Abstract:
A light guide comprises two or more straight light guide sections and one or more light coupling elements between sections to transport light from one section to another section. By having at least two TIR (totally internally reflecting) surfaces aligned with the light guiding direction of one of the light guiding sections, a light coupling element of the invention transports light from one light guide section to another with little or no light loss and little or no increase of etendue.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   U.S. patent application Ser. No. 10/161,753, filed 4 Jun., 2002, and assigned to the same Assignee as the present application, relates to a projection display system employing light guides of the invention of the present application. 
   BACKGROUND OF THE INVENTION 
   This invention relates to light guides, and more particularly relates to rigid light guides exhibiting low or no light loss while substantially preserving etendue of the guided light beam, regardless of changes in direction of the guided light beam or intersection of the guided light beam with other beams. 
   Rigid light guides such as solid or hollow light pipes, are an attractive, low-cost means for light collection, manipulation and transportation. Such light guides provide such functions in a much more compact volume than is possible with conventional optics. Unfortunately, with present light guide technology, bends or folds in the guides cause severe loss of light and increased etendue (cone angle), as illustrated in  FIGS. 1A through 1D . 
     FIG. 1A  shows schematically a straight light guide  10  and a guided light beam represented by outer ray traces  1  and  2 . Note that the beam maintains the same cone angle as it travels along the guide, as indicated by the invariant angle α formed at the intersections of the traces. When a bend  11  is introduced into the guide  10 , as shown in  FIG. 1B , the cone angle is increased, as indicated by comparing angle β formed by the intersection of outer ray  3  and central ray  4  prior to encountering the bend, and angle γ formed by the intersection of these rays after the bend. 
   The increased cone angle of the guided beam means that fixed aperture collection devices may not be able to collect all of the light from the beam as it exits the light guide. 
     FIG. 1C  shows another way in which light may be lost. When a second bend  12  is added to the light guide  10 , some of the light, represented by outer ray traces  5  and  6 , is reflected backward, as illustrated by ray  6 , which reflects from an area just beyond the second bend to travel back along the bend  12  in the reverse direction, as ray  7 . 
     FIG. 1D  shows that a smooth bend  13  has an effect similar to that of the sharp bend  12  in  FIG. 1B , ie, the cone angle of the guided beam is increased, as illustrated by the angle δ between rays  8  and  9  after the bend, compared to their parallel path (0 angle) before the bend. 
   Attempts to couple light guide sections with mirrors also leads to light loss, as shown in  FIGS. 2A through 2C . In  FIG. 2A , for example, the cone angle φ is such that the guided beam (indicated by outer rays  15  and  16 ) after exiting through exit aperture  17  of light guide  18  and being reflected by mirror  19 , has an etendue too large for collection at entrance aperture  20  of light guide  21 , resulting in significant coupling loss. Moving mirror  19  as close as possible so that it actually touches the edges of exit aperture  17  of light guide  18  and entrance aperture  20  of light guide  21 , as illustrated in  FIG. 2B , minimizes but does not eliminate the coupling loss.  FIG. 2C  shows the virtual image of the guided beam  22  after exiting from light guide  23  and reflection and before entry into light guide  24 , illustrating the dependence of coupling loss on the cone angle θ. 
     FIG. 2D  illustrates that coupling loss can be reduced or eliminated by inserting relay lenses  25  and  26  into the path of the guided beam before and after reflection by mirror  19 . These relay lenses  25  and  26  limit the extent of the guided beam so that it fits within the entrance aperture  20  of light guide  21 , avoiding coupling losses. However, such relay optics are expensive and prevent the desired compact arrangement. 
