the heterogeneous interface  414  and the core-cladding interface. Some the light rays leak out ( 428 ) of the core since they do not meet the total-internal reflection criteria either at the core cladding interface or the heterogeneous interface  414 . 
       FIG. 5  shows the ray traces for the optical pipe  102  with the use of a super-cladding layer  504  on top of the cladding layer  506  and the core  108 . As shown in  FIG. 5 , a semi-conical core and a semi-cylindrical cladding layer with semi-conical inclusions. The purpose of the super-cladding layer  504  is to convert the input rays incident at an angle much further from the normal (with respect to the longitudinal axis  110 ) to an angle closer the normal (With respect to the longitudinal axis  110 ). This strategy therefore leads to a wider angular aperture of our light guide apparatus. In  FIG. 5 , light rays  508 ,  510  and  512  are incident on the optical pipe  102 . These light rays pass through the super-cladding layer  504  to enter the cladding layer  506  where the light rays undergo total-internal-reflection when they hit the air inclusion of the cladding layer  506  in the shape of parabolic cone. These reflected rays then enter the dense medium  516  of the core  108 . Inside the core  108 , the light rays undergo total internal reflection at the heterogeneous interface  520  between dense medium  516  and rare medium  518 . The semi-conical shape of the heterogeneous interface changes the angle of the reflected ray with respect to the longitudinal axis  110 , thus allowing the rays to meet the total-internal reflection at the core-cladding interface. This results in light rays  514  which have been trapped inside the dense medium  516  or the core  108 . 
     As can be seen from the  FIG. 5 , the light ray  508 , which is incident at an angle −22.5 degrees from the normal, gets converted to an angle closer to the normal when it hits the cladding layer and therefore gets trapped and guided into the core  108 . Similarly, the light ray  512 , which is incident at an angle of +12 degrees with respect to the normal is also converted to an angle closer to the normal after going through the super-cladding layer  302 . 
       FIGS. 6( a ) through 6( c )  illustrate ray tracing diagrams for a light guide apparatus having a core, in accordance with various embodiments of the present disclosure. The embodiment shown in  FIG. 6( a )  is similar to the core embodiment shown in  FIG. 2( c ) . As shown in  FIG. 6 ( a ) , light entering the core in a predetermined range of angles with respect to a normal to the longitudinal axis is reflected in a manner that retains a light propagating inside the core. 
     One such ray is shown in  FIG. 6( a )  as ray  608  that enter the core at a grazing angle of incidence with respect to the longitudinal axis of the core  108 . The core  108  as shown in  FIG. 6( a )  includes two optical media  606  and  604  with the heterogeneous interface  612  in the shape of an nested cones repeated along the longitudinal axis  110 . As can be seen from the side view and top view of the core  108  in  FIG. 6( a ) , the incident light  608  gets trapped inside the core  108 . The trapped and propagating ray  610  undergoes multiple refractions and reflections at the heterogeneous interface  612  and total internal reflection at the core surface. In this particular ray trace, the light ray  608  is incident at an angle of 20 degrees with respect to the longitudinal axis  110 . The refractive index of optical medium  606  is 1.5. The refractive index of optical medium  604  is 1.6. The half angles of the two nested cones are 14 and 26 degrees, respectively. 
     The embodiments shown in  FIGS. 6 ( b ) and 6( c )  are similar to the core embodiment shown in  FIGS. 2( c )  and  2  ( d ). 
       FIG. 6 ( b )  shows the side view and top view of the core  108  as shown in  FIG. 2( d ) . The core  108  includes three optical media  614 ,  616 , and  618 . The heterogeneous interface between  614  and  616  is in the shape of nested cones repeated along the longitudinal axis  110 . The shape of the interface between  614  and  618  is a cylinder around a longitudinal axis  110 . In this particular ray tracing simulation, optical medium  618  is chosen to be air. However, the medium can have a refractive index higher or lower than medium  614  and in the range of 1.0 to 2.2. A light ray  620  is incident on the core  108  at a grazing angle with respect to the longitudinal axis  110 . Three more rays are shown to be symmetrically placed around the circumference of the core  108 , which are at the same grazing angle with respect to longitudinal axis  110 . The light ray  620  after entering the core gets trapped and propagates as light ray  622  after going through a series of refractions and reflections at the heterogeneous interface  619 , reflections at the  614 / 618  boundary, and reflections at the surface of the core  108 . 
       FIG. 6( c )  shows the effect of the cladding in providing incident radiation at angles closer to the normal to the longitudinal axis  110  into the core  108  the construction of which is similar to that shown in  FIG. 6( b ) .  FIG. 6( c )  shows the top view ray trace of light ray  632  entering the core  108  after being reflected by a parabolic conical air inclusion  630 . On entering the core  108 , the light ray  632  undergoes refraction and reflection at heterogeneous interface  619  between the two optical media  614  and  614 , reflection at the cylindrical interface of  614 / 618  and reflections at the surface of the core. All these processes trap and propagate the light ray along the length of the core and is labeled in the figure as  634 . For this particular simulation same refractive indices and design of the core was chosen as was used in  FIG. 6( b ) . 
       FIG. 7  illustrates a perspective view of a solar panel including an array of light guide apparatuses, in accordance with one embodiment of the present disclosure. In one embodiment, the solar panel includes an array of parallel optical pipes, as described above, and an array of solar cells coupled to an end of the parallel optical pipes. In one embodiment, the solar panel includes an optional back reflector. The array of parallel optical pipes shown in  FIG. 7  can be side-pumped to achieve accumulated light to be collected at an end of the optical pipes. A back reflector may be placed below the array of light pipes to ensure that any light that gets lost form the light pipe can be pumped back in again. 
     As shown in  FIG. 7 , an array of solar cells is placed at the end of the optical pipe array. The array of light concentrators in  FIG. 7  is one of the embodiments in  FIGS. 1 ( a ) - 1  ( d ). Although a back reflector is shown in  FIG. 7 , embodiments in which the back reflector is absent are also within the scope of these teachings. An array of solar cells receives the light concentrated by the optical pipe array. These solar cells can be a continuous strip or individual solar cells at the end of each optical pipe/fiber, and/or packaged together in serial or parallel arrangements. These solar cells can be attached to the array of light pipes by means of an optical transparent adhesive materials, such as an epoxy resin or silicone with an index matched or closer to that of light pipe. 
     If light from the sun is used as a source, the embodiment of the optical pipe array shown in  FIG. 7  brings the benefit of pumping light into individual light pipes even if the position of the sun changes in the sky. For example, if this array is placed such that the longitudinal axis of the light pipe in oriented north-south direction, the various positions of sun going from morning to evening are equivalent in terms of the geometry of pumping the light and hence the efficiency of guiding light into the light pipe owing to the symmetry of our optical elements. Also, as described above, the additional cladding layers can bring the functionality that light with wider angles of aperture normal to the axis of the light pipe can also be guided into the light pipe. Thus, light can be pumped into the light pipe even when there is a seasonal variation (about 45 degrees) in the position of the sun. Therefore, the light guide apparatus of the present disclosure allows for the use of this optical pipe array as a non-tracking solar concentrator to collect the light at one end of the array. This concentrated light may be used for various purposes. One example is for power generation by placing an array of solar cells at the edge. It is appreciated that other uses are possible and are described further below. 
     As shown in  FIG. 7 , an array of individual cylindrical or half cylindrical concentrators (also referred to as optical pipes) can be assembled together, in one embodiment, to construct a light guide apparatus that can concentrate light with a wide angular aperture for incoming light. However, in other embodiments, it is also possible to use the elements of the core  108  and cladding  104  in a planar arrays such that the interface between core and cladding is also planar.  FIG. 8( a )  shows such a construction, with the cladding layer  802  in a cuboid shape consisting of an array of semi-conical air inclusions  806 . Each column of the semi-conical inclusions has a common longitudinal axis  803 . The core layer  804  as shown in  FIG. 8( a )  has the shape of a prismatic wedge with the angle of the wedge to be between 0.5 to 20 degrees. Instead of the semi-conical air inclusions, it is also possible to use an interface between two materials in this cladding layer  802  such that a higher refractive index medium is on top and lower refractive index materials on the bottom. The shape of this interface is defined by an array of semi-cones. 
     The top view of the cladding layer  802  in  FIG. 8( b )  array includes a row and column array of semi-conical air inclusions  806 . Each column includes an array of semi-conical inclusions that have a common longitudinal axis  803 . The separation between each longitudinal axis equals the larger diameter of the identical conical inclusions. This leads to gaps  808  in the array. When light enters the cladding layer  802  through the gaps  808 , there is a chance that it might not hit any of the conical inclusions  806  and hence would be transmitted instead of being guided into the core. 
       FIG. 8( c )  shows top view of an array  810  of conical inclusions  812  which resembles the design of fish-scale. In this case, all the properties of individual conical inclusions are same as that described previously. However, the conical inclusions in adjacent columns are staggered and brought closer together such that the separation between longitudinal axes  114  for adjacent columns is less than the larger diameter of individual conical inclusions. This design ensures that all the light that hit the cladding layer is incident on one of the conical inclusions  812  and hence gets guided. 
     All the design variations to the conical inclusions as described previously, including the use of conical heterogeneous interfaces between a dense medium and rare medium, holds true in the case of the planar cladding layer as shown in  FIGS. 8A-C . 
     While  FIG. 8( a )  shows the planar light concentrator using the embodiments of the light guide described in this invention, the use of a core in the shape of a prismatic wedge limits the concentration ratio (ratio of input aperture area to output area). However, as shown in  FIG. 8( d ) , a planar core  818  includes a planar array of semi-conical heterogeneous interface  822  between optical media  818  and  820  can be used. Such a heterogeneous conical interface  822  can be obtained by using an array of longitudinal cross-section of conical heterogeneous interface  214  described in  FIGS. 2( c ) and 2( d ) . This conical heterogeneous interface  822  can also be in the fish-scale design array as described for the conical inclusions in  FIG. 8( c ) . Furthermore, all the design variation to the core mentioned in previous sections, including all the optimal values of various design parameters, hold here as well. 
     In one instance, the light guide apparatus of the present disclosure allows for light trapping and propagation over long distances, substantially without loss, in a planar geometry of the core described in  FIG. 8( d )  and continuous pumping along the length of the structure leads to high concentration of light to be accumulated in the core. The heterogeneous interfaces in this planar core can be a repetition of a “nested semi-cone” motif with the semi-cone angles for the two cones are about 0.5-60 degrees and about 5-85 degrees. It is found that optimal performance of waveguiding can be achieved in this structure, and optical performance is achieved when the difference in angles of the two cones is in the range of about 1 to 20 degrees with the best value obtained around 12 degrees. Also, the refractive index difference between the two materials should be small (in the range of about 0.01 to 0.3) with best results obtained for a difference of about 0.07. 
