Abstract:
An optical system having an optical waveguide for outputting light, a receiver for receiving the light, and redirecting optics for outputting the light from the optical waveguide. The optical system can be used for light illumination. The optical system can further be used for diffusing light in illumination applications by replacing the receiver with a light source such that the light flows in the reverse of the concentration system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of Ser. No. 13/282,147 filed Oct. 26, 2012, which claims the benefit of U.S. Provisional application No. 61,407,772. Ser. No. 13/282,147 is also a continuation-in-part of U.S. application Ser. No. 12/939,348 filed Nov. 4, 2010, which is a continuation of U.S. Ser. No. 12/705,434 filed Feb. 12, 2010, now U.S. Pat. No. 7,925,129, which is a continuation-in-part of U.S. application Ser. No. 12/207,346 filed Sep. 9, 2008, now U.S. Pat. No. 7,664,350, which is a continuation-in-part of U.S. application Ser. No. 11/852,854 filed Sep. 10, 2007, now U.S. Pat. No. 7,672,549, all of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention is directed to the field of optics for light collection and delivery. Applications include concentration of sunlight onto photovoltaic or thermal receivers, and diffusion of light for illumination applications. 
       BACKGROUND AND SUMMARY OF THE INVENTION 
       [0003]    Edge collectors or optical waveguides are used for collection and concentration of light; in particular, sunlight. An edge collector or optical waveguide is defined for this application as an optical device that receives light from a top surface, and delivers the concentrated energy to the edge of the device.  FIG. 1   a  shows a simple schematic of the cross section of an optical waveguide  10 .  FIG. 1   b  is a 3D representation of the same optical waveguide  10 . 
         [0004]    In practice, these types of optical waveguides  10  are generally of the type described in U.S. Pat. Nos. 7,664,350 and 7,672,549. Other types of optical waveguides include luminescent solar concentrators, or dye luminescent solar concentrators.  FIG. 1   c  shows an optical system of the former type. Input light  20  falls on multiple concentrating units  40  across the aperture, and the waveguide  10  collects the concentrated light from all the units and delivers it to an edge  30  of waveguide  10 . 
         [0005]    However, there are many advantages to having a secondary set of optics  50  (see  FIGS. 2 and 3   a ) at the edge  30  to redirect the light  20  in a favorable manner. In  FIG. 2 , the light  20  is delivered to the edge  30  of the waveguide  10  and is redirected approximately 90 degrees towards a receiver  60  placed parallel to the base of the waveguide  10 . The invention articulated herein describes a variety of methods to design these secondary redirecting optics  50 . The invention helps make the optical waveguide  10  more useful. Key commercial criteria for the optical waveguide concentrating systems include compactness, efficiency, level of concentration, and manufacturability. Different methods for the redirection impact these criteria in different ways. 
         [0006]    It should also be noted that the applications for this optical waveguide  10  or device are several. The light energy can be delivered to a variety of receivers.  FIGS. 3   a  to  3   d  show some examples of receivers  60 , including further concentrating or diffusing optics such as lenses, compound parabolic concentrating optics, photovoltaic cells, or heat exchangers which will be described in more detail hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1   a  illustrates a cross-sectional view of a conceptual optical waveguide; 
           [0008]      FIG. 1   b  illustrates a three-dimensional view of a conceptual optical waveguide; and 
           [0009]      FIG. 1   c  illustrates an optical waveguide used in solar concentration; 
           [0010]      FIG. 2  illustrates an embodiment of the optical system; 
           [0011]      FIG. 3   a  illustrates an embodiment with a lens receiver;  FIG. 3   b  illustrates an embodiment with a CPC receiver;  FIG. 3   c  illustrates an embodiment with a photovoltaic cell receiver; and  FIG. 3   d  illustrates an embodiment with a heat exchanger receiver; 
           [0012]      FIG. 4   a  illustrates another embodiment of the optical system;  FIG. 4   b  illustrates another embodiment of the optical system;  FIG. 4   c  illustrates multiple optical systems adjacent each other with redirecting optics; and  FIG. 4   d  illustrates multiple optical systems adjacent each other without redirecting optics; 
           [0013]      FIG. 5   a  illustrates an embodiment where the redirecting optic is integrated into the waveguide; and  FIG. 5   b  illustrates an embodiment where the redirecting optic is separated from the waveguide; 
           [0014]      FIGS. 6   a - b  illustrate an embodiment where the optical system is mirrored about a central axis; 
