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.
Look inside a taillight or a turn signal lens. You’ll see the same stepped pattern. Almost invisible from the outside. Same concept.
The coolest thing about lighthouses (to my mind) is that the lens floats on a trough filled with mercury. It’s so clever on so many levels (ha ha). Firstly, it’s level. It won’t evaporate. Anything sitting on top, such as the lens assembly, won’t sink. Finally, it’s essentially friction free.
On the minus side, well, it’s made of mercury I suppose.
If you’re ever close to where you can go to museum and see one of these amazing lenses be sure you do it. They could pass very well just as art, but to think something so beautiful and well done helped keep sailors safe at sea gives them a special aura in my mind that makes them somewhat otherworldly.
There are stories and legends about during wartime taking these precious things out of the light tower and burying them or carrying them off somewhere to safety to make sure they didn’t suffer damage from gun fire.
Lightouses were not just “bright light means big rocks, keep away”: they were positioning devices. The flashes encoded by the rotating shield were used to identify which lighthouse was which. Combined with a compass (to get a bearing from your vessel to the lighthouse) you could get a bearing from the lighthouse to your vessel. With two lighthouse in view, this gave you an exact position. Some lighthouses also added directional colour filters for different bearings to aid in rapid navigation (e.g. if the light was red you had strayed to port, if it was green you had strayed to starboard, if it was white you were on the correct approach). The distance of the light above the horizon (apparent height at range) during the day (or at night, the distance at which the light was just on the horizon line and the light ‘twinkled’ by wave action) also gave you your distance from the lighthouse, which combined with the bearing gave an absolute position reference.
The flashes encoded by the rotating shield _are_ used to identify which lighthouse was which.
FTFY
“The lens works like a series of prisms”
Nah. It is more like a lens with cutting out rectangular pieces.
A lens works by bending the light when passing from a medium with one density into a medium with a different density. This is caused by different speeds of light (or any other el.mag. wave) in different materials. If the wave hits rectangular all waves arrive at the same time, so they are bend with the same difference and so in sum nothing happens. But if the beam hits with an angle, some waves are bend earlier because they arrive earlier. And some arrive later, so they are bend later. And this in sum results in a bend beam. Have a look at “active electronically scanned arrays” to understand how lots of punctual emitters create a beam.
A prism works due its material property of having different light speeds for different wave length. So different wave length are bend with different speeds additional to the normal bending process from above.
It is called chromatic aberration when happening in lenses and you don’t want to have that happen. But unfortunately this is how the world works. And opens a whole new can of worms for lens designs…
So only the surface is needed, all the other glass (material) only transmits the light (wave). Cutting a lens in small stripes and remove the rectangular stuff will give you something flat.
Aliex has flat magnification feets sized 180 mm x 120 mm wiith 3 times magnification for some Euros incl. shipping. Aging hackers love that trick.
Check out the video at 24 seconds in. The center section of the lens is a Fresnel, refracting the light, but the upper and lower sections are prisms used to reflect and redirect the light across much larger angles.
I took the fresnel out of a discarded overhead projector, it can light fireworks on a sunny day.
Or burning atomically enhanced giant ants… I gotta stop watching old SciFi movies.. 😄
The lighthouse lens in the video is not remotely like the Fresnel lens we know and love from discarded overhead projectors and flat magnifiers.
Those flattened “Fresnel” lenses work (like ordinary lenses) by refraction, “bending” light.
The lighthouse lens uses a series of prisms that redirect the light through total internal reflection: they are really mirrors, and don’t refract the light at all, except in the center section which is just an ordinary convex lens.
Oh, that is so wrong. See Jan Praegert’s reply, he nailed it.
No. It’s plain. Have a look at the embedded video. Note the cross section of the prisms in the CAD. Have a close look at the prism assembly. Even the top photo in this post shows it plainly: the center section is indeed a Fresnel-type lens. The upper and lower prismatic parts are plainly not oriented to refract light — that portion of the lens operates by total internal reflection in the prisms.
Total internal reflection is more efficient than trying to refract the light because you can arrange the light to enter the prism always normal to the surface, to minimize loss. It also isn’t subject to chromatic aberration. Even more importantly, you can get much better numerical aperture out of a reflective assembly than a refractive lens, so the resulting optic is much more efficient.
So there is a misunderstanding in terms.
We have fresnel lenses, prisms and total internal reflection elements (that do not need to look llke prisms). These three things are different.
A lighthouse lens assembly consist of a fresnel lens and two sections with total internal reflection elements.
Bingo.
But note that (as I said) the lens in the embedded video contains no Fresnel lens section – just a plain convex lens and the TIR components.
Can you give an example of a total-internal-reflection optical element that would not be called a prism? pentaprism, Dove prism, corner prism, erecting prism… I’m certain there are many more.
Hyper-radial lenses.
https://uslhs.org/hyper-radial-lenses