A beacon covering the whole Paris region

History and culture

Monday 25 May 2020

Modified the 27/12/23

Since the Tower was erected, a powerful beacon has swept the horizon from it’s summit. How does it work? By Bertrand Lemoine.

The summit of the Eiffel Tower was equipped with a beacon as soon as it was completed. Four semi-circular, metal arches joined together atop the third floor to support the small platform that held the lantern. To reach it, you had to climb a spiral staircase and then a ladder. The lantern sat on two levels, each surrounded by a narrow balcony. A cylindrical, glassed-in cage housed the beacon’s access ladder, and above it was the lantern itself, topped by a dome and utilizing a rotating optic with a Fresnel lens. This was supposed to be the most powerful beacon in the world, with a reach of 80 kilometers (50 miles).

©SETE/JérômeSchlichter

The illusion of a rotating beacon!

The many modifications made at the Tower’s summit, including the broadcasting mast installed in 1957, led to this beacon’s removal and replacement in 1999 by one with a very different design. Four motorized, synchronized projectors with a reach of 80 kilometers project a double rotating beam on opposite sides of the Tower. Each projector sweeps a quarter of the horizon before returning to its initial position. Their perfect timing gives the illusion of a rotating beacon. During its renovation in 2017, the beacon was equipped with new 6,000-watt xenon lamps.

©SETE/JérômeSchlichter

Bertrand Lemoine is an architect, engineer and historian. He was a research director at the CNRS and general manager of the Atelier International du Grand Paris. He is an internationally recognized specialist in the history and current events of architecture, construction, the city and heritage in the 19th and 20th centuries, particularly in Paris, Greater Paris and the Eiffel Tower. He is the author of forty-three books and several hundred articles on these subjects. He is currently a consultant on architectural, urban, digital and energy issues.