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Lighting the Way: Fresnel Lens

All About Glass

Fresnel lens lighthouse

The most dangerous part of a sailing trip used to be returning to shore. Lighthouses were built to signal the safest route, but often the weak light from their lamps was not visible until too late. The large, thick lens that was supposed to project the light absorbed much of the signal.  Hollowing out the interior of the lens helped, but not enough.

French physicist Augustin Fresnel knew that light rays spread out in all directions from their source. He spent eight years perfecting a system to capture those divergent rays and redirect them into a strong, concentrated, horizontal beam. In 1822, he unveiled his invention: a lighthouse lens made up of several thin lenses, each surrounded by concentric rings of prisms.

How a Fresnel lens works

Travel has changed since Fresnel’s day, but travelers still rely on Fresnel-type lenses for safety. They’re used in everything from car headlights and traffic lights to airport beacons and ships’ lights.

First-order Fresnel lens for the Makapuu Point Lighthouse

Up to standard

red railroad lantern

By the 1890s, the railroad industry was booming. Fresnel-type signal lenses were being used to control the increasing train traffic. The trouble was that each rail line was using its own shades of each color for lenses. Was a signal that appeared orange meant to be red—or yellow?

It was time to establish standards. Working with a color blindness expert, Corning chemist William Churchill identified the glass compositions that produced the shades of red, green, and yellow that the human eye finds most distinctive. The shades he developed would appear the same under any weather conditions. By 1908, the railroad industry had adopted Corning’s colors as its standard.

Published on October 24, 2011