What is the third equation trying to illustrate?
Professor Pat mentioned that we want to specify power as a parameter in pbrt. Using the third equation, we can make the light brighter or darker for the same value of s.
@alpaca, I agree. This can be seen if we substitute in equation 3 into equation 2 we will get that $\phi = ... = \phi$, thus in the third equation we can directly set the power as a user parameter.