LCD Screen Tester: Free Online Screen Test for Dead Pixels
LCD Screen Tester. Click Start Full Screen Test to run this LCD screen tester and find dead pixels, stuck pixels and uneven backlight. You can test your refresh rate without any software by using the browser-based measurement tool directly.
Interactive Tool
Make the rectangle closer to the background until you can no longer tell them apart.
At 256 steps the ramp should look continuous; at fewer steps every bar should be distinct.
Use this LCD screen tester to cycle through solid colors, grayscale shades, and a checkerboard pattern fullscreen to spot dead pixels, stuck pixels, and backlight uniformity problems.
A new monitor or laptop usually comes with a short return window, and the cheapest time to find a bad pixel is before it closes. This LCD Screen Tester fills your liquid crystal display with one solid color at a time, so dead pixels, stuck dots, backlight bleed and patchy brightness have nowhere to hide. It is a quick display test that runs in your browser, with no download, showing you which defects are worth reporting.
How to Run the LCD Screen Tester
Wipe the screen with a soft cloth first: a speck of dust on a white field looks just like a dead pixel. The bandwidth calculator covers DSC compression and chroma subsampling so you can find workarounds when bandwidth is borderline.
- Click a color tile to paint the whole screen in full screen.
- Step through the colors with the arrow keys or the space bar; these keyboard shortcuts keep the cursor off the screen.
- Use the color picker or a hex code to add a custom color, such as mid grey.
No install, no account; nothing leaves your browser.
Go Fullscreen with F11 and Exit with Esc
Fullscreen mode hides the browser chrome (tabs, toolbars and the address bar), which would otherwise cover the top edge, where bleed and bad pixels often sit. If the page does not switch over on its own, press F11. Press Esc or the escape key to return to the page.
Testing Phones, Tablets and TVs
The same sweep works on phones, tablets, desktop monitors and smart TVs, OLED included. Android browsers paint edge to edge after a tap; on iOS Safari, which blocks fullscreen, scroll to hide the toolbar or rotate to landscape.
What Each Color in the Screen Test Reveals
Each color field exposes a different fault, because every pixel is three subpixels (red, green and blue), and a fault in one channel hides until that channel lights up. Run the full color cycle once, pausing on each screen. The burn in test is particularly useful for checking gaming displays that have been used with static HUD overlays.
| Test screen | What to look for | Likely defect |
|---|---|---|
| Black | A lit dot, or glow at the edges | Stuck dot, light leaking at the edges |
| White | A dark dot that stays put | Dead pixels, or a speck on the glass |
| Red, green, blue | A dark dot on one color only | Missing subpixels in that channel |
| Yellow | A dim or tinted spot | Weak red or green subpixels |
| Mid grey | Blotches or a colored cast | Clouding, uneven brightness |
Against solid colors, your eye picks out one wrong dot among millions on the display.
The Black Screen Pass for Stuck Pixels
The black screen is where stuck pixels give themselves away: a subpixel jammed on shows as a bright dot of red, green or blue while everything around it stays dark. Scan the corners slowly, then switch to the white screen for the opposite fault.
Defective Pixels Explained: A Dead-Pixel Check Guide
The type of fault decides whether you can fix it or should send the screen back. A dead-pixel check sorts what you find into three groups:
- Dead pixel: its transistor has failed, so the liquid crystal never passes light. These dark pixels stay black on every color.
- Stuck pixel: one channel is always on, so it shows as a bright pixel on black.
- Partial sub-pixel fault: one channel is dim rather than off, visible only on its own color.
A stuck one sometimes recovers after pixel-massage or rapid color flashing; dead pixels never do.
Dead Pixel or Dust? Confirming With a Magnifier
On a Retina or other hi-dpi screen, subpixels read as a faint tint rather than a dot. Hold a magnifier or your phone camera close: a speck on the glass wipes away, a real fault stays in the grid.
Checking a Refurbished 27-Inch Monitor with a Color Cycle
The question is simple: keep a refurbished 27-inch 2560 × 1440 monitor bought online, or send it back inside the 14-day cancellation window UK distance-selling rules allow. It arrived yesterday, so there are 12 days left. The answer, after one evening, is to start the return.
