Barndoor

Adjustable doors attached to the front of stage lanterns to control the area of light covered by a particular beam. (UK)

barndoor

[n] - an opaque adjustable flap on a lamp fixture
Found on http://www.webdictionary.co.uk/definition.php?query=barndoor

barndoor

noun an opaque adjustable flap on a lamp fixture; used in photography to cut off light from particular areas
Found on https://www.encyclo.co.uk/local/20974
No exact match found