The Fresnel Lens used in Lighthouses
The history of it and how it works

Augustin Jean Fresnel (1788 - 1827)

Fresnel (pronounced: Fur-nel)

An accomplished engineer and scientist, Fresnel changed our understanding of how light behaves and can be manipulated and intensified. His revolutionary system of lens optics replaced the system of multiple parabolic reflector and lamp assemblies with a single lamp inside of a single lens. Fresnel lenses are used today in traffic signals, infrared motion detectors, theatre projection systems, photographic equipment, industrial optics, environmental monitoring equipment, astronomical equipment and automobile lights.

The basic idea behind a Fresnel lens is simple. Imagine taking a plastic magnifying glass lens and slicing it into a hundred concentric rings (like the rings of a tree). Each ring is slightly thinner than the next and focuses the light toward the center. Now take each ring, modify it so that it's flat on one side, and make it the same thickness as the others. To retain the rings' ability to focus the light toward the center, the angle of each ring's angled face will be different. Now if you stack all the rings back together, you have a Fresnel lens. You can make the lens extremely large if you like. Below is a cut out section of a Fresnel Lens to give you an example.

To bend and focus the rays to form a single, concentrated beam of high intensity light, the catadioptric prisms refract and reflect; the dioptric prisms and center bull's eye lens refract. With just a 1000 watt bulb, a first-order Fresnel lens can generate a 680,000 candlepower beam visible up to 21 miles out to sea if set high enough.

The sizes of the Fresnel Lneses used in Lighthouses are catagorized by the order of the lens, the largest of which is a "First Order lens". In looking at lighthouses they will often tell you what order lens is in that spisific Lighthouse such as it has a First Order Lens, or a Third Order Lens etc. below is a chart to give a rough idea of how big the lens is that a lighthouse houses.

Fresnel lens order sizes
First 7' 10" 6' 1"
Second 6' 1" 4' 7"
Third 4' 8" 3' 3"
Third and 1/2 3' 0" 2' 5 1/2"
Fourth 2' 4" 1' 8"
Fifth 1' 8" 1' 3"
Sixth 1' 5" 1' 0"
Source: Guardians of the Golden Gate: Lighthouses and Lifeboat Stations of San Francisco Bay
Ralph Shanks and Lisa Woo Shanks, editor. 1995, Costano Books, ISBN: 0-930268-08-3.

My son MAC standing in front of a Third Order Fresnel Lens
This 3rd Order, flashing (rotating) lens served in the "New" Point Loma Lighthouse from 1891 until 1997.

This small Sautler & Lemonnier 5th Order, fixed lens
lighted the Ballast Point Lighthouse from 1890 until 1960.

MAC standing beside a very small lens.