What you say is absolutly correct. The inverse square law for radiation
assumes a point source. While nothing is a true point source you can
get pretty close to one. A good example is an electronic flash unit. Anyone
who has used an enlarger knows that using the inverse square law is a
fair approximation for determining exposure times. A bank of FL light
bulbs would certainly not be a point source. I do not know the exact
relationship for a diffuse and large source other than doubling the distance
would give you less than half the light intensity. I guess I should have
written a more accurate statement. I was, more or less, speaking metaphorically.Let me give this one a little more thought.
Bob Schramm