Understanding the Principles of Light Behavior - Photographic Rendering with V-Ray for SketchUp - page 166

Understanding light decay
When working with lights in a computer graphics application, typically speaking, there
are two options available to help us control the distance that light is allowed to travel
in a scene. One is attenuation, which is generally a forced cut off point beyond which
light isn't allowed to travel. A typical set of controls for attenuation would involve val-
ues that can be set for both near and far cut off points or distances in the scene.
One important thing we need to keep in mind regarding attenuation, however, is that it
is not a naturally occurring phenomenon. In the real world, light has no built-in mech-
anism whereby we can just tell it to stop travelling beyond a fixed point in space. If
you have ever tried to create a completely blacked out environment, you will no doubt
have wished that such a control was available, as light seems to find a way to travel
in an environment no matter how hard you work to block it.
Decay or falloff, in contrast to attenuation, is a natural phenomenon and refers to
something that decreases or declines gradually in size, quantity, activity, or force. In
the real world, light follows a very specific decay or falloff pattern; this is often referred
to as Quadratic or Inverse Square falloff and is governed by the Inverse Square
Law .
Note
Not that the Inverse Square law applies only to the way that light works. It is ac-
tually at work in quite a number of areas including gravity, electricity, and sound
to name just a few. This law states that a physical quantity or strength is inversely
proportional to the square of the distance from the source of that physical quantity
or strength.
Let us put this in a practical example: at 2 meters from its source, a travelling
sound has already lost one quarter of the energy or volume that was present at
the 1 meter point, and so, it is already only one quarter as loud as it was at its
original source. By the 3 meter point, it has lost one-ninth of its intensity, and so
on.