Patent Application
20060290285
Metal halide discharge lamps have been favored for their high efficacies and high color rendering properties which result from the complex emission spectra generated by their rare-earth chemistries. Particularly desirable are ceramic metal halide lamps which offer improved color rendering, color temperature, and efficacy over traditional quartz arc tube types. This is because ceramic materials can operate at higher temperatures than quartz and are less prone to react with the various metal halide chemistries. The preferred ceramic material is polycrystalline aluminum oxide (polycrystalline alumina or PCA).
Various shapes have been proposed for ceramic discharge vessels ranging from a right circular cylindrical shape to an approximately spherical (bulgy) shape. Examples of these types of arc discharge vessels are given in European Patent Application No. 0 587 238 A1 and U.S. Pat. No. 5,936,351, respectively. The bulgy shape with its hemispherical ends is preferred because it yields a more uniform temperature distribution, resulting in reduced corrosion of the discharge vessel by the metal halide fill materials.
One limitation to introducing ceramic metal halide lamps into broader markets (such as residential applications) is the time that it takes for the lamp to warm-up and reach its steady-state operating condition with full light output or steady-state operating voltage. For a typical ceramic metal halide lamp, this warm-up period may take several tens to hundreds of seconds, depending on the amount of power delivered and the heat capacity of the lamp. Larger lamps have greater mass and heat capacity and thus require a longer time to absorb enough energy to raise their temperature to the point where the metal halide salts are sufficiently vaporized to produce the desired light output. Besides limiting the applications for ceramic metal halide lamps, slow warming can also result in sputtering of the tungsten electrodes leading to blackening of the lamp and a decrease in light output.
One method that has been used to decrease the warm-up period is to overpower the lamp for an initial period until the lamp is fully operational. For example, automotive lamps which normally operate at 35 W are routinely ignited and operated at about 90 W for several seconds because of the need for instant lighting of the roadway. However, this approach requires a different ballast to operate the lamp and is practical only when new fixtures are installed. In addition, the over-wattage condition risks cracking and explosive failure of the ceramic discharge vessel from the thermal shock.
U.S. Pat. No. 6,294,871 describes doping ceramic bodies, primarily polycrystalline alumina arc tubes, with a UV-absorbing additive selected from europium oxide, titanium oxide and cerium oxide to provide UV attenuation. The doping is preferably done at a level below about 5000 ppm in order to preserve translucency. Other oxides of rare earth metals including lanthanum, dysprosium and neodymium are also cited as possibly providing UV attenuation. Another effect attributed to the dopants is allowing the arc tube to run at a higher temperature. However, the patent contains no information on the effect on the warm-up time of the arc tubes.
Thus, it would be advantageous to provide a rapid warm-up ceramic metal halide lamp that could be used in existing fixtures or other applications where rapid warm-up is desired.
We have discovered that the warm-up time of ceramic metal halide lamps may be dramatically shortened, by at least about 50%, by making the discharge vessel out of polycrystalline dysprosium oxide (dysprosia), Dy2O3. The reason for the shorter warm-up time is believed to be a result of the strong absorption bands of polycrystalline dysprosia in the range of 275-475 nm in combination with a heat capacity that is lower than PCA. These strong absorption bands, which are not present in undoped PCA, absorb UV and blue radiation emitted by the discharge which is then converted to heat causing to a quicker warming of the discharge vessel and the components of the metal halide fill. The lower heat capacity means that less heat is needed to increase the vessel temperature.
In a conventional metal halide lamp containing mercury, the emitted radiation from the discharge during the warm-up phase is typically Hg atomic emission with strong lines at 254 nm, 365 nm, and 436 nm. In effect, the low power phase during warm-up produces blue and UV radiation which previously exited the PCA discharge vessel. The instant invention captures this radiation and converts it into heat in the ceramic body of the discharge vessel. Essentially, the amount of power available for heating the discharge vessel is increased during the warm-up phase with no overt electrical overpowering of the ballast.
A metal halide lamp made with a polycrystalline dysprosium oxide discharge vessel has a warm-up time that is less than about 50%, and preferably less than about one-third, of the warm-up time of a similarly constructed and operated lamp made with a PCA discharge vessel. For example, a 70 W ceramic metal halide lamp can have a warm-up time of less than about 20 seconds with a Dy2O3 discharge vessel compared to greater than 50 seconds for the same lamp with a Al2O3 discharge vessel when operated under normal, i.e., not over-wattage, conditions. Since the rapid warm-up is achieved only by a change in the ceramic material, the metal halide lamps according to this invention can be operated in existing fixtures without the need for changing the electrical ballast. As used herein, the term “ceramic metal halide lamp” also includes lamps with a ceramic discharge vessel that contains substantially only metallic mercury as a fill.
