A dead pixel is black on every background because its transistor has failed. A stuck pixel shows one colour at full strength because a subpixel is jammed on, so it appears red, green or blue rather than black.
The difference decides what to do next. A dead pixel is permanent; a stuck one occasionally frees itself, and it is the one worth spending a few minutes on before accepting it.
How do you tell them apart?
By cycling the screen through solid colours and watching one spot.
| On white | On black | Verdict |
|---|---|---|
| black dot | invisible | dead pixel |
| invisible | coloured dot | stuck pixel |
| invisible | invisible | dust on the surface |
| dark patch | invisible | dirt or a mark |
The test needs a full-screen colour with nothing else on it, which is the entire reason a screen test exists as a tool. A pixel that vanishes on a black background is stuck rather than dead.
It is worth cleaning the screen first. A surprising share of suspected dead pixels are specks that survived an earlier wipe.
Will a manufacturer replace one?
Usually not for a single pixel. Display policies allow a number of defective pixels before a panel counts as faulty, and the thresholds come from an international standard that defines classes of acceptability.
The thresholds vary by class and by manufacturer, and the count is what matters rather than the visibility. A single bright pixel in the centre of an expensive monitor can be within policy while being intolerable to look at.
Buying from a retailer with a generous return window is the practical answer, because a return does not require a defect to be classified.
What else does a screen test show?
Uniformity problems that are invisible on ordinary content. A solid grey fills the screen with one value, which makes any variation across the panel obvious, and a solid black in a dark room shows backlight bleed at the edges.
Gradient tests show banding, where a smooth transition renders as visible steps. Some of that is the panel and some is the source, and comparing the same gradient at different sizes distinguishes them.
None of these is worth chasing on a screen you are happy with. They are worth running on a new one inside the return period.
What is keyboard ghosting?
The failure to register a key because two others are already held down. Keyboards read a grid, and on a shared-matrix design certain three-key combinations are electrically indistinguishable from a different combination — so the keyboard drops one rather than reporting the wrong one.
It is a design limit rather than a fault, and it is why the specification for gaming keyboards mentions how many simultaneous keys are guaranteed. Ordinary typing rarely triggers it because typing is sequential.
A keyboard test also catches the more mundane problems: a key that repeats, a key that needs a hard press, and modifier keys that stick. All three are easier to confirm on a page showing every keypress than by trying to notice them while working.
Can a screen substitute for a ruler?
Once it is calibrated, for small measurements. The problem is that a browser has no reliable way to know a display’s physical size, so an on-screen ruler has to be scaled against a real object.
A payment card is the standard reference because its dimensions are set by an international standard: 85.60 by 53.98 millimetres, the same for every card in your wallet. Matching the on-screen scale to a card gives an accurate ruler for anything that fits on the screen.
The same calibration problem applies to a protractor, where the angles are correct regardless of scale — an angle does not need calibrating, only a length does.
Why run these tests at all?
Because the faults they find are ones ordinary use hides. A dead pixel disappears against busy content, backlight bleed is invisible except on a dark scene in a dark room, and a failing key is easy to blame on your own typing for weeks.
The tests take a couple of minutes each and they all run in the browser, which means the check costs nothing and needs no installation on a machine you may be about to return.
Questions people ask
How many pixels are on a screen? A common desktop resolution holds around two million, and a high-density laptop panel several times that — which is the context for a policy allowing a handful of failures.
Should I test a second-hand screen? Before paying, ideally. Every one of these tests runs from a browser on the machine in front of you.
Can a stuck pixel be fixed? Sometimes, by rapidly cycling colours in that area. Gentle pressure is also suggested and carries its own risk.
Do dead pixels spread? Not usually. A pixel failing does not affect its neighbours.
Does a webcam test send video anywhere? It should not. The stream stays in the browser, which is checkable by watching the network activity.
Why does my microphone show no level? Usually the wrong input device selected, or permission not granted to the page.
Run the test before the return window closes. The dead pixel test and monitor test cover the display, the keyboard tester, mic test and webcam test check the inputs, and the online ruler and protractor turn the screen into a measuring tool once it is scaled.