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Significant efficacy enhancement of low wattage metal halide hid lamp systems by acoustically induced convection configuration

Modern metal halide high intensity discharge (HID) lamps today represent the class of light sources providing white light with the highest conversion efficiency (36-40%) at a very good colour quality (CRI = 80-95) for general lighting applications. Recent progress on producing ceramic arc tubes with very small tolerances in geometric shape allow to utilize lamp vessels as acoustic resonators. Investigations on the specific acoustic lamp spectra have revealed that the use of arc tubes with enhanced aspect ratio provide a sufficient selectivity of the acoustic modes useful for arc stabilization. However, high aspect ratio arc tube metal halide lamps show the drawback of colour separation by elemental segregation effects especially in vertical burning orientation. We have investigated new schemes of acoustic excitation by applying tuning procedures on the electrical operation power spectrum for lamp operation . By this, a long term stabilized arc straightening and plasma mixing effects can be achieved simultaneously which are counteracting to elemental segregation in any burning orientation. Excitation of a set of well defined acoustic resonances in the lamp plasma leads to stable multi-cell convection structures in the arc, which showed to be beneficial for high aspect ratio HID ceramic lamps in terms of arc stability, thermal loss and luminous efficacy. With the use of metal halide plasma gas compositions based on Na and the rare earth Ce very high electrical-to-visible efficiency of up to 50% can be achieved also for low wattage metal halide systems at 90-120 W. Lamps with warm-white colour (CCT =3000-3500K) show luminous efficacy levels up to 150 lmW-1 at a good general colour rendering index (CRI) in the range of 80-85. In combination with adequate electronic control gears based on a lean and low loss circuit topology combined with intelligent programming, new energy-saving HID systems for general lighting applications with high sustainability potential are feasible.