       FIG. 8( e )  shows a specific design variation such that the planar core  816  has a heterogeneous interface  828  in a prismatic shape instead of the semi-conical shape. However, the longitudinal cross-section in the  FIG. 8( d )  and  FIG. 8( e )  is the same. In one exemplary embodiment of  FIG. 8( e ) , the two optical media  824  and  826  have the refractive index in the range of 1.3 to 2.4. However, in other embodiments, the requirement is that the refractive index difference between the two optical media is in the range of 0.01 to 0.3. The angles from horizontal of the two faces of the prismatic heterogeneous interface  828  are, in one exemplary embodiment, in the range of 0.5 to 70 degrees and 5 to 90 degrees respectively. The requirement, in other embodiments, is that the two angles are close to each other and their difference should be in the range of 1 to 40 degrees. In one instance, the difference in the angles is in the range of 4 to 20 degrees is preferred. 
       FIGS. 8 ( e   1 ) to  8 ( e   4 ) show design variations to the cross-section of the prismatic planar core described in  FIG. 8( e ) . In  FIG. 8 ( e   1 ) shows the design of the cross-section showing the hetero-interface between the two optical media in the shape of trapezoids array rather than a scalene triangle array. The heterogeneous interface is depicted in the figure by repetitions of flat portions  844  and angular surfaces  846 . The angular surfaces  846  have angles in the range of 1 to 70 degrees from horizontal and the angle difference between the two adjacent ones being equal to 1 to 50 degrees. The best values are obtained when this angle difference is in the range of 0 to 30 degrees. 
       FIG. 8 ( e   2 ) shows another design variation to the cross-section of the prismatic planar core described in  FIG. 8( e ) . In this case the heterogeneous interface  848  between two optical media  824  and  826  is defined as consisting on angular faces such that each face is defined in cross-section by lines of different slopes instead of a single slope. The angles of these faces from the horizontal are in the range of 1-70 degrees such that the angle difference between the adjacent faces is in the range of 0-30 degrees. 
       FIG. 8 ( e   3 ) shows another design variation to the cross-section of the prismatic planar core described in  FIG. 8( e ) . In this case the heterogeneous interface  850  between two optical media  824  and  826  is defined to be convex in the direction of propagation of light and either a section of a parabola or ellipse or a circle or another conic section. The interfacial structure can also be a free form curve which is y=Ax n  where n is a real number between 0.1 to 10 etc. 
       FIG. 8 ( e   4 ) shows another design variation to the cross-section of the prismatic planar core described in  FIG. 8( e ) . In this case the heterogeneous interface between two optical media  824  and  826  is depicted by repetitions of flat portions  852  and curved section  854 . The curved section  854  is intended to be convex in the direction of propagation of light and either a section of a parabola or ellipse or a circle or another conic section. The curved section can also be a free form curve which is y=Ax n  where n is a real number between 0.1 to 10 etc. 
       FIG. 8( f )  shows the ray tracing simulation of the planar core  816  with a heterogeneous interface  828  as described in  FIG. 8( e ) . As can be seen, the light beam  830  propagates through the planar core  816  without any loss of light from the core. For this particular ray tracing simulation, it is possible to obtain a substantially lossless propagation over a length of planar core greater than 1 meter. The input light is incident on the core  816  at an angle of 20 degrees from the cladding layer. The angles of the two faces of the prism are 14 and 26 degrees from horizontal. The index of optical medium  824  and  826  is 1.6 and 1.5 respectively. When the angle from horizontal of the incident light on the planar core  816  is increased, the angles of the faces of the prismatic heterogeneous interface  828  also need to be increased proportionally to ensure that the light is trapped and propagates in the core  816  without any loss. 
       FIG. 8( g )  shows a specific design variation of the planar core in which the longitudinal cross-section looks similar to that described in  FIG. 8( d )  and  FIG. 8( e ) . However, the planar core  832  as shown is  FIG. 8( g )  has a circular cross section in the third dimension as seen in the top view. The incident ray  838 , in one exemplary embodiment, enters at an angle of 70 degrees from normal on the top surface and has a radial symmetry with respect to the circular cross-section. The input light aperture is the top surface while the output aperture is the cylindrical shape  842 . It is evident that dramatically high concentrations can be achieved if the diameter of the cylinder  842  is chosen to be very small compared to the diameter of the circular input aperture. 
     It is important to note here that all the design variation to the cross-section of planar core  FIG. 8( e )  as depicted in  FIG. 8 ( e   1 ) to  8 ( e   4 ) are also applicable to the cross-section of the core design in  FIG. 8( g ) . 
     Further, it is observed that this core design works even if the structure is prismatic as shown in the  FIG. 8( f ) . The criteria for light trapping and propagation in this exemplary structure is that the angle of light incident on the core be lower than about 40 degrees (optimal results for 20 degrees), that the angles of the prisms should be in the range of about 2-30 degrees and about 7-40 degrees, and that the refractive index difference between two materials be in the range of about 0.02-0.3 (0.07 difference optimal). 
     For the case of planar core, the concentration ratio can be in the range of 100 to 1000× and can go even higher if the angular spread of the incoming light is not wide. When the angle aperture of incoming light (seasonal variation) is greater, a slight global taper in the range of 0.05 degree to 2 degrees can be used to trap and propagate light into the core. In order to address the angular spread of incoming light, one can utilize the concept of chirping where the two angles are varied systematically to achieve optimal waveguiding. This means that angular difference of the two nested semi-cones in the planar core is varied periodically, in small magnitude. In one instance, for an optimal case, the difference between the angles is increased by 0.5 degrees in each successive pair of nested-semi-cones until the difference is 14 degrees. The then the difference in angles is reduced back to 12 degrees in successive decrements of 0.5 degrees. The same concept of chirping also holds for planar core with prismatic faces as described in  FIG. 8( e ) ,  8 ( e   1 ) through  8 ( e   4 ), and  8 ( f ). 
       FIGS. 9( a ) through 9( e ) , including  FIGS. 9 ( a   1 ) through  9 ( a   3 ) illustrate a planar light guide apparatuses, in accordance with various embodiments of the present disclosure. As shown in  FIG. 9 , optical elements of cladding, core and supercladding are embedded in a planar material to make a solar panel. It is evident that a planar device in which light can be pumped in at a wide angular aperture and guided to the edge is easy to manufacture and can find multiple uses. In one aspect, the present disclosure provides a fabrication method of a planar light guide device that uses side pumping of light pipes. 
       FIG. 9( a )  shows a scheme of operation of such a planar light guide device ( 901 ) that guides incident light incident at multiple angles of the sun (180 degrees mornings to evening variation, and 45 degree seasonal variation). As such, the solar energy concentrator of the present disclosure does not need to track the sun. 
     This embodiment shown in  FIGS. 9 ( a ) - 9  ( c ) includes a cladding layer having an array of semi-conical inclusions. This layer is used to turn the incoming light into a grazing incidence. The top surface of this layer can be semi-cylindrical or flat. The material in this exemplary layer can have a refractive index in the range of about 1.3 to 2.0, but is always lower than at least one of the materials in the core. 
     All the variations in the shape of the cone described previously shown can also be used here as well. Instead of using an air inclusion, one can also use a conical hetero-interface between two optical materials with the material at the bottom having a lower refractive index. The index difference and the angle of the cone may be chosen in such a way that there is total internal reflection at this interface and hence an angle of light exiting this layer is a grazing angle. 
     In other embodiments, a whispering gallery mode circular ring resonators can be used for the core to replace the wedges at the bottom, as shown in  FIG. 9( c ) . This allows for a grazing incidence light at wider range of angles to be guided into the core. The diameter of circular hollow spaces are much greater than that of the wavelength of light, and typically are in the range of about 1-500 microns. 
       FIG. 9( a )  illustrates a planar light guide  901  using various embodiments of the light guide apparatus as described in this disclosure. This concentrator has a wide angular aperture which is represented by the angles  912  and angle  914  and can be used as a non-tracking solar concentrator for concentration in the range of 2× to 16× and a single-axis tracking in the range of 17×-200×. The angle  912  can be in the range 0 to 180 degrees and its maximum value of 180 degrees would represent the variation in the angle of the sun from morning to evening. The angle  914  can be in the range of +/−45 degrees from the normal. Its typical value of +/−22.5 degrees would represent the seasonal angular variation of the sun during the year. Any ray of light within this solid angle bounds represented by  912  and  914  will be guided into the planar light concentrator and gets propagated to its edge.  910  represent rays of light incident within this angular aperture.  911  represents the light that gets guided and propagates inside the planar light concentrator  901 . The planar light concentrator consists of a multi-layer stack: a super-cladding layer  908 , a cladding layer  904 , a core layer  906  and a light guide layer  902  which can be an extension of the core layer  906 . An array of packaged solar cells  916  connected to each other in series and parallel configuration are attached to the edge of the planar light guide  901  and produce electrical power using the concentrated light as the input. This assembly including  901  and  916  can be used as a solar panel which uses only small area of active solar cell and does not need to track the sun. 
     The cladding layer  904  consists of various embodiments as described previously in this invention and is similar in construction to the planar cladding layer described in  FIGS. 8( a ), 8( b ) and 8( c ) . The planar core layer  906  has similar construction to that described in  FIGS. 8( d ), 8( e ) ,  8 ( e   1 ),  8 ( e   2 ),  8 ( e   3 ),  8 ( e   4 ) and  8 ( f ). The light guide layer  902  can also be an extension of the one medium of the core as stated earlier. This layer is an optically transparent layer of glass or plastic with refractive index matching that of the dense medium of core and is higher in refractive index than the cladding layer  904 . The super-cladding layer  908  is a collimating layer that takes light rays with a angular variation represented by angle  912  and converts them the a beam if light around the normal with an angular variation of +/−2.5 degrees. The super-cladding layer  908  includes optical elements described in  FIG. 3  and the planar array of such optical elements has been described in detail in  FIG. 9( d ) , and  FIG. 9( e ) . 
