Barn doors are metal shutters or flaps mounted on lamps used in filmmaking
. There are generally four flaps in a set.
They are usually mounted on a ring to allow them to be turned in relation to the light. You open or close them to adjust the size and shape of the beam of light emitted. They are painted matte black to minimize reflections.
Often the electrician will clip filter gels to the barn doors with clothes pegs, across the light beam, if colour effects are desired.
Barn doors are also used in theater lighting.