The angle in degrees, as viewed from the specimen, between
the normal to the polarization plane and the laboratory Y
axis as defined in the AXIS category.
Note that this is the angle of polarization of the source
photons, either directly from a synchrotron beamline or
from a monochromater.
This differs from the value of
in that _diffrn_radiation.polarisn_norm refers to
polarization relative to the diffraction plane rather than
to the laboratory axis system.
In the case of an unpolarized beam, or a beam with true
circular polarization, in which no single plane of
polarization can be determined, the plane should be taken
as the XZ plane and the angle as 0.