Abstract:
An efficient downlight system for directing light includes a light source and a generally tubular, hollow coupling device with an interior light-reflective surface for receiving light from the source at an inlet and transmitting it as a generally diverging light beam through an outlet. The coupling device is shaped in accordance with non-imaging optics and increases in cross sectional area from inlet to outlet in such manner as to reduce the angle of light reflected from the surface as it passes through the device. A thermal-isolating region has an inlet positioned in proximity to an outlet of the coupling device and has an outlet for passing light to an optical member. An arrangement for splitting the light from the outlet of the thermal-isolating region comprises a plurality of light guides. Each light guide includes an inlet end for receiving light from the thermal-isolating region and an outlet end for directing light to a remote location. The inlet ends are substantially coplanar with each other and form a substantially solid shape. The coupling device and thermal-isolating region are shaped so as to distribute a respective, substantial (i.e., useful) amount of light to each of the plurality of light guides. Efficiencies of about 30 lumens per watt may be achieved.

Description:
This is a continuation-in-part of application Ser. No. 09/561,365; filed on Apr. 28, 2000 for “Efficient Fiberoptic Directional Lighting System,” by the Roger F. Buelow II, John M. Davenport and Juris Sucls, the same inventors as for this application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an optical system including an arrangement for splitting light into a plurality of light guides for directing light, e.g., downwardly from a ceiling fixture. 
     BACKGROUND OF THE INVENTION 
     Fiberoptic systems offer many advantages over conventional electric lighting directional lighting systems. These include delivery of light without the heat generated by the light source, the absence of ultraviolet light, controllability, the use of simple and compact lighting fixtures, the absence of electrical wiring at the lighting point, increased life, etc. Unfortunately, improved efficacy over conventional lighting is not yet one of the advantages of commercially available fiberoptic systems. The following example illustrates the present situation: A conventional MR-16 Track lighting system using four 50-watt light sources would consume 200 watts (lamps) and 40 watts (transformer power supply) while delivering about 600×4 (or 2400) lumens, for an overall delivered system efficacy of about 10 lumens per watt. State of the art 150-watt metal halide fiberoptic systems might deliver 363 lumens in each of four remote points (e.g., 3 meters) consuming 150 watts (lamp) and 60 watts (power supply and fan) for total efficacy of about 7 lumens per watt. 
     Because of increased international concern over reduction in energy usage—and thus the desire to foster the use of energy efficient lighting—as well as the practical concern of lowering operating costs, it would be desirable for fiberoptic systems to achieve increased efficiency over conventional directional lighting sources. This should be possible, in principle, since metal halide light sources now commonly in use in fiberoptic applications typically have four times the efficiency of conventional halogen light sources. The inherent efficiency gain in using a metal halide light source, however, is lost in the inefficiencies of conventional imaging collection (e.g., elliptical reflector) and distribution approaches (e.g., bundled fibers) used in the architecture of such systems. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the invention comprises an efficient system for directing light, which includes a light source and a generally tubular, hollow coupling device with an interior light-reflective surface for receiving light from the source at an inlet and transmitting it as a generally diverging beam through an outlet. The coupling device is shaped in accordance with non-imaging optics and increases in cross sectional area from inlet to outlet in such manner as to reduce the angle of light reflected from the surface as it passes through the device. A thermal-isolating region has an inlet positioned in proximity to an outlet of the coupling device and has an outlet for passing light to an optical member. An arrangement for splitting the light from the outlet of the thermal-isolating region comprises a plurality of light guides. Each light guide includes an inlet end for receiving light from the thermal-isolating region and an outlet end for directing light to a remote location. The inlet ends are substantially coplanar with each other and form a substantially solid shape. The coupling device and thermal-isolating region are shaped so as to distribute a respective, substantial (i.e., useful) amount of light to each of the plurality of light guides. 
     The foregoing system typically achieves a high degree of efficacy (e.g., 30 lumens per watt) while being compact. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side plan view, partially in block form, of a fiberoptic lighting system according to the invention. 
