Complete Screen Test Guide
What Are Screen Dead Pixels?
A dead pixel is a pixel on a liquid crystal display panel that cannot function properly. Each pixel consists of three sub-pixels: red (R), green (G), and blue (B). An abnormality in any sub-pixel will cause the pixel to display incorrectly. Dead pixels are generally classified into the following categories:
- Bright/Stuck Pixel: One or more sub-pixels of a pixel remain permanently on, appearing as a continuously lit colored dot against a black background. Bright pixels may appear red, green, blue, or white. In some cases, bright pixels can potentially be restored using professional pixel repair software.
- Dead Pixel: The pixel is completely unable to light up, appearing as a black dot against a white or bright background. Dead pixels are typically caused by permanent damage to the transistor and are generally irreparable.
- Sub-pixel Defect: Only one or two sub-pixels are abnormal and can only be observed against specific color backgrounds.
According to the international standard ISO 13406-2, dead pixels on LCD panels are classified into four grades based on severity. Generally, consumer-grade monitors are considered acceptable if they have 3-5 dead pixels. However, with advancements in manufacturing processes, more and more brands are beginning to offer zero-bright-pixel guarantees.
The best method for dead pixel detection is to check against solid-color backgrounds one by one. Set the screen to solid red, solid green, solid blue, solid white, and solid black respectively, and carefully observe for any abnormal pixels under each color. It is recommended to perform the test with the screen brightness set to maximum and in moderate ambient lighting conditions.
What Is Backlight Bleed? Why Are IPS Panels Prone to It?
Backlight bleed refers to uneven bright areas appearing at the edges or corners of an LCD monitor's screen when displaying a completely black image. Due to the arrangement of liquid crystal molecules in IPS panels, light control at the screen edges is relatively weaker, making them more susceptible to backlight bleed. This typically manifests as pale yellow or white halos around the corners of the screen, known as "IPS Glow".
It's important to distinguish between backlight bleed and IPS Glow: backlight bleed results from poor panel assembly leading to light leakage and is considered a quality defect; IPS Glow is an inherent characteristic of IPS technology. VA panels offer higher contrast ratios and exhibit less backlight bleed. OLED panels do not have backlight bleed issues at all.
IPS vs OLED: Differences in Dead Pixels
IPS and OLED are fundamentally different display technologies, and their dead pixel behaviors and causes differ significantly:
- IPS Dead Pixels: Typically caused by damage to liquid crystals or the Thin Film Transistor (TFT). A dark dot appears as a small black spot on light backgrounds, while a bright dot appears as a continuously lit colored spot on dark backgrounds.
- OLED Dead Pixels: Each pixel is an independent organic light-emitting diode. A defective pixel may appear as a completely non-emitting black dot or a dot with abnormal brightness or color. OLEDs are also susceptible to screen burn-in.
- Pentile Arrangement Impact: Many OLED screens use a Pentile sub-pixel arrangement, where a single defective pixel can affect a larger display area.
Grayscale Test Principle
Grayscale refers to the range of gray levels from pure black to pure white. Standard 8-bit panels can display 256 levels of grayscale (0 to 255), each corresponding to a different brightness value. The grayscale test verifies whether the monitor can clearly distinguish each level of grayscale variation.
A well-calibrated monitor should exhibit a smooth and uniform grayscale transition from pure black (gray level 0) to pure white (gray level 255). If noticeable "steps" (indistinguishable gray levels) or "breaks" (abrupt jumps) are observed, it indicates issues with grayscale performance. 10-bit panels can display 1024 levels of grayscale for even finer transitions.
Gamma Correction Explained
Gamma correction is a fundamental concept in digital display. The human eye's perception of brightness is non-linear — we are more sensitive to changes in dark areas than bright areas. The standard sRGB color space uses approximately Gamma 2.2. The formula is: Output Brightness = Input Value^(1/Gamma).