       FIG. 9 ( a   1 ) shows a variation in the construction of the planar light concentrator  901  which consists of a multi-layer stack: a super-cladding layer  908 , a cladding layer  904 , a core layer  906  and a light guide layer  902  which is an extension of the core layer  906  which can also a substrate/mechanical support, a reflector layer/film/mirror  913  that is attached or laminated to the side edges, a transparent protective layer  909  and a protective layer  907  underneath the light guide layer  902 . The layer  909  can be glass or polymer and may have an ultraviolet absorber as a part of it. The layer  907  is a glass or polymer has refractive index less than the light guide layer and typically has a refractive index in the range 1.2 to 1.45. An array of packaged solar cells  916  arranged in series and parallel configuration are attached to the edge of the planar light concentrator  901  and produce electrical power using the concentrated light as the input. This assembly including  901  and  916  can be used as a solar panel which uses only small area of active solar cell and does not need to track the sun during that day or with seasons. 
       FIG. 9 ( a   2 ) shows an construction of the planar light concentrator compared to the construction in  FIG. 9 ( a   1 ) such that the packaged solar cells are not attached on the edge. Here the array of packed solar cells  917  are sandwiched between the optical layers, either  902 / 906  or  906 / 904  or  902 / 907  or affixed underneath the whole assembly. 
       FIG. 9 ( a   3 ) shows an alternate construction of the planar light concentrator such that the bottom surface close to the light collection edge is tapered as illustrated. The purpose of this approach is to provide an alternate way to couple light from the planar optical concentrator onto a solar cell whose width is larger than the edge of the waveguide. In the figure only layer  902  is tapered but the whole optics assembly can be tapered at the edge. The angle of taper can be in the range of 10-80 degrees. The array of packaged solar cells  915  is attached to this tapered edge. The concentration ratio is the ratio of the area of top surface (input light) and the area of taper (light output). By changing the angle of the tapered edge, this concentration ratio can be controlled. 
       FIG. 9( b )  is a longitudinal cross-section of the planar light concentrator  901  shown in  FIG. 9( a ) . The cladding layer  904  has been shown here as a conical heterogeneous interface between a dense optical medium  918  and rare optical medium  920 . The incoming light from the super-cladding layer  908 , which, in one exemplary embodiment, is +/−2.5 degrees from the normal, hits this heterogeneous interface and is converted to grazing incidence with respect to the horizontal before entering the light guide layer  902  and the planar core layer  906 . The planar core layer  906  traps and propagates the light in the light guide layer  902 . The light guide layer  902  is made of glass or plastic, provided mechanical support to the concentrator, has refractive index higher than cladding layer  904  and a refractive index closer to the dense medium of the planar core  906 . 
       FIG. 9 ( b   1 ) shows the cross-section of an exemplary design of the planar optical concentrator which we have optimized for &gt;80% optical efficiency of light collection over 0.5 m×1 m area of input surface and 6 mm×1 m for output surface. Typical values of refractive index n1, n2 and n3 are 1.5, 1.4 and 1.6 respectively. The design consists of a protective layer  909 , a super-cladding layer  908 , two layers constituting the cladding layer  904 . The angles of the faces in top layer of  904  are 57° and 90° from horizontal whereas in the bottom layer of  904  the angles are 30° and 46° from horizontal. The core layer  906  is defined by interfaces with angles of individual faces being 17.5° and 29.5° from horizontal. The light guide layer  902 , which acts as a substrate as well, is chosen to have a refractive index of 1.5. But it can be in the range 1.47 to 1.6. It is clarified here that many other combinations of refractive indices and angles of the faces for each scalene prism array interface can be used to achieve effective waveguide and this description is one such optimal combination. In the 3 rd  dimension, orthogonal to this cross-section the design of the interfaces can be straight line (prismatic) or curved (leading to a complete circle or an arc of a circle) or semi-conical interfacial structure as described previously in this disclosure. 
       FIG. 9 ( b   2 ) shows one exemplary design of a planar light concentrators along with ray tracing simulations. This design is similar to the design shown in  FIG. 9 ( b   1 ) except that this design is created for light sources with a divergent beam of +/−0.5 degree and also uses near normal incidence between interfaces to achieve maximum light bending in the cladding layers. As can be seen in the Figure, we use 4 sets of cladding layers to bend the divergent light gradually to a grazing incidence. As the beam bends to lower angles its divergence increases from +/−0.5 degree at source to a divergence of +/−5 degree right before entering the core. The detailed ray traces at each interface in this design are shown in the  FIG. 9 ( b   3 ) and the results of the large scale ray tracing simulation is shown in the  FIG. 9 ( b   4 ). As shown in the  FIG. 9 ( b   3 ), we use a TIR as a mechanism for light bending in the CL1 and CI2 layers. For the CI3 and CI4 layers, near-TIR is used as a mechanism to achieve large light bending since the use of TIR demands that the refractive indices difference between the two optical media is 0.005 which is would bring practical difficulties. The divergent beam enters the core at grazing incidence and undergoes near-TIR to achieve light trapping in the core as has been described in the previous sections in this disclosure. For this design, the optimal values of n1, n2 and n3 as shown in the  FIG. 9 ( b   3 ) are 1.54, 1.41 and 1.57 respectively. As can be seen in  FIG. 9 ( b   4 ), a large scale ray tracing simulation shows a light collection efficiency of 81% when multiple light sources with a beam divergence of +/−0.5 deg illuminate the entire 300 mm length of the light guide apparatus. The width of the core is 5 mm and most of the light is collected in the core leading to a concentration ratio of 60×. 
       FIG. 9 ( b   5 ) shows a ray tracing simulation of an exemplary design of a planar light concentrator consisting of a single scalene prism array interface between two optical media (n1=1.54, n2=1.41) with near normal incidence input beam. This design can be seen as an extension of  FIG. 8( e )  in which the light is grazing incidence. This design in  FIG. 9 ( b   5 ) is much simpler than the multilayer stack but has the problem that it needs a very collimated beam (beam divergence=0.1 deg) to obtain high optical efficiencies of light collection over 0.5-1 m distance. 
       FIG. 9(C)  illustrates the use of a micro ring resonator layer  926  sandwiched between cladding layer  904  on top and light guide layer  902  and planar core layer  906  on the bottom. The micro ring resonator layer is to address the inefficiency of the super-cladding and cladding layer to turn incoming light ray beyond the angular range represented by angle  914  into a grazing incidence with respect to the horizontal. The ring resonator consists of the annular tube of an optical medium with high refractive index embedded in an optical medium with lower index medium (preferably air). In this configuration of the planar light concentrator  901 , the angles of the faces of the heterogeneous interface planar core array  906  are reversed from positive angles to negative. Thus the path of trapped light in this case is inverted in the −x direction. 
       FIG. 9( d )  illustrates the design of the planar super-cladding layer  908  as labeled in previous  FIG. 9( a )-9( c ) . The purpose of the super-cladding layer  908  is to act as a passive collimating layer that converts any light within the angular variation described by angle  914  into a narrow range of angles (preferably +/−2.5 degrees) around the vertical. The optical element shown here is an extension of that shown in  FIG. 3 . In  FIG. 3 , the heterogeneous interface in the super-cladding layer was shown to have a repetitive bi-conical shape around the cladding layer and its longitudinal cross-section looks like an array of steep isosceles triangles. When we move to a planar light concentrator  901 , the symmetry around the longitudinal axis is no longer critical and hence this layer can have a prismatic shape. As shown in  FIG. 9( d ) , the performance of the super-cladding layer can be enhanced by adding a parabolic hetero-interface right below the triangles resulting in the shape of a “oil drop”. The optical medium  934  on top of the “oil drop” motif  936  is a optical medium if refractive index lower than optical medium  936  and is similar in refractive index to optical medium  938 . In one embodiment, a criteria for the above design of super-cladding layer is the difference in refractive index between dense optical medium  936  and each of the rare optical media  934  and  938  is in the range of 0.02 to 0.2. In one instance, the preferred value is around 0.02 to 0.07. The isosceles triangle needs to be steep such that, in one exemplary embodiment, the vertex angle is in the range 1 to 25 degrees with the optimal value around 5 to 7 degrees. The parabola right below the triangle should be a steep parabola with the design strategy that its focus should be in close proximity to converging point of the light beam being refracted by the triangular hetero-interface. 
     It is also possible to have the angular faces  940 , defining the isosceles triangle in the cross-section described above, to be a segment of a curve rather than a straight line. This segment of a curve can be a section of a parabola or ellipse or a segment of a curve defined by y=Ax n  where A is a constant and n is a real number between 0.1 and 10. 
       FIG. 9( e )  shows a cross section of the super-cladding layer used to collimate incident light,  950  from incoming angles of ±22.5 degrees to a near-normal (i.e. normal to the surface of the panel,  952 ) orientation. The output of the cladding is near-normal light,  962  with a small angular spread (±2.5 degrees or less) for use by the planar cladding layer and planar core optics as described in  FIGS. 9( a ) and 9( b ) . 
     The super-cladding includes four main elements; three layers of truncated triangular prism arrays  954 ,  954 ,  958 , followed by a parabolic lens microarray  960 , all of which are embedded inside a material of low refractive index,  964 . 
     Each triangular prism layer is a periodic array, with a repeating unit comprising one small and one large triangle, both truncated at the top to expose a horizontal surface that allows normal (or near-normal) light to pass through undisturbed. This makes the unit a trapezoid. However, the unit is still referred herein as a triangle or truncated triangle for simplicity. This configuration of alternating small and large triangles is used to ensure that one repeating unit is not in the ‘shadow’ of another. 
     Incoming light strikes the angular faces of the prism and undergoes refraction, moving towards the normal. The shape of the triangles in the individual prism layer as well as the relative alignment of the layers are so chosen to only affect the path of light rays with a large angular deviation from the normal, while letting near-normal light pass undisturbed. 
     There are three key requirements for the performance of the super-cladding layer:
         (a) There is an offset between the repeating elements of one triangular prism layer and the next. In the absence of this offset, incoming light rays are not collimated monotonically towards the normal orientation, but instead alternate between moving towards and away from a normal orientation.   (b) Each triangular prism layer is scaled up to be twice as large as the layer above it. In the absence of this up-scaling, initially-normal light rays are deflected away from the normal.   (c) A small difference in the refractive indices between the materials making up the triangular prisms and the surrounding medium. In one particular instance, there refractive indices are 1.552, 1.563, 1.571, and 1.583 for the materials making up 954, 954, 958 and 964 respectively. While different materials can be used to make the different triangular prism layers, it is not a requirement and all the layers could be made out of the same two material set. The only key factor is their refractive index difference is small (in the range of 0.01 to 0.1).       

     The three triangular layers are followed by a parabolic concave lens microarray that further “collimates” the light to a more normal orientation. The lens faces  966  and  968  are nominally parabolic in this construction, although other surface shapes are also equally valid. This parabolic layer is used to collimate the light beam even further and is similar to the parabolic heterointerface described in  FIG. 9( d ) . 
