Besides mercury vapor, the lamp contains iodides or bromides of different metals. Iodine and bromine are of the halogen group of the periodic table, and so are termed "halides" when ionized. Scandium and sodium are also used in some types, with thallium, indium, and sodium in European "Tri-Salt" models. More recent types use dysprosium for high color temperature and tin for lower color temperature. Holmium and thulium are used in very high power movie lighting models. Gallium or lead are used in special high UV-A models for printing purposes. The mixture of the metals used defines the color of the lamp. Some types, for festive or theatrical effect, use almost pure iodides of thallium, for green lamps, and indium, for blue lamps. An alkali metal, (sodium or potassium), is almost always added to reduce the arc impedance, allowing the arc tube to be made sufficiently long and simple electrical ballasts to be used. A noble gas, usually argon, is cold filled into the arc tube at a pressure of about 2 kPa to facilitate starting of the discharge. Argon filled lamps are typically quite slow to start up, taking several minutes to reach full light intensity; xenon fill, as used in automotive headlamps, start up relatively faster.