Glossary - B

B 

back light illumination from behind (and usually above) a subject to produce a highlight along its edge and consequent separation between the subject and its background. See side-back light. 

backing lighting the illumination provided for scenery in off-stage areas visible to the audience. 

baffle a single opaque or translucent element to shield a source from direct view at certain angles, to absorb or block unwanted light, or to reflect and redirect light. 

balcony lights luminaires mounted on the front edge of an auditorium balcony. 

barn doors a set of adjustable flaps--usually two, four, or eight--which can be attached to the front of a luminaire (usually a Fresnel spotlight) in order to partially control the shape and spread of the light beam. 

base light uniform, diffuse, near-shadowless illumination sufficiently intense for a television or film picture of acceptable quality at a desired lens opening. Acceptable base level of unaccented stage illumination. 

beacon a light (or mark) used to indicate a geographic location. See aerodrome beacon, aeronautical beacon, airway beacon, approach-light beacon, hazard or obstruction beacon, identification beacon, and landmark beacon. 

beam angle the angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90°, typically the maximum and minimum angles. 

Note In certain fields of application, the beam angle was formerly measured to 10% of maximum intensity. 

beam axis of a projector a line midway between two lines that intersect the intensity distribution curve at points equal to a stated percentage of its maximum (usually 50%). 

beam lumens the total flux in that region of space where the intensity exceeds 50% of the maximum intensity. 

beam projector a luminaire with the light source at or near the focus of a paraboloidal reflector, producing near-parallel rays of light in a beam of small divergence. Some are equipped with spill rings to reduce spill and glare. In most types, the lamp can be moved toward or away from the reflector to vary the beam spread. 

beam spread (in any plane) the angle between the two directions in the plane in which the intensity is equal to a stated percentage of the maximum beam intensity. 

blackbody a temperature radiator of uniform temperature whose radiant exitance in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it absorbs all the radiant energy that falls upon it. All other temperature radiators can be classed as nonblackbodies. Nonblackbodies radiate less in some or all wavelength intervals than a blackbody of the same size and the same temperature. 

Note The blackbody is practically realized over limited solid angles in the form of a cavity with opaque walls at a uniform temperature and with a small opening for observation. It is variously called a standard radiator, an ideal radiator, or a complete radiator. 

blackbody (planckian) locus the locus of points on a chromaticity diagram representing the chromaticities of blackbodies having various (color) temperatures. 

black light the popular term for UV energy near the visible spectrum. 

Note For engineering purposes the wavelength range 320 to 400 nm has been found useful for rating lamps and their effectiveness upon fluorescent materials (excluding phosphors used in fluorescent lamps). By confining black light applications to this region, germicidal and erythemal effects are, for practical purposes, eliminated. 

black-light flux radiant flux within the wavelength range 320 to 400 nm. It is usually measured in milliwatts. See fluoren. 

Note The floren is used as a unit of black-light flux and is equal to one milliwatt of radiant flux in the wavelength range 320 to 400 nm. Because of the variability of the spectral sensitivity of materials irradiated by black light in practice, no attempt is made. 

blending lighting general illumination used to provide smooth transitions between the specific light areas on a stage. 

blinding glare glare that is so intense that for an appreciable length of time after it has been removed, no object can be seen. 

bollard† luminaires having the appearance of a short, thick post, used for walkway and grounds lighting. The optical components are usually top-mounted. 

borderlight a long continuous striplight hung horizontally above a stage and aimed down to provide general diffuse illumination or to light the cyclorama or a drop; usually wired in three or four color circuits. Also available in portable versions. 

bracket (mast arm) an attachment to a lamp post or pole from which a luminaire is suspended. 

brightness (of a perceived aperture color) the attribute by which an area of color of finite size is perceived to emit, transmit, or reflect a greater or lesser amount of light. No judgment is made as to whether the light comes from a reflecting, transmitting or self-luminous object. See also brightness of a perceived light source color, luminance, subjective brightness, and veiling brightness. 

brightness contrast threshold when two patches of color are separated by a brightness contrast border as in the case of a bipartite photometric field or of a disk-shaped object surrounded by its background, the border between the two patches is a brightness contrast border. The contrast that is just detectable is known as the brightness contrast threshold. 

brightness of a perceived light source color† the attribute in accordance with which the source seems to emit more or less luminous flux per unit area.