The beam of the Saint Mathieu lighthouse travels over 29 nautical miles, about 55 km. It flashes a white light every 15 seconds thanks to the lenticular lenses of Augustin Fresnel.  Its main function is to guide vessels towards the entry of the Brest Channel. The lamp is activated by a photoelectric cell located at the foot of the lighthouse. As soon as this cell is in shadow, the lamp is activated. At the top of the lighthouse, we also have a steady white beam with a range of 14 miles, aligned with that of Kermorvan to indicate the mouth of the Chenal du Four south pass. The sector lamp (small lighthouse to the right of the abbey) is an auxiliary lamp. This  is equipped with one white sector, inserted between two red and green sectors. It was brought into use in January 1894.