Characteristics of the getter materials used in High Intensity Discharge Lamps

In the field of discharge lamps many efforts are focused on improving qualities and performances of the devices. It is well known that gaseous impurities, possibly present inside the burner and inside the outer bulb of high intensity discharge (HID) lamps, are one of the main causes of the deterioration of their performances. Usually getter materials are adopted to absorb the gaseous impurities that may be generated by the residues of lamp fabrication process or that are released during lamp operation. These can be efficiently removed by means of a simple getter component, so as to create and maintain the very clean atmosphere necessary to assure high quality characteristics of the lamp for all its life. Getters are used in HID lamps with outer bulbs both under vacuum and with nitrogen atmosphere; in this second type of lamps it is necessary that the getter works properly without being affected by, or interfering with, the jacket filling gas. The getter material must be able to sorb various gaseous species generated inside the lamp envelope, such as hydrogen, water vapour, carbon monoxide and hydrocarbons. In this paper we have presented and discussed properties and sorption characteristics of the main getter materials adopted in HID lamps, such as the Zr-Al, Zr-Fe and Zr-Co-rare earths alloys. The sorption performances of the materials were measured in experimental systems simulating possible lamp conditions. Experiments were carried out to study the ability of the materials to capture hydrogen that is the most deleterious gaseous contaminant in discharge lamps. Moreover, in order to understand the complex sorption mechanisms that take place on the getters when different gaseous impurities are coexisting inside the lamps, specific tests were performed to determine the simultaneous sorption of different gas species, such as sorption of hydrogen and nitrogen, hydrogen and carbon monoxide, hydrogen and carbon dioxide