Bioreactor vessel having an optical foam sensor

A bioreactor vessel has an optical foam sensor (36) with a foam contact surface for contacting the foam to be detected. The foam contact surface is an inner side of a window (38) transparent to light from the visible and ultraviolet spectral range in an outer wall of the bioreactor vessel (10). An outer side of the window (38) is coupled to an illumination and detection unit (52) with at least one first light source (56) of visible light and at least one photodetector (58) to detect light from the first light source (56) that is reflected in the bioreactor vessel (10). The foam contact surface has a titanium dioxide coating (44) superhydrophilizable by photoactivation with ultraviolet light and the illumination and detection unit (52) has at least one second light source (66) of ultraviolet light.