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The Fresnel lens was an optical instrument invented by the French scientist Augustin-Jean Fresnel (1788-1827).  Fresnel determined that light rays could be bent through an array of prisms to greatly increase the intensity of an ordinary lantern wick, so that it in clear weather it could be seen as far away as 20 miles.  This was an unheard of distance at the time for any light.

Fresnel_lighthouse_lens_diagram
       Image 1: Diagram of optics of a Fresnel lens, 1872.
Over time Fresnel's invention came to be manufactured into lighting apparatus for lighthouses.  These optics came in different sizes, called "orders", ranging from 6 (smallest) to the largest, the First Order lens.  There were many different subtypes.  

Also closely linked to the lens design was the property of the light called its "characteristic", that is, each light had to have a unique lighting pattern, such as a flash in another color, typically red, or a particular pattern of flashes.  The Minot's Ledge Light, for example, became known as the "I love you" lighthouse for its flash pattern which was I-4-3.  

To accomplish the pattern of the characteristic, the lens would typically be mounted on a rotating turntable.  Originally these turnables were driven by clockwork, and later by electricity.  In some cases, typically the largest lighthouses, the fresnel lens was mounted on a vat of mercury.

The light itself was generated by a specialized lantern burning oil, typically whale oil, or some times lard, which left a big mess that had to be cleaned up.  Later lights burned kerosene, which when it was invented, generated the brightest light of any fuel.  Of course it was quite a lot of work hauling kerosene cans up to the top of a tall lighthouse.  

One sees the round ball-like structure on the center of roofs of lighthouses, this is a cover for the chimney through which the lamp smoke would escape.  With electrified lights, this chimney became unnecessary but it remains as an architectural feature.

The table below gives some idea of the basic characteristics of the lens orders.

Lens
Order
Oil used per hour (gm) Brightness (6th Order = 1) Height
(Inches)
Weight (lbs.) Use
1st 750 17.69 102 12,800 Largest seacoast lights
2nd 500 11.54 81 3,530 Great Lakes lighthouses, sea coasts, islands, sounds
3rd 200 3.85 62 1,985 Sea coasts, sounds, river entry, bays, channels, range lights
4th 150 2.31 28 440 to
660
Shoals, reefs, harbors, river and harbor islands
5th 90 1.23 21 265 to
440
Breakwaters, channels, rivers, small islands in sounds
6th 90 1 17 220 Pier or breakwater lights

398px-Mukilteo_Light-house_lens_01
       Image 2: Fourth order Fresnel lens, Mukilteo Lighthouse,
       Edmonds, WA © Joe Mabel, limited reuse per CC BY-SA 3.0
Image 2 shows the Fourth Order lens installed in the Mukilteo Lighthouse in Edmonds, Washington in 1924.  Still in use today, this lens was made in Paris in 1852, and it was actually the second Fresnel lens to be installed in the lighthouse.  

The longevity of this optic is a testament both to the skill of its makers and the dedication of the many people who have cared for it and maintained it for the past 160 years.

The Mukilteo light, visible from 12 nautical miles away, has a  characteristic consisting of a white flash every 5 seconds.  The lighthouse itself is built quite close to the beach, and the light's focal plane, that is, the difference between the level of the sea to the center of the lens, is only 33 feet.  

This charming light station, which includes two classic keeper's houses on immaculate grounds, is located very close to the Edmonds ferry terminal, and is open for tours from April to September.  

Close to Seattle, this would make a fine day trip for anyone interested in lighthouses.  (Lighthouse nuts ... er ... fans in the state of Washington are reminded that they may purchase a Washington Lighthouse plate to support lighthouse restoration efforts in the state!)

Split Rock Lighthouse - Celebrating 100 Years
       Image 3: Third order bi-valve Fresnel lens, Split Rock
       Lighthouse, near Duluth, Minn. © Pete Markham,
      limited reuse per CC BY-SA 2.0
Image 3 shows the Third Order bivalve lens installed in the magnificent Split Rock Lighthouse on Lake Superior near Duluth, Minnesota.  While the lighthouse is no longer in use as an active aid to navigation, on the first Friday of every month, the Minnesota Historical Society lights the station for an hour or two.  The light itself is now generated by a 1,000 watt incandescent bulb.  

