The quantity (Ip-In)/(Ip+In), where Ip is the intensity (amplitude squared) of the electric vector in the plane of polarization and In is the intensity (amplitude squared) of the electric vector in the plane of the normal to the plane of polarization. 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 is to be taken as the XZ plane and the normal is parallel to the Y axis. Thus, if there was complete polarization in the plane of polarization, the value of _diffrn_radiation.polarizn_source_ratio would be 1, and for an unpolarized beam _diffrn_radiation.polarizn_source_ratio would have a value of 0. If the X axis has been chosen to lie in the plane of polarization, this definition will agree with the definition of 'MONOCHROMATOR' in the Denzo glossary, and values of near 1 should be expected for a bending-magnet source. However, if the X-axis were perpendicular to the polarization plane (not a common choice), then the Denzo value would be the negative of _diffrn_radiation.polarizn_source_ratio. See http://www.hkl-xray.com for information on Denzo and Otwinowski & Minor (1997). This differs both in the choice of ratio and choice of orientation from _diffrn_radiation.polarisn_ratio, which, unlike _diffrn_radiation.polarizn_source_ratio, is unbounded. Reference: Otwinowski, Z. & Minor, W. (1997). 'Processing of X-ray diffraction data collected in oscillation mode.' Methods Enzymol. 276, 307-326.