Attenuation
In real life, radiance reaching us from a light source decreases with distance as 1/r2 (the stars are much less bright than the sun). However, due to the nature of approximations used in graphics, the inverse-square law typically results in pictures that are too dark; the fix is to allow the programmer to control how fast the decay is. Ii in the formulas is replaced not by Ii/r2, as physics suggest, but by
and the most common choice of constants is
a = 1, b = c = 0, that is, no attenuation!