Glossary - E

E 

effective ceiling cavity reflectance, a number giving the combined reflectance effect of the wall and ceiling reflectance of the ceiling cavity. See ceiling cavity ratio. 

efficacy† See luminous efficacy of a source of light and spectral luminous efficacy of radiant flux. 

efficiency† See luminaire efficiency, luminous efficacy of a source of light, and spectral luminous efficiency of radiant flux. 

egress lighting† emergency lighting for egress. 

electric discharge† See arc discharge, gaseous discharge, and glow discharge. 

electric-discharge lamp a lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or gas. See carbon arc lamp, cold-cathode lamp, fluorescent lamp, glow lamp, high intensity discharge (HID) lamp, and hot cathode lamp. 

Note Electric-discharge lamps can be named after the filling gas or vapor that is responsible for the major portion of the radiation, for example, mercury lamps, sodium lamps, neon lamps, and argon lamps. A second method of designating the electric-discharge lamps is by physical dimensions of operating parameters, for example, short-arc lamps, high-pressure lamps, and low-pressure lamps. A third method of designating electric discharge lamps is by their application. In addition to lamps for illumination there are photochemical lamps, bactericidal lamps, blacklight lamps, sun lamps, and others. 

electromagnetic spectrum† a continuum of electric and magnetic radiation encompassing all wavelengths. See regions of the electromagnetic spectrum. 

emergency† any condition, external or internal to the premises, that compromises the effectiveness of the lighting in an occupied area for safe movement within and out of that area and safe operation of equipment within the space. An emergency can include any or all of the following: 

Utility power failure; Utility power voltage reduction (brownout) below the minimum required to support the arc in fluorescent or HID lamps   Power interruption within the premises including total power loss or individual phase or branch circuit failure   Fire or smoke. 

emergency lighting designed to supply illumination essential to the safety of life and property in the event of failure of the normal supply. 

emissivity, ε the ratio of the radiance (for directional emissivity) or radiant exitance (for hemispherical emissivity) of an element of surface of a temperature radiator to that of a blackbody at the same

emittance, ε The ratio of radiance in a given direction (for directional emittance) or radiant exitance (for hemispherical emittance) of a sample of a thermal radiator to that of a blackbody radiator at the same temperature. 

enclosed and gasketed† See vapor-tight luminaire. 

equal interval (isophase) light a rhythmic light in which the light and dark periods are equal. 

equipment operating factor the ratio of the lumens of a high-intensity discharge (HID) lamp-ballastluminaire combination in a given operating position to the lumens of the lamp-ballast-luminaire combination (a) operated in the position for rating lamp lumens, and (b) using the standard (reference) ballasting specified for rating lamp lumens.  Note If the given lamp operating position is not the same as the lamp rating position, the lumens ratio for the operating ballast to standard rating ballast is determined in the given operating position. This ratio is multiplied by the lamp position factor to obtain the equipment operating factor. 

explosion-proof luminaire a luminaire that is completely enclosed and capable of withstanding an explosion of a specific gas or vapor that can occur within it, and preventing the ignition of a specific gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within. It must operate at such an external temperature that a surrounding flammable atmosphere is not ignited thereby.