Patent ID: 8198792

Claim:
An electrodeless discharge lamp, comprising: a bulb made of a light-transmitting material, having a noble gas and mercury sealed therein, and provided with a spherical portion and a neck portion extending from said spherical portion; a base connected to said neck portion; a protrusion that is formed at an apex of said spherical portion, which is on an opposite side to said neck portion, and that protrudes out of said spherical portion; and an induction coil is supplied with a high-frequency current to apply an electromagnetic field to the bulb, and said electromagnetic field triggering discharge in the bulb to cause light emission, wherein when defining the relations: B=W /(4×π×( D/ 20) 2 ), S =π×( d/ 20) 2 , L =π×( d/ 10), X =( B×S )/( L×A ), where W (W) denotes the lamp input power, D (mm) denotes the diameter of said spherical portion, d (mm) denotes the diameter of a portion at a joint surface between said neck portion and said base, and A (mm) denotes the distance from a largest-diameter portion of said spherical portion to said joint surface, the electrodeless discharge lamp is formed in such a manner that “X” takes on a value obtained on the basis of the (Formula) below: t− 6≦10959× X+ 25≦ t+ 6 (Formula) where t is the temperature (° C.) at the tip of said protrusion during downward stable lighting of the electrodeless discharge lamp, so that the electrodeless discharge lamp is allowed to obtain a constant light output without changing the luminous flux value depending on the lighting direction, and wherein the electrodeless discharge lamp is configured that the temperature of the coldest spot of the bulb during upward stable lighting is substantially the same as the temperature of the coldest spot of the bulb during downward stable lighting.