During a lab dinner at Kabuki, Professor Shuhai Xiao and I started discussing about fluorescent watches - what was the principle behind these watches. So I read up a bit about glow in the dark watches. This is what I found at some sites -
"Tritium paint on watches is a mixture of tritium and phospor. Tritium is naturally radio-active and needs no external source of light or charge to work. Tritium does not glow. As it decays, tritium emits beta radiation, which is a bunch of excited electrons that in turns excite the electron in the phosphor atoms making them emit photons, or light, as they return to their ground (non-excited) state: the phosphor GLOWS. Phosphor can also be excited by UV light from the sun or other light sources. Thus, the tritium paint relies on tritium radioactivity to make the phosphor glow in the dark, not any charge from external light source."
Thus I think the watches glow based on the principle of phosphorescence which is slightly different from fluorescence. And the energy source is the radioactive material.
Another explanation at How Things Work: http://science.
Occasionally you will see something glowing but it does not need charging. The most common place is on the hands of expensive watches. In these products, the phosphor is mixed with a radioactive element, and the radioactive emissions energize the phosphor continuously. In the past, the radioactive element was radium, which has a half-life of 1600 years. Today, most glowing watches use a radioactive isotope of hydrogen called tritium (which has a half-life of 12 years) or promethium, a man-made radioactive element with a half-life of around three years.
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