The polarization state of an electromagnetic wave can conveniently be described by a set of Stokes parameters. On the left, the polarization ellipse describes the motion of the optical electric field () in a plane transverse to the light propagation direction. On the right are the corresponding Stokes parameters defined in terms of intensity difference measurements

,

,

,

where the intensities of the polarization components (: horizontal, : vertical, ±45: along diagonals, : circular left, : circular right) are normalized to the total light intensity. The parameters that you can vary are the orientation of the ellipse, its degree of linear polarization , where and are the intensities measured along the major and minor axes, and its handedness (direction of rotation). For 100% polarized light, the endpoint of the Stokes vector lies on a unit sphere called the Poincaré sphere.