There are two common UV wavelengths, referred to as "long" and "short". I forget which is which, but one of them produces a visible purple light, whereas the other is not visible to the human eye (thus, the reason those are called "black lights"). The UV spectrum, is just that, a spectrum from a low to high frequency, so it is possible there is a specific wavelength within that spectrum that a special ink is made to ONLY fluoresce when that wavelength hits it.
There are two common UV wavelengths, referred to as "long" and "short". I forget which is which, but one of them produces a visible purple light, whereas the other is not visible to the human eye (thus, the reason those are called "black lights"). The UV spectrum, is just that, a spectrum from a low to high frequency, so it is possible there is a specific wavelength within that spectrum that a special ink is made to ONLY fluoresce when that wavelength hits it.
that's crazy but pretty cool. i've never heard of this.