  In a series of articles we have shown that all parametric-down- conversion
processes, both of type-I and type-II, may be described by a positive Wigner
density. These results, together with our description of how light detectors
subtract the zeropoint radiation, indicated the possibility of a completely
local realist theory of all these processes. In the present article we show how
the down-converted fields may be described as retarded fields, generated by
currents inside the nonlinear crystal, thereby achieving such a theory. Most of
its predictions coincide with the standard nonlocal theory. However, the
intensities of the down converted signals do not correspond exactly with the
photon pairs of the nonlocal theory. For example, in a blue- red down
conversion we would find 1.03 red "photons" for every blue one. The theory also
predicts a new phenomenon, namely parametric up conversion from the vacuum.
