Active Paint

The painted lane markings on highways usually have tiny (around 500 micron) glass beads sprayed on top. Their refractive index is carefully chosen so that when light shining on them refracts and reflects inside them, most of it is reflected directly back at the source, so the paint glows brightly in your headlights.

Glass beads covered in rainwater effectively have their refractive index reduced by 3/4 (the inverse of the refractive index of water), ruining the effect. This is why you have such trouble seeing the lines on the pavement on a rainy night. You'd think you could fix this with two populations of glass spheres with different refractive indices, except that there is no glass with a high enough index (note that except for a fiscal difficulty, diamonds should work for this.)

Active paint takes this simple idea and gussies it up with too much speculative technology, producing a truly frightening result.

In any particular direction, we can simulate the effect of a retroreflective sphere with a simple circuit containing a photodetector, a light emitting diode and means for energy storage (a battery or capacitor). These gadgets are designed to photoelectrically charge up their little batteries during the day, and to shine their LEDs at night whenever they detect incoming light -- the photodetector and LED have little lens systems to make them directional.

Using modern integrated circuit technology we can make these into tiny specks (a few dozen microns on a side) and disburse (or disperse!) them in a paint binder. The result is retroreflective paint that works even in the rain.

But wait, there's more! We can put a microcontroller in each speck and program these gadgets to do all sorts of things. I suggest using coded pulses of light to program them. We can use a video projector to program the paint differently in different places. We can use this to make traffic signs that are only readable from the appropriate direction, or that give different messages to people in different places. Imagine a rotating ball that presents a different image in each direction, so the surface appears to animate as it rotates.

We could make walls that only show images when they receive an appropriate light-pulse code, perhaps reacting to small lapel-worn LED beacons. If the specks can communicate with one another, they can be made to do image recognition and figure out who is looking at them and beam an image back only in the appropriate direction, allowing you to make messages that are truly For Your Eyes Only.