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Chroma-key is a well-established technique in TV production. It allows actors and props to be extracted from a camera image and superimposed onto another image to form a composite image showing the actors against a 'virtual' background. The actors stand in front of a background of a uniform colour. The background colour must not be present anywhere in the foreground; for that reason, blue is often chosen. A keying unit detects those parts of the camera image that contain this 'key' colour, and replaces them with the background image.

In order to generate a good key signal with low noise, it is necessary to have the coloured background illuminated brightly and evenly. This gives rise to several problems, particularly when used in large studios:

It is often time-consuming and difficult to provide bright and even illumination over a large area.

	It is sometimes difficult to light the actors to give the desired artistic effect, since the lighting is primarily determined by the technical requirements of the key generation. Scenes with low lighting levels pose particular problems.
	Areas where dark shadows cannot be avoided, such as under tables, do not produce clean key signals.
	Coloured light scattered from the brightly-lit background and floor lands on actors and props, changing their hue and giving an unnatural appearance. This is known as 'spill'.

BBC R&D has developed a keying method that overcomes all of these problems. The system

	places no constraints on studio lighting (and will even work in complete darkness!);
	eliminates ‘spill’;
	allows clean keying even in difficult areas such as dark shadows;
	works with conventional chroma-keyers and allows shadows to be extracted.

Fig. 1 shows the method in use in a virtual studio production. It uses a background made of a special retro-reflective material, illuminated by a light of the chosen key colour mounted on the camera. Retro-reflective materials reflect nearly all incident light back in the direction from which it comes. They are usually made out of tiny glass beads with a reflective coating on half their surface. Such materials are commonly used in road signs, to make them appear bright to a driver viewing them by his headlights. In this application, the material reflects the coloured light straight back into the camera, ensuring that the background appears brightly lit regardless of the setting of the studio lights. The material appears to have a dark grey colour under normal illumination, so coloured spill onto actors is eliminated.

Ideally, the source of coloured light should be placed coincident with the camera lens. This may be done using a semi-silvered mirror in front of the camera, which reflects a blue light to make it appear to come from the centre of the lens (as in a front-axial projection system). A more compact alternative which gives results almost as good is to use a ring of bright LEDs around the camera lens (Fig. 2). To change the key colour, it is only necessary to change the lights on the cameras; there is no need to change the whole cyclorama.