A yarn sensor for optically scanning a yarn (3), traveling longitudinally through a measurement gap (19), includes a light source (20) projecting a beam of light into the measurement gap (19), at least one receiver (23) for directly transmitted light, and light transmitting elements (24, 25, 26, 27) transmitting the light between the light source (20), measurement gap (19), and receivers (21, 22, 23). The light source (20) is an emitter with Lambert's emission characteristics. The light-transmitting element (27) includes a diaphragm and a lens, and the diaphragm is projected approximately into infinity. A diffusor (39) located between the light source (20) and the diaphragm generates radiation that passes through the diaphragm and is symmetrical to the optical axis of the lens. The yarn sensor can be used in the textile industry in spinning or bobbin winding machines with improved quality of the measurement results.