In copending application Ser. No. 912,268, filed June 5, 1978 by Daniel M. Cap and William H. Lake, titled High Pressure Metal Vapor Discharge Lamps of Improved Efficacy, which is a continuation-in-part of an earlier application Ser. No. 812,479, filed July 5, 1977 similarly assigned, useful and efficient high pressure discharge lamps are disclosed having much smaller sizes than have been considered practical heretofore, namely discharge volumes of one cubic centimeter or less. In preferred form achieving maximum efficacy, these high intensity lamps utilize generally spheroidal thin-walled arc chambers which may vary in shape from slightly oblate to substantially prolate. Remarkably high efficacies are obtained by raising the metal vapor pressure above 5 atmospheres and to progressively higher pressures as the size is reduced. In such miniature lamps, the convective arc instability usually associated with the high pressures utilized is avoided, and there is no appreciable hazard from possibility of explosion. Practical designs provide wattage ratings or lamp sizes starting at about 100 watts and going down to less than 10 watts, the lamps having characteristics including color rendition, efficacy, maintenance and life duration making them suitable for general lighting purposes.
A less desirable characteristic of these miniature high pressure metal vapor lamps is the very rapid deionization to which they are subject. In operation on 60 Hz alternating current, deionization is almost complete between half cycles so that a very high restriking voltage is required to be provided by the ballast. Particularly in metal halide lamps, during lamp warm-up within the first few seconds after arc ignition, the reignition voltage reaches extremely high levels. In view of these deionization limitations associated with low frequency operation of miniature metal halide lamps, the use of conventional 60 Hz ballasts has many disadvantages.
The object of the invention is to provide an improved method or operating system for miniature metal halide lamps which overcomes the limitations imposed by rapid deionization at low operating frequencies and which permits the design of compact, practical and efficient high frequency ballasts.