The most important key terms and abbreviations from the temperature, humidity and pressure measurement technique are explained in this glossary.
Letter - A
Absolute humidity

The absolute humidity is the mass of water vapour per unit volume of humid gas.

Absolute zero

The temperature of -273.15 °C, -459.69 °F, or 0 K;thought to be the temperature at which molecular motion vanishes and a body would have no heat energy.


The maximum deviation in a set of measurements between the temperature indicated by a radiation thermometer and the known temperature of a reference source, including the uncertainty of the reference temperature source. The accuracy can be expressed in a variety of ways including temperature, percentage of temperature reading, or percentage of full scale temperature of an instrument.

Ambient derating

Derating or decrease in accuracy of an instrument due to changes in its ambient temp from that at which it was calibrated. See also temperature coefficient.

Ambient operating range

Range in the ambient temperature over which the instrument is designed to operate.

Ambient temperature

The temperature of the instrument. Can also refer to the temperature that gives rise to the background. See Background Radiation.

Ambient temperature compensation (TAMB)

See Reflected Energy Compensation.


American Society for Testing and Materials.

ASTM E 1256

ASTM E1256 – 88, Standard Test Methods for Radiation Thermometers (Single Wave-band type). A standard by which Raytek products are tested and calibrated for accuracy, repeatability, resolution, target size, response time, warm-up time, and long-term drift.

Atmospheric windows

The spectral bands in which the atmosphere least affects the transmission of Radiant energy. The spectral bands are 0.4 to 1.8, 2 to 2.5, 3 to 5, and 8 to 14 micrometers.