PURPOSE: To obtain a light source for generating the fluorescent screen of a cathode-ray tube with the uniform illuminance distribution by forming a bulb sealed with the mercury vapor into a nearly spherical shape and arranging an electron emitting side electrode and an electron absorbing side electrode at the preset positions.

CONSTITUTION: A cathode 12 on the electron emitting side is arranged in a spherical bulb made of quartz, with its tip slightly biased from the center of the bulb, an anode 13 on the electron absorbing side is faced to it at the preset distance, and leads 14, 15 are connected. A fine wire made of 2%ThO2-W is wound on the cathode 12 and anode 13 for the heat radiation. Since the bulb is spherical, the illuminance distribution is symmetrical on the right and left and uniform with no irregularity. The tip of the cathode 13 is biased from the center of the bulb to the anode 13 side, thus the deterioration of the cathode is prevented.