The rendering equation is

$$
I(x,x') = g(x,x') \left[ \epsilon(x,x') + \int_{S} \rho(x,x',x'') I(x',x'') dx'' \right]
$$

where:

: $I(x,x')$ 
is the related to the intensity of light passing from 
point $x'$ to point $x$

: $g(x,x')$
is a “geometry” term 

: $\epsilon(x,x')$
is related to the intensity of emitted light from $x'$ to $x$ 

: $\rho(x,x',x'')$
is related to the intensity of light scattered from $x''$ to $x$ by a
patch of surface at $x'$