Glossary - E

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

effective floor cavity reflectance pFC a number giving the combined reflectance effect of the wall and floor reflectance of the floor cavity. See *floor cavity ratio.*

effective intensity Ie The effective intensity (*I*e) of a flashing light is, conventionally,

where the source has constant magnitude *I*0 over a time duration *t*. For a time-varying source *I(t*):

expressed in candelas. The times *t*1 and *t*2 are so chosen as to maximize the calculated effective intensity.It is then found that the actual intensity *I*t at time *t*1 or *t*2 is equal to the calculated equivalent intensity *I*e. 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† See means of egress.

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.

electroluminescence the emission of light from a phosphor excited by an electromagnetic field.

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

elevation the angle between the axis of a searchlight drum and the horizontal. Elevation is positive for angles above the horizontal, and negative below the horizontal.

ellipsoidal reflector spotlight a spotlight in which a lamp and an ellipsoidal reflector are mounted in a fixed relationship directing a beam of light into an aperture, where it can be shaped by a pattern, iris, shutter system, or other insertion. The beam then passes through a single or compound lens system that focuses it as required, producing a sharply defined beam with variable edge definition.

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 exit† a way out of the premises that is intended to be used only during an emergency.
emergency lighting 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 temperature.
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-ballast- luminaire 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.
equivalent contrast ˜C a numerical description of the relative visibility of a task. It is the contrast of the standard visibility reference task giving the same visibility as that of a task whose contrast has been reduced to threshold when the background luminances are the same. See visual task evaluator.
equivalent contrast ˜Ce the actual equivalent contrast in a real luminous environment with nondiffuse illumination. This actual equivalent contrast ˜Ce is less than the equivalent contrast due to veiling reflection. ˜Ce  = ˜C × CRF. See contrast rendition factor. equivalent luminous intensity of an extended source at a specified distance the intensity of a point source that would produce the same illuminance at that distance. Formerly, apparent luminous intensity of an extended source.
equivalent sphere illumination (ESI) the level of sphere illumination that would produce task visibility equivalent to that produced by a specific lighting environment.
equivalent veiling luminance the luminance of the reflected image of a bright surface that is superimposed on a test object to measure the veiling effect equivalent to that produced by stray light in the eye produced by a disability glare source. The disability glare source is turned off when the reflected image is turned on.
erythema a temporary reddening of the skin such as produced by exposure to actinic UV radiation. UV- induced erythema is due to actinic action and is a delayed effect occurring several hours after exposure. This differs from IR-induced erythema, a thermal effect occurring only for the duration of time that the skin temperature is elevated. Note The degree of erythema is used as a guide to dosages applied in UV therapy.
erythemal effectiveness the capacity of various portions of the ultraviolet spectrum to produce erythema.
erythemal efficiency of radiant flux (for a particular wavelength) the ratio of the erythemal effectiveness of a particular wavelength to that of wavelength 296.7 nm, which is rated as unity. Note This quantity formerly was called relative erythemal factor.
erythemal exposure the product of erythemal flux density on a surface and time. It usually is measured in µW × min/cm2.
Note For average untanned skin a minimum perceptible erythema requires about 300 µW × min/cm2 of radiation at 296.7 nm.
erythemal flux radiant flux evaluated according to its capacity to produce erythema of the untanned human skin. It usually is measured in microwatts of UV radiation weighted in accordance with its erythemal efficiency. Such quantities of erythemal flux are said to be in erythemal microwatts. See erythemal efficiency of radiant flux and erythemal unit. Note A commonly used practical unit of erythemal flux is the erythemal unit (EU) or E-viton (erytheme), which is equal to the amount of radiant flux that produces the same erythemal effect as 10 µW of radiant flux at wavelength 296.7 nm.
erythemal flux density the erythemal flux per unit area of the surface being irradiated. It is equal to the quotient of the incident erythemal flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in µW/cm2 of erythemally weighted UV radiation (erythemal µW/cm2). See *finsen.* Note A suggested practical unit of erythemal flux density is the finsen, which is equal to one E-viton per square centimeter.
erythemal threshold† See minimal perceptible erythema.
erythemal unit (EU)? a unit of erythemal flux that is equal to the amount of radiant flux that produces the same erythemal effect as 10 μW of radiant flux at wavelength 296.7 nm. Also called
E-viton.
E-viton (erytheme)? See erythemal unit.
exit ? the portion of a means of egress that segregates all other spaces in the building or structure by fire- resistant construction in order to provide a protected way of travel to the exit discharge. Exits include exterior exit doors, exit passageways, horizontal exits, and separated exit stairs or ramps.
exit access ? the portion of a means of egress that leads to an exit.
exit discharge ? the portion of a means of egress between the conclusion of an exit and a public way.
exit sign ? a graphic device including words or symbols that indicates or identifies an escape route or the location of, or direction to, an exit or emergency exit.
exitance ? See *luminous exitance* and *radiant exitance.*
exitance coefficient , *EC*  the ratio of the average (time zero) wall or ceiling cavity exitance to the quotient of the total lamp flux divided by the floor area. Note (i) Exitance is measured in lumens per unit area, where the units of area agree with those of the floor area. (ii) Average wall or ceiling cavity luminances can be determined by noting the underlying assumption of lambertian room surfaces where *L = M/π; L* is in candela per unit area, where the units of area agree with those of M. (iii) Exitance coefficients and former luminance coefficients are numerically identical.
excitation purity of a light , *p*e  the ratio of the distance on the CIE chromaticity diagram between the reference-point and the light-point to the distance in the same direction between the reference-point and the spectrum locus or the purple boundary. See colorimetric purity.
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.
externally illuminated exit sign ? an exit sign with an externally mounted light source. The exit legend style="padding-top: 2pt;padding-left: 5pt;text-indent: 0pt;text-align: left;">and background are typically opaque and rely on reflected light for visibility.
eye light illumination on a person to produce a specular reflection from items such as eyes, teeth, and jewelry without significantly increasing the total illumination of the subject.