Combined Model With Multiple Lights and Metallic Highlights

I_[l] = I_amb[l] + sum[i]( I_diff[l] + I_spec[l] ) for N lights
= k_a[l] I_a[l] O_d[l] + sum[i]( k_d[l] I_li[l] (N^.L) O_d[l] + k_s[l]([l], [t]) I_li[l] (N^.H)ⁿ O_d[l] )

For even more accurate highlights, specular reflection coefficient depends on both wavelength and angle of incidence K_s([l], [t]).