A catheter is adapted to exchange heat with a body fluid, such as blood, flowing in a body conduit, such as a blood vessel. Charges from one row of photosensors are transferred through another row of photosensors to the charge transfer register, and charges from both rows of photosensors are optionally added together in the charge transfer register. The layer sequence includes a underlayer, a buffer layer and a recording layer made of a magnetic material. A core end of the pressure exerting roller is rotatably supported by a shaft bearing. The collector lens collects a back flux which is returned by the returning optical system and which is re-reflected by the selective reflection optical system, and forms a light source image of the light source.