The quantity (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
This is an average or other representative sample of the
This is the second of the Stokes polarization parameters,
I, Q, U, V [also known as I, M, C, S; see Berry et al. (1977)].
If the absolute intensity is not known, the value 1.0
is assumed for I, and all four Stokes parameters are
dimensionless. When the absolute intensity is known,
all four Stokes parameters are in units of watts per
Berry, H. H., Gabrielse, G. & Livingston, A. E. (1977).
'Measurement of the Stokes parameters of light',
Appl. Optics, 16:12, 3200 -- 3205.