Màn hình trắng và kiểm tra màn hình

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• Khoảng trắng — bắt đầu / thoát toàn màn hình màu
• ← / → — màu trước / tiếp theo
• C — bật/tắt xoay vòng tự động
• F — toàn màn hình của trình duyệt
• Esc — thoát

The dead-pixel problem in detail

Every full-colour pixel on a modern LCD is built from three subpixels — one red, one green, one blue — typically arranged side-by-side in a horizontal stripe. Each is an independently addressable liquid-crystal cell that modulates how much of the backlight passes through a coloured filter. The pixel's apparent colour is the additive sum of its three subpixel intensities. OLED panels also use a tri-colour structure, but the red/green/blue elements are themselves the light source (organic LEDs) rather than filtered backlight. Samsung's PenTile RGBG matrix shares green subpixels between adjacent pixels, which is one reason OLED text can look slightly different from LCD at the same nominal resolution.

Because each subpixel is independently driven, a fault can affect just one of the three. This is why solid red, green and blue test screens reveal defects that a pure white screen hides:

• Dead pixel — a pixel (or subpixel) that doesn't emit any light. On an LCD, a transistor failure leaves the cell permanently opaque, so the spot stays black on every test colour. Generally not field-repairable.
• Stuck pixel — a pixel stuck in the on state, showing a fixed colour (red, green, blue, white, or any combination). Electrically alive but not responding to commands. Often correctable.
• Hot / bright pixel — a stuck-on white or coloured pixel particularly conspicuous on a black background.
• Partial subpixel defect — only one of the three subpixels is dead or stuck, so the pixel shows a tinted colour instead of the intended one.

Test logic falls out of those definitions: pure white reveals stuck-OFF (dead) pixels as black dots; pure black reveals stuck-ON pixels as glowing dots; pure red, green, blue in turn reveal subpixel-level faults — a dead red subpixel makes a tiny cyan-tinted dot on the pure red test, for instance. The Auto-cycle feature steps through all five in sequence so you can sweep an entire panel in 10–15 seconds.

How to fix a stuck pixel

Stuck pixels (not dead ones) can sometimes be coaxed back to life. Three approaches, in order of safety:

• Pixel-cycling software — utilities that flash R/G/B (and sometimes white/black) at high frequency over the affected area, "exercising" the cell. The best-known free option is JScreenFix, a browser-based draggable HTML5 patch of rapidly-changing pixels; the project recommends running it for around 10 minutes and reports anecdotal success on stuck pixels.
• Gentle physical pressure — cover the pixel with a slightly damp cloth and press very lightly with a fingertip or capped pen. This sometimes re-seats the liquid-crystal cell. Too much pressure can spread the defect or crack the panel — emphasis on very lightly.
• Heat or cold — wrapping the screen briefly with a warm cloth has occasionally been reported to revive stuck pixels. Effectiveness is anecdotal.

Dead pixels (no light at all) almost never come back — the transistor is gone.

Manufacturer tolerance — ISO 9241-307

The international standard for pixel-defect tolerance is ISO 9241-307, which defines four panel classes. Most consumer monitors are sold to Class II — meaning a brand-new panel can legitimately ship with up to two bright dead pixels, two dark dead pixels, and five stuck subpixels per million pixels and still meet spec. Premium and medical-grade panels are sold to Class I (zero defects). Apple's repair policy is notably asymmetric: bright stuck-on pixels are treated as more disturbing than dark dead ones because they're more visible against typical content, and a single bright subpixel on a Retina display has historically been enough to qualify for warranty replacement while several dark ones might not.

Practical takeaway: test a new monitor (or a used one before purchase) before the return window closes. The whole-rainbow sweep takes 30 seconds; finding a defect on day 31 is usually too late.

Backlight bleed, clouding, IPS glow

LCDs are not self-illuminating: behind the liquid-crystal sandwich sits an LED backlight. When the LCD displays pure black, every cell rotates to its maximum-blocking state, but no liquid-crystal cell is perfectly opaque. Light leaks through, and at the panel edges where the backlight diffuser meets the bezel the leakage shows as bright patches or bands of grey-blue glow — that's backlight bleed. Clouding is a related artefact: uneven brightness in the centre of the panel, often shaped like splotches of slightly-brighter grey on the black field, usually caused by uneven pressure on the panel during assembly or shipping.

IPS glow is a third closely-related artefact specific to IPS-type LCDs: a corner-of-the-panel sheen visible when viewing a dark image off-axis. Unlike bleed, IPS glow is angle-dependent — it disappears when you look straight on. It's a property of the IPS panel design and isn't a defect, although it can be more or less pronounced from sample to sample.

