ICBO—International Conference of Building Officials. Author of The Uniform Building Code.

Impact load—An assumed dynamic load resulting from the motion of machinery, elevators, craneways, vehicles, and other similar moving forces.

Impact wrench—An electric or pneumatic device used to tighten nuts on bolts.

Impedance—The ratio of voltage to current in an alternating current circuit.

Incline—The slope of a roof expressed in percent or in the number of vertical units of rise per horizontal unit of run.

Inelastic scattering—Scattering of particles as a result of collisions in which part of the kinetic energy is lost as heat or radiation. (See Thermalizing)

Infrared spectrum—Those wave lengths of the electromagnetic spectrum which are by convention called infrared, generally considered to be of wave lengths from just beyond the visible (.77 microns) to about 3000 microns. (Longer wavelengths)

Inorganic, adj.—Being or composed of matter other than hydrocarbons and their derivatives, or matter that is not of plant or animal origin.

Insulation—See Thermal insulation.

Insulation, thermal—Any material used in building construction to reduce heat transfer.

Interlace—The intermixing of more than one complete field of scan lines. Instead of scanning each line sequentially from the top to the bottom, some lines are left as spaces and are filled in on the display by a subsequent field.

Internal pressure—Pressure inside a building which is a function of wind velocity, and number and location of openings.

Internal rate of return (IRR)—The compound rate of interest that, when used to discount study period costs and benefits of a project, will make the two equal.

Inverse square law—Applies to a process where the intensity of radiation decreases as the square of the distance—that is if one went twice the distance, the intensity would be 1/4.

Inverted display—The transposition of the grey scale (i.e., black for white) in a CRT display.

Isocyanate—A highly reactive chemical grouping composed of a nitrogen atom bonded to a carbon atom bonded to an oxygen atom;=N=C=O; a chemical compound, usually organic, containing one or more isocyanate groups.

Isotherm—Contours or lines depicting equal apparent temperatures on the display screen. On imaging scanners, these areas are electrically highlighted on the display as bright white.

Isotherm thermogram—A picture of a thermal image showing areas of equal apparent temperature.

Isothermal units—A unit of thermal measurement common to a particular IR system. It must be converted to temperature by correcting for instrument settings, detector output, emissitives, and ambient conditions.