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Acrylic replica 4th Order Fresnel for Pottawatomie Light

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Postby Terry_Pepper » Thu Mar 31, 2005 6:19 pm


The present Pottawatomie lighthouse was erected in 1858 after the mortar in the original 1836 structure failed, causing an impending crumbling of the original structure

Automated in 1945 with the installation of a battery-powered flashing light, the lantern was removed from atop the dwelling after a new solar-powered acrylic optic was installed atop a steel framework tower to the west of the dwelling in 1988.

The Wisconsin Department of Natural Resources obtained ownership of the entire island, and strapped for funds, the lanternless station sat in a declining state of repair until a local group of preservationists gathered to form the non-profit Friends Of Rock Island in 1994, with their bold mission being the complete restoration of the unique structure.

Over the ensuing years, the motivated group installed a replica nonagonal lantern, and completely restored the station interior, returning the building to its pristine original condition to a point that volunteers are now able to comfortably live in the building while serving as docents to the many visitors who make the enjoyable day-long trek to Rock Island which includes two ferry rides and a 1 ½-mile walk from one end of Rock Island to the other. With the dedication of the completed restoration on May 15, 2004, the group turned its attention to the restoration of the station outbuildings, consisting of a stone privy and brick oil storage building.

However, there was still one thing missing in the lighthouse, itself – the Fourth Order Fresnel lens.

With the whereabouts of the original lens undetermined, in partnership with the DNR, the group managed to obtain a grant to obtain a replica of the original Fourth-Order Fresnel from Dan Spinella and his company Artworks Florida. Dan’s company has been attaining considerable success in manufacturing exacting replicas of classical Fresnel lenses using CNC-machined and polished optical acrylic in place of the original Crown glass.

As of today, Artworks Florida has finished the machining the framework for the lens assembly, and is currently working on the catadioptric and dioptric panels. Dan expects to have the entire assembly and pedestal complete and shipped to Rock Island in time for the assembly to be installed in the reconstructed lantern in time for the 2005 Door County Lighthouse Walk, which will be held on May 21 and May 22.

Below, you will find four images of the work in process courtesy of Mr Spinella, along with a 3D computer generated model, which was created from the high tolerance machining geometry input into the CAD system, and thence exported to the CNC machinery which created the high tolerance brass and acrylic components.

Image
The image above shows a 3D rendering of how the
finished lens will appear after final assembly.


Image
The image above shows the assembled brass frame,
ready to receive the machined acrylic panels.


Image
The image above shows the catadioptric prisms in their
as-machined condition, awaiting final polishing.


Image
The above image shows the dioptric panels in their
as-machined condition, awaiting final polishing.


For more information on Artworks Florida, visit Dan's website at: http://www.artworks-florida.com/
For iformation on The Friends of Rock Island, visit: http://uniontel.net/~cmarlspc/
For more history on Pottawatomie, visit my page on the station at: http://www.terrypepper.com/lights/michigan/pottawatomie/pottawatomie.htm

My thanks to Dan Spinella for granting his permission to display his photographs of the work in process.
Terry Pepper
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Postby Hersh » Thu Mar 31, 2005 10:22 pm


That's great, if you can't get a real lens, this would be the next best thing !!! I always get depressed when I see an empty lantern room.
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Postby beachbum1616 » Fri Apr 01, 2005 4:48 pm


Glad to see another lantern room filled, even if it is not by an original lens.
Stephen

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Postby Pharoslvr » Fri Apr 01, 2005 5:35 pm


Are the refraction properties identical in an acrylic prism to that it in the type of glass that Fresnel used...or do adjustments have to be made to the angle of the prism in order to compensate accordingly?

Thanks,
Brent
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Postby cawteener » Fri Apr 01, 2005 5:36 pm


Me, too!
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Postby Terry_Pepper » Fri Apr 01, 2005 6:20 pm


Apparently the refraction, reflection an dispersion properties of the optical acrylic material are close enough to the crown glass originally used by the French glass manufacturers that Mr. Spinella is able to use Augustin Fresnel's original formulae in calculating the angles and assembled positions for the prisms.

From a purely personal prsepctive, I feel the jury is still out as to the long-term viability of the acrylic material under long-term exposure to the high levels of ultraviolet light and heat experienced in lighthouse lanterns. In discussung these lenses with others with considerable experience with lighthouse optics, concern has been raised that the acrylic may turn milky in appearance, and even deform as a result of heat, causing the assembly to warp out of critical alignment.

Since the use of machined and polished optical acrylic in this application is new, only time will tell how successful it turns out to be over the long-term.

Of course, it must be kept in mind that an untended lantern is the worst place to display a true classical Fresnel lens for the same reasons.

The ultraviolet light can discolor the crown glass as it reacts with chemicals added during the glassmaking process to reduce striae and other imperfections, resulting in the green and magenta casts frequently seen in old lenses which were displayed in untended lanterns for long periods.

