green-screenThe green or blue screen effect is widely used in modern cinematography for generating compositions with two video streams. The technique relies on the color hue, or chroma range, hence the chroma key naming. It is used to remove the background from a subject shot, which is later made transparent with the computer, allowing the post production team to place a different background behind the actor. The technique can be used with many different coloring background, but green and blue are used because these colors are more distinct in hue from the majority of skin colors so there’s less risk of making mistakes.

An example that will be familiar to most: weather forecast presenters are shot on a green or blue screen, which is replaced live with a large computer-generated map. Note that if the presenter will wear blue or green at the moment of the broadcast, these will also be replaced by the background video.

Larry Butler invented this method in the 1930s. The escaping from a bottle genie scene from The Thief of Bagdad, released in 1940, is the first known scene to be shot on a proper blue screen. The movie won the Academy Award for Best Special Effect.

The Warner Brothers Studio began a collaboration in 1950s with the researcher Arthur Widmer. The collaboration perfected the blue screen technique.

Engineer Petro Vlahos observed that green and blue colors have similar intensities in the real world scenes. For this refinement, he was awarded the Academy Award in 1964. The 70s and 80s added several improvements to the blue and green screens technique. A camera film would split its beam into two projects, with the help of an optical printer. This allowed the post processing to combine one frame at a time, one of the actor with one frame of the desired background. The Empire Strikes Back, one of the Star Wars saga movies, released in 1980s, made use of a quad optical printer, which allowed a faster and cheaper process. The brain behind this innovation, Richard Edlung, also received the Academy Award.