The invention relates to an instrument (100) that can at least partially be inserted into an internal cavity (2) of an object (1), particularly a catheter or an endoscope. The instrument (100) comprises an optical system (OS) for collecting light coming from external objects through a viewing corridor (VC). The optical system comprises an OLED (110) for illuminating said external objects which is disposed in the viewing corridor (VC). In a particular embodiment, the OLED (110) may at least partially be transparent. By arranging the OLED (110) in the light corridor, an optimal illumination can be achieved together with a compact design of the whole instrument (100).