An optical sensor for operating along a catheter (103), the optical sensor comprising a light pattern generator configured to project at least one light pattern at a radial projection angle with respect to the optical sensor length onto the inner surface of an elongated volume into which the optical sensor is inserted, wherein the light pattern generator comprises an illumination system for providing a first light beam having a component of its direction along the sensor length and a first light redirection element (1007) configured to redirect the light beam to generate the at least one light pattern (1009) at an oblique and/or right angle to the optical sensor length, a second light redirection element (1012) for redirecting a reflected version of the light pattern from the inner surface of the elongated volume to provide a second light beam and an imaging device (1013) having a field of view (1011) with a central axis substantially parallel to the length of the optical sensor for receiving the second light beam.