Abstract:
The seal temperature of a reflector lamp having a ceramic metal halide light source is reduced by a light absorbing layer which is provided in a region of the outer jacket adjacent to the electrode seal. Light reflected within the neck cavity of the reflector lamp impinges on the light absorbing layer and is absorbed before it can reach the electrode seal which is at least partially located in the neck of the reflector. The heat from the absorbed light is conducted into the base of the lamp to be dissipated in the socket.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is related to copending application Ser. No. 10/120,958, filed Apr. 11, 2002. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention is related to light sources that are mounted within a reflector body. More particularly, this invention is related to reflector lamps having a ceramic metal halide light source.  
       BACKGROUND OF THE INVENTION  
       [0003]     Ceramic metal halide light sources are comprised of a ceramic discharge vessel (commonly referred to as an arc tube) that is generally made of polycrystalline-alumina. Typical metal halide fills may include mercury, alkali- and alkaline-earth iodides, in particular NaI and CaI 2 , and rare-earth iodides such as DyI 3 , TmI 3  and HoI 3 . Xenon or argon are typical gas fills. Tungsten electrodes are used to generate an arc within the discharge vessel. Because electrical power must be supplied to the electrodes, the electrode assemblies must extend through the arc tube wall. In a conventional construction, capillary tubes hold the electrode assemblies and a frit material is used to form a hermetic seal between the electrode assembly and its respective capillary. The ceramic arc tube is often jacketed in another envelope, called an outer jacket, to protect the metal parts from oxidation. These outer jackets are usually thermally isolated from the arc tube and contain a vacuum or are filled with a partial pressure of an inert gas and a getter material, e.g., an aluminum or zirconium compound, to getter hydrogen and oxygen.  
         [0004]     In recent years, ceramic metal halide light sources have become increasingly favored because of their efficiency and color rendering properties. As a result, the applications for ceramic metal halide light sources have expanded into traditional incandescent lighting applications, such as parabolic reflector (PAR) lamps, which must be adapted-to accommodate these high-intensity, high-temperature light sources. For example, a typical failure mode for ceramic metal halide sources occurs as a result of chemical attack by the metal halide fills on the frit materials used to make the electrode seals. In a conventional reflector lamp structure, this problem is exacerbated because some of the emitted visible radiation is reflected back onto the ceramic metal halide source, and in particular, the electrode seal located in the neck portion of the reflector. The construction of the electrode assembly and the seal make it a particularly good absorber of visible light. The absorbed energy causes the electrode seal to overheat which in turn increases the rate of chemical attack by the fill on the seal leading to premature lamp failure. Therefore, it would be advantageous to keep the electrode seal from overheating in order to extend the operating life of the lamp. It would be a further advantage to accomplish this without significantly affecting the performance or cosmetic appearance of the reflector lamp.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is a reflector lamp having a ceramic metal halide light source wherein a particular portion of the outer jacket of the ceramic metal halide source is provided with a light absorbing layer. Light reflected within the neck cavity of the reflector lamp impinges on the light absorbing layer and is absorbed before it can reach the electrode seal which is located at least partially in the neck cavity of the reflector lamp. The absorbed light raises the temperature of the outer jacket, but not the electrode seal. The heat generated by the absorbed light is conducted into the base of the reflector lamp to be dissipated in the socket.  
         [0006]     In a preferred embodiment, the temperature of the electrode seal during operation of the reflector lamp is at least about 50° C. lower than the temperature would have been if the lamp were constructed without the light absorbing layer on the outer jacket. This invention is particularly applicable to those discharge vessels which have elongated seal structures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross-sectional view of a preferred embodiment of the reflector lamp of this invention.  
         [0008]      FIG. 2  is a side view of the light source of the reflector lamp shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.  
         [0010]      FIG. 1  is a cross-sectional view of a preferred embodiment of the reflector lamp of this invention. The reflector lamp is comprised of reflector body  10  and light source  12 . A lens  40  may or may not be attached to the forward edge of the reflector to enclose the light source  12  within the reflector body  10 . The lens may be fused, glued, or similarly coupled to the reflector body as is known in the art. The reflector body  10  is comprised of concave shell  29  and neck  24  which defines neck cavity  6 . The shell  29  surrounds the light source  12  to reflect light from the light source  12  onto a field to be illuminated during lamp operation. The reflector body  10  has a reflective surface  11  to reflect the light emitted from light source  12 .  
