The standard uncertainty (estimated standard deviation, e.s.d.)
of _diffrn_radiation.polarizn_Stokes_Q,
(Ip-In)*cos(2*theta), where Ip is the intensity
(amplitude squared) of the electric vector in the plane of
polarization, In is the intensity (amplitude squared) of
the electric vector in the plane of the normal to the
plane of polarization, and theta is the angle as viewed
from the specimen, between the normal to the polarization
plane and the laboratory Y-axis as defined in the
AXIS category.