Abstract:
A light pipe with directional side-light extraction comprises a light pipe and light-extraction structure applied to the light pipe over only a part of the cross-sectional perimeter of the light pipe and over an active section of the length of the light pipe in which directional side lighting is desired. The light-extraction structure comprises any of (i) material, other than a light-carrying portion of the light pipe or any fluoropolymer cladding on the light-carrying portion, including light-scattering material, (ii) surfaces treated to have light-scattering properties, and (iii) material with a reflective property. The foregoing light pipe eliminates the need for using a reflector, as with fluorescent lamps, by extracting the light only in the desired direction, towards a target area to be illuminated. Other embodiments of the invention promote uniformity in side light emission from a light pipe.

Description:
[0001]    This application is a divisional application of, and claims priority from, co-pending U.S. application Ser. No. 10/796,830 filed Mar. 9, 2004. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a light pipe used for side-light illumination purposes. More particularly, the invention relates to directional extraction of side light from a light pipe. 
       BACKGROUND OF THE INVENTION 
       [0003]    Light pipe is used in two main ways. In an end-light application, the light pipe is optimized to carry light along its length, and transmit it at the output face of the light pipe. In a side-light application, light is extracted out the side of the light pipe and provides illumination along its length. The present invention relates to applying light-extraction means over only a part of the circumference, or cross-sectional perimeter, a light pipe, less than 360 degrees, in order to extract light in a directional manner rather than over the full 360 degrees around the light pipe. 
         [0004]    Often, light extracted from the side of a light pipe over the full 360 degrees around the light pipe is undesirable because a reflector would be needed to redirect a significant portion of the light towards the intended area to be illuminated. Some of the redirected light impinges on the light pipe and may be either absorbed into the light pipe so as to reduce side-light output, or is scattered into unintended directions. This is the same drawback associated with fluorescent lighting and results in an inefficient fixture for delivering light onto the target surface. 
         [0005]    It would thus be desirable to eliminate the inefficient fixture and reflector combination for use with a light pipe by extracting the light only in the desired direction, towards the intended target to be illuminated. 
         [0006]    It would be further desirable to obtain uniformity in light output along a section of light pipe in which side light is extracted. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment of the invention provides a light pipe with directional side-light extraction comprising a light pipe and light-extraction means applied to the light pipe over only a part of the cross-sectional perimeter of the light pipe and over an active section of the length of the light pipe in which directional side lighting is desired. The light-extraction means comprises any of (i) material, other than a light-carrying portion of the light pipe or any fluoropolymer cladding on the light-carrying portion, including light-scattering material, (ii) surfaces treated to have light-scattering properties, and (iii) material with a reflective property. 
         [0008]    The foregoing light pipe eliminates the need for using a reflector, as with fluorescent lamps, by extracting the light only in the desired direction, towards a target area to be illuminated. 
         [0009]    Other embodiments of the invention promote uniformity in side light emission from a light pipe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a simplified, schematic side view of a sidelight illumination system according to the present invention. 
           [0011]      FIGS. 2   a - 2   c  are isometric views of light pipes, with  FIGS. 2   a  and  2   b  showing prior art light pipes, while  FIG. 3   c  shows a light pipe according to the present invention. 
           [0012]      FIGS. 3-12  are side plan views of light pipes showing preferred geometries of light-extraction means,  FIG. 5  being a cross sectional view taken at Lines  5 - 5  in  FIG. 4 . 
           [0013]      FIG. 13   a  is a side plan view of a light pipe, and  FIG. 13   b  is a cross-sectional view of the light pipe of  FIG. 13   a , in which a light-extraction means comprises light-reflective means. 
           [0014]      FIGS. 14   a  and  14   b  are cross-sectional views of a light pipe with a core and clad and a light pipe with a core but no clad, respectively. 
           [0015]      FIGS. 15 and 16  are simplified, isometric views, partially cutaway, of a co-extrusion die for inserting different types of light-extraction means between a core and a clad of a light pipe. 
           [0016]      FIGS. 17   a - 17   b  show fragmentary, partial cross sections of light pipes having different light-extraction means. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    This description describes the three areas of (1) general principles of the invention, (2) preferred geometries of light-extraction means, and (3) methods of manufacturing the geometries of the light-extraction means. 
