[David Cooper (OP)]

I've known for decades that highlighter pens have fluorescent ink that's brighter than the ink in normal pens, but had no way of exploring it properly, until now. I wouldn't have thought of trying it either, but by an accident of luck I had some of my highlighted notes under some Christmas lights when the normal room lights were switched off, and I noticed a greater fluorescence than normal. These particular Christmas lights have four colours of LEDs: red, orange, green and blue. If you point an individual light at the paper with the fluorescent ink on it, for three of the colours of LED it does nothing special, just lighting up the whole page in the colour of that LED. However, one colour of LED does something startling, making all the different colours of the ink light up, even though only one single frequency of light is falling on them. It's nice to see, so give it a go.

[evan_au]

I assume that it is the blue LED that produces the fluorescence?

Gallium Nitride LEDs emit at the blue end of the spectrum (near ultraviolet), and the chemicals in the highlighter pen can downconvert these photons to longer wavelength photons where your eye is more sensitive.

See: https://en.wikipedia.org/wiki/Light-emitting_diode#Blue_and_ultraviolet

[David Cooper (OP)]

I assume that it is the blue LED that produces the fluorescence?

Indeed, and it works best with the orange ink - it's weird seeing the orange glowing brightly when there's only blue light landing on it.