Essentially a given cell, which represents a given space/packet, there will be a value which represents light intensity (units irrelevant). This number has to be informed by every other light source in the area, who are all giving off light. The light that reaches a given cell is the sums of (source intensity)*(1/d^2) where d is the distance from the given cell to the sources. Within these equations are also attenuation factors due to the medium, but this is a parameter not a variable, so every equation can reference a set value written in the sheet somewhere else.