In the 1820s, a new kind of lens saved ships | Hacker News Reader

In the 1820s, a new kind of lens saved ships

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83 pointsclouddrover49 comments

This article seems to paint Fresnel lenses as some new kind of optics. They're not. The point of Fresnel lenses is to save weight and thickness. In an ordinary lens, its thickness is proportional to its diameter. In a Fresnel lens, thickness is (almost) constant, irrespective of diameter. So if you need a lens that's a meter in diameter (e.g. for a lighthouse) the weight is enormous if you use ordinary lens technology. Fresnels are lighter because Mr. Fresnel figured out that all the glass in the middle was unnecessary. The angles at which the rays meet the lens surface are what matter, and in his lens the angles are preserved without all that heavy glass in the middle.

A rough ASCII sketch to illustrate this:

Regular lens:

        _____
       /     \
      /       \

Fresnel lens:

        _____
     /|/     \|\

That does not work at all and just confused me. The angles must change on the interrupted parts to follow the general curve of the lens' surface.

https://commons.wikimedia.org/wiki/File:Fresnel_lens.svg is a very good illustration.

As a counterpoint the GPs diagram was useful for me (perhaps because I have a background involving some study of optics).

Granted the parents picture is way better than the rough ASCIIart

It sounds like the Fresnel lens was an engineering breakthrough rather than scientific.

Exactly. You said it more succinctly than I did.

Was it an engineering breakthrough in the sense that people had realized it was a desirable design before, but didn't know how to make it before Fresnel (I assume) figured that out?

Good question. When you tour these old lighthouses, the gigantic glass lenses are certainly impressive in their construction. The technology may be well understood today, but I doubt you could procure such a thing. (Well maybe if you're the NRO.)

Yeah, pretty much. That said the ability to shape glass like that was pretty cool. In theory, you can keep making the rings smaller and smaller (and the lens thinner and lighter) until you hit the wavelength of the light you're trying to focus.

I would consider the insight required to realize that Fresnel lenses work at all to be very much a new step in optics. Geometrically speaking it is not intuitive at all.

They have their disadvantages though, since the angles between incident light and surface are only mostly preseved in practice: e.g. scattering from single facet radii (due to limited manufacturing precision), the necessity of draft facets that do not serve any optical function and introduce artifacts through unwanted refraction/reflections (and even lateral light transport due to total internal reflection and refraction towards a neighbouring facet). These effects are what usually makes fresnel lenses with a large amount of cuts appear a little blurry or milky, depending on the manufacturing precision.

You can reduce the influence of the draft facets to a degree but only if you have a point-like source (e.g. large lens diameter compared to light source size). Artifacts will be more or less visible for different object points, compared to a smooth lens.

It's a tradeoff between form factor, efficiency and quality - you wouldn't use them in high-power laser optics or telescope mirrors.

The benefit of a Fresnel lense is that it only needs to roughly collimate the light, not to focus an image, whereas for a telescope mirror or lense you want as much incident light to come to the focal plane at the same focal length.

https://en.wikipedia.org/wiki/Fresnel_lens

Thanks, saved me from the clickbait title.

A whole long article about a lens design, with zero images or diagrams of the lens. I know what they look like so I don’t need it, but I just don’t understand how or why someone would publish this without a picture of the subject of the article.

Fresnel lens are everywhere now... I most recently came across them embedded as a planar lens in a semiconductor part used for DNA sequencing:

http://41j.com/blog/2018/12/quantumsi/

They aren't!

Years ago they used to be common on the backs of cars, so you could look where you were parking. Vans and estate cars invariably had them.

But now?

Backup cameras.

Same with the original lighthouse application. They don't use Fresnel lenses any more.

The lighting tech used for aircraft has superceded the Fresnel tech of old. So lighthouses are done differently now.

Even in home applications Fresnel lenses don't last the distance, You don't have hacky Fresnel lenses in products to make up for them having a naff CRT for the image.

But then again, Fresnel lenses do live on, but in better ways. There are many compound lenses for lights that are not a classic Fresnel lens but work nicely.

Nikon makes fresnel telephoto lenses, and they save size + weight vs their classic counterparts

In practice though most lenses and applications move away from being Fresnel. The lights get better or a camera or other screen technology replaces it.

Fresnel lenses are like motorbike sidecars, they are a cheap hack. But if I was to post 'you don't see motorbike sidecars these days' then some nitpicker on HN would Google search a new Kickstarter project of a motorbike sidecar and - two seconds later - mod me down for being generally but not universally correct.

As I understand it the Nikon lenses are not 'Fresnel' as we know it. The idea is to combine a regular lens with a Fresnel lens to remove chromatic aberration. The weight saving isn't what it is about since there is still a regular lens that the Fresnel lens works with.

> But if I was to post 'you don't see motorbike sidecars these days' then some nitpicker on HN would Google search a new Kickstarter project of a motorbike sidecar and - two seconds later - mod me down for being generally but not universally correct.

I'm of course not going to downvote you, but just to help you out with your point IMZ-Ural still manufacture motorcycles with sidecars to this day, and in fact are working on a fully electric model.

> In practice though most lenses and applications move away from being Fresnel.

Tungsten fresnel lights (and now LED fresnel lights) are very common in videography. The lens provides a simple spot/flood mechanism.

The fresnel element version of the 300mm f/4 lens reduces the weight by half and the size by one third compares to the non-fresnel version

True! "Proprietary Phase Fresnel technology" on the 500 PF. It looks very different though.

