From: luweitest@address.invalid   
      
   Hi all,   
   I am doing some transmission measurements using spectrometer PerkinElmer Lamda   
   750. It uses halogen lamp. The sample is normal soda-lime float glass. I find   
   a weird phenomenon: The result varies with the sample' position on the light   
   path. Specifically,    
   in the middle of the sample chamber where the light is more focused, the   
   measured transmission is smallest; move the sample towards the entrance or   
   exit, where the light spot gets bigger, the result gets bigger (continuously).   
   The maximum difference is    
   about 0.5%.   
      
   I cannot think of any reason caused this. The parallel light is ideal;  yet   
   near parallel light should be OK to use. The incidence angle should be the   
   same regardless of the position. The reflectance and absorption should be the   
   same regardless of the    
   light spot size. Should they?   
      
   The optical diagram of could be the spectrometer could be found here:   
   http://www.perkinelmer.com/CMSResources/Images/44-74449BRO_LAMBD   
   -750-UV-Brochure.pdf   
      
   --   
   Regards,   
   Lu Wei   
   PGP key ID: 0x92CCE1EA   
      
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