in single-use bioreactors are generally based on the optical principle of oxygen quenching. The sensor is thereby illuminated with a filtered light source 20. The dye (often an oxygen sensitive fluorophore) emits light that differs in wavelength, phase and intensity from the source and contains less energy. If oxygen is near the fluorophore, the oxygen molecule takes up the spare energy which results in a reduction of the fluorescence signal (quenching). A photo diode catches the emitting fluorescence and separates it from the radiated light with the help of dichroic mirrors 40. The extent of the quenching is dependent on the concentration of oxygen. Ambient light and background noise can disturb the measuring signal of the optical oxygen sensors 66. However, they can be used for small concentrations and allow measurements in very small volumes (less than 1 mL). See sensors.