Dead Pixel & Screen Tester

A screen is millions of tiny coloured dots, and a few can fail at the factory or over time. A dot that never lights up, or one that never turns off, is easy to miss on a busy picture and impossible to ignore on a plain colour. The check takes a minute: fill the screen with one colour at a time and look. It is worth doing on a new monitor, phone or laptop while the return window is still open.

The test screens

Press Sample to cycle through the screens automatically every two seconds, or choose one with the buttons. The sequence is Black, White, Red, Green, Blue, Yellow, Cyan, Magenta, Mid grey, Gradient and Checkerboard, eleven in all. Each has a short tip under the picture. Black reveals stuck bright pixels and bright backlight leaks at the edges. White reveals dead pixels and dirt. Red, green and blue show a stuck sub-pixel in a single channel, the dots that stay another colour. Yellow, cyan and magenta combine two channels.

Running the test

Go full screen with the button or by pressing F, and dim the room. Sit at a normal distance, then move close. Step through the colours with the arrow keys or space, or click the screen. In automatic mode the page changes colour for you at a rate of 1 to 10 seconds. Escape leaves full screen. Look at the corners and edges as well as the middle. Wiping the screen with a soft dry cloth first removes marks that look like pixels.

A worked example

On a laptop, set the page to full screen and the first screen is black. A single bright green dot that is also visible on the black, white, red and blue screens is a stuck pixel, since it never changes. A dot that is black on every screen, including white and yellow, is a dead pixel. If the dot disappears when you wipe the screen, it was dust. The note shown for each colour reminds you what that colour is best at revealing.

Beyond pixels

Mid grey shows uneven backlight, banding and tinted corners. The gradient should be a smooth ramp; visible steps, called banding, are common on cheap panels. The checkerboard shows image retention and flicker on a high-contrast pattern. These are not defects in the strict sense, but they are good to know about before you buy or return a screen.

Warranty and policy

Manufacturers allow a small number of faulty pixels, and the number depends on the brand, the panel class and the screen size. Some set the limit by type, bright or dark, and by location. Check the policy for your model before arguing a return, and take a photo of the evidence. This page cannot tell you how many pixels your warranty allows.

Edge cases

Colours are drawn at full strength, but they look different on every screen, depending on its settings and its panel type. Some phones show a notch or rounded corners that cut off the colour. Screen protectors can hide or imitate faults.

Limits

It shows colours; it does not detect faults automatically or repair anything. You are the sensor. Nothing is sent anywhere.

Related tools

• Chroma Key Screen shows a single colour full screen for a backdrop.
• Refresh Rate Test checks that the display really runs at the rate on its box.
• Touch Screen Test checks the digitiser of a touch display.
• Keyboard Test checks the other part of a laptop that wears out.