- Maritime empires used light towers at least 2,000 years ago. Ancient Egyptian history recorded that priests tended beacon fires. The Romans kept fires burning at ports throughout the empire, even as far away as the 80-foot Dover light tower in Britain. It burned smoke signals during the day and a bonfire at night. In the mid-1700s, lighthouses used fires fueled by whale oil. Only an estimated 3 percent of the light was actually visible out at sea. The only way to amplify the light output was to increase the size of the flame by using more fuel. By the early 1800s a parabolic mirror reflector behind the flame collected the light, concentrating the beam so that it was brighter. In the early 1820s, the French government commissioned Augustine Jean Fresnel, a physicist-engineer to design an improved lighting system. Fresnel’s solution was his more efficient Fresnel lens.
- The Fresnel lens.lighthouse lens 1 image by Aaron Kohr from Fotolia.com
As a physicist, Fresnel was known for his studies in theories of light refraction and reflection. Refraction refers to how a beam of light changes direction when it passes through one substance to another. You can see this in a swimming pool when you stand on the bottom and see that your feet seem distorted. Fresnel studied ways that a lens could bend light rays through reflection and refraction to focus them into a single, narrow, concentrated horizontal beam. A single lens is composed of a clear material like glass that allows light to pass through it and focus in a point. He discarded the idea of one huge glass lens because it would have to be too big, but he realized that multiple lenses could capture light rays and bend them into one single beam. - A combination of small lenses and prisms arranged in a stair-step configuration bends and focuses light rays in to a single concentrated beam. Fresnel devised a system of lenses that surround the light source in a barrel-shaped configuration. Multiple lenses above and below the light source collect light rays and focus them into a single light beam on the same horizontal plane as the lenses directly in front of the flame. These lights could be seen for some 20 miles out to sea. Fresnel’s invention focused approximately 80 percent of the light into a single concentrated beam. Before the Fresnel lens came into play, sailors could not distinguish one light from another along the shoreline. Out at sea they might see a light but not know which port it represented. By putting red or green panels of glass in front of the flame and flashing the light at different intervals, ports could set up their own unique identification signals.
- Fresnel classified his lenses according to their size into divisions called “orders.” The first order lens is the largest at 7 feet 10 inches in height and an inside diameter of 6 feet 1 inch. The smallest is the sixth order, which is 1 foot 5 inches tall and has an internal diameter of 1 foot. The lenses are very difficult to maintain, clean or repair. Now most Fresnel lenses can be seen not in lighthouses but preserved in museums. The only first order lens in service now is at the Sequin Island lighthouse near Boothbay, Maine. The Coast Guard has tried to remove it, but the general public has resisted this attempt.
- A Fresnel lens can concentrate light but the trade-off is in image quality. The lenses are presently used in overhead projectors, projection televisions and some magnifying glasses. They also concentrate sunlight in solar collectors. The lenses are widely used in automobile headlights, tail lights, and backup lights, as well as retina identification cameras and magnifiers for small CRT screens. Fresnel lenses also serve to produce gentle reading lights on some airplanes.
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