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| Fresnel's early 1st Order lens |  |
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Augustin Fresnel
Augustin Fresnel, a French physicist, invented a lens in 1819 that would revolutionize lighthouse illumination worldwide. The first lens installed at Cordouan Lighthouse in 1823 consisted of eight panels of annular lenses placed around the lamp at a focal distance of 920 mm. To collect the light that would otherwise escape above the lenses, Fresnel placed a series of plane silvered mirrors on which the light was thrown by another system of lenses. Before electricity, these massive lenses were rotated by a clockwork mechanism similar to the concept of a grandfather clock. A weight would drop down the center shaft of the lighthouse attached to a cable, which was wound around a drum. The drum was linked to a series of gears. The ratio of the gears was designed to allow the weight to drop at a certain speed. A pinion gear from the clockwork would interface with a large ring gear attached to the pedestal and rotate the lens. Augustin Fresnel |
St. Augustine's 1st Order lens
The St. Augustine lens was the first restoration of a Fresnel lens in the United States. First installed in 1873, this lens contains 370 individual pieces of glass. The lens was restored in 1993 replicating 13 of the hand cut prisms. This lens contains a fixed or belt lens segment, noted by its horizontal prisms, flanked by two rotating or bulls-eye style lenses. The reflecting prisms above and below the center belt are also fixed horizontal prisms. This is called a fixed/flashing lens. The lens exhibits a steady glow with a flash every 30 seconds. St.Augustine Lighthouse website |
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| Inside St. Augustine's fixed/flashing lens | |
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Belt Lens
This damaged center belt lens is the largest glass segment in the lens assembly. It is 11 inches high, 36 inches wide, and 1-1/2 inches thick. The replacement lens was machined from a large block of glass that was 4 inches thick. It took several weeks of machining with diamond cutters, sanding, and polishing to complete this lens segment. |
Bullseye Lens
This damaged bullseye lens assembly contains the most accurate glass segments in the entire lens. The lens consists of a center plano convex lens with concentric rings that are all individual pieces assembled together. The outside diameter of each ring is machined to thousands of an inch. |
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Restored Belt Lens
The image to the left is the restored St. Augustine belt lens. The center lens and the first prism just above, were machined, ground and polished from blocks of glass.
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Restored Bullseye Lens
The image to the right is the St. Augustine restored bullseye lens. Dioptric ring no. 4 and 5 along with upper dioptric no. 6 and 7 were machined, ground and polished to precise dimensions to allow them to be assembled in the existing lens.
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