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First, apply a UV-absorbing (or reflecting) coating on the car's windshield.
Then, on top of that (on the side towards the inside of the car), apply a coating that is clear with respect to visible light, but which will visibly flouresce when illuminated with UV light.
Put whatever images you want
onto this surface with conventional light projection techniques, but using UV light instead of visible light. The projector can be anywhere in the car, since the flourescent chemical is omnidirectional.
To get a full color, one would use three types of clear but flourescent dye, which glow red, green, and blue respectively; these would be applied as dots on the windshield. The projector still only uses one color -- UV light, but it picks which dots to illuminate.
I have no idea how they work (I briefly tried to find out), but you can see through them from the inside of the car. [Zimmy, May 04 2009]
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||/First, apply a UV-absorbing (or reflecting) coating on the car's windshield./
||I suggest flour. Which is on my mind for some reason. Or maybe Bisquick.
||...and in other news, owners of [goldbb] Industries UV HUD devices are being advised to return their cars to the dealer for a refit and complementary cataract operation.
||UV at wavelengths just short of visible is not dangerous (or at least not more dangerous than blue light, which is a little dangerous to the eyes. I am not sure why this is not already used. [+]
||Zimmy, the window advertisements are made using a piece of plastic which is black on the interior, and printed on the exterior.
||The plastic is perforated to allow outside light in, so one can see through them from inside the car.
||Since the interior of the car is so much darker than the picture printed on the outside of the plastic, observers outside the car cannot see in.
||The problem here is cracking the first park. There are
selective mirrors, called dichroics that exist, these can be
engineered such that they reflect UV and pass visible light.
The problem is they're not completely efficient. They pass
say 95% of visible light and reflect say 90% UV. The problem
is the 10% remaining. If that's from the sun, then you have
say 300W m2 of UV hitting the screen. Say 100W on your
area of interest with 10W making it through. That's a lot, a
big background fluorescence to get over. To compete, you'll
need Watts of UV on the inside, and that's scary.