Method of making a plasma lamp

#The present invention generally relates to electric lamps and methods of manufacture. More specifically, the present invention relates to lamps wherein the light source includes a light emitting plasma contained within an arc tube (i.e. plasma lamps) having dichroic thin film coatings to improve the operating characteristics of the lamp.

#Plasma lamps such as mercury lamps or metal halide lamps have found widespread acceptance in lighting large outdoor and indoor areas such as athletic stadiums, gymnasiums, warehouses, parking facilities, and the like, because of the relatively high efficiency, compact size, and low maintenance of plasma lamps when compared to other lamp types. A typical plasma lamp includes an arc tube forming a chamber with a pair of spaced apart electrodes. The chamber typically contains a fill gas, mercury, and other material such as one or more metal halides, which are vaporized during operation of the lamp to form a light emitting plasma. The operating characteristics of the lamp such as spectral emission, lumens per watt (“LPW”), correlated color temperature (“CCT”), and color rendering index (“CRI”) are determined at least in part by the content of the lamp fill material.

#The use of plasma lamps for some applications has been limited due the difficulty in realizing the desired spectral emission characteristics of the light emitting plasma. For example, metal halide lamps were introduced in the United States in the early 1960's and have been used successfully in many commercial and industrial applications because of the high efficiency and long life of such lamps compared to other light sources. However, metal halide lamps have not as yet found widespread use in general interior retail and display lighting applications because of the difficulty in obtaining a spectral emission from such lamps within the desired range of CCT of about 300-400 K and CRI of greater than about 80.

#FIG. 1 is an illustration of a formed body arc tube for plasma lamps.

#FIG. 2 is an illustration of the transmissivity characteristics of a multilayer coating according to one aspect of the present invention.

#FIG. 3 is an illustration of the variability of the CRI of the light transmitted by filters as a function of the location of the filter center.

#The present invention finds utility in the manufacture of all types and sizes of plasma lamps. As discussed above, plasma lamps have found widespread acceptance in many lighting applications, but the use of plasma lamps in some applications may be limited due to the difficulty in realizing the desired spectral emission characteristics of the light emitting plasma in such lamps. It has been discovered that multilayer thin film optical interference coatings designed so that a significant portion of the light that is selectively reflected by the coating is absorbed by the plasma provide a means for obtaining the desired spectral emission characteristics while maintaining or improving the overall operating characteristics of plasma. By way of example only, certain aspects of the present invention will be described in connection with obtaining the desired spectral emission characteristics in sodium/scandium metal halide lamps to raise the CRI of such lamps.

#FIG. 1 illustrates a formed body arc tube suitable for use in sodium/scandium metal halide lamps. With reference to FIG. 1, the arc tube 10 is formed from light transmissive material such as quartz. The arc tube 10 forms a bulbous chamber 12 intermediate pinched end portions 14. A pair of spaced apart electrodes 16 are sealed in the arc tube, one in each of the pinched end portions 14. The chamber 12 contains a fill gas, mercury, and one or more metal halides.

#During operation of the lamp, an arc is struck between the electrodes 16 that vaporizes the fill materials to form a light emitting plasma. According to the present invention, a multilayer thin film coating may be applied to any surface in the lamp which substantially surrounds the plasma, e.g., the arc tube, an arc tube shroud, the outer lamp envelope, or a reflector. According to certain aspects of the present invention, the number and thickness of the layers comprising the coating are determined so that a significant portion of the light emitted from the plasma that is selectively reflected by the coating is absorbed in the plasma. In the coatings of the present invention directed to plasma absorption, the properties of the coating (including reflectance, transmittance, and absorption) are determined as a function of several plasma and lamp characteristics including the spectral emission characteristics of the plasma, the spectral absorption characteristics of the plasma, the physical dimensions of the plasma, the angular distribution of the light emitted from the plasma on the coating, and the geometry of the coated surface.