So we have been using these 0.3 ND (1/2 power) inconel   
   filters as beam splitters and 90 degree phase shifters   
   in a michelson interferometer... we also pick off the   
   reflected beam to get the other phase (sine and cosine).   
   Sometime several years ago someone-sigma changed the coating   
   to chromium, but didn't tell us.  We can still get inconel   
   ND from elsewhere.  So that OK   
      
   There is some uncertainty about where the   
   phase shift is, at the inconel/ air(glass) interface,   
   or is it the absorption in the inconel itself.   
   The fact that a 90 degree shift happens at 0.3ND   
   seems to point directly at absorption to me....   
   Is there something I might be missing?   
   0.3 chormium has less absorption...   
   hah! use some numbers to test theory,   
   what a concept.   
      
   George H.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)