From: rrllff@yahoo.com   
      
   On Thursday, December 10, 2015 at 3:12:50 AM UTC-5, Lu Wei wrote:   
   > 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_LAM   
   DA-750-UV-Brochure.pdf   
   >    
   > --    
   > Regards,   
   > Lu Wei   
   > PGP key ID: 0x92CCE1EA   
      
   The only possibility that comes to mind is that there may be some wedge or   
   power in your sample.   
      
   http://www.richardfisher.com   
      
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