KAPTN  KEN           

   The Fresnel Lens

   A Marvel of Glass, Brass and Mathmatics

 In 1788, lighthouse lenses with a diameter of 21 inches and a thickness of 5 inches were made by Thomas Rogers in England. They were made from thick slabs of poor quality window glass and were not very successful.

A Frenchman, Pierre Guinand in 1790 was able to make 9 inch glass lenses of good quality but he concentrated on small, high optical quality lenses with two German glass makers and steered away from the lighthouse requirements. 

1810 saw Winslow Lewis in America making lenses from bottle glass but again not very successfully.

In 1819 a young French engineer, Augustin Jean Fresnel developed and demonstrated a dioptric glass lens which consisted of dioptric (reflecting) mirrors placed above and below a glass bulls eye lens. Later the mirrors were replaced by catadioptic (reflecting/refracting) glass prisms. In 1842 the first complete catadioptic lens was a fixed (non rotating) appararus, a 3rd order Fresnel lens on the lighthouse at Gravelines, France.

By the mid 1900s there were several manufacturers in France, Henry Lapaute, Francois Soeil & Tabouret, Sautter, Barbier & Fenestre, Cie, and one in England, Chance Brothers. A high number of lighthouses in America have lenses made by one of these manufacturers.

The accepted name for these types of lighthouse lenses is Fresnel lens (pronounced Fray-nel) in honor of Augustine Jean Fresnel for his work in developing the system. His first lens system was installed in the Cordouan lighthouse in France in 1823 which had mirrors above and below the bullseye lens. Fresnel then went on to develope glass prisms which replaced the mirrors with the result of increased efficiency. The prisms became a permanent part of the lens system and can be found on lenses around the world.

 Lenses are devided into Orders, the First Order being the largest at 10 feet tall and the Sixth Order being the smallest at about two feet in height. There are two types of systems, fixed and rotating. Fixed obviously is a stationary lens and gives a steady beam of light. The rotating system makes the light beam circle around for 360' appearing to the sailors as a flash of light. Depending on the speed of rotation, the time between each flash of light can be varied and used to identify each lighthouse, this is called the Signature  or characteristic of the lighthouse and is recorded on nautical charts.

The early mechanisms to rotate lenses was a clockwork device similar to a grandfather clock where a heavy weight is cranked up and as it decends, provides the motive power to turn the lens. There are not very many original clockworks left, most of them were replaced by electric motors between 1935 and 1960.  Some are in nautical museums and once in a while you may find one in a lighthouse; Point Reyes lighthouse in Marin County, California, still has a working clockwork mechanism. Call ahead to make sure that it is open as there is a 15 mile drive from highway 101  and 300 steps down to the lighthouse.

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