From: sam@repairfaq.org   
      
   Lu Wei  writes:   
      
   > On 2015-10-14 5:58, Samuel M. Goldwasser wrote:   
   >> ...   
   >> To the original poster: You can probably pretyy much ignore what Phil   
   >> and I have said.  If your path length difference is small compared to   
   >> 150 mm (the cavity length of the 1107 tube), then it will be fine. ;-)   
   >>   
   > Thank you for all responded with helpful comments. I measure a glass   
   > substrate with the ellipsometer, , and my concern is whether the laser   
   > reflected from the front side and back side will interfere and take into   
   > account as one light. Glass thickness will not exceed 5mm, so I can   
   > safely take that for granted. But the ellipsometer still does not show a   
   > regular pattern (eye-glass shaped) like one-side reflection, which I am   
   > still confused.   
      
   They will interfere, but the result won't be detectably different than   
   if the laser had an infinite coherence length because it is such a small   
   difference.   
      
   Not knowing the specs of your instrument, I can't say whether that's a   
   problem.  Do you have the manual?   
      
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