Dead Pixel & Backlight Bleed Test

About Dead Pixel & Backlight Bleed Test

The test fills the whole screen with black, white, red, green, blue, cyan, magenta, yellow and 50% gray (#808080), then adds gray steps, gradients, a 1-pixel checkerboard and a grid. It starts on black on purpose: let your eyes adapt to the dark and look for backlight bleed first, then move to the bright fields for pixel faults.

A faulty pixel is classified by how it looks against each background. The same dot can be black on white and green on red, and that combination tells you whether the whole pixel is off or a single subpixel is stuck. That is why the test cycles through every primary and secondary color rather than just white and black.

Dead, stuck and hot pixels

Each pixel on an LCD or OLED panel is three subpixels: red, green and blue. If all three are stuck off, you get a dead pixel, a black dot on a white field. If all three are stuck on, you get a hot pixel, a white dot on black. If only one or two are stuck, it is a stuck pixel.

A stuck pixel changes color with the background. Say the red subpixel is stuck on. On black it shows as a red dot, on green it shows as yellow (green plus red), and on the red screen it disappears because it matches its neighbors. Many people check only white, find nothing, and miss a fault that is obvious on the green screen.

Dust and bubbles under a screen protector are easy to mistake for pixels. Look at the dot from an angle: if it seems to float and shift relative to the pixel grid, it is on the surface, not in the panel.

Pixel fault classes: ISO 13406-2 and ISO 9241-307

The classes manufacturers quote come from ISO 13406-2 (2001). It sorted faults into Type 1 (always-on bright dot), Type 2 (always-off dark dot) and Type 3 (stuck subpixel), and set a maximum count per million pixels for each class. That standard was withdrawn and replaced by ISO 9241-302, 303, 305 and 307 in 2008, but spec sheets still say things like "ISO 9241-307 Class II" and use the same class names.

In the ISO 13406-2 table, Class I allows zero faults of any type. Class II allows 2 Type 1, 2 Type 2 and 5 Type 3 faults per million pixels. Class III allows 5, 15 and 50. Consumer monitors are almost always rated Class II.

The key phrase is "per million pixels". A 4K panel has over eight million pixels, so a single bright dot is well within Class II. Whether you can return the monitor depends on the retailer's return window and the manufacturer's own dead-pixel policy, not on the standard. Those policies differ by brand (some count only bright dots, some count only faults in the central area, some offer a zero-bright-dot guarantee on certain lines), so read the actual policy for your model. Test the day it arrives: a no-questions return window is usually far easier than a warranty claim.

Bleed and uniformity are not pixel faults

Cloudy bright patches along the edges or corners of the black screen are backlight bleed, caused by pressure from the bezel or assembly variation. It is not covered by the pixel classes. IPS glow, a soft sheen visible at an angle, moves when you move your head; bleed stays put. For a fair judgment, sit straight on, use your normal brightness, and dim the room only moderately. A phone photo of a black screen at full brightness in a pitch-dark room will always look far worse than what you see in use.

OLED panels have no backlight and no bleed. On OLED, use the 50% gray screen to look for banding, vertical lines, and burn-in shaped like a taskbar or status bar.

What the stuck-pixel fixer can and cannot do

The fixer box cycles red, green, blue, white and black roughly every 90 ms (every 500 ms if your system asks for reduced motion). Rapidly switching a stuck liquid-crystal cell is reported to free it in some cases, but nothing guarantees it. A dead pixel whose driving transistor has failed will not come back. Give it 10 to 30 minutes; if nothing changes, move on to the return or warranty route. The box flashes quickly, so do not use it if you have a history of photosensitive seizures.

Worked examples

How to use it

1. Wipe the screen, dim the room a little, and start the full-screen test.
2. Step through the colors with → or Space; note the position of any odd dot and which colors it shows on.
3. Check the edges on black for bleed, and the gray steps and gradients for banding.
4. For a suspected stuck pixel, drag the fixer box over it and leave it for 10–30 minutes.

Frequently asked questions