Fresnel Lantern

A Fresnel lantern (pronounced fraa-nell) is a common lantern used in theatre, which employs a Fresnel lens to wash light over an area of the stage. The lens produces a wider, soft-edged beam of light, which is commonly used for back light and top light.

The distinctive lens, named after Augustin-Jean Fresnel, has a 'stepped' appearance instead of the 'full' or 'smooth' appearance of other lenses. This allows the lens to have a much greater curvature than would otherwise be practical. The lens focuses the light by tilting each ring of glass slightly more towards the center as the distance is increased from the center of the lens. If the glass were completely flat, this would cause a corresponding pattern of circles of light, so Fresnel lenses are usually stippled on the flat side. This pattern of small bumps helps to break up the light passing through the lens and gives Fresnels their characteristic soft beam. This means that the intensity of the light is consistent across the spread of the beam of light, as opposed to being less intense around the edges as in an ERS. The stepped lens design causes less heat buildup than a plano-convex lens of the same angle.