   Alternatives such as fiber optic bundles are also expensive. Moreover, fiber optic bundles suffer significant insertion loss because of a relatively low packing density. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a light guide comprises at least first and second light guide sections and one or more light coupling elements between the sections to transport light from one section to another section. By having at least two TIR (totally internally reflecting) surfaces, a first TIR surface adjacent to the exit aperture of the first light guide section and aligned with the light guiding direction of the second light guide section, and a second TIR surface adjacent to the entrance aperture of the second light guide section and aligned with the light guiding direction of the first light guide section, a light coupling element of the invention transports light from the first light guide section to the second light guide section with little or no light loss and little or no increase in etendue. 
   In one embodiment, two light guide sections are arranged with their light guiding directions at an angle to one another, for example, at a right angle, and are coupled with a wedge-shaped light coupling element having two TIR surfaces, a first TIR surface adjacent to the exit aperture of a first light guide and a second TIR surface adjacent to the entrance aperture of the second light guide. The light coupling element also has a reflecting surface set at an angle to the TIR surfaces, for reflecting light from the first light guide section to the second light guide section. The second TIR surface guides the light from the first light guide section to the reflecting surface, while the first TIR surface guides the reflected light to the second section. In such an arrangement, little or no light escapes during transport across the coupling element from one light guide to the other and little or no increase in etendue occurs. 
   By using multiple light coupling elements and light guide sections coupled at the same or different angles, transportation of light energy along an arbitrary path is enabled. 
   In another embodiment, a light coupling element with three or more TIR surfaces enables coupling three or more light guide sections. For example, a light coupling element with four TIR surfaces coupling four light guide sections in a crossed pattern makes it possible to independently transport light over two intersecting guide paths without cross coupling energy from one to the other and with little or no loss and little or no increase in etendue. 
   In another aspect of the invention, a light coupling element of the invention may incorporate one or more dichroic elements or surfaces having selective transmission and/or reflection passbands, enabling color splitting and/or recombination, with little or no light loss and little or no increase in etendue of the guided beam(s). 
   For example, in the intersecting-guide-path embodiment, crossed dichroic elements in the light coupling element enable splitting of a light beam from one light guide section into three components, which are each guided along one of the remaining three light guide sections. Alternatively, three component beams, each guided along a different light guide section toward the light coupling element, can be recombined by the crossed dichroic elements into a single beam, which is then guided along a remaining light guiding section. 
   The light guide sections of the invention may be solid elements of an optically transparent material, or may be hollow pipes having sidewalls with interior reflecting surfaces. A light guide of the invention may use only solid or only hollow sections, or a combination of solid and hollow sections, depending on the application. The cross-sections of the light guide sections will normally be rectangular, but may also be square or other regular polygon having an even number of sides. 
   As will be appreciated, light may pass through any of the light guides of the invention in either direction with no difference in outcome, except in those cases in which dichroic or polarizing elements or surfaces are employed. It should therefore be understood that the terms “entrance” and “exit” with respect to the faces and apertures of the elements of the various light guides described and claimed herein are used for convenience of description only. These terms are meant to be interchangeable and thus not to limit the utility of the invention to light travel in one direction only. 
   The invention is useful in a variety of light collecting, processing, transporting and distributing applications, in fields as diverse as illumination, displays, information processing and high power applications. Specific examples of applications within these fields include projection displays, backlighting for liquid crystal displays, automotive illumination, and welding. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIGS. 1A through 1D  are schematic illustrations of rigid light guides of the prior art, having a straight configuration, a single sharp bend, two sharp bends and a smooth bend, respectively; 
       FIGS. 2A through 2D  are schematic illustrations of rigid light guide sections of the prior art, having conventional optical couplings of mirrors ( FIGS. 2A and 2B ), air ( FIG. 2C ) and a combination of lenses and a mirror ( FIG. 2D ); 
       FIGS. 3A through 3D  are schematic illustrations of rigid light guides of the invention,  FIGS. 3A and 3B  showing a solid and a hollow light guide, respectively, each having a right angle bend, and  FIGS. 3C and 3D  showing a solid and a hollow light guide, respectively, each having a 45 degree bend; 
       FIGS. 4A through 4C  are schematic illustrations of intersecting, cross-coupling and beam-splitting rigid light guides of the invention, respectively; 
       FIG. 5  is a detail view of the coupling region between a light guide section and a light coupling element, showing an adhesive layer filling the space between these elements; and 
       FIG. 6  is a cross section view of one of the light guide sections. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 3A  of the drawing, there is shown one embodiment  30   a  of a light guide of the invention, including light guide sections  31  and  32 , each with an entrance aperture ( 31   a ,  32   a ), an exit aperture ( 31   b ,  32   b ), and a side wall ( 31   c ,  32   c ). Positioned between exit aperture  31   b  of light guide section  31  and entrance aperture  32   b  of light guide section  32  is a wedge-shaped light coupling element  33 , having two TIR (totally internally reflecting) faces, entrance face  33   a  and exit face  33   c , and internally reflecting face  33   b , extending at an angle to the TIR faces  33   a  and  33   c.    
