A BRDF (Bidirectional Reflectance Distribution Function), written f(l,v), gives the amount of incoming light energy from direction l that turns into outgoing light energy in direction v. It is written with respect to a surface patch whose normal points upward.

Some good notes about BRDFs used in physically-based shading systems:

uLightEnergy r:1.0 g:1.0 b:1.0 * intensity:2.0
uLightPosition x:0.0 y:0.0 z:5.0