PURPOSE: To prevent any decrease in luminosity at the startup, by enlarging the spaces under the electrodes where mercury is stored in larger amount than that flows out due to a temporary temperature difference occurring between the inner tube and the outer bulb.

CONSTITUTION: The illustrated outer bulb structure is the same as the conventional one except the recess 7 under the electrode. The external diameter of the outer bulb 1 is 700mm, its length is 100mm, and the inner tube 4 is 12mm in its external diameter and 60mm in its length. The recess 7 under the electrode is a pipe made of nichrome alloy, whose external diameter is 2mm and length is 8mm. By virtue of the mercury stored in the pipe 7, the outflow of the mercury to the outer bulb 1 due to the temperature rise of the inner tube 4 with the turning on of the lamp, which might lower the partial pressure of mercury, is prevented; as a result, the emission of ultraviolet radiation to excite the phosphor material is prevented from decreasing.