   The light coupling element  33  is of an optically transparent material. As used herein, the term “optically transparent” means that the material is able to transport light with little or no absorption of light. Suitable materials include, for example, glass and plastic, although any optically transparent material having an index of refraction higher than that of the external medium in contact with the surfaces  33   a  and  33   c  is suitable. The higher index of refraction enables these surfaces to behave as total internal reflection (herein “TIR”) surfaces. 
   The TIR faces  33   a  and  33   c  of the light coupling element  33  are parallel to the light guiding direction (defined by the direction of the side walls) in the non-adjacent light guide sections  32  and  31 , respectively, and in this embodiment, because the light guide sections  31  and  32  are arranged with their light guiding directions at right angles, the TIR faces  33   a  and  33   c  are also at right angles to one another. 
   In this embodiment, light guide sections  31  and  32  are of a solid optically transparent material, such as plastic or glass. Because of the higher refractive index of such a material relative to the surrounding medium, usually air, a light beam (represented by the outer rays  36  and  37 ) is confined within the light guide by the phenomenon of total internal reflection (herein “TIR”). 
   Essentially, because light can travel from one medium to another only if it strikes the interface at or above a critical angle (relative to the surface), as determined by Snell&#39;s Law, light which strikes the interface below the critical angle is reflected. Because of the relatively small cone angle of the guided light beam, and the difference in refractive index of the light guide medium and its exterior surroundings (eg, air), the guided beam is totally internally reflected as it moves along the light guide. 
   When the beam (as indicated by the outer rays  36  and  37 ) reaches the exit aperture  31   a  of light guide  31 , the steep angle at which it encounters this aperture allows it to exit the light guide section and enter the light coupling element  33  through entrance face  33   a . As ray  36  encounters TIR exit face  33   c  at a low angle, it is reflected back to the reflecting surface  33   c , and then is reflected back to encounter exit face  33   c  again, but this time at a steep angle, whereby the ray is enabled to enter light guide section  32  through entrance aperture  32   a . Unlike ray  36 , ray  37  first encounters reflecting surface  33 , where it is reflected back toward TIR entrance face  33   a  at a low angle, and is thence reflected at a steep angle toward TIR face  33   c , enabling it to pass into light guide section  32  through entrance aperture  32   a.    
   Although the medium in the spaces ( 34 ,  35 ) between the light guide sections and the light coupling element can be air or other gas, this space may be filled with any other substance having a lower index than that of the light coupling element, for example, an adhesive layer such as a UV curing acrylic or epoxy. 
     FIG. 3B  shows another embodiment of a light guide  30   b  of the invention, in which the solid light guiding sections  31  and  32  have been replaced with light pipes  38  and  39 , each having an entrance aperture ( 38   a ,  39   a ), an exit aperture ( 38   b ,  39   b ), and a side wall ( 38   c ,  39   c ) having an interior reflective surface ( 38   d ,  39   d ). Thus, unlike the case of the solid light guide sections of  FIG. 3A , in which rays  36  and  37  are reflected at the interface of side walls  31   c  and  32   c  with the surrounding medium by the phenomenon of TIR, rays  41  and  42  are reflected from the interior reflective surfaces  38   d  and  39   d . In other respects, the operation of the light guide  30   b  is similar to that of  30   a.    
