Abstract:
A single-ended ceramic discharge lamp is described which has an integral optical surface such as a parabolic or elliptical reflector. The single-ended configuration eliminates the need for the mounting structures found in double-ended lamps that can interfere with the light emitted from the lamp, particularly in focused beam applications.

Description:
BACKGROUND OF THE INVENTION 
     Double-ended ceramic discharge lamps, i.e. lamps in which the electrodes enter the ceramic discharge vessel from opposite sides, are well known. For example, U.S. Pat. No. 5,721,465 describes a xenon arc lamp with a cylindrical ceramic body into which an elliptical reflector is molded and a quartz window is mounted opposite the reflector. The lamp has opposed electrodes: one which extends into the discharge vessel from the base of the reflector, the other from the opposite side where the window is located, i.e., the light-emitting end. Similar configurations are found in U.S. Pat. Nos. 6,200,005, 6,285,131, 6,351,058, 6,597,087, 6,602,104 and 6,316,867. However, a common disadvantage with these lamps is that the window-side electrode and its mounting structure obstruct a portion of the light exiting the window. 
     Also known are singled-end ceramic discharge lamps, i.e., lamps in which the electrodes enter the discharge vessel from the same side. For example, European Patent Application No. EP 1 111 654 A1 describes several single-ended configurations. Although one embodiment is shown with an integral lens in the dome to enhance light intensity distribution, the discharge vessels do not otherwise attempt to focus the arc as in the above-described double-ended lamps. Examples of other single-ended lamps are shown in U.S. Patent Publication Nos. 2005/0211370 and 2005/0212433 which describe different electrode/capillary configurations but also do not provide a means for focusing the arc. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to obviate the disadvantages of the prior art. 
     It is another object of the invention to provide a ceramic discharge lamp having an integral optical surface. 
     It is a further object of the invention to provide a ceramic discharge lamp that does not have a mounting structure that obstructs the emitted light. 
     In accordance with an object of the invention, there is provided a single-ended ceramic discharge lamp that comprises a discharge vessel formed of a ceramic material. The discharge vessel has a cupulate body portion and a stem. The cupulate body portion is rotationally symmetric about a central axis and has an inner surface, an outer surface, a rim and a base. The stem extends outwardly from the base and has two electrode assemblies. A light-transmissive end cap is sealed to the rim of the cupulate body portion. The end cap and the cupulate body portion enclose a discharge cavity that contains a discharge medium. Each electrode assembly has an electrode tip that protrudes into the discharge cavity, a feedthrough portion that is sealed in the stem, and a lead end for connecting to a source of electric power. The electrode tips of the electrode assemblies define an arc gap. At least one of the inner surface or outer surface of the cupulate body portion comprise an optical surface wherein the arc gap is positioned at a focus of the optical surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional illustration of an embodiment of the ceramic discharge vessel of the single-ended ceramic discharge lamp of this invention. 
         FIG. 2  is a cross-sectional illustration of an embodiment of the singled-ended ceramic discharge lamp of this invention. 
         FIG. 3  is an enlarged cross-sectional view through line A-A of the stem of the lamp shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional illustration of a first alternate embodiment of the lamp shown in  FIG. 2 . 
         FIG. 5  is a cross-sectional illustration of a second alternate embodiment of the lamp shown in  FIG. 2 . 
         FIG. 6  is a cross-sectional illustration of a third alternate embodiment of the lamp shown in  FIG. 2 . 
         FIG. 7  is an enlarged cross-sectional view through line B-B of the stem of the lamp shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
       FIG. 1  is a cross-sectional view of a preferred embodiment of the discharge vessel  10  of the single-ended ceramic discharge lamp of this invention. The discharge vessel  10  is constructed of a ceramic material, preferably polycrystalline alumina (PCA), although other ceramic materials such as yttrium aluminum garnet, aluminum oxynitride, or aluminum nitride may be used. The discharge vessel  10  has a cupulate (cup-shaped) body portion  5  and stem  11  which extends outwardly from base  23  of cupulate body portion  5 . The cupulate body portion  5  is rotationally symmetric about central axis  20  and defines discharge cavity  12 . Flange  16  extends outwardly from rim  4  of open end  21 . The flange  16  is shown with a rabbet  14  on the inner edge for accepting and sealing to a light-transmissive end cap  8  as shown in  FIG. 2 . Openings  3  are provided in the stem  11  for receiving electrode assemblies as shown in  FIG. 2 . 
