How is the falloff for a a real life light source? I know that in different 3d APIs it can be calculated in several different ways, but do these in anyway relate to how it is in real life?

Light, in real life, follows the inverse square law, so it really never gets to zero.

In a vacuum, the falloff is inverse quadratic as a function of distance from the light source (1 / x2), however in Real Life™ you're rarely in a complete vacuum. There are other obstacles that absorb the energy of the light and cause it falloff much sooner, such as fog or dust. Most 3D API's provide you with access to constants that let you control linear, quadratic, and constant falloff of the light. Those are there so you can adjust your light and make it look better, as opposed to making it behave more realistically. Unless you have a full radiosity solution (and even when you do), it's extremely difficult to mimic light behavior in the real world, so those constants let you easily "fake" somewhat decent lighting, since quadratic falloff alone usually causes your scene to be way too dark and doesn't distribute the light very well.

Quote:Light, in real life, follows the inverse square law, so it really never gets to zero.

"never" is a bit strong. Light is quantized, so it can get to zero. Event horizons can also do the trick. I realize this is being anal retentive.