They have "Transitions" face shields for helmets. They are the same "Transitions" (I think that is a brand name) that you can get from your eye doctor. They turn dark when they sense UV.
So you can wear your prescription glasses under your helmet and then you'll get the sunglasses effect when the UV is out. I think the shield is $175.
Just like the transitions eye glasses, they only turn dark when they sense UV rays. So on an overcast day where there is alot of glare but not alot of sun, they'll stay clear. Some people prefer this while others would prefer they got dark, or would wear sunglasses on those days. Kinda personal preference.