recently, i've started to learn more about GI and the techniques that we can use to implement.
one of the main basic things that i've learned in this topic is Radiance and Irradiance.
Radiance is the spatial density of radiation incident at a surface, regardless of the direction
and Irrediance is the same thing but with direction and orientation dependency.
in Irradiance the numerator is d?(p)
but in Radiance, the numerator is d^2?(p,w)
so my question is why the power of Radiance's derivative is 2 ?