- What's the difference between a dead pixel and a stuck pixel?
- A dead pixel gets no power at all, so it stays black no matter what the screen is showing. You find it on the white screen, where it reads as a small dark dot. A stuck pixel is the opposite: one or more of its subpixels is locked on, so it keeps glowing even when the screen is black. That's why this test opens on black — a stuck pixel is the easiest fault to spot and the one most people actually have. The practical difference is what you can do about it. Stuck pixels sometimes unstick on their own, or with a flashing-noise trick that rapidly cycles the subpixels. A dead pixel is a hardware fault and doesn't come back.
- Why nine colors instead of just black and white?
- Because a pixel is three subpixels — red, green, and blue — and a single failed subpixel doesn't show up on black or white. Black finds anything stuck on. White finds anything fully dead. Red, green, and blue each light exactly one subpixel across the whole panel, so a dot that stays dark on the red screen has a dead red subpixel specifically. Cyan, magenta, and yellow light two subpixels at a time, which catches pairs. Gray at 50% is the uniformity pass: it's where you notice clouding, a bright corner, or backlight bleed, which are panel problems rather than pixel problems.
- Why does this go fullscreen instead of testing inside the page?
- Because the strip of screen under the browser's toolbar and address bar never gets tested otherwise, and that's real screen area with real pixels in it. A test that runs inside a page also leaves navigation, headers, and often an ad unit sitting on top of the very surface you're inspecting. Real fullscreen through the browser's Fullscreen API removes all of it. After two seconds of no input the indicator fades to nothing and the cursor hides, so the display is showing exactly one color and nothing else.
- How many dead pixels does a manufacturer have to replace?
- Fewer than you'd hope, and it depends who made it. Most manufacturers write their warranty against ISO 9241-307 pixel-fault classes, which allow a certain number of faults per million pixels before a panel counts as defective. Class II — the common consumer tier — tolerates a small number of always-on, always-off, and partially-faulty subpixels before the panel fails the standard. One dead pixel in the middle of a Class II monitor is usually not a warranty case. What often works better is the retailer's return window, which typically doesn't care about pixel counts at all. Check the display the day it arrives, not three weeks later.
- Can a stuck pixel be fixed?
- Sometimes. The common approach is to flash rapidly changing noise over the stuck pixel for 10 to 30 minutes, which forces the subpixels to cycle and can shake one loose. It works often enough to be worth trying and it costs nothing but time. Gentle pressure on the pixel with a soft cloth while the screen cycles is the other frequently repeated trick, and it carries real risk of making things worse — you can damage the panel permanently. Neither approach does anything for a dead pixel, and no software can restore a subpixel that isn't receiving power.
- Does this work on a phone or tablet?
- Yes, and it's one of the better uses for it — checking a secondhand phone before you hand over the money. Tap anywhere to advance. iOS Safari doesn't allow the Fullscreen API, so the test falls back to an overlay that fills the visible viewport instead, and tells you so. In that mode the browser toolbar can still cover a strip at the top or bottom; scroll once to hide it before you check those edges. On OLED phones, look carefully at the black screen — OLED pixels genuinely switch off, so a stuck pixel there is a bright dot on a perfectly black field.
- I see a smudge, not a dot. Is that a dead pixel?
- Probably not. A pixel fault is sharp-edged and exactly one pixel wide — at normal viewing distance it looks like a speck of dust that won't wipe off. A soft, spread-out patch is usually something else: clouding or mura from uneven backlight, pressure damage from something pressing on the panel, or bleed at the edges where the backlight escapes around the frame. The gray screen at the end of the sequence is the one that shows those clearly. If the mark moves when you tilt the screen, it's a reflection. If it wipes off, it was dust.
- Does anything get uploaded or stored?
- No. The whole test is nine CSS background colors rendered by your browser. There's no image processing, no camera, no analysis, and nothing to send anywhere. Nothing is written to storage and there's no account. Close the tab and there's no trace that you ran it.
- Why check with the room lights off?
- Reflections hide faults and create false ones. A lit room bounces off the glass and adds bright patches that look like stuck pixels and dark patches that hide dead ones. Dim the lights, clean the screen first with a dry microfiber cloth so you're not chasing dust, and sit about an arm's length away — close enough that a single pixel is visible, far enough that you can scan a whole quadrant without moving your head. Take the black and white screens slowly; those two find most faults.
