An apparatus 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, which opens into the chamber, and at least one slit shaped outlet opening opposite the inlet opening, which extends over the length of the chamber; a tube element, which extends in a longitudinal direction through the chamber, the tube element being at least partially transparent to UV radiation, wherein the tube element is arranged in the chamber such that a flow space is formed between the tube element and the wall of the chamber, the flow space being symmetric with respect to a longitudinal centre plane of the chamber, the longitudinal centre plane dissecting the outlet opening in its middle, and such that the tube element extends into the slit shaped outlet opening in the housing and thereby forms two longitudinally extending outlet slits (37, 186) between the tube element and the housing; and at least one UV-radiation source (32, 132, 210, 220, 222) in the tube element, which is arranged to emit UV-radiation in the direction of the flow space and through the outlet opening out of the housing; characterized by: means (200, 226) for causing UV radiation to be emitted primarily in a first wavelength range through a first section of the tube element into the flow space and for causing UV radiation to be emitted primarily in a second wavelength range through a second section of the tube element through the outlet opening of the housing, wherein the second section as seen from a center point of the tube element covers at least an angular region corresponding to an angular region of the slit shaped opening in the housing as seen from the center point of the tube element, wherein the first and second wavelength ranges differ, and wherein for at least the second section, a maximum of 20%, preferably a maximum of 5%, of the radiation power emitted through the second section comes from the first wavelength range. a housing (14, 114) having an elongated chamber (22, 122), at least one inlet opening, which opens into the chamber, and at least one slit shaped outlet opening opposite the inlet opening, which extends over the length of the chamber; a tube element, which extends in a longitudinal direction through the chamber, the tube element being at least partially transparent to UV radiation, wherein the tube element is arranged in the chamber such that a flow space is formed between the tube element and the wall of the chamber, the flow space being symmetric with respect to a longitudinal centre plane of the chamber, the longitudinal centre plane dissecting the outlet opening in its middle, and such that the tube element extends into the slit shaped outlet opening in the housing and thereby forms two longitudinally extending outlet slits (37, 186) between the tube element and the housing; and at least one UV-radiation source (32, 132, 210, 220, 222) in the tube element, which is arranged to emit UV-radiation in the direction of the flow space and through the outlet opening out of the housing; characterized by: means (200, 226) for causing UV radiation to be emitted primarily in a first wavelength range through a first section of the tube element into the flow space and for causing UV radiation to be emitted primarily in a second wavelength range through a second section of the tube element through the outlet opening of the housing, wherein the second section as seen from a center point of the tube element covers at least an angular region corresponding to an angular region of the slit shaped opening in the housing as seen from the center point of the tube element, wherein the first and second wavelength ranges differ, and wherein for at least the second section, a maximum of 20%, preferably a maximum of 5%, of the radiation power emitted through the second section comes from the first wavelength range.