The quantity (Ip-In)*sin(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. This is an average or other representative sample of the frame. This is the third 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 square metre. Reference: Berry, H. H., Gabrielse, G. & Livingston, A. E. (1977). 'Measurement of the Stokes parameters of light', Appl. Optics, 16:12, 3200--3205.