From: astroelectronic@t-online.de   
      
   Phil,   
      
   > If you only rotate one side, you're then in mixed coordinates, which is   
   > very rarely what you want.   
      
   What I wanted to do is rotating the observer by 90 degrees, so that horizontal   
   polarized light becomes vertical polarized light, and vice versa.   
   I thought that in this case only one rotation is required. If the input   
   polarization state is expressed as a vector, the results are as expected.   
   But if the input polarization state is expressed as a matrix (formula 6.20),   
   the result is wrong.   
   I thought that rotating the matrix from both sides is only required if a   
   component (polarizer or waveplate) is rotated.   
      
   Michael   
      
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