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.
Referring now to
Two opposed capillary tubes 2 extend outwardly from the body 6 along a central axis. The capillary tubes in this embodiment have been integrally molded with the ceramic body. The discharge chamber 12 may contain a buffer gas, e.g., 30 torr to 20 bar Ar, Ne, Kr, Xe or mixtures thereof, and a metal halide fill 8, e.g., mercury plus a mixture of metal halide salts, e.g., NaI, CaI2, DyI3, HoI3, TmI3, and TlI. Lamp fills are not limited to these specific salts. Other rare earth, alkali, and alkaline metal salts may also be used, such as PrI3, LiI, or BaI2. The metal halide fill may also be mercury-free in which case the metal halide salt mixture may also contain other easily volatilized components such as InI and ZnI2. The fill 8 may also be substantially only mercury in sufficient quantity to produce approximately a 200 bar operating pressure.
Electrodes assemblies 14 are sealed to capillaries 2 with a frit material 9. The discharge tips 3 of the electrode assemblies 14 protrude into the discharge chamber 12 and the opposite ends 5 extend beyond the distal ends 11 of the capillaries in order to supply electrical power to the discharge vessel. Electrical power may be supplied by a number of ballast types (not shown) including lead or lag, 50 or 60 Hz conventional magnetic ballasts, or an electronic ballast at a suitable frequency to operate the lamp in frequency regions clear of undesirable acoustic resonances, e.g., a 90 Hz square wave.
In a preferred structure, the electrode assemblies are constructed of a niobium feedthrough, a tungsten electrode, and a molybdenum coil that is wound around a molybdenum or Mo—Al2O3 cermet rod that is welded between the tungsten electrode and niobium feedthrough. A tungsten coil or other suitable means of forming a point of attachment for the arc may be affixed to the tip 3 of the tungsten electrode. The frit material 9 creates a hermetic seal between the electrode assembly 14 and capillary 2. In metal halide lamps, it is usually desirable to minimize the penetration of the frit material into the capillary to prevent an adverse reaction with the corrosive metal halide fill.
A glass outer envelope 30 surrounds the discharge vessel and its associated components and is sealed to stem 47 to provide a gas-tight environment. Typically, the outer envelope is evacuated, although in some cases it may contain up to 400 torr of nitrogen gas. A getter strip 55 is used to reduce contamination of the envelope environment.
Referring to
The Dy2O3 discharge vessels were slightly smaller than the standard 70 W PCA discharge vessel, however, the dimensional differences are not thought to be related to the observed rapid warm-up of the Dy2O3 vessels. This is because a relatively slow warm-up is present in all sizes and wattages of metal halide lamps with PCA discharge vessels. The dimensions of the vessels are given in Table 1.
The lamps are “warmed-up” to their steady-state operating condition when there is no longer a substantial change in Vimax. With reference to the curves in
With the values of Vss determined it is possible to directly compare the warm-up performance of these lamps. As defined herein, the warm-up time of the lamp is the time following the initial arc ignition at which Vimax reaches 90% of the steady-state operating voltage, Vss. This threshold point is plotted in
This effect is not to be expected if one considers that Dy2O3 when compared to PCA has a lower thermal diffusivity (about 5 times lower at 500° C.) and a lower thermal conductivity (about 7 times lower). If heat conduction in the ceramic were the sole mechanism of heat transport, then it would be expected that there would be a slower heating of the cold end of the Dy2O3 vessel leading to a slower warm-up. Therefore, as stated earlier, radiation absorption must have played an important role in the observed rapid warm-up in the Dy2O3 vessel. The absorption properties of Dy2O3 can been seen in
A further consideration is the lower heat capacity of Dy2O3. In terms of voluminous heat capacity, PCA is actually 1.5 times higher than Dy2O3. Thus, on the basis of heat capacity alone, it would take less heat to raise the temperature of a Dy2O3 vessel at a given volume. This is also believed to be an important contributor to the rapid warm-up of the Dy2O3 vessel.
While there have been shown and described what are 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.
