Before revision: The reason the function is defined as a quotient of two differentials and not directly as a quotient between the undifferentiated quantities, is because other irradiating light than , which are of no interest for , might illuminate the surface which would unintentionally affect , whereas is only affected by .
After revision: The reason the function is defined as a quotient of two differentials and not directly as a quotient between the undifferentiated quantities, is because irradiating light other than , which are of no interest for , might illuminate the surface which would unintentionally affect , whereas is only affected by .