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I'm not the brightest person and I was wondering if it was possible, in the same way that it's possible to make ink that's visible under UV light, if it's possible to make ink that requires a distinct combination of colours of light to work. If there are different coloured UV lights would it be possible to make an ink (or buy one) that reacts only when all three are present and overlapping?

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You could make a pair of dyes that only glows when mixed. For example, you could mix a dye that absorbs deeper UV and emits longer UV with a dye that absorbs longer UV and emits blue, for example. Neither alone would glow very much under deep UV illumination, but when the two are mixed it would glow more brightly. I don't know how effective it would be, though, since fluorescence processes are inherently lossy.

However, to make something which only glows in response to a mixture of different light colors would be difficult through chemistry alone, primarily because it would presumably have to involve some kind of light-mixing to take place; the excited state lifetimes of typical molecules are on the order of nanoseconds or shorter, and as a result the light densities required for multiphoton activity to occur are absurdly high.

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