Abstract: Disclosed is an apparatus for obtaining geometrical data relating to an internal surface of a cavity. The apparatus comprises a probe having an end portion insertable into the cavity in a direction of insertion, radiation directing means for directing electromagnetic radiation from the end portion to at least one location on the internal surface to cause the radiation to be reflected from said location, detection means for detecting the reflected radiation from the at least one location, and means for determining from the detected radiation a distance between the probe and the internal surface in a direction having at least a transverse component relative to the direction of insertion. The radiation directing means is adapted to direct the radiation at an acute angle relative to the direction of insertion.