BULB GEOMETRY FOR LOW POWER METAL HALIDE DISCHARGE LAMP

A low-wattage metal-halide discharge lamp (10) has a tube (12) of the double ended type that forms a bulb (14) or envelope, a pair of electrodes, e.g., an anod e (30) and a ca- thode (32), which penetrate into an arc chamber (16) inside the envelope (14 ), and a suit- able amount of mercury plus one or more metal halide salts. The electrodes a re each formed of a refractory metal, i.e., tungsten wire, extending through the respective necks (22, 24) into the arc chamber (16). Heat transfer along the tube wall and through the necks (22, 24) is controlled by designing the bulb (14) wall so that the cross sectiona l areas at the necks (22, 24) and at quarter-chamber planes (52, 54) halfway between the necks (22, 24) and a midplane (50) have a respective quarter chamber loading factor and a neck loading factor each within a desired range. Lamps of this design achieve high effica cy at relatively low power, i.e., below 40 watts.