Photoluminescent materials (commonly called “glow-in-the-dark”) has the ability to absorb light sunlight, fluorescent, incandescent, etc) and then to emit light. When ambient darkness occurs, it becomes highly visible, lasting at least 10hours. Upon removal of the light source, the stored light is gradually released; the strongest glow is produced during the first 30 minutes of darkness, the most critical period following a power failure or other emergency. Then it fades over a period of time, when there is light again, it stores energy again. So the photoluminescence process is circulatory.