I'm reading a text on ray tracing. There is this section about radiometric quantities where radiance is defined as
$L = \frac{d^2\Phi}{dA cos\Theta d\omega}$
$\Phi$ is the radiant flux
$\Theta$ is the solid angle (sr) subtended by the observation or measurement
$\omega$ is the incidence angle measured from the surface normal
This is just one of many equations using $d$ and $d^2$. I'm pretty sure that $d$ has something to do with differential equations. I already read some texts on differential equations but I still don't understand the meaning of $d$ and $d^2$ in this context.
Can someone explain this to me or point me to some reference/resource/book whatever?
Especially the $d^2$ puzzles me.