Light source unit for exposure apparatus

A light source unit for an exposure unit is provided. The unit comprises a cylindrical light-emitting tube and a screen sleeve which has a slit in the circumferential direction and with which the tube is covered through water filled therebetween. According to the invention, the light source unit has the two opposite edges of the slit in the screen sleeve that are in the axial direction partly extended downward to the outer periphery of the light-emitting tube so that a flat exposure pattern is obtained that is small in the radial direction at the periphery of the surface to be exposed to light. Further, a more favorable effect can be obtained by the provision of an attenuating filter on the outer periphery of the screen sleeve for the purpose of shielding a part of the light from the light source in the axial direction.