An apparatus for coupling radiation into and out of an optical fiber (4, figure 1) consists of an optical element, such as lens 6； a radiation system such as light source 10； and a sensor system such as colour sensor 16. The optical element is for coupling radiation into and out of the optical fiber. The radiation system is operable to produce input radiation, the radiation system arranged such that the input radiation is at least partially received by the optical element. The sensor system is for receiving output radiation from the optical element and is operable to generate a signal indicative of at least one characteristic of the output radiation. The optical element, the radiation system and the sensor system are generally coaxial and the radiation system and the sensor system are both disposed on the same side of the optical element. The apparatus may be used for medical tubes such as nasogastric and nasointestinal feeding tubes for the correct placement of the portion of the tube through which the fluid exits.
