Old airport departure boards had a spooky habit. After years of showing the same words, you could sometimes still make out "GATE" or "DEPARTURES" faintly on the screen even when it was showing something else. The screen had, in a sense, remembered.

This is called burn-in, and it's not only a relic of old technology. The gorgeous OLED screen on many modern phones, watches and TVs can do a gentle version of the same thing. The good news: once you understand why it happens, it's easy to avoid.

Every pixel is its own tiny lamp

On a traditional LCD screen, there's one big backlight behind everything, and the pixels act like little shutters that let more or less of that light through. On an OLED screen there's no backlight at all. Every single pixel makes its own light using thin layers of organic (carbon-based) material that glow when electricity runs through them.

That's what makes OLED look so good. Each pixel can be fully bright or fully off, right next to each other, which gives that inky, endless contrast.

Lamps wear out

Here's the catch. Like any light source, those organic materials slowly get dimmer the more they're used. That's normal and happens to every pixel over the life of the screen.

The problem arises when some pixels work much harder than others for a long time. Imagine a bright white clock or a navigation bar sitting in exactly the same spot for hundreds of hours. Those pixels age faster than their neighbours. Eventually they're a touch dimmer, and when you show a plain grey background you can see a faint ghost of the old image.

The colours don't all age at the same speed either. Blue light-emitting material has historically been the hardest to make long-lasting, which is why screen makers put so much effort into managing it. When aging is uneven between colours, a ghost can show a slight colour tint as well.

Burn-in vs. a temporary afterimage

Not every ghost is permanent. Sometimes a screen shows a faint afterimage that fades away after a few minutes of normal use. That's called image retention, and it's usually harmless. True burn-in is the slow, permanent kind that comes from uneven wear over a long period.

Why pure black is the healthiest colour

On an OLED screen, a truly black pixel (colour code #000000) isn't just dark. It's switched off. It makes no light, so it isn't wearing out, and it uses essentially no power. That's why a pitch-black background is so friendly to OLED phones: most of the screen is resting, and the battery thanks you too.

It's also why clocbox starts on pure black and only offers very dark "midnight" themes. A dark grey background might look almost the same to your eye, but every one of those pixels is still lit.

The sneaky trick: moving things very slightly

Screen makers have a clever weapon against burn-in: they nudge things around. Many phones, TVs and smartwatches with always-on displays shift static elements by a few pixels every so often. You don't notice, but the wear gets spread across a wider group of pixels instead of hammering the exact same ones.

clocbox has its own version called Pixel Shift, switched on by default in the ☰ menu. Once a minute it moves the clock and controls a few pixels in a random direction. It's small enough that you'd never spot it from across the desk, but over a long night it keeps any one set of pixels from doing all the work.

How to run a desk clock all night, kindly

  • Keep the background black. The darker the screen, the less wear and the less battery used.
  • Lower the brightness. Brightness speeds up wear. At night, you rarely need much. Try clocbox's Night Mode.
  • Leave Pixel Shift on. It's free insurance.
  • Mind the heat. Warm screens age faster. Keep the phone out of direct sun and somewhere air can reach it, especially while charging.
  • Give it a rest sometimes. Turning the screen off when you don't need it is the best protection of all.

Every screen is different, so think of these as good habits rather than guarantees. Used sensibly, an old phone can make a lovely bedside or desk clock for a long time.

Open the clocbox clock to try Night Mode and Pixel Shift.