Specific radiative intensity
Specific (radiative) intensity is a quantity used in physics that describes electromagnetic radiation. It is a term used in much of the older scientific literature. The present-day SI term is spectral radiance, which can be expressed in base SI units as W m−2 sr−1 Hz−1.
It gives a full radiometric description of the field of classical electromagnetic radiation of any kind, including thermal radiation and light. It is conceptually distinct from the descriptions in explicit terms of Maxwellian electromagnetic fields or of photon distribution. It refers to material physics as distinct from psychophysics.
For the concept of specific intensity, the line of propagation of radiation lies in a semi-transparent medium which varies continuously in its optical properties. The concept refers to an area, projected from the element of source area into a plane at right angles to the line of propagation, and to an element of solid angle subtended by the detector at the element of source area.
The term brightness is also sometimes been used for this concept. The SI system states that the word brightness should not be so used, but should instead refer only to psychophysics.
The specific (radiative) intensity is a quantity that describes the rate of radiative transfer of energy at P1, a point of space with coordinates x, at time t. It is a scalar-valued function of four variables, customarily written as
where:
I (x, t ; r1, ν) is defined to be such that a virtual source area, dA1 , containing the point P1 , is an apparent emitter of a small but finite amount of energy dE transported by radiation of frequencies (ν, ν + dν) in a small time duration dt , where
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