How the test works
The page fills the whole screen with pure black, the darkest value the panel can show. A small timer sits in the corner and disappears after a moment without mouse movement or touch, so the screen carries nothing but black. After thirty seconds the caption reminds you that your eyes have adapted, which is the moment to look at the edges and corners. Move the mouse or touch the screen to bring the caption back, and press Escape, swipe down or tap the cross to leave.
The page works for two very different panels. On an OLED, black means the pixels are switched off, so anything you see on the screen is either a stuck pixel or a reflection of the room. On an LCD, black means the backlight is being blocked, never perfectly, so a faint grey glow is expected; the interesting question is whether it is even.
- Do it in a dark room. Daylight and lamps reflect off the glass and hide everything.
- Set brightness to the level you actually use, then try the maximum once.
- Give your eyes half a minute. Bleed and glow only appear once the pupils have opened.
Nothing about your screen is measured or transmitted. The page draws a colour and nothing else.
What the result means
The result card records how long the page was open. The result you care about is what the panel showed:
- A completely dark, even field. On an OLED this is normal. On an LCD it is a very good panel, or the room was not dark enough.
- A tiny dot of red, green, blue or white. A stuck pixel: one sub-pixel is frozen on. Black is the best page to find them, and the dead pixel test confirms which colour is stuck.
- A faint glow along one edge or in a corner that stays put. Backlight bleed, typical for edge-lit LCDs. Run the backlight bleed test to judge how much.
- A sheen that shifts as you move your head. IPS glow. Not a defect, and it disappears when you sit straight on.
- Lighter clouds in the middle of an LCD. Clouding or "dirty screen effect", a uniformity issue with the diffuser. It is easier to see on the grey levels of the bleed test.
What to do about a problem
A stuck pixel can sometimes be freed by running a fast-changing pattern over it for ten to twenty minutes; the dead pixel test page explains the method and its limits. Bleed and glow cannot be corrected by software, but their visibility depends on brightness. Many people who notice bleed at maximum brightness in a dark room never see it at the brightness they use for work. If a small amount only shows on a black test page, it is not worth a return.
Reflections are the most common false alarm. A lamp behind you, a window or a bright phone on the desk all appear as glow on a black screen. Cover them or turn them off before you decide anything.
When to return or claim the warranty
Check a new screen on the first day, while a no-questions return is still possible; that is far easier than a warranty claim later. A single stuck pixel is often below the manufacturer's replacement threshold, so read the panel class policy before you file. For bleed, photograph the screen in a dark room with the camera exposure locked and include an object of known brightness in the frame, otherwise the photo will exaggerate what the eye sees and the claim will be dismissed as normal.
Frequently asked questions
Why is my black screen grey instead of black?
On an LCD the backlight is always on and the liquid crystal only blocks it, never completely. A typical panel lets through one part in a thousand, which looks dark grey in a dark room and black in daylight. That is the contrast ratio in the specification. On an OLED every pixel is its own light source and switches off, so black is truly black.
Does a black screen save battery?
On an OLED phone or laptop, yes: pixels that show black draw almost no power, which is why dark themes help. On an LCD the backlight stays at the same level regardless of the picture, so the black page saves nothing; only turning the brightness down does.
What is the difference between this and the backlight bleed test?
This page is the quick check and a screen-off page: one colour, one tap. The backlight bleed test adds two near-black greys, which reveal clouding that pure black hides, and a 30-second adaptation countdown, because your eyes need that long in the dark before the corners can be judged.
Why does a corner glow when I look from the side?
That is IPS glow, a property of the panel type rather than a defect. It is a silvery or yellowish sheen that moves and fades as you change your viewing angle. Backlight bleed is different: it stays in the same place however you look at it. Sit straight on at normal distance and judge only what remains.
Can I leave the black page on to rest my eyes?
Yes. It draws nothing, sends nothing and, where the browser allows it, keeps the screen awake for as long as the page is open. On an OLED it is close to having the screen off. On an LCD, lower the brightness as well, because the backlight still shines. Press Escape or tap the cross when you are done.