[dsg-eic_dirc] exit sign inside the dark room

[Beni Zihlmann]

Hi All,
I just want to understand the purpose of the "EXIT" light in the dark room.
If I understood correctly this sign is there for safety reason in case 
of power loss. This means this light requires
separate power (ups/battery)? Also, since the purpose of the light is to 
be on when there is power loss, does that
mean that if there is power the light is not required to be on?
If that is correct than one can install a relay that switchers on power 
loss and turns on the light. while when there
is power the light is NOT on.
is that an acceptable operation mode?

cheers,
Beni