  The well-known Hong-Ou-Mandel effect is revisited. Two physical reasons are
discussed for the effect to be less pronounced or even to disappear: differing
polarizations of photons coming to the beamsplitter and delay time of photons
in one of two channels. For the latter we use the concepts of biphoton
frequency and temporal wave functions depending, correspondingly, on two
frequency continuous variables of photons and on two time variables $t_1$ and
$t_2$ interpreted as the arrival times of photons to the beamsplitter. Explicit
expressions are found for the probability densities and total probabilities for
photon pairs to be split between two channels after the beamsplitter and to be
unsplit, when two photons appear together in one of two channels.
