Abstract:
An optical coupler for distributing light in a 360-degree pattern at a small angle is disclosed. The coupler comprises a solid rod with a first end region for receiving light and a second end region for distributing light. The rod tapers up in cross-sectional dimension between the first and second regions so as to decrease the angular distribution of light reflecting internally from the sides of tapered portions of the rod. An axis extends from the second end region towards the first end region. The second end region includes a generally conically shaped void pointing towards the first end region and defining a surface for deflecting light in a 360-degree pattern about the axis at a small angle compared to light entering into the first end region.

Description:
This application claims priority from provisional application Ser. No. 60/072,168, filed on Jan. 6, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an optical coupler and system for distributing light in a 360-degree pattern, and more particularly to such a coupler and system for distributing light in the mentioned pattern at a small angle above and below a plane. 
     BACKGROUND OF THE INVENTION 
     Boats and other water-going vessels are often required to display a light on the highest part of the vessel. Because the highest point is usually the top of a mast which may support sails or radio antenna, for instance, it may be difficult to replace a failed lamp in that location. It, therefore, would be desirable to employ a lamp located at the base of the mast, where it is easily reached, and to direct the light to the top of the mast with a fiber-optic, or light, guide. However, the problem arises of how to distribute the light reaching the upper end of the light guide to meet a required beam pattern. The required pattern for at least one class of ships is to provide 94 candela for 360 degrees in a horizontal plane and which extends 7½ degrees above and 7½ degrees below the horizontal plane. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the invention comprises an optical coupler for distributing light in a 360-degree pattern at a small angle. The coupler includes a solid rod with a first end region for receiving light and a second end region for distributing light. The rod tapers up in cross-sectional dimension between the first and second regions so as to decrease the angular distribution of light reflecting internally from the sides of tapered portions of the rod. An axis extends from the second end region towards the first end region. The second end region includes a generally conically shaped void pointing towards the first end region and defining a surface for deflecting light in a 360-degree pattern about the axis at a small angle compared to light entering into the first end region. 
     The foregoing embodiment provides an output device for distributing light in the required pattern which minimizes light loss and is simple to manufacture and compact. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an optical coupler in accordance with the invention. 
     FIG. 2 is a plan view of a light input port of the coupler of FIG.  1 . 
     FIG. 3 is a plan view of a light output port of the coupler of FIG.  1 . 
     FIG. 4 is a side view of the coupler of FIG. 1 showing the paths of various light rays and various dimensions. 
     FIG. 5 is a block diagram of a complete light system incorporating a coupler such as shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment for distributing light from the end of a light guide (not shown) in the required pattern is illustrated in FIGS. 1-3. FIG. 1 shows an optical coupler  10  with an input end region  12  and an output end region  14 , each preferably centered about a longitudinal axis  15 . Side wall  16  of coupler  10  may taper from input region  12  to output region  14 , and, if desired, may also taper in regions  12  and  14 . The tapers are preferably uniform. An input light port  18 , shown also in FIG. 2, receives light from a light guide (not shown) and passes it into the body of coupler  10 . An output port  20 , preferably conically shaped, also shown in FIG. 3, is used for distributing light in a desired pattern. 
     FIG. 4 shows a further side view of coupler  10 , illustrating various light rays with respect to axis  15  and a plane  22  orthogonal to the axis and representing a horizontal plane when the coupler is attached to the mast of a boat or other vessel, for instance. “Horizontal” and other orientations mentioned herein are merely exemplary. A light ray  24   a  received through port  18  totally internally reflects from angled surface  20  to propagate as light ray  24   b . As FIG. 4 illustrates, light ray  24   b  is oriented at only a small angle with respect to plane  22 , e.g., 7½ degrees above or 7½ degrees below plane  22 , or a resulting angle of 15 degrees. “Small” angle refers herein to angles generally less than approximately 40 degrees, and preferably less than approximately 20 degrees. Totally internally reflected light rays such as ray  24   b  provide the desired distribution of light. 
     Light ray  26  illustrates the maximum angle  28  of light that is not reflected from wall  16  of the coupler, the light entering a peripheral portion of input light port  18  and exiting a peripheral portion of output port  20 . Angle  28  may be 7½ degrees, for instance. In contrast, light received through input port  18  typically has an angular spread of 30 to 40 degrees half angle. In this connection, the taper of coupler wall  16  results in an angle-to-area conversion for reducing the angle of light received from the input port. Where an especially large angle-to-area conversion is desired, the taper may define a compound parabolic reflector made in accordance with non-imaging optics, a technology known per se in the art. Preferably, a substantial axial length, or the full axial length, of coupler wall  16  is tapered in output region  14  from the axial center  29  of output port  20  to the top of coupler  10 . This causes such axial length of the coupler to function both as an area-to-angle coupler and as a means of directing light (e.g., ray  24   b ) into a desired pattern. 
     FIG. 5 shows a light system incorporating a coupler  30 , which may be as shown, for instance, in FIG. 1. A light source  32  may typically comprise a small source placed in an elliptical reflector so as to image the source onto the input end (not shown) of a light guide  34 , such as a ½-inch diameter large core plastic optical fiber (LCPOF). A preferred light source is an XMH60P1 source made by GE Lighting of Cleveland, Ohio. It can introduce more than 2000 lumens into a ½-inch diameter LCPOF. An SEL500LCPOF produced by Lumenyte International Corporation of Irvine, Calif. transmits light with an efficiency of more than 0.985 per foot, so a typical mast of 25 height will transmit more than 0.685 of the incident light. With light provided to the light guide of 2000 lumens, light output from the light guide will therefore be 1370 lumens for 25 feet of light guide. If optical coupler  30  reduces the angular spread of light from 30-40 degrees to 7½ degrees in the vertical direction, the 1370 lumens are available to produce a desired light pattern. The total solid angle in a light beam 15 degrees wide and covering all 360 degrees is 1.64 steradian. If there are no other losses, the available flux is 1370/1.64 lumens per steradian or 836 candela, which is well in excess of the required 94 candela. 
     Referring again to FIG. 1, the tapering of coupler wall  16  reduces the angle of the light entering input port  18 . The input port is preferably just slightly larger in cross-sectional dimension than the core of light guide  34  (FIG.  5 ), or slightly more than ½-inch in diameter for the mentioned LCPOF. An appropriate size for the output end of the taper can be determined from the known angle-to-area relationship wherein the mathematical product of the area of an emitter (or output) and the solid angle of the emitted light, and the same factors for a receiver (or input), are the same. In the mentioned example, the area of input end times the solid angle corresponding to 40 degrees plane angle equals the area of output times the solid angle corresponding to a 7½ degree plane angle. Diameter D (FIG. 4) of the output end of coupler  10  can be expressed by equation 1 as follows: 
     
