Microwave excitation of metal halide plasma lamps

High pressure metal halide plasma (HID) lamps as compact light sources are characterized by high efficiency, excellent color rendering and long lifetime. Usually the lifetime is mainly influenced by electrode erosion leading to wall blackening and a reduction of the luminous flux. Therefore it is of actual research interest to have an electrodeless power input into HID lamps. More then ten years ago there was a high power electrodeless sulfur lamp with a microwave excitation commercially available. New trends with lower power HID units below 50 W including research on electrodeless HID lamps could expand the field of application and break into new markets. This work discusses the high frequency power input and the plasma behavior in high pressure lamps. Therefore several lamp geometries and lamp fillings were tested. The lamps were made of quartz and ceramics containing Ar as an ignition gas and several metal halides as light emitting substances. The lamps were ignited by an external high voltage pulse and operated in a special resonator configuration at frequencies around 2.45 GHz. The main energy input by inductive coupling was varied between 5 and 30 W. The power supply was built on a semiconductor basis. Input power, spectral distribution in the visible spectral range and vessel temperature were observed by a vector network analyzer, a fiber spectrometer and an infrared camera respectively. To enhance the vapor pressure of the additives, the lamps were also operated in a vacuum chamber. Variations in the filling, the input power and the wall temperature lead to changes in the spectral output, color temperature, color rendering and to changes in the coupling with the circuit. Further experiments are planed to study the influence of other resonator configurations and lamp shapes.