The background light is used to illuminate the background area of a set

Theatrical scenery

Theatrical scenery is that which is used as a setting for a theatrical production. Scenery may be just about anything, from a single chair to an elaborately re-created street, no matter how large or how small, whether or not the item was custom-made or is, in fact, the genuine item, appropriated...

. The background light will also provide separation between the subject and the background. In the standard 4-point lighting setup, the background light is placed last and is usually placed directly behind the subject and pointed at the background.

In film

Film

A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

, the background light is usually of lower intensity. More than one light could be used to light uniformly a background or alternatively to highlight points of interest.

In video

Video

Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 and television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

, the background light is usually of similar intensity to the key light because video cameras are less capable of handling high-contrast ratios. In order to provide much needed separation between subject and background, the background light will have a color filter, blue for example, which will make the foreground pop up.