Understanding the Principles of Light Behavior - Photographic Rendering with V-Ray for SketchUp - page 167

Light decay - exercise one
Now, while our sound example may help us understand the technical aspect of how
the Inverse Square law works, it doesn't really give us an easy-to-use and real world
method for testing it. This is because, our ability to evaluate sound levels or volume
in an environment depends on quite a number of shifting variables that includes both
physical (our own hearing ability) and environmental (acoustics of the area) factors.
Because Inverse Square decay affects all types of radiation, we can in fact use heat
as a much more measurable example of this law in action. All we need to do is stand
at a reasonable distance from a decent heat source such as a fire, oven, or radiat-
or, and then slowly walk towards it. Initially, we will cover a large percentage of the
distance between ourselves and the heat source without feeling an equally strong
increase in the heat coming from it. It is only once we get really close to the heat
source that those levels start to climb quite dramatically, with the final quarter of the
travel distance yielding the biggest change of all.
This is the Inverse Square law at work, and this is how light decay works. As light
travels away from its source, it essentially spreads itself over a wider and wider area,
and so the level of illumination it can provide in the environment decreases, becom-
ing less and less powerful.
Light decay - exercise two
At nighttime, take a candle into the smallest room of your home and with all other
light sources turned off, take note of how much detail in the room you can see (in
other words, note how much of the surrounding environment is illuminated).
Then take that same candle into the largest room in your house and compare the
level of illumination or amount of the surrounding environment that you can now see.
Even with only a reasonable increase in room size, the change in illumination levels
will usually be quite obvious. The bigger the difference in room size of course, the
bigger the difference in the candle's ability to illuminate it. Again, this is the Inverse
Square law at work.