Lighthouse Science: How and Why the Fresnel Lens Costs a Million Dollars
In the 1700 and 1800’s open water shipping was the lifeline of commerce – it was the only way to transport goods between the continents, but it wasn’t easy. Among the many difficulties was the problem of ships, and all of their precious cargo running aground and being smashed on unseen rocks.
This necessity brought about the creation and design of many lighthouses on the coastlines, which would let ocean navigators see when they were getting too close to dangerous outcrops. The issue was, the lights were essentially a large candle or oil lamp, and many times they were so dim that when the boat Captain saw the light, it was already too late. So scientists of the day searched for ways to focus and project the light much further.
The Fresnel Lens (from Wikipedia)
French physicist and engineer Augustin-Jean Fresnel developed the multi-part Fresnel lens for use in lighthouses. His design allowed for the construction of lenses of large aperture and short focal length, without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can also capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one, to be visible over greater distances.
The first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary; its light could be seen from more than 20 miles (32 km) out. Fresnel’s invention increased the luminosity of the lighthouse lamp by a factor of 4 and his system is still in common use.
Fresnel Lenses Sizes (from Lighthouse Ratings)
Through the years seven classifications of Fresnel lenses were developed. These classifications, know as “orders,” were developed to meet all the needs of the many different lighthouses and the diverse coastal environment each lighthouse stood within.
The first-order Fresnel lens was the largest of the Fresnel lenses and could exceed 12 feet in height and one ton in weight. Because of their size and strength, these lenses primarily were used in lighthouses that needed an extremely focused and far-reaching signal. These lights were usually utilized in lighthouses along oceans.
The smaller Fresnel lens orders, the fourth through sixth orders, were often situated on lakes and harbors because of their limited nature of lens signal and visibility.
Why a Million Dollars?
The glass for the lens was formed and machined in a specially designed factory – lost to bombing in World War II. Due to various reasons, it has never been replicated. This makes original glass Fresnel lenses extremely rare and valuable. The glass cannot be reproduced, when repairing or restoring a lens, fabricators use acrylic to gain the same effect at much less cost, but they’re not the real thing.
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