Before the invention of the high-powered LED, and even really before the widespread adoption of electric lights in general, lighthouses still had the obligation of warning ships of dangers while guiding them into various safe harbors. They did this with gas lights and impressive glass lenses known as Fresnel lenses which helped point all available light in the correct direction while reducing weight and material that would otherwise be used in a conventional lens.
Now, a company in Florida is using acrylic in reproductions of antique Fresnel lenses. At first glance, it seems like acrylic might not be the best substitute for glass, but the company is able to achieve extreme precision using a CNC machine and then polishing and baking the acrylic which makes it transparent and excellent for use in lighthouse lenses like this. The reproduction lenses are built out of brass, and the lens elements are glued in place with a special adhesive. It’s a convincing replication worthy of use in any lighthouse.
Be sure to check out the video below to see how these lenses are built, and although we’re not entirely sure what exactly is being sprayed on the lenses when they are being polished, perhaps someone in the comments section can illuminate that for us. Of course, there are other uses for Fresnel lenses than in lighthouses, and we’ve seen some great examples of them put to use for many different applications.
Impressive work! The result is awesome.
I would love to have details about how to polish clear plastic too. It would be useful to restore scratched parts (like large clocks front “glass”).
BrassO works.
There are loads of different plastic buffing compounds out there. You can use wet dry paper if all you care about are cosmetics but cheap abrasive may leave grit embedded in the plastic. Most auto stores should carry headlight polish kits which are nothing more than marked up plastic buffing compound and some tack cloth.
There’s also Applied Science:
https://www.youtube.com/watch?v=7na8kQ78vkQ
I’ve polished a lot of headlight lenses, and from what I’ve found, the best combination of cheap/effective/available is regular automotive buffing/rubbing compound, the kind you find at wal-mart called “scratch remover” or similar. I keep some on hand, and a rag and some of that compound will buff up a headlight in no time.
After 141 years (2018-1877) what will acrylic lenses look like exposed to direct sunlight ? My guess is, if exposed to UV, that they will start to yellow, .after a decade. But maybe the final coating they sprayed on, after polishing, was a UV protective coating to slow it down.
You have to be totally amazed by the original Fresnel lenses, pure genius.
The lens is very thin, compared to a more traditional lens with the same focal point, so only a fraction of the light is absorbed in transmission. It is a much cooler lens, pun intended.
Fresnel lighthouse lenses is more about weight and ability to capture more light from the lightsource, and easier to produce. Yes more pieces, but each has a simpler design and can be perfected for its part of the light angle. A complete lens would litterally weight a ton, be hard to move and handle for the mechanics of the era, and be very big and hard to make and transport. And if it breaks…
The lens was a fifth order. First order lenses are hugh.
These are made to go into museums, and rich people’s houses, not working light houses.
There are multiple types of acrylic, surprisingly most of them are completely transparent to UV rays. Unlike things like Polypropylene, or High Density Polyethylene, most types don’t break down in sunlight readily. I learned this when research cheap alternatives to glass panels for a solar distillery project (ultimately went with Polycarbonate, due to its relatively UV resistance, and its flexibility).
Interesting. I generally think of polycarbonate as being quite rigid and not very flexible. I suppose when it comes in thin enough sheets it has some flexibility?
Indeed, specifically the corrugated roof/wall panel sheets. I could nearly bend it in half, and still have it spring back to shape. I don’t recall the mill thickness, but i got a 2’x7′ panel from Home Depot for $16, much more budget friendly compared to the alternatives.
The light through the lenses on the hinged door seemed less bright than the other segment.
It is different. This is what provides the recognition pattern when the lens assembly is rotated around the lamp. This makes it possible to identify the lighthouse (the pattern is shown on charts) and helps figure out where you are. You can think of it as a predecessor to Loran, but the math is strictly geometric construction on the chart using data from visual sights.
You can see its a different lens setup on two facets, its part of the light characteristic of the lens. Different lighthouses, different patters/colours.
https://en.wikipedia.org/wiki/Light_characteristic
Nice to see ‘new’ tech used for such a staple of old times, hats off to this company.
Dan Spinella’s company (Artworks Florida) has some other lovely examples on their web page (not connected with them…I’m just a Fresnel lens geek). Here’s a pretty good rundown on the back story:
http://www.lighthousedigest.com/Digest/StoryPage.cfm?StoryKey=4670
Installation at the Port Clinton light – time will tell how the acrylic holds up.
https://youtu.be/Oq45uNVjbrI
last product is pledge, we used it for helicopter windscreen. also wonder what the suns uv rays would do…
Back of the envelope on raw material costs: ~$600 in acrylic per quarter, and $500 in brass. So roughly $3k in raw materials. Thats got to be a $10-20k piece of art. Must be nice to have buyers for that!!
You can direct machine optical finishes in some clear plastics with a precision lathe and a single crystal diamond tool bit. No post machining polish required. The same equipment can also make a die with optical finishes so plastic can be injection molded. Overhead projector Fresnel lenses are made that way.
The black stuff they sprayed on looks like a guide coat to me. You can see where the low spots and high spots are to sand something flat.
What about casting in non-yellowing clear urethane then machining to final size?
Wonderful to see that the history of lighthouses is not being lost, I’ve been to a few lighthouse’s in Australia and had a opportunity some 40 yeas back to acquire the upper lenses and mid lenses of a 1st order lighthouse out of one our lighthouses, the Government body sold off all the non required items at a auction I attended way back, ive carted the lenses’ from house to house over the years