Method for optical fiber use, wherein light that is generated by means of a light generating apparatus, is led by exploiting an optical fiber by directing light to the end of the optical fiber for leading the light further by the optical fiber, wherein inside a uniform and essentially closed casing structure are arranged: a cooling space which is placed at one end of the casing structure for cooling an input end for the light of the optical fiber, a lamp space inside of which said light generating means are placed and which is separated by an intermediate wall from the cooling space, a light opening in the intermediate wall whereby an input end for the light of the optical fiber is provided at the light opening, and at least one blower arranged opposite to the cooling space at the other end of the casing structure and leading the cooling air flow (w) out from the casing structure, preferably as a flow-through, through an end wall of the casing structure opposite to the cooling space, characterized in that the cooling space has a perforated back wall at a point of the cooling space which enables passage of air through the same into the cooling space, that the cooling space is cooled by means of a Peltier-unit acting as a cooling device, wherein the cold side of said Peltier-unit is arranged in connection with one or several internal surfaces of said cooling space, and the warm side of the Peltier-unit is arranged in connection with one or several outer surfaces of the casing structure, and in that a guiding assembly is arranged to the intermediate wall, carried out by edges of the light opening directed towards said light input end, and provides a passage of the cooling air flow (w) in the cooling space to the light input end of the optical fiber, whereby a cooling air flow (w) is led by the blower by means of forced convection from the cooling space via the guiding assembly to the lamp space. a cooling space which is placed at one end of the casing structure for cooling an input end for the light of the optical fiber, a lamp space inside of which said light generating means are placed and which is separated by an intermediate wall from the cooling space, a light opening in the intermediate wall whereby an input end for the light of the optical fiber is provided at the light opening, and at least one blower arranged opposite to the cooling space at the other end of the casing structure and leading the cooling air flow (w) out from the casing structure, preferably as a flow-through, through an end wall of the casing structure opposite to the cooling space, characterized in that the cooling space has a perforated back wall at a point of the cooling space which enables passage of air through the same into the cooling space, that the cooling space is cooled by means of a Peltier-unit acting as a cooling device, wherein the cold side of said Peltier-unit is arranged in connection with one or several internal surfaces of said cooling space, and the warm side of the Peltier-unit is arranged in connection with one or several outer surfaces of the casing structure, and in that a guiding assembly is arranged to the intermediate wall, carried out by edges of the light opening directed towards said light input end, and provides a passage of the cooling air flow (w) in the cooling space to the light input end of the optical fiber, whereby a cooling air flow (w) is led by the blower by means of forced convection from the cooling space via the guiding assembly to the lamp space.