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Close-up of a Fresnel lens.

Fresnel Lenses

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by Chris Woodford. Last updated: November 6, 2016.

If you're unlucky enough to be caught in a storm at sea, there's nothing more reassuring than the friendly wink from a nearby lighthouse. But have you ever stopped to think how that light can travel so far across the ocean? It's largely down to lenses—amazing, curved, Fresnel lenses (pronounced "Fre-nel," with a silent "s") that concentrate light into a super-powerful beam. Let's take a closer look at how they work!

Photo: If you see a light shining through a piece of glass or plastic with this weird pattern of concentric circles on its surface, you can be sure you're looking at a Fresnel lens.

How does a lighthouse work?

A lighthouse uses similar science to a telescope, but works in exactly the opposite way—with the help of a Fresnel lens. The glass lenses in a telescope refract (bend) light rays from distant objects so they seem to be much nearer. But in a lighthouse, the Fresnel lens wrapped around the lamp concentrates the light rays into a powerful and parallel beam so people can see it, with just a naked eye, as far as 30 km (about 20 miles) away or more!

Three cunning tricks make this possible:

  1. Lighthouses use amazingly powerful xenon lamps (a little bit like neon lamps) that are hundreds of thousands of times brighter than the lamps in your home.
  2. They mount their lamps in towers high above the sea level, which makes them visible roughly five times further away.
  3. They use specially shaped lenses and prisms to concentrate their light into a super-powerful beam.

Closeup of a lighthouse tower showing the lamp and lens inside.

Closeup of a lighthouse lamp showing the Fresnel lens and prisms.

In theory, you could make a lighthouse beam with just an ordinary glass lens, but it would need to be enormous and heavy and that would make it incredibly expensive and quite impractical. That's why lighthouses use hollow, lightweight Fresnel lenses, which have a very distinctive "stepped" surface that bends the light as much as a thick, heavy glass lens. They're named for Augustin-Jean Fresnel, (1788–1827), the French physicist who pioneered them in the early 19th century. Car headlamps use Fresnel lenses molded from plastic in much the same way.

Look closely at a lighthouse and you'll see the Fresnel lens surrounding the lamp. The concentric rings are actually "steps" (thick ridges) in the lens surface. Each step bends the light slightly more than the one beneath it, so the light rays all emerge in a perfect, parallel beam that travels many kilometers/miles across the ocean.

The lamp in this particular lighthouse rotates so it sweeps across a much greater area of the sea. The rotation also means the light seems to flash every 10 seconds when you're far away from it. That makes the lamp much more noticeable and, because different lighthouses flash at different rates, sailors can time the flashes to figure out which lighthouse they're looking at and where they are.

Photos: The Fresnel lens at Anvil Point lighthouse near Swanage in Dorset, England, which was originally built in 1881 and fully automated over a century later in 1991.

It's hard to get close enough to the Fresnel lens in a lighthouse to see exactly what it's like, but if you're near a science museum you might just be lucky—they sometimes have old Fresnel lenses on show. Here are some photos I took of the working Fresnel lens at Think Tank, the science museum in Birmingham, England. Note how there is a Fresnel lens on each side of the lamp (making eight in total) with prisms (curved chunks of glass) mounted above and below the lens to pull in light rays that deviate further from the central axis, making an even brighter beam.

Lighthouse Fresnel lens Closeup of a dummy lighthouse lamp showing the Fresnel lens and prisms.

Photos: The Fresnel lens exhibit at Think Tank, the science museum in Birmingham, England. The silvery thing at the bottom is the electric turntable that makes the whole lamp and lens assembly rotate very slowly.

How do you make a Fresnel lens?

Lenses work by bending (refracting) light beams. The bending happens when light enters the glass (passing from the air into the glass) and when it leaves again (passing from the glass back into the air). It follows that the only part of a lens that really matters is the border between the glass and the air (in other words, the outer edge of the lens); most of the lens doesn't really do that much at all. What if we could cut a lens right down so that all we had left was the useful outer part? That's the basic theory of the Fresnel lens—and here's how it works in practice.

A simple animation showing how a Fresnel lens can be cut from an ordinary lens.

  1. Suppose our lens is made of some material we can cut easily, like yellow jell-o (jelly). Let's try and cut away the useless inner part of the lens leaving behind the useful outer curve!
  2. Let's say we want our Fresnel lens to be about a third the thickness of the original lens. We can divide its height by drawing three equally spaced horizontal lines.
  3. Where each horizontal line touches the upper curve of the lens, we draw a vertical line down to the base.
  4. The interesting bits of the lens are the orange upper segments, so let's save these and lose the rest of the lens.
  5. Drop the orange segments down to the baseline and what we have is a finished Fresnel lens!

Glass or plastic?

Close-up of a plastic Fresnel lens in a car headlamp.

Photo: Car headlamps often use inexpensive, plastic Fresnel lenses to throw light off into the distance. Although plastic Fresnel lenses are cruder and produce poorer quality images than traditional glass lenses, it doesn't matter in this case: all that's important is concentrating light into a beam at a reasonably well focused point on the road ahead.

Fresnel lenses are all about making big, powerful beams of light that stretch very long distances. Unlike the conventional lenses in something like a telescope, the optical quality of the light beam emerging from a Fresnel lens often doesn't matter very much: if you're operating a lighthouse, all you're trying to do is throw light off into the distance; it's not important if sailors see a "blurred" image of the lighthouse... as long as they can see something! That means Fresnel lenses can be made from relatively inexpensive plastic, such as acrylic or polycarbonate, as well as glass. You simply need a mold containing the lens pattern in reverse—and then you can make as many identical Fresnel lenses as you want! Plastic Fresnel lenses are smaller, thinner, weigh less, and cost less than comparable glass lenses.

How a Fresnel lens produces a large magnified image in a projection TV, from RCA's 1984 patent US4,482,206.

If you want to use a Fresnel lens the other way—to collect light rays from a distance and bring them into a sharply focused image—you need to be more precise. Inexpensively made Fresnel lenses make poorer quality images than traditional glass lenses because of a problem called spherical aberration: light rays traveling through a Fresnel lens at different angles will come to a focus at slightly different points, giving a blurred image. You get quite a bit of distortion because the surface of a Fresnel lens is discontinuous: unlike with a smoothly curving lens, there are sudden jumps from one segment of a Fresnel lens to the next. You'll probably also find that different colors are refracted by the lens to different degrees, giving you unwanted color fringes in your image (a problem called chromatic aberration). Although it's possible to adjust the angle of the steps in a Fresnel lens to minimize aberrations, generally you'd use a conventional lens (probably made from optical quality glass) for higher optical performance.

Artwork: Lighthouses and car headlamps aren't the only places where you'll find Fresnel lenses. Projection TV systems sometimes use them to make large, magnified images. In this 1980s design by RCA, an image generator (green, 102, maybe a traditional CRT or LCD screen), fires its picture onto a back-projection screen via a prism and mirrors (blue, 103/104), through a compound Fresnel lens (red, 10) and focusing lens (9, pink). Artwork from US Patent 4,482,206: Rear projection television screen having a multi-surface Fresnel lens by Bertram VanBreemen, RCA, November 13, 1984, courtesy US Patent and Trademark Office.

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Text copyright © Chris Woodford 2008, 2016. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008/2016) Fresnel lenses. Retrieved from http://www.explainthatstuff.com/fresnel-lenses.html. [Accessed (Insert date here)]

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