     FIG. 3C  shows yet another embodiment of a light guide  30   c  of the invention in which solid light guide section  44  is positioned with its light guiding direction (defined by the direction of the side wall  44   c ) at an angle of 45 degrees to the light guiding direction of the light guide section  43 , instead of 90 degrees, as is the case in light guides  30   a  and  30   b . As in these previously described embodiments, the TIR faces  45   a  and  45   c  of the light coupling element  45  are parallel to the light guiding direction in the nonadjacent light guide sections  44  and  43 , respectively. However, in the present embodiment, this determines an angle of 135 degrees between the TIR faces  45   a  and  45   c  of the light coupling element  45 . In order that the spaces  46  and  47  between TIR faces  45   a  and  45   c  and the adjacent light guide apertures  43   b  and  44   a  of light guide sections  43  and  44  are of equal thickness d (see  FIG. 5 ) across the width of the apertures, these apertures  43   b  and  44   a  form an angle of 45 degrees with respect to the sidewalls  43   c  and  44   c.    
     FIG. 3C  shows yet another embodiment of a light guide  30   c  of the invention in which solid light guide section  44 , having an entrance face  44   a , an exit face  44   b  and sidewall  44   c , is positioned with its light guiding direction (defined by the direction of the side wall  44   c ) at an angle of 45 degrees to the light guiding direction of the light guide section  43 , instead of 90 degrees, as is the case in light guides  30   a  and  30   b . The light coupling element  45  has a reflecting face  45   b  and TIR faces  45   a  and  45   c . As in these previously described embodiments, the TIR faces  45   a  and  45   c  of the light coupling element  45  are parallel to the light guiding direction in the non-adjacent light guide sections  44  and  43 , respectively. However, in the present embodiment, this determines an angle of 135 degrees between the TIR faces  45   a  and  45   c  of the light coupling element  45 . In order that the spaces  46  and  47  between TIR faces  45   a  and  45   c  and the adjacent light guide apertures  43   b  and  44   a  of light guide sections  43  and  44  are of equal thickness d (see  FIG. 5 ) across the width of the apertures, these apertures  43   b  and  44   a  form an angle of 45 degrees with respect to the sidewalls  43   c  and  44   c . Outer ray traces  48  and  49  indicate the path of a light beam through the light guide. 
     FIG. 3D  shows another embodiment of a light guide  30   d  of the invention in which solid light guide sections  43  and  44  have been replaced by hollow light pipe sections  50  and  51 , having outer walls  50   c  and  51   c , and inner walls  50   d  and  51   d , respectively, which operate in the same manner as the hollow light pipe sections  38  and  39  in light guide  30   b . Light pipe sections  50  and  51  are coupled by solid light coupling element  52 . Outer ray traces  53  and  54  indicate the path of a light beam through the light guide. 
   In accordance with another embodiment of the invention, a light coupling element with four TIR surfaces enables the intersection of two independent light beams without cross coupling.  FIG. 4A  shows such an arrangement  60   a  in which a cube-shaped light coupling element  65  has TIR surfaces  65   a  through  65   d . Four light guide sections  61  through  64  each have an entrance aperture ( 61   a ,  62   a ,  63   a ,  64   a ) and an exit aperture ( 61   b ,  62   b ,  63   b ,  64   b ) and a side wall ( 61   c ,  62   c ,  63   c ,  64   c ). The light guide sections  61  and  63  are positioned with their exit apertures  61   b  and  63   b  adjacent to TIR faces  65   a  and  65   c , forming spaces  66  and  68 , while light guide sections  62  and  64  are positioned with their entrance apertures  62   a  and  64   a  adjacent to TIR faces  65   b  and  65   d , forming spaces  67  and  69 . 
