Effect of impurities on discharge lamps and beneficial action of getters

In discharge lamps a clean gaseous atmosphere is an important requisite to assure satisfactory operating characteristics and lifetime. During lamp manufacturing it is not possible to obtain a completely clean atmosphere in the discharge tubes and in the outer bulbs, if present, because of the presence of residual air not removed by the pumping systems and of impurities released by the components. Residual gases after device sealing typically consist of oxygen, nitrogen and water vapour (air components). In addition some gaseous contamination is induced during lamp operation because impurities are outgassed by internal lamp components. Hydrogen is the most dangerous impurity for the discharge lamps: presence of small amounts of this gas in the arc tube can significantly increase the starting and re-ignition voltage. This phenomenon occurs because hydrogen molecules interact with the excited filling gas atoms carrying off their energy that should contribute to the discharge; in order to compensate this energy dissipation a higher initial voltage is necessary to start the discharge and also a higher operation voltage is requested. Due to the higher requested voltage heavier ions bombardment mechanisms may occur on the electrodes and higher sputtering phenomena take place, thus causing blackening on the lamp parts close to the electrodes. Other gaseous impurities like nitrogen, carbon monoxide and carbon dioxide have a similar negative effect but to a less extent than hydrogen. In addition the presence of H₂ and of other impurities in the outer bulb of high intensity discharge lamps, typically sealed under vacuum or filled with argon and nitrogen, can induce thermal losses and difficulties in the achievement of suitable, stable thermal conditions for the arc tube. Usually a getter material able to capture gaseous impurities is placed inside the arc tube of low pressure discharge lamps to maintain a high purity of the filling gas and high performances during - ll the lifetime. In High intensity discharge lamps a getter is mounted in the outer bulb to sorb impurities in the envelope and to remove hydrogen even from the discharge tube: as the active getter material is able to maintain a low hydrogen partial pressure in the bulb, during lamp operation, the hydrogen evolved inside the arc tube tends to permeate the walls and to diffuse into the outer bulb where it is sorbed. The paper will describe the negative influence induced by the impurities on the performances of discharge lamps and the positive effects provided by the use of getters.