Abstract:
A glass extension ( 24 ) with a cavity ( 32 ) for an amalgam ( 22 ) is added to an exhaust tube ( 12 ) or a flare ( 14 ) of a fluorescent lamp. The mouth ( 36 ) of the cavity (32) is provided with a bimetal valve having a cover portion ( 40 ) to close the cavity ( 32 ) when the lamp is cold and to open the cavity when the lamp is hot. The amalgam ( 22 ) is a metallic amalgam of, for example, mercury or other know suitable materials for establishing a metallic vapor equilibrium during operation of a discharge lamp. The amalgam holder helps to maintain optimum mercury vapor equilibrium at start-up, and to reduce the effects of mercury vapor starvation resulting from off-times.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electric lamps and, in particular, to a holder for an amalgam in a gas discharge lamp. 
     Maintaining mercury vapor pressure equilibrium within a gas discharge lamp such as a fluorescent lamp is necessary to maintain optimum lumen output during extended lamp on periods. In conventional fluorescent lamps, the mercury vapor pressure increases to an optimum pressure allowing the lamp to reach maximum lumen output. As time passes, the mercury vapor pressure increases to a level above the most preferable pressure causing the luminous flux to decrease. 
     To help maintain the pressure at the optimum pressure, amalgams are introduced to maintain the mercury vapor pressure within an optimal range during lamp operation. Upon lamp ignition, the amalgam is heated which causes mercury to diffuse out of the solid and is released into the lamp as vapor. The amalgam achieves mercury vapor equilibrium during lamp operation by supplying the same amount of mercury atoms to the interior of the lamp envelope as are spent. However, when the lamp is switched off, the decrease in temperature causes the mercury vapor to navigate to and diffuse into the amalgam causing mercury starvation. The lack of mercury vapor in the lamp envelope during off periods results in low lumens at lamp startup. In order to obtain peak lumens upon lamp ignition without any startup time penalty, an adequate dose of mercury vapor is required to remain in the lamp envelope during lamp off periods. 
     A restricted fixed orifice in an amalgam capsule can be used to reduce the starvation effect for short off times. U.S. Pat. No. 5,828,169 uses such a technique. However, during extended off times, virtually all of the mercury can still return to the amalgam. 
     SUMMARY OF THE INVENTION 
     An amalgam holder for a discharge lamp having an exhaust tube and a flare pinched about the tube where the holder includes an extension of the tube or flare having a cavity for an amalgam, the cavity having a mouth; and a cover for the mouth, the cover being thermally actuated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an amalgam holder according to the invention. 
     FIG. 2 is a perspective view of a detail of the holder of FIG.  1 . 
     FIG. 3 is a plan view of an amalgam holder according to an additional embodiment of the invention. 
     FIG. 4 is a perspective view of a detail of the holder of FIG.  3 . 
     FIG. 5 is a plan view of an amalgam holder according to another embodiment of the invention. 
     FIG. 6 is a perspective view of a detail of the holder of FIG.  5 . 
     FIG. 7 is a plan view of an amalgam holder according to still another embodiment of the invention. 
     FIG. 8 is a perspective view of a detail of the holder of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an amalgam holder  10  for a gas discharge lamp such as a fluorescent lamp includes an exhaust tube  12 , a flare  14 , two lead wires  16 ,  18 , a cathode  20  between the lead wires  16 ,  18 , an amalgam  22 , an extension  24  and a cover or valve  26 . The exhaust tube  12 , the flare  14 , the extension  24  may be formed, for example, from glass or a glass-like material. The leads  16 ,  18  and the cathode  20  are electrically conductive materials well-known in the art. The amalgam  22  is a metallic amalgam of, for example, mercury and other materials known in the art to be suitable for establishing a mercury (or other metallic ion) equilibrium during operation of a discharge lamp. 
     Typically, the flare  14  is pinched over the leads  16 ,  18  and the exhaust tube  12  while the material of the flare and tube are in a softened state. The area where the flare  14  is pinched is called the pinch  28 . The pinch  28  typically includes a vent  30  in communication with the interior of the exhaust tube  12 . A discharge lamp typically has a tubular envelope (not shown) that is closed at the end by inserting the flare  14  into the envelope cathode-first and sealing the flare  14  to the envelope. 
     The extension  24  can be advantageously formed from an extended portion of the tube  12  or it may be formed as an extension of the flare  14 . If desired, the extension  24  could be formed from a separate piece of material. After pinching, there is little apparent difference. 
