Fresnel Lenses | How They Work

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.

ConvergeLens1

 

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.

ConvergeLens2

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.

NASA

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.

University of Chicago

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.

University of Chicago

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.

Fresnel

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.

Fresnel2

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.

FresnelCut

 

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.

fresnel-lenses2

Click this image for much more information.

 

A wealth of information is available in The Fresnel Lens Makers. Thanks to shoreacres for this.

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.

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15 thoughts on “Fresnel Lenses | How They Work

  1. Your excellent post on fresnel lenses was one of your most interesting to this old sailor. I first went to sea in 1956, at which time there was no such thing as GPS and navigation was done the old-fashioned ways. I still remember my Midshipman training trip down the Chesapeake Bay at night when we were required take sightings on lighthouses, it being essential to identify each according to color, frequency and spacing. We used a stopwatch to be certain. Even at the time I noticed, curiously, that the lights usually didn’t disappear between flashes, but merely almost so. Now I know why. Navigation was exacting work, complicated by weather and unforgiving of any error or inattention. Now they have GPS. Buncha pansies! 😆

    Liked by 2 people

    • That makes me feel good, Jim. Your feedback is really appreciated.

      I first thought the post would be relatively short with a couple of diagrams showing rays traced through shapes. That quickly changed as I discovered more information.

      I grew up in the midwest. Oceans for me were fields of corn and beans. Navigation was all NSEW on the grid of the roads. Everything was orthogonal.

      Thanks for being a good sailor and sharing with me about this part or your experience.

      Like

  2. I loved this. The third-order Fresnel lens from the old Matagorda lighthouse at Indianola is at the Calhoun County museum in Port Lavaca. (The station’s logbook is there, too.)

    The lighthouse was rebuilt a couple of times, and moved farther inland at one point. During the Civil War, an earlier, fifth-order lens was removed. Some accounts say it was crated for storage, while other say that “storage” actually meant it was buried in the sand to make things as difficult as possible for the Union forces.

    In any event, the lighthouse’s final, third-order lens was from L. Sautter, in Paris. The first lens the company sent to the U.S. was a third-order made for Alcatraz Island (1853). There’s as much information about the factories as you could want in this article, which adds:

    “In 1854, two years after his arrival as the head of the company, Louis Sautter submitted his first patent relative to the construction of lighting for lighthouses: “a modification in the construction of the mechanical part of the rotating devices, for lighthouses using flashing lenses, and a new system of lamps for the lighting of these lighthouses.” Many other industrial patents followed this first submission. According to one of his engineers, Sautter also developed designs that allowed improvements in the size of the glass that could be delivered by Saint Gobain.”

    “Sautter worked with Leon Foucault in 1860 to develop an improved rotation clockwork regulator. Sautter became a major supplier of lighthouse equipment by 1861. Monsieur Reynaud, Director of the French Lighthouse Service, stated that: “The administration always desires to maintain two construction establishments for lighthouses in Paris (on a plan of equality) and therefore distributes the work as equally as possible.” The two firms were Henry-Lepaute and Sautter.”

    Speaking of lighthouses, here’s how “my” lighthouse looked at sunset last night: one storm gone, and another developing.

    Liked by 4 people

  3. I’ve often wondered why those lights had the shapes which I now know to be fresnel lenses. Very nice article written quite understandably to this layperson who wasn’t curious enough to find out on my own. Thanks for the enlightenment.

    Liked by 2 people

    • I can’t either. I have used holograms, taught about them, have them on my credit card, and even made some in workshops. These ideas are so far beyond my understanding.

      Thanks for the link. It was interesting.

      Like

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