The standard uncertainty (estimated standard deviation, e.s.d.)
of _diffrn_scan_frame.polarizn_Stokes_I,
Ip+In+Inp, 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 Inp is the intensity (amplitude squared) of the
non-polarized (incoherent) electric vector.