     The cupulate body portion  5  has a substantially uniform wall thickness T, in the region between flange  16  and stem  11 . The thickness T is preferably between 0.3 and 2.0 mm, and more preferably 0.8 mm. Some thickening of the wall immediately adjacent to the flange and stem may be desirable increase the robustness of the discharge vessel. 
     At least one of inner surface  7  or outer surface  29  of the cupulate body portion  5  is formed as an optical surface that may be designed to reflect and/or focus a portion of the light emitted by the arc discharge. Preferably, the optical surface comprises a parabolic surface of revolution formed about central axis  20 . The parabolic surface is intended to function as a parabolic reflector for directing at least a portion of the light emitted by the discharge out of the open end  21  of cupulate body portion  5 . The optical surface may be polished to enhance its reflectivity or coated with a reflective material. The optical surface may also be formed as an elliptical reflector depending on the particular optical characteristics desired for the lighting application. For example, a parabolic reflector would be useful in forming a more uniform beam pattern for flood lighting or automotive headlamp applications whereas an elliptical reflector would be useful to focus the light into a light guide or for projection applications. Other useful optical surfaces include aconic and spherical reflectors. 
     It is preferred to form the discharge vessel as a unitary piece (as shown) using a conventional ceramic molding process such as injection molding, gel-casting, or isostatic pressing. However, the discharge vessel may be formed as multiple ceramic pieces which are then joined by conventional methods. Preferably, the ceramic material of the discharge vessel is opaque in order to reduce the amount of light exiting the lamp through the walls of the discharge vessel. The ability to use an opaque ceramic for the discharge vessel rather than a translucent or transparent ceramic as is required for other discharge lamps should reduce the manufacturing cost of lamp since lower purity alumina powders may be used. 
     Referring now to  FIG. 2 , there is shown a cross-sectional illustration of an embodiment of the singled-ended ceramic lamp. A light-transmissive end cap  8  is shown sealed to rim  4  of cupulate body portion  5  thereby enclosing discharge cavity  12 . Preferably, the light-transmissive end cap  8  is a flat, circular sapphire window having a thickness on the order of 1 mm. However, other transparent or translucent ceramic materials may also be used, e.g., polycrystalline alumina, quartz, or aluminum oxynitride. The end cap  8  may further have a dome shape ( FIG. 4 ) or a lenticular shape ( FIG. 5 ) to further influence the distribution of light passing out through the end cap. The end cap  8  may be sealed to the rim  4  with a frit material or by an interference fit caused by differential shrinkage of the ceramic parts as is well known in the art. In this embodiment, the end cap  8  is a flat, circular sapphire window that sits in the rabbet  14  formed in the inner edge of flange  16 . The window is then sealed to the flange by differential shrinkage during sintering of the discharge vessel. When the manufacturing process requires that the electrode assemblies be inserted into openings  3  through the open end  21  of the discharge vessel, then the use of a frit material to seal the window to the flange is preferred. This method of insertion is particularly advantageous when the electrode tips  9  are angled toward each other. Other seal configurations are shown in  FIGS. 4 and 5  using frit or eutectic materials. Although preferred, it is not necessary to form a flange at the rim  4  of the open end  21  for sealing to the end cap  8 . For example, the flat, circular sapphire window could be sealed directly to a flat annular PCA rim surface with a eutectic material such as Y 2 O 3 —Al 2 O 3  as is known in the art. 
     Referring again to  FIG. 2 , stem  11  extends outward from base  23  and has two openings  3  which permit electrode assemblies  2  to pass through. Each electrode assembly  2  typically has three sections: an electrode tip  9 , a feedthrough section  17 , and a lead end  15  for connecting the lamp to a source of electric power (not shown). The stem  11  is preferably centered on central axis  20 . An enlarged cross section of stem  11  through line A-A is shown in  FIG. 3 . In this embodiment, the cross-sectional profile of stem  11  is generally oval, however, it is possible to use other stem geometries ranging from cylindrical or fluted columnar shapes to rectilinear shapes including wedge-shaped stems. 
