How a screen makes a picture : I Wish I Knew

The short version

A screen is a grid of tiny dots called pixels. Each pixel is made of three even tinier lights, red, green and blue, and by changing how bright each one is, it can show almost any colour. The whole grid is redrawn many times a second, and your eyes blend it into a smooth picture.

I always thought a screen showed a picture, like a window.

Up close it is only a grid of coloured dots.

Your eye does the blending.

Three colours are enough

The eye has three kinds of colour sensor, one most sensitive to red light, one to green and one to blue. Because of this, mixing only those three lights in the right amounts can fool the eye into seeing nearly any colour. Red and green together look yellow. All three at full brightness look white. All off is black.

Each pixel has a red, a green and a blue part, called subpixels, so small that the eye merges them. Look through a drop of water on a screen and you can see them.

How many pixels

Common screen sizes, in millions of pixels.

Old standard TV (480 lines) 0.3 million

HD (1080p) 2 million

4K 8 million

Many pictures a second

A screen redraws the picture many times a second, called its refresh rate, usually 60 times. Each picture differs slightly from the last, and your brain reads the sequence as movement. Fast refresh rates, such as 120 times, look smoother, which helps with games and scrolling.

33 million

The number of pixels on an 8K screen, four times as many as 4K. On a phone-sized screen you could not tell the difference.

More pixels always means a better picture.

Past a certain point your eye cannot see the individual dots. At normal distance, the quality of the screen's colours, contrast and brightness matters more than the pixel count.

Two ways to light the pixel

The pixels need light. Most screens have a backlight behind a layer that blocks it to different degrees, which is how LCD and OLED differ. Many screens now also sense touch, as described in touchscreens.

Sharpness depends on distance

What matters is how many pixels fit into your view, not how many there are in all. A phone is held close, so it needs many pixels in each inch to look smooth. A television is watched from further away, so fewer pixels per inch look equally sharp.

The eye can separate detail down to roughly one sixtieth of a degree. Beyond that, extra pixels cannot be seen. That is why a very high-resolution television only looks better than a good HD one if it is large or you sit close.