Radian: A unit of angular measurement equal to 57.296 degrees.
Radiation: The transfer of electro-magnetic energy, generally (in the context of imaging systems) in the form of light of any wavelength.
Radiometric Imaging: The quantitative measurement of light using an imaging system.
Rankine: A temperature unit in the English system based on absolute zero. One degree Rankine is equal to one degree Farenheit.
Raster: A pattern of scanning one row after another, similar to the way in which most languages are read (including English). Some video systems form images using a raster pattern.
Rayleigh Scattering: The scattering of light by particles which are smaller than the wavelength of the light that is scattered. Rayleigh scattering is the reason that clear daylight skies appear blue in visible color.
Recognition Range: The maximum range at which a specified target can be recognized as belonging to a general class of targets, such as human or vehicle, but without being able to distinguish between subclasses of like targets. See Probability of Detection/Recognition/Identification.
Reflectance: The fraction of incoming energy (at a given wavelength or in a given waveband) that is reflected off of a target. See Conservation of Energy.
Resolution: A general term, often casually used to describe the performance of a thermal camera, meaning “the smallest detail that can be perceived”. Two broad categories of resolution are thermal resolution and spatial resolution (defined separately).
RF: Acronym for Radio Frequency, referring to electromagnetic radiation of frequencies between about 3 Hz to 300 GHz.