First the readouts. The browser reports 2048 × 1152, which looks wrong until you remember Windows is set to 125% scaling: 2048 × 1.25 = 2560 and 1152 × 1.25 = 1440, so the native grid matches the spec sheet. Skipping that sum would have sent a working monitor back for the wrong reason.
Then the sweep, at about 60 cm, after a 20-minute warm-up. Black shows one green dot, upper left, about 9 cm from each edge. It stays green on blue and red, so it is a stuck green subpixel, not dust. White shows nothing; the dead-pixel check comes up clean.
The tester found the dot; whether it justifies a return depends on how large it looks from my chair. Against ISO 13406-2 Class II, no: 3,686,400 pixels allow 7 always-on faults. Against your eyes, yes. I worked out the pixel pitch, 25.4 ÷ 108.8 PPI = 0.233 mm, which at 60 cm spans 1.34 arcminutes, above the roughly 1 arcminute that 20/20 vision resolves. The same panel size at other resolutions shows why this one is borderline:
| 27-inch grid | Pixel pitch | Arcminutes at 60 cm | Class II always-on limit |
|---|---|---|---|
| 1920 × 1080 | 0.311 mm | 1.78 | 4 |
| 2560 × 1440 | 0.233 mm | 1.34 | 7 |
| 3840 × 2160 | 0.156 mm | 0.89 | 16 |
The caveat: a bright point on black is visible even below 1 arcminute, because the eye detects light, not shape, so the 4K row is not a free pass either. The opposite holds for a dark dot on white, which really does fade out below that line.
The plan follows from the numbers: run rapid color flashing over the dot for 30 minutes tonight, repeat the black pass tomorrow, and if the dot is still green on day 4, file the return with its position noted, leaving 8 days of margin. The standard says keep it; the seating distance says return it, and the input that matters most here is the pitch, not the defect count.
Backlight Bleed vs IPS Glow on a Black Screen Display Test
An LCD is lit from behind, and light escaping at the edges is backlight bleed. Check it in a dark room at max brightness on a black screen, standing back.
- Backlight bleeding is a warm-grey wash creeping in from the bezel that stays put however you sit.
- IPS glow is a silvery corner sheen that shifts as you move your head; it is normal for that technology, not a defect.
- Clouding is blotchy light across the middle of the screen.
Only bleed you still see at everyday settings is worth raising with the seller.
Native Resolution and Pixel Math for Your LCD Panel
The tool reports three numbers. The viewport is the area actually painted. The screen resolution is what your operating system reports. The native screen resolution multiplies that by devicePixelRatio to give the physical pixels:
Worked example: a 14.2-inch laptop reports 1512 × 982 at a device pixel ratio of 2. Its native resolution is 3024 × 1964, a pixel count of 5,939,136 (17,817,408 subpixels), which works out to 253.9 PPI and a pitch of 0.100 mm. On a high-dpi screen like this, scaling draws each interface pixel with four physical ones; only a full-screen color reaches all of them, and the viewport shrinks while toolbars show. Check it against the spec sheet.
How Many Defective Pixels Justify a Warranty Return
The old ISO 13406-2 standard (replaced by ISO 9241-307) set a common yardstick: a Class II screen may have 2 always-on, 2 always-off and 5 faulty subpixels per million pixels. Scaled and rounded down:
| Screen | Pixels | Always-on | Always-off | Sub-pixel faults |
|---|---|---|---|---|
| 1080p (1920 × 1080) | 2,073,600 | 4 | 4 | 10 |
| Worked example (3024 × 1964) | 5,939,136 | 11 | 11 | 29 |
| 4K (3840 × 2160) | 8,294,400 | 16 | 16 | 41 |
Each manufacturer sets its own, often stricter policy, and premium lines may promise zero-defect screens. Run a pre-purchase check on used monitors, and test new ones inside the return window.
Uniformity, Gradients and Gamma in a Monitor Test
Pixels are only half of a monitor test. A mid gray tone field shows whether brightness is even, with no tinted patches. A test pattern with circles and fine frequency patterns checks overall image quality. Smooth gradients should run without abrupt changes, and a gamma check, where you blend a grayscale logo into its background, shows whether shadows and highlights sit where they should.
