Glossary - 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.
back-up lamp a lighting device mounted on the rear of a vehicle for illuminating the region near the back of the vehicle while moving in reverse. It normally can be used only while backing up.

bactericidal (germicidal) effectiveness the capacity of various portions of the ultraviolet (UV) spectrum to destroy bacteria, fungi, and viruses.

bactericidal (germicidal) efficiency of radiant flux the ratio of the bactericidal effectiveness of that wavelength to that of wavelength 265.0 nm, which is rated as unity.

Note Tentative bactericidal efficiency of various wavelengths of radiant flux are given in Chapter 5, Nonvisual Effects of Radiant Energy.

bactericidal (germicidal) exposure the product of bactericidal flux density on a surface and time. It usually is measured in bactericidal μW × min/cm2 or bactericidal W × min./ft2.

bactericidal (germicidal) flux radiant flux evaluated according to its capacity to produce bactericidal effects. It usually is measured in microwatts of UV radiation weighted in accordance with its bactericidal efficiency. Such quantities of bactericidal flux would be in bactericidal microwatts.

Note Ultraviolet radiation of wavelength 253.7 nm usually is referred to as "ultraviolet microwatts" or "UV watts."

bactericidal (germicidal) flux density the bactericidal flux per unit area of the surface being irradiated. It is equal to the quotient of the incident bactericidal flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in μW/cm2 or W/ft2 of bactericidally weighted UV radiation (bactericidal μW/cm2 or bactericidal W/ft2).

bactericidal lamp a UV lamp that emits a significant portion of its radiative power in the UV-C band (100 to 280 nm).

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.

ballast a device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current, and waveform) for starting and operating. See *reference ballast*.

ballast factor the fractional flux of a fluorescent lamp operated on a ballast compared to the flux when operated on the standard (reference) ballast specified for rating lamp lumens.

Note The lamp is at specified ambient temperature conditions for photometric testing.

ballast-lamp photometric factor ratio of fluorescent luminaire lumen output using given ballast and lamp types (under photometric test conditions) to the lumen output using the lamp and ballast types used to generate a photometric test.

Note This factor is applicable when "energy-conserving" lamps and ballasts are used in a luminaire photometered with standard lamps and conventional ballasts; it is also applied in the converse situation.

bar (of lights) a group of three or more aeronautical ground lights placed in a line transverse to the axis, or extended axis, of the runway. See *barrette*.

bare (exposed) lamp a light source with no shielding.

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.

barrette (in aviation) a short bar in which the lights are closely spaced so that from a distance they appear to be a linear light.

Note Barrettes are usually no longer than 4.6 m (15 ft) in length.

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.

biconical reflectance p(mi;mr) ratio of the reflected flux collected through a conical solid angle to the incident flux limited to a conical solid angle.

Note The directions and extent of each cone must be specified; the solid angle need not be a right circular cone.

biconical transmittance τ(mi;mt) ratio of transmitted flux collected through a conical solid angle to the incident flux limited to a conical solid angle.

Note The directions and extent of each cone must be specified.

bidirectional reflectance p(8i,。i;8r,。r) ratio of reflected flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux.

Note The directions of incidence and collection and the size of the solid angle "element" of collection must be specified. In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone, but can be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles.

bidirectional reflectance distribution function (BRDF), fr the ratio of the differential luminance of a ray dLr(8r,。r) reflected in a given direction (8r,。r) to the differential luminous flux density dEi(8i,。i) incident from a given direction of incidence, (8i,。i), that produces it.

where *d*n 三 *d*m cos 8 Notes (i) This distribution function is the basic parameter for describing (geometrically) the reflecting properties of an opaque surface element (negligible internal scattering). (ii) It can have any positive value and approaches infinity in the specular direction for ideally specular reflectors. (iii) The spectral and polarization aspects must be defined for complete specification, since the BRDF as given above defines only the geometric aspects.

bidirectional transmittance τ(8i,。i;8t,。t) ratio of incident flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux.

Note The direction of incidence and collection and the size of the solid angle element must be specified.

bidirectional transmittance distribution function (BTDF), ft the ratio of the differential luminance dLt(8t,。t) for a ray transmitted in a given direction (8t,。t) to the differential luminous flux density dEi (8i,。i) incident from a given direction of incidence (8i,。i) that produces it:

where *d*n 三 *d*m cos 8 Notes (i) This distribution is the basic parameter for describing (geometrically) the transmitting properties of a thin scattering film (with negligible internal scattering) so that the transmitted radiation emerges from a point that is not significantly separated from the point of incidence of the incident ray(s). The governing considerations are similar to those for application of the *bidirectional reflectance distribution function (BRDF)*, rather than the *bidirectional scattering-surface reflectance distribution function (BSSRDF)*. (ii) It can have any positive value and approaches infinity in the direction for regular transmission (possibly with refraction but without scattering). (iii) The spectral and polarization aspects must be defined for complete specification, since the BTDF as given above defines only the geometrical aspects.

bihemispherical reflectance, p(2n; 2n) ratio of reflected flux collected over the entire hemisphere to the flux incident from the entire hemisphere.

bihemispherical transmittance, τ(2n; 2n) ratio of transmitted flux collected over the entire hemisphere to the incident flux from the entire hemisphere.

binocular portion of the visual field that portion of space where the fields of the two eyes overlap.

biological rhythm a characteristic periodic change in a living organism or life-related process. Some biological rhythms are induced and/or synchronized by light.

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 to evaluate black-light flux according to its capacity to produce effects.

black-light flux density black-light flux per unit area of the surface being irradiated. It is equal to the incident black-light flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in milliwatts per unit area of black-light flux.

black-light lamp an ultraviolet lamp that emits a significant portion of its radiative power in the UV-A band (315 to 400 nm).

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.

Blondel-Rey law the ratio of the thresholds of a square-form flashing light (*E*a) and of a steady light (*E*o), in point vision conditions at night. The ratio depends on the duration in seconds of the flash (*t*):

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.

borderline between comfort and discomfort (BCD) the average luminance of a source in a field of view that produces a sensation between comfort and discomfort.

boundary lights aeronautical ground lights delimiting the boundary of a land aerodrome without runways. See *range lights*.

bowl an open-top diffusing glass or plastic enclosure used to shield a light source from direct view and to redirect or scatter the light.

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.

bulb? See *lamp*.