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Quote from a friend: "Actually they aren't invisible, they look like shitty white ink tattoos under normal conditions and you can't really make it out a lot of the time, looks more like a scar. At least what I've seen, heard."
Yeah, the natural light photos make it look like the ink produces some fairly strong scarring, or at least raised skin. Still a cool effect, though, assuming you're under blacklights often enough to make it worthwhile.