Abstract:
A light pipe with side-light extraction by light pipe-surface alteration includes an optical light pipe with a plastic light-carrying portion covered with a fluoropolymer cladding. A plurality of light-extraction devices is spaced along an active section of the light pipe for emission of side light over only a range from about 2 to 270 degrees of the cross-sectional circumference of the light pipe in the active section. The light-extraction devices have inlets passing through the cladding and optically contacting the plastic light-carrying portion. Another form of light pipe with side-light extraction by light pipe-surface alteration includes an optical light pipe with a plastic light-carrying portion. An active section of the light pipe for side light emission comprises an axisymmetrical change of area of cross section of the light pipe that generally diminishes from an inlet end of the active section, through the active section.

Description:
[0001]    This application claims priority from Provisional Application No. 60/453,368 filed on Mar. 10, 12003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to light pipes including means for extracting light from the side of the 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 the light is extracted out the side of the light pipe and provides illumination along its length. Often, light extracted from the side of a light pipe over the full 360 degrees about the light pipe axis 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 desired surface.  
           [0004]    It would 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 a target area to be illuminated.  
           [0005]    Alternatively, in a second embodiment of the invention, with a light pipe lacking a fluoropolymer cladding, it would be desirable to provide novel ways of extracting the light from the side of the light pipe.  
         SUMMARY OF THE INVENTION  
         [0006]    A first embodiment of the invention provides a light pipe with side-light extraction by light pipe-surface alteration, which includes an optical light pipe with a plastic light-carrying portion covered with a fluoropolymer cladding. A plurality of light-extraction devices is spaced along an active section of the light pipe for emission of side light over only a range from about 2 to 270 degrees of the cross-sectional circumference of the light pipe in the active section. The light-extraction devices have inlets passing through the cladding and optically contacting the plastic light-carrying portion.  
           [0007]    The foregoing light pipe eliminates the inefficient fixture and reflector combination of fluorescent lamps, for instance, by extracting the light only in the desired direction, towards a target area to be illuminated.  
           [0008]    A second embodiment of the invention provides a light pipe with side-light extraction by light pipe-surface alteration, which includes an optical light pipe with a plastic light-carrying portion. An active section of the light pipe for side light emission comprises an axisymmetrical change of area of cross section of the light pipe that generally diminishes from an inlet end of the active section, through the active section the further the distance along the light pipe from a light source.  
           [0009]    The foregoing embodiment of the invention provides novel ways of extracting the light from the side of the light pipe. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a simplified side view of a sidelight illumination system in accordance with the present invention.  
         [0011]    [0011]FIG. 2 is an enlarged, perspective view, partially cutaway, of a light pipe incorporating a series of optical elements for directional side light-extraction.  
         [0012]    [0012]FIG. 3 a  is a side plan view of a light pipe having optical elements as in FIG. 2 but arranged in a different pattern.  
         [0013]    [0013]FIG. 3 b  is a side plan view of a light pipe having optical elements as in FIG. 2 but having differing sizes.  
         [0014]    [0014]FIG. 4 a  is an enlarged, perspective view, partially cutaway, of a light pipe incorporating pieces of material with a refractive index at least as high as that of the core of the light pipe for directional side light-extraction.  
         [0015]    [0015]FIG. 4 b  is a fragmentary, perspective view showing an alternative piece of material for use in the light pipe of FIG. 4 a.    
         [0016]    [0016]FIG. 5 a  is a side plan view of a light pipe incorporating pieces of material as in FIG. 4 a  but that are arranged in a different pattern.  
         [0017]    [0017]FIG. 5 b  is a side plan view of a light pipe incorporating pieces of material as in FIG. 4 a  but that have differing sizes.  
         [0018]    [0018]FIG. 6 is a perspective view of a light pipe having discrete regions in which side-light is extracted in accordance with a second embodiment of the invention.  
         [0019]    [0019]FIG. 7 is a perspective view of a light pipe having a tapered region for extracting side light in accordance with a second embodiment of the invention.  
         [0020]    [0020]FIG. 8 is a fragmentary side view of a portion of a tapered region of FIG. 6 or  7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    This description covers the three points of (1) a first embodiment of the invention in which side light is extracted only in a desired direction, (2) a second embodiment in which side light is extracted in a novel way around the entire 360 degree about the light pipe, and (3) manufacturing details for the foregoing embodiments.  
