Point Reyes National Seashore
California
Inside the Lighthouse and Lens Room
The Fresnel Lens: The lens in
the Point Reyes Lighthouse is a "first order" Fresnel (fray-nel) lens, the
largest type of Fresnel lens. Augustine Jean Fresnel of France
revolutionized optics theories with his new lens design in 1823.
Before the Fresnel lens, lighthouses used
mirrors to reflect light out to sea. These old mirrored lights could only
be seen eight to twelve miles away. After the invention of the
Fresnel lens, the brightest lighthouses could be seen all the way to the
horizon, about twenty-four miles.
The Fresnel lens intensifies the light by
refracting and magnifying the source light through crystal prisms into
concentrated beams. The Point Reyes lens is divided into twenty-four
vertical panels, which direct the light into twenty-four individual beams.
A counterweight and gears similar to those in a grandfather clock rotate
the 6000-pound lens at a constant speed, one revolution every two minutes.
This rotation makes the beams sweep over the ocean surface like the spokes
of a wagon wheel, and creates the Point Reyes signature pattern of one
flash every five seconds.
Above - lighthouse light
lens and clockworks
Below - old fog horn