     FIG. 2 is a perspective view of a fiberoptic arrangement for efficiently splitting light from a thermal-isolating region. 
     FIG. 3 is a perspective view of the fiberoptic arrangement shown in FIG. 2, partially cut away. 
     FIG. 4 is an end view of thermal-isolating region of FIG. 3 for illustrating considerable uniformity of light intensity produced with the coupling devices of FIG.  1 . 
     FIGS. 5A-5D are plan views of inlet ends of light guides. 
     FIG. 6 is an end view of a prior art arrangement of a bundle of round fibers. 
     FIG. 7 is a side plan view of light guide  28  of FIG.  2 . 
     FIG. 8 is a perspective view of a plurality of light guides according to a different embodiment of the invention. 
     FIG. 9 is a perspective, exploded view of a light guide with a straight side and a receiving light guide of round cross section, each light guide being partially cut away. 
     FIG. 10 is a simplified view of the two light guides of FIG. 9 placed against each other. 
     FIG. 11 is a perspective view of a collar holding the different light guides of FIG.  9 . 
     FIG. 12 is a perspective view illustrating a preferred method of making a light-splitting arrangement for use in the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a fiberoptic lighting system according to the invention. The lighting system employs a light source  10  and one or more light coupling devices  12  and  14 . Light source  10  may be a metal halide lamp as shown, or a filament-type halogen lamp, or an electrodeless lamp, by way of example. 
     Each of coupling devices  12  and  14  is generally tubular and has a respective, interior light-reflecting surface  12   a  or  14   a  for receiving light at an inet end, nearest the light source, and for transmitting it to an outlet end. The coupling devices may adjoin each other at boundary  13 , with divergence to accommodate the upper and lower ends of light source  10 . Each coupling device increases in cross-sectional area from inlet to outlet in such manner as to reduce the angle of light reflected from its interior surface as it passes through the device, while transmitting it as a generally diverging light beam through the outlet. By “generally diverging” is meant that a substantial number of light rays diverge from main axis  15  of light propagation, although some rays may be parallel to the axis. Preferably, substantially all cross-sectional segments of surfaces  12   a  and  14   a  orthogonal to main axis  15  substantially conform to a compound parabolic collector (CPC) shape. A CPC is a specific form of an angle-to-area converter, as 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). 
     Traditionally, reflectors (not shown) control light from light sources in a so-called “imaging” method. Elliptical reflectors, for example, image the light source, positioned at a first focus of the reflector, onto a second focus. The controlled light converges from the surface of the reflector to the second focus as the light exits the reflector. Parabolic reflectors are another example of optics using imaging. In a parabolic reflector, the controlled light is collimated so that light rays exit in a generally parallel fashion. In contrast, the coupler of the present invention uses “non-imaging” optics, and, in preferred embodiments, realizes small size and superior light-mixing properties possible with such optics. As the light leaves a non-imaging collector (e.g., coupling device  12  or  14 ), most of the light is controlled so as to be generally diverging at a directionally useful angle (typically up to 35 degrees) as it leaves the reflector. This is an important aspect of a lighting system since the light is most highly concentrated at the exit of the non-imaging collector (e.g., coupling device  12  or  14 ). In contrast, in an elliptical system the light is most highly concentrated at the second focus. For a parabolic system, the light concentration is practically the same wherever it is collected. Although the light emitted by a parabolic system may have a high angular uniformity, its imaging quality typically precludes high spatial uniformity in light intensity (and color as well for discharge sources). 
     Light rays (not shown) passing through coupling devices  12  and  14  are respectively received by thermal-isolating regions  16  and  18  before reaching the diagrammatically shown fiberoptic splitting arrangements  20  and  22  for directing the light to desired locations. Thermal-isolating regions may comprise generally tubular rods  16   a  and  18   a  of quartz, by way of example, and air gaps  16   b  and  18   b.    
