PATENT ABSTRACT
A solar light concentration plate comprises a plurality of holograms diffracting incident light wherein each of the plurality of the holograms has a thickness, at least one intermediate light guide plate disposed between the plurality of the holograms, and a pair of external light guide plates disposed on outer surfaces of outermost holograms of the plurality of the holograms, wherein at least one of the pair of the external light guide plates has an inner surface and an outer surface inclined relative to the inner surface.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Korean Patent Application No. 10-2010-0124955, filed on Dec. 8, 2010, and all the benefits accruing therefrom under 35 U.S.C. §119, the content of which in its entirety is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1) Field 
         [0003]    Provided is a solar light concentration plate with high concentration efficiency and wavelength separation. 
         [0004]    2) Description of the Related Art 
         [0005]    A main energy source that is currently used is a fossil fuel such as coal and petroleum. However, continued use of the fossil fuel causes problems such as global warming and environmental pollution as well as resource exhaustion. Accordingly, use of renewable energy sources that do not cause environmental pollution such as solar light, tidal power, wind power, and geothermal heat has been suggested as an alternative energy source for replacing the fossil fuel. 
         [0006]    Among the renewable energy sources, technology of converting the solar light into electricity is most widely used. Various materials and devices are being developed for the efficient conversion of the solar light into electricity, and for example, recently suggested technology based on the multi-layered p-n junction structure and III-V Group materials accomplishes light conversion efficiency of about 40%. 
         [0007]    Furthermore, the solar light can be directly used instead of being converted into electricity. For example, direct use of the solar light as an indoor illumination by collecting the solar light by a light-collecting device installed on a rooftop of a building and transmitting the solar light inside the building using light guide has been suggested. The direct use of the solar light transmitted from the rooftop as an indoor illumination may greatly reduce electricity consumption. However, in general, natural lighting is insufficient to be used inside the building and thus artificial illuminations are used even in the daytime. 
         [0008]    Therefore, efficient light concentration is the core technology that can be applied to various fields that utilize the solar light. A currently-available light concentration plate usually includes large number of silicon photoelectric conversion devices, thereby having a large area which may not be suitable for a mass production due to high cost. 
         [0009]    Therefore, it has been suggested that an optical device such as lens is used for focusing the solar light on a photoelectric conversion device to increase an amount of light in a given area and to reduce a size of a photoelectric conversion device, and a prism or a diffraction lattice is used for separating wavelengths so as to utilize a photoelectric conversion device suitable for each wavelength. 
         [0010]    However, the above-described technology may increase a space of the light concentration plate in a direction toward the solar light. For a concentration system using a lens or a hyperbolic mirror, a photoelectric conversion device is spaced apart from the lens or the mirror by a focal distance, and thus an additional space for the focal distance may be required by the system. In the case of a prism, a distance for spatially separating light according to wavelengths may be required. The above mentioned spatial limitations may make it hard to implement a photovoltaic power generation system. 
       SUMMARY 
       [0011]    One embodiment of the present invention provides a solar light concentration plate that occupies a small space, is inexpensive, has high concentration efficiency, and may separate wavelengths. 
         [0012]    In an embodiment of the present invention, a solar light concentration plate is provided that includes a plurality of holograms diffracting incident light and having different thicknesses, at least one intermediate light guide plate disposed between the holograms, and a pair of external light guide plates disposed on outer surface of outermost holograms among the plurality of holograms, wherein at least one of the external light guide plates has an inner surface and an outer surface inclined to the inner surface. 
         [0013]    In an embodiment of the present invention, among the holograms, an uppermost hologram may have larger angular selectivity than other holograms. 
         [0014]    In an embodiment of the present invention, the hologram may diffract light having a wavelength of a range. 
         [0015]    In an embodiment of the present invention, the wavelength of a range may be about 10 nanometers (nm) to about 300 nm. 
         [0016]    In an embodiment of the present invention, each of the outer surfaces of both external light guide plates may have an outer surface inclined to an inner surface. 
         [0017]    In an embodiment of the present invention, the angle made with the outer surfaces of the external light guide plates may be about 1 degree to about 10 degrees. 
