Fahrenheit - A temperature scale based on 32°F as the freezing point of water and 212°F as the boiling point of water at standard atmospheric pressure; a relative scale related to the Rankine scale [0°F = 459.67.R; 1 F° (DT) = 1 R (DT)].

Field of View [FOV] - The angular subtense (expressed in angular degrees or radians per side if rectangular, and angular degrees or radians if circular) over which an instrument will integrate all incoming radiant energy. In a radiation thermometer this defines the target spot size; in a scanner or imager this defines the scan angle or picture size or total field of view (TFOV).

Information on Measuring Field of View.

Fiber Optic, Infrared - A flexible fiber made of a material that transmits infrared energy, used for making non-contact temperature measurements when there is not a direct line of sight between the instrument and the target.

Filter, Spectral - An optical element, usually transmissive, used to restrict the spectral band of energy received by an instrument’s detector.

Flame Filter - A filter of a specific waveband used to minimize the effects of flame, enabling the IR camera to “see” through the flame. The specific waveband is a region where the transmittance of flame approaches unity. Center wavelengths are typically 3.9 mm for shortwave instruments and 10.6 mm for longwave.

Focal Plane Array [FPA] - A linear or two-dimensional matrix of detector elements, typically used at the focal plane of an instrument. In thermography, rectangular FPAs are used in “staring” (non-scanning) infrared imagers. These are called IRFPA imagers.

Focal Point - The point at which the instruments optics image the infrared detector at the target plane. In a radiation thermometer, this is where the spot size is the smallest. In a scanner or imager, this is where the instantaneous field of view (IFOV) is smallest.

Foreground Temperature [See Instrument Ambient Background] - Temperature of the scene behind and surrounding the instrument as viewed from the target.

Frame Repetition Rate - The time it takes an infrared imager to scan (update) every thermogram picture element (pixel); in frames per second.

Full Scale - The span between the minimum value and the maximum value that any instrument is capable of measuring. In a thermometer, this would be the span between the highest and lowest temperature that can be measured.