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 monochromator. This differs from the value of _diffrn_radiation.polarisn_norm 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. See _diffrn_radiation.polarizn_source_ratio.