On Thursday, October 15, 2015 at 4:21:37 AM UTC-4, Lu Wei wrote:   
   > On 2015-10-15 4:19, Samuel M. Goldwasser wrote:   
   > > 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?   
   > >   
   > Yes I have the manual.  The situation I am considering is not common in   
   > ellipsometry, because normally an optically semi-infinite substrate is   
   > used. If the substrate have a thickness less than coherent length, then   
   > it should be treated as an layer. But in my situation, that layer is too   
   > thick (mm instead of normally nm), and laser is the light source, so the   
   > reflection beams from front and back side actually split apart. So   
   > interference may just happen at their intersection, and the signal   
   > analyzer may get is the superposition of two incoherent lights and a   
   > interfered light.  That seems really complicated.   
   >   
   > The measurement situation:   
   > http://postimg.org/image/44b1ze9hb/   
      
   Can you put a small wedge on the substrate?   
   Roughen (sand blast) the bottom to get rid of the reflection?   
      
   Careful placement of a bit of black tape to block the second reflection.   
   (Yeah that's hard when the angle is changing...)   
      
   George H.   
   >   
   > --   
   > Regards,   
   > Lu Wei   
   > PGP key ID: 0x92CCE1EA   
      
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