       FIG. 9( f )  shows data for one embodiment of the structure in  FIG. 9( e ) .  FIG. 9( f )  shows angular range of the output rays for one embodiment of the structure in  FIG. 9( e )  when the input angular range is +/−22.5 degrees form normal. As can be seen from the  FIG. 9( f ) , the angular divergence of the output beam is +1-22.5 degrees. In this particular simulation, we found that the 97.5% of the input light in the angular range of +/−22.5 degrees is collimated into a output beam of angular range+/−2.5 degrees. 
     The materials used to fabricate the light guide apparatus as described in this invention can be glass, carbon-backbone based polymers, plastics, small organic molecules, polymers based on silicon-atom-backbone such as silicones and siloxanes. One may also choose to use mixtures of siloxanes with other polymers and small molecules to achieve specific mechanical properties as well as optical properties such as refractive index and dispersion. As described herein below, these materials may be used in the liquid form and cross-linking agents added in small amounts to achieve a solid, semi-solid, elastomeric or gel-like layer after application of heat or UV light. Also, fluorinated polymers and fluorinated silicones and siloxanes may be specifically used to achieve low refractive index than the base non-fluorinated versions of the same polymers. Similarly, a sulfurized version of the polymers may be used to achieve a higher refractive index than the non-sulfurized versions of the same polymers. In some uses of the present disclosure, some liquids with different refractive indices such as water, mineral oil, organic solvents, fluorinated liquids etc may be also used. Also, for various embodiments described in this disclosure, we may use glass of various refractive index (1.4 to 2.2). In some instances we may use glass that may have low melting temperature to enable molding of glass. 
     Refraction at a polymer (or glass)/air interface can lead to splitting of the white light due to variation in the refractive index of the dense medium at various wavelengths (dispersion relation) while the index of air remains constant. However, if refraction happens at the interface of two optically dense materials with different refractive indices (n1 and n2) such that the ratio of n1/n2 is constant for each wavelength, then light of each wavelength is refracted by the same magnitude and hence no chromatic aberration takes place. In optical simulations of the present disclosure, the choice of materials such that the material of higher index has a higher dispersion (low Abbe number) while a material of lower index has a lower dispersion (higher Abbe number) is made. This strategy is also leads to a much wider choice of available materials since many of the higher index materials have low Abbe numbers.  FIG. 10( a )  shows the prescriptions for Materials Selection in the light guide apparatus of the present disclosure. 
     Materials with engineered refractive index can also be used by selective doping the optical media with small quantities (0.01% to 0.00001%) of dyes and chromophores that absorb light in specific parts of the spectrum. This strategy creates a tailored refractive index vs. wavelength distribution for one or both materials which can lead to wave-guiding for only certain parts of the spectrum. Materials whose refractive index vs. wavelength distribution has been engineered by mixing two materials with different refractive index vs. wavelength distributions can also be chosen.  FIG. 10( b )  shows an example of wave-guiding using above described strategy of material selection. Here we have chose to set a “cutoff” by choosing a materials combination for the cladding layer such that the n 1 /n 2  decreases slowly with wavelength going from 400 nm to 800 nm wavelength. At 800 nm, the n 1 /n 2  ratio falls below the threshold that is required for the incident light to undergo total-internal-reflection at the heterogeneous interface. Therefore the 800 nm light (and beyond) gets transmitted through the cladding layer with slight deviation in its path and hence does not contribute to the wave-guiding process.  FIG. 10C  shows the data for wave-guiding and transmission of light as a function of wavelength of light for predetermined material combination in the cladding layer. As can be seen from the figure, 100% of the light beyond 800 nm gets transmitted (no wave-guiding at all) while there is clear wave-guiding over 1 m length and ˜60% collection of light at the edge for light in 400-800 nm range. 
     A similar strategy can be used in the core layer such that the n 1 /n 2  ratio is different for the materials in the core layer and this would result in leakage of selected wavelengths of light from the core and wave-guiding of rest of the wavelengths of light in the spectrum. 
     The light guide apparatus as described in this invention can be made by widely used injection molding process or mold casting or stamping or imprinting process. A mold which is negative of the design of the part that needs to be made. A cross-linkable liquid polymer or gel can be poured into the mold them peeled off once it solidifies. The mold can also be in the shape of a cylindrical drum, the rotating action of which can be used to prepare the parts in a continuous mold-and-peel, high speed roll-to-roll process. The process of fabrication can also involve injection molding of polymers on glass substrate or another polymer substrate. It may also involve use liquid silicone rubber (LSR) molding on glass substrates. A combination of the above mentioned methods can be used to obtain a multi-layer stack of optical materials on glass. 
     In one embodiment, the present disclosure provides a method for fabricating the cladding layer or core in the light guide apparatus described above. As shown in  FIG. 11( a ) , a first optically transparent material has a surface having a male portion  1102  (a plurality of protrusions) formed thereon. The first optical transparent material can be engaged with a second optically transparent material. The second optically transparent material has a surface having a female portion  1104  (a plurality of recesses or indentations) formed thereon. In one embodiment, the male and female portions are configured to have similar shapes, and the female portion is configured to be bigger than the male portion to leave a space for air inclusions  1106  in between after assembling them together. A location of each protrusion corresponds to a location of each indentation, thereby forming a number of inclusions. This fabrication method can be used to produce any light guide assembly that contain air inclusions, such as the core  108  and the cladding layer  104  as shown in  FIG. 1( a ) . 
     In an exemplary embodiment, for the cladding layer, the male portion would have semi-conical protrusions on its surface and female portion will have semi-conical recesses on the surface. The recesses, in that exemplary embodiment, are slightly bigger in size (5-50 microns) in each dimension. When both the parts are assembled together to make one piece, there are air inclusions trapped inside the finished piece whose dimensions are equal to the difference in the dimensions of protrusions and recesses. 
     In one embodiment, the method further includes depositing a layer of a third optically transparent material over the surface of the first optically transparent material. The layer of the third optically transparent material may have a substantially constant or varying thickness, which is configured such that a shape of the surface of the first optically transparent material, after deposition, is substantially congruent with a shape of the surface of the second optically transparent material. After assembling, the third optically transparent material is disposed in the space between protrusions and indentations. 
     In one embodiment, the male portion  1102  has semi-conical protrusions on its surface, and the female portion  1104  has semi-conical recesses on its surface. The recesses are slightly bigger in size (5-50 microns) in each dimension. When the first and second optically transparent materials are assembled together to make one piece, there are air inclusions trapped inside the finished piece whose dimensions are equal to the difference in the dimensions of protrusions and recesses. 
     To have a complete cone assembly, two such parts with semi-cones can be assembled together to form part with full cone inclusions. In order to have air inclusions in finished part with special geometry, such as spiral air inclusions, the conical protrusions and recesses can have surface texture. The protrusions have concave surface texture while the recesses have convex surface texture. If the cross-sections of the texture is desired to be a full circle, each of the concave and convex surface texture may be a semicircle. Further, it is also possible to have convex and concave surface textures which are arcs of a circle such a 90 degree arc, 45 degree arc, and so on. 
     In another instance, where the inclusions are not air inclusions but are inclusions of a third optically transparent material different from the first and second optically transparent materials, a layer of the third optically transparent material of predetermined thickness is deposited on the male portion. The predetermined thickness is equal to about the difference in size between the protrusions in male portion and the recesses in the female portion. When the two parts are assembled together, the inclusions of the third optically transparent material are disposed between the male and female portions. 
     The first and second optically transparent materials and the size of the inclusions are selected such that a predetermined deflection of light, incident on the assembly within the range of angles with respect to a normal to a longitudinal axis, is obtained. 
     In yet another embodiments, the embodiment includes two different materials with a specific shape of the boundary of the two materials (heterogeneous interface). In fabricating such an embodiment, the first optical layer is prepared using molding method described above using a mold that has the shape of the heterogeneous interface to be prepared. The molded layer is then filled with the second optical material to create a final layer consisting of two different optical materials with a heterogeneous interface in a predetermined shape. 
     Although the light guide apparatus of the present disclosure has been described as a solar light concentrator, it is appreciated that other uses of the present disclosure are possible. 
     The planar light collector in various versions of  FIG. 9( a )-9( e )  uses various layers of optically transparent materials with the heterogeneous interface between to materials in a pre-determined geometry. While it is assumed that the layers in this light guide apparatus be solid to maintain mechanical integrity, one or more of the optically transparent materials may be liquid. These liquid layers may be sandwiched between solid layers, physically encapsulated to prevent their leakage and perform same optical functions as the solid materials of the same refractive index. 
     Also, in some manifestations of this invention, there can be air inclusions. Injection and withdrawal of optical fluids from the air inclusions in the cladding and the core cavity can result in loss of optical wave-guiding since the total internal reflection needs an interface between dense and rare medium. This concept can be used in our design to make smart windows which produce power when waveguiding is on (no liquid in cavity) and act as transparent windows when waveguiding is off (cavities filled with liquid). 
     In a manifestation of this invention where a optical liquid layer is used as a lower refractive index, the TIR happens at the interface of the dense solid medium and the rare liquid medium. If this liquid is pumped out of the device and replaced by another liquid whose refractive index is equal to the dense medium, the TIR at the interface of two materials is prevented and light is transmitted instead of being guided. Therefore a power producing smart window with an active control of light transmission (and guiding) of light using can be demonstrated using the influx and out-flux of optical fluids in our light guide apparatus. 
       FIG. 12( a )  illustrates the concept of the smart window using the light guide apparatus as described in this invention. An alternate version of this application could be the use of a laminate on a window which consists of one or more optical layers as described in this invention. The design of the optical layers can be optimized that light travels to the edge of the window where an array of solar cells can be used to produce power. Alternately, the design of optical layers can be such that the light from the sun (at a grazing incidence) enters the light guide, travels a small distance and then exits the light guide whereas the ambient light is transmitted through the optical layers laminated on window glass. This would result in an effect of reduction in glare from the sunlight while the visual quality of the window is preserved. 
     Using the principle of reversibility of light, the light guide apparatus of the present disclosure can be used as a lighting device. A source of light is placed on one of the edges of the light guide apparatus such that the light emitted by the light source gets guided into the core and progressively leaks out of the light guide apparatus. In this case, the output of the light guide apparatus is its top surface that provides uniform illumination with a wide angular divergence of the output light rays.  FIG. 12( b )  shows an array of light emitting diodes acting as light sources place on the edge of the light guide apparatus. 
