Description
Fresnel lenses were classified by order referring to the strength of light emitted. These ranged from Huge first order lenses, which were visible to twenty-one miles, to smaller sixth- and seventh-order lenses, which were visible at lesser distances. Lenses of the second and fourth order were used at Portland Head Light. The lenses, combined with changes in the design of the lanterns that held them, vastly improved the navigational lighting system.
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