1. Field of the Invention
The present invention generally relates to electric lamp assemblies, and more particularly, to electric lamp assemblies having an improved structure for supporting a shroud.
2. Description of Related Art
Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica lamp capsule or arc tube that is hermetically sealed within a borosilicate glass bulb or outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes press sealed in opposite ends and has a bulb portion containing fill material including mercury, metal halide additives, and a rare gas to facilitate starting. In some cases, both in low and high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud which comprises a generally cylindrical tube of light-transmissive material, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp outer envelope with the arc tube located within the shroud. The shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters. The shroud allows the lamp outer envelope to remain intact by dissipating the energy of a shattering arc tube. The presence of a shroud expands the market for metal halide lamps into open-type (absence of an expensive cover plate) lighting fixtures. The shroud can also be used for color correction of the discharge source. For such color correction, the shroud includes a wavelength selective reflector or absorber or phosphor, such as a multilayer titania-silica dichroic reflector.
Sodium is an important constituent in metal halide arc discharge lamps, usually in the form of sodium iodide. Sodium is used to improve the efficacy and color rendering properties. It has long been recognized that arc tubes containing sodium lose sodium during operation by movement or migration through the arc tube wall. The iodine originally present in a metal halide lamp as sodium iodide is freed by sodium loss, and the iodine combines with mercury in the arc tube to form mercury iodide. Mercury iodide leads to increased reignition voltages, thereby causing starting and lamp maintenance problems, and shortening lamp life.
There is evidence that most of the sodium loss is due to a negative charge on the arc tube walls caused by photoelectric emission from electrified side rods used to support the arc tube and shroud within the outer envelope. One solution to the problem has been various electrically insulated, isolated, or "floating" mounting supports attached or clipped to the outer surface of the shroud and the press seals of the arc tube in combination with a current return line for the outer end electrode of a fine molybdenum wire, known as a flying lead, spaced as far away from the arc tube as possible and hugging the curve in the outer bulb. For example see U.S. Pat. Nos. 5,270,608, 5,252,885, 5,136,204, 5,122,706, and 4,963,790 the disclosures of which are expressly incorporated herein in their entirety. While such lamp constructions provide an improvement, the elements located outside of the shroud limit the outer diameter of the shroud and thus constrain the physical size or wattage of the arc tube that can be used with a given outer envelope, the press seals of the arc tube must have tight manufacturing tolerances and are prone to damage during assembly, and the structures require a relatively high number of parts and welds.