       1. General Principles of the Invention 
       [0018]      FIG. 1  shows a sidelight illumination system  10  according to the present invention. System  10  includes a light source  12 , a light pipe  14 , and a target surface  16  to be illuminated. Arrows  18  show directional illumination of target surface  16  from a section  20  of light pipe  14 , referred to herein as the “active section.” Section  20  may comprise either a fraction of the length of light pipe  14 , or the entire length of light pipe  14 . 
         [0019]    To put the sidelight illumination system of  FIG. 1  into perspective,  FIGS. 2   a  and  2   b  show prior art light pipes of two different types.  FIG. 2   a  shows a prior art light pipe  22  designed to receive light  24  at one end, transport it through the light pipe with minimum attenuation, and provide illumination  25  at the other end. Light pipe  24  includes a core  26  and a cladding  28  having a lower index of refraction relative to the core. 
         [0020]      FIG. 2   b  shows a length of prior art light pipe  30  constructed for sidelight emission, which is designed to extract light along its length and around its entire circumference. Thus, light  32  entering one end of light pipe  30  is extracted as sidelight  34  around the entire circumference of the light pipe. Residual light  35  passes through the other end of the light pipe. Similar to  FIG. 2   a , light pipe  30  has a core  32  and cladding  36 . 
         [0021]    In accordance with the invention,  FIG. 2   c  shows one example of a light pipe  40  in which light is extracted in a manner that favors one side of the pipe. Thus, light  42  entering core  44  of the light pipe is extracted from the side of the pipe in strip-like region  46  of cladding  48 . Light pipe  40  of  FIG. 2   c  provides directional side-light extraction, and so can be used as light pipe  14  of  FIG. 1 . 
       2. Preferred Geometries of Light-Extraction Means 
       [0022]      FIGS. 3-12  show preferred geometries of light-extraction means according to the invention. In all these figures, the light pipes may have a fluoropolymer cladding over-a core as shown in  FIG. 14   a , for instance, or may be free of a fluoropolymer cladding as shown in  FIG. 14   b , for instance. 
         [0023]      FIG. 3  shows an active section of a light pipe  50  that receives light  52  at one end, extracts light  54  from the side of the pipe along a strip  56  of uniform width that contains light-extraction means (described in the next section). Any light not extracted then exits the other end of the light pipe as light  58 . 
         [0024]    As can be seen in  FIG. 3 , a greater density of light rays  54  are extracted near input light  52  than near output light  58 . This would result from having a uniform density of light-extraction means along the length of strip  56 , and also from the fact that less light is available in the light pipe as the distance from input light  52  increases. To counteract this phenomenon, the light-extraction means could have a differential strength along the light pipe, with more light-extraction capability in the pipe the further away from input light  52 . This phenomenon is illustrated in  FIG. 4 , in which extracted side-light rays  54   a  from light pipe  50   a  along strip  56   a  are uniform in density along the entire length of light pipe shown. As used herein, “uniformity” in side-light emission means that the lumen output as between inlet and outlet portions of a side-light emitting section of the light pipe is within plus or minus 20 percent of the average value of each other. More uniformity than this may also be desirable in some circumstances. 
         [0025]    One way to increase the light-extraction strength along a light pipe to achieve uniform side-light extraction is shown in  FIG. 5 . Thus, in  FIG. 5 , a light pipe  50   b  having a core  60  and a cladding  62  includes a strip  56   b  of light-extraction means—such as a substrate with light-scattering material—interposed between the core and cladding. Strip  56   b  increases in thickness from input light  52  to output light  58 . This achieves a uniform distribution of side-light  54   b  extracted from the light pipe. As used herein, “light-scattering material” includes material that scatters light by reflection, material that scatters light by refraction, or material that scatters light by a combination of refraction and reflection. 
         [0026]    Rather than increasing the density of light-extraction means along the length of a light pipe—or in addition to such increase in density,  FIG. 6  shows a light pipe  50   c  in which a strip  56   c  increases in width along the length of the light pipe. This increases light-extraction efficiency as the strip widens. Thus, the side-light rays  54   c  are uniform along the length of light pipe shown. 