Canon makes them too -- look for the DO lenses.

For many years now I've owned a 70-300 DO very very compact zoom lens and always been happy with it. There are some rare optical artifacts, in some lights, on some subjects, but it's never been a problem in real life.

Very common in theatrical lighting, where a somewhat-focused beam is wanted but the not-quite-crisp edges characteristic of a Fresnel lens are an asset rather than a liability.

For example: https://www.strandlighting.com/global/products/rama-led-fres...

Vive (SteamVR) headsets use fresnel lenses. (vive vs vive pro comparision) https://i.imgur.com/SvxVRUL.jpg

It could be a zone plate. They look similar, but focus based on diffraction instead of refraction.

https://en.wikipedia.org/wiki/Zone_plate

The diagram could be either, but isn't a perfect match for either. Perhaps a hybrid is possible. The 4-level right-angle construction (stair step) is weird.

The Zone plates are very cool. Thanks for the link.

Zone plates work great for radio waves. I don't know why they aren't used more than parabolic dishes, since they're easier to manufacture and mount. Perhaps the gain is smaller for a given diameter?

A third choice is the phased array.

There are two dimensions to focus, and they don't have to be done the same way. For example, the trough antenna is parabolic in one direction but uses something else in the other direction. Viewed from along the resulting beam, it is a rectangle. Manufacture the reflector involves a simple 1-dimensional bend of the metal.

My guess would be twofold: adding delay if the source is not straight though enough to screw communication or diffusing the image too much if your focal point is not a point but a sensor.

Zone plate focal length is wavelength dependent, unlike parabolic dishes.

Very common example: Focusing lenses in front of smartphone camera flash LEDs.

The "meatball" https://en.wikipedia.org/wiki/Optical_landing_system

The system described is for aircraft carriers. I know about it because I maintained a smaller version on a fast frigate in the seventies. It was slaved to the ship's gyrocompass, which provided a stable horizontal reference.

(Besides navigation, the gyrocompass also provided that stable reference to the ship's gun.)

Here's one from Amazon for $8 that's A4-sized and probably weighs a few grams. Great for magnifying or building a solar oven. If you can find one a meter square, you could probably weld with it in full sun.

[0] https://www.amazon.com/11-75-Fresnel-Magnifier-Projection-PR...

> I had one of these for years and somehow lost it. I recently replaced it. One of the uses I put it to was starting fires. It will ignite a pine board in seconds. Don't get your hand at the focal point with the sun shining through it or you will burn the peewadden out of your hand. It's not as good as a magnifying glass for reading. I have taken pictures through one to blow a small picture up into a larger one.

You’ve got to love Amazon reviews sometimes.

Googling “peewadden” also has an interesting story from the American civil war.

I used to run a site that showed people how use these to create a projection tv (a bad one!) from a fresnel lens and a CRT tv. Was a fun and simple project for kids.

Anyone remember AcidWarp? It was a DOS program from the early '90s, distributed through BBSes, that generated lovely flowing color patterns. It came zipped with a text file with instructions for building a Fresnel-based projector for your PC monitor.

Looks like it's still online: http://www.noah.org/acidwarp/

"Big Face Box" was a fun project I saw at a Makers Faire: https://makezine.com/2017/05/25/bigfacebox-a-simple-way-to-g...

I'm not sure where they got the plastic sheet lenses from... I know old rear-projection TVs used to have giant ones.

(They had a warning on the booth: "Do not take outside". I'd hate to learn that lesson the hard way.)

For SV folks who want to see a (not operational) Fresnel lens, there is one nearby at https://en.m.wikipedia.org/wiki/Pigeon_Point_Lighthouse

Point Bonita is closer, if you're in SF.

Alcatraz and the Faralones are technically within SF city limits.

http://www.terragalleria.com/images/us-ca/usca30023.jpeg

https://californiathroughmylens.com/wp-content/uploads/2018/...

http://www.lighthousefriends.com/light.asp?ID=100

I believe the old lighthouse near Daytona Beach (Ponce Inlet) in FL has a few too on the grounds.

Or they could just put on pretty much any of the major VR headsets since most of them use fresnel lenses.

There’s also one in the Maritime NHP visitor’s center:

https://www.nps.gov/safr/planyourvisit/the-visitor-center.ht...

https://www.nps.gov/safr/planyourvisit/images/lgthouselenscm...

>"Were I a Roman Catholic, perhaps I should on this occasion vow to build a chapel to some saint; but as I am not, if I were to vow at all, it should be to build a lighthouse.” — Benjamin Franklin, July 1757

I had a look out of curiosity and cannot find any record of him subsequently building a lighthouse. He is a politician however and didn't actually say that he would vow, just if he were to vow. I did find out that he made money writing poetry about dead lighthouse keepers at the age of twelve however, just nothing about building any.

> He is a politician however and didn't actually say that he would vow, just if he were to vow.

That's really harsh for a sentence that has the word perhaps in it! It's a perfectly ordinary hypothetical, not a misleading trickery of words.

I first came across Fresnel lenses in a cute place. I bought a "Where's Wally pocket edition" book for my daughter and since the book was very small (A6), they included a small plastic Fresnel lens in a little pocket inside the front cover. This made it practical to find Wally.

I have one in my card wallet for "emergencies" :)

I always thought it was tight lips that saved ships.

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