     Fiberoptic arrangement  20  or  22  (FIG. 1) may be realized, by way of example, as arrangement  24  shown in FIG.  2 . Arrangement  24  comprises two light guides  26  and  28  that may be symmetrical with each other along a main direction of light propagation. The inlets of the guides are preferably aligned with the outlet of the thermal-isolating region about a main axis (not shown) of light propagation. The light guides are preferably flexible. For protection against combustion as required by various building codes, one or more light guides (e.g.,  26  and  28 , FIG. 2) can be sheathed with aluminum or other fire-resistant material. 
     FIG. 3 shows details of inlet ends  26   a  and  28   a  of light guides  26  and  28 . Preferably, these ends are substantially coplanar with each other and form a substantially solid shape (e.g., generally circular) for receiving light from thermal-isolating region  16  (FIG.  2 ). By including separate light guides, arrangement  24  avoids the difficulty of making a properly formed V-shape  30  where the light guides separate from each other where the light guides are integrally formed with each other. 
     Splitting arrangement  24  can efficiently and reliably split light between light guides  26  and  28  owing to considerable uniformity of light intensity provided by coupling devices  12  and  14  (FIG.  1 ). FIG. 4 shows an outlet end of thermal-isolating region  16 , with a center  17   a.  At half radius  17   b  from the center, the light intensity (1) is at least about 60 percent of the maximum light intensity and (2) may reach upwards of 75 percent; (3) is at least about 60 percent of the light intensity at the center; (4) is typically greater than the average light intensity across the entire outlet of the thermal-isolating region; and (5) typically is symmetrically distributed about the center. Further, the uniformity of light intensity is independent of the length of thermal-isolating region  16 . 
     In contrast, the light intensity for a light system (not shown) using a conventional elliptical reflector at half radius from a beam of light is typically no greater than about 50 percent of the maximum light intensity. Further, the point of maximum intensity is often displaced from the center of the beam in the conventional arrangement due to assembly tolerances. Additionally, non-uniformity in light intensity can arise due to focusing and defocusing that may alternately occur along the length of a thermal-isolating region such as quartz (not shown) used with the conventional reflector. Although very careful selection of the length of such region (which adds another manufacturing tolerance difficulty) may reduce such non-uniformity, it is often difficult to eliminate. In addition to precluding reliable light-splitting, such non-uniformity may cause excessive, localized heat that can damage a plastic light guide. 
     The considerable uniformity of light intensity allows splitting arrangement  24  (FIG. 3) to efficiently and reliably split light into light guides  26  and  28 . FIGS. 5A-5D show preferred shapes of inlet ends to light guides for achieving a reliable split of light among the guides. All shapes form a substantially solid shape and preferably cover at least about 90 percent of the outlet of the thermal-isolating region (FIG.  2 ). FIG. 5A shows above-described inlet ends  26   a  and  28   a  arranged symmetrically about center  32   a  of the generally circular (e.g., oval or circular) shape formed. A respective perimeter portion of each light guide substantially coincides with center  32   a . The same is true for the  3 ,  4  and  6  light guides shown in FIGS. 5B,  5 C and  5 D with respect to centers  32   b ,  32   c  and  32   d . This tends to assure that the light received by each light guide is at least substantially proportional to the ratio of its inlet area to the combined areas of the light guides. 
     The inlets shown in FIG. 5A join each other at abutting planes. The inlets of FIG. 5A each have a cross section that is substantially one-half of a substantially solid shape; the inlets of FIG. 5B, one-third of a substantially solid shape; the inlets of FIG. 5C, one-fourth of a substantially solid shape; and the inlets of FIG. 5D, one-sixth of a substantially solid shape. 
     Preferably, the light guides of FIGS. 5A-5D are substantially arranged symmetrically about the respective centers, with the light guides covering more than approximately 90 percent of the outlet of the thermal-isolating region (e.g.,  16 , FIG.  2 ). Preferably, the inlet areas of the light guides for each of FIGS. 5A-5D are substantially the same to provide substantially equal light. 
     The inlets shown in FIGS. 5A-5D could be individually clad with a protective covering, and each inlet and associated light guide could be hollow. 
