XPost: sci.physics, sci.physics.electromag   
   From: gah@ugcs.caltech.edu   
      
   In sci.physics.electromag anorton  wrote:   
      
      
   (snip, I wrote)   
   >> That is for directly viewing an object. How about for an indirect   
   >> case, like imaging onto a ground glass viewing screen, and then   
   >> looking at the screen? In that case, it seems to me that a larger   
   >> lens does get you a brighter image, and also that it better   
   >> represents the case for IR viewers and image intensifiers.   
   (snip)   
      
   > Whether it is a CCD or ground glass, it does not matter.   
   > The energy per unit area in the image for an extended source   
   > depends only on the source brightness and the f/# at the   
   > image plane.   
      
   Yes. But for the direct view case, it is limited by the f/#   
   of the eye.   
      
   Say, for example, you image onto a screen that scatters equally   
   in all directions. (Ground glass probably isn't that equal.)   
      
   Now, in the indirect case the image as seen by the eye will   
   not be as bright as the direct case, as much of the light will   
   go the wrong direction. The light intensity on the screen,   
   though, is not limited by the eye f/# but only the imaging lens.   
      
   In the case of image intensifiers or IR viewers, the light is   
   imaged onto a photocathode or CCD array, and again not limited   
   by the eye f/#. The signal is then electronically amplified,   
   and converted back to a visible image. That image can have   
   higher intensity than the source, due to power supplied to   
   the conversion unit.   
      
   > (Actually that above link mainly does deal with the case of   
   > a detector rather than observing with the eye.   
   > They eye is not so different, it just introduces an extra   
   > aperture that cuts off light at a minimum f/# )   
      
   -- glen   
      
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