           [0015]      FIGS. 7   a - c  illustrate embodiments where the redirecting optics modify the level of concentration; 
           [0016]      FIG. 8   a  illustrates a redirecting optic employing total internal reflection; and 
           [0017]      FIG. 8   b  illustrates a redirecting optic employing partial refraction; 
           [0018]      FIG. 9  illustrates a redirecting optic employing reflection; 
           [0019]      FIG. 10   a  illustrates a redirecting optic comprised of a parabolic curve;  FIG. 10   b  illustrates a redirecting optic comprised of a parabolic curve with partial refraction;  FIG. 10   c  illustrates a redirecting optic comprised of a parabolic curve with a partial reflective material; and  FIG. 10   d  illustrates a redirecting optic comprised of a parabolic curve and a flat facet; 
           [0020]      FIG. 11   a  illustrates an embodiment of redirecting optics showing TIR leaking; 
           [0021]      FIG. 11   b  illustrates another embodiment of redirecting optics showing TIR leaking; and 
           [0022]      FIG. 11   c  illustrates another embodiment of redirecting optics showing TIR leaking; 
           [0023]      FIG. 12   a  illustrates an embodiment of redirecting optics with air between the redirecting optics and the receiver; and  FIG. 12   b  illustrates an embodiment of redirecting optics with cladding material between the redirecting optics and the receiver; 
           [0024]      FIG. 13   a  illustrates an embodiment of the optical system;  FIG. 13   b  illustrates an embodiment of the optical system with an angled waveguide; and  FIG. 13   c  illustrates an embodiment of redirecting optics with cladding material between the redirecting optics and the receiver; 
           [0025]      FIGS. 14   a - c  illustrate various embodiments of light pipe redirecting optics; 
           [0026]      FIG. 15   a  illustrates an embodiment of redirecting optics with cladding material between the redirecting optics and the receiver; and  FIG. 15   b  illustrates an embodiment of redirecting optics with cladding material and glass between the redirecting optics and the receiver; 
           [0027]      FIG. 16   a - e  illustrates various embodiments of the optical system; 
           [0028]      FIG. 17   a  illustrates a linearly symmetric optical system;  FIG. 17   b  illustrates an axially symmetric optical system; and  FIG. 17   c - f  illustrates various embodiments of axially symmetric optical systems and associated redirecting optics; and 
           [0029]      FIG. 18  illustrates an embodiment of the optical system constructed for illumination applications. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    In a preferred embodiment, as shown in  FIG. 4   a , light from the waveguide  10  travels roughly perpendicular to the plane of collection. The redirecting optics  50  redirects this light  20  to a receiver  60  so as to change the angle of propagation of the cone of the light  20 . 
         [0031]    In a preferred embodiment, the optical waveguide  10  redirects the light  20  substantially perpendicular to its angle of propagation in the waveguide  10 , as shown in  FIG. 4   b . This enables the receiver  60  to lie parallel to the plane of collection, as in  FIG. 4   c .  FIG. 4   d  shows how the waveguide  10  and receivers  60  interfere with one another if the optical waveguide  10  does not have redirecting optics associated therewith. The “optics layer” (the redirecting optics  50 ) and “receiver layer” (the receiver  60 ) thereby form horizontal slab sections that can be easily assembled and mated together. The two layers can form a laminate construction that is sturdy and durable. Connections between receivers  60  can be more easily made; for example, electrical interconnects between photovoltaic cell receivers. As the receiver  60  (e.g., photovoltaic cell) heats up, having a separate receiver layer enables more effective heat transfer off a back plane heat sink. 
         [0032]    Each of the following sections outlines a specific problem being addressed by the redirecting optics  50  and some preferred methods of solving that problem. 
       1. Redirector as Same or Separate Part 
       [0033]    The redirecting optics  50  can be constructed to be a feature of the same manufactured part as the waveguide optics  10 , as shown in  FIG. 5   a . It can also be a separate part in another embodiment, as shown in  FIG. 5   b.    
         [0034]    Being made in the same part has advantages for the waveguide  10 —less manufacturing steps, no alignment required between parts, and potentially greater efficiency from no losses between interfaces. 
         [0035]    Being made as a separate part for the components of the waveguide  10  and the redirecting optics  60  can help preserve or enhance the level of light concentration. The interface  70  can act as a totally internally reflecting (TIR) surface and thus better contain the light  20  within the waveguide  10 . 
