Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. "Radiant exitance" or "radiant emittance" is used when the radiation is emerging from the surface. The SI units for all of these quantities are watts per square metre (W·m-2), while the cgs units are ergs per square centimeter per second (erg·cm-2·s-1, often used in astronomy). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units.

All of these quantities characterize the total amount of radiation present, at all frequencies. It is also common to consider each frequency in the spectrum separately. When this is done for radiation incident on a surface, it is called spectral irradiance, and has SI units W·m-3, or commonly W·m-2·nm-1.

If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source.

See also

*Photometry (optics) Main Photometry/Radiometry article - explains technical terms
* Albedo
* Illuminance
* Fluence
* Insolation

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  • irradiance — ● irradiance nom féminin Littéraire. Qualité de ce qui irradie ; rayonnement. irradiance [i(ʀ)ʀadjɑ̃s] n. f. ÉTYM. 1874, Verlaine; de irradiant, d après radiance. ❖ ♦ Littér. et rare. État, quali …   Encyclopédie Universelle

  • irradiance — ir*ra di*ance ([i^]r*r[=a] d[i^]*ans), irradiancy ir*ra di*an*cy ([i^]r*r[=a] d[i^]*an*s[y^]), n. [From {irradiant}.] [1913 Webster] 1. The act of irradiating; emission of rays of light. [1913 Webster] 2. That which irradiates or is irradiated;… …   The Collaborative International Dictionary of English

  • irradiance — (adj.) 1660s, from irradiant (1520s), from L. irradiantem (nom. irradians), prp. of irradiare (see IRRADIATE (Cf. irradiate)). Related: Irradiancy (1640s) …   Etymology dictionary

  • irradiance — [i rā′dē əns] n. 1. an irradiating; radiance 2. the amount of light or other radiant energy striking a given area of a surface; illumination: Also irradiancy irradiant adj …   English World dictionary

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  • Irradiance — L’irradiance ou éclairement énergétique est un terme utilisé en radiométrie pour quantifier la puissance d un rayonnement électromagnétique par unité de surface, dans le système international d unités, elle est exprimée en watt par mètre carré… …   Wikipédia en Français

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  • irradiance — ə̇+ noun 1. obsolete : emission of rays (as of light) 2. : something (as intellectual or spiritual illumination) that is emitted like rays of light informed … with so splendid a spiritual irradiance J.P.Bishop 3. : radiant flux density on a given …   Useful english dictionary

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