Green Screen Research

Green Screen or 'Chroma Key' is a special effects/post production technique for compositing two images or videos together based on colour hues. The technique has been used heavily in many fields to remove a background from the subject of a video or photo , particularly in newscasting, motion picture and video gaming industries.
  A green screen is manly used to differentiate between characters and the background. A green or blue screen is used, as they are easy to differentiate  between skin tones and clothes.

One of the first uses of chromakey was used in the 1940 film 'The Thief of Bagdad'. This was done by Larry Butler, who is credited for the development of blue screen. He invented blue screen and travelling matte techniques in order to achieve the visual effects which were unprecedented in 1940. He was also the first special effects man to have created these effects in Technicolor, which was in its infancy at this time.
In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began working on an ultra violet travelling matte process. He also began developing blue screen techniques. One of the first films to use them was the 1958 adaptation of 'The Old Man and the Sea'

This is an example of green screen used in the 2012 movie 'The Avengers'. Green screen is needed here in order for live action characters to interact with CGI, such as lasers and fire and even characters like the Hulk and Iron man. This use of green screen is convincing, in the fact that the background image looks as though it is really there.