Any polarisation state of light can be represented by a Stokes' vector, red, lying on (fully-polarised) or in (partially-polarised) the sphere. The cartesian co-ordinates of the vector are given by Stokes' parameters [1], {S1,S2,S3}, purple; the end points of the axes are the horizontal/vertical {h,v}, diagonal/anti-diagonal {d,a} and right/left-circular {r,l}, polarised states, respectively. Alternatively, the vector can be represented in spherical co-ordinates by a length, P, and two angles, θ, φ, green. The vector length is the degree of polarisation, ; θ is the longitude, and φ is the latitude. For linearly polarised light, φ = 0°; for circularly polarised light. φ = 90°.