The blackening is due to tungsten transferred from the filament to thewall
The present invention relates to an electric lamp with high efficiency, good color rendering, and high lamp lumen maintenance. It finds particular application in connection with a ceramic metal halide lamp with halides of barium or strontium inthe fill and will be described with particular reference thereto.
Discharge lamps produce light by ionizing a vapor fill material such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucentor transparent discharge vessel which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to beingexcited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Ceramic metal halide lamps have been developed with efficiencies in the range of about 90-100 lumens per watt (LPW), color rendering indices Ra of 85-95, or higher, and lumen maintenance values of 80%, or higher, and color temperatures ofbetween about 2600 and 4000K at wall loadings of from about 20 to 50 W/cm2. However, premature failure of the lamps may occur due to blackening of the discharge vessel walls. The blackening is due to tungsten transferred from the filament to thewall. The presence of oxygen and/or water vapor in the lamp atmosphere has been found to contribute to the wall blackening. Water vapor is particularly harmful because even trace amounts increase the evaporation of the tungsten filament coil by meansof the well-known “water cycle.” In the water cycle, the temperature of the tungsten coil is thermally sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with the tungsten from the coil to form volatile oxides,which migrate to cool parts of the lamp and condense. These oxide deposits are reduced by the gaseous hydrogen to yield black metallic tungsten and reformed water, which causes the cycle to repeat.
Tungsten halogen lamps, which comprise a hermetically sealed, light transmissive discharge vessel enclosing a tungsten filament and containing a fill comprising a halide or halogen gas are widely used in a variety of applications. Some of theselamps operate on a tungsten halogen cycle which is a regenerative, continuous process in which a halogen-containing tungsten compound is produced when the halide combines chemically with particles of tungsten evaporating from the incandescent tungstenfilament. Subsequent thermal decomposition of these so-formed halogen-containing tungsten compounds at the filament returns the tungsten particles back to the filament. Halogen compounds used for the fill include bromine and bromides, such as hydrogenbromide, methyl bromide, dibromomethane, and bromoform. Lamps that operate at low wall loadings (WL), e.g., below about 30 W/cm2, and thus low temperatures, i.e. below about 200° C. interior wall temperatures, generally do not support thetungsten halogen cycle. Additionally, if WL is too low then the halide temperature tends to be too low leading to reduced halide vapor pressure and reduced performance.
It has been proposed to incorporate of a calcium oxide or tungsten oxide dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO 99/53523 to Koninklijke Philips Electronics N.V. U.S. Pat. No. 6,844,676 to Alderman,et al. discloses an arc tube fill comprising metallic mercury, a mixture of noble gases and, optionally, radioactive 85Kr, and a salt mixture such as a mixture composed of sodium iodide, calcium iodide, thallium iodide, and several rare earthiodides.
In one aspect of the exemplary embodiment, a ceramic metal halide lamp includes a discharge vessel formed of a ceramic material which defines an interior space. An ionizable fill is disposed in the interior space. The ionizable fill includes aninert gas, mercury, and a halide component. The halide component includes an alkali metal halide, an alkaline earth metal halide component, and optionally at least one of a rare earth halide and a Group IIIA halide. The alkaline earth metal halidecomponent includes at least one of a barium halide and a strontium halide. At least one electrode is positioned within the discharge vessel so as to energize the fill when an electric current is applied thereto. The lamp having a wall loading, whenenergized, which is sufficient to maintain the tungsten halogen cycle.
In another aspect, a ceramic metal halide lamp includes a discharge vessel formed of a ceramic material which defines an interior space. An ionizable fill is disposed in the interior space. The ionizable fill includes an inert gas, mercury, anda halide component. The halide component includes, expressed as mol % of the total halide component of the fill, at least about 5 mol % of sodium halide, optionally, from about 1% to about 10% of a group IIIA metal halide, from about 10% to about 95% ofan alkaline earth metal halide, the alkaline earth metal halide comprising at least one of barium halide and strontium halide, and optionally from about 1% to about 15% of a rare earth metal halide. The lamp has a wall loading of at least 30 W/cm2.
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