Can somebody explain briefly to me how a LED screen works?

In an LED screen the backlight is generated with white LEDs. White LEDs are just like normal monochrome LEDs, except that they contain an additional phospher compound to convert some light of a certain frequency into light of another frequency. Therefore they appear to be white.

The development of the blue LED was the key in the lock for white LEDs, "The existence of the blue LED and high efficiency quickly carried to the first white LED, which employed a Y3Al5O12:Ce, or "YAG", phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white."

Here is the emission spectra of a white LED:

http://en.wikipedia.org/wiki/Image:White_LED.png

For comparison here is an emission spectra for three monochrome LEDs:

http://en.wikipedia.org/wiki/Image:Red-YellowGreen...

If on the other hand you mean one of the GIANT screens that are used to display football matches in outdoor venues and the like, those do just use red, green and blue LEDs for the display. They can get away with that because the people are so far away from the screen that the three individual LEDs of the pixel cannot be seen, and instead appear to be a solid colour.

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