Abstract: An optical head having a photoelectric converter at one end and a socket for a light guide plug at the opposite end, has a spring loaded apertured diaphragm adapted to press lightly against the front end of the plug when it is inserted into the socket. The diaphragm has a cylindrical aperture at its side turned toward the plug and an opening defined by a frustoconical surface widening in the direction of the converter. This surface and the rear wall of the diaphragm can be provided with a light absorbent coating or the surface can be reflective and the rear wall provided with a light absorbent coating.