A lens lantern is a small, self-contained lamp structure which may sometimes be used to serve as a
lighthouse
A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lens (optics), lenses and to serve as a beacon for navigational aid for maritime pilots at sea or on inland waterways.
Ligh ...
. Unlike a regular
Fresnel lens
A Fresnel lens ( ; ; or ) is a type of composite compact lens (optics), lens which reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections.
The simpler Dioptrics, d ...
, the lantern requires no housing to protect it from the weather; its glass sides would
refract
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
and magnify the light in the same fashion as would the lens. Lens lanterns were popular alternatives to lighthouses in the nineteenth century; they required less care, were cheaper to erect, and could be fairly easily placed.
Development
The need for beacon lights in the late 19th century led to the development of the "eight-day tubular lens-lantern" in 1885 by Joseph Funck. By using a large, circular oil tank around the top of the lantern, it could operate for up to eight days without additional maintenance. This lantern design, however, was susceptible to being blown out by winds. It was improved upon by David Heap, resulting in the five-day lens lantern in 1889.
See also
*
F. Ross Holland, Jr.
*''
America's Lighthouses: Their Illustrated History Since 1716''
References
Lighthouse fixtures
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