Difference between Close and Distance Light Sources:

In this simple diagram, the intensity of light falling on a surface may be thought of simply as the number of rays, which hit it. It may be seen that more rays fall on cube A than on cube B and it can quite easily be shown that the number is proportional to the reciprocal of the square of the distance between the light source and the object (i.e. 1/r where r is the distance between source and surface).

Parallel Rays : It may also be seen in Figure that the light rays crossing cube B are nearly parallel whereas the rays crossing cube A are highly divergent. It means for distant illumination, there is little variation in intensity between one side of an object and the other (which means we only need to do one calculation of intensity for the whole surface), whereas this is not true for close illumination. If the need exists to implement a physically accurate illumination model, we could not make this assumption and would have to take account of these effects, but for most purposes, the simple model will suffice.