Battery Backup
Products with battery backup contain a battery so in the case of a power outage the lights or signs with stay illuminated for roughly 90 minutes.
Steel
Steel Exit Signs
Concealed
Concealed or Hidden emergency lights will be completely hidden from view and in the case of a power outage or emergency they will open and turn on. If you do not need or want to completely hide your lighting you might just want to look at recessed emergency lights.
Black
Most product that have black housing color are made of a thermoplastic material.