   In operation, the guided light beam indicated by outer ray traces  70  and  71  is guided by light guide section  61  into light coupling element  65 , where it is totally internally reflected by TIR faces  65   c  and  65   d  into light guide section  62 . Simultaneously, the guided light beam indicated by outer ray traces  72  and  73  is guided by light guide section  63  into light coupling element  65 , where it is totally internally reflected by TIR faces  65   a  and  65   b  into light guide section  64 . Thus, two guided light beams can have intersecting paths without cross coupling. 
     FIG. 4B  shows a modification  60   b  of the arrangement of  FIG. 4A  in which two dichroic filter elements  79  and  80  are arranged in a crossed pattern in the light coupling element  78 , having an entrance face  78   a  and exit faces  78   b ,  78   c  and  78   d . Dichroic element  79  reflects red light and transmits green light while dichroic element  80  reflects blue light and transmits green light. Light guide sections  74 ,  75 ,  76 ,  77  each have an entrance face ( 74   a ,  75   a ,  76   b ,  77   a ) and an exit face ( 74   b ,  75   b ,  76   a ,  77   b ).In operation of this light guide  60   b  as a beam splitter, a white light beam indicated by outer ray tracings  81  and  82  is guided by light guide section  74  to light coupling element  78 , where it encounters dichroic elements  79  and  80 , and is split into a green component (ray traces  83  and  84 ), a red component (ray traces  85  and  86 ), and a blue component (ray traces  87  and  88 ), which are guided away from the light coupling element  78  by light guiding elements  75 ,  77  and  76 , respectively. 
   In operation of the light guide  60   b  as a beam combiner, the directions of travel of the guided light beams is reversed, so that green, red and blue components entering light guide section  75 ,  77  and  76 , respectively, are combined into a white light beam by dichroic elements  79  and  80 , and the white light beam is carried away by light guide element  74 . 
     FIG. 4C  shows a modification  60   c  of the arrangement  60   a  of  FIG. 4A  in which a polarizing element  93  is arranged in the light coupling element  92 . Light coupling element  92  has an entrance face  92   a  and exit faces  92   b  and  92   c , and reflecting face  92   d . Polarizing element  93  reflects S polarized (TM) light and transmits P polarized (TE) light. In operation of this light guide  60   c  as a polarizing beam splitter, a light beam indicated by outer ray tracings  94  and  95  is guided by light guide section  89 , having entrance face  89   a  and exit face  89   b , to light coupling element  92 , where it encounters polarizing element  93 , and is split into an S component (ray traces  98  and  99 ), and a P component (ray traces  96  and  97 ), which are guided away from the light coupling element  92  by light guiding elements  91  and  90 , having entrance faces ( 90   a ,  91   b ) and exit faces ( 90   b ,  91   a ), respectively. 
   Many other combinations are possible. For example, in the light guide  60   b , TIR surface  78   b  may be given a reflective surface and light guide section  75  eliminated, so that in the beam splitter mode, white light entering light guide section  74  is split into two components, rather than three. For another example, in any of the embodiments of  FIG. 3 , the reflective surface may be a dichroic surface, so that its reflectivity is selective for a certain wavelength band, enabling a narrowing of the wavelength range of the guided light beam as it passes through the light guide. Thus, for example, white light entering one light guide section can exit the coupled light guide section as red, green or blue light. 
     FIG. 5  is a detailed view of an adhesive layer  500  filling the space between a light guide section  501  and a light coupling element  502  of the invention. The adhesive layer has a thickness d and a refractive index lower than that of the light coupling element  502 . 
     FIG. 6  is a cross section view  600  of light guide section  31 , showing a square shape. Light guide sections of the invention can also have a rectangular or other polygonal shape with an even number of sides. 
   The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims.