FRESNEL LENS

When Iain and I attended the Royal Society of Edinburgh conference marking the 200th anniversary of the building of the Bell Rock Lighthouse (more here when I talked about the actual lighthouse) the second topic that caught my interest was Andrew Walker's talk on the optics of the actual lighthouse light.

I had not realized how the light was designed. So I read up on it and made an attempt to describe it in our yachting organisation's newsletter.

The lenses used to focus the beam from lighthhouse lamps are called Fresnel lenses. They take their name from the man who invented them in the early nineteenth century: Augustin-Jean Fresnel (pronounced fray nell), a French commissioner of lighthouses.

Professor Walker showed an image illustrating the idea of how they got their shape. I used Photoshop to make a drawing below. The resolution here is low but I have archived the original high resolution image.

The diagram shows how a spherical glass lens is formed into concentric rings which are actually an array of separate prisms that convert the light into a parallel beam. To retain the rings’ ability to create the beam, the slope of each ring’s angled face is different. The rings are stacked together to form the Fresnel lens.

These lenses are less bulky, flatter and can capture more light which means that the light can be visible over longer distances.

Lismore lighthouse, west of Scotland.

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Photograph of Fresnel light is Wikipedia. Stunning images are on Ian Cowe's Flickr website here.