function L=Ltransf(x)
% transformation for Stokes parameter,see Liou book, 322p
% angles in radians

L=[1, 0,        0,        0;...
   0, cos(2*x) ,sin(2*x) ,0;...
   0, -sin(2*x),cos(2*x) ,0;...
   0, 0,        0,        1];