Also, the combination of ultraviolet light and violent temperature swings experienced in untended lanterns causes the litharge to embrittle and break-down, allowing the prisms to loosen, and in the worst case, to fall from the framework with disastrous consequences as the prisms hit the cast iron lantern floor.

The Lighthouse Service was well aware of these problems, and keepers were stringently forced to draw curtains across the lantern windows at all times during daylight hours to reduce ultaviolet penetration, and to make constant adjustments to the tower/lantern ventilators in an attempt to moderate the temperature in the tower.

Regardless, I would personally far prefer to see one of these new acrylic replicas displayed in any lantern, with the precious true Fresnel lenses safely displayed under properly maintained museum conditions, where it can be ensured their beauty and precision can be enjoyed for many generations to come.
Last edited by Terry_Pepper on Sat Apr 02, 2005 7:40 am, edited 1 time in total.
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Postby Pharoslvr » Sat Apr 02, 2005 6:27 am


Thanks Terry! And thanks for mentioning the possible effects of ultra-violet light and heat, in the medium and long term, as well. I was wondering about that, but failed to include it in my initial question.

One additional question for you.....
In lieu of the old lead based litharge, what type of caulking are they presently using? My bet would be some type of urethane based material, however either silicones or butyl rubber have some advantages, as well.

Brent
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Postby Terry_Pepper » Sat Apr 02, 2005 8:18 am


Brent,

I have not heard of the specific material being used to secure the acrylic components, but I would assume that one of your suggested adhesives is likely on-target.
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Postby Weasel58 » Sat Apr 16, 2005 5:23 pm


It's funny I should happen to read this topic today. I have just returned from the quarterly Florida Lighthouse Association meeting, and guess who our guest speaker was? Dan Spinella from Artworks Florida. Our meeting was at the Anclote Keys lighthouse off Tarpon Springs, and the group that helped restore the tower two years ago, just happened to have been the first group to purchase one of his lenses for installation in their tower. He had sold other lens before, but they were all going to be displayed in museums. The fourth order for Anclote was the first lens he was approached for, whose use was going to be an active aid to navigation. He showed us a video of some of the other lenses he has worked on, from the restoration of the first order in St Augustine, to the manufacturing and testing of the Anclote Keys Lens. All in all, Dan’s was a very informative presentation. With some help from the local Coast Guards ATN team we have been able to determine that the current lens has the same range (sixteen miles) as the original lens that was in Anclote before it was electrified during the 1950s. The current light source is a 50-watt bulb. There had been some talk of using a 100-watt bulb, but the ATN reps stated that with 100-watts the range would be between 25 and 30 miles, and could not be allowed as this could cause confusion with the Egmont Key light which marks the entrance into Tampa Bay.

Also with Dan was a representative from Engberg Mold, the company that actually manufactures the lenses to Dan’s specifications. They brought with them a fixed sixth order lens that they displayed to the association during our meeting.
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Postby Terry_Pepper » Sat Apr 16, 2005 5:41 pm


Eric,

Thank you for the input on the active acrylic Fresnel replica in the lantern at Anclote. Such a location would certainly be subjected to a far higher level of heat than at the Pottawatomie light in Wisconsin. Of course, it would not be subjected to the rigors of below zero winter temperatures experienced here in the Great Lakes.

This years, the Great Lakes Lighthouse Keepers Association (GLLKA) will be installing a historically correct lantern on our St. Helena Island lighthouse (for which the iron components were cast at a foundry in Florida). We have been in contact with Dan for a couple of years relative to his recreating a replica of the fixed Third and half order lens which was historically installed at the station before it was removed during the installation of a solar-powered Tidelands Signal optic, and subsequently “lost.”

As such, I am happy to hear that there is some growing experience with these replica lenses in the harsh environment of an untended lantern.
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Postby Pharoslvr » Sun Apr 17, 2005 9:58 am


'Just curious, Terry....what kind of price are we talking about for a 3.5 Order replica? Do you have a ballpark figure?
Also, is the "finished weight" about the same as a regular Fresnel or is it a little bit lighter?

Brent
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Postby Hersh » Sun Apr 17, 2005 10:54 am


Thanks for the info Eric, it really is amazing that one of those lenses can make a 100 watt bulb visible for nearly 30 miles !!! Fresnel was truly a genius.
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Postby island » Sun Apr 17, 2005 2:23 pm


It is amazing, Hersh. With an electric bulb the filiment can be positioned right at the lens focal point you can get more concentrated output from the lens than was possible with the IOV lamps.
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Postby Weasel58 » Mon Apr 18, 2005 6:49 pm


To answer Brent's question, the same thing was asked of Dan at our meeting last Saturday. His answer was that the acrylic part of the lens was about 1/10th the weight of a glass lens. Of course the brass weighs the same.

One of the videos Dan showed was the testing of the lens. It was positioned on a picnic table only about 5 ft above sea level and was visable about 7 miles away.
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Postby Pharoslvr » Tue Apr 19, 2005 5:36 am


I had no idea that the acrylic prisms were that much lighter.
Amazing! 8O

Thanks Eric
Brent
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