Abstract:
A light collection system comprises a light source with a bulbous section for emitting radiant energy in the direction of first and second portions of the bulbous section. A reflecting surface directs visible light from the light source to the first portion. Another reflecting surface directs UV energy from the light source to the second portion. A first angle-to-area converter receives visible light in the first portion and decreases the angle of the visible light to a desired angle. A second angle-to-area converter receives UV energy in the second portion and decreases the angle of the UV energy to a desired angle. A phosphor layer receives UV energy downstream of the second angle-to-area converter and converts the UV energy to visible light. A third angle-to-area converter receives visible light from the phosphor and reducing the angle of such light to an angle optimized for entering a fiber optic cable.

Description:
This application claims priority from U.S. Provisional Application No. 60/452,822 filed Mar. 7, 2003. 

   FIELD OF THE INVENTION 
   The present invention relates to a light collection system for extracting more visible light from a light source by converting ultraviolet (“UV”) energy into visible light. 
   BACKGROUND OF THE INVENTION 
   Using non-imaging optics, it is currently possible to achieve over 90% collection of visible light from a light source such as a metal halide lamp. The light can be used to couple into a fiber optic illumination system, but typically any UV energy is filtered from light directed to a fiber optic cable using absorbing or reflecting filters. This is to prevent degradation of the fiber optic cable. 
   One way prior art lighting systems used UV energy is by employing phosphor to convert UV energy to visible light. Phosphor conversion systems have been used in fluorescent lamps, metal halide lamps, and light emitting diodes to produce light or change the color of light. However, phosphors do not conserve the angular distribution of the driving light—the light they emit has a broader angular distribution than the light absorbed. This increase in angular distribution often reduces the brightness of phosphor-based lamps sufficiently so as to be difficult to use in efficient, compact optical coupling systems such as lighting fixtures or fiber optic illuminators. What is needed is a compact way of improving light-output quantity and quality using phosphors. 
   It would be desirable to provide a light collection system in which UV energy is captured and converted into visible light with phosphor. It would be desirable to provide a compact way of doing so while improving both light-output quantity and quality. This would increase the visible light provided from a light source that produces UV energy in addition to visible light. The additional visible light could be directed to a fiber optic cable without causing UV damage to the cable. 
   SUMMARY OF THE INVENTION 
   The current invention seeks to improve quantity of light collected from a metal halide lamp and directed into a fiber optic cable of compact size by using phosphors. 
   Some light sources used in a fiber optic illumination system, especially metal halide light sources, produce a significant amount of UV energy. The inventive system extracts more visible light from such a light source, as set forth in the following specification. The claimed designs utilizes so-called reflecting surfaces made from “thin film” coatings and non-imaging optics to collect visible light and UV energy, and then uses phosphor to convert the UV energy into additional visible light. In this way, the UV energy is harnessed and converted into usable visible light and the system has a net increase in delivered visible light. This net increase can be sufficient to overcome coupling optics inefficiencies such that the system delivers more light than the bulb produces. 
   In accordance with a preferred form of the invention, a light collection system comprises a light source with a bulbous section for emitting radiant energy in the direction of first and second portions of the bulbous section that are preferably opposite each other. A reflecting surface directs visible light from the light source to the first portion. Another reflecting surface directs UV energy from the light source to the second portion. A first angle-to-area converter receives visible light in the first portion and decreases the angle of the visible light to a desired angle. A second angle-to-area converter receives UV energy in the second portion and decreases the angle of the UV energy to a desired angle. A phosphor layer receives UV energy downstream of the second angle-to-area converter and converts the UV energy to visible light. A third angle-to-area converter receives visible light from the phosphor and reducing the angle of such light to an angle optimized for entering a fiber optic cable. 
   The foregoing light collection system captures UV energy and converts it into visible light with phosphor. The light collection system is compact and It does so in compact and improves both light-output quantity and quality. This increases the visible light provided from a light source that produces UV energy in addition to visible light. The additional visible light can be directed to a fiber optic cable without causing UV damage to the cable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, in which like reference numerals refer to like parts: 
       FIG. 1  is a simplified, side plan view of a light source with a bulbous section having respectively different optical coatings on opposite sides of the bulbous sections; with the coatings being shown with vertical or horizontal cross-hatch lines for convenience although a cross-sectional view of the coatings is not being shown; 
       FIG. 2  is a side plan view, partially in cross section, of the light source of  FIG. 1  and a pair of angle-to-area converters respectively associated with the two different optical coatings on the bulbous section of the light source; 
       FIG. 3  is a side plan view the same as  FIG. 2  but also including a solid angle-to-area converter for further conditioning of UV energy; 
       FIG. 4  is a side plan view the same as  FIG. 3  but also including a further angle-to-area converter at the output of the solid angle-to-area converter of  FIG. 3 ; and 
       FIG. 5  is a side plan view similar to  FIG. 4  but with some rearrangement or changes of parts, with vertical and horizontal cross-hatch lines corresponding to those discussed above for  FIG. 1 , and with a dotted vertical line on a light source added for convenience to delineate right-and left-hemispheres of a bulbous section of the light source. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1–4  show sequential stages of construction of a preferred system of light-system elements, starting with an arctube and proceeding to a complete system. As mentioned above, like reference numerals refer to like parts. This means, for instance, in some of the later figures, description of a part to which reference is made by a reference numeral will be found in connection with an earlier figure. 