     The extension  24  may be, for example, generally cylindrical with a generally cylindrical cavity  32  within. The amalgam  22  may be retained within the cavity  32  by a dimple  34 , or the like, in the wall of the extension  24 . The mouth  36  of the cavity  32  may also be narrowed to constrict mercury vapor flow and to allow more efficient covering of the mouth  36  with the cover  26 . 
     Referring also to FIG. 2, the cover  26  is a generally L-shaped piece of bimetal having a mounting portion  38  and a cover portion  40 . The mounting portion  38  is attached to the lead wire  16  by, for example, welding. The cover portion  40  closes the mouth  36  when the cover  26  is cold. During operation of the lamp, the cover  26  is heated and the cover portion  40  lifts off of the mouth  36 , thereby opening the cavity  32  to the atmosphere inside the envelope. The cover  26  thus operates as a thermally actuated valve. 
     When the lamp is at full operating temperature, the cover  26  has been heated by the cathode  20  and is open and the amalgam  22  establishes a desired mercury equilibrium inside the envelope. When the lamp is turned off, the cover  26  closes off the amalgam  22  from the atmosphere of the envelope thereby preventing the mercury in the envelope atmosphere from recombining with the amalgam  22 . In this way, sufficient mercury remains in the envelope atmosphere to prevent mercury starvation during lamp startup. 
     While the cover  26  in the preferred embodiment is heat actuated using a bimetallic element, it would also be possible to actuate the cover  26  with electrical, chemical, or other motive techniques. 
     Referring to FIGS. 3 and 4, an additional embodiment of an amalgam holder  10 ′ includes many of the previously described elements and operates in a similar manner. The bimetal cover  26 ′ includes a mounting portion  38 ′ and a cover portion  40 ′. The mounting portion  38 ′ is a generally cylindrical sleeve and the cover portion  40 ′ is a flap attached to the mouth end of the sleeve. The extension  24 ′ includes a rolled in groove  42  to assist in retaining the cover  26 ′ by engaging an annular lip  44  inside the mounting portion  38 ′. 
     Similar to the previous embodiment, the cover portion  40 ′ closes the mouth  36  when the cover  26 ′ is cold. During operation of the lamp, the cover  26 ′ is heated and the cover portion  40 ′ lifts off of the mouth  36 , thereby opening the cavity  32  to the atmosphere inside the envelope. The cover  26 ′ thus operates as a thermally actuated valve. 
     Referring to FIGS. 5 and 6, an additional embodiment of an amalgam holder  10 ″ includes many of the previously described elements and operates in a similar manner. The bimetal cover  26 ″ includes a mounting portion  38 ″, a cover portion  40 ″ and a strip portion  46 . The mounting portion  38 ″ is a band seated in the groove  42  and the cover portion  40 ″ is a flap connected to the mounting portion  38 ″ by the strip portion  46 . If the strip portion  46  is considered to be a remaining portion of a cylindrical sleeve, it would represent between 1 and 10 degrees of the sleeve, with 5 degrees being preferred. 
     Similar to the previous embodiment, the cover  40 ″ closes the mouth  36  when the cover  26 ″ is cold. During operation of the lamp, the cover  26 ″ is heated and the cover portion  40 ″ lifts off of the mouth  36 , thereby opening the cavity  32  to the atmosphere inside the envelope. The cover  26 ″ thus operates as a thermally actuated valve. 
     It should be noted that the embodiments of FIGS. 3,  4 ,  5  and  6  each have a mounting portion that clasps the extension. While these embodiments completely encircle the extension, it is also possible to clasp the extension with embodiments that do not entirely encircle the extension. For example, if the mounting portion encircles greater than half of the circumference of the extension, the extension will still be clasped by the mounting portion. 
     Referring to FIGS. 7 and 8, an additional embodiment of an amalgam holder  10 ′″ includes many of the previously described elements and operates in a similar manner. The bimetal cover  26 ′″ includes a mounting portion  38 ′″ and a cover portion  40 ′″. The mounting portion  38 ′″ is a V-shaped member that compresses when inserted in the mouth  36  and then expands to retain the cover  26 ′″ on the extension  24 . The mounting portion  38 ′″ includes an aperture  48  to minimize the blocking of the mouth  36  with the mounting portion  38 ′″. The cover portion  40 ′″ is a flap attached to the top of one leg of the V of the mounting portion  26 ′″. 
     Similar to the previous embodiments, the cover portion  40 ′″ closes the mouth  36  when the cover  26 ′″ is cold. During operation of the lamp, the cover  26 ′″ is heated and the cover portion  40 ′″ lifts off of the mouth  36 , thereby opening the cavity  32  to the atmosphere inside the envelope. The cover  26 ′″ thus operates as a thermally actuated valve. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.