Abstract: The invention relates to an optical sensor (1) for determining particle and/or dye concentrations in liquid or gaseous media and to a method for operating the same. The optical sensor (1) comprises at least one measuring head. The measuring head consists of an emitter unit (2) with a semiconductor emitting element (9), which emits visible emission light beams (8), and with a receiver unit (3) with a semiconductor receiving element (10). The portion of the emission light beams (8), which pass through an absorption section containing liquid or gaseous medium, is guided onto the receiving element (10). An evaluating unit (6) is coupled to the measuring head via electric leads (4, 4?), and the received signals, which are present at the output of the semiconductor receiving element (10), are evaluated inside said evaluating unit in order to determine the particle or die concentration.