              Polarisation ratio of the diffraction beam incident on the\
               crystal. It is the ratio of the perpendicularly polarised to the\
               parallel polarised component of the radiation. The perpendicular\
               component forms an angle of _diffrn_radiation.polarisn_norm to\
               the normal to the diffraction plane of the sample (i.e. the plane\
               containing the incident and reflected beams).