  Quaternions have been used to represent polarization states and polarization
operators. But so far, only polarizers, dichroic or non-depolarizing devices
have been represented in that way. We propose a quaternionic representation of
perfect as well as partial depolarizers. It leads us to design actual setups
for producing natural (unpolarized) light from an arbitrary partially or
completely polarized wave or for reducing the degree of polarization of a wave.
We conclude by making the link with Azzam's polarization orthogonalization
problem.
