Bob, I have not really looked into photochromic (technical name) face shields for helmets, so I cannot give you much insight there. I believe most shields are made of poly(carbonate) and until recently, maybe 4 years +/-, transitions (brand name of plastic photochromic lenses for your glasses) were not available for poly. We now can put transitions in poly, though, so I don't see why it couldn't be put in a shield.
Transitions lenses have been continuously updated and improved in the last 15 or so years. They are now almost, but not quite as good as the old/original photochromic, glass photogrey. I mean they don't quite darken as much as the old photogrey lenses but are close. They do lighten indoors or at night as much or more than photogreys to nearly 100% clear. IMO, neither is as dark or comfortable to wear as a pure sun lens that is dark all the time.
Up until this year I wore a 3/4 helmet and wrap around type glasses with transitions lenses in them, so I was good to go in sunlight or dark. I've switched to a full face this year and now just carry a separate pair of sunglasses (I prefer grey polaroid lenses) if it is real bright. If a person wore prescription glasses, I suppose you could wear transitions under a FF helmet, however, I'm not sure how well they would darken in sunlight with the shield closed. You must keep in mind the mode of action of any photochroic lens, it needs direct UV exposure to darken the lens. If your shield blocks a significant %age of the UV, and it probably does, you won't get as much darkening of the lens under it. Same thing happens in a car, unless you drive a convertible
Whew, sorry for the long-winded reply, hope that helps.