       1. First Embodiment—Directional Side-Light Extraction  
       [0022]    [0022]FIG. 1 shows a sidelight illumination system  10  in which a light source  12  delivers light to a light pipe  14 , which has a section  15  that does not emit side light and an active section  16  for emitting side light. In a first embodiment of the invention, the side light emitted shines in the direction of arrows  18  toward a target area  20  to be illuminated. In a second embodiment of the invention, the side light is directed around the full 360 degree swath around the longitudinal axis of the light pipe in a novel manner. In either embodiment, active section  16  for emitting side light comprises either a fraction of, or the entirety of, light pipe  14 . In the latter circumstance, light pipe  14  does not include a section  15  that does not emit side light.  
         [0023]    [0023]FIG. 2 shows a light pipe  14  with an active section  16  for side-light emission. Active section  16  contains a series of optical elements  24  that penetrate a fluoropolymer clad  26  of the light pipe and optically contact a core  28  of acrylic polymer, for instance, of the pipe. Such optical contact may be better appreciated from the cutaway portion  28   a  of the core. By “optically contact” is meant that elements  24  are connected with the core in such a way that a light ray (not shown) can travel effortlessly from core to optical element without passing through an intermediate material having a refractive index different from that of the core or of the optical element. Elements  24  may be connected to the core with an appropriate index-matching adhesive. Elements  24  may be hollow and with or without an interior reflective surface  25 , or they may be solid. Preferably, they are made by die casting or injection molding of plastic for economy.  
         [0024]    [0024]FIG. 2 also shows how a light ray  30  travels down light pipe  14  with optical elements  24  present. When light ray  30  makes contact with an optical element  24 , it is extracted out the side of the pipe and is directed according to the shape of the optical element. The collective distribution of side light in active section  16  is determined by the shape and location of optical elements  24  along the length of active section  16 . The shape and concentration of the elements could be constant or vary along the length of the light pipe.  
         [0025]    Regarding FIG. 2, for a typical light pipe  14  having a 6- to 25-mm core diameter (e.g., 19 mm), an optical element may have a diameter (along the length of the pipe) of about 5 mm. Further, optical elements  24 —only a few of which are shown—might occupy a range of between about 2 and 270 degrees around a longitudinal axis of the light pipe. More preferably the range is below about 180 degrees, more preferably below about 90 degrees, and still more preferably below about 45 degrees. On the other hand, the range is preferably greater than 10 degrees, and more preferably more than about 30 degrees. One shape for an optical element would be that of a so-called angle-to-area converter, in which each element increases in cross-sectional area from inlet to outlet in such manner as to reduce the angle of light passing through the device. A preferred angle-to area converter comprises a compound parabolic concentrator (CPC) shape. A CPC shape is described in detail in, for instance, W. T. Welford and R. Winston, High Collection Nonimaging Optics, New York: Academic Press, Inc. (1989), chapter  4  (pp. 53-76).  
         [0026]    To achieve uniform side-light emission in active section  16 , optical elements  24  may be distributed in a sequentially increasing density along the active section, from the inlet to such section (which receives light ray  30 ). For instance, FIG. 3 a  shows such an arrangement of sequentially increasing density of optical elements  24   a  in active section  16  of light pipe  14 . Alternatively, as shown in FIG. 3 b , to achieve uniform side-light emission, the sizes of optical elements  24   b  may increase along the length of active section  16  of light pipe  14  from inlet to outlet. The approaches of FIGS. 3 a  and  3   b  can be combined to achieve uniform side-light emission. 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. The increases in size or density of the optical elements does not need to occur along the entire length of the active side-light section.  
         [0027]    Beneficially, the optical elements of FIG. 2, for instance, can be concentrated in a relatively short section of the light pipe, so that the elements extract a majority of light from the light pipe over a relatively short section. This creates a bright region of directed illumination.  
