$$a = s / r$$
Arc length ($s$) over radius ($r$).
$$\omega = \frac{A}{r^2}$$
Area subtended ($A$) over squared radius ($r^2$).
$$dA = r^2 \sin(\theta) d\theta d\phi$$
$$d\omega = \frac{dA}{r^2} = \sin(\theta) d\theta d\phi$$
Energy of electromagnet radiation.
$$\Phi = \frac{dQ}{dt}$$
Energy per unit time.
Unit: Watt $[W]$
Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".[1]
$$I_\Omega = \frac{d\Phi}{d\omega}$$
Radiant flux per unit solid angle.
Unit: Watt per steradian $[W * sr^{-1}]$
Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.[1]
$$E = \frac{d\Phi}{dA}$$
Radiant flux per unit projected area.
Unit: Watt per square meter $[W * m^{−2}]$
Radiant flux received by a surface per unit area.[1]
$$L_\Omega = \frac{d^2 \Phi}{d\omega dA \cos(\theta)}$$
Radiant flux per unit solid angle per unit projected area.
Unit: Watt per steradian per square meter $[W * sr^{-1} * m^{−2}]$
Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".[1]