         [0018]    In an embodiment of the present invention, the holograms may include phase difference holograms. 
         [0019]    In an embodiment of the present invention, the hologram may have a thickness of about 1 micron or more. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other aspects, advantages and features of this disclosure will become more apparent by describing in further detail embodiments thereof with reference to the accompanying drawings, in which: 
           [0021]      FIGS. 1 and 2  are cross-sectional views of exemplary embodiments of solar light concentration plates. 
           [0022]      FIGS. 3 and 4  are schematic diagrams respectively explaining Bragg diffraction and Raman-Nath diffraction of an exemplary embodiment of a diffraction lattice. 
           [0023]      FIGS. 5 and 6  are schematic diagrams respectively explain angular selectivity and wavelength selectivity of an exemplary embodiment a diffraction lattice. 
           [0024]      FIG. 7  is a graph showing parameters Q, Δθ and Δλ as a function of a diffraction lattice thickness, when setting a center wavelength as 500 nanometers (nm) and Bragg angle as 22 degrees. 
           [0025]      FIGS. 8 and 9  are schematic cross-sectional views explaining operation of the exemplary embodiment of the light concentration plate shown in  FIG. 1 . 
           [0026]      FIGS. 10 and 11  are schematic cross-sectional views explaining operation of the exemplary embodiment of the light concentration plate shown in  FIG. 2 . 
           [0027]      FIG. 12  is a schematic cross-sectional view explaining operation of the exemplary embodiment of the light concentration plate shown in  FIG. 1 . 
           [0028]      FIG. 13  is a graph showing change in angular selectivity in three cases of diffraction lattices. 
           [0029]      FIG. 14  is a graph showing a thickness of a diffraction lattice satisfying about 150 nm wavelength selectivity and angular selectivity corresponding thereto. 
           [0030]      FIG. 15  is a graph showing calculation result for thickness design of upper and lower holograms. 
           [0031]      FIGS. 16 and 17  are a schematic diagram and a graph respectively explaining angle multiplexing. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope. In the drawing, parts having no relationship with the explanation are omitted for clarity, and the same or similar reference numerals designate the same or similar elements throughout the specification. 
         [0033]    It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0034]    It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
         [0035]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
         [0036]    Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower,” can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
         [0037]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0038]    Embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims. 
         [0039]    Hereinafter, embodiments of the present invention will be described in further detail with reference to the accompanying drawings. 
         [0040]    An exemplary embodiment of a solar light concentration plate is described in detail with reference to  FIGS. 1 and 2 . 
         [0041]      FIGS. 1 and 2  are cross-sectional views of exemplary embodiments of solar light concentration plates. 
         [0042]    First, a solar light concentration plate  100  shown in  FIG. 1  includes two volume phase holograms  110  and  120 , and three light guide plates  130 ,  140 , and  150 . The concentration plate  100  may have various shapes such as a triangle, a quadrangle, an oval, for example. 
         [0043]    Each of upper, middle, and lower light guide plates  130 ,  140 , and  150  may include a transparent plastic film, for example. In one exemplary embodiment, the plastic film may have a refractive index of about 1.5, and in the present exemplary embodiment, a total reflection angle for light toward air from the light guide plates  130 ,  140 , and  150  is about 42 degrees. In general, a refractive index of a plastic light guide plate is within a range of about 1.3 to about 1.7, and a total reflection angle is determined in the range of about 50 degrees to about 36 degrees according to Snell&#39;s law. However, a material included in the light guide plates  130 ,  140 , and  150  is not limited there to as long as the material may guide light. 
         [0044]    Although both sides of the middle light guide plate  140 , i.e., one side of the middle light guide plate  140  which contacts  110  and the other side of the middle light guide plate  140  which contacts  120  are substantially parallel to each other, the upper and the lower light guide plates  130  and  150 , respectively, are substantially sloped such that outer surfaces of the upper and the lower light guide plates  130  and  150  are inclined relative to inner surfaces of the upper and the lower light guide plates  130  and  150 . Specifically, the outer surface (or upper surface) and the inner surface (or lower surface) of the upper light guide plate  130  are inclined to each other, and the outer surface (or lower surface) and the inner surface (or upper surface) of the lower light guide plate  150  are also inclined to each other. In one exemplary embodiment, only one of the upper and the lower light guide plates  130  and  150  is substantially sloped such that one of the outer surfaces of the upper and the lower light guide plates  130  and  150  is inclined relative to inner surfaces of the upper and the lower light guide plates  130  and  150 . An angle between the outer surface of the upper light guide plate  130  and the outer surface of the lower light guide plate  150  may be greater than about 0 degree and equal to or less than about 10 degrees, and in one preferred exemplary embodiment, the angle between the outer surface of the upper light guide plate  130  and the outer surface of the lower light guide plate  150  may be about 1 degree to about 5 degrees. 
