Lighthouse Glasses ~ Lenses of the Old Lighthouse at Dungeness, England photographed by Tim Anderson (more). ©Tim Anderson, shown with permission.

Early lighthouse lamps with Argand burners, whale oil, kerosene, acetylene, were dim and every lumen needed capture for the mariners’ beam.   A simple focussing lens would be prohibitively large and heavy because of the size of the lamps. A single lens would in any event capture only a small fraction of the available light.

The greatest advance was in 1822 with the combined lenses and prisms of the Parisian brothers Leonor and Augustin Fresnel.

In their refined form a central lens focussed light into a beam. Weight was reduced by constructing the lens from several separate rings, each one much thinner and lighter than its equivalent single lens section ~ a Fresnel lens.

The zonal lens alone still missed light going downwards and upwards. This was captured by further series of curved prism sections. In those the light underwent two refractions plus an internal reflection. The overall effect was a very fast and comparatively lightweight optical system.

To give timed flashes for easy identification, several lenses were arranged in a weight driven rotating cylindrical cage. The Dungeness light used white, green and red coloured beams.

With their mesmerizing ranks of glass, glints and reflections these ancient optics are objects of beauty.

Early composite optics were possible because only the light direction had to be managed and not its phase. Preserving phase is more involved!

Human lenses, for all their considerable artifice, are unstable in a profound sense that their focus is precarious. The slightest misfiguring or misalignment causes it to dissolve into a blur. In contrast, Nature's focussing is stable and always sharp ~ More about that in an OPOD to come.


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