1. A polarisation image detection system comprising: a lens system having an optical axis and arranged for receiving incident radiation from a viewed scene and for producing a focused image thereof; imaging detector means positioned on the optical axis at a focused image location for detecting and outputting a radiation level of said focused image; linear polarizer means and retarder means both located on the optical axis and each having a maximum electric vector direction associated therewith transverse to the optical axis; means for varying the optical setting of the system by doing at least one of a) rotating the electric vector direction of the polarizer means relative to the detector means, b) rotating the electric vector direction of the retarder means relative to the detector means, and c) varying the retardance of the retarder means, the imaging detector means for detecting and outputting radiation levels of said focused image corresponding to said varied optical settings; storage means for recording the outputs of the imaging detector means corresponding to the varied optical settings of the system; and computing means for calculating, from the recorded outputs, the degree of polarisation, the polarisation factor and the polarisation phase angle of the viewed scene.