Fluorescent lamp extension tube amalgam holder

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.

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.

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.