### radiance

(measure of EMR to/from a solid angle through and area)

The common meaning of the term **radiance** is the rate of electromagnetic radiation
energy (power) emitted, radiated, or transmitted through and area
to or from a solid angle, e.g., watts (joules per
second) per steradian per square meter. It is defined in
terms of area and solid angle, but it can be thought of as something
like the EMR-power along a straight line, and because of the way
it is quantified, it does not decrease with distance. It can be
thought of as the power being transmitted from a point on the source
to a point on the receiving surface.

The terms used for EMR measurement vary across science and technology
(sometimes with conflicting meanings for the same term) and in
astronomy, term intensity is commonly used for this value,
as described above, as well as the terms **surface brightness** and
just **brightness**.

The terms **spectral radiance**, specific intensity
and **spectral brightness** indicate the analogous values per
frequency-interval or wavelength-interval, e.g., per hertz
or per meter. Be warned that conversion between the per-wavelength
and per-frequency spectral radiance is made more complicated by the
differentials.

(*EMR,measure*)
**Further reading:**

http://en.wikipedia.org/wiki/Radiance

**Referenced by pages:**

intensity

irradiance

Planck function

radiative flux

radiative transfer (RT)

Index