Stage lighting instruments (or lanterns / Luminaires in Europe) are used in stage lighting to illuminate theatrical productions, concerts, and other performances taking place in live performance venues. They are also used to light television studios and sound stages.
Many stagecraft terms vary between the United States and the United Kingdom. In the USA, lighting fixtures are often called "instruments" or "units". In the UK, they are called "lanterns" or "luminaires". This article mainly uses terms common to the USA.
The Electronic Theatre Controls (ETC) Source Four (also known unofficially as Source 4 or S4) is an ellipsoidal reflector spotlight (ERS) used in stage lighting. First released in 1992, the Source Four was invented by David Cunningham and features an improved lamp and reflector compared to previous ERS designs, tool-free lamp adjustment, and a rotating, interchangeable shutter barrel. The Source Four is widely used by professional theaters in the United States.