Welcome to our newest blog WP_20151104_15_36_08_Proseries where we will examine the technological innovations that improved lighthouses and made coastlines around the world safer. Each installment of the blog series will cover an invention or technological application introduced at the St. Augustine Lighthouse in its history. This first installment will focus on the powerhouse of the lighthouse – the first order Fresnel lens.

Lighthouse Illumination

The first lighthouses, as we recognize them now, were simply bonfires set atop high ground to improve their visibility. Eventually, people placed these fires on elevated platforms to extend their range. Technological advances replaced these fires with oil lanterns that provided a strong, steady light source. Despite these improvements in light sources, lighthouses still required some sort of magnification to project their beacon out to the horizon.

A light source like an oil lantern or an electric bulb sends its light in all directions, unless altered in some way. As the light extends from the source, the rays get farther apart. The farther away from the light source you are, the farther apart the rays of light become. This is why a light appears dimmer as you move away from it and it was one of the factors limiting the usefulness of early lighthouses.

Lighthouse Magnification
Illustration courtesy of Old Baldy Foundation
Illustration courtesy of the Old Baldy Lighthouse Foundation

Some early attempts to magnify a lighthouse light source included a mirror or mirrors, first flat then curved, to redirect more of the light’s rays out to sea. The image to the right shows a diagram of what these concave magnification devices looked like. This kind of reflector would magnify the oil lantern, but light diffusion would still limit the intensity of the lighthouse beacon. Despite the mirrored reflection, the light from this lantern would still spread apart (though not as much) as it moved away from the light source causing it to dim at a distance.

Fresnel lens diagramFrench physicist/mathematician Augustín Fresnel developed a lens system in the early 19th century to redirect more light from a light source than ever before. His invention, called the Fresnel lens, redirects more light than the previous mirror-based devices.

The Fresnel lens includes prisms above and below the light source, which redirect the light that would be lost into the sky or down into the base of the lighthouse, straight out to the horizon. The rays leave the lighthouse in parallel lines, eliminating diffusion and magnifying the light source out to the horizon, often 20 to 30 miles away depending on the height of the lighthouse tower.

The St. Augustine Lighthouse Fresnel Lens

The St. Augustine Lighthouse, built in 1874, received a first order Fresnel lens from Parisian lens maker Sautter, Lemonnier & Cie. Louis Sautter had been making lenses since 1852, though he merged with Lemonnier in 1870. Sautter reconfigured the company again in 1890 to become Sautter, Harlé & Cie. Our 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 our lighthouse has three WP_20151104_15_36_08_Pro“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.

Click here for the next installment in our lighthouse technology series, where we discuss the clockwork mechanism for turning the Fresnel lens.

Our Fresnel lens is still turning, sending its light 20 miles out to sea. Come visit the lighthouse and see our historic lens at the top of the tower.

Paul Zielinski is Director of Interpretation for the St. Augustine Lighthouse & Museum. He received his master’s degree in Public History from the University of West Florida and joined the lighthouse family in 2011.