The Saint Augustine Lighthouse with the sun directly behind the lamp room showing the sunlight being reflected through the Fresnel lens. The original Saint Augustine Lighthouse received a first order Fresnel lens from Parisian lens maker Sautter, Lemonnier & Cie. The lens has 370 individual prisms and weighs roughly 2 tons. A 1000-watt light bulb currently serves as it light source, as they have since the tower was electrified in 1936.
The lens built for the Saint Augustine lighthouse has three bulls-eye panels that send out beams of light. When the lens rotates, the rotating beams create the illusion of flashes to observers, a unique timing characteristic that differentiates our lighthouse from others and makes it identifiable at night. When first installed in 1874, a clockwork mechanism rotated the lens once every nine minutes, displaying a flash every three minutes. Today, with an electric motor, these flashes appear every 30 seconds.
The Fresnel lens, created by Augustin-Jean Fresnel in the 1800s uses a succession of concentric rings, each consisting of an element of a simple lens, assembled in proper relationship on a flat surface to provide a short focal length. The Fresnel lens is used particularly in lighthouses and searchlights to concentrate the light into a relatively narrow beam.
There are two main types of Fresnel lens: Imaging and non-imaging. Imaging Fresnel lenses use segments with curved cross-sections and produce sharp images, while non-imaging lenses have segments with flat cross-sections, and do not produce sharp images. As the number of segments increases, the two types of lens become more similar to each other. In the abstract case of an infinite number of segments, the difference between curved and flat segments disappears.