DW> I have a pair of phototropic glasses, quite some time ago. I
 DW> wasn't very happy with them. (Is "phototropic" the right
 DW> word? I assume you mean materials that darken in strong
 DW> light, and vice versa.) I found they were very
 DW> temperature-dependent. The process of fading from dark back
 DW> to transparent happened much faster when they were warm than
 DW> cold, so *very* strong light was needed to keep them dark in
 DW> hot conditions, but they became dark in dim light when they
 DW> were very cold. Maybe they've improved.

Glass lenses of this type perform better in warm weather.  Plastic is the
opposite, performing better in cold weather.  Alas, I have the plastic
lenses and live in a warm climate.  Glass lenses are a special order item
now, and almost no one wears them anymore.  I prefer them for their scratch
resistance, but they are less safe and heavier.

 DW> At present, I use a pair of magnetic clip-on dark glasses,
 DW> which have polarizing lenses. Under most conditions, they
 DW> work very well. There are problems, though, when I try to
 DW> look at Liquid Crystal Displays, which also use polarized
 DW> light. I have to take them off to gas up my car, since the
 DW> display on the pump is a LCD one. Through the glasses, it
 DW> just looks black. I would have expected the designers of the
 DW> displays to think of this, and rotate the plane of
 DW> polarization 90 degrees.

I have a separate pair of polarized glasses that I use only for driving. 
The photo-sensitive lenses don't get dark enough when I'm driving, partly
because there is no direct sunlight striking the lens, and partly because
the car windows block UV rays.

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