Grand Rapids, Michigan, USA: Summer 2003: In 1822, French physicist Augustin Fresnel (pronounced Fruh-nell) invented a lens that so dramatically increased the candle power of lighthouse beacons, his name is synonymous with the lens itself.

In its simplest form, the Fresnel lens is a barrel-shaped array of lenses designed to encircle a light source. With Fresnel''s optic array, as much as 80 percent of the original light source can be transmitted over a distance of 20 miles. Previously-existing lamps transmitted only 39 percent of their original light source. By the time a vessel saw the light, its crew had little time to avoid the danger it signaled.

With Fresnel's design, circular lenses in the area immediately horizontal to the light source magnified and concentrated the visible light as it passed through them. At the same time, multiple prisms mounted around the periphery of the barrel above and below the light source each collected and intensified the light and redirected it in the same directional plane as the lenses.

Originally, seven types of lenses, called "orders," were developed. The design was eventually refined to 11 orders. Each order was a different size and had varying degrees of light capture and intensification capabilities. The three largest orders were used for seacoast lights. Orders four, five and six were smaller and used for harbor or bay lights. A 3.5 order lens was used primarily in the Great Lakes, although the fourth order lens was the most frequently used on Lake Michigan.

A few rare Fresnel lenses are still in use today, including in the tower at Little Sable Point just south of Silver Lake. Examples of the lenses can be viewed by visitors circling Lake Michigan at several lighthouse museums, including Grand Traverse Lighthouse, Sand Point Lighthouse and Old Michigan City Lighthouse.

Source: Terry Pepper (www.terrypepper.com)