An apparatus (8; 108) for applying a liquid medium irradiated with UV radiation onto a substrate, the apparatus comprising: a housing (14; 114) having an elongated chamber (22; 122), at least one inlet opening (24; 164), which opens into the chamber (22; 122), and at least one slit shaped outlet opening (21; 121) opposite the inlet opening (24; 164), which extends over the length of the chamber (22; 122); a tube element (30; 180), which extends in a longitudinal direction through the chamber (22; 122), the tube element (30; 180) being at least partially transparent to UV radiation, wherein the tube element (30; 180) is arranged in the chamber (22; 122) such that a flow space is formed between the tube element (30; 180) and the wall of the chamber (22; 122), the flow space being symmetric with respect to a longitudinal center plane of the chamber (22; 122), wherein the longitudinal center plane dissects the outlet opening (21; 121) in its middle, and such that the tube element (30; 180) extends into the slit shaped outlet opening in the housing and thereby forms two longitudinally extending outlet slits (37; 186) between the tube element (30; 180) and the housing (14; 114); and at least one UV-radiation source (32; 182; 220, 222) in the tube element (30; 180), which is arranged to emit UV-radiation in the direction of the flow space and through the outlet opening (21; 121) out of the housing (14; 114) in order to generate radicals in the liquid and to bring the radicals to the surface of the substrate; and means for adjusting the radiation exiting the outlet opening through the tube element (30; 180) such that the intensity of the radiation increases towards the longitudinal center plane of the chamber, and characterized in that the means comprise a plurality of at least three UV radiation sources, said UV radiation sources being located within the tube element (30; 180), adjacent to each other and adjacent to a portion of the tube element (30; 180) which is arranged in the outlet opening (21; 121) or outside the chamber (22; 122), wherein the adjacent radiation sources emit UV radiation with different radiation intensities. a housing (14; 114) having an elongated chamber (22; 122), at least one inlet opening (24; 164), which opens into the chamber (22; 122), and at least one slit shaped outlet opening (21; 121) opposite the inlet opening (24; 164), which extends over the length of the chamber (22; 122); a tube element (30; 180), which extends in a longitudinal direction through the chamber (22; 122), the tube element (30; 180) being at least partially transparent to UV radiation, wherein the tube element (30; 180) is arranged in the chamber (22; 122) such that a flow space is formed between the tube element (30; 180) and the wall of the chamber (22; 122), the flow space being symmetric with respect to a longitudinal center plane of the chamber (22; 122), wherein the longitudinal center plane dissects the outlet opening (21; 121) in its middle, and such that the tube element (30; 180) extends into the slit shaped outlet opening in the housing and thereby forms two longitudinally extending outlet slits (37; 186) between the tube element (30; 180) and the housing (14; 114); and at least one UV-radiation source (32; 182; 220, 222) in the tube element (30; 180), which is arranged to emit UV-radiation in the direction of the flow space and through the outlet opening (21; 121) out of the housing (14; 114) in order to generate radicals in the liquid and to bring the radicals to the surface of the substrate; and means for adjusting the radiation exiting the outlet opening through the tube element (30; 180) such that the intensity of the radiation increases towards the longitudinal center plane of the chamber, and characterized in that the means comprise a plurality of at least three UV radiation sources, said UV radiation sources being located within the tube element (30; 180), adjacent to each other and adjacent to a portion of the tube element (30; 180) which is arranged in the outlet opening (21; 121) or outside the chamber (22; 122), wherein the adjacent radiation sources emit UV radiation with different radiation intensities.