Step Two: Find p for a Directional Light Source

Using this α we find

p = s + αl
  =
s −
nTs
nTl
l

If we multiply both sides of this equation by β = nTl we find

p' = βp = (nTl)s − (nTs)l

which is exactly the same equation we found for the positional light source, except for the different value of β.