A screen doesn't show a picture. It lights up, switches off or masks millions of tiny dots fast enough for the eye to take them for a picture.
This subject reads in three parts: this sheet, which explains what all screens share, then two chapters, The liquid crystal display (LCD) on liquid crystals and The electronic ink screen on electronic ink. LEDs, which light most screens and make up some on their own, have their own sheet: LEDs.
The pixel
Every screen is a grid of pixels. On a colour screen, each pixel is made of three sub-pixels, red, green and blue. By setting the brightness of each, you get every colour: all three at full give white, all three off give black.
Resolution is the number of pixels: 1920 × 1080 for "Full HD", 3840 × 2160 for 4K. Density is pixels per inch (ppi): beyond about 300 ppi at phone reading distance, the eye can no longer tell the dots apart.
The same number of pixels looks sharp on a phone and coarse on a big TV: density decides sharpness, not resolution alone.
Three ways of making light
Every screen falls into one of these families.
| Family | Principle | Examples | In sunlight | In the dark |
|---|---|---|---|---|
| Emissive | Each dot makes its own light | OLED, giant LED walls, old tube TVs, plasma | Average | Perfect |
| Transmissive | A lamp behind, shutters in front | Computer, TV and phone LCD screens | Average to poor | Good |
| Reflective | No lamp: they bounce back ambient light, like paper | Electronic ink, calculator and watch LCDs | Perfect | Unreadable without a light |
This table decides power use. An emissive or transmissive screen spends energy every second it is on, and the backlight is often the biggest drain in a phone or a laptop. A reflective screen uses almost nothing: an LCD calculator runs for years on a button cell, an e-reader for weeks.
Refreshing the picture
A computer screen redraws its picture 60 times a second or more (60 Hz, 120 Hz). That's what makes motion smooth. An electronic ink screen only changes the picture when told to, and takes a fraction of a second to do it: perfect for a page, useless for video.
How a screen gets the picture
- Large screens receive the picture through a video cable: HDMI, DisplayPort, and older VGA (analogue, still very common on old monitors and projectors).
- Small screens in electronic devices are driven directly by a small controller, through a serial link with a few wires (SPI, I2C) or a parallel bus. This is what you use to build a device yourself with a microcontroller, see The Phenix devices.
- The controller is a chip bonded behind the panel, which keeps the picture in memory and redraws it on its own. Each model has its own commands: without its documentation, a salvaged screen is very hard to bring to life.
What you learn by taking them apart
- Old phone screens are hard to reuse. Their connectors have pins 0.4 mm apart or less, their controller often has no public documentation, and their backlight sometimes needs more than 10 volts. For a build, a screen module sold for microcontrollers, already mounted on a small board, costs a few euros and saves days.
- A broken laptop screen contains a flat, diffusing light panel behind the liquid crystals: an excellent lighting panel once powered. See The liquid crystal display (LCD).
- A screen that stays black isn't necessarily dead: often only the backlight has failed. Shine a torch at the screen at an angle, and the picture appears.
- Touchscreens are one more layer, glued in front of the panel: you can crack a phone's touch glass without harming the display, and the other way round.