Patent ID: 7977886

Claim:
A mercury-free discharge bulb for a vehicle, the discharge bulb comprising: an arc tube comprising: a light-emitting portion which is formed of a ceramic tube and comprises a light-emitting material and a starting xenon gas filled therein, wherein a filling pressure of the starting xenon gas is in a range from about 6 atm to about 18 atm; thin tube portions which are formed at respective ends of the light-emitting portion; and electrodes which are fixed inside the thin tube portions and which are provided in the light-emitting portion so as to face each other, wherein the arc tube is configured to satisfy a relationship of 2≦Ea/Da≦4, in which Ea denotes a length of a region of the light-emitting portion and Da denotes an inner diameter of the light-emitting portion, a wall load value of the arc tube is about 45 W/cm 2 or more, the mercury-free discharge bulb is operated at a rated lamp power smaller than 35 W in an environment in which an inside temperature of the light-emitting portion is 1000° C. or higher, the light-emitting material comprises at least a sodium halide and a rare-earth metal halide, and excludes a scandium halide, and a difference between a vapor pressure of the sodium halide and a vapor pressure of the rare-earth metal halide under an environment of about 1000° C. is about 10 kPa or less.