Here is a small sound clip I recorded of Bob Gale's audio commentary on BTTF 1 blu-ray, talking about the glasses. Not much info, but this is what he says.
http://vid22.photobucket.com/albums... future sunglasses sound clip_zps6dyppgws.mp4
'' My recollection is that we had some kind of a promotional deal with whatever company made these sunglasses. It's the only time in the movie we see him wearing sunglasses and we put them in here to do some kind of a contractual obligation and we never see them again, because shooting with mirrored sunglasses is a major pain in the ass.''
And I still have the same question I had three years ago from page 1, is Bob referring to the Zeiss frames or the Porsche eyewear as originally scripted? Either way, the word mirror could also describe a highly reflective gloss black surface, which would also be a pain to shoot. He only remembers the production difficulties and not the product specifics. Where is the Zeiss advertising for Back to the Future?
Mirrored outside? Definitely looks like it. Mirror inside? Not so much.