I don't have a UV light source (this is how i always exposed them while at uni) and I don't really have any money to buy one.
It doesn't have to be expensive, you can buy 2 fluorescent tubes and an armature for less than 40£. Those tubes last a long time. Don't bother with black light tubes though, they are too shielded to be of any good use, you need the ones they also use in aquariums and the like, maybe check a shop that sells stuff like that. I had excellent results with a setup like that and still use it. Alternatively you could pick up one of those facial tanning beds, those aren't terribly expensive if you look around (maybe even second hand will do).
Sure, I could give you a boring explanation who I really am but I rather let the Origami do the talking.