       2. Mirror Image 
       [0036]      FIG. 6   a  shows an optical waveguide  10  and redirecting optics  50  as previously discussed, with the collection area of input light  20  and the height of the optics components noted. 
         [0037]    In  FIG. 6   b , the combination of the waveguide  10  optics, redirecting optics  50 , and the receiver  60  is mirrored about a central axis. The receiver  60  can then be constructed as one contiguous part. The collection area delivering light to a single receiver  60  is thus doubled while the height of the total construction is not changed—therefore the construction is twice as compact for a given area of receiver  60 . 
         [0038]    Additionally, the entire mirrored waveguide and redirecting combination can be manufactured in one part, thereby simplifying manufacturing. 
       3. Secondary Concentration 
       [0039]    The optical waveguide  10  delivers a certain level of concentration, shown as C 1  in the embodiment of  FIG. 7   a , defined by the collection area A 1  divided by the area of the edge A 2 . The redirecting optic  50  receives the light  20  at this level of concentration and may change the level of concentration upon delivery to the receiver  60 . 
         [0040]    The secondary concentration caused by the redirecting optic  50  is shown as C 2  and defined as the ratio of A 2 , the input area for the redirecting optic  50 , and A 3 , the output area for the redirecting optic  50 . The final level of concentration is shown as Cfinal and defined as A 1 /A 3 . 
         [0041]    In  FIG. 7   a , the redirecting optic  50  increases the level of concentration from the optical waveguide  10 . Cfinal is therefore greater than C 1 . When concentration factor is a critical parameter, such as in reducing the area of photovoltaic cell material in order to reduce costs of solar panels, secondary concentration from the redirecting optic  50  can be beneficial. 
         [0042]    In the embodiment of  FIG. 7   b , the redirecting optic  50  reduces the level of concentration from the optical waveguide  10  (Cfinal&lt;C 1 ). In the particular embodiment of  FIG. 7   b , a simple flat facet  80  is disposed at an appropriately shallow angle with respect to the horizontal base of the optical waveguide  10 , or the edge collector, and can deliver a reduction in concentration. In  FIG. 7   c  this embodiment provides a further example. Even though the optical waveguide  10  delivered an appreciable level of concentration, the redirecting optic  80  reduced the final concentration factor to about 2×. This can be beneficial when, for example, it is desired to reduce solar cell material by a factor of two, and also to have large areas of contiguous solar cell, all the while having the overall optical system be highly compact. 
       4. Limitations of Total Internal Reflection 
       [0043]    Total internal reflection (TIR) can be employed to reflect the light  20  for redirection. It is superior to a reflective coating as it is nearly lossless, while a coating will absorb some of the energy falling on it. It is also cheaper as it removes the extra manufacturing step and material cost of a reflective coating. 
         [0044]    However, total internal reflection takes place only for light rays providing angles of incidence larger than the critical angle formed by the refractive indices of the optic and the surrounding material. 
         [0045]    In the embodiment of  FIG. 8   a  is shown a flat redirecting optic  50  at the TIR limit. The angle theta formed between the light ray  90  and the normal to the interface is just larger than the critical angle.  FIGS. 14   a - c  show what happens when the angle of the light ray  90  is smaller than the critical angle—the ray is refracted through the redirecting optic  50  surface and is not delivered to the receiver  60 . A requirement to employ TIR thus places a constraint on how much additional secondary concentration can occur while preserving efficiency. 
       5. Reflective Coatings 
       [0046]    In another embodiment reflective coatings may be employed to concentrate the light further than the TIR limit allows.  FIG. 9  shows this embodiment. In this case, the angle of incidence is similar to that in  FIG. 8   b  where the light  90  was essentially lost. Here, the light ray  90  is instead reflected down towards the receiver  60 . A reflector  100  thus allows the redirecting optics  50  to have a form which provides a steeper incline with respect to the optical waveguide  10  which can enhance secondary concentration without losing light rays through the interface. (However, as described in section 4 hereinafter, the tradeoff on efficiency is that a reflector absorbs some of the energy falling on it). 
         [0000]    6. Curved redirecting optics 
         [0047]    A curved section such as a parabolic section  110  in  FIG. 10   a  can be more effective than a flat surface at delivering secondary concentration. A curve, such as a parabola, is a focusing optical surface and therefore can take a light cone from the optical waveguide  10  and perform a secondary level of concentration on it, delivered to a receiver  60 . 
