Barn doors, or occasionally a set of barn doors, are an attachment fitted to the front of a Fresnel lantern, a type of lantern used in films, television, and theatres.
The attachment has the appearance of a large set of barn doors, but in fact there are four leaves, two larger and widening on the outside, two smaller and getting narrower towards the outside. They facilitate shaping of the beam of light from the fixture, and prevent the distinctive scatter of light created by the Fresnel lens from spilling into areas where it is not wanted, such as the eyes of audience members.
Barn doors are mounted with a ring that fits inside of the color gel slot on the instrument. Because of this, barn doors have a gel slot built into them, so the light can still be colored. Depending on the size and local practices, barn doors may be attached to the pipe or the instrument with their own safety cable.