Displays

Look very closely at a screen, ideally with a magnifying glass. The smooth image falls apart into a grid of tiny glowing dots. Each dot is called a pixel, short for picture element. A screen can do nothing more than make a very large number of pixels glow in the right colour. Everything else, every photo and every video, is created in your head.

Red, green and blue

A pixel actually consists of three even smaller glowing patches: a red one, a green one and a blue one. They are called subpixels. From a distance, their colours merge in your eye into a single shade. If all three glow at full strength, you see white. If all are off, you see black. Red and green together make yellow. Each subpixel usually has 256 brightness levels, which gives 256 times 256 times 256 combinations, over 16 million colours per pixel.

LCD and OLED

How do you make millions of pixels glow? Two technologies share almost all of today's screens. In an LCD, a white layer of light glows behind the whole picture, and in front of each subpixel sits a kind of tiny blind made of liquid crystals that lets the light through or blocks it. The colour comes from a tiny red, green or blue filter above each blind. In an OLED, each subpixel glows by itself, like a microscopic lamp. That is why an OLED can show perfect black: the pixel is simply switched off. In an LCD, some backlight always shimmers through.

Resolution: why you cannot see pixels

The resolution says how many pixels a screen has. Full HD means 1920 pixels per row and 1080 rows, together over 2 million pixels. Phone displays line up 400 or more pixels along a stretch of one inch, which is about 2.5 centimetres. From roughly 30 centimetres away, your eye can no longer tell such fine dots apart, and the pixels blur into a smooth surface. That is exactly why modern displays look like printed paper instead of a grid of dots. Motion is chopped into steps too: a screen typically rebuilds the picture 60 times per second, fast models 120 times. At such short intervals the single frames merge into smooth motion for your eye.

Touch on top

The reason you can also tap on the picture is an extra, invisible layer above the pixels. Inside it runs a fine grid of transparent conductive tracks that creates a weak electric field. Your finger conducts electricity and disturbs this field in exactly one spot. The touch controller measures the disturbance, works out the position and reports it to the device. That is why the screen often fails with thick gloves: the insulating fabric keeps your finger away and the field stays undisturbed. Touchscreen gloves get around this with conductive threads in the fingertips.