What does emittance mean?
Emittance is a technical noun referring to the act of emitting something, or more specifically in physics, to the amount of radiant energy — particularly thermal or infrared radiation — given off by a surface. It appears chiefly in scientific and engineering contexts such as thermodynamics, materials science, and aerospace design, where engineers compare the thermal emittance of different coatings or materials to control how much heat they radiate. The word should not be confused with the more common 'emission', which broadly covers any discharge of energy or matter, nor with 'emissivity', which expresses a surface's radiating efficiency as a dimensionless ratio relative to a perfect blackbody. In ordinary conversation, 'emittance' is rare; most speakers would say 'emission'. Its value lies in precise technical writing where the distinction between actual emitted radiation and idealized radiative properties matters.
nounThe act or process of emitting; the amount of radiation or energy emitted, especially thermal radiation emitted from a surface.
- The emission of radiant energy, especially thermal radiation, from a surface (physics).
- The act or fact of emitting something (general use, now rare).
"The satellite's instruments measured the emittance of infrared radiation from the planet's surface."
"The coating was chosen to maximize the thermal emittance of the radiator panel."
"Careful measurements were taken of the emittance from the heated metal surface."
Rarely pluralized; typically used as an uncountable mass noun in technical writing.
"The report tabulated the emittances of various materials at different temperatures."
In physics, 'emittance' and 'emissivity' differ by just two letters — but one measures what a surface actually radiates, and the other scores it against a perfect blackbody.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of emittance
Emittance derives from the Latin verb emittere ('to send out'), formed from e- ('out') and mittere ('to send'). It entered English in the late 16th or early 17th century, following the pattern of other nouns ending in -ance derived from verbs. It is closely related to 'emission' (from the Latin past participle emissus) and shares its root with 'transmit' and 'mission'. In modern physics usage, particularly American technical literature, it came to denote measured radiated energy distinct from the ratio-based term 'emissivity'.
How emittance is actually used
Predominantly a technical term used in physics and thermodynamics. In everyday English, 'emission' is far more common for general senses. In physics contexts, 'emittance' is often distinguished from 'emissivity' (a dimensionless property), though usage varies across subfields.
Easily confused with emittance
Emittance is the actual radiated flux from a real surface, while emissivity is a dimensionless ratio comparing that surface to an ideal blackbody.