|If you don't have a UV black-light, the postcards will still glow in the dark and of course be visible in normal light.
For better glow-in-the-dark results place postcards near a bright light source and allow at least 30 minutes for charging.
The brighter the light and the more time the postcards are exposed to it, the more energy they absorb and the more they will glow.
Additionally, the darker the room when you switch off the lights, the more pronounced the night glow effect will appear.
A black light source will charge the postcards better and make them glow more than a normal light source would.
Alternatively, you can use fluorescent lighting for charging (FT tubes or small CFL economy lamps).
Old incandescent lamps cannot provide sufficient charging as they do not emit UV light.