Left watch under desk lanp for three full 8 hr. nights!
At one time luminescent dials were made with Radium Luminous paint with a half-life of 1600 years so the "glow" didn't go away soon. And getting enough of the material to cause it to glow sufficiently was dangerous to the wearer. Since then Tritium mixed with zinc sulfide was substituted. Please see: http://wornandwound.com/2013/08/23/seeing-in-the-dark-luminescence-in-watches/
Later, non-radioactive materials were used. Unfortunately, all these materials decay and are less efficient. If you have a small flashlight you can hold it to the face of the watch and it will then get brighter...for a while. But, soon it will fade again. Holding the watch under a light for a long time won't help. The reason is that the material can't hold a "charge" and the excited materials have lost much of their ability to luminesce.
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