Dead pixels, stuck pixels and what they look like
Each pixel on an LCD or OLED screen is made of red, green and blue sub-pixels. A dead pixel never lights up, so it shows as a black dot on a white or coloured background. A stuck pixel is permanently on in one or more sub-pixels, so it shows as a bright red, green, blue or white dot, most visible on black. Solid full-screen colours make both easy to see, which is why this test cycles through black, white, the three primaries and their mixes. Check a new monitor, laptop or phone within the return window, because many shops and manufacturers only replace screens with a certain number of faulty pixels.
Other problems the patterns reveal
- Backlight bleed and clouding: on the black screen in a dark room, LCDs may show lighter patches near the edges. A little is normal; large bright areas are not.
- Uniformity and tint: the white and 50% grey screens should look even, without pink or green zones.
- Banding: the grey and colour gradients should look smooth. Visible steps suggest a 6-bit panel, aggressive compression or the wrong colour settings.
- Burn-in on OLED: faint shapes of status bars or logos are easiest to see on the grey screen.
- Sharpness: the fine checkerboard and grid patterns show scaling blur if the display is not at its native resolution.
Can a stuck pixel be fixed?
Sometimes. Stuck pixels occasionally recover after hours of rapidly changing colours or gentle pressure with a soft cloth while the screen is on, but there is no guarantee and pressing too hard can make things worse. Dead pixels usually cannot be repaired. If your device is under warranty, take a photo of the pixel on the test colour.
About the screen details
The page also reads what your browser reports: the screen size in CSS pixels, the device pixel ratio (how many physical pixels make up one CSS pixel), an estimate of the physical resolution, colour depth, colour gamut and whether HDR is active. Browsers round some of these values, and operating-system scaling changes them, so treat them as a guide.