Restoring the lenses required making a new set of lenses
and a frame in which to hold them. The fact that it was a
Fresnel lens, made out of many concentric rings of glass fit
together with a microscopic amount of space between them,
meant that a specialist had to be employed to make sure the
lenses were workable and accurate.
That specialist was Dan Spinella, owner of lighthouse lens
restoration company Artworks-Florida. As one of the few
people who does work on reproductions of Fresnel lenses,
he was a perfect fit for the needs of the Charlotte-Genesee
Spinella explained the history of Fresnel lenses, which can
be traced back to France. They were invented in the 1800s
by French physicist Augustin Fresnel. He revolutionized the
lighthouse industry, making lenses which could catch a single
beam of light and redirect it with more luminosity than a single
lens. The first Fresnel lens was installed in the Cordouan
Lighthouse in France in 1823.
Spinella reached out to several companies before he found
one which was as accurate and experienced with optics as
he wanted. It’s easy to make a lens that turns light wavy if it
isn’t manufactured correctly. In the end, he coordinated with
Star Prototype, a low-volume manufacturing and prototyping
company, to create the lenses.
Reproducing the beacon consisted of two major tasks:
fabricating the bronze frame, and the lens itself.
For the lens material, Spinella chose to use an acrylic
plastic instead of the flint glass used in the original lenses.
Flint glass was favored by 19th-century lighthouse lens
makers because it is highly refractive and has high diffraction
and can also be combined with convex lenses to reduce color
distortion. It was made using powdered flint or lead for added
clarity. But forming flint glass lenses requires glass making
technologies that are not in common use today.
“I think back to that day and think I don’t know how they
did it! But of course we’re not geared up to do it [today],”
In contrast, Acrylic good optical properties and can be
worked with modern high-precision machine tools that are
commonly available. Using acrylic was a matter of necessity
but also convenience. Flint glass lenses were extremely heavy,
up to 1,200 pounds. The replacement acrylic PMMA lenses
weigh about 10 percent of the original weight.
To determine the lenses’ shapes, Spinella used the original
optical formula invented by Fresnel, changing it slightly to
ensure that light would to refract through acrylic, which will be
used instead of glass. From there he built a 3D model, which
he sent to Star Prototype for fabrication on a very precise
3-axis CNC mill.
The standardization of lighthouse lens sizes made it
relatively easy to make the initial 3D models. Spinella has
original drawings and cross sections that tell him what the
focal length of the length should be, where the prisms should
be placed, and other essential information. Because the sizes
were standardized, he did not have to measure the lenses
by hand. He made the 3D models using AutoCAD, which
he says is the program he uses most often. He will also
sometimes use 3D Studio Max if a more artistic approach is
needed for a particular project.
Finding the correct dimensions to model the 3D version
of the lenses on was relatively simple. Lighthouse lenses are
made according to standardized sizes, with the largest being
the first order and the smallest the sixth order. The Charlotte
lighthouse uses a fourth order lens, which is 36 inches high
and about two feet in diameter.
Star Prototype could also conform to these standardized
plans. Their work was especially good because it was
exceptionally accurate, Spinella said. They also knew how
to use a coating that makes the acrylic look greenish, like
antique glass. Clear acrylic can be easily identified as plastic,