The quantum description of optical phase

It is shown that the Susskind-Glogower phase states and the Pegg-Barnett Hermitian phase operator lead to equivalent descriptions of optical phase, via a (well defined) positive-operator-valued measure. Thus, the observable conjugate to the photon number exists only in a generalized sense. The Pegg-Barnett formalism is clarified and some misapplications in the literature pointed out.