There is no requirement to carry backup lenses. I suppose if an employer has a requirement any lens would work but it would be easier to just put on a pair of glasses.
I believe it's an ICAO thing and you do need them in Canada, but yeah, I remembered hearing a long time ago that the FAA had no such requirement. I wasn't going to argue it with my company, tough. I guess I'll have to figure out whether or not contacts as backups works for them..
I operate in Europe and have been told that perception sunglasses fulfill the requirement. This was from one of our first officers that is very by the book.
Sent from my GT-I9100
Since we go to Canada I always have my backup glasses. One day leaving Medford, OR in the early AM, one of my contacts just started bugging the crap out of me. I pulled it off, and it was somehow slightly torn. My eye still felt a little uncomfortable, and the absolute last thing I wanted to do was stick another contact in there.
I put my glasses on, and everything was fine. I HIGHLY recommend not having contacts as your backup.