Typically the re-emitted light is not the same color as the absorbed light, and Step 6 has some pictures, and explanation, of how that works. But it doesn't matter if you slept through Quantum Mechanics, or even if you never had to take that class in the first place. For this 'ible, it is enough to know the color of the absorbed light is blue, provided by a blue LED, and the color of the re-emitted light from the GITD material is green. Visually the effect is very pretty. Especially if you like the colors blue and green.
Aside from the underlying physics, and using big words like "phosphorescent", the next hardest part is actually finding phosphorescent (aka GITD) material, in the form of a flat sheet big enough to doodle on. The best thing I found was this: glow-in-the-dark printer paper. Yes, they actually make GITD printer paper. However there's no monger who sells it near where I live, so I had to buy it from an online retailer. There's more about sources for GITD paper in Step 4.
Another trick is fitting a blue LED into the tip of the body of what used to be a ballpoint pen. As an added complication I want the LED to turn on only when it is pressed against the pad. That way it "marks" in the same manner as a pen or pencil, which marks only when pressed against the paper. If the LED is always on, I get an effect that is more like spray-paint, and that isn't the effect I want.
Another feature I want is for this contraption to be powered from a single AA battery. To facilitate this, I hack apart a cheap single-cell LED flashlight, and pull out its little magic LED-driver module, and then insert said module into my circuit. See Steps 8, 10 and 11, for more on the magic LED-driver module.
When I put all these elements together: GITD sheet, plus clipboard, plus page protector,plus blue LED, plus hacked LED driver module, plus pressure actuated switch... the result is the Phosphorescent Notepad.
It's a toy that I can use to draw doodles in the dark. And it's great fun at parties! Particularly those parties that happen at night.
The last feature/flaw: Drawings on the Phosphorescent Notepad erase themselves!
Given a few minutes the GITD material fades back into equilibrium with the ambient light/darkness. The drawings don't last, unless I take a picture of them, and that is somewhat tricky because it must be done in low light. A picture of a few "saved" doodles are attached to this Intro, and Step 13 which explains the trick to taking these pictures. (Note: If the camera flash goes off, the resulting flood of white light obliterates the drawing. Thus, pictures of phosphorescent doodles must be taken with the camera's flash function turned off.)