Greyscale and Banding Checks
A 256-step greyscale ramp should look like one continuous fade. In the tester, step through a few custom greys by hex code; each should look even. Stripes on a ramp are banding, common on screens faking extra shades.
Color Accuracy, Color Gamut and Color Distances
Two near-identical color patches test color distances. Color accuracy and color gamut (sRGB versus DCI-P3) can only be judged roughly by eye: the tester's primary fields reveal an obvious tint or a weak channel, but for real figures and calibration, use a colorimeter, which also confirms color depth.
Response Time, Ghosting and Refresh Rate: Motion in a Panel Test
A moving-block panel test shows response time: slow pixels leave ghosting streaks behind each block, while too much overdrive adds a bright halo. VA screens smear most in dark scenes. Refresh rate sits outside what a color sweep shows: for a 165 Hz monitor stuck at 60 Hz, check settings, the cable (HDMI versus DisplayPort) and the GPU.
Viewing Angle and Sharpness
View a mid-grey field from off-centre: tone and tint should barely shift, which is viewing angle stability. A sharpness check needs small text rather than a solid field, so run it beside the color sweep.
LCD Test vs OLED: Burn-In, Blooming and LED Backlights
An LCD test looks for bleed because the display is lit from behind. Mini LED screens add local dimming, which can cause blooming: halos around bright objects on dark backgrounds. OLED lights each pixel itself, so a black screen is truly black and bleed cannot occur. Its risks are burn-in and temporary image retention instead, plus PWM flicker when dimmed. The color sweep stays the same.
Laptop and Monitor Tips for an Accurate Pixel Test
- Allow a 20-minute warm-up before judging tone.
- Turn off vivid mode, dynamic contrast and HDR.
- Run the display at its native grid so scaling cannot blur a fault away.
- Judge bleed from your normal seating distance; a flaw seen only up close rarely matters at a usual viewing distance.
- For a laptop screen test, compare battery and mains power.
Whether you call it a free online dead pixel test, a free online screen tester, an LCD screen test or a monitor screen test, the method is the same: even color, a steady eye and notes on anything that stays put, with no extra hardware.
Frequently Asked Questions
- What is the difference between a dead pixel and a stuck pixel?
- A dead pixel never lights, so it shows as a dark dot on white and on every bright color. A stuck pixel has a subpixel that is always on, so it glows red, green or blue on the black screen. Stuck pixels sometimes recover after rapid color flashing; dead ones do not.
- How many dead pixels are acceptable before a return?
- It depends on the manufacturer. Many follow the old ISO 13406-2 Class II yardstick of 2 always-on, 2 always-off and 5 subpixel faults per million pixels, while premium lines often promise zero bright pixels. A single bright dot on black is hard to live with, so test within your return window.
- Why should I run the test in full screen?
- Without full screen, the tabs, address bar and taskbar cover the edges of the screen, which is exactly where backlight bleed and dead pixels are most often found. Press F11 on a desktop or use the full screen option on a phone, and press Esc to exit.
- Do I need a dark room?
- Only for backlight bleed. Use a dark room, max brightness and the black field for that check. Run the other tests in normal lighting, because a dark room exaggerates small differences you would never notice in everyday use.
- Does the tester work on phones, tablets and TVs?
- Yes. It runs in any modern browser. Android browsers enter full screen after a tap; iOS Safari does not give web pages true full screen, so scroll to hide the toolbar or turn the phone to landscape. On a TV, use the remote's arrow keys.
- Why doesn't my screen resolution match the monitor's spec sheet?
- Browsers report a scaled resolution. Multiply it by the device pixel ratio to get the native screen resolution: 1512 × 982 at a ratio of 2 is 3024 × 1964. The viewport is smaller again because the browser chrome takes part of the window.
- Does the same test work on OLED screens?
- Yes, the color sweep finds dead and stuck pixels on OLED too. OLED has no backlight, so there is no bleed, but watch for burn-in and image retention on the gray and white fields.
- Is anything uploaded when I run the test?
- No. The colors are drawn in your browser and nothing is recorded or sent anywhere. There is no download, no install and no account.