XPost: sci.physics, sci.physics.electromag   
   From: jos.bergervoet@xs4all.nl   
      
   On 2/11/2012 2:19 AM, RLW wrote:   
   > On Fri, 10 Feb 2012 16:12:59 +1100, Peter Webb wrote:   
   >   
   >> "glen herrmannsfeldt"  wrote in message   
   >> news:jh26nn$ngj$1@speranza.aioe.org...   
   >>> In sci.physics.electromag RichD  wrote:   
   >>>> How do IR, night vision gogglles work?  I'll guess, much like digital   
   >>>> cameras - lenses, sensor, viewscreen, amplifiers and necessary   
   >>>> electronics.  My question is, could a unit be designed to operate   
   >>>> completely passively, just lenses and filters, i.e. no battery   
   >>>> required, like hand held binoculars?   
   >>>   
   >>> 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.   
   >   
   > There's a limit to how much the larger objective helps at a given   
   > magnification.  Run a Web search for "exit pupil".   
      
   No, don't. The IR is not coming out so the exit pupil   
   is irrelevant. IR-to-visible conversion would require   
   the IR to be sensed by some image detector. And then   
   there is *no* limit to how much a larger objective   
   helps (because the sensor has no pupil).   
      
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   Jos   
      
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