Stefano Paolo Giorgi

Materials to Improve Performance of Discharge Lamps

Lighting is responsible for the consumption of more than 2650 TWh of energy per year, corresponding to about 20 of the global electricity consumption. All the strategies and international policies pushing for the adoption of more efficient light sources are assuming a real important role for energy saving and for the reduction of greenhouse gas emissions. This paper will illustrate technological approaches that can contribute at improving the performance of low-pressure mercury lamps and high-intensity-discharge lamps. Advancements and optimization of materials, such as getters, active amalgams, and starting amalgams, can allow to produce light sources with superior characteristics. Concerning the aspect related to innovative materials, it is worth underlining that many of the fluorescent lamps today available on the market are using advanced technological solutions based on stable mercury compounds for accurate and reliable Hg dosing, down to very low levels.

Mercury Dosing in Fluorescent Lamps

Low pressure mercury lamps are very efficient light sources, but for the radiation generation they exploit mercury that has a strong negative effect on the environment. Worldwide regulations are pushing for the reduction of mercury content in fluorescent lamps in order to minimize their environmental impact. The trend of Hg content reduction is requiring advanced technological solutions for safe and precise Hg dosing in lamp down to low level. The old liquid mercury dispensing technology, involving risks of contamination with new more reliable and more environmental friendly mercury dosing solutions. The innovative technologies exploiting Hg dispensers based on stable mercury compounds are very reliable solutions for a safe and controlled delivery of small amounts of mercury. The paper will present main methods used to introduce mercury in flourescent lamps, especially focusing on the characteristics of the new Hg dispensers based on inter-metallic compounds that allow to accurately dose mercury even at very low level.

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

Materials to improve performances of discharge lamps

Lighting is responsible for the consumption of more than 2650 TWh of energy per year, corresponding to about 20% of the global electricity consumption. All the strategies and international policies pushing for the adoption of more efficient light sources are assuming a real important role for energy saving and for reduction of greenhouse gas emissions.

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.