It comes from a product developed specifically for motion picture chromakey work, normally bluescreen production. Keylight is used on chromakey shots in virtually any major feature film you can think of. Keylight� is a powerful plug-in from UK's The Foundry, (maker of the high-end Nuke compositer/VFX program). KeyLight 1.2 in After Effects CS4, under OSX 10.6 Snow Leopard, (but not 64-bit) with virtually identical controls.
Keylight 2.0 in Final Cut Pro 7 under OSX 10.5 Leopard.