         [0045]    The volume phase holograms  110  and  120  include an upper hologram  110  disposed between the light guide plates  130  and  140 , and a lower hologram  120  disposed between the light guide plates  140  and  150 . Each of the volume phase hologram  110  and  120  diffracts incident light having a wavelength of a determined range which enters at an incidence angle of a determined range, at an angle of a determined range. The two holograms  110  and  120  may have wavelength selectivity of about 10 nanometers (nm) to about 300 nm as a whole range, and may diffract incident light with an incidence angle of about 0 degree to about 10 degrees at any diffraction angle. In one exemplary embodiment, the volume phase holograms  110  and  120  may include a diffraction lattice, and may be recorded using interference of light such as laser, for example. According to the present exemplary embodiment, the volume phase holograms  110  and  120  may be recorded with laser, and the incident light is solar light when using the light guide plate. 
         [0046]    In one exemplary embodiment, the holograms  110  and  120 , and the light guide plates  130 ,  140 , and  150  may be attached with an index matching adhesive to prevent scattering at the interface therebetween. 
         [0047]    The concentration plate  100  may be connected to an optical fiber  200 , which may be connected to a photoelectric conversion device  310  and/or a lighting instrument  320 . The concentration plate  100  collects incident solar light and sends it to the photoelectric conversion device  310  or the lighting instrument  320  through the optical fiber  200 , and the light may be converted into electricity by the photoelectric conversion device  310  or directly used as a direct lighting by the lighting instrument  320 . 
         [0048]    In one exemplary embodiment, the concentration plate  100  may be directly connected to the photoelectric conversion device  310 . 
         [0049]    Referring to  FIG. 2 , a solar light concentration plate  400 , similarly to that the exemplary embodiment of the concentration plate  100  shown in  FIG. 1 , includes three light guide plates  430 ,  440 , and  450 , and two volume phase holograms  410  and  420  disposed therebetween. 
         [0050]    However, differently from the exemplary embodiment of  FIG. 1 , the upper and the lower volume phase holograms  410  and  420  have different thicknesses from each other. In  FIG. 2 , a thicker left portion of the solar light concentration plate  400  is connected to an optical fiber (not shown). 
         [0051]    Hereinafter, operating principles of exemplary embodiments of the solar light concentration plates are described in detail. 
         [0052]    First, the operating principle of an exemplary embodiment of a volume phase hologram is described in detail with reference to  FIGS. 3 to 7 . 
         [0053]      FIGS. 3 and 4  respectively illustrate Bragg diffraction and Raman-Nath diffraction of an exemplary embodiment of a diffraction lattice,  FIGS. 5 and 6  respectively illustrate angular selectivity and wavelength selectivity of the exemplary embodiment of the diffraction lattice, and  FIG. 7  is a graph showing parameters Q, Δθ and Δλ as a function of a thickness of the diffraction lattice when setting center a wavelength as 500 nm and Bragg angle as 22 degrees. 
         [0054]      FIGS. 3 and 4  show plate-like diffraction lattices  600  and  700 , respectively, which are one-dimensional phase holograms. Grating axes  610  and  710  of the diffraction lattices  600  and  700 , respectively, are substantially perpendicular to surfaces  620  and  720  of the diffraction lattices  600  and  700 , respectively. Two kinds of diffractions are generated by the diffraction lattices  600  and  700 . One is Bragg diffraction shown in  FIG. 3 , which is predominantly generated by a thicker diffraction lattice  600 , and the other is Raman-Nath diffraction shown in  FIG. 4 , which is predominantly generated by a thinner diffraction lattice  700 . 
         [0055]    Referring to  FIG. 3 , Bragg diffraction allows incident light  630  which enters only at a given incident angle called Bragg angle (θ B ) relative to the grating axis  610  to be diffracted, and only one outgoing light  650  is admitted. The outgoing light  650  complies with a law of diffraction to make an angle substantially equal to Bragg angle (θ B ) relative to the grating axis  610 . Thus, an angle between an extension  640  of the incident light  630  and the outgoing light  650  becomes twice the Bragg angle (2θ B ). 
         [0056]    Referring to  FIG. 4 , according to Raman-Nath diffraction, a specific incidence angle is not required to generate diffraction, and a plurality of diffracted outgoing lights  750  is generated from one incident light  730 . 
         [0057]    To distinguish a type of diffraction performed by a diffraction lattice, a parameter Q is introduced. The parameter Q is defined by the following equation 1; 
         [0000]    
       
