Topics in Particle and Dispersion Science

Radiometry: Irradiance

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Irradiance, E [W m^-2] of electromagnetic radiation (ER) (for example, Köhler R 1998, Mobley 1994, Morel A and Smith 1982, Raschke 1978), also known as areance (for example, Wyatt 1991), is the power, Φ, of ER, incident from a direction, ξ, per unit area, A, at a point r in space.

E(r, ξ) = ∂Φ / ∂A

(1)

The definition of irradiance, as stated here, accounts for the effect of orientation of the area at point r in respect of the direction, ξ, of the incident radiation. Indeed, consider a unidirectional ER source radiating power Φ in direction ξ. Irradiance due to that source, at a plane perpendicular to ξ is different (greater) than that at any oblique plane. That maximum value is typically used when the irradiance due to a unidirectional beam is referred to.

Note that various definitions of irradiance are in use, for example, "the flux per unit area incident onto a surface from all directions in space." Hence, when the irradiance is referred to in a publication, one should consult its intended definition.

It has been tacitly assumed here that irradiance, E(r, ξ), represents an integral over a wavelength range of the spectral distribution (spectrum) of irradiance, E_λ(r, ξ) [W m^-3], where λ is the wavelength of ER.

The radiometric definition of irradiance corresponds to the definition of intensity as used in some other branches of physics (for example, Hecht 1987). The irradiance is also denoted by I, when the notation E conflicts with that for the electric field, E, and - in the older literature - by H.

Irradiance can be regarded an integral of radiance over a solid angle (note that radiance is defined in reference to an area of a surface normal to ξ at r, i.e. projected area), as shown in Vector irradiances and Scalar irradiances.

CITATION: Jonasz M. 2006. Radiometry: Irradiance (www.tpdsci.com/Tpc/RdmIrdER.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 22-Mar-2006 Modified: 06-Mar-2008 Peer-reviewed: 03-Sep-2003

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