     The preferred inlet shapes of FIGS. 5A-5D (and obvious variations) cover more area of the outlet of a thermal-isolating region than would occur through the use of a bundle of round fibers. FIG. 6 shows a prior art bundle  36  of round fibers assumed to be without individual claddings, with a thermal-isolating region  37  shown in dashed lines. The voids between the fibers prevent the fibers from covering more than about (3*Π)/6 (i.e., about 90 percent) of the outlet of a thermal-isolating region. The voids likewise limit the amount of light that can be received from a thermal-isolating region. In practical systems, the cladding further limits the amount of light that can be received. Accordingly, the preferred shapes (and obvious variations) can receive more light than can bundle  36  (FIG.  6 ), in some cases receiving substantially all the light from the thermal-isolating region. 
     FIG. 7 shows a preferred shape for light guide  28  (FIG.  2 ). The left-shown inlet end has a substantially flat surface  40  and a surface  42  that is half of a generally round (e.g., oval or round) shape. The right-shown outlet end tapers upwardly in cross-sectional area in the main direction of light propagation so as to reduce the angular distribution of light passed therethrough. The tapering preferably occurs in such manner that successively greater angular distributions of light can be selected by respectively trimming away successively greater axial portions of the outlet end. For instance, trimming away the right-most axial portion up to boundary  46  yields a greater angular distribution than trimming to boundary  48 . 
     A central region  50  (FIG.  7 ), between the inlet and outlet ends, is substantially smaller in cross section than the ends. The central region is thus easier to bend than the inlet and outlet ends of the light guide. 
     Preferably, the cross section of the light guide of FIG. 7 changes shape smoothly from having a substantially straight side an its inlet end, to generally circular at a point distant from its inlet end. This maximizes light propagation efficiency. By “smoothly” is meant that the cross section at any point along the main direction of light propagation transitions to the next point without any substantial discontinuities. 
     The shape of the light guide of FIG. 7 achieves high efficiency as well as a high preservation of the brightness of the light. Other shapes will achieve comparable efficiency although with less brightness, such as shown in FIG.  8 . As shown in that figure, the entire cross sections of light guides  60 ,  62 ,  64  and  68  each comprise a fraction (i.e., one-fourth) of a substantially solid shape such as generally circular (as shown) along a main direction of light propagation. At the left-shown inlet ends, the inlets would appear as shown in FIG.  5 C. The right-shown outlet ends diverge from each other so as to respectively direct light to different locations. 
     FIG. 9 shows the coupling of light guide  60  (FIG.  8 ), having a cross section of one-fourth of generally round, to a light guide  70 , having a round cross section. Round cross-sectioned light guides are typically used for distributing light up to considerable distances. Preferably the perimeter of the inlet end of light guide  70  substantially coincides with the perimeter of the outlet of light guide  60 , as shown in FIG. 10, although it can be larger or smaller without great detriment. 
     Light guide  60  (FIG. 9) may be held adjacent to light guide  70  with a collar  72  shown in FIG.  11 . Each of holes  74  may receive a light guide with a substantially straight side (e.g.,  70 ) in its left-shown side and a light guide with a generally round cross section (e.g.,  70 ) in its right-shown side. Collar  72  can accommodate the four light guides shown in FIG. 8, for instance. Collar  72  may be made of aluminum, for instance. 
     FIG. 12 illustrates a preferred method of making a light-splitting arrangement having light guides as shown, for instance, in FIGS. 5A-5D. An elongated, generally cylindrical piece of flexible acrylic or other fiberoptic material  88 , preferably warmed above 60 C. to improve its ability to be cut, is provided. Suitable fiberoptic materials are described in U.S. Pat. Nos. 5,485,541 and 5,406,641. The elongated material  88  is then cut along its longitudinal direction by a cutting blade  90 , such as a knife or razor blade (e.g., part no. 62-0165 sold under the trademark GEM, registered in the U.S. Patent and Trademark Office), preferably coated with TEFLON material. Alternatively, other cutting means such as a laser can be used. 
     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.