         [0028]    [0028]FIG. 4 a  shows a light pipe  14  having pieces  40  of material in active section  16  for side-light emission. For a typical light pipe  14  having a 6- to 25-mm core diameter (e.g., 19 mm), a piece  40  may have a dimension (along the length of the pipe) of about 5 mm. Pieces  40 —only a few of which are shown—might occupy a range of between about 2 and 270 degrees around a longitudinal axis of the light pipe. More preferably the range is below about 180 degrees, more preferably below about 90 degrees, and still more preferably below about 45 degrees. On the other hand, the range is preferably greater than 10 degrees, and more preferably more than about 30 degrees. Pieces  40  of material penetrate clad  42  and optically contact core  44 , of acrylic polymer, for instance, as shown best at the cutaway portion  44   a  of the core. The term “optically contact” is defined above. Preferably, pieces  40  optically contact the core in the same way as described above with respect to optical elements  24  of FIG. 2. Pieces  40  have an index of refraction at least as high as that of core  44 , and may comprise die-cast, or injection-molded, plastic such as acrylic polymer  24 . When a light ray such as  46  travels down light pipe  14  and reaches the interface between core  44  and a piece  40 , the piece extracts the light ray by refraction from the side of the light pipe.  
         [0029]    In FIG. 4 a , the distribution of side light from light pipe  14  depends on the geometry of the pieces  40 , their index of refraction, and their location. The shape and concentration of pieces  40  could be constant or vary along the length of the light pipe. Preferably, surfaces  40   a  of pieces  40  that face radially away from the light pipe are rectangular (e.g., square). Preferably, the side surfaces of the pieces, e.g., surfaces  40   b , form right angles with surfaces  40   a , so as to limit total internal reflection within the pieces, which might reflect the light back into the light pipe. Preferably, surfaces  40   a  constitute flat surfaces, parallel to the longitudinal axis of light pipe, as shown in FIG. 4 a.    
         [0030]    Alternatively, FIG. 4 b  shows a light pipe  14  having a piece  41  of material that is cylindrical, and oriented orthogonal to the length, or longitudinal axis, of the light pipe. Preferably, piece  41  has a flat, upper surface  41   a , as shown, that is parallel to the longitudinal axis of the light pipe. Of course, more than one piece  41  may be used.  
         [0031]    To achieve uniform side-light emission in active section  16 , pieces  40  may be distributed in a sequentially increasing density along the active section, from the inlet of such section (which receives light ray  46 ). For instance, FIG. 5 a  shows such an arrangement of sequentially increasing density of pieces  40  in active section  16  of light pipe  14 . Alternatively, FIG. 5 b  shows pieces  40  that increase in size from inlet to outlet of active section  16  to achieve uniformity of light emission. Or, some combination of the approaches of FIGS. 5 a  and  5   b  could be used to achieve uniformity of side-light emission. The increases in size or density of the optical elements does not need to occur along the entire length of the active side-light section. The foregoing distribution of pieces  40  applies equally well to pieces  41 , for instance.  
         [0032]    With a predetermined placement of pieces  40  material along the length of light pipe  14 , side-light section  16  extracts a given amount of light along the length of the light pipe which is directed towards a surface to be illuminated. This is useful in illuminating a specific surface (e.g.,  20 , FIG. 1).  
       2. Second Embodiment—Novel Side-Light Extraction for 360° 
       [0033]    A second embodiment of the invention concerns a novel way to extract light from the full 360 degrees about the longitudinal axis of a light pipe.  
         [0034]    In general terms, light can be extracted from a light pipe by altering the shape of the light pipe and creating points or areas where light traveling in the light pipe will exceed the critical angle for total internal reflection (TIR) at the surface of the core. Light exceeding the critical angle for TIR will exit from the side of the light pipe instead of transmitting along the length of the pipe. Light can be extracted in a uniform and even manner along the length of a given section of light pipe, through suitable selection of the spacing and size of predetermined decreases in diameter. At each area where the diameter is decreased (in a stepwise manner), light will be removed from the light pipe. Design of these areas-which will be apparent to those of ordinary skill in the art from the present specification-will dictate the amount of light extracted at each diameter decrease as one moves along the length of the light pipe.  
