The rear-view mirror in my car has got one of those flick switches which moves the angle; there is a secondary, semi-reflective coating so that you can see the ghost of the headlights behind you, without the full glare. Theoretically useful, but practically pointless as, when correctly adjusted, all the light comes in from the wing mirrors anyway, so you still get dassled. Headlights are getting brighter as we move from incandescent technology, so I find this an increasing problem.
So, my idea is this. First, in front of the mirror, we place a light-reactive surface, perhaps segmented off, but something that generates a (very low) voltage when light hits it. This is then connected to a liquid crystal film in from of it, which darkens when the voltage is applied to it. A little bit of plumbing and we should end up with a system which dims overly-bright headlights, specifically in the area where the headlights are. The rest of the mirror would be unaffected. Because the light-reactive film is behind the liquid crystal, it should even have self-regulating feedback system.
Questions left in my mind: is there enough energy in the light to produce a voltage that will switch a liquid crystal; would the darkened headlights just look further away and confuse the driver; how to stop the wing-mirrors going black in the day?