The standard test: pick Black, drop brightness to around 30–50%, fullscreen in a dark room, look for bright patches. The brightness slider is built precisely for this — testing at 100% often hides the bleed because the eye adapts to the bright surface. OLED panels don't have backlights at all, so they have neither bleed nor clouding (each pixel emits its own light). Their analogous problem is burn-in — see the OLED note below.

Webcam fill light — colour temperature primer

Using a monitor as a fill light for a webcam call is a real practical use case — it's a free, instant softbox. The three "webcam white" presets are calibrated for the three standard photographic colour temperatures:

• Warm (2700 K) — incandescent / "soft white" bulb temperature. Flatters skin tones with a slight orange tint; can look dim on-camera and feel like sunset lighting.
• Neutral (4000 K) — between warm and cool, the most natural-looking on most webcams. The default recommendation for video calls.
• Cool (6500 K) — daylight / D65 (the international colour-science standard). Brightest and crispest, can wash out skin tones if your webcam's auto white-balance isn't locked.

For best results: pick a colour, set brightness to a comfortable level, fullscreen, sit roughly 40–80 cm from the screen so the light spreads evenly across your face, and tilt the monitor up slightly to lift shadows under the eyes. The Wake Lock API keeps the screen on for the whole call so you don't get a sudden dim halfway through.

A note about LCD vs OLED for fill-light use

If you're planning to leave the screen on a uniform white for an hour-long meeting every day, the underlying display technology matters. LCD backlights are always on and unaffected by the displayed image — leaving white up indefinitely doesn't damage the panel. OLED emits per-pixel, and prolonged static images cause burn-in: the pixels age unevenly and a faint ghost of the static content stays visible afterwards. Modern OLEDs (since around 2020) include burn-in mitigations (pixel-shift, peak-brightness limiters), but a fill-light routine that runs hours a day on an OLED screen is genuinely risky.

A safety note about flashing

Photosensitive seizures. Rapidly-flashing screens — especially red — can trigger seizures in people with photosensitive epilepsy. The infamous 1997 broadcast of the Pokémon episode "Dennō Senshi Porygon" used red strobing at roughly 12 Hz and hospitalised 685 viewers in Japan. WCAG 2.3.1 (Three Flashes or Below Threshold) sets a hard limit at three flashes per second for general-purpose web content, with stricter rules for red flash. The Auto-cycle feature on this page is intentionally limited to a 1-second minimum interval and uses smooth colour transitions, but if you have a history of photosensitive seizures, set the interval to 5 seconds or longer and avoid the red-cycling sequence.

More questions

My monitor has one bright pixel — is it worth claiming warranty?

Often yes for premium brands, sometimes no for budget brands. Apple, Dell UltraSharp, LG UltraGear and several professional lines treat a single bright pixel as warranty-replaceable. Mass-market budget monitors typically follow ISO 9241-307 Class II tolerances and won't replace until you exceed the count. Check the manufacturer's published "zero bright pixel" or "pixel perfect" guarantee before buying — it's often a meaningful product differentiator.

What's the difference between this and the Flashlight tool?

Different intent, different defaults. The Flashlight tool is built for emitting light in a useful colour or mode for reading, camping or signalling — a single bright surface optimised for "I need light." The White Screen is optimised for testing your display and for work-oriented lighting (video calls, photography, calibration) — multiple test colours, brightness control, calibrated white-balance presets, fullscreen behaviour. They share underlying tech but the UX is built around different jobs.

Why does my screen look slightly tinted at low brightness?

Most LCD backlights aren't perfectly white — they have a slight blue-cyan or green-yellow tint that becomes more visible as brightness drops. Modern monitors include compensation circuits that adjust the white point at lower brightnesses, but cheap panels and very old ones often skip this. If the tint bothers you, manual white-balance calibration (via the monitor's OSD or via system colour management) usually helps; for serious work a hardware colorimeter (X-Rite i1Display, Datacolor Spyder) is the right tool.

Will this work on my phone or tablet?

Yes — every modern mobile browser supports the Fullscreen API and Wake Lock API the page uses. On iOS Safari, fullscreen still leaves a thin status bar at the top; tap the address bar to hide it for true edge-to-edge. On a tablet, the page is large enough to be a useful makeup-application or video-call light source on its own.

Does anything get sent to a server?

No. The page paints a colour into the viewport and toggles fullscreen via the browser's Fullscreen API. There's nothing to upload — your colour choice never leaves your device. The page works offline once it's loaded.