     FIG. 1  shows a light source  10 , such as a metal halide arctube lamp, having arms  12   a  and  12   b  and a bulbous section  14 . Preferably, the right-hand half, or hemisphere, of bulbous section  14  has a coating  16  that passes visible light but reflects UV energy back into bulbous section  14 . Preferably, the left-hand half, or hemisphere, of the bulbous section has a coating  18  that passes UV energy but reflects visible light back into the bulbous section. The left- and right-hand hemispheres are preferably positioned opposite to each other. 
   Coating  16  is designed to allow visible light (e.g, rays  15 ) to pass but to reflect UV energy back into bulbous section  14 . Coating  16  preferably also contains anti-reflective (“AR”) material optimized for visible light, to promote better transmission of visible light through coating  16 . Coating  18  is designed to allow UV energy (e.g, rays  19 ) to pass but to reflect visible light back into bulbous section  14 . Coating  18  preferably also contains AR material optimized for UV energy, to promote better transmission of UV energy through coating  18 . An idealized version of light source  10  would have the right-shown approx. hemisphere of the bulbous section emitting all of the visible light from the arctube and the left-shown approx. hemisphere would emit all of the UV energy. In reality, the visible light- and UV energy-reflecting materials in coatings  16  and  18  are not perfect reflectors, and various light absorption elements such as the quartz arc tube reduce transmission so that about 80% of visible light and 80% of UV energy respectively pass through coatings  16  and  18 . 
   Coatings  16  and  18  are known in the art as “thin film” coatings. Such coatings are also known as multi-layer optical interference coatings. The material used for the coating would include those high index materials that are transparent to most wavelengths of UV energy and can handle the high temperatures encountered near the light source. HfO2, ZrO2, and AlO2 are all good choices for this coating. Coating could be done, for example, by sputtering machines like the MICRODYN sputtering machine, or by LPCVD machines like the ISODYN LPCVD machine, both sold by DSI of Santa Rosa, Calif. 
     FIG. 2  shows how angle-to-area converter  20  preferably using non-imaging optics, reduces the half angle of visible light  22  to below about 50 and preferably to about 38 degrees. This reduction of the half angle of visible light  22  allows the light to be coupled to typical fiber optic cables (not shown).  FIG. 2  also shows how angle-to-area converter  24  reduces the half angle of UV energy  25  to below about 75 and preferably to about 60 degrees. Further processing of the UV energy occurs as shown in the subsequent figures. 
   Angle-to-area converters  20  and  24  preferably comprise a non-imaging collectors attached to respective left- and right-hand halves of arctube  10 . Both converters are similar to current shape used in Product No. EFO-4+4-NC-120 sold by Fiberstars Corporation of Fremont, Calif. On the visible side converter  20 , a cold mirror coating is optimized for reflecting visible light, instead of UV protection, so that efficiency is improved over the foregoing Fiberstars&#39; product. 
   Converter  24  on the UV side of lamp  10  is designed for shorter (UV) wavelengths than the visible light converter  20  on other side; that is, the shape and length of the converter, as well as a UV-reflecting coating  27  on the interior of the converter are optimized for UV energy. Converter  24  preferably has a different shape than the visible light converter  20 , and could be shorter than converter  20 . A coating  27  for the UV converter is made to reflect UV energy, and could be made of thin film or metallic construction. The UV energy leaving converter  24  is controlled to an angle that allows efficient coupling to a solid converter  30 , as shown in  FIG. 3 . 
   Referring to  FIG. 3 , a solid angle-to-area converter  30  that is preferably made of quartz is placed at the output side of UV converter  24 . Converter  30  preferably follows the laws of non-imaging optics and increases the maximum half angle of the UV energy to a high angle that improves efficiency. The input end of converter  30  preferably has an AR coating  32  optimized for UV transmission at the determined higher angle. UV energy  25  is shown leaving converter  24  and passing into converter  30  as UV energy  26 , via AR coating  36 . The smaller, output end of converter  30  is coated with a thin film, cold mirror  34 , for instance, that passes UV energy and reflects visible light. Atop or near mirror  34 , there is a layer  36  of phosphor, which converts the UV energy into visible light, such as light rays  35  directed to the left in  FIG. 3 . Light is emitted by the phosphor in all directions. The light that travels to the right in  FIG. 3  is reflected to the left by mirror  34 . 
