Abstract: The invention concerns an optical sensor for measuring the property of a gaseous or liquid medium, particularly the opaqueness of a washing or rinsing liquid in a washing or rinsing machine, with at least one radiation source, a radiation receiver and a light-transmitting sensor body, with an optical boundary surface, which, depending on the nature of the medium present outside the boundary surface of the sensor body, to a greater or lesser degree transmits to the radiation receiver and/or reflects the radiation directed on to it, resulting at the radiation receiver in different measurement signals which characterise the reflectivity and the refractive index conditions at the boundary surface. Unambiguous measurement signals for process control of a washing or rinsing machine are obtained through separate sensing of the transparency and reflection of the medium.