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The Origin of the Fresnel Lens [Hackaday]
If you are a Hackaday reader, you probably know what a Fresnel lens is. You find them in everything from overhead projectors to VR headsets. While it seems commonplace now, the Fresnel lens was an important invention for its day because it revolutionized maritime navigation and, according to a post over at IEEE Spectrum, that was the driving force behind its invention. In fact, the lens has been called “the invention that saved a million ships“.
The problem stems from issues in navigation. Navigating by the sun and the stars is fine, but not workable when you have heavy cloud cover, or other reasons you can’t see them. A lighthouse often marked an important point that you either wanted to navigate towards or, sometimes, away from. Sure, today, we have GPS, but for a long time, a lighthouse was your best bet.
The problem is that in those days, a lighthouse was an oil lamp, a concave mirror, and an ordinary lens. This made the lighthouses difficult to spot. Napoleon started the Commission of Lighthouses as part of the Corps of Bridges and Roads. This is the Corps that employed optical genius, Agustin-Jean Fresnel. Although some lighthouses were already using lenses, they weren’t using the special Fresnel-style lenses. There had been speculation about building this type of lens, but Fresnel was apparently unaware of them when he proposed his lens for lighthouse use in 1819. His proposed lens was a bit different than earlier proposals, too.
The lens works like a series of prisms, the ones on the edges bending light more sharply and the center bending it hardly at all. Compared to a conventional lens, a Fresnel will be thinner and lighter or — conversely — for the same thickness and weight, the Fresnel can have better properties. However, the distortions make them less suitable for imaging where regular lenses still reign supreme. A thinner lens, of course, should let more light through, which is important when you are trying to shoot a beam a long distance. Fresnel’s lenses let through 98% of the lamp’s output, which could signal ships up to 32 km away.
By 1823, the lenses were appearing on lighthouses. By 1860, all lighthouses in the United States were using the improved optics. One of those things that you don’t really give a lot of thought to, but at one time it was major high tech. You have to wonder in 200 years what we are using today that will be relegated to the ordinary and commonplace.
You can see replica lighthouse lenses in the video below. If you make your own, though, be careful not to get a big head.