     The planar solar concentrators as described in  FIG. 9( a )  can be used for indoor lighting applications by replacing the solar cells on the edge of the light guide apparatus by an array of optical fibers. The concentrated light that is collected at the edge of the light guide apparatus can be pumped into individual optical fibers coupled to the edge. These optical fibers can then take light to the lighting fixtures inside a building to illuminate the interiors with natural day lighting. 
     Solar Thermal: The planar optical concentrators can be used to guide concentrated light onto and evacuated tube solar thermal water heater. The concentrated light makes the tube much hotter than in the regular water heaters. This will make the water heater more efficient because of the higher thermodynamic efficiency at a higher temperature of the tube. This scheme can also be used for solar thermal application where a thermal fluid flows inside a tube which runs through the edge of a cascade of planar concentrator. As an example, our calculation show that a cascade of 10 panels of 1 m×1 m area in this scheme can heat the thermal fluid at a temperature to 400 degrees if the mass flow rate of the thermal fluid is kept as a 0.06 kg/s through the tube. 
       FIG. 12( e )  shows the schematic of the light trapping optics that we have demonstrated in our light guide apparatus to be used on top of a photovoltaic device to trap the light that leaves the photovoltaic device without getting absorbed. Such light trapping optics relies on the total internal reflection at the heterogeneous interface of the two optical materials. The optical laminate layer redirects the light back onto the photovoltaic device by converting the angle of the high angle photons to an angle that lies within the total internal reflection cone of the dense material/air interface. The optical laminate can also have a design of the planar core layer as depicted in  FIGS. 8( e )  and  8 ( e   1 ) through  8 ( e   4 ). 
     One of the problems of luminescent concentrators is that the luminescent material emits light at all angles but only the emitted light that lie within the TIR cones is contained within the slab. The light emitted beyond the TIR cone is lost resulting in lower performance of the luminescent concentrators. These TIR losses will be overcome by laminating a multilayered stack of engineered optical layers which modify the angle of the light emitted beyond the TIR cone to shallower angles using the principles used in the design of the core in the light guide apparatus described in this invention. The design of optical layers as shown in  FIG. 12F , consists of two optical polymers with a refractive index difference &lt;0.1 and an interfacial structure consisting of an array scalene triangular prism with an included angle in the range of 10°-15°. The angles of the scalene triangles are chosen to achieve the TIR at the interface of the two polymers and trap the light emitted beyond the regular TIR cone. The small included angle and low refractive index difference is effective in propagating the trapped light with minimal loss. 
     The embodiments disclosed herein have not relied on Birefringence of any optical media, or modifying the refractive index of any of the layers or sections or on the anisotropy of the refractive index of any layer or section. Embodiments without Birefringence of any optical media, or modification the refractive index of any of the layers or sections or the anisotropy of the refractive index of any layer or section are within the scope of these teachings. 
       FIG. 12( g )  illustrates the use of the concentrated light output of light guide apparatus as described in this invention as an input for the optical pumping of the lasing medium in a laser device. The input to the light guide can be sun light or an array of diode lasers incident on the top surface of the planar optical concentrator. Various schemes have been proposed in the art to achieve optical pumping of the lasing medium beyond its lasing threshold. A high power source in a compact geometry is required to optically pump the laser. The concentrated light output at the edge of the light guide apparatus can be an effective method to achieve this goal as shown in  FIG. 12( g ) . 
       FIG. 12 ( b   1 ) shows the a manifestation of the light guide as described in previous sections and specifically in  FIG. 12 b    as a collimating device when the light is input from the edge of the light guide. A light source  1201   b   1 , an LED in one embodiment, emits input light  1203   b   1  into the light guide. In this particular figure, the light guide consists of scalene prism array interface  1205   b   1  such that both the faces of the prism are an angle in the range of 0 to 89 deg from the horizontal. The two optical media  12009   b   1 ,  12011   b   1  have refractive indices in the range of 1.3 to 2.4. Medium  12091   b   1  has a higher refractive index compared to medium  1201   b   1 . An output substantially collimated beam  1207   b   1  emerges from a surface of the light guide. Substantially collimated as used here in refers to collimated within +/−10°. 
       FIG. 12 ( b   2 ) shows the example of ray trace in the light guide described in  FIG. 12 ( b   1 ) . . . . A light source  1201   b   2 , an LED in one embodiment, emits input light into the light guide. The light rays propagate in light guide until the light meets the criteria to total internal reflection at the interface between the two optical media (i.e. medium  1209   b   2  and medium  1207   b   2  and exits the light guide at a particular angle. Rest of the light rays keep moving in the light guide until they also meet this TIR criterion. By choosing the angles of different faces of the scalene prism array interface,  1203   b   2  we can control the specific angle at which the outcoupled collimated light  1205   b   2  leaks the front face of the light guide. If we choose the outcoupling face to be shallow, the angle if emitted light from front face would also be shallow. For an emission of collimated light at an normal angle form the front face, the outcoupling face of the interfaces should be in the range of 60 deg (+/−5 deg) 
     The light which is emitted by the light source at a wider angle with respect to longitudinal axis, meets the TIR criteria at a shorter distance from light source compared to the light that is emitted from light source at a narrower angle with respect to longitudinal axis. Thus, light emitted at different angles from source is spread out at different distance from the light guide. However, this relationship between angle and distance of emission is not linear and hence for a lambertian light source, the emission intensity from front surface as a function of distance from the edge (and hence light source) is non-uniform. 
     Another aspect of this process is that when light propagates inside the light guide, it undergoes refraction at the interface between the two optical media and sometimes also encounters near-TIR criteria. During these interactions, Fresnel reflections also occur. The Fresnel reflections are nominal (around 0.1%) when the light is incident close to normal but it tends to become higher intensity when the angle of incidence at these interfaces is higher. These Fresnel reflections can cumulatively be a source of substantial optical loss if the angle of light after Fresnel reflection is different from the angle of outcoupling or when angle of Fresnel reflection is not shallow enough to keep propagating forward in the light guide. The scalene prism array design of interfaces as shown in  FIGS. 12 b     1  and  12   b   2  ensures that the fresnel reflections are either in the angle range of outcoupled light or be shallow enough to keep propagating forward in the light guide. This maintains collimation of outcoupled light in the range of +/−10 degree from the mean ray. 
     While we have described here in this invention that out-coupling of light is through TIR, this is not the only mechanism possible in this given configuration of interfaces. Refraction and Near-TIR, as described previously, coupled with Fresnel Reflections also be chosen as a mechanism for light outcoupling from the light guide. This near-TIR can occur at one of the internal interfaces in the light guide or at the planar interfaces. The interfaces can also be designed such that there is near-TIR at the scalene prism interface, the refracted light stays within the light guide but the Fresnel reflections are out-coupled from the light guide as collimated emission. 
     A drawback of just having the scalene prism array interface in the light guide is that there is non-uniform emission of out-coupled light along the length of the light guide. Also, some of the light from source whose angle is very close to longitudinal axis never gets non-optimal enough to be opt-coupled from the light guide.  FIG. 12 b     2 - a  shows one approach in which asymmetric and symmetric V groove interfaces may be created to make the light progressively non-optimal in the light guide. As shown in this alternative design, the edge-lit light guide consist of a multilayer stack of polymers. The design is characterized by a optical medium bounded by planar interface which may be called a core. One of the interfaces is a scalene prism array interface for out-coupling of light and another interface for making light non-optimal in the light guide. The latter interface can be a symmetric V-groove array interface, an asymmetric V-groove array interface or a scalene prism array interface. 
     While the scalene prism array interface shows high performance of light collimation, one drawback is that it is somewhat difficult to fabricate with high-throughput manufacturing methods.  FIG. 12 ( b   3 ) shows an alternate configuration of interfaces inside the edge-lit light guide such that the light out-coupling interface is a V groove interface (symmetric or asymmetric) . . . . A light source  1201   b   3 , an LED in one embodiment, emits input light  1203   b   1  into the light guide. The V groove interfaces separate two media  1203   b   3 / 1205   b   3 ,  12011   b   3 / 12013   b   3 . The scalene prism array separates to other media  1207   b   3 / 1209   b   3 . The advantage is that V-groove interfaces are much easier to fabricate. But the drawback is that the fresnel reflections are not necessarily in the direction of light emission or shallow enough for light propagation in the forward direction. Some of the fresnel reflections are sent in the backward direction 
       FIG. 12  ( b   5 ) shows a use of edge-lit light guide collimator as a backlight for an LCD display. The LCD display assembly  1207   b   5  is the standard LCD configurations. There is an optional reflector  12011   b   5  underneath the light guide  1209   b   5  to reflect off any scattered light back into the light guide. A light source  1201   b   5 , an LED in one embodiment, emits input light into the light guide  1209   b   5 . An output substantially collimated beam  1205   b   5  emerges from a surface of the light guide. 
       FIG. 12 ( b   6 ) shows an alternative use case for edge-lit light guide collimator as described in this patent disclosure. In this case, the collimating light guide of these teachings  1207 B 6  is sandwiched between the LCD layer  1205   b   6  and the traditional light guides described in the prior art which emit light at a wider range of angles. This traditional light guide configuration typically consists of light guide  12015   b   6 , diffuser  12013   b   6 , microprism films  1209   b   6 ,  12011   b   6  etc. Instead of this traditional wide angle light source there can be another illumination source which emits light at wide angle range. This use case takes advantage of the fact that our light guide is transparent to light coming behind it. So, when our collimating light guide is turned on, the LCD display is illuminated with very directional light and hence offers a combination of energy-efficiency, brightness and privacy. When our light guide is turned off, and light source below it is illuminated, the LCD is illuminated with diffused light and hence lower brightness and can be viewed at a wide range of angles. 
       FIG. 12 ( b   7 ) shows an alternative use case for the edge-lit light guide collimator as described in this patent disclosure. In this case, a polarization recycling film (for example, but not limited to, DBEF film sold by 3M) is placed between the LCD layer and the edge-lit light guide collimator. The s-polarized light passes through this layer but the p-polarized light is reflected back. This reflected p-polarized light passes through the light guide (due to its transparent characteristics) and encounters the polarization rotator and a reflector layer underneath the light guide. The p-polarized light is converted to s-polarization going through the layers underneath. Thus, on being reflected back this light also passes through the polarization recycling film. Therefore, in this method almost all the collimated light is converted to a s-polarization before it enters the LCD layer. 
     An LED emits light that is divergent in both dimensions (both x-axis and y-axis). With all the description mentioned in previous sections, light is collimated on only one axis if microstructure prism array interface is linear. For many practical applications, light needs to be collimated on both axes. Therefore modifications in the design of the light guide and/or the arrangement of the microstructures have to be made to collimate the light on both axes.  FIGS. 12 ( b   8 )- 12 ( b   13 ) describe the such methods to collimate light on both axes. 
