A geometrical representation of the state of polarization of a wave of monochromatic radiation. The Stokes' parameters s1, s2, s3 can be regarded as the Cartesian coordinates of a point P on a sphere S, which has a radius s0. This representation means that every possible state of polarization for a plane monochromatic wave with a given intensity (meaning that s0 = c, where c is a constant) corresponds to one point on S and vice versa. This representation was introduced in 1892 by the French physicist Henri Poincaré (1854–1912) with S being called the Poincaré sphere. Applications of the Poincaré sphere include discussions of the polarization of light in crystals.