  Recently, Torgerson and Mandel [Phys. Rev. Lett. 76, 3939 (1996)] have
reported a disagreement between two schemes for measuring the phase difference
of a pair of optical fields. We analyze these schemes and derive their
associated phase-difference probability distributions, including both their
strong and weak field limits. Our calculation confirms the main point of
Torgerson and Mandel of the non-uniqueness of an operational definition of the
phase distribution. We further discuss the role of postselection of data and
argue that it cannot meaningfully improve the sensitivity.
