Inverse Square Law Revisited

For my daughter's 6th grade Science Fair experiment she chose to explore the Inverse Square Law as it relates to light falloff. Technically the ISL only relates to point light sources, and it will 'fail' when the lights are not point light sources, but just how badly does it fail?

Any light is approximately a point light if you get far away from it. So how accurate is the ISL in real world cases? I was actually pretty curious to find out, myself.

So without further ado, here are the results of our experiments, courtesy of my 12-year-old daughter...