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.
-- td, Feb 02 2001

Scotchlite http://www.3m.com/market/safety/scotch/
3M Corporation markets a line of micro-beaded materials (fabric, adhesive tape, etc.) called Scotchlite that are used to make highway signs, safety clothing, etc. [td, Feb 02 2001, last modified Oct 21 2004]

Pressure sensitive paint http://www.halfbake...20sensitive_20paint
Paint whose color changes as pressure is applied to it. [td, Feb 02 2001, last modified Oct 04 2004]

Temperature Sensitive Paint http://www.halfbake...20Sensitive_20Paint
Made with thermochromic pigments? [td, Feb 02 2001, last modified Oct 04 2004]

Programmable House Paint http://www.halfbake...ble_20House_20Paint
Adjust the color of your paint after the fact. [td, Feb 02 2001, last modified Oct 04 2004]

Smart Dust http://robotics.eec.../~pister/SmartDust/
They tend to use modulated corner reflectors. [egnor, Feb 02 2001, last modified Oct 21 2004]

A rotating ball that presents a different image in each direction--KEWL. I need a better monitor to see that, graphically.
-- reensure, Mar 16 2001


Have you read Vernor Vinge's _A Deepness in the Sky_?
-- wiml, Mar 16 2001


Definitely speculative, but interesting (once I finally figured out what the heck you were describing).

There are pictures that change depending on the angle you use to look at them; as I understand it they're made from vertical cylinders of plastic (rather than spheres), but otherwise much like you describe (without the miniaturized electronics).

If you just want a better retroreflector, there are other options, of course (and they are already generally deployed in situations which warrant them).
-- egnor, Mar 16 2001


In Australia they are trialling some new Cat-Eye reflectors which flash to warn of slippery/icy roads. (BlackIce) I assume they have little humidty/temperature sensors in them and tiny solar cell and LEDs. Being a motorcycle rider, I think they're really cool.
-- goodie, Mar 16 2001


That sounds interesting, [goodie]; can you dig up a link?
-- egnor, Mar 16 2001


Mom! The lane markers are staring at me again!
-- nick_n_uit, Mar 18 2001


The multidirectional display can be implemented via holography (albeit with no way of controlling who sees it). My vote would be to make the lane indicators thicker (higher off the pavement) in addition to more visible so that even if you can't see them, you can feel them through the tires.

By the way, anybody know who the genius was who thought it would be a good idea to make lane indicators *more* difficult to see on rainy nights?
-- phoenix, Jun 25 2001


//In Australia they are trialling some new Cat-Eye reflectors which flash to warn of slippery/icy roads//

In the parking lot where I work there are idicators that turn blue when there is a risk of "black ice." So far as I can tell they are just temperature reactive chemicals. This is in Southern Canada.

This all sounds great, but if //fiscal difficulty// is a problem would all this microprocessing not cost more than diamond dust?
-- LED Prism, Oct 02 2006



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