Two different products get called emergency exit lights. One is an illuminated sign that tells a person where the exit is. The other is a luminaire that puts light on the floor so the person can get there. A route needs both, and buildings frequently have one and assume it covers the other.
Illuminated and photoluminescent signs
An exit sign has to be visible when the normal lighting has failed. That is achieved either by an internally illuminated sign with its own battery, or by a photoluminescent sign charged by the normal lighting and readable in the dark for a period, or by an externally lit sign covered by an emergency luminaire. Which is appropriate depends on the space and on how long the route has to remain identifiable.
Escape route luminaires
These put a specified illuminance along the centre line of the escape route so that people can see the floor, the stairs and the obstacles. A sign that shows where the door is, in a corridor too dark to walk down, has not solved the problem. The standard sets where luminaires are required, and stairs and changes of level are the locations where their absence is most dangerous.
Where each is required
Signs at every final exit and at every point where the route direction is not obvious. Luminaires at each exit door, at stairs so each flight is lit, at changes of direction and level, at intersections, outside the final exit, and at fire fighting equipment and call points. Open areas of any size need anti panic lighting so people can find the route at all.
Testing, which is the same for both
Both are on the same monthly function and annual duration regime, because both depend on a battery. An illuminated sign that has lost its battery is a sign that is dark exactly when it is needed, and it looks entirely normal while the mains is on.
Questions people ask about emergency exit lights
Are photoluminescent signs enough on their own?
They identify the exit for a period after the lights go out. They do not light the route, so they supplement escape route luminaires rather than replace them.
Do emergency lights need to be on all the time?
Maintained luminaires are; non maintained ones are dark until the supply fails. Both are acceptable and the choice follows the use of the space.
Does the outside of a final exit need lighting?
Yes. A route that ends in an unlit yard has not reached a place of safety that anybody can use.