         
           
             
               
                 
                   
                     Q 
                     = 
                     
                       
                         2 
                          
                         πλ 
                          
                         
                             
                         
                          
                         d 
                       
                       
                         
                           Λ 
                           2 
                         
                          
                         
                           n 
                           0 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   〉 
                 
               
             
           
         
       
     
         [0058]    wherein λ is a wavelength, d is the thickness of a diffraction lattice, Λ is a pitch distance of a refractive index (or absorption) modulation, and n 0  is average refractive index. Generally, a diffraction lattice with Q value equal to or greater than 10 shows Bragg diffraction, and a diffraction lattice with Q value of significantly less than 1 shows Raman-Nath diffraction. 
         [0059]    To easily control diffracted light, in the present exemplary embodiment, a Bragg diffraction lattice which has angular selectivity and wavelength selectivity may be used. The characteristics will be described in detail with reference to  FIGS. 5 and 6 . 
         [0060]      FIGS. 5 and 6  show a diffraction lattice  800  wherein a grating axis  810  is inclined at a Bragg angle (θ B ) relative to a surface normal  825  of a surface  820 .  FIG. 5  shows incident light entering at various incidence angles, and  FIG. 6  shows incident light of various wavelengths. 
         [0061]    Referring to  FIG. 5 , angular selectivity refers to a characteristic that diffraction is generated only for an incidence angle of a specific range around Bragg angle (θ B ), wherein a minimum deviation angle at which diffraction is not generated or a maximum angle range (Δθ) where diffraction may be generated is defined by the following equation 2; 
         [0000]    
       
         
           
             
               
                 
                   
                     Δθ 
                     = 
                     
                       
                         n 
                          
                         
                             
                         
                          
                         λ 
                       
                       
                         2 
                          
                         d 
                          
                         
                             
                         
                          
                         sin 
                          
                         
                             
                         
                          
                         
                           θ 
                           B 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   〉 
                 
               
             
           
         
       
     
         [0062]    wherein λ is a wavelength, d is the thickness of a diffraction lattice  800 , and θ B  is Bragg angle. Angular selectivity (Δθ) can be varied from about 0.001 degrees to about 10 degrees. However, it is noted that substantially too large an angular selectivity (Δθ) may make the thickness of a diffraction lattice  800  small, thereby making Q value substantially too small to enter into a Raman-Nath diffraction region. 
         [0063]    Referring to  FIG. 6 , wavelength selectivity refers to a characteristic that diffraction occurs only for a specific wavelength range, wherein the wavelength range (Δθ) that can be diffracted is defined by the following equation 3; 
         [0000]    
       
         
           
             
               
                 
                   Δλ 
                   = 
                   
                     
                       
                         
                           λ 
                           2 
                         
                          
                         cos 
                          
                         
                             
                         
                          
                         
                           θ 
                           B 
                         
                       
                       
                         2 
                          
                         d 
                          
                         
                             
                         
                          
                         
                           sin 
                           2 
                         
                          
                         
                           θ 
                           B 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   〉 
                 
               
             
           
         
       
     
