They use Luminescent material such as on the hands of the watch to line up the reflector portion of the bulb. Since the light stays on, the material is being "charged" and when light goes out it will emit the "stored" energy. The idea is that since the Luminescent is the most productive right after it was charged and office buildings keep the lights on all the time so then when somethign happens (like blackout) you will get the brightest light you can get for free. It is safe (don't know about biodegradability and disposal) and looks good and prevents panic. I think the biggest catch is that it is not suitable for home use since we dont keep our lights on all the time and last blackout cought NYC at 4 pm in full daylight.