         [0035]    Thus, FIG. 6 shows a light pipe  50  having respectively uniform cross-sectioned regions  52   a ,  52   b  and  52   c , and linearly tapered regions  54   a  and  54   b  in side-light extraction areas  16   a  and  16   b . Light pipe  50  comprises a plastic core, such as acrylic polymer, and is free of a fluoropolymer clad. Instead, ambient air  56  with a lower refractive index than the plastic core enables the light rays to totally internally reflect (TIR) as they propagate down the light pipe, at least in regions  52   a - 52   c  of uniform cross-section.  
         [0036]    A light ray  57  entering region  52   a  reflects on the side of regions  52   a  and  52   b  by total internal reflection (TIR), but upon reaching tapered region  54   a  the ray exceeds the critical angle for TIR and is extracted from the side of the light pipe. In this way, tapered regions  54   a - 54   b  cause light to be extracted from the side of the light pipe. A coating (not shown) such as an anti-reflection coating may be applied to tapered regions  54   a  and  54   b  to aid the extraction of light. Alternatively, a coating (not shown) could be applied to these tapered regions to hinder the extraction of light. Such coatings will be apparent to those of ordinary skill in the art based on the present specification.  
         [0037]    In the embodiment of FIG. 6, the distribution of side light is determined by length of the transitions of tapered regions  54   a  and  54   b  from larger to smaller diameters, the change in diameter of light pipe  50  before and after the transition, and the frequency of such transitions along the length of the light pipe. The shape and concentration of the transitions could be constant or vary along the length of the light pipe.  
         [0038]    [0038]FIG. 7 shows a light pipe  60  having a region  62  with a downwardly tapering cross section in section  16  for side-light emission. The taper in region  62  is non-linear, preferably to achieve uniformity in side-light emission. Light pipe  60  operates in similar manner to light pipe  50  of FIG. 6, but instead has one continuous taper in region  62 . Portions of region  62  with the greatest taper extract more light assuming the same input light level. However, as light is extracted along the length of a light pipe, less light is available to be extracted, and hence an increasingly greater taper is desired the greater the distance from an inlet to a side-light emitting section.  
         [0039]    Light pipe  50  of FIG. 6 could incorporate non-linear tapers such as shown in FIG. 7, to achieve a desired light output.  
         [0040]    [0040]FIG. 8 shows a portion of a tapered region  70 —from FIG. 6 or  7 —on which a coating  72  which either hinders light emission or helps light emission. Coating  72 , which may be placed on selected tapered regions, gives the designer an additional factor to control light emission. Selection of a suitable coating  72  will be apparent to persons of ordinary skill in the art.  
         [0041]    Two issues arise for selecting a discrete tapers versus a continuous taper for the light pipe of FIGS. 6 and 7. In a light pipe with a relatively short length (e.g., 1 meter) of active side-light section, for instance, it would be desirable for uniform output to have a non-linear taper over the entire active section. However, if discrete points of light are desired, such as in a long ceiling fixture, the discrete tapers would be required. For uniformity of the amount of light extracted from various points of extraction, however, the amount of taper or the linearity of the taper may need to be varied according to the principles expressed herein for both the first embodiment and the current (second) embodiment. Thus, for instance, with reference to FIG. 5 a , the density of spacing of regions of side-light extraction can become higher, the further the distance from the light input.  
       3. Manufacturing Concerns  
       [0042]    Optical elements  24  of FIG. 2 and pieces  40  of material of FIG. 4 a , for example, could be manually inserted onto a core of a light pipe. Preferably, however, optical elements  24  and pieces  40  are inserted onto the core of a light pipe in a continuous extrusion process for making light pipe. A cross-head die or co-extrusion die could be used, for instance. In such process, a wheel (not shown) could punch holes through still-molten clad material, downstream of the die to remove the clad. A second wheel (not shown) then continuously positions optical elements or pieces of material directly on the still-molten core, downstream of the die. At this point, the core is still relatively fluid, so the elements or pieces can adhere, preferably directly to core. Thereafter, the core could undergo further steps such as cross-linking if that process has not occurred earlier.  
         [0043]    The second embodiment of the invention, involving multiple discrete downward taperings, or a single continuous, downward tapering, of the cross section of a light pipe can be made by casting or injection molding. This also applies to light pipes such as pipe  14  of FIG. 1 that includes a section  15  that does not emit side light and an active section  16  for emitting side light.  
         [0044]    While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled 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.