         [0064]    wherein λ is a wavelength, d is the thickness of a diffraction lattice  600 , and θ B  is a Bragg angle. 
         [0065]    As shown in the above-described equations 2 and 3, the angular selectivity and wavelength selectivity depend on the wavelength (λ), the thickness (d) of a diffraction lattice  800 , and a Bragg angle (θ B ). Particularly, as the diffraction lattice  800  becomes thicker, the angular selectivity and wavelength selectivity becomes larger, resulting in smaller Δθ and Δλ. 
         [0066]      FIG. 7  is a graph showing the parameters Q, Δθ and Δλ as a function of the thickness of a diffraction lattice, when a center wavelength is equal to 500 nm, and Bragg angle is equal to 22 degrees. 
         [0067]    In one exemplary embodiment, when Δλ is 150 nm, for example, a thickness of a diffraction lattice for satisfying it is about 5.5 microns (μm). In the present exemplary embodiment, Δθ is about 11 degrees, and Q is about 25. If Δλ is 100 nm, the thickness of a diffraction lattice is about 8 microns, Δθ is about 7 degrees, and Q is about 20. 
         [0068]    An exemplary embodiment of a light-concentration plate includes the above described diffraction lattice or volume phase hologram. 
         [0069]    An exemplary embodiment of an exemplary embodiment a solar light concentration is described in detail with reference to  FIGS. 8 and 9 . 
         [0070]      FIGS. 8 and 9  are schematic cross-sectional views of an exemplary embodiment of a solar light concentration plate. 
         [0071]    Referring to  FIG. 8 , Bragg angle of a volume phase hologram  110  is determined larger than ½ of an angle of total reflection of the light guide plates  120  and  130  attached to both sides, i.e., upper side and lower side, of the volume phase hologram  110 , and a grating axis of the hologram  110  is made inclined at Bragg angle relative to a surface normal of the hologram  110 , as shown in  FIGS. 5 and 6 . A center wavelength may be selected by controlling a wavelength or incidence angle of recording light of hologram. Specifically, from white solar light, a red wavelength may be exclusively diffracted and other wavelengths may be transmitted. A wavelength range where diffraction can be generated and a minimum deviation angle may be selected by controlling a thickness of hologram. An effective range of the minimum deviation angle will be described below. 
         [0072]    Supposing that solar light  21  including blue light  11 , green light  12 , and red light  13  perpendicularly enters into a concentration plate  100 , the incident light  21  passes through the upper light guide plate  120  without changing its angle to reach the hologram  110 . The hologram  110  selects red light  13  from the incident light  21  to diffract it at twice the Bragg angle (2θ B ) (22), and passes blue light  11  and green light  12  without changing an incident angle thereof. Since Bragg diffraction has wavelength selectivity, it may diffract only a specific wavelength range. Bragg diffracted light  23  reaches an interface  132  between the lower light guide plate  130  and air, wherein since the incidence angle 2θ B , is larger than the angle of total reflection of the light guide plate  130 , the light  23  is totally reflected at the interface  132  ( 24 ). 
         [0073]    Light  25  totally reflected at the interface  132  between the light guide plate  130  and air meets the hologram  110  again, and passes through the hologram  110  without diffraction ( 26 ), which is further described in detail with reference to  FIG. 9 . 
         [0074]    Referring to  FIG. 9 , when a grating axis  41  is inclined at Bragg angle (θ B ) to a direction  42  perpendicular to a surface  112  of a hologram  110 , the incidence angle at which the hologram  110  causes diffraction includes two angles making Bragg angle (θ B ) with reference to the crystal face  41 . One of them is a perpendicular direction  42 , the other is a direction indicated by a reference numeral  43  (hereinafter also referred to as a “reference direction”). Light entering at an incidence angle other than the perpendicular direction  42  and the reference direction  43  is not diffracted. Therefore, referring to  FIG. 7 , since the light  25  totally reflected at the interface  132  between the lower light guide plate  130  and air has no relationship with the two directions, it passes through the hologram  110  without diffraction. 
         [0075]    The light  25  that passed through the hologram  110  enters into the upper light guide plate  120  and advances to meet an interface  122  between the upper light guide plate  120  and air. Here, since the incidence angle is equal to 2θ B  and larger than an angle of total reflection of the light guide plate  120 , the light  25  is also totally reflected at the interface  122  ( 27 ). As described above, the perpendicularly incident light  21  entering into the concentration plate  100  begins to be guided toward one direction, i.e., leftward direction, of the concentration plate  100 . 
         [0076]    However, when light  28  totally reflected at the interface  122  between the upper light guide plate  120  and air meets the hologram  110 , diffraction occurs ( 29 ) and light may  30  go toward a lower direction, which gets out of the concentration plate  100 . 
         [0077]    Referring to  FIGS. 8 and 9 , the direction where the light  28  totally reflected at the interface  122  between the upper light guide plate  120  and air enters the hologram  110  is substantially identical to the reference direction  43 . Therefore, the light  28  diffracted by the hologram  110  goes along the perpendicular direction  42  according to the law of reflection. Therefore, the diffracted light  30  goes toward the lower direction  30  to get out of the concentration plate  100 . 
         [0078]    In order that the light  28  totally reflected at the interface  122  between the upper light guide plate  120  and air does not get out of the concentration plate  100  to be continuously guided, the light  28  may go straight toward the direction indicated by reference numeral  31  without being diffracted by the hologram  110 , which will be further described in the following embodiment. 
         [0079]    Another exemplary embodiment of a solar light concentration plate is described with reference to  FIGS. 10 and 11 . 
         [0080]      FIGS. 10 and 11  are schematic cross-sectional views of another exemplary embodiment of a solar light concentration plate  400 . 
         [0081]    Referring to  FIG. 10 , a tilt angle (θ T ) of upper and lower light guide plates  420  and  430  relative to a horizontal axis, e.g., upper and lower surfaces of a hologram  410  is larger than ¼ of the angular selectivity (Δθ) of the above-described diffraction lattice or hologram  410 . 
         [0082]    In the present exemplary embodiment, perpendicularly entering incident light  51  passes through the upper light guide plate  420  without significant change in the progressing direction to reach the hologram  410 , and it is diffracted to a reference direction  63 . Since an upper surface of the upper light guide plate  420  is slightly tilted relative to the horizontal axis, slight refraction may occur when the incident light  51  enters the upper light guide plate  420  from the air, and an incident angle the incident light  51  entering the hologram  410  may be substantially slightly out of the perpendicular direction, but such refraction may be ignored for better comprehension and ease of description because the incident angle the incident light  51  entering the hologram  410  may be corrected when recording the hologram  410  or be within angular selectivity range. 
         [0083]    Light  52  diffracted by the hologram  410  passes through the lower light guide plate  430  and totally reflected at an interface  432  between the lower light guide plate and air ( 53 ). Totally reflected light  54  passes through the hologram  410  without diffraction, enters the upper light guide plate  420 , reaches an interface  422  between the upper light guide plate and air, and is totally reflected at the interface  422  ( 55 ). The twice totally reflected light  56  meets the hologram  410  again, and at this time, since the incidence angle of the twice totally reflected light  56  entering the hologram  410  is larger than angular selectivity around the reference direction  63 , the light  56  passes through the hologram  410  without diffraction. Finally, the incident light repeats this process and is guided to one end of the concentration plate  400 , i.e., leftward direction. 
         [0084]    This process is further described in detail with reference to  FIG. 11 . 
         [0085]    Referring to  FIG. 11 , supposing that a direction perpendicular to the upper surface  412  of the hologram  410  is a perpendicular direction  62  to the hologram  410 , the light  52  diffracted by the hologram  410  is in the reference direction  63 , an angle between the reference direction  63  and the grating axis  61  of the hologram  410  is θ B , and an angle between the reference direction  63  and the perpendicular direction  62  is 2θ 6 , wherein θ B  is a Bragg angle. 
         [0086]    Supposing that a direction perpendicular to the interface  432  between the lower light guide plate  430  and air is a perpendicular direction  64  to the lower light guide plate  430 , the perpendicular direction  64  to the lower light guide plate  430  is tilted at an angle (θ T ) to the perpendicular direction  62  to the hologram. The angle θ |1  at which diffracted light  52  enters the interface  432  should be determined with reference to the perpendicular direction  64  to the lower light guide plate  430 , as represented by the following equation 4; 
         [0000]      θ |1 =2θ B +θ T    &lt;Equation 4&gt;
 
