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How to build a digital billboard which is placed in the city, readable at day and night, uses a minimum of energy and is totally cheap? I dont know but there are some ideas:
1) Phosphorescence
Also known as glow-in-the-dark. The whole billboard is made of phosphorescent material. The graphics are written by a laser. The writing can be either as continuous vector shapes (typical laser show) or like an old TV line by line. Usually the latter method needs a very fast movement of the laser to generate a constant image. The fact that the material keeps the light for some time makes it ideal for that because it can take some time (seconds) to write the image. (and is maybe a nice effect by the way)
Although its meant to glow for some hours, directly after the exposure to light it fades away really fast. And is also brighter. But thats perfect. So the display can be overwritten just after a short time.
But there is actually a way to remove the glow manually! Exposing it to IR (infrared) light pushes the material to emit the “stored light” faster. A big IR washer can be used as an eraser.
2) magnetic field viewing film

Magne-View® Film
3) Elektrochrome Gläser
