Screen Pixels
Red, green and blue: the secret of every colour on a screen
Summary
The picture on a screen is made of millions of tiny dots called pixels. Each pixel is really three tiny lights: red, green and blue subpixels. Every colour on the screen is made by changing the brightness of these three lights; red and green together look yellow, and all three together look white. In LCD screens a backlight is always on and liquid crystals set how much of it gets through; in OLED screens each subpixel makes its own light.
In depth
Screens make colours by additive mixing: because the three kinds of cone cells in our eyes are most sensitive to red, green and blue, different mixtures of these three lights seem to the brain like thousands of different colours. Each subpixel usually takes 256 brightness levels with 8 bits, and one pixel can show about 16.7 million colours. In an LCD, the backlight passes through a first polarizer and becomes light vibrating in one direction; the liquid crystal layer turns that direction depending on the voltage, and a second polarizer lets through only light arriving in the right direction; a colour filter gives the subpixel its colour. In an OLED, an organic layer gives off light as a current passes through it, which is why black pixels switch off completely.
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