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Creating the Lights

 

Lighthouse keepers have kept our lights of the coast aglow through a variety of ways over the years. In time, the great bonfires were replaced by devices that burned many different fuels such as lard, oil derived from sperm whales, mineral oil, and kerosene. Prior to the invention of the Fresnel (pronunciation: frA-'nell) lens, the effectiveness of a lighthouse was somewhat limited by these more primitive light sources. One such light source was the Argand lamp, which used a hollow, loop-shaped wick that provided more light than a solid wick. Argand lamps were most often used in conjunction with a parabolic (bowl-shaped) reflector to help increase the light's intensity. Multiple lamps and reflectors were used when even greater intensity was required.


(photo above: The light source inside 1st order Fresnel lens in Point Arena Lighthouse, California)

OutlensDeveloped by Augustin Fresnel of France and introduced in 1822, the Fresnel lens signaled the arrival of the golden age of lighthouses. Through a sophisticated arrangement of prisms, the Fresnel lens focuses light into a concentrated beam that, in some cases, could be seen from more than 20 miles away. Designed in a series of seven sizes (called orders, 1st through 6th, with a three-and-a-half order), the Fresnel lens could accommodate a variety of lighthouse sizes and functions. A sixth-order lens was the smallest in size and therefore had the smallest focal distance, while a first-order lens was the largest and, in some cases, reached a height of up to 12 feet. Though he died only five years after inventing the lens, Augustin Fresnel's contribution to the lighthouse was, and still is, utilized to a great extent. From wood fires and candles to oil lamps and incandescent bulbs, no technological advance contributed more significantly to lighthouses than the Fresnel lens.

below the light sourcePoint Arena, California

 

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Reflecting the Light:
Exploring How the Fresnel Lens Works

Light refracts, or bends, at the interface of two materials (air and lens). When light hits one outside surface on a prism, its direction is changed. When the light passes through the prism and hits another outside surface, its direction is changed again. By carefully crafting prisms to the right specifications, the Fresnel lens takes advantage of these properties so that the glow of a light source can be focused into a solid, more intense beam of light.

(animation above: how the prisms of a Fresnel lens refract light )

Prisms

(photo above: a close look at the prisms of the 1st order Fresnel lens in Point Arena Lighthouse, California)