Glossary - I

ice detection light an inspection light designed to illuminate the leading edge of the aircraft wing to check for ice formation.

ideal radiator† See blackbody.

identification beacon an aeronautical beacon emitting a coded signal by means of which a particular point of reference can be identified.

ignitor a device, either by itself or in association with other components, that generates voltage pulses to start discharge lamps without preheating of electrodes. illuminance E
= dφ/dA the areal density of the luminous flux incident at a point on a surface

illuminance (footcandle or lux) meter an instrument for measuring illuminance on a plane. Instruments that accurately respond to more than one spectral distribution are color-corrected, that is, the spectral response is balanced to *V*(λ) or *V'*(λ). Instruments that accurately respond to more than one spatial distribution of incident flux are cosine-corrected, that is, the response to a source of unit luminous intensity, illuminating the detector from a fixed distance and from different directions, decreases as the cosine of the angle between the incident direction and the normal to the detector surface. The instrument is comprised of some form of photodetector with or without a filter driving a digital or analog readout through appropriate circuitry.

illumination an alternative but deprecated term for illuminance. It is frequently used because "illuminance" is subject to confusion with "luminance" and "illuminants," especially when not clearly pronounced. Note The term illumination also is commonly used in a qualitative or general sense to designate the act of illuminating or the state of being illuminated. Usually, the context indicates which meaning is intended, but occasionally it is desirable to use the expression "level of illumination" to indicate that the quantitative meaning is intended.

incandescence the self-emission of radiant energy in the visible spectrum due to the thermal excitation of atoms or molecules.

incandescent filament lamp a lamp in which light is produced by a filament heated to incandescence by an electric current. Note Normally, the filament is of coiled or coiled-coil (doubly coiled) tungsten wire. However, it can be uncoiled wire, a flat strip, or of material other than tungsten.

incomplete diffusion (partial diffusion) that in which the diffusing medium partially redirects the incident flux by scattering while the remaining fraction of incident flux is redirected without scattering; that is, a fraction of the incident flux can remain in an image-forming state.

index of sensation (M) (of a source) a number that expresses the effects of source luminance (*L*s), solid angle factor (*Q*), position index (*P*), and the field luminance (*F*) on discomfort glare rating. See Chapter 9, Lighting Calculations. See *discomfort glare rating (DGR),* where *L*s and *F* are expressed in cd/m2.

For an equation defining *Q* see *solid angle factor.* Note A restatement of this formula lends itself more directly to computer applications.

indirect component the portion of the luminous flux from a luminaire that arrives at the workplane after being reflected by room surfaces. See direct component.

indirect lighting lighting involving luminaires that distribute 90 to 100% of the emitted light upward.

infrared lamp a lamp that radiates predominately in the infrared; the visible radiation is not of principal interest.

infrared (IR) radiation for practical purposes any radiant energy within the wavelength range of 770 to 106 nm is considered infrared energy.

inhibition (visual) reduction in magnitude of the sensation aroused by the stimulus (or a reduction in visual sensitivity) caused by some other stimulation that is adjacent spatially or temporally.

initial luminous exitance This term can be used in two different ways. In flux transfer it is the density of luminous flux leaving a surface within an enclosure before interreflections occur. In lighting calculations it is the total exitance at time zero before depreciation (light losses) occur. Note For light sources this is the luminous exitance as defined in *luminous exitance*. For nonself-luminous surfaces it is the reflected luminous exitance of the flux received directly from sources within the enclosure or from daylight.

instant-start fluorescent lamp a fluorescent lamp designed for starting by a high voltage without preheating of the electrodes. Note In the UK, a cold-start lamp.

integrating photometer a photometer that enables geometrically total luminous flux to be determined by a single measurement. The usual type is the Ulbricht sphere with associated photometric equipment for measuring the indirect illuminance of the inner surface of the sphere. (The measuring device is shielded from the source under measurement.)

intensity† a shortening of the terms *luminous intensity* and *radiant intensity*. Often misused for level of illumination or illuminance.

intensity (candlepower) distribution curve a curve, often polar, that represents the variation of luminous intensity of a lamp or luminaire in a plane through the light center. Note A vertical intensity distribution curve is obtained by taking measurements at various angles of elevation about a source in a vertical plane through the light center; unless the plane is specified, the vertical curve is assumed to represent an average such as would be obtained by rotating the lamp or luminaire about its vertical axis. A horizontal intensity distribution curve represents measurements made at various angles of azimuth in a horizontal plane through the light center.

internally illuminated exit sign† a transilluminated exit sign containing its own light source.

interreflected component That portion of the luminous flux from a luminaire that arrives at the workplane after being reflected one or more times from room surfaces, as determined by the flux transfer theory. Also called interflectance. See flux transfer theory.

interreflection The multiple reflection of light by the various room surfaces before it reaches the work plane or other specified surface of a room. Also called interflectance.

interrupted quick-flashing light a quick-flashing light in which the rapid alternations are interrupted by periods of darkness at regular intervals.

inverse square law A law stating that the illuminance E at a point on a surface varies directly with the intensity I of a point source and inversely as the square of the distance d between the source and the point. If the surface at the point is normal to the direction of the incident light, the law is expressed by E = I/d2. Note For sources of finite size having uniform luminance, this gives results that are accurate within 1% when d is at least 5 times the maximum dimension of the source as viewed from the point on the surface. Even though practical interior luminaires do not have uniform luminance, this distance d is frequently used as the minimum for photometry of such luminaires when the magnitude of the measurement error is not critical.

iris an assembly of flat metal leaves arranged to provide an easily adjustable near-circular opening, placed near the focal point of the beam (as in an ellipsoidal reflector spotlight) or in front of the lens to act as a mechanical dimmer as in older types of carbon arc follow spotlights.

irradiance, E † the density of radiant flux (power) incident on a surface. isocandela line a line plotted on any appropriate set of coordinates to show directions in space, about a source of light, in which the intensity is the same. A series of such curves, often for equal increments of intensity, is called an isocandela diagram.

isolux (isofootcandle) line a line plotted on any appropriate set of coordinates to show all the points on a surface where the illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.

Kelvin† the unit of temperature used to designate the color temperature of a light source. A temperature scale where each degree is the same size as a centigrade degree, but the Kelvin scale has its zero at -273°C.

key light the apparent principal source of directional illumination falling upon a subject or area.

kicker a luminaire used to provide an additional highlight or accent on a subject.

klieg light a high-intensity carbon arc spotlight, typically used in motion picture lighting.