       FIG. 12 ( b   8 ) shows the top view of alternate design of the rectangular light guide with beveled corners as shown. The LEDs used to input light into the light guide are placed on these four beveled corners. The lines shown as arcs in the figure represent the microstructure of interfaces whose cross-section is described in detail the  FIGS. 12 ( b   1 )- 12 ( b   4 ). These interface cross-section can be scalene, V-groove or asymmetric V-groove design. Due to the centro-symmetric nature of these interface with respect to the LED light source the light is collimated on both axes. 
       FIG. 12 ( b   9 ) shows the top view of an alternate design of rectangular light guide. As shown in the figure, the rectangular light guide is divided into trapezoidal segments which are assembled together to make a rectangle. The very thin spacing between adjacent trapezoids consists of either air or a medium with refractive index lower than the trapezoid. The LEDs are placed at the smaller edge of trapezoid. The dotted lines the figure shaped as arcs represent the microstructure of interfaces whose cross-section is described in detail the  FIGS. 12 ( b   1 )- 12 ( b   4 ). These interface cross-section can be scalene, V-groove or asymmetric V-groove design as detailed previously. The collimation of light on the first axis happens due to the microstructure of the light guide. The collimation of light on the second axis happens by two means: first by the arc shape of the groove and secondly by undergoing TIR at the edges of the trapezoid. Thus collimation of light on both axes is achieved. 
       FIG. 12 ( b   10 ) shows the top view of an alternate method of collimating light on both axes. The basic premise is that we collimate the light on each axis sequentially rather than simultaneously. As shown in the figure, the LEDs are placed on the edge of the first light guide than collimates light on one axis. The inset shows the microstructure on this light guide. The light output from this light guide is input on the second light guide. The design of the second light guide is the same as that described in  FIGS. 12 ( b   1 ) to  12 ( b   4 ) such that the light is collimated on the second axis as well when it is out-coupled from the top-surface of the light guide. 
     It is envisioned that these two light guides can also be stacked on top of each other with an optically dense medium in between rather than placed sequentially. The microstructural features consist of linear scalene prism arrays, V-groove or asymmetric V-grooves as described in  FIGS. 12 ( b   1 ) to  12 ( b   4 ). The microstructural features of each light guide layer are the designed such that they are orthogonal to that in the other layer. 
       FIG. 12 ( b   11 ) shows the top view of a circular edge-lit light guide where the LEDs are placed on the outer circumference of circular the light guide. The dotted lines shown as concentric circles in the figure represent the microstructure of interfaces whose cross-section is described in detail the  FIGS. 12 ( b   1 )- 12 ( b   4 ). These interface cross-sections can be scalene, V-groove or asymmetric V-groove design. Due to the centro-symmetric nature of these interfaces with respect to the LED light source the light is collimated on both axes. 
       FIG. 12 ( b   12 ) shows the top view of a circular light guide which a variation of the design is shown in  FIG. 12 ( b   11 ). An additional feature unique to this design variation is that there is an array of trapezoids on the outer annulus of the light guide which have higher refractive index compared to the area adjacent to them as shown in the figure. The LED are specifically places at the edge of these trapezoids only. The dotted lines shown as concentric circles in the figure represent the microstructure of interfaces whose cross-section is described in detail the  FIGS. 12 ( b   1 )- 12 ( b   4 ). These interface cross-sections can be scalene, V-groove or asymmetric V-groove design. The light collimation of achieved firstly due to the TIR at the trapezoid interface and later due to the centro-symmetric nature of these interfaces with respect to the LED light source. 
       FIG. 12 ( b   13 ) shows the top view of an edge-lit light guide in the shape of an annulus. The key differentiating feature of this design compared to the  FIG. 12 ( b   11 ) and  12 ( b   12 ) is that the LEDs are placed on the smaller, inner circumference of the annulus. The dotted lines shown as concentric circles in the figure represent the microstructure of interfaces whose cross-section is described in detail the  FIGS. 12 ( b   1 )- 12 ( b   4 ). These interface cross-sections can be scalene, V-groove or asymmetric V-groove design. Due to the centro-symmetric nature of these interfaces with respect to the LED light source the light is collimated on both axes. 
     Embodiments in which a light sources switch between the light guide apparatus of these teachings and another illumination structure are within the scope of the art and can use and actuate a shutter, such as disclosed in U.S. Pat. No. 3,614,211, which is incorporated by reference herein in its entirety and for all purposes, or more recently using an actuated micromirror and a second mirror. Actuated micromirrors are disclosed for example in STATIC AND ELECTRICALLY ACTUATED SHAPED MEMS MIRRORS, Ph. D. Thesis by Bin Mi, submitted to the Department of Electrical Engineering and Computer Science, Case Western Reserve University, on May 2004, which is incorporated by reference herein in its entirety and for all purposes. 
       FIG. 13( a )  shows a light bending device ( 1301 ) that consists of a multilayer stack of optically dense media ( 1301 - 1311 ) which have refractive indices in the range of 1.3 to 2.5 with either textured of flat interfaces ( 1312 - 1319 ) as boundaries between adjacent optical media. This optical device successively bends incident light using refraction, Total Internal Reflection (TIR), near-Total Internal Reflection (near-TIR) or their combination at each of these interfaces ( 1312 - 1319 ). The device stack is arranged such that the optical media form alternate layers of higher and lower refractive index. This is not a mandatory condition as described in mechanism below. 
     Mechanism: The mechanism of operation relies on progressive bending of light through a stack of textured interfaces between a medium of higher refractive index and a medium of lower refractive index. At the next interface the light is transferred from lower refractive index medium to higher refractive index medium with nominal change in direction. Thus the light is at a higher refractive index medium at a greater angle from normal and these two combined processes are repeated multiple times to bend the light to desired angles. 
     In the  FIG. 13( a ) , the input light  1302 , incident on the optical device  1301  at a predetermined range of angles, bends at the 1 st  interface ( 1312 ) between 1 st  medium ( 1303 ) and 2 nd  medium ( 1304 ) either as a result of refraction or a combination of TIR and refraction thereby entering the 2 nd  optical medium ( 1304 ) at a deviation from the original direction of incident light. In this particular example 1 st  medium ( 1303 ) has a higher refractive index than 2 nd  medium ( 1304 ). The interface 2 ( 1313 ) is an array of V-groove prism and is designed such that the light traveling in the 2nd optical medium ( 1304 ) is near-normal to one of the faces of the V-groove and near-parallel to the other face of the V-groove. Thus light travels from Medium 2 with lower refractive index to Medium 3 with higher refractive index with nominal deviation. The light bends further at the flat interface between Medium 3 with higher refractive index and Medium 4 with lower refractive index. Once the light is in Medium 4, it is incident at the interface 4 which is designed on the same principle as Interface 3 i.e. one face of the V-groove is near-normal to light and the other face is near-parallel to incident light. Thus there is nominal light bending when light travels from Medium 4 with lower RI to medium 5 with higher RI. This alternate sequence of higher and lower refractive optical media with alternate flat and V-groove interfaces is repeated to achieve higher bending of light. 
     Higher number of layers is needed to bend light by higher magnitude. Also the refractive index contrast between adjacent higher refractive index and lower refractive index optical medium dictates how many layers are need to bend the light by a specific amount. The last optical medium (in this case Medium 9) can be an optically dense medium with RI between 1.3 and 2.5 or it can be air in case we want to preserve the angle of the bent light when it exits the optical device. 
     When the angle of light in the higher index medium is greater than 45 degree, Fresnel reflections start to occur at the flat interface and some of the light is reflected and hence doesn&#39;t follow the intended path. In order to circumvent the problem, we can use a stack of multiple optical media with flat interfaces such that their refractive index is intermediate to the higher and lower refractive index mediums and are stacked such that there is a gradient in refractive indices from higher to lower values. 
     While we have used flat interfaces ( 1314 ,  1316 ,  1318 ) to bend the light from higher RI to lower RI medium, it is envisioned that this interface can also be textured with a specific groove design. Furthermore, the textured interfaces ( 3013 ,  3015 ,  3017 ,  3019 ) can also be used to bend the light by a small amount rather than their primary purpose of transferring the light from a lower refractive index medium to a higher refractive index medium. One or more of the optical media in this stack may be birefringent. 
       FIG. 13( b )  shows a light bending and trapping device ( 1320 ) that consists of a multilayer stack of optically dense media ( 1303 - 1311 ) which have refractive indices in the range of 1.3 to 2.5 with either textured of flat interfaces ( 1312 - 1319 ) as boundaries between adjacent optical media. This optical device successively bends incident light using refraction, Total Internal Reflection (TIR), near-Total Internal Reflection (near-TIR) or their combination at each of these interfaces ( 1312 - 1319 ). The dotted lines starting at  1302  and ending at  1321 . 
     The device stack is arranged such that the optical media form alternate layers of higher and lower refractive index ( 1303  being higher refractive index and  1304  being lower refractive index and so on). This is not a mandatory condition as described in mechanism below. The mechanism of this light trapping device ( 1320 ) involves bending light progressively to shallower angles using mechanism described in  FIG. 13( a )  and then recycling it as described further. In this particular example shown in  FIG. 13( b ) , the interfaces  1312 - 1318  successively bend the light to shallow angles. This shallow light in medium  1310  enters the medium  1311  (which has higher refractive index compared to  1310 ) through the interface  1319 . The interface  1319  is configured such that one face of the micro-prism is parallel to incident light and the other face is normal (or near-normal) to the incident light. The shallow light hits the bottom surface ( 1322 ) of the medium  1311  and undergoes Total internal reflection at the surface and is incident on interface  1319  again. This incident light undergoes multiple refractions at the interface  1319  and eventually recycled back into medium  1311 . This recycling of light between interfaces  1322  and  1319  can repeat multiple times to effectively trap the light in the light trapping device ( 1320 ) for a long distance. It is worth mentioning that, instead of air, there could be another optically dense medium below medium  1311  which has refractive index lower than  1311  such that TIR happens at the boundary  1322  and light is recycled back into medium  1311 . 
       FIG. 13( c )  Show the Zemax simulation of this optics with zoomed-in version of light trapping at the interface. 
       FIG. 13( d )  shows the use of light trapping waveguide ( 1320 ) to concentrate incident light ( 1323 ), redirect this concentrated light ( 1324 ) to edge of the waveguide and produce power by placing a solar cell ( 1325 ) on the edge of the waveguide. 
