Re:Don't copy machine have this feature too? (Score 2, Informative)

The government has mandated that all copiers user a particular frequency of light when they copy. Every copier/scanner is supposed to use that frequency. When that frequency of light is put onto our US currency, I have forgetten whether it's because of the ink, or perhaps a pattern they have on the bills, but in any case, they won't show up.

There's all sorts of hidden things in the currency, especially the new one, hidden steganographic details etc, to make them harder to counterfeit.