         [0011]     The reflective surface  11  is preferably formed by applying a reflective coating  22  to the reflector body&#39;s inner surface  25 . The reflective coating  22  substantially covers the inner surface  25  including the neck  24  of the reflector body  10 . Preferably, the inner surface  25  of the reflector has been aluminized or silvered to provide the reflective coating. Other typical reflective coatings include multi-layer dichroic coatings which are designed to reflect only certain portions of the spectrum of light emitted from the light source.  
         [0012]     The reflector body is rotationally symmetric about a reflector axis  28 . The concave shell may have a parabolic, elliptical or other similar optically functional cross section. The inner surface  25  of the reflector body  10  may be smooth, faceted or otherwise contoured to reflect the light in a preferred direction to yield a desired beam pattern. The neck  24  of reflector body  10  is provided with an electrical connection or connections  30  for providing power to light source  12 ; and a mechanical support or supports, which may be the same as the electrical connections. Preferably, the mechanical support holds light source  12  such that the axis  18  of the light source is substantially co-axial with the reflector axis  28 .  
         [0013]     The light source  12  is a ceramic metal halide source comprised of a double-ended discharge vessel  7  having an axially symmetric body  14  with two capillaries  3 ,  5  extending outwardly from the body  14  in opposite directions along axis  18 . The axially symmetric body  14  defines an arc discharge cavity  4 . In this embodiment, the body  14  has the shape of a right circular cylinder. However, other known shapes include spheroid geometries such as the one described in U.S. Pat. No. 5,936,351. Preferably, the discharge vessel  7  is composed of a polycrystalline alumina. Other ceramic vessels are feasible, e.g., yttrium aluminum garnet.  
         [0014]     Each capillary  3 , 5  of the discharge vessel  7  contains an electrode assembly  26  which passes through it. Electrode seals  8 ,  9  are used to hermetically seal the electrode assembly  26  to its respective capillary  3 ,  5 . The electrode assembly is usually composed of multiple metal sections, viz. niobium, molybdenum wire coiled on a molybdenum mandrel, and a tungsten rod with a tungsten coil at its end. The electrode seals are formed with a frit material which is a lower melting temperature ceramic composition, e.g., Al 2 O 3 , Dy 2 Q 3 , and SiO 2 . The coefficient of thermal expansion of the polycrystalline alumina, frit, and niobium are similar to minimize thermal stress in the seals. The frit is melted during a sealing operation and covers about 5 mm of the electrode assembly, including 3 mm of the niobium and 2 mm of the molybdenum coil on the molybdenum mandrel. It is the tight winding of the Mo wire over the Mo mandrel that provides a good absorber of visible light. Details of various electrode structures and seals are described in U.S. Pat. No. 5,424,609. The electrode assemblies protrude into discharge cavity  4  for the purpose of striking an electric arc between the opposing ends of the two electrode assemblies. At their opposite ends, the electrode assemblies extend beyond the ends of the capillaries to provide external electrical connections. The discharge cavity  4  contains mercury, a metal halide fill and a buffer gas. The metal halide fill typically comprises 5-10 mg of a mixture of metal halide salts, e.g., NaI, CaI 2 , HoI 3 , DyI 3 , and TmI 3 . The buffer gas may be Ar, Kr, Xe, or a mixture thereof, with a fill pressure of 10 to 400 torr.  
         [0015]     Outer jacket  33  is transparent to visible light and is constructed of glass, e.g., fused silica (quartz) or an aluminosilicate glass. In this embodiment, outer jacket  33  has a tubular shape which is closed at both ends. The upper end  35  of the outer jacket is domed and the base end  39  contains press seal  37 . The electrical leads  45  are connected to the external portion of the electrode assemblies  26 . The leads  45  are welded to molybdenum foils  47  which are in turn welded to electrical connections  30  thereby providing a conductive path for connecting the light source  12  to an external power source (not shown). The molybdenum foils  47  are sealed within press seal  37  to provide a closed environment within the outer jacket  33 . Eyelets may be located in the base of the neck to duct the electrical connections through the reflector body. The electrical connections may then be soldered in place. The outer jacket  33  may have a vacuum environment or be filled with a gas such as nitrogen gas. Filing the outer jacket with nitrogen gas has the effect of cooling the seal area, but it also cools the rest of the discharge vessel resulting in an undesirable color shift in the light output.  