     The feedthrough sections  17  of the electrode assemblies  2  are sealed in their respective opening  3  with a frit material  19 . A preferred frit material for this purpose is a 65% Dy 2 O 3 -10% Al 2 O 3 -25% SiO 2  frit (% by weight). The electrode assemblies  2  may be comprised of separate sections that have been welded or otherwise joined together, or may be formed as a single piece, e.g., a tungsten or molybdenum wire. Electrode tips  9  shown in  FIG. 2  have a coil welded to the tip which forms the point of arc attachment. However, the electrode tip  9  may be formed without the coil as shown in  FIG. 6 . In this embodiment, inner surface  7  and outer surface  29  are formed as a parabolic reflectors, the gap  6  between the electrode tips  9  where the arc discharge occurs is positioned approximately at the focus of the inner parabolic surface. However, in some embodiments it may be desirable to place the arc gap at an intermediate position between the focii of the two parabolic surfaces or even at the focus of the outer parabolic surface. A narrow arc gap is preferred in order to take better advantage of the optical properties of the parabolic reflector. 
     A discharge medium is contained in the discharge cavity  12 . Preferably, the discharge medium comprises a solid fill  25  and an inert gas such as argon or xenon. More preferably, the solid fill contains at least one metal halide e.g., NaI and some combination of DyI 3 , Tm 3 , HoI 3 , TlI, and LiI. The metal halide fill also may be combined with a small amount of mercury. Other discharge media include high pressure xenon gas or mercury, depending upon the desired spectrum of light to be emitted by the lamp. 
     In a first alternate embodiment shown in  FIG. 4 , the singled-ended ceramic lamp has a domed end cap  8 ′ which is sealed to flange  16 ′ by a frit material  30 . The frit material  30  is contained in a groove  32 . End cap  8 ′ extends to the outer edge of flange  16 ′ and is sealed to the top surface of flange  16 ′ instead of being seated in a rabbet. The frit material  30  may be a conventional frit such as Dy 2 O 3 —Al 2 O 3 —SiO 2  or it may be a eutectic material such as Y 2 O 3 —Al 2 O 3 . 
     In a second alternate embodiment shown in  FIG. 5 , the end cap  8 ″ has a lenticular portion  38  for focusing the light emitted by the lamp. Having a lens formed in the end cap  8 ″ is particularly advantageous for applications wherein the light needs to be focused into a light guide such as a fiber optic bundle. Like the embodiment shown in  FIG. 4 , the end cap  8 ″ extends to the outer edge of flange  16 ″. However, the frit material  30  is contained in a rabbet  14 ′ formed in the outer end of flange  16 ″. In addition, the outer surface  29  of the cupulate body portion  5  has been provided with a coating  27 . The coating may be a dark, light-absorbing coating such as a tungsten/alumina cermet that is designed to further reduce the amount of light exiting out the back of the lamp. The coating  27  also may be a reflective coating that is designed to reflect light back toward the discharge cavity  12  thereby increasing the amount of light exiting end cap  8 ″. The reflective coating may also comprise a multi-layer dichroic coating that is designed to reflect visible radiation and allow infrared radiation to pass through and out the back of the lamp. It may also be desirable in some cases for the coating to reflect infrared radiation back into the discharge vessel to increase efficiency. A reflective coating may also be applied to the inner surface of the discharge vessel. Such a coating must be capable of withstanding the environment inside the discharge vessel, particularly when the lamp is in operation, while maintaining its reflective properties. 
     In a third alternate embodiment shown in  FIGS. 6 and 7 , the stem  11 ′ has a wide single opening  37  that accepts a ceramic insert  35 . In this embodiment, the electrode assemblies  2 ′ comprise tungsten or tungsten alloy wires that have been sealed directly to insert  35  without a frit material. This is better seen in  FIG. 7  which is a cross section of the stem  11 ′ though line B-B. This stem configuration allows the orientation of the electrodes to be fixed prior to inserting them into the discharge vessel. In addition, it permits the electrode tips  9 ′ to be angled towards each other to prevent migration of the arc down the electrode assemblies. Because of the improved ability to fix the arc location, a narrower arc gap  6  may be realized. Once the electrodes have been fixed in the insert  35 , the insert  35  may then be sealed in opening  37  either with or without a frit material. This embodiment of the single-ended lamp is further shown with a close-fitting metal reflector  40  which is mounted on stem  11 ′ using collar  42 . Also, no solid fill is used. Instead, the discharge cavity  12  only contains a gaseous fill such as xenon gas. 
     While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.