       
           [pi (0.25) 2 ]/4×2 pi (1−cos 30)=[ pi ( D ) 2 ]/4×2 pi (1−cos 7½)  (eq. 1) 
       
     
     Solving equation 1 for D shows that D =1.97 inches. 
     In determining the length L of coupler  10  (FIG.  4 ), it is preferred that the marginal rays such as ray  26  (FIG. 4) which do not reflect from surface  16  not lie at more than 7½ degrees from the axis. FIG. 4 shows this to occur where length L of coupler  10  is at least 8½ inches. 
     Referring again to light ray  24   b  in FIG. 4 that is oriented in a desired pattern, conically shaped output port  20  changes the direction of ray  24   a  due to total internal reflection at this surface of the output port. This is because the angle of incidence for almost all of the rays reaching the output port exceeds the critical angle expressed by sine (critical angle) equals 1/n, where n, the index of refraction, for acrylic is about 1.49. If coupler wall  16  tapers over its entire length, the angle at which the output port is cut should not be exactly 45 degrees (half angle) because refraction at outer wall  16  shifts the light rays slightly upwards. To compensate for this effect, the cone angle of the output port may be increased to about 47½ degrees. Alternatively, the upper part of coupler wall  16  in the vicinity of output end region  14  can be made cylindrical so no refraction will occur for the axial rays. In this case, the cone may be oriented at 45 degrees from the axis. 
     In a working example of the embodiment of FIG. 1, a prototype tapered rod with a conical void in the larger end was fashioned from acrylic, with input port  18  of ½-inch diameter, output port  20  of 2-inch diameter, and a length L (FIG. 4) of 8½ inches. While machining of the acrylic was employed, molding would be more economical for large scale production. A 45-foot length of SEL500 large core plastic optical fiber from Lumenyte International and an XMH60P1 lamp from GE Lighting were employed. Output light was measured 20 feet from the tapered rod, indicating an intensity of 560 candela, or 1.4 foot-candles (lumens per square foot) at 20 feet. Since candela equals foot candles times distance squared, the mentioned 560 candela results from 1.4 times (20) 2 . The width of the output beam of light at 20 feet was approximately 6 feet. The tan (half angle) equals 3/20. The half angle equals 8.5 degrees, or more than required. 
     The dimensions in the foregoing design example are linearly scalable. Thus, if a fiber-optic cable of ¼-inch diameter is used rather than the ½-inch diameter cable mentioned above, the other mentioned dimensions are also halved. 
     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. For instance, while the foregoing refers to a Xenon metal halide light source and a LCPOF, an incandescent illuminator directing light into glass fibers, for example, can be used instead to provide a suitable light pattern where the optical coupler is appropriately dimensioned. 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.