       FIG. 13( e )  shows a variation of the scheme proposed in  FIG. 13( d ) . Here the incident light  1323  is trapped into the light trapping optical waveguide  1320  resulting in concentrated light  1324  travelling along the length of the waveguide. This concentrated light hits the beveled edge  1326  of the waveguide which redirects the light to a solar cell  1325  placed on the bottom face of the waveguide as shown in the  FIG. 13( e ) . 
       FIG. 13( f )  Show the use of light trapping to design a hybrid solar panel that uses both single junction solar cells and multi-junction solar cells to produce greater power from both the devices. Sunlight at any location on earth surface contains some amount of diffused light in addition to the direct normal irradiance (DNI). Since the light trapping optics described in this invention works effectively for a certain predefined range of angles, we lose the ability to harness the diffused portion of the solar insolation. As shown in  FIG. 13( f ) , we use the light trapping optics waveguide to guide the concentrate and redirect the direct portion of the sunlight to the multijunction solar cell by the method described in  FIG. 13( e ) . The majority of diffused sunlight passes through the waveguide, hits the single junction solar cell placed underneath the light trapping optics and gets converted to electricity. Typically, the light trapping optics need not be in direct contact with the single junction cell and there can be air as a separator between the two. This would also prevent the heat accumulated as a result of light concentration to affect the performance of single junction solar cell underneath. However, an air gap can lead to accumulation of dirt and moisture which can be avoided by making one monolithic stack. However, since the single junction solar cell has materials with higher refractive index than the light trapping waveguide, light leaks into the solar cell. To avoid this, we need to put an intermediate layer of lower refractive index than the waveguide so that the light travelling inside the waveguide undergoes TIR at the interface and thus remains contained within the waveguide. However, the diffused light would be incident at the interface of waveguide/low refractive index material at higher angles and thus pass through and hit the single junction solar cell. Using this method of harnessing light, we can make solar panels with large area power conversion efficiencies around 30% for majority of locations worldwide. 
       FIG. 13( g )  show the use of light trapping optics to recycle light in a solar panel. In a typical solar cell, some of the photons are absorbed and then re-emitted thus contributing to losses. Also some of the photons are scattered by various interfaces present in the solar cells. Also, many times the absorber layer in a solar cell is an indirect semiconductor (e.g. Silicon) which requires larger thickness of the absorber to absorb all photons and convert them to electron-hole pair. As shown in  FIG. 13( g ) , the light trapping optics as described in this invention can be used to bend light to shallow angles and recycle that light multiple times within the waveguide. By the virtue of shallow angles, the photons gets absorbed and much smaller depths in the semiconductor absorber layer thus boosting efficiency. Furthermore, light that is absorbed and re-emitted also gets trapped and recycled within the waveguide and gets few more chances of reabsorption by the semiconductor absorption layer. Thus the light trapping optics can be used on top of solar cells to boost their power conversion efficiency by cutting down on various loss mechanisms. 
       FIG. 14( a )  shows an achromatic flat lens ( 1401 ) that can bend incident light ( 1402 ) over a predetermined range of angles using a multilayered stack of optical media ( 1403  to  1409 ) separated from each other by interfaces ( 1411  to  1416 ) such that the incident light after going through lens converges at a focus ( 1410 ). The optics can also be structures such that there is a virtual focal point i.e. the incident rays diverge after hitting the flat lens. The trajectory of the incident light  1402  going through different layers and interfaces of flat lens  1401  is shown by dotted lines. 
     Traditionally, lenses have chromatic aberration due to the dispersion relationship of the lens material i.e. variation in refractive index with wavelength. Because of this dispersion phenomenon, light of different wavelengths bend by a different angle going through the two surfaces of the lens 
     The mechanism of operation of flat lens shown in  FIG. 14( a )  is similar to the light bending optics shown in  FIG. 13( a ) . The only difference between the two is that in the case of flat lens  1401  the angles of the two faces of the micro-textured interface cross-sections ( 1411  to  1416 ) vary with position so that there is a focus ( 1410 ) at which the rays of light converge (or seem to diverge from). 
     Incident light ( 1402 ) bends at the interface  1411  separating optical media  1403  and  1404  (which may both be dense optical media with different refractive index) either by refraction or a combination of Total internal reflection and refraction. In one instance the medium  1403  can be air instead of dense optical medium in which case the light bending at interface  1403  happens via refraction. The cross-section of interface  1411  can be either a right angled prism array or a V-Groove prism array or a scalene prism array. 
     This light in optical medium  1404  is now incident on interface  1412  which is structured such that one face of the micro-textured cross-section is parallel and the other face is perpendicular to the incident refracted light from previous interface. This ensures that light transfers from medium  1404  to  1405  with nominal bending. The light in medium  1405  encounters the flat interface  1413  such that light bends further away since medium  1405  is chosen to be higher refractive index than medium  1406 . 
     The light in medium  1406  then encounters interface  1414  where light gets transferred to medium  1407  with nominal light bending since the interface  1414  is structured such that incident light is parallel to one face of the micro-textured prism cross-section and normal (or near-normal) to the other face. The light in medium  1407  bends again at flat interface  1415  since the refractive index of medium  1408  is chosen to be lower than medium  1407 . The light in medium  1408  encounters interface  1416  and thus enters medium  1409  with nominal bending since one face of the micro-prism cross-section of interface  1416  is structured parallel to the incident light and the other face is structured normal (or near normal) to the incident light. Finally after numerous light bending events, the incident light converges at the focal point  1410  inside medium  1409  which can be an optically dense medium or an air medium. 
     Thus, by using multiple light bending-and-transfer events, we can successively bend light by much larger magnitude compared to other flat lenses technologies or Fresnel lenses. The flat lens described in this invention is achromatic since most light bending happens at the interface between two dense optical media. As shown in the  FIG. 14( a ) , the refractive index of both optical media change with refractive index, however their ratio remains in a close range for all wavelengths. Since the ratio of refractive indices of the optical media dictate the light bending at the interface as per the Snell&#39;s law, each wavelength of light bends by an equal magnitude. While the cross-section has been shown in the  FIG. 14( a )  remains the same, the flat lens can either be prismatic such that it only bends light on one coordinate axis or be centro-symmetric such that it bends light on both coordinate axes. 
     The achromatic flat lens described in this invention can be used imaging and for non-imaging application such as (a) Light collimation for LED lighting, general lighting, automotive exterior and interior lighting etc. (b) lenses used in optical instrumentation such as various kinds of microscopes, interferometers, telescopes etc. (d) imaging lenses for use in photography, virtual reality systems, augmented reality and various other imaging applications. 
     Typically the light emitting diodes (inorganic semiconductor based or organic emitter based) and other light sources emit light at wide range of angles. However, many times it is desirable to deliver a narrower the output emission of the light to the user.  FIG. 14( b )  shows the use of a flat lens array ( 1417 ) to manage emission cone of emitted light ( 1419 ) from an array of light emitting diodes ( 1418 ) such that the light output ( 1420 ) from the exit aperture of the flat lens is emitted at a predetermined, narrower range of angles with respect to the flat lens array ( 1417 ). The structure of the flat lens arrays is similar to the unit structure of a single flat lens shown in  FIG. 14( a ) . Here the medium in which the light is emitted from light source  1418  can be air or a dense optical medium with refractive index between 1.3 and 2.5. When the wide angle light is emitted in dense medium, some of the light whose angle is beyond the total internal angle of the dense medium/air interface gets waveguided within the dense medium and doesn&#39;t reach the user. Hence a loss of energy efficiency/optical efficiency. When the multilayer optics with textured interfaces that constitute the flat lens array  1417  is used, the wide angle light gets bent to a narrower angle going through the optics such that the angle of light at the exit surface of the flat lens is much lower than the total internal reflection condition and hence this light is also able to reach the user. Furthermore, the narrower range of angles of the exit also means that there is an increase in brightness. Such a flat lens array can be used for display applications such as those based micro-LEDs arrays or Organic Light emitting diodes (OLEDs) or any other technology that uses direct emission of light from each pixel of the display. Such flat lens array can also be for illumination applications. 
       FIG. 15 ( a )  shows an Angle selective reflector device  1501  that reflects light incident at shallow angles while transmitting light coming at angles closer to the normal. The device  1501  consists of a two layers stack consisting on dense optical media  1504  and  1505  separated by a micro-textured prism array interface ( 1506 ) with the light being incident on optical medium  1504 . The device is designed such that optical medium  1504  has lower refractive index compared to that of optical medium  1505 . When shallow angle light ( 1502 ) is incident on the medium  1504 , it bends closer to normal due to refraction and is incident on the interface  1506 . The interface  1506  is designed such that one face of the micro prism is almost parallel to the incident light and the other face is near normal (or normal) to the incident light. As a result, the incident light enters medium  1505  with nominal bending. The light in medium  1505  is incident on the bottom surface of the medium and meets the criteria for total internal reflection (TIR) at this interface since the medium  1505  has higher refractive index compared to medium  1504 . The light reflected from the bottom surface of medium  1505  is again incident on the interface  1506  and enters medium  1504  with nominal bending. When light on it outward path in medium  1504  hits the top surface of medium  1504  it undergoes refraction condition since it&#39;s refractive index is lower than medium  1505 . Thus shallow angle light is reflected from device  1501  as if this is a mirror. This phenomenon happens only when light comes at shallow angles. The upper limit of angle which gets reflected depends on the refractive index contrast between medium  1504  and  1505 . When the refractive index ratio between  1505  and  1504  is high, light of steeper angles also gets reflected. When light is incident at steeper angles close to the normal ( 1507 ), the light does not meet TIR criteria at the bottom surface of medium  1505  and hence gets transmitted ( 1508 ). 
     Since the two layer stack can reflect light only at shallow angles, it is of interest to develop angle selective reflector that can bend light at steeper angles.  FIG. 15( b )  shows the angle selective reflector  1522  which can reflect light at steeper angles: light incident at 40 deg to 90 deg from normal is reflected whereas the light incident in the angle range 0 to 40 deg from normal is transmitted. These angle ranges are representative and can be tuned by the specific construction of the angle selective reflector  1522 . The path of light going through various optical media and interfaces that constitute device  1522  is shown by dotted lines. 
     As shown in the  FIG. 15( b ) , incident light  1520  enters medium  1509  via refraction and encounters micro-textured prism interface  1515 . This interface is designed such that one face of the micro-textured prism is parallel to the incident light and the other face is perpendicular to incident light. This results in light being transferred into medium  1510  with nominal bending. In some cases, the medium  1509  can be chosen to be air as well. In such cases also the incident light is parallel to one face of the micro-prism interface  1515 . 
