Blue screen technology is a process known throughout professional circles as chroma keying. During production of a film or video, it involves replacing certain elements of the screen (usually a background), which is dictated by bright blue coloring (sometimes green), with other elements. For example, let's say an actor needed to be in the arctic, but budgetary constraints prevent filming there; then blue screen technology is enacted. An actor stands in front of a blue screen, which is easy for computers to distinguish once the film/video is fed into the machine. That blue background can then be replaced by footage of the arctic landscape and, voila, the actor has successfully shot a sequence in a location thousands of miles away.
Originally, the process was known as travelling matte, since before the onset of digital composition, putting blue screen shots together was long, costly and complicated. It was first developed by Larry Butler for the 1940 film "The Thief of Bagdad" and involved shooting the actor against a blue screen, then rephotographing it several times using different filters before the images are finally combined one frame at a time. In the 1980s, mini-computers made the process a little easier and were used to aid the effects in George Lucas's "The Empire Strikes Back." In the mid-1990s and early 2000s, digital composing via computers became the standard, and chroma keying has become an enormous part of any production involving special effects. In 2007, the film "300" was shot almost entirely against blue screen.
Chroma keying works by identifying a single color and virtually erasing it from the frame. Blue and green are often selected because of their contrasts to natural human skin tone. The actor is filmed against the blue or green background, and when the image is fed into an editing program, that color is "keyed" out and replaced with another image. This process has been simplified to the point where it is not uncommon in home video editing software.
Though the term "blue screen" encompasses the use of chroma key technology in general, green screens are more often used today. This is because of most cameras' sensitivity to the color, making it easier to isolate than blue. However, if the actor's clothing is green, then a blue screen must be used, since the camera could easily mistake the green clothing for a part of the background, making it invisible on the screen.
Chroma keying is not just used for replacing backgrounds; in fact, it's become such a common tool in the filmmaker's arsenal that other uses have been invented for it. In the 2004 film "I, Robot," actors wore tight suits made of bright green material, then acted in the sequence. Later, the green-suited actors were easily replaced with digital robots. Blue screen technology can also be used to turn actors invisible, with a primitive (yet effective) version of it used in 1933's "The Invisible Man." In that film, a dark cloth was wrapped around the actor's head, so that, when transposed against a background of similar color, it seemingly vanished.