Imager, Infrared - An infrared instrument that collects the infrared radiant energy from a target surface and produces an image in monochrome (black and white) or color, where the gray shades or color hues correspond respectively to target exitance.

Image Display Tone - Gray shade or color hue on a thermogram.

Image Processing, Thermal - Analysis of thermal images, usually by computer; enhancing the image to prepare it for computer or visual analysis. In the case of an infrared image or thermogram, this could include temperature scaling, spot temperature measurements, thermal profiles, image manipulation, subtraction and storage.

Imaging Radiometer - An infrared thermal imager that provides quantitative thermal images.

Indirect Thermography - Thermal imaging and measurement of a surface which is indirectly affected by the target of concern. That is, the target of concern is thermally decoupled from the surface due to thermal insulation, such as an air gap or a radiant barrier.

Indium Antimonide [InSb] - A material from which fast, sensitive photo-detectors used in infrared scanners and imagers are made. Such detectors usually require cooling while in operation.

Inertia, Thermal - See thermal effusivity.

Infrared [IR] - The infrared spectrum is loosely defined as that portion of the electromagnetic continuum extending from the red visible (0.75 mm  to about 1,000 mm) . Because of instrument design considerations and the infrared transmission characteristics of the atmosphere, however, most infrared measurements are made between 0.75 and 20 mm.

Infrared Focal Plane Array [IRFPA] - A linear or two-dimensional matrix of individual infrared detector elements, typically used as a detector in an infrared imaging instrument.

IRFPA Imager or Camera - An infrared imaging instrument that incorporates a two-dimensional IRFPA (focal plane array) and produces a thermogram without mechanical scanning.

Infrared Radiation Thermometer - An instrument that converts incoming infrared radiant energy from a spot on a target surface to a measurement value that can be related to the temperature of that spot.

Infrared Thermal Imager - An Instrument or system that converts incoming infrared radiant energy from a target surface to a thermal map, or thermogram, on which color hues or gray shades can be related to the temperature distribution on that surface.

Instantaneous Field of View [IFOV] - The angular subtense (expressed in angular degrees or radians per side if rectangular and angular degrees or radians if round) found by the ratio of the detector dimension divided by the instrument focal length; the projection of the detector at the target plane. In a radiation thermometer this defines the target spot size; in a line scanner or imager it represents one resolution element in a scan line or thermogram and is a measure of spatial resolution.

Isotherm - A pattern superimposed on a thermogram or on a line scan that includes or highlights all points that have the same apparent temperature.