Exposure to actinic light is best, although usually impractical.
Actinic light is high in UV component, and sunlight is a good source. Not so helpful if you want to go to bed at night and that's why you need the lume fully charged, since the sun has probably gone down hours ago.
Alternatives are fluorescent light, which doesn't have a huge UV component but it's better than most incandescent sources (bulbs with a filament). However I believe that halogen lamps are pretty good.
It's all about getting energy into that phosphor, the lume pigment. The more intense the light is, the better, so remember the inverse square law (light intensity decreases by the square of the distance from the source).
For photography, charge the lume with a blacklight, as suggested above. One of the small handheld units will do the job okay, even the little keyring units do an allright job. If it's a big UV lamp, pay close attention to the safety dirrections -- you do not want to get that UV light in your eyes.
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