     The light in medium  1510  encounters the flat interface  1516  such that light bends away from normal since the medium  1511  is chosen to be lower refractive index than medium  1510 . The light in medium in  1511  encounters interface  1517  and is transferred into medium  1512  with nominal bending since the interface  1517  is designed such that one face of the micro-prism is parallel to incident light and the other face is normal (or near-normal) to incident light. The light in medium  1512  bends at flat interface  1518  to enter medium  1513  since the latter is chosen to be lower refractive index than medium  1512 . The light in medium  1513  then encounters the micro-textured prism interface  1519  and passes through with nominal light bending since the interface  1519  is designed such that one face of the micro-prism interface is parallel to the incident light and the other face is perpendicular the interface. Thus the light that enters medium  1514  has undergone sufficient light bending that it meets criteria for total internal reflection at the bottom surface of medium  1514  and hence gets reflected. 
     On it return path, the light (a) transfers into medium  1513  without bending at interface  1519 , (b) then bends towards normal at interface  1518 , (c) then transfers again into medium  1511  without bending at interface  1517 , (d) then again bends towards normal at interface at  1516 , (e) then transfers again into medium  1509  without bending at interface  1515  and (f) finally exits the medium  1509  undergoing refraction at the top surface. 
     Thus the method of successive light bending and transfer into higher refractive index medium can be utilized to fabricate an angle selective reflector that reflects light incident at predetermined range of angles while transmitting light at certain other predetermined range of angles which are closer to the normal. 
     The light that is incident closer to the normal gets transmitted and has a higher beam divergence than the incident light. As a result the device  1522  is diffusive to light incident at an angle closer to normal and hence the image of an object behind the device  1522  gets distorted.  FIG. 15( c )  shows an angle selective reflector device  1523  that circumvents this issue by modifying the interfaces as described below. 
     As shown in  FIG. 15( c ) , the device  1523  consists of a multilayer stack of optical media ( 1528  to  1533 ) separated from each other by interfaces ( 1534 - 1538 ). The difference between this device  1523  and device  1522  described in previous section is that here the interfaces  1534 ,  1536  and  1538  have micro-textured prism array regions which are intermittent with flat regions in between. The flat interfaces ( 1535  and  1537 ) are the same as that in device  1522 . Furthermore, the device  1523  is assembled such that the flat regions of interfaces  1534 ,  1536  and  1538  are aligned with each other. The presence of flat regions in these interfaces which are aligned with each other ensures that incident light closer to normal ( 1526 ) gets transmitted through the device without any light bending and hence the image quality of an object behind the device  1523  is undistorted. If the incident light that is closer to normal, hits the micro-prism regions of the interfaces  1534 ,  1536  and  1538 , the transmitted light will have higher beam divergence and hence diffusive effect in terms of image quality. When the length of flat regions is less than what the eye can resolve, it is possible to see a clear image of an object behind the device  1523 . 
       FIG. 15( d )  shows the use of an angle selective reflector  1545  as a window film overlaid on the window glass  1544  such that it reflects direct sunlight ( 1540 ) but transmits ambient diffused light ( 1542 ). As shown in the figure, the direct light from the sun ( 1540 ) is incident on the angle selective window film device ( 1545 ) at high angles from the normal. As a result this light is reflected by the device  1545 . The ambient diffused light which is closer the normal to the window film ( 1545 ) gets transmitted. Either device  1501  or device  1522  or device  1523  can be used as a window film to achieve above mentioned purpose. In the case device  1501  or device  1522  is used as window film, the user has the benefit of privacy while direct sunlight is reflected. In the case device  1523  being used as a window film, the user has the benefit of seeing through the window clearly while the direct sunlight is reflected. Such window films ( 1501 ,  1522  or  1523 ) can reduce the heat inside the building due to sun&#39;s insolation and hence improved the energy efficiency by reducing the energy required to cool the building. 
     The angle selective reflector device as described in  FIGS. 15( a ), ( b ), ( c )  can also be used as horizontal or vertical window blinds. The angle selective reflectors using either the device  1501  or  1522  or device  1523  can be used as a projection surface such that light from a projector coming at higher angles from the normal is reflected to the user while ambient light is transmitted. In case the device  1523  is used as a window film, we can have the ability to maintain the ability to see clearly through the window without distortions while also reflecting away the direct sunlight. 
     The angle selective reflector devices as shown in the  FIG. 15( a ) ,  FIG. 15( b )  and  FIG. 15( c )  can be used in a display device, signage device or lighting/illumination device to manipulate the path of light. 
       FIG. 16  shows the an apparatus for light bending and trapping optics in air medium to concentrate light and use it to heat fluid flowing in a receiver tube for solar thermal applications. The apparatus a glass sheet  1601  with a lamination of light bending optics  1602  (similar to that shown in  FIG. 13( a ) ), a secondary concentrator  1603  and a receiver tube  1604 . The light bending optics  1602  consists of a multilayer stack of polymers with different refractive index with textured interfaces such that the incident light  1605  is progressively bent to shallow angles and released into the air medium similar to the mechanism shown in  FIG. 13( a ) . The shallow angle light  1606  that enters the air medium encounters the reflective boundaries of the secondary concentrator  1603  which reflect and accumulate this shallow angle light over large area into a smaller area exit aperture thereby concentrating the light by 10-1000 times. This concentrated light  1607  hits the receiver tube  1604  where it gets absorbed and converted to heat. This heat gets use to raise the temperature of a flowing thermal fluid inside the receiver tube. This thermal fluid can be used to run a thermodynamic cycle to convert thermal energy into useful work, mechanical energy or electricity. The component  1604  can also be a solid state thermoelectric device or a thermo-voltaic device that can be used to convert thermal energy directly into electrical energy. 
       FIG. 17( a )  shows a device ( 1701 ) to collimate, homogenize and achieve glare-reduction from a light source that emits light at wide range of angles. As shown on the figure, the device  1701  consists of multilayer stack of optical media  1702  to  1707  with alternate high and low refractive indices with  1702  being the high refractive index (RI) medium. These optical media  1702  to  1707  are separated from each other by interfaces  1708  to  1712 . The interface going from high RI to low RI being symmetric or asymmetric micro-prism array interface while interface going from low RI to high RI medium is a flat interface. For example, going from high RI medium  1702  to lower RI medium  1703  the interface  1708  is textured while going from low RI medium  1703  to high RI medium  1709  is flat interface. The stack so built has alternate textured and flat interfaces. 
     As shown in  FIG. 17( a )  the light source  1713  emit light at wide range of angles ( 1716 ). This wide angle light  1716  is incident on device  1701  which collimates the light leading to output light  1717  emitted in a narrow range of angles. In the process, some of the light is recycled backwards (shown as  1715 ) and hits the reflector  1714  and reflected back to the device  1701  to go through the optics again. The reflector  1714  can either be a specular reflector or a non-specular reflector (also referred to as diffuse reflection). The process of collimation leads to substantial reduction in the intensity of output light  1717  emitted in angles between 45° to 90° from the normal to the output surface. The light in this angle range is the reason for glare from light sources and hence the collimation also achieves glare reduction. Since some of the light is recycled we achieve a homogenization of the light sources. 
     Mechanism of operation: To simplify the mechanism of light collimation in  FIG. 17( a ) , we will describe the portion of input light  1716  that is emitted to the right. Since the optics shown here is symmetric, similar process will happen for the light emitted to the left. With respect to this input light emitted to the right, we will define the two faces of the micro-prism array interface as “onward face” which faces the light beam directly and the “leeward face” that is facing away from the light beam. This is illustrated in the  FIG. 17( b ) . As shown in  FIG. 17( b ) , the light that is incident of onward face passed through the interface to low RI medium with nominal bending since the angle of light is close to normal to that onward face. This light in low RI medium is now incident on the flat interface that bends the light towards the normal. Successive such interactions of light beam with onward faces of the prism leads to light being bent to angles close to normal. This light comes out of the device collimated. 
     The light that is incident on the leeward face undergoes TIR at this face followed by refraction as light enters the subsequent low RI medium. Due to TIR this light becomes quite shallow. The angle of this light becomes so shallow that it remains shallow even when it bends towards the normal at the subsequent flat interface. This shallow light most likely encounters the onward face for the next textured interface, passes through with nominal bending and then bends toward the normal. This process repeats one more time in the stack. However it still remains quit shallow, undergoes TIR at the bottom surface of the device  1701  and is recycled backwards where it hits the reflector  1714  and is incident on device  1701  again to have another chance at light collimation. 
     The light that initially interacted with the onward face of the interface  1708  and then bent towards normal at flat interface  1709  can subsequently interact with the leeward side of the interface  1710  or  1712 . Such interaction will increase the angle of light with respect to the normal to the flat surface. At times when this interaction leads to TIR or near-TIR, the light will become shallow enough that it will undergo TIR at the flat bottom surface of the device  1701  and recycled back to reflector  1714  to have another chance at interaction with the device  1701  for light collimation. 
       FIG. 17 ( c )  shows the ray tracing simulation of a wide angle light source. As can be seen in the figure the light output is collimated and there is light recycled back towards the reflectors. Some of the recycled light gets collimated at the second pass through  1701  while some of the light gets collimated at the third or fourth pass through the optics. Some of the recycled light is trapped within the multilayer optical stack only to be redirected back to the output at collimated angle. Sometimes this trapped light is redirected to the output at shallow angles contributing to the “leakage” in the collimated light. 
       FIG. 17( d )  shows the intensity vs angle for the output light as it goes through the device  1701  and shown in ray tracing simulation of  FIG. 170 . As can be seen from the figure, the output light is in the angle range+/−36 deg from normal while the input light beam is incident at +/−60 deg. 
     It is possible to design the angles of the micro-prism interfaces minimize the unwanted interactions such that the signal to noise ratio is high and most of the light is collimated. Also, we have described symmetric optics to achieve collimated output light which is symmetric around the normal. It is possible to achieve asymmetric angular range of the output light if we choose asymmetric micro-prism interfaces for interfaces  1708 ,  1710  or  1712 . 
     Unless otherwise stated in this disclosure, the refractive indices of the optical media described in this invention are in the range from 1.2 to 4.0. These optical media can be transparent to light in all or some portions of electromagnetic spectrum. 
     For the purposes of describing and defining the present teachings, it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     Although embodiments of the present disclosure have been provided in detail, it is to be understood that the method and the apparatus of the present disclosure are provided for exemplary and illustrative purposes only. Various changes and/or modifications may be made by those skilled in the relevant art without departing from the spirit and scope of the present disclosure as defined in the appended claims.