PURPOSE: To improve the efficiency of an incandesent lamp and maintain the average use life by increaseing the volumetric ratio of a sealed-in gas (argon or krypton), and specifying the relationship between the pitch of a double-coil filament and the coil length.

CONSTITUTION: The volumetric ratio of the seal-in amount of argon or krypton to that of an inert gas such as nitrogen is increased up to 90W95% so as to enhance the efficiency of an incandescent lamp by at least about 0.5%. A double-coil filament is so designed that the relationship between the number of turns of the coil and the coil length in lighting condition satisfies the following: Y≥8.86x + 0.32, where y=arc voltage/number of turns of double coil, x=coil length in lighting condition/number of turns of double coil. This maintains the sufficient arc voltage and prevents reduction of average use life.