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 d E i ( ω ω i ) , which are of no interest for f r ( ω ω i , ω ω r ) , might illuminate the surface which would unintentionally affect L r ( ω ω r ) , whereas d L r ( ω ω r ) is only affected by d E i ( ω ω i ) .