Intensity in astronomy (commonly called Radiance outside astronomy) is a measure of Electromagnetic Radiation (EMR) striking a surface from a given solid angle, i.e., source. Alternately, electromagnetic radiation from a surface radiating within a solid angle can use the same measure. A common unit is watt per steradian per square meter.
d²Φ L = ———————— dAdΩcosθ Φ ≈ ——————— AΩcosθ
This is the common way the term Intensity is used in Astronomy. In physics, the term is often used all electromagnetic radiation striking a surface in watts per square meter. Thus the term "Radiance" to distinguish meanings.
Specific Intensity or Spectral Radiance is the Intensity at a specific Wavelength.
The Mean Intensity is the average intensity in all directions from a surface (perhaps within a solid angle), i.e., integrating it over the angle and dividing by 4π. It can be "specific", i.e., per wavelength. Using the mean is a useful way to simplify models.
Imaging Fourier Transform Spectroscopy (IFTS)
Kramers' Opacity Law
Line Shape Function
Optical Depth (τ)
Rosseland Mean Opacity
Equation of Radiative Transfer (RTE)
Source Function (S)
Spectral Power Distribution (SPD)
Stellar Temperature Determination
Wien's Displacement Law