Glossary - I

I

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

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

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. 

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. 

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. 

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.