         [0027]      FIG. 7  shows a light pipe  50   d  having a strip  56   d , whose width increases in the direction from input light  52  to output light  58 . Strip  56   d  is similar to strip  56   c  of  FIG. 6 , although strip  56   d  exists only in active section  70 ; that is, a section of the light pipe for side-light extraction. This configuration allows the maximum amount of light to be delivered to a remote area and then be extracted through use of light-extraction means at the desired area to be illuminated. 
         [0028]    As an alternative to providing a single strip of light-extracting material  56  in  FIG. 3 ,  FIG. 8  shows a light pipe  50   e  in which a series of rectangular strips  56   e  of light-scattering means are placed along the light pipe. Using a series of constant-width strips decreases the light extraction along a section of light pipe relative to using a single strip of material with the same light-extraction strength per unit area.  FIG. 9  shows a similar series of strips  56   f  of light-extraction means, but with a higher density the further the distance from input light  52 . 
         [0029]    Similar to  FIG. 8 ,  FIG. 10  shows round configurations of light-scattering means  56   g  along the length of a light pipe  50   g.    
         [0030]    Somewhat similar to  FIG. 9 ,  FIG. 11  shows round configurations of light-extraction means  56   h  at a higher density the further the distance from input light  52 . Light-extraction means  56   h , however, are bunched together in groups of differing sizes to achieve a higher density the further the distance from input light  52 . 
         [0031]      FIG. 12  shows another pattern of light-extraction means  56   i  for a light pipe  50   i  comprising a series of progressively larger triangular shapes. This illustrates that the shapes can be the same, but simply increase in size. 
         [0032]    From the various approaches illustrated herein for achieving an increase the strength of light-extraction the further away from input light, a person of ordinary skill in the art will find combinations of various approaches to be obvious. 
         [0033]      FIG. 13   a  shows a light pipe  50   j  incorporating light-extraction means  56   j  comprising reflective material. Suitable reflective materials include barium sulfate, titanium dioxide, calcium carbonate, zinc oxide or a metallic foil. As shown in  FIG. 13   b , light rays  72  are extracted from light pipe  50   j  by reflection from light reflective material  50   j , shown greatly enlarged. This occurs when the angle of incidence of light (not shown) propagating down a light pipe and striking the reflective surface is high enough to cause the light to be extracted from the opposite side of the light pipe. 
         [0034]    The various geometries of light-extraction means described in connection with  FIGS. 3-4  and  6 - 12  also apply to the embodiment of  FIGS. 13   a - 13   b.    
         [0035]    Unless otherwise noted, the various geometries of light-extraction means described in connection with  FIGS. 3-13  apply to construction of a light pipes having a core with or without a fluoropolymer dad. Thus,  FIG. 14   a  shows a light pipe  73  having a core  74  and fluoropolymer clad  76 , while  FIG. 14   b  shows a light pipe  75  having a core  76  but no fluoropolymer clad. Whether to include a fluoropolymer dad or not depends on the composition of the core and the type of light-extraction means used, which means are discussed under point (3) below. 
         [0036]    To summarize some of the foregoing considerations under this point (2) on preferred geometries of light-scattering means—without referring to the drawings—, by applying a strip of light-scattering material along one side of a light pipe, light can be extracted where the material is located in a directed manner. A uniform piece of constant width and thickness would be the easiest to manufacture. However, over a long length of light pipe, such construction would be difficult to achieve even illumination along the length of the light pipe. 
         [0037]    As the distance along a light pipe from the light input increases, there is less and less light available for extraction. However, by making the light-extraction efficiency in the light pipe increasingly higher, the further the distance from the light input, the number of raw (i.e., non-adjusted) lumens per unit length extracted from the side of the light pipe remains substantially constant along the length and produces uniform illumination. One way to increase light-extraction efficiency is by tapering a strip containing light-scattering material, so that at increasing distances from the light input, the strip increasingly widens to increase its extraction efficiency. Alternate methods of achieving increased extraction efficiency are to vary the density of light-scattering material present within the strip, or to vary the thickness of the strip. A combination of all three of the foregoing approaches may provide the optimum design for a particular application. 