- Input 0.0 (black) → Output 0.0 (black)
- Input 0.5 (mid-gray) → Output 0.73 (lighter gray)
- Input 1.0 (white) → Output 1.0 (white)
This tool provides "Linear" and "Gamma 2.2" modes. In Linear mode, grayscale values are proportional to pixel values; Gamma 2.2 simulates the output characteristics of a standard monitor.
Refresh Rate and Display Smoothness
The screen refresh rate refers to how many times per second the display updates the image, measured in Hertz (Hz). Common refresh rates include 60Hz, 75Hz, 120Hz, 144Hz, 165Hz, and 240Hz. Higher refresh rates mean smoother motion and lower input latency.
This tool uses the requestAnimationFrame API to measure the rendering interval of 120 frames using high-precision performance.now() timestamps. Factors that may affect results include vertical sync (V-Sync), system load, power saving mode, and variable refresh rate technologies (G-Sync/FreeSync).
Ghosting and Response Time
Ghosting refers to the blurring or trailing effect behind moving objects in fast-moving content. Response time is typically measured as Gray-to-Gray (GtG) in milliseconds. Modern IPS panels have GtG response times of 4-8ms, high-end gaming monitors can achieve 1ms, and OLED panels typically measure below 0.1ms.
VA panels, while excellent in contrast, tend to have slower response times (especially in dark-to-dark transitions), making ghosting more noticeable. This tool simulates motion with a white square on a black background at 1x, 2x, and 4x speeds.
Subpixel Arrangement and Text Rendering
Each pixel in an LCD monitor consists of three subpixels: red, green, and blue. The arrangement affects text clarity and edge smoothness:
- RGB Stripe: The most common arrangement. Windows ClearType and macOS subpixel antialiasing are optimized for RGB.
- BGR Arrangement: The reverse of RGB. If the OS subpixel rendering is set to RGB, text edges may show colored fringes.
- Pentile Arrangement: Common in OLED panels with only two subpixels per pixel. May cause jagged text edges at lower resolutions.
This tool draws 1px-wide red, green, and blue vertical stripes to help identify your display's subpixel arrangement. Using a magnifying glass or phone macro mode gives a clearer view.
Screen Aging and Maintenance Tips
- LCD (IPS/VA/TN) Aging: Manifests as reduced backlight brightness and decreased color saturation. LED backlight lifespan typically exceeds 30,000-50,000 hours. LCD panels do not suffer from burn-in.
- OLED Aging: Organic light-emitting materials degrade over time. Blue subpixels deteriorate fastest, potentially causing color shift. Prolonged display of static content can cause burn-in.
Maintenance recommendations:
- Avoid prolonged operation at maximum brightness.
- Enable screensavers or auto-sleep when not in use.
- For OLED screens, avoid static high-contrast images and use pixel refresh regularly.
- Clean with professional screen cleaner and microfiber cloth. Avoid alcohol or ammonia-based cleaners.
- Maintain appropriate ambient temperature and humidity. Avoid direct sunlight.
How to Use This Tool
- Check Device Info: Your device's resolution, pixel ratio, color gamut and other parameters are automatically displayed at the top of the page.
- Dead Pixel Test: Select a color and click "Fullscreen Test". Carefully observe the solid-color screen for any abnormal pixels. Test with red, green, blue, white, and black in sequence.
- Grayscale Test: Choose "Linear" or "Gamma 2.2" mode and observe whether the grayscale gradient is smooth and uniform.
- Refresh Rate Test: Click "Start Test" and wait for the 120-frame calculation to complete.
- Ghosting Test: Select a speed and click "Fullscreen Test" to observe trailing behind the moving square.
- Subpixel Test: View the fullscreen red, green, blue stripe pattern. Use a magnifier for detailed observation.
- Backlight Bleed Test: Turn off room lights and enter fullscreen pure black mode. Observe screen edges for light halos.
All tests run locally in your browser using Canvas real-time rendering. No data is uploaded. Press ESC or click the screen to exit fullscreen mode.