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
Reference: Otwinowski, Z. & Minor, W. (1997). 'Processing of
X-ray diffraction data collected in oscillation mode.' Methods
Enzymol. 276, 307-326.