         [0087]    Light  54  reflected at the interface  432  according the law of reflection also makes an angle of θ |1  relative to the perpendicular direction  64  perpendicular to the lower light guide plate  430 . 
         [0088]    An incidence angle θ R1  at which the light  54  reflected at the interface  432  enters the hologram  410  again should be calculated with reference to the perpendicular direction  62  to the hologram instead of the perpendicular direction  64  to the lower light guide plate  430 , as represented by the following equation 5; 
         [0000]      θ R1 =θ |1 +θ T =(2θ B +θ T )+θ T =2θ B +2θ T    &lt;Equation 5&gt;
 
         [0089]    Since the incidence angle θ R1  is distant from the perpendicular direction  62  and the reference direction  63 , the light  54  passes through the hologram  410  without diffraction. 
         [0090]    An incidence angle θ |2  at which the light  54  passing through the hologram  410  enters the interface  422  between the upper light guide plate  420  and air should be determined with reference to a perpendicular direction  65  to the upper light guide plate  420  which is perpendicular to the interface  422  between the upper light guide plate  420  and air. And, since the direction  65  perpendicular to the upper surface of the upper light guide plate  420  is tilted at an angle θ T  with reference to a perpendicular direction  62  to the hologram  410  in the opposite direction to the perpendicular direction  64  to the lower surface of the lower light guide plate, the incidence angle θ |2  is calculated by the following equation 6; 
         [0000]      θ |2 =θ R1 +θ T =(2θ B +2θ T )+θ T =2θ B +3θ T    &lt;Equation 6&gt;
 
         [0091]    And, the light  56  reflected by the interface  422  also makes an angle of θ |2  relative to the perpendicular direction  65  to the upper surface of the upper light guide plate  420 . 
         [0092]    The incidence angle θ R2  at which the light  56  reflected at the interface  422  enters the hologram  410  again is calculated with reference to the perpendicular direction  62  to the hologram, as represented by the following equation 7; 
         [0000]      θ R2 =θ |2 +θ T =(2θ B +3θ T )+θ T =2θ B +4θ T    &lt;Equation 7&gt;
 
