False-colour: The use of one colour to represent another; eg, the
use of red emulsion to represent infrared light in colour infrared film.
False-colour image: A colour image where one or more channels which
indicate non-visible reflectance are expressed as one or more of the red,
green and blue additive colour components, so the resulting colours do not
correspond to normal visual experience; also called a false-colour composite.
The most commonly seen false-colour images display the near infrared as
red, red as green and green as blue.
Far infrared: A term for the longer wavelengths of the infrared region,
25 µm-1 mm, and generally accepted shorter wavelength limit of the
microwave part of the electromagnetic spectrum. Severely limited in terrestrial
use, as the atmosphere transmits very little radiation in this range.
Feature class: A class in which the channels may be other than radiance
data and is therefore a more general term than spectral class.
Feature extraction: The process in which an initial measurement pattern
or some sub-sequence of measurement patterns is transformed to a new pattern
feature.
Feature selection: The process by which the features to be used in
the pattern recognition or discrimination problem are determined.
Field of view (FOV): The solid angle through which an instrument
is sensitive to radiation. Owing to various effects, diffractions, etc.,
the edges are not sharp. In practice they are defined as the 'half-power'
points, that is, the angle outwards from the optical axis, at which the
energy sensed by the radiometer drops to half its on-axis value.
File: A variable number of records grouped together and treated as
a main division of data.
Film speed: The property of film that determines how much exposure
must be allowed for a given light source to secure a negative of correct
density and contrast.
Filter: (1) noun: Any material which, by absorption or reflection,
selectively modifies the radiation transmitted through an optical system.
(2) verb: To remove a certain component or components of radiation, usually
by means of a filter, although other devices may be used. (3) Kernel used
in filtering transformation.
Filtering: In analysis, the removal of certain spectral or spatial
frequencies to highlight features in the remaining image.
Fluorescence: A property of some materials where electromagnetic
energy of one wavelength is absorbed and then re-emitted at another (usually
longer) wavelength.
Focal length: The distance measured along the optical axis from the
optical centre (rear nodal point) of the lens to the plane of critical focus
of a very distant object.
Format: (1) The arrangement of descriptive data in descriptors, identifiers,
or labels. (2) The arrangement of data in bit, byte, and word form for computer
processing.
FOV: Field of view.
Frequency (n): Number of oscillations per unit time or number of
wavelengths that pass a point per unit time.
Frequency spectrum: Function representing the image components for
each spatial frequency.