What's at the top of a lighthouse?

What's at the top of the lighthouse? If you are very lucky, you will find a Fresnel Lens.

This type of lens is composed of arrays of prisms that produce a parallel beam of light many times brighter, and so travels farther, than light from the conventional convex lens, while using far less glass and saving weight. The first operational lens array was invented by the French physicist Augustin-Jean Fresnel in the 1820s. It was improved upon over time by Fresnel and others, and widely adopted in lighthouses (and theatrical lighting) across the 19th century. Today, very few Fresnel Lens survive in lighthouses and those that exist typically have plastic replica prisms. For the most part, Fresnel Lens were replaced by radio navigational devices. (Today, ships navigate by GPS.)

We were very lucky to find a Fresnel Lens atop the Pensacola Light House in FL. It is a little bit of a climb if you want to see the lens. You first go through a door in the light keeper’s house and through a tunnel that connects the house and tower.

Then it’s up an iron staircase that winds a few hundred feet up the interior of the narrowing tower to a service room.

A ladder-type set of iron steps leads from the service room to the lantern room, above. It is there that you see it above you. – a Fresnel Lens.

For photos of a few other lighthouses, please see my 9/1/18 post.