   Light rays  35  emitted by phosphor  36  in the 2Pi steradians towards the left in  FIG. 3  reach a final angle to area converter  40  as shown in  FIG. 4 , and are immediately transformed to a usable angular distribution. Mirror  34  on converter  30  reflects to the left in  FIG. 3  visible light that is emitted by phosphor  36  in the 2Pi steradians towards solid converter  30 . This light is reflected by mirror  34  so that it can be transmitted through phosphor  36  or absorbed in the phosphor and re-emitted as visible light, so that most of the UV energy is converted to visible light. 
   In brief,  FIG. 3  shows how UV energy is converted to visible light. UV energy leaving the converter  24  enters a preferably solid, preferably quartz converter  30 . This solid converter increases the half angle of the UV energy, preferably to 90 degrees. The output side of converter  30  is coated, preferably with thin film coatings to create mirror  34  that reflects visible light and passes UV energy. The output side of the collector is further coated with phosphor material  36  that converts UV energy to visible light. 
   Preferably, converter  30  is solid so that mirror  34  and phosphor layer  36  can be applied to the outlet of the converter. Alternatively, only the outlet of converter  30  could be solid, with the inlet to the converter being hollow. Further, converter  30  could be omitted, although this would reduce the conversion efficiency of UV energy to visible light, and would require the use of another substrate to hold mirror  34  and phosphor  36 . 
   Additionally, mirror  34  could be omitted from any of the embodiments shown herein, with some loss to the total collection efficiency. This would require another substrate to hold phosphor  36 . 
     FIG. 4  shows how the visible light from phosphor coating  36 , at a preferably 90 degree half angle, is converted to a half angle of preferably 38 degrees using a preferably non-imaging angle-to-area converter  40 . Converter  40  may have substantially the same shape as converter  20  used on the half of the light source that produces visible light. 
   Converter  40  has an interior coating  41  that is optimized for reflection of visible light. Converter  40  receives visible light at a 90 degree half angle, for instance, and converts it to a preferably 38 degree-half angle light  50 . Because etendue is essentially preserved, converter  40  could be the same size and shape, and have the same coatings, as converter  20  used on the other half of the light source. Etendue is essentially maintained because converters  20 ,  24 ,  30  and  40  are properly designed as non-imaging optics with angle-to-area conversion, and because the angle of light is only slightly increased by phosphor  36 , if at all. 
   The UV energy striking the phosphor has a broad angular distribution with extents at or near 90 degrees. That is, the phosphor absorbs light from all angles and then emits light that has a broad angular distribution with extents at or near 90 degrees. In this way, the phosphor may not have a large effect on the angular distribution of the light. 
   The foregoing light collection system increases efficiency of collection for visible light over existing system because substantially all of the visible light is maintained and new light is generated by converting UV energy into visible light. 
   The angular distribution of energy at both the arctube surface and output of solid converter  30  is very broad, with significant energy up to 80–90 degrees half angle. This requires a suitably designed thin film coating that responds differently to different groups of wavelengths (visible light vs. UV energy). This is within the ordinary skill of the art. 
     FIG. 5  shows a preferred variation of the light collection system shown in  FIGS. 1–4 . In  FIG. 5 , a light source  60  is shown without coatings  16  and  18  as in  FIG. 1 . Rather, coating  16   a  on the inlet to a light-collecting rod  62 , of solid quartz, for instance, fulfills the same general function as coating  16  of  FIG. 1 . That is, coating  16   a  allows visible light energy to pass to the right in  FIG. 5 , while reflecting UV energy back through a hemisphere  60   a  of a bulbous section  63  of the light source so that it passes to the left in  FIG. 5 . Similarly, coating  18   a  fulfills the same general function as coating  18  of  FIG. 1 . That is, coating  18   a  allows UV energy to pass to the left in  FIG. 5 , while reflecting visible light back through another hemisphere  60   b  of bulbous section  63  of the light source so that it will pass to the right in  FIG. 5 . A dotted line  61 , added to  FIG. 5  for convenience, delineates hemispheres  60   a  and  60   b  from each other. 
   Collectors  65  and  67  in  FIG. 5  are designed to function well for both UV and visible light energy. For this reason, they may likely have a different shape than a collector designed for improved collection of only one of these two energy types, such as collector  20  in  FIG. 4 . Such collector  20  is designed, in contrast, for improved collection of visible light as opposed to UV energy. Also in contrast to collectors  65  and  67 , collector  24  in  FIG. 4  is designed for improved collection of UV energy as opposed to visible light. Thus, in practice, collector  65  in  FIG. 5 , for instance, will differ from collectors  20  and  24  in  FIG. 4 , since collector  65  is designed to function well for both visible and UV energy, likely resulting in design compromises that reduce the collection efficiency for visible light below that realized with collector  20 . 
   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.