Astrophysics (Index)About


(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.

Further reading:

Referenced by pages:
Planck function
radiative flux
radiative transfer (RT)