Absolute Zero: The temperature at which all molecular motion ceases. This is the lowest theoretically possible temperature. On the Kelvin scale, absolute zero occurs at 0 K. In Celcius, absolute zero is at -273.15°C. In Farenheit, absolute zero is at -459.67°F
Absorptance (Absorptivity): The fraction of incoming energy (at a given wavelength or in a given waveband) at a target that is absorbed by the target. Within the limits of Kirchoff’s Law, absorptance is equal to emittance. See Conservation of Energy.
Airy Disk: The Airy disk is the bright “inner fringe” of the diffraction pattern produced by an ideal (diffraction limited) optical system. This central spot comprises about 84% of the total energy.
Ambient: Local conditions around the object of interest.
Analog Signal: A continuously varying signal, as opposed to a discretely varying (digital) signal.
Angstrom: A length unit equal to 10-10 meters.
Anomaly: A continuously varying signal, as opposed to a discretely varying (digital) signal.
Anti-Reflective (AR) coating: A coating on an optical element (usually, a lens or window) intended to reduce the light energy reflected off the front surface, and thereby increase the amount of light transmitted through the element.
Arcminute: A small unit of angular measurement equal to 0.0167 degrees.
Arcsecond: A very small unit of angular measurement equal to 2.77 x 10-4 degrees.
Athermalized Lens: An optical lens design that stays in focus regardless of the temperature.
Atmospheric Attenuation: Absorption of the infrared energy by the atmosphere between the target object and the detector. Usually only a factor over very long distances.
Azimuth: The horizontal angular distance from a reference direction.
