Along with the Kermorvan Lighthouse [fr], it signals the direction of the Chenal du Four, which transient ships used to follow on a north-south axis before the creation of the Rail d'Ouessant.

This bizarre juxtaposition (some suggestions were made to restore the abbey, while deconstructing the lighthouse and rebuilding it elsewhere) is explained by the connections these structures shared throughout Saint-Mathieu's history.

The end of the 17th century, however, brought with it a need for accessibility to a new naval base in Brest and the necessity of more serious methods of navigation for western Brittany.

However, the cost of ignition was expensive, and the light of Saint-Mathieu was thus only lit on particularly dark nights in the autumn and winter.

French naval commander Anne Hilarion de Tourville complained that the lighthouse was not regularly lit.

Monks of the abbey offered their services in exchange for a droit de bris (a right to the goods from ships wrecked on one's property), and were charged with illuminating the light from 1 January 1694.

An internal staircase led to sixteen half-lenses placed 55 metres (180 ft) above sea level, which were protected by a glass border of 81 centimetres (32 in), 9 millimetres (0.35 in) thick.