Optical source unit for an endoscope

In an illumination device for photographing a portion of a body cavity to be examined with an endoscope having a light source having a discharge tube for illuminating the portion to be examined and a mechanical shutter arranged in the optical path of illuminating light from said light source, an optical source unit comprising:

• a light quantity detecting circuit for detecting information on the brightness of illumination of said light source;
• circuit means for determining a correct exposure time and circuit means for estimating correct exposure time, each coupled to said light quantity detecting circuit;
• wherein the closure of said mechanical shutter is controlled by the control operations of said circuit means for determining a correct exposure time estimating circuit, the quantity of light emitted by said light source controlled according to information provided by said circuit means for estimating correct exposure time;
• said circuit means for estimating correct exposure time classifying an estimation value in the initial period of exposure detection, whereby when the estimation value is classified, a control signal based on a first estimation value representative of a short time is utilized to control both of said light source and said mechanical shutter, and a control signal based on an estimation value other than said first estimation value is utilized to control only said light source.