In a recent post about Maine lighthouses, I included two photos of the Fresnel lenses used to project the bright light beam across the water. One of the readers is a man I’ve enjoyed working with before in the blog world. He suggested I add a post with some description of how the Fresnel lens works. Here it is.
Basics of Converging Lenses
The converging, or convex lens, is able to bring parallel rays of light toward a focal point. As a child, I played with a magnifying glass lens to burn leaves, grass, and other things.
The lens can also be used in a different way to project light rays parallel to each other in a beam. Simple projectors work on this basic principle. A lighthouse is designed to do this.
Large Lens Applications
A problem arises when the optical instrument using a convex lens becomes very large. The lens is very heavy and difficult to mount and support. An example of this is in the refracting telescope. At the front end of the telescope is a convex lens. It focuses the parallel light rays from a source toward the focus near the eyepiece end.
The largest lens ever used successfully for this type of telescope is at the Yerkes Observatory in Williams Bay, Wisconsin. Note the person at the lower left for scale. The lens at the upper end is 40 inches (1.02 m) in diameter.
Here it is lowered for close inspection. The large amount of glass needed adds tremendous weight. The shape of the lens changes slightly because it sags, distorting images. It is difficult to cast a lens that large with no flaws in the glass. Any glass flaw also distorts images.
Melanie reminded me of her experience with theater lights. Spotlights need to project a beam to the stage. Big spotlights would be very heavy and hard to operate with a large convex lens up front. We watched an Andy Williams college concert in 1980. The spotlight operator was all the way to the back of the auditorium. He had trouble tracking Mr. Williams with the huge spotlight. Andy was good natured about it when he called up to the guy saying ‘I’m over here.’
A lighthouse also needs a large diameter convex lens to project a wide beam across the water. But, there is limited space available at the top of the lighthouse. A set of huge and heavy glass lenses won’t fit the small space.
The Fresnel Lens Design
Augustin-Jean Fresnel is credited with development of the Fresnel lens for lighthouses. The first Fresnel lens was used in 1823 says Smithsonian Magazine. His design was able to eliminate much of the heavy glass from a large convex shape. It maintained the curved surfaces of the lens where the refraction of the rays occur.
Part (a) of this image shows the full amount of glass in a typical convex shape. Part (b) shows the large amount of glass that is not needed thus saving a great deal of weight. Part (c) places the curved segments onto a plane.
The shape of part (c) replaces the shape of part (a). The curved surfaces are the same giving nearly the same changes to the rays as before. The thinner version weigh much less.
Today, the shape in part (c) is easily molded from glass or plastic. It can be made into vary large sizes for many applications at much less cost. Here is a Fresnel lens from a spotlight which has been cut to show the ridges. The molded shape is not able to produce the same high quality imaging and precision as a standard lens. But that tradeoff for light weight and flexibility is worth it in many applications.
Art and Technology
Lighthouse applications come in a variety of designs. I won’t try to explain the rationale for the various ones. Their size and shape often dictates which application is best. I find them beautiful works of art and technology. More images can be found at the Fresnel Lens Museum.
This twelve minute video by Artworks Florida Classic Fresnel Lenses, LLC, beautifully describes the processes used today to repair and restore old lighthouse lenses. The glass is replaced with acrylic to save weight and give the lighthouse the same look as before.