intensity

Intensity in astronomy (commonly called radiance outside astronomy) is a measure of electromagnetic radiation 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

• L - intensity
• Φ - power in watts
• Ω - solid angle in steradians
• θ - angle between source and normal of the source

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.