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Rubidium halide cathodes for low-pressure, mercury-free discharge lamps with metal halide filling

IP.com Disclosure Number: IPCOM000033194D
Publication Date: 2004-Dec-01
Document File: 3 page(s) / 27K

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Rubidium halide cathodes for low-pressure, mercury-free discharge lamps with metal halide filling


Low-pressure, mercury-free, Indium-halide and Copper-halide discharge lamps are known. The electric discharges of low-pressure gas fillings of Indium-halides and/or Copper-halides generate the optical radiation. The spectrum of atomic Indium and Copper, and also of Indium-halides and Copper-halides lie in blue to near-UVA range, of which the radiation is to be converted to white light radiation with good colour rendering via phosphors. Due to the smaller Stokes´shift in the radiation conversion from UVA photons to the visible photons, the electrical input energy is more efficiently used in generating white light. The luminous efficacy of these lamps reaches above 100 lm/W. With regard to environmental issues, lamps containing no harmful substances are limited in their usages. In this aspect, the mercury-free lamps are "environmental-friendly" products.

The low-pressure discharge lamps conventionally equip inner electrodes inside the lamp bulbs for sustaining electrical discharges. The widely-used electrode for the low-pressure discharge lamps consist of a tungsten wire and electron emitters. In order to enhance electron emission from the electrodes, materials with lower work function, alkali-earth oxides (Calcium oxides, Strontium oxides, and Barium oxides) are used as the electron emitters.

During operation of the above mentioned metal-halide lamps, the gas temperature in the lamp bulbs reaches 150-500oC. When alkali-earth oxides electron emitters are used in the lamps, the metal halides then react strongly with the alkali-earth oxides and form metal-oxides and alkali-earth halides. The required metal-halide gas filling within the lamp bulb do not exist any more and the electron emission from the emitter is drastically reduced or stopped. The metal halide discharge is then terminated and no optical radiation from the metal-halides can be emitted. Therefore, it is not appropriate to use the alkali-earth oxides as the electron emitter materials of the electrodes. Nevertheless, the discharge operation using the inner electrodes use the existing electrical ballasts for the mercury-rare-gas fluorescent lamps and sodium lamps. For these reasons, providing the electrode concept for low-pressure, metal-halide lamps is therefore of the critically important issue to be solved. This publication contains a new concept for the electron emitters capable of being used in low-pressure metal-halide lamps

Findings and discussion

Rubidium-halides have work functions between 0.1 and 1.2eV (RbF: 0.1eV; RbCl: 0.5eV; RbBr: 0.4eV; RbI: 1.2eV), comparable to values of BaO (0.6eV). In accordance with the Richardson-Dashman theory for thermal electron current emission, the emitted electron current increases exponentially with decreasing work function. Therefore, in terms of thermal electron emission, it is expected that Rubidium-halides have an el...