         [0038]    This light scattering strip does not need to cover the entire length of the light pipe. If made from a longer piece of light pipe, the first section can be optimized to transmit light, such as end-light, and then the scattering material could be placed so that it extracts light at the far end of the light pipe. This would produce an integrated light pipe with a section of light pipe optimized to transport light, and a section optimized to extract light towards a target area. Several pieces of the light-scattering material could be placed along the length of a light pipe to produce more than one area of side illumination along the length of a long light pipe. 
         [0039]    It some cases a single run of light-scattering material may extract too much light too quickly or in an undesirable distribution. To avoid this, multiple smaller pieces of light scattering strips may be applied in various patterns to produce the desired output distributions. 
         [0040]    These light-scattering materials could be applied to many various types of light pipes. 
       3. Methods of Manufacturing the Above-Described Geometries 
       [0041]    Light-extraction means of the invention include (i) material inserted between the core and clad of a light pipe, (ii) surfaces of the core of a light pipe treated to have light-scattering properties. 
         [0042]    As to (i) material inserted between the core and clad of a light pipe, co-extrusion die  80  of  FIG. 15  could be used.  FIG. 15  shows a reservoir  82  for material for a clad  84  of a light pipe  86 , but omits a reservoir for a core  87  for simplicity. A nozzle for core  87  is shown at  87   a . Clad  84  is shown partially cutaway. In a molten state, the clad is shown at  85 , shown partially cutaway. At this point, molten clad  85  has just been injected from a nozzle  85   a . A strip  88  of material which may include regions  90  of light-extraction means, such as light-scattering material, is inserted between core  87  and clad  84  in a co-extrusion process. Alternatively, strip  88  could comprise reflective material for the embodiment described above in connection with  FIGS. 13   a  and  13   b.    
         [0043]      FIG. 16  shows a co-extrusion die  94  for extruding light-extracting material  96  between a core  98  and clad  100 . In a molten state, the clad is shown at  101 , and is partially cutaway. At this point, molten clad  101  has just been injected from a nozzle  101   a , and molten core material  99  for forming core  98  has just been injected from a nozzle  99   a .  FIG. 16  shows a reservoir  82  for material for clad  100 , but omits a reservoir for core  98  for simplicity. Multiple streams  102  of material form light-extracting material  96 , which streams can be analogized to the operation of an ink-jet printer. Material  96  may comprise, as shown, a series of strips of material whose length—and hence density and light-extraction effectiveness—vary along the length of the light pipe. More generally, the light-extracting material  96  could be co-extruded in the desired shape, size, thickness, and/or density between the core and cladding material. 
         [0044]    Two alternatives for extruding light-scattering material between the cladding and core are, first, that light-scattering material may be extruded as part of the cladding material. This can be done using multiple streams (not shown) of dad material, similar to the multiple streams of light-extracting material  102  in  FIG. 16 . In this case co-extrusion of the core material is not necessarily needed, as the cladding and sheathing could be extruded as a hollow tube with the desired pattern of scattering material in the cladding already. The core material could then be poured into the tube and the light pipe would be cast in the traditional manner. Second, a strip or strips of material (not shown), similar to strip  88  shown in  FIG. 15 , with regions  90  of light-scattering means, could be inserted into a preformed cladding in tubular form. 
         [0045]    For light pipes which do not have a cladding, light-scattering material can be applied in the proper size and shape with a simple adhesive sticker (not shown) that is adhered to the light pipe. Alternatively, the light pipe&#39;s surface could be etched (i.e., roughened) by mechanical or chemical means, or even painted to produce the desired pattern in the surface of the light pipe. Further, organic solvents in oil-based paints can chemically etch the surface of a polymer light pipe to create light scattering, in addition to any light-scattering properties of the paint itself. 
         [0046]    Finally,  FIGS. 17   a - 17   b  show some of the different types of light-extraction means discussed in this specification. 
         [0047]      FIG. 17   a  shows a light pipe  120  with an etched, or roughened, surface  122  of a core  124 . Light rays  126  reaching roughened surface  122  are extracted from the side of the light pipe. 
         [0048]      FIG. 17   b  shows a light pipe  130  with a paint layer  132  on a core  134 . Paint layer  132  contains light-scattering material, such as titanium dioxide or barium sulfate. 
         [0049]    While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.