
In the 1820s, a new kind of lens saved ships - clouddrover
http://www.bbc.com/travel/story/20190620-the-invention-that-saved-a-million-ships
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dreamcompiler
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.

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

~~~
BurningFrog
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?

~~~
flomo
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.)

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davidivadavid
[https://en.wikipedia.org/wiki/Fresnel_lens](https://en.wikipedia.org/wiki/Fresnel_lens)

~~~
gruez
Thanks, saved me from the clickbait title.

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s1mon
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.

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new299
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/](http://41j.com/blog/2018/12/quantumsi/)

~~~
Theodores
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.

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

~~~
Theodores
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.

~~~
hug
> 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.

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dmckeon
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](https://en.m.wikipedia.org/wiki/Pigeon_Point_Lighthouse)

~~~
dredmorbius
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](http://www.terragalleria.com/images/us-ca/usca30023.jpeg)

[https://californiathroughmylens.com/wp-
content/uploads/2018/...](https://californiathroughmylens.com/wp-
content/uploads/2018/11/point-bonita-lighthouse-16-640x427.jpg)

[http://www.lighthousefriends.com/light.asp?ID=100](http://www.lighthousefriends.com/light.asp?ID=100)

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inflatableDodo
> _" 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.

~~~
Dylan16807
> 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.

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noufalibrahim
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" :)

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unstatusthequo
I always thought it was tight lips that saved ships.