         [0093]    Therefore, if 4θ T  is Irger than angular selectivity (Δθ) of the hologram  410 , the light  56  passes through without diffraction by the hologram  410 . 
         [0094]    In one exemplary embodiment, only one of the two light guide plates  420  and  430  may be inclined using the above principle. 
         [0095]    Meanwhile, wavelength selectivity Δλ of 150 nm means that diffraction efficiency of light deviating 150 nm from the center wavelength becomes 0. Specifically, the entire range including a shorter wavelength range and a longer wavelength range with reference to the center wavelength is 300 nm. However, since substantially effective amount of light is about an half of maximum diffraction efficiency and thus the wavelength range decrease by half, light within about 150 nm range is substantially diffracted and satisfies guide condition. Thus, reference to wavelength selectivity is determined as Δθ of about 11 degrees, and the tilt angle (θ T ) shown in  FIG. 10  is about 3 degrees. Supposing that the length of the concentration plate  400  is 300 millimeters (mm), difference between the thicknesses of the thicker part and the thinner part in one light guide plate of  420  and  430 , respectively, is about 15 mm, and a thickness difference between a sum of the thicker part of the two concentration plates  420  and  430  and a sum of the thinner part of the two concentration plates  420  and  430  is about 30 mm. Therefore, the concentration plate  400  has a thicker thickness of one side compared to the length or area of the other side. 
         [0096]    A thick concentration plate  400  may cause loss when transmitting light from the concentration plate  400  to optical fiber  200 , and increase manufacture cost. 
         [0097]    The tilt angle (θ T ) should be substantially decreased to reduce the thickness of the concentration plate  400 , and angular selectivity of the hologram  410  should be substantially reduced to substantially decrease the tilt angle (θ T ), 
         [0098]    The exemplary embodiment shown in  FIG. 1  includes a plurality of holograms to reduce angular selectivity. 
         [0099]    Then, the operation of the exemplary embodiment of the solar light concentration plate as shown in  FIG. 1  is described in detail with reference to  FIGS. 12 to 15 . 
         [0100]      FIG. 12  is a schematic cross-sectional view explaining the operation of the exemplary embodiment of the concentration plate of  FIG. 1 ,  FIG. 13  is a graph showing change in angular selectivity of diffraction lattices in cases  1  to  3 ,  FIG. 14  is a graph showing the thickness of a diffraction lattice satisfying about 150 nm wavelength selectivity and corresponding angular selectivity, and  FIG. 15  is a graph showing a calculation result for thickness design of upper and lower holograms. 
         [0101]    Referring to  FIG. 12 , since upper and lower volume phase holograms  510  and  520  are disposed between upper, middle and lower light guide plates  530 ,  540 , and  550 , perpendicular incident light  71  is primarily diffracted by the upper hologram  510 , and primarily diffracted light  72  passes through the middle light guide plate  540 , and then, secondarily diffracted again by the lower hologram  520 . Since total reflection condition of the light guide plate is satisfied through the secondary diffraction, Bragg angles of the upper and lower holograms may be decreased to reduce a minimum deviation angle. The secondarily diffracted light  73  is totally reflected at an interface  552  between the lower light guide plate  550  and air ( 74 ), totally reflected light  75  passes through the two holograms  510  and  520  without diffraction to reach an interface  532  between the upper, light guide plate  530  and air ( 76 ). The light  75  is totally reflected at the interface  532  between the upper light guide plate  530  and air, and totally reflected light  77  meets and passes through the upper hologram  510  without diffraction. Light  78  passing through the upper hologram  510  passes through the middle light guide plate  540 , and then meets the lower hologram  520 , and also passes through it without diffraction. Light  79  passing through the lower hologram  520  is totally reflected at the interface  552  between the lower light guide plate  550  and air again, and progresses to a leftward direction with repeating the above described process 
         [0102]    In  FIG. 12 , a diffraction reference direction of the upper hologram  510  is indicated by a reference numeral  81 , and a diffraction reference direction of the lower hologram  520  is indicated by a reference numeral  82 , wherein Bragg angle of the hologram  520  is smaller than those of the exemplary embodiments shown in  FIGS. 11 and 12 . If the Bragg angle of a single hologram  410  is 22 degrees, for example, outgoing light of a same angle as the outgoing light of the single hologram  410  may be obtained from the lower hologram  520  by setting each Bragg angle of two holograms  510  and  520  as 11 degrees. However, a direction of the grating axis of the lower hologram  520  is determined so that light entering in a diffraction reference direction  81  of the upper hologram  510  instead of a perpendicular direction is diffracted. 