         [0016]     Light source  12  is oriented so that the base end  39  of outer jacket  33  is situated within the neck cavity  6  of the reflector body  10 . Consequently, an end of the discharge vessel  7  containing an electrode seal, in this case defined by capillary  3  and electrode seal  8 , is also at least partially situated within the neck cavity  6 . In order to reduce the temperature of the electrode seal  8  during the operation of the lamp, a region of the outer jacket  33  adjacent to the electrode seal  8  is provided with light absorbing layer  15  which masks the electrode seal  8  from reflected radiation  2  within the neck cavity. Since the amount of radiation reaching the electrode seal in the neck cavity is reduced, the temperature of-the electrode seal is lower. Preferably, the light absorbing layer  15  is continuous and extends circumferentially around the outer jacket  33  and lengthwise towards base end  39  from a point between 0.1 to 2 mm below the body  14  of the discharge vessel  7  covering the length of capillary  3 . More preferably, the light absorbing layer starts 1 mm below the body  14  of the discharge vessel  7  so as not to interfere with the light gathering power of the reflector lamp.  
         [0017]     The placement of the light absorbing layer  15  may be better observed in  FIG. 2  which is a side view of the light source  12  without the reflector body. The distance D represents an offset between the lower boundary  50  of the body  14  of discharge vessel  7  and the beginning edge  52  of the light absorbing layer  15 . As stated above, this distance D is preferably 0.1 mm to 2 mm and more preferably 1 mm. The light absorbing layer  15  is preferably opaque and may be formed as an integral part of the glass outer jacket  33 , e.g., dyeing or impregnating a region of the glass outer jacket with ions to alter the light absorbing properties of the glass, or incorporating a section of an opaque glass in the transparent outer jacket. The light absorbing layer  15  may be applied by brushing, spraying, dipping, electroplating, silk-screening or deposited by CVD or PCVD. Preferably, the light absorbing layer  15  is applied as an opaque coating to the exterior surface of the jacket as shown in  FIGS. 1 and 2 . The opaque coating comprises a refractory light-absorptive paint such as the automotive blacktop coating which is commonly used on the tips of halogen headlamps to control glare. Examples of the automotive blacktop coating may be found in U.S. Pat. Nos. 3,784,861 and 4,288,713. When cured, the black top coating forms a matte, dark gray or black surface. Such blacktop compounds may consist for example of an emulsion of Kaolin clay, silicon powder, aluminum phosphate and water, which cures to a durable coating upon baking. Other formulations may contain silicon, carbon, and iron powders dispersed in butanol and glycerin. Alternatively, the coating may be a high-temperature black paint capable of 315° C. (600° F.) continuous operation, for example Krylon BBQ and Stove paint, sold by Sherwin Williams of Cleveland, Ohio. The neutral gray or black absorptive coatings have very little reflection in the visible and so do not alter the color of the primary beam by scattering selective wavelengths.  
         [0018]     The effectiveness of the light absorbing layer at reducing the temperature of the electrode seal in the neck cavity was measured by operating two groups of 70W PAR30 ceramic metal halide lamps in a vertical, base-up orientation. The first group consisted of control lamps that did not have the light absorbing layer on the outer jacket. The second group of lamps were fabricated according to this invention. The light absorbing layer was an automotive blacktop coating that had been painted onto the exterior surface of the outer jacket. A stripe about 3 mm wide and parallel to the capillary was removed to permit infrared viewing of the seal area in order to measure its temperature. On average, the temperature of the electrode seal in the neck cavity of the PAR 30 lamps was reduced by about 50° C. from about 890° C. to about 840° C. by the addition of the light absorbing layer to the outer jacket. Furthermore, by placing the light absorbing layer on the outer jacket instead of on the reflector body the cosmetic appearance of the reflector lamp is less affected.  
         [0019]     While there has been shown and described what are at the present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.