A metal halide discharge lamp comprises a light-emitting tube provided with a pair of electrodes arranged to face each other. A rare gas, a halide of a light-emitting metal, and mercury are sealed in the light-emitting tube to form the metal halide discharge lamp. The metal halide discharge lamp is classified into a short arc type and a long arc type. In a short arc metal halide lamp with bilateral sealed terminations, at least one metal halide is encapsulated as the emission metal in an arc tube which is made of quartz glass and in which a pair of electrodes is located, at a distance of roughly a few millimeters from one another, together with mercury and a starting rare gas. Seal areas are joined in one piece to the two ends of the arc tube. In the respective seal area, a molybdenum foil is inserted, to the ends of which an electrode and one end of an outer lead pin are welded for purposes of power supply. The short arc type metal halide discharge lamp is used in projectors such as liquid crystals projector in which light rays emitted from a lamp are collected so as to be projected onto a screen, and an overhead projector, and is also used for illumination of shops in the form of down light and spot light. Also, a small short arc type metal halide discharge lamp has come to be used in recent years as a headlamp of a vehicle in place of a halogen lamp. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. In a metal halide lamp, mercury, rare gas and metal halide are encapsulated in an arc tube for purposes of emission with color reproduction. Scandium, sodium, dysprosium, neodymium, tin, thulium, cerium or the like is used as a compound of iodine or bromine for this metal halide. These metal halides are present as a liquid in the vicinity of the wall of the arc tube during luminous operation of the lamp. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including metal halide additives and a rare gas to facilitate starting. Mercury may also be included. Tungsten electrodes are sealed in opposite ends of the arc tube to enable energization of a discharge arc within the arc tube. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated. High intensity discharge (HID) product range consists of mercury vapor (MV), high pressure sodium (HPS), and quartz metal halide (MH) lamps. For conventional metal halide lamps, silica is used as a material for the arc tube. Recently, however, ceramic is getting attentions to replace the silica, since the silica-made arc tube has not yielded enough strength against the high temperature and high pressure resulting from the light emission. A ceramic metal halide lamp includes a discharge tube defining an interior region. Upon application of sufficient voltage across a pair of electrodes positioned within the interior region of the discharge tube, a high pressure arc is ignited in the discharge tube interior region. Ceramic metal halide lamps provide several advantages over conventional metal halide lamps including a more uniform color spectrum. One of the key advantages of ceramic metal halide discharge lamps is that they can provide good color uniformity. Achieving good color uniformity requires good control over arc tube voltage, which in turn is strongly dependent on the length of the arc gap, or the distance between the electrode tips. Compared to the conventional HID lamps, these ceramic metal halide lamps display excellent initial color consistency, superb stability over life, high luminous efficacy of >90 lumens/watt and a lifetime of about 20,000 hours. These highly desirable characteristics are due to the high stability of the polycrystalline alumina (PCA) envelopes and a special mixture of salts, which emits a continuous-spectrum light radiation close to natural light. By adjusting the composition of salts used in said lamps, color temperatures of 3800 4500K, and a color rendering index (CRI) of above 85 can be achieved.

Operating the metal halide lamps which make use of luminescence of the encapsulated materials in the arc discharge requires an electric circuit (generally including a starter and a ballast) for starting and stably maintaining the discharge. Metal halide lamps start upon application of a high voltage between two main electrodes or to an inductive start system. The initial application of an excitation source across the electrodes causes the rare gas to ionize and produce light. Continued application of the excitation source causes the vaporization and the ionization of the mercury and metal halide to produce light. When the lamp is turned on by applying a voltage to the lead wires from the ballast, a part of or the entire metal halide evaporates. The evaporated metal halide is dissociated to metal atoms and halogen atoms by arc discharge occurring between the pair of electrodes, and thus the metal atoms are excited so that light is emitted. In the vicinity of the wall of the arc tube, the dissociated metal atoms are recombined with the halogen atoms, and return to a metal halide. This cycle phenomenon is repeated to allow the lamp to be stably on. Although the metal halide has a lower vapor pressure than that of mercury, the metal halide is readily excited and emitted, so that there is a tendency that emission caused by an added metal mercury is stronger than emission caused by mercury in metal halide lamps. Therefore, mercury primarily serves as a buffering gas to determine a voltage in the arc tube. The mercury controls the current-voltage characteristics of the lamp, and the alkali metal iodides adjust the color quality, and contribute to lumen output through strong emissions. The photometric performance parameters of metal halide lamps are dependent on the partial pressures of the enclosed metal halide salts. Their vapor pressures are primarily controlled by the arc tube wall temperature in the region where the metal halide vapors condense. The luminous efficacy, color rendering index and other lamp output characteristics may be varied, depending upon the particular composition of the metal halides in the arc tube. Most commercial ceramic metal halide lamps contain a complex combination of metal halides, particularly iodides. In general, iodides are more favored than fluorides because of their lower reactivity and are more favored than chlorides or bromides because they tend to be less stable at higher temperatures. Thallium iodide is a common component which is mainly used to adjust the (x,y) color coordinates so that they lay on the blackbody curve. A metal halide lamp having an arc tube filled in which is a combination of metal iodides such as dysprosium (Dy) iodide and thallium (Tl) iodide or a combination of metal iodides such as dysprosium (Dy) iodide and neodymium (Nd) iodide, is characterized in its high luminous efficacy and high color rendering property. Metal halide arc discharge lamps are made to operate either vertically or horizontally, with some horizontal arc discharge lamps having an arched or bowed arc discharge tube.