The incandescent lamps in every exit light I've ever seen have used either 15T6 candelabra base or 20T6.5 intermediate base lamp as the primary lamps (besides the occasional ones that take a medium base.)
The backups have used either #909 wedge base or #1594 double contact bayonet base 6 volt lamp.
Just replacing the entire unit isn't an option when you have a building standard to maintain.
I can't for the life of me figure out where a 2" diameter MR16 would be suitable for an exit fixture.
