Definitions:

• I_l,a - Ambient light intensity (same old hack)
• k_a - Ambient surface reflectance (hacks beget hacks)
• I_l,i - Luminous intensity of light source i
• k_s - percentage of light reflected specularly (notice terms sum to one)
• r_l - Diffuse reflectivity
• l_i - vector to light source
• n - average surface normal at point
• D - microfacet distribution function
• G - geometric attenuation Factor
• F_l(q_i) - Fresnel conductance term
• v - vector to viewer