Combined Model With Multiple Lights and Metallic Highlights

I[l] = Iamb[l]  + sum[i]( Idiff[l]        + Ispec[l]        ) for N lights
= ka[l] Ia[l] Od[l] + sum[i]( kd[l] Ili[l] (N.L) Od[l] + ks[l] Ili[l] (N.H)n Od[l] )

For metallic highlights, specular reflection must be wavelength dependent.