Fresnel lantern

This allows the lens to focus the light by tilting each successive ring of glass slightly more steeply as its distance increases from the center.

This pattern of small bumps helps to break up the light passing through the lens and gives Fresnels their characteristic soft beam.

[1] Theatrical Fresnel lanterns are typically made in three-, six-, or eight-inch lens diameters, with lamps ranging in power from 150 W to 2,000 W. The three-inch variety is referred to as an inkie.

While the focus can alter the size of the beam, the distinctive scatter of light that the Fresnel lens emits also requires a way of controlling its shape.

Since Fresnels cannot use internal shutters, such as those found in an ellipsoidal spotlight, they are often fitted with distinctive barn doors to control the spill and shape the beam of light.

A Fresnel with the lens open to show the ridges. There is no lamp in the instrument
1: Cross section of a Fresnel lens 2: Cross section of a conventional plano-convex lens of equivalent power
Fresnel lanterns in use at a classical concert; they can be seen silhouetted against the stage and each sports a barn door