         [0048]    However, the curve  110  can become steep enough to have the incident angle of light  90  exceed the critical angle for total internal reflection, as shown in  FIG. 10   b . Two solutions are possible. First, a reflective coating  120  may be applied to the section of the curve  110  that would otherwise see light leaking out, as shown in  FIG. 10   c . Second, the parabola  110  can be truncated at the TIR limit, and a flat facet  130  be used at the very same angle as the end of the curve, such that TIR is always achieved along the redirecting optic. This feature is shown in  FIG. 10   d .    
       7. TIR “Leaking” 
       [0049]    In yet another embodiment when the optical waveguide  10  and redirecting optic  60  are mirrored, some leaking of light  140  from the redirecting optic  50  surface may be tolerated, since it is collected by the opposite redirecting surface and delivered to the receiver  60 , as shown in  FIG. 11   a . This can increase the secondary concentration achievable because the redirecting optic  50  surface can be placed at a steeper angle and therefore shrink the required area of the receiver  60 . 
         [0050]    In another embodiment a cascade of facets  160  are possible to take advantage of this effect, with the steeper ones of the facets  160  located closer to the bottom surface, as shown in  FIG. 11   b . Each facet  160  can be placed at the appropriate angle to maximize both concentration and efficiency given the range of angles of light rays incident upon it. Alternatively, a curved surface  170  that approximates the cascade of facets  160  may be applied to achieve the same, as shown in the embodiment of  FIG. 11   c . The redirecting optic  50  can also be comprised of multiple curves, such as parabolic section  170  and an arc  180 , as shown in  11   c.    
       8. Cladding on Base 
       [0051]    In another embodiment if air  190  is used between the base of the optics  50  and the top of the receiver  60 , then light  210  exiting the redirecting optic  50  will refract to increase the cone angles, as shown in  FIG. 12   a.    
         [0052]    A cladding material  200  may be applied to the base of the waveguide-redirecting optic construction, as shown in  FIG. 12   b . The cladding  200  is of a lower refractive index than the medium of the redirecting optic  50 . This allows the cone of light  210  at the receiver  60  to be narrower as the refraction off the base of the optic system is mitigated (the rays  210  will refract by larger angles traveling from an optic to air which has the lowest possible refractive index, versus travelling from an optic to another material). Mitigating the cone angles out of the redirecting optic  50  can help preserve the level of concentration achieved by the overall optical system. 
         [0053]    Cladding will also provide an efficiency advantage. Fresnel reflection occurs at interfaces of different refractive indices, with losses being larger for greater differences in index. Having air between the optic and the receiver  160  will result in the greatest Fresnel reflection losses. 
         [0054]    Cladding can also provide structural and reliability advantages. It can encapsulate a sensitive material that needs environmental protection, like a photovoltaic cell. It can also decouple stresses between the optic and the receiver  160 , for example as a result of differing rates of expansion under temperature increases. 
       9. Angled Light Guide 
       [0055]    In an alternate embodiment the direction of propagation of the light cone in the waveguide  10  need not be exactly perpendicular to the input light, as shown in  FIG. 13   a . In a variation the waveguide  220  may be constructed at an angle to the horizontal, as shown in  FIG. 13   b.    
         [0056]    An angled form of the waveguide  10  is advantageous for the redirecting optic  50 , since it allows for greater secondary concentration to remain with the TIR limit. The reason is because light has a smaller angle of required redirection.  FIG. 13   b  shows this advantage—the light ray  90  can be reflected back in via total internal reflection over a larger angle than as shown in  FIG. 13   a.    
         [0057]      FIG. 13   c  shows the change in angle of facets  230  required when an angled waveguide  220  combines with the cladding  200  on the base. In order to deal with extreme rays reflecting off the focal area  25 , the final facet  24  (“Facet  2 ”) needs to occur at a shallower angle than the previous facet  23  (“Facet  1 ”), otherwise light rays  235  reflected off the focal area will refract through the Facet  2  and escape upwards. 
       10. Redirecting Light Pipes 
       [0058]    An alternative embodiment involves increasing the aspect ratio in order to win greater secondary concentration.  FIG. 14   a  shows the use of a curved light pipe  260  to redirect the light  265 . The aspect ratio of the optical waveguide  10 -redirecting optic  260  is made significantly larger.  FIG. 14   b  shows the same but achieving secondary concentration by tailoring the curved surfaces of the redirecting optics, or the pipe feature type of redirecting optics  260  to taper.  FIG. 14   c  shows a similar approach but with flat facets for the redirecting optics  260  instead of curved sections. 
