From: oh2hvj@sral.fi   
      
   "anorton"  writes:   
      
   > A related quantity is etendue. The overly simple definition is that it   
   > is the product of the area of a light beam and the solid angle   
   > diverging from it. As you trace rays that are not vignetted through   
   > the optical system, the etendue can not decrease. If you find that it   
   > is, then you are vignetting rays somewhere. Conservation of etendue   
   > basically says you can not squeeze light energy into a smaller   
   > "volume" in area*angle space.    The conservation is law can be proven   
   > by conservation of energy.   Etendue can increase due to scattering   
   > for, example, but if it does, then it means your downstream optical   
   > system needs to handle more etendue (e.g. larger aperture, smaller   
   > f/#) if you do not want to lose light.   
      
   Actually conservation of etendue what I was thinking of; how to explain in   
   simple terms   
   that light from multiple large-area LEDs cannot be focused through a   
   small-diameter fiber. I remember having read the proof by conservation of   
   energy, I just can't get it into my head anymore or find it from anywhere!   
   I think that one was quite simple and should be understandable for my   
   client!   
      
   --   
   Mikko   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)