-> G'morning Herman and David, 
 
-> And thanks for the conversation on glasses. 
 
-> I used to have a pair of super-duper poloroids in my youth (before  
-> LCD displays!), but when they were stolen I'd got over their  
-> status-enhancing benefits and never replaced them. 
 
-> Worrying now about UV damage is a bit pointless, I guess, but the  
-> idea of phototropic lenses is intriguing;  I shall enquire of my  
-> trusty optician of the current capabilities of present-day  
-> offering (a new series of ads claiming on-demand coffee, fame and  
-> girls as well as instant protection set off my original mention  
-> here). 
 
-> Temperature variation had not occured to me,  much less the UV  
-> filtering effect of car windows... 
 
-> A hat with a proper brim might be a better aid. 
 
-> :-) 
  
I found that the correct word is "photochromic" - becoming 
(dark-)coloured in response to light. "Phototropic" is used to describe 
plants that bend toward a source of light. 
  
Many types of glass are pretty good at absorbing UV, even ones that 
appear completely transparent to visible light. I'm not sure if 
photochromic lenses become better UV-absorbers when they darken, or 
whether they are just always like that. 
  
In England, in my youth, it was possible to buy a special kind of 
window-glass called "Vitaglass", which *transmitted* UV. This allowed 
you to get a tan indoors, which was considered to be a good thing back 
then. 
  
Brimmed hats are good at protecting the face from UV coming from above, 
but are useless against UV that is scattered from the ground, snow, 
water, etc.. I often wear a hat *and* dark glasses, except when the 
temperature is so high as to make wearing a hat uncomfortable. Then I 
carry a parasol. 
  
Polarizing sunglasses have never gone out of style here. I think 
they're a great idea. 
  
                        dow 
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