The Eiffel Tower's beacon has four lamps: its four motorized projectors create the sweeping double beam seen above Paris.
The modern beacon replaced an earlier lantern arrangement in 1999. Each projector sweeps through a 90-degree section of the horizon, and computer synchronization makes the four movements appear to form a single rotating, cross-shaped beam.
These are not the Tower's famous sparkling bulbs. The beacon is a separate summit installation designed to project light outward, while 20,000 low-power bulbs are attached to the structure for the hourly sparkle effect. The beacon's projectors use 6,000-watt xenon lamps, each chosen for an approximate 1,200-hour lifespan.
The system can reach roughly 80 kilometres, making the Tower function as a giant nighttime landmark as well as a tourist attraction. A common mix-up is answering 20,000—the number of decorative sparkle lights—instead of four, the number of beacon projectors.