The Split Rock lens is a bivalve or clam shell variety, which, as its name indicates, is shaped like a giant clam shell rather than a polyhedron as in the more common Fresnel optics.  

A staff member of the Minnesota Historical Society, attired as a lighthouse keeper, is shown inside the lantern room, giving a good perspective on the size of the lens, which was the original optic installed in the lighthouse.
The lens rotates on a tub containing 7 1/2 quarts of mercury.  

Originally the rotation was driven by clockwork mechanism, which was replaced with electric drive in the 1940s, but in the 1980s the clockwork drive was restored, and it remains in place to this day.

380px-CapeBlancoFresnelLens
      Image 4: Second order Fresnel lens, Cape Blanco
      Lighthouse, near Sixes, Oregon © Cacophony,
      limited reuse per CC BY-SA 3.0
Image 4 shows the second order Fresnel lens currently in use at the Cape Blanco Lighthouse on the coast of southwestern Oregon. Manufactured by Henry LePaute, the lens is 80 inches high (6 feet 8 inches), and is sometimes considered to lie midway between a 1st order and a 2nd order lens.  

Although this light station was established in 1870, this lens was installed in the lantern room in 1936, replacing a fixed 1st order lens, which had a flash induced by a curve panel which would rotate around the fixed lens, this being called an acculting screen.

If you are ever in the vicinity of Sixes, Oregon (Pop. A few seagulls and a whole bunch of cranberry bogs), take a little time and drive west out on Cape Blanco to the lighthouse.  It's quite a considerable distance from the coast highway, as Cape Blanco is actually the westernmost point of Oregon.  

The light tower and the adjacent workroom are all that remain of the original light station, but a nice but small modern visitor's center has been established.  Docents will take you on a tour of the lighthouse.  The view is splendid, but try not to pick a foggy day for your visit!

1st Order Fresnel Lens
      Image 5: First order Fresnel lens from Point Sur, Mon-
     terey County, CA © Oliver Lopena, limited reuse per
      CC BY-NC-ND 2.0
Image 5 shows the first order Fresnel lens which formerly was installed in the Point Sur Lighthouse south of Monterey in the Big Sur country on the coast of California.  

This lens was built in France in 1887 by the firm of Barbier & Fenestre.  It is almost 9 feet (3 m) tall and weighed 4,330 pounds. The entire structure, including the pedestal and clockworks was 18 feet (5 m) tall and weighed 9,570 pounds (4,341 kg).  It operated in the lighthouse from August 1, 1889 to November 1972.  

When the optic was replaced with a dull aerial beacon, it remained inactive in the lighthouse for a long time, until it was taken apart and eventually, many years later reassembled and placed on exhibit in the Maritime History Museum of Monterey, where it was eventually lit for the first time again in October 1994.

354px-Hyperradial_Lens
      Image 6: Hyperradiant lens built 1909 for Manora
     Lighthouse at Karachi
Image 6 shows the hyperradiant lens built in 1909 by Chance Brothers, an English optics firm, for the Manora Point Lighthouse, in Karachi, then in India, now in Pakistan.  The hyperradiant lens was so big that it was "off the chart" in terms of the traditional Fresnel orders.  

Because of the expense only a very few were built.  The one shown in image 6 weighed over 6 tons, and floated on bed of mercury, which reduced friction so much that it could be turned with a finger.  The inside diameter alone was 2.66 meters.

The only hyperradiant lens ever installed in the United States was at the Makapuu Lighthouse in Hawaii, on the island of Oahu.  On account of the enormous size of the optic (over 12 feet tall!) it became known in local folklore as "The Bulging Eye."

Automated in 1974, the lens is still in use, and is the largest lighthouse lens in the United States.  Some vandal saw fit to fire a bullet into the light in 1984, making a jagged hole in the priceless main lens.

Properly cared for, there is really no limit to the useable lives of these optics.  They convey a sense of grace and power, and they astound us with the skill of their makers.

Well, that's all for now.  What do you think?

Pax.

Originally posted to Plan 9 from Oregon on Fri Mar 23, 2012 at 02:26 PM PDT.

Also republished by DKOMA, History for Kossacks, DK GreenRoots, J Town, and Community Spotlight.

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