PURPOSE: To protect the sealing section from crack caused by repetitation of restarting, by securing both ends of filament with metal shaft having specific diameter and burrying such that the shaft will project more than specific length from silica glass.
CONSTITUTION: Filament is composed of metal wire coil 30 and shafts 31, 32 composed of metal wire having one ends secured to both ends of said metal wire coil 30 and the other ends fixed to metal foils 4a, 19 are provided. Then a portion of said shafts 31, 32 and metal foils 4a, 19 are burried in silica glass and sealed where the length of shafts 31, 32 not burried in silica glass is made more than 15 times of the wire diameter (d) of metal wire coil 30 while diameter (D) of shafts 31, 32 is made more than 1.5 times of the wire diameter (d). Consequently blackening at the root of filament through repetitive restarting and occurrence of crack at the sealing section can be prevented.
WATABE KEIJI
DOBASHI MASAHIRO
