It fills the screen with test patterns, one at a time, so you can inspect a new monitor, laptop or phone before the return window closes, or check an old one for faults. The eighteen patterns are:
- Solid colours: black, white, red, green, blue, yellow, cyan, magenta and 50% grey, to find dead pixels (always dark), stuck pixels (always lit in one colour), dust under the panel and backlight bleed.
- Grey steps, grey gradient and RGB gradients, to judge tint and banding.
- Near-black and near-white steps, to check that shadows and highlights are not crushed or clipped.
- A one-pixel checkerboard and one-pixel lines, to see whether the image is shown at its native resolution or scaled.
- Text in sizes from 9 to 22 pixels, including Vietnamese diacritics, to judge sharpness.
- Motion: blocks moving a whole number of pixels every frame, to see smearing (ghosting) and overshoot halos.
Below the patterns, the tool reports what the browser knows about the screen: size in CSS pixels, device pixel ratio, an estimate of the physical resolution, colour depth, wide-gamut and HDR support, and an estimated refresh rate.
How to run the test
Pick a pattern and press Start fullscreen test. Arrow keys or Space move between patterns; Esc or a click ends the test. On a phone or tablet, swipe left or right to change pattern and tap to exit. The overlay naming the pattern fades after a few seconds so it does not hide anything. If the browser does not allow fullscreen for a page (iPhone Safari, for example), the test fills the window instead; hide the browser bars by scrolling or use landscape.
Clean the screen first with a dry microfibre cloth: dust looks exactly like a dead pixel on white. View from straight ahead at a normal distance, then come closer for anything suspicious. A magnifier or a phone camera zoomed in helps to see single subpixels.
Dead pixels, stuck pixels and backlight bleed
A dead pixel stays dark on every colour; it is easiest to see on white. A stuck pixel stays lit in one colour, or a subpixel stays on: a red dot on black that is invisible on red and yellow is a stuck red subpixel. That is why the test cycles through primaries and secondaries rather than just black and white.
Stuck pixels sometimes recover: some people run rapidly flashing colours on the spot for a while, or press very gently with a soft cloth while the screen is off. Results vary and pressing can damage the panel, so try this only after deciding not to return the screen. Dead pixels generally do not come back.
Backlight bleed and clouding show on black and dark grey in a dim room as brighter patches near the edges or corners of LCD panels. Some is normal; judge it at your usual brightness, not at maximum. OLED screens have no backlight, so they show neither, but they can show image retention: look at mid grey for faint outlines of taskbars or logos.
Banding, black level and scaling
On the gradient you should see a smooth ramp. On a 1920-pixel-wide screen each of the 256 grey levels of an 8-bit signal is about 7.5 pixels wide; if the panel shows only 64 levels (6-bit without dithering), the bands become four times wider and easy to see. Banding can also come from colour profiles, night-light modes or contrast settings, so check with those switched off.
The near-black steps run from 0 to 32. On a well set-up screen you should tell the patch at 4 or 6 apart from pure black; if the first several merge, brightness or the black level is set too low. The near-white steps from 255 down to 223 show the opposite problem.
The checkerboard and lines are drawn in device pixels. At 100% scaling and native resolution they look like an even grey; moiré, blur or crawling lines mean the operating system, the browser zoom or the monitor's own scaler is resizing the picture. Set the monitor to its native resolution.
What the motion test can and cannot tell you
The blocks move a fixed number of device pixels every frame, timed by the browser's animation frames, so the motion is perfectly regular as long as no frame is dropped. Look at the trailing edges: a smeared tail means slow pixel response, a bright or dark halo means overshoot from aggressive overdrive.
A web page cannot measure response time, input lag or the exact refresh rate. The refresh rate shown is how often the browser paints, estimated from the median interval between animation frames and snapped to a common rate when it is within 3%. It is usually the display's rate, but power saving, background tabs, battery mode and variable refresh rate displays can lower it. The physical resolution is the CSS screen size times the device pixel ratio, which is right at 100% browser zoom. The rest of what your browser reports about the display and device is on the browser info page. For what to do with a faulty screen under warranty, see finding dead and stuck pixels.
