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Backdrop
A vertical surface with painted designs which is used to form the background for a theatrical type setting, usually made of heavy cloth drawn tight to achieve a smooth, flat surface. It may sometimes be used as a cyclorama when painted as a sky.

Back Light
Used behind the subject and pointing toward the camera for separation.

Background Light
Aimed at the background for separation of subject and background.

Balance
An abridged version of Color Balance.

Ballast
An electrical apparatus that limits the electrical current in a particular circuit, usually a circuit containing an arc source.

Barn Doors
An apparatus with adjustable flaps, usually 2, 4 or 8, that attaches to the front of a luminaire. It is used to block or shape the beam.

Beam
In Photometry, the circular area of the base of a cone shaped beam where the intensity is at least 50% of the maximum intensity. The maximum intensity is ideally located at the center of the base. It should be noted that some luminaires, such as ellipsoidal spotlights and follow spots, can be adjusted or designed such that the light emanating from them does not include the entire beam, i.e. the edge of the beam is greater than 50% of its center.

Beam Angle
The angle of the vertex of a cone shaped beam where the perimeter of the base is defined by where the intensity is 50% of the maximum intensity.

Beam Diameter
The diameter of the base of a cone shaped beam where the perimeter of the base is defined by where the intensity is 50% of the maximum intensity.

Boom
In the film and video industries, an adjustable arm mounted at an angle from a vertical pipe or stand, often counter-balanced, used to hold lighting or sound equipment.

Borosilicate Glass
A type of glass commonly used for lenses and mirrors.

Breakup
See the definition for Cucoloris.

Broad Light
A light unit with a wide beam spread.

Bulb
Quite often this term is used interchangeably with Lamp.

Butterfly
A large fabric scrim, diffuser, reflector or opaque fabric panel used for dimming, softening, bounce lighting and shading, respectively. They are usually 4' to 8' square and mounted on yoke supported frames.

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