How to use the dead pixel test
Wipe the screen first. A speck of dust or a fingerprint can look exactly like a faulty pixel, so use a dry microfibre cloth and check again in good light. Then press Start full-screen test: the dead pixel test opens in full screen with solid red, and the mouse cursor disappears so it cannot hide a defect.
Move through the colours in a fixed order: red, green, blue, white and black. Click or tap anywhere, or press the right arrow key, to go to the next colour; the left arrow key takes you back. Press Esc whenever you are done. Nothing changes on its own, which gives you as long as you need on each colour and avoids the rapid flashing that some testers use. That is deliberate, because flashing full-screen colour can be uncomfortable or risky for people with photosensitive conditions.
On each colour, scan the panel slowly in strips from top to bottom. Sitting a little closer than usual helps, and a magnifying glass is handy on very dense phone displays.
How it works
Every pixel on an LCD or OLED panel is made of red, green and blue subpixels. A solid red screen asks only the red subpixels to light up, so a broken red subpixel shows up as a dark dot. White switches all three on at once, while black switches them all off, which is where stuck subpixels and light leaks stand out.
At the top of the test you will also see your screen resolution and device pixel ratio, for example 1920 × 1080 @2x. The first figure is the size the browser reports in CSS pixels; the ratio tells you how many physical pixels sit behind each of those, which helps when you compare the result with the panel’s advertised specification.
Reading the results
| What you see | Likely cause |
|---|---|
| Black dot on every colour | Dead pixel |
| Coloured dot that stays lit | Stuck subpixel |
| Glow along the edges on black | Backlight bleed |
| Faint image left behind on grey or white | Burn-in or image retention |
A stuck pixel sometimes recovers after the screen has been running for a while, but a dead pixel is permanent. Backlight bleed is easiest to judge in a dark room on the black screen with brightness at a normal level; a little glow is typical of many IPS panels. On white, look for uneven patches or tinted zones, which point to poor uniformity.
If you find a defect on a new display, take a photo with the colour that shows it and contact the seller quickly. Return windows are often short, and pixel policies differ widely between manufacturers and retailers, so ask the shop what applies to your purchase before you send anything back.