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We investigate on the properties of the P-representation of the density matrix of a two-mode e.m. field which simulates a quantum configuration in which the two field components have very peculiar properties concerning the phase-difference between them. This analysis gives the opportunity to introduce and to verify the effectiveness of a visibility operator which in our opinion is able to describe in the quantum case the phase correlation existing between the two fields and to lead to a degree of internal coherence which allows to evaluate this correlation in a quantitative way.