TPDSci Intensity

Radiometry: Intensity

Intensity, I [W sr^-1] of electromagnetic radiation (ER) (for example, Köhler R 1998, Mobley CD 1994, Morel A and Smith 1982, Raschke E 1978), also referred to as radiant intensity and known as pointance (for example, Wyatt CL 1991, note that pointance is rarely used) at a point r in space and along a direction ξ is the power, Φ, of ER per unit solid angle, ω:

I(r, ξ) = ∂Φ / ∂ω

(1)

It has been tacitly assumed here that the intensity, I(r, ξ), represents an integral over a wavelength range of the spectral distribution (spectrum) of intensity, I_λ(r, ξ, λ) [W sr^-1 m^-1], where λ is the wavelength of ER.

Intensity can be regarded an integral of radiance over projected area.

The radiometric definition of intensity conflicts with that used in several other disciplines. In some branches of physics, intensity is the power of ER per unit area (for example, Hecht E 1987). The latter is the irradiance in radiometry. In others, intensity is the power of ER per unit area and unit projected solid angle and has the unit of radiance. This may lead to confusion, unless the dimension or definition of that quantity is explicitly stated.

Intensity is an SI base quantity with the unit of candela (cd). This unit is defined as follows: 1 candela is the luminous intensity, in a given direction, of a source that emits monochromatic ER of frequency 540×10¹² Hz (λ ≈555 nm) and that has a radiant intensity in that direction of 1/683 W sr^-1 (Resolution 3 of the 16th CGPM 1979, see: Palmer JM 1993 and SI brochure). Using the term intensity to denote quantities with the units of W m^-2 or W m^-2 sr^-1 does not conform with the proper SI usage.

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

HISTORY:
Published: 13-Jan-2006
Modified: 24-Mar-2007
Peer-reviewed: 03-Sep-2006

Topics in Particle and Dispersion Science