Calculating Reflection Vector

where L and N are unit length => projection of L on N is N cos [theta] S = N cos [theta] - L

R is L mirrored about N
R = N cos [theta] + S = 2N cos [theta] - L
= 2N (N^.L) - L => where I_spec = k_s I_l ( (2N (N^.L) - L )^.V )ⁿ
(2N (N^.L) - L )^.V = R^.V

Alternatively: use halfway vector H
H = (L + V) / |L + V|