         [0059]    The pipe feature  260  can in principle achieve greater secondary concentration than the redirecting optics discussed thus far, because it preserves the level of concentration given by the optical waveguide  10 , orients the light cone to face the receiver  60  directly, and then can achieve the maximal level of secondary concentration. Previous redirecting optics faced total internal reflection constraints that prevented achieving the maximum allowable levels of secondary concentration. 
         [0060]    However, previous redirecting optics  260  do retain the compactness of the optical waveguide. Hence the following tradeoffs are seen:
       TIR non-pipe approaches that achieve highest compactness and highest efficiency but not highest concentration   Reflector non-pipe approaches that achieve highest compactness and highest concentration but not highest efficiency   TIR pipe approaches that achieve highest concentration and highest efficiency but not highest compactness       
 
       11. Base Glass 
       [0064]    Another embodiment also increases the aspect ratio somewhat in order to win greater concentration.  FIG. 15   a  shows a design already discussed—the redirecting optics  50  with the cladding layer  200 .  FIG. 15   b  shows how a sheet of optical material like glass  270  can be placed at the base of the waveguide-redirecting optic construction. The additional height enables greater secondary concentration as the light  275  has a greater distance to travel, and the angles of the edge rays are such that greater travel shrinks the final focal area. 
         [0065]    The base glass  270  can also act as a mechanical and environmental barrier, protecting the receiver  60  (e.g., solar cell) from dirt and moisture that may enter the voids in the optical components, and from mechanical stresses from thermal and other expansion and contraction. 
       12. Supporting Optics for Central Redirection 
       [0066]    Since the overall waveguide-redirecting optic construction is designed to efficiently collect light  20  from the top surface, the light  20  falling on the central region of the construction (above the redirecting optic) must also be collected for optimal efficiency. 
         [0067]      FIG. 16   a  shows an embodiment with the complete waveguide 10-redirecting optic  50  combination, including supporting optics for the central redirection. It is designed so that input light  20  falling across the entire front surface of the optical waveguide  10  is delivered to the receiver  60 . 
         [0068]      FIG. 16   b  shows a close-up of the central redirecting region. A light ray  280  from the optical waveguide  10  is depicted, with a path to the receiver  60  as has been discussed previously. 
         [0069]    The question is how light rays  20  from the central redirecting  50  region can make their way to the receiver  60 . Three approaches are possible, preferably in combination. First, the design of sections of the optical waveguide  10  may be modified to accommodate the different angled surfaces on the redirecting optic  50 . In  FIG. 16   c , the system through which the light ray  280  travels has larger lens  290  and facet  300  features to accommodate the tapering of the redirecting optic surface  310  towards the receiver  60 . The light ray  280  therefore travels through a different pathway than the rays  20  from other embodiments described before in the optical waveguide  10  system. 
         [0070]    Second, features can be placed in the top element to direct the light  280  to an appropriate place on the redirecting optic  50  such that it is delivered to the receiver  60 . In  FIG. 16   d , the tooth feature  320  on the base of the top element redirects the light ray  280  such that it intersects with the redirecting optic surface  310  in the region directly above the receiver  60 . Without that tooth feature  320  , the light ray  280  would not hit the receiver  60 , limiting efficiency, or the receiver  60  would have to be wider to accommodate the ray  280 , limiting concentration. 
         [0071]    Third, rays sufficiently near the center of the system of optical components are allowed to pass through with no change in direction. They undergo some refraction upon hitting the redirecting surface optic  310 , but they are sufficiently near the center of the system so as to ensure they end up on the receiver  60 . 
         [0072]    Options two and three above require that the entire redirecting optic  310  be constructed with no reflector—i.e., pure TIR is employed. A reflective coating on the redirecting optic  310  would block incoming rays from the central redirecting region, limiting efficiency. 
