->  DW> I am sure that this would work, as would several other things
I dreamed 
->  DW> up, *if* there were lenses that would darken and clear fast enough. 
 
-> Err, I'm far from up-to-date in these fields,  but isn't there  
-> some sort of fast-switching polaroid that'd be worth trying ? 
  
There are lenses that will switch very fast, but they have to be 
electrically driven. Basically, an electric field is applied across a 
material that rotates the plane of polarization of light, the amount of 
rotation depending on the electric field. This whole thing is put 
between two polarizers. Depending on the electric field, light can 
either pass through both polarizers or not. This sort of thing is used 
in, for example, high-quality 3-D movies and TV. The two images, for 
the two eyes, are put on the screen in rapid alternation, and the 
viewer wears glasses that are switched in synchronization, so each eye 
sees only the images that are meant for it. This sort of thing can 
certainly work up to frequencies in the kilohertz range. I don't know 
if it could handle megahertz. 
  
But the electric glasses are cumbersome and expensive. Fast 
photochromic ones would be a lot better, for the applications I had in 
mind. 
  
                         dow 
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