A cukoloris is a flat opaque object with holes in it. It is held in front of a light to make a pattern of shadows on the subject, such as tree branches. It can be constructed out of styrofoam, wood, or cardboard, or if it is meant to be mounted very close to the light, even metal. It's sometimes spelled cucoloris, kookaloris, cookaloris, or cucalorus. The word cukoloris is Gaelic and means "ghost charm." How a Gaelic word became a standard term in film production is unknown.
The name is often shortened to "cookie," a usage which is probably reinforced by the fact that a cukoloris "cuts holes" in a beam of light much like a cookie cutter cuts cookies from a sheet of dough. A similar device in theater stagecraft is called a "gobo," supposedly short for "goes before optics." A gobo is inserted between the actual lamp and a focusing lens, permitting a much more focused image from the gobo, in the manner of a slide projector. A "cookie" would instead be hung after the lens and produces an indistinct shadow.
It can also refer to a flickering light source or reflection, to suggest, say, a pile of gold or glowing orb of power lighting up the faces of the actors. The gold itself need not be visible to the audience.
Used in any number of high tech shows and movies in which a computer display seems to be projecting its screen image onto the user's face. It's not entirely accurate, but that's not why they use it in the first place.
- 2001: A Space Odyssey was the first movie to show computer monitors projecting their images onto the user's face. This is pure Rule of Cool, because in order to get this effect in real life you'd have to be staring straight into the bulb of a projector. There were 16mm projectors behind all the flatscreens on the sets, so all Kubrick had to do was take the screens off.
- In City of the Dead, inside the Raven's Inn, a pronounced "roaring fire" effect is applied to the walls.
- In the flashback scene of Fangs of the Living Dead, weird patterns are projected on the walls of Malenka's laboratory.
- A "flickering light source" version is the Stargate in the Stargate-verse. To avoid CGI costs, the open gate is in many shots offscreen but its flickering light — produced by a stagehand warping a flexible mirror — illuminates the rest of the scene, and sound effects do the rest.