Editing Lighthouse (section)

===Lens===
{{see also|Fresnel lens}}
[[File:Cape Meares Lighthouse lens - Oregon.jpg|thumb|[[Cape Meares Lighthouse]] in Oregon; first-order Fresnel lens]]
Before modern [[strobe light]]s, [[Lens (optics)|lenses]] were used to concentrate the light from a continuous source.  Vertical light rays of the lamp are redirected into a horizontal plane, and horizontally the light is focused into one or a few directions at a time, with the [[light beam]] swept around. As a result, in addition to seeing the side of the light beam, the light is directly visible from greater distances, and with an identifying [[light characteristic]].

This concentration of light is accomplished with a rotating lens assembly. In early lighthouses, the light source was a [[kerosene lamp]] or, earlier, an animal or vegetable oil Argand lamp, and the lenses rotated by a weight driven clockwork assembly wound by lighthouse keepers, sometimes as often as every two hours. The lens assembly sometimes floated in [[mercury (element)|liquid mercury]] to reduce friction.  In more modern lighthouses, electric lights and motor drives were used, generally powered by diesel electric generators. These also supplied electricity for the lighthouse keepers.<ref name="Crompton"/>

Efficiently concentrating the light from a large omnidirectional light source requires a very large diameter lens. This would require a very thick and heavy lens if a conventional lens were used.  The [[Fresnel lens]] (pronounced {{IPAc-en|f|r|eɪ|ˈ|n|ɛ|l}}) focused 85% of a lamp's light versus the 20% focused with the parabolic reflectors of the time. Its design enabled construction of lenses of large size and short focal length without the weight and volume of material in conventional lens designs.<ref name=mhp/>

Fresnel lighthouse lenses are ranked by <!-- "order" should be linked somewhere useful -->''order'', a measure of refracting power, with a first order lens being the largest, most powerful and expensive; and a sixth order lens being the smallest.  The order is based on the focal length of the lens. A first order lens has the longest focal length, with the sixth being the shortest. Coastal lighthouses generally use first, second, or third order lenses, while harbor lights and beacons use fourth, fifth, or sixth order lenses.<ref name="nps"/>

Some lighthouses, such as those at [[Cape Race Lighthouse|Cape Race]], Newfoundland, and [[Makapuu Point Light|Makapuu Point]], Hawaii, used a more powerful [[hyperradiant Fresnel lens]] manufactured by the firm of [[Chance Brothers]].