What an LCD test actually checks

Published April 30, 2026

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An online LCD test - sometimes labelled screen test, display test, or monitor test - is a sequence of full-screen color and pattern fills that reveals defects you can't see in normal use. The test runs in your browser at full resolution; you watch the screen and look for spots that don't match the expected pattern. This guide explains what each check is testing for, what the result means, and when running the test is worth your time.

The four things a real LCD test checks

1. Dead pixels. A dead pixel is a single point on the screen that no longer lights up. On a black background it's invisible (everything is black already); on a solid red, green, blue, or white background it shows as a tiny dark dot. Modern LCD and OLED panels have millions of pixels - a 1080p monitor has 2,073,600. One or two dead pixels in a corner are common and rarely warranty-eligible by themselves; clusters or center-of-screen dead pixels usually qualify. Run LCD Test, cycle through the white, red, green, and blue full-screen tests, and inspect each carefully.

2. Stuck pixels. A stuck pixel is permanently lit at one color (typically red, green, or blue), regardless of what should be on screen. On the white test it appears as a colored dot; on the matching color test it disappears. Stuck pixels are sometimes recoverable with a "pixel exerciser" - a rapidly cycling pattern that stresses the cell - which the longer LCD-test sequences include. If a stuck pixel survives 30 minutes of the exerciser pattern, it's almost certainly a hardware fault.

3. Color uniformity / banding / dirt. When you look at a perfectly solid red, green, or blue, the entire screen should be uniformly that color. Real-world results: backlight bleed (lighter patches near the edges), VA-panel "vignetting" (corners darker than center), panel dirt or finger oils that show only on solid color. None of these is fatal; uniformity issues are quality-of-display findings rather than warranty triggers (unless the deviation is severe).

4. Burn-in / image retention (OLED only). If the screen is OLED (most newer phones, some monitors, all current Apple Watches and many high-end TVs), repeated long-display of static content can leave a faint ghost. The check is the gray full-screen test - any visible ghost of menus, taskbars, or UI elements is image retention, and persistent retention is burn-in. LCDs technically don't burn in (the backlight is uniform), so on a regular LCD this check is a no-op.

How to run the test in 30 seconds

Step 1. Open LCD Test on the device whose screen you want to test. Open the page on the actual screen - testing a phone screen with the laptop screen tells you nothing about the phone.

Step 2. Click the test pattern button (typically labelled with a color or pattern name). The browser fills the entire screen with that pattern.

Step 3. Look closely at every region of the screen. Move your eyes systematically: top-left corner, top edge, top-right corner, right edge, etc. Use a clean cloth to wipe off finger oils first - they look exactly like stuck pixels.

Step 4. Press Esc or click again to exit the full-screen pattern. Repeat for every test the tool offers (white, black, red, green, blue, gray are the standard set).

What the results mean

When to run an LCD test

Before returning a screen for warranty. Most warranty policies define dead-pixel thresholds in terms of count and location (e.g. "5 or more dead pixels" or "2 or more dead pixels in the central viewing area"). Without test patterns you can't reliably count them; with the test, you can document each defect and submit a clean RMA case. Run the test, capture screenshots or photos for each color pattern, and include them in the warranty submission.

Before buying a used monitor. The seller may insist the screen is fine; a 30-second LCD test will surface any dead-pixel cluster, severe backlight bleed, or panel vignetting. Cheaper to run before paying than to deal with a return later.

After unpacking a new screen. Modern panels arrive with strict per-batch QC, but even one-in-a-thousand failures slip through. Within the return-policy window (typically 14-30 days) is the moment to find them.

After dropping a phone. A drop can crack the panel internally without a visible exterior crack; the LCD test reveals dead lines or large dead regions immediately.

After a long stretch of work. Eye fatigue can make stuck or dead pixels fade into the background. Running the test on a fresh start of the day surfaces things you'd otherwise miss.

What an LCD test does NOT check

Be honest about scope - an in-browser test is limited by what JavaScript can paint on a canvas. It does not measure:

Color accuracy - that requires a colorimeter (Spyder, ColorMunki, X-Rite). The test fills with sRGB primaries; it can't tell whether your panel renders them within Delta-E 3.

Brightness uniformity in candela - same reason; needs a luminance meter.

Refresh-rate stability - JavaScript runs at the browser's animation cadence, not the display's actual refresh frequency. Use a tool like testufo.com for refresh-rate-specific tests.

Input lag - requires hardware timing (high-speed camera + reference signal). The browser test runs at the OS's frame cadence, masking display latency.

For everyday "do I have dead pixels, is the panel intact" checks, the in-browser test is the right tool. For pro-grade calibration or competitive-gaming latency tuning, you need dedicated hardware.

Phone screens vs laptop screens vs external monitors

The same pattern fills work on all three, but practical differences:

Phone screens are often OLED (especially on iPhones since X and on most flagship Android since 2018). The OLED-specific burn-in / image-retention check matters here. The phone screen also has rounded corners and notch / cutout regions; dead pixels near the radius are typically not warranty-eligible.

Laptop screens are usually IPS LCD. Backlight bleed at the edges is the most common cosmetic issue; dead-pixel clusters in the central region are the most common warranty trigger. Open the test in full-screen browser mode (F11 on most browsers) so the OS taskbar doesn't cover bottom rows.

External monitors usually have stricter pixel policies. Top-tier brands sometimes offer zero-dead-pixel guarantees; budget-line panels often allow up to 5 dead pixels before warranty engages. Check the brand's exact pixel policy before submitting an RMA.

Quick recap

1. An LCD test reveals dead pixels, stuck pixels, color uniformity issues, and (on OLED) burn-in.
2. Run LCD Test, cycle through every solid-color pattern, and inspect each region.
3. Document any defect with a photo before contacting the manufacturer for warranty.
4. The browser test does NOT measure color accuracy, brightness in candela, refresh-rate stability, or input lag - those need dedicated hardware.

Related tools and guides

• LCD Test - the tool this guide walks through.
• Camera Test - test your webcam or phone camera in the browser.
• Microphone Test - check input level and noise floor.
• Keyboard Test - verify N-key rollover and stuck keys.
• Device Test Tools - the full hub of device-test pages.
• Dead-pixel testing guide - companion guide focused on dead-pixel policy specifics.
• Device-test checklist for remote work - pre-meeting 60-second sequence.