  As known, the degree of entanglement of biphoton states with respect to the
transverse components of photon wave vectors (momenta) or coordinates can be
characterized either by the parameter K associated with the Schmidt
decompositions, or by the parameter R, defined as the ratio of the widths of
the unconditional and conditional single-particle distributions of biphoton
states. As entanglement is a fundamental characteristics of a state, it must be
independent of a choice of its different representations, i.e. its entanglement
parameters must be identical in the coordinate and momentum representations.
Likewise, entanglement itself and its characterization parameters must remain
constant along the photon propagation length. It's known, however, that only
the parameter K obeys these requirements, whereas the parameter R in the
coordinate representation deviates from K beyond the near zone and, thus,
becomes inapplicable as a measure of the degree of entablement. But, as shown
below, there is a way of saving the parameter R by means of modifying its
definition in such a way that the modified parameter R turns out to be as good
as the parameter K both in the momentum and coordinate representations, as well
as in the near and far zones and everywhere between them. We will discuss also
possible ways of measuring this newly defined parameter R experimentally.
