XPost: sci.physics, sci.physics.electromag   
   From: gah@ugcs.caltech.edu   
      
   In sci.physics.electromag Peter Webb  wrote:   
      
   (snip on IR night viewers)   
   >> They also do amplification, so, no, they can't be passive.   
      
   > Sure they can. You just need a lens or a mirror larger than the human pupil   
   > at the front end, and you will get gain. If you don't believe me, try   
   > looking at the moon through a large astronomical telescope at low   
   > magnification. Unfortunately you generally also get magnification, which   
   > depending on the application may be a bad thing. But passive devices can   
   > definitely amplify the light that you see.   
      
   But IR is lower frequency than visible light, so you can't use   
   something like fluorescence to do the conversion. There is a conversion   
   system with very low efficiency, but even that needs a power source.   
   (It is used for testing IR lasers, and other high power IR sources.)   
      
   The passive way to do up-conversion is with a frequency tripler,   
   but that has even lower efficiency. It works for a laser source,   
   not a reflection laser illuminated, and not from natural IR   
   sources.   
      
   -- glen   
      
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