Abstract: A method and a test device are disclosed for testing whether the relative position and orientation of a distal end of an optical fiber relative to its electrode and/or handle piece of a medical instrument is correct. Light is coupled into a proximal end of optical fiber and is emitted at the distal end thereof onto a projection surface. At the proximal end the light of the light source is coupled eccentrically and/or asymmetrically relative to a center of a proximal face of the optical fiber. A light pattern is created on the projection surface that can be tested based on at least one test criterion in order to test the correct arrangement or orientation of the distal end. The test can be carried out automatically by a camera and a central unit or by a user by observing the light pattern on the projection surface.