Infrared light signal through a light wave line (6) illuminates an examining area in which a catheter (2) is inserted. Evaluation units (10a,10b) convert the light signal into image information. Imaging may be performed with the help of OCT using light having a wavelength that is selected to penetrate fabric, with the help of OCT using light having a wavelength that is selected to penetrate blood or with the use of an infrared camera that takes radiation optical pictures.