Turn the whole screen pure black and look at what's left. On an LCD you'll see backlight bleed and corner glow; on an OLED, true black means the pixels are off entirely. Bright dots that survive are stuck pixels — and every smudge and speck of dust stands out for cleaning.
How this test works
The test is a single static frame of pure black — #000000, every subpixel driven to zero — held across the whole screen. What you're evaluating is everything that still reaches your eyes: on an LCD, the glow that leaks through from the backlight; on any panel, bright dots where a stuck subpixel refuses to turn off; and on the glass itself, the dust and fingerprints that black makes visible better than any other background. A dark room and a minute of adaptation make all three easier to judge.
Fullscreen comes from the browser's Fullscreen API, with a plain full-viewport overlay as the fallback on devices without it — tap the ✕ in the corner, press Esc, or use the browser's own gesture to get back. The black is rendered by the panel, not simulated: the page sets one CSS background color and does nothing else — no animation, no cycling, no measurement loop. The readout strip reports the hex value, your display resolution as reported by the operating system, the viewport size, and the live fullscreen state, all read locally in your browser without anything leaving your device.
The main limit is panel technology, and it's most of what you'll see. An OLED turns pixels off for black, so a perfect panel shows literally nothing — which also means this test can't judge OLED uniformity; near-black grays are what expose that. An LCD can only block its backlight, so some glow is normal, and only pronounced clouding or heavy corner bleed is worth a warranty conversation. Ambient light matters as much as the panel: reflections will read as glow, so judge bleed in a dim room at the brightness you actually use.
Frequently asked questions
What is a black screen test for?
A pure black field shows you everything a screen does when it is supposed to be showing nothing. On an LCD you can judge backlight bleed and corner glow, any bright dot that survives is a stuck pixel, and in a dark room you'll see exactly how gray your black really is. It also doubles as a cleaning aid — dust and fingerprints stand out on black better than on any wallpaper.
Why isn't the black screen perfectly black on my monitor?
Because most LCDs cannot switch their backlight off for a black frame. The liquid-crystal layer blocks as much of the always-on backlight as it can, and what leaks through is the dark gray you're seeing — usually worse near the edges, where the light is fed in. An OLED has no backlight; each pixel emits its own light and simply turns off for black, which is why an OLED showing this page in a dark room can look powered down.
Will this test find dead pixels?
It finds stuck pixels — subpixels frozen on — because they glow as bright colored dots against the black. A fully dead pixel is black itself, so it's invisible here; you need a white or colored field to see it. Run the white screen or the full dead pixel test afterwards to cover both failure types.
Can I use the black screen while cleaning my monitor?
Yes — it's one of the two things this page is best at. Dust, smudges, and fingerprints are far easier to see against black than against a bright desktop. Wipe with a dry or slightly damp microfiber cloth and no solvents. Powering the display off entirely shows dirt even better and removes any risk of pressing keys, but the black screen is the practical middle ground when you want to keep working.
Does showing black save power or help the battery?
On an OLED, yes: pixels showing black are off, so power drops with the amount of black on screen. On a typical LCD it makes almost no difference, because the backlight — the main consumer — stays on at the same level regardless of the picture. Either way this page adds no load of its own: the color is painted once and nothing animates.