         [0103]    In one exemplary embodiment, a number of holograms may be three or more, and as a number of holograms increases, the Bragg angle of each hologram decreases in proportion to the number of the holograms. 
         [0104]      FIG. 13  shows change in angular selectivity of diffraction lattices (or holograms) in cases  1  to  3 . As shown in the graph, as a number of diffraction lattices increases, angular selectivity of each diffraction lattice decreases to 11 degrees in case  1 , 5 degrees in case  2 , and 3 degrees in case  3 . If angular selectivity decreases, a tilt angle of a light guide plate may decrease, and thus the thickness of the light guide plate may be reduced or an area of the light guide plate may increase. 
         [0105]    If the thickness of the thicker side of the light guide plate is determined as 10 mm, for example, length may increase to 104 mm in a single diffraction lattice structure, 227 mm in a double diffraction lattice structure, and 385 mm in a triple diffraction lattice structure, which means that an area of the concentration plate also increases. 
         [0106]      FIG. 14  shows a thickness of a diffraction lattice satisfying about 150 nm wavelength selectivity and corresponding angular selectivity. As a number of diffraction lattices increases, Bragg angle decreases and a thickness of the diffraction lattice increases. Increase in the thickness of the diffraction lattice may result in increase in diffraction efficiency. 
         [0107]    So far, exemplary embodiments of a method of efficiently guiding solar light entering in a perpendicular direction to the concentration plate have been described. However, an incidence angle of solar light changes every hour by the rotation of earth and every season due to the revolution of the earth. Therefore, it is important to maintain high efficiency concentration performance during change in the location of the sun. In this regard, angular selectivity (Δθ) is a substantially important parameter. If Δθ is substantially large, it may be difficult to avoid diffraction of the light inside the concentration plate by a diffraction lattice, and if Δθ is substantially small, entering solar light may become sensitive to incidence angle and high efficiency concentration may be achieved only at substantially perpendicular angle relative to the concentration plate. Therefore, there is a need to relax solar light incidence conditions while eliminating light loss by interaction between guided light and a diffraction lattice. 
         [0108]    Therefore, in one exemplary embodiment, the upper hologram related to incidence condition may be designed to have large Δθ, and the lower hologram may be designed to have small Δθ. The exemplary embodiment is shown in  FIG. 2 , wherein the thickness of the upper hologram  410  is smaller than the thickness of the lower hologram  420 . 
         [0109]    Relationship between angular selectivity and thickness is described in detail with reference to  FIG. 15 . 
         [0110]      FIG. 15  is a graph showing a calculation result for thickness design of the upper and lower holograms  410  and  420 , wherein the two holograms  410  and  420  in combination should provide angle displacement of the incident light perpendicularly entering the surface at an angle of total reflection of the light guide plates  430 ,  440 , and  450 , for example, 42 degrees or more in the concentration plate. This means that the sum of Bragg angles of the two holograms  410  and  420  should be 21 degrees or more. And, the upper hologram  410  indicated by 1 st  grating in  FIG. 15  provides comparatively weak angular selectivity (Δθ&gt;7 degrees), and the lower hologram  420  indicated by 2 nd  grating provides strong angular selectivity (Δθ&lt;3 degrees). If Bragg angle of the upper hologram  410  is 17 degrees and the thickness is 10 microns, for example, relatively gentle incidence condition may be obtained with angular selectivity of 8 degrees. If Bragg angle of the lower hologram  420  is 5 degrees and the thickness is 100 microns, for example, angular selectivity will be 3 degrees, thus eliminating light loss due to rediffraction occurred by guide in the concentration plate. 
         [0111]    To further relaxing incidence condition, angle multiplexing may be introduced in the upper hologram  410  to form multiple holograms. Referring to  FIGS. 16 and 17 , to introduce angle multiplexing, incidence angles θ 1  to θ 4  of reference beam is controlled while fixing signal beam when recording hologram. For design and correction of recording time or angle, known method in holographic memory may be used. The multiple holograms recorded by the above method identically diffract light entering in various directions at progress course of the signal beam. The light is diffracted again by the lower hologram  420  beyond the angle of total reflection, and finally guided in the concentration plate  400 . 
         [0112]    As described above, according to the present exemplary embodiments, high concentration efficiency and wavelength separation may be enabled while using inexpensive and less space occupying light guide plate. 
         [0113]    While this disclosure has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.