Fresnel Lens

This flat, flexible, transparent green disc forms a fairly powerful lens, creating a fisheye effect, similar to looking through the wrong end of a telescope. The tiny concentric circular ridges look like a giant fingerprint, but are a marvel of optical engineering. Compared to conventional spherical lenses, this type of lens, called a ‘Fresnel’, reduces the amount of material required by creating these ridges known as ‘Fresnel zones’. Closer to the centre they form more oblique angles, getting thinner, flatter and more widely spaced, capturing more light. The angled ridges focus and condense the light into one point, and magnify without the need for a thick and heavy lens.

The Fresnel lens is named for its inventor, French physicist Augustin Jean Fresnel, and is pronounced frayNEL. He studied light and optics in the 19th century, inventing lighthouse lenses, which could throw a beam for many miles. Glass Fresnel lenses also are used in theatre lamps, called simply Fresnels. Their use for image projection reduces quality, so they are often used only where image quality is not so important, or as an alternative to the bulk or cost of a solid lens. Cheap Fresnel lenses like this are stamped or moulded on transparent plastic and light enough to stick to a window or light source with tape. They are familiar to us from their use in overhead projectors and on the back windows of transit vans. Since these plastic lenses can be made far larger than glass ones, they are increasingly used to concentrate sunlight for heating in solar devices.