Ip+In, where 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.
This is an average or other representative sample of the
This is the first of the Stokes polarization parameters,
I, Q, U, V (also known as I, M, C, S). See
H. H. Berry, G. Gabrielse, A. E. Livingston (1977),
"Measurement of the Stokes parameters of Light",
Applied Optics, 16:12, 3200 -- 3205.
If the absolute intensity is not known, the value 1.
is assumed for I, and all 4 Stokes parameters are
dimensionless. When the absolute intensity is known,
all 4 Stokes parameters are in units of Watts per