Fresnel Lens

The lens on display in the center of this room is a fourth-order lens that was in the tower at Beavertail until it was replaced in 1991 by a rotating beacon.

The lens can be open to allow visitors to see inside.

Touching the glass prisms is prohibited

Notes on the Fresnel Lens

This type of lens was developed in 1823 by a Frenchman named Augustin Fresnel(pronounced frey - NEL).The arrangement of the prisms bends the light rays thus making the beam more intense and visible further out to sea.

Typical lenses in use before the Fresnel lens lost nearly 97 % of the light from a flame, and a flame with a reflector behind it still lost about 83 % of its light.The Fresnel lens was able to capture all but about 17 % of its light.At one time all the lighthouses in America were equipped with a Fresnel lens.

The size of a Fresnel lens is designated by order: a 1 st order lens is the largest with an average height of about 12 feet.The 6 th order lens is the smallest in height, about 1.5 feet tall. You may want to invite visitors to observe the height of the light from the lens. Then open the door of the lens and show them the lighted bulb. The magnification is due to the optics of the lens.The larger bulb inside the lens(1000 watts) was used in the tower and is too bright to put on display here because of safety reasons. Therefore, a 20 watt bulb is used for demonstration purposes.