Abstract: The invention concerns a method for feeding an optical cored wire into a molten metal bath and an immersion system and an immersion nozzle to carry out the method. The optical cored wire (6) is decoiled, a feeding and straightening device (4) with a plurality of rollers (20, 21) conducts feeding of the optical cored wire (6) in a feeding direction towards the metal bath (11) as well as a first straightening of the optical cored wire (6), and subsequently a separated further plurality of non-motor driven nozzle straighteners (13) arranged between the feeding and straightening device (4) and the metal bath (11) conducts a second straightening of the optical cored wire (6). Very high precision of temperature measurement can thereby be achieved.