         [0073]      FIGS. 16   a - c  combine many elements discussed in the previous sections in a preferred embodiment:
       From Section 1 the redirecting optic  310  is made as one part with the optical waveguide  10 , for simplified manufacturing.   From Section 2 the optical waveguide 10-redirecting optic  50  is mirrored about a central axis, increasing collection area and compactness of the overall device.   From Section 3 the optical waveguide  10  achieves secondary concentration such that Cfinal&gt;C 1 , reducing the receiver  60  area and thereby receiver cost.   From Section 4 the redirecting optic  310  employs total internal reflection to maximize efficiency   From Section 6 the redirecting optic  310  employs a parabolic curve to increase secondary concentration, and then truncates the curve with a flat facet attached to it as the TIR limit is reached. (The parabolic curve may be approximated by several flat facets to achieve a comparable result.)   From Section 7 the redirecting optic  310  allows “TIR leaking”—some rays are allowed to breach the critical angle and refract over to the mirror image surface, where they are nevertheless collected by the receiver  60 . This allows the redirecting surface  310  to have steeper angles enabling further secondary concentration.   From Section 8 cladding  200  is used between the redirecting optic and the receiver, to enhance concentration, efficiency, structural support, and reliability   From Section 9 the optical waveguide  10  is an angled waveguide—i.e., the waveguide  10  is not perfectly perpendicular to the input light  20 . This allows the redirecting optic  310  to have steeper angles which increases secondary concentration achievable.   From Section 12 supporting optics  290 ,  300 ,  320  are constructed in the optical waveguide  10  in the central redirecting region above the redirecting optic  310 . These supporting optics ensure that light incident on the central region is delivered to the receiver  60  through the redirecting optic  310 , maximizing efficiency and concentration. As has been illustrated above, the redirecting optic  50  can be comprised of any combination of a parabolic surface, an elliptical surface, a hyperbolic surface, an arc, a flat reflective surface, a tailored shape reflective surface, a total internal reflecting surface, a component parabolic concentrator optic, a light pipe, and a refractive component.         
         [0083]    The previous sections described ways to design various embodiments of the redirecting optics  50 . The following describe two alternative implementations for the redirecting optics  50 . All of the descriptions in the previous elements apply to the following two implementations. 
       Linearly Symmetrical Optics Versus Axially Symmetrical Optics 
       [0084]    Since the design lies in the cross-section, the optical components may be rendered in a linear extrusion as shown in  FIG. 17   a , or a rotational extrusion as shown in  FIG. 17   b . All the elements described in this application are applicable to either extrusion. 
         [0085]    However, axially symmetric optics in rotational extrusion face an additional challenge.  FIG. 17   c  shows a familiar case—a light ray  20  from the optical waveguide  320  is redirected downwards towards a central receiver  60 . However, in the rotational extrusion, the light ray  20  would have to be in perfect alignment with the radius of the disc in order to strike the redirecting optic  330  for redirection as designed (note that the tip of the redirecting optic  330  comes down to a point in the rotational extrusion). In  FIG. 17   d , a light ray  340  that is slightly off center misses the redirecting optic  330  and does not arrive at the receiver  60 . 
         [0086]    In  FIG. 17   e , this is ameliorated by shifting the axis of rotation away from the tip of the redirecting optic  330 . Thus the light ray  340  faces not a point but a wall  350 . The distance of the axis of rotation from the tip of the optic  360  can be tuned so that substantially all of the waveguide light  20  is captured, maximizing efficiency. The disadvantage is weaker secondary concentration as compared to the linear extrusion of the same cross-sectional design. 
         [0087]      FIG. 17   f  shows an embodiment in another approach to solving the problem. The redirecting optic tip  360  is treated as the axis of rotation. However, the redirecting region  50  has vertical slits  380  sliced into it at periodic intervals about the axis. These slits  380  may be filled with air or a low index cladding material, or the walls of the redirecting region  50  may be coated with a reflector. A ray of light  340  that is off center will hit one of the walls  380  and be reflected back in towards the center. After one or several reflections of the “guide wall” the ray  340  will finally interact with the redirecting surface of the redirecting optic  370 , and be redirected downwards to the receiver  60 . This approach can preserve the secondary concentration achievable in a linear extrusion of the same cross-section. If no reflective coating is used, i.e., total internal reflection is the sole mechanism, then this approach maximizes efficiency as well. 
       Optical Path in Reverse for Light Diffusion or Illumination 
       [0088]    In yet another embodiment the optics described in this application are so far for light collection and concentration. However, the optical system in reverse is an effective diffuser of light. 
         [0089]    In  FIG. 18 , light  400  enters the device from a light source  290  where the receiver  60  has so far been—the central region at the base. The light  400  is redirected into the optical waveguide  10  via the redirecting optics  50 , and the optical waveguide  10  takes the light travelling substantially horizontal and diffuses it to output rays  410  that are substantially normal to the surface of the waveguide  10 . The top surface of the waveguide  10  may be tuned with lenses or other optical elements to emit light at any range of angles. Thus the device is a highly compact and efficient light diffuser. Applications include but are not limited to LED optics, luminaires, spotlights, and automotive headlights and taillights. 
         [0090]    The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated.