Dead Pixel & Flicker Test Suite

How to check a new monitor for dead pixels

A dead pixel is a display cell that has permanently failed. A stuck pixel is powered but locked to a single colour, while a bright subpixel stays fully lit. A dead pixel receives no power and appears as a small fixed black dot on a bright background, and a bright subpixel can appear as a white or coloured dot on a dark screen.¹

The correct method: run the Pure Black test and scan every corner slowly. Press Space to show/hide the cursor. Left-click or use ← / → to switch colors. Right-click to mark any suspect pixels. Switch to Pure White, where pixels stuck at black show clearly. Repeat with each primary colour. A stuck subpixel appears as a coloured dot on the other colour screens and vanishes on its own.²

Backlight bleed and IPS glow

When a monitor shows bright patches or silvery corners on a dark screen, the cause is usually one of two things: backlight bleed or IPS glow. They look similar at a glance but have completely different origins, and knowing which one you are dealing with determines whether the panel is actually defective or just behaving as its technology dictates.

How backlight bleed happens

LCD panels shine a backlight through a liquid crystal matrix. When the backlight leaks around the panel edges because the frame clamping is uneven, bright irregular patches appear on dark screens. This is a build-quality issue that varies from panel to panel and can worsen if the monitor is subjected to pressure during shipping or storage.³

On a Pure Black test screen, bleed usually shows as brighter corners or cloudy patches along one or more edges. Because it comes from physical light leaking through the panel assembly, its location and intensity stay fixed regardless of where you sit or how you tilt your head, which is the simplest way to tell it apart from IPS glow.

Why IPS glow is different

IPS glow is related but distinct. It is an inherent optical property of IPS panels that produces a silvery shimmer in the corners at oblique viewing angles, and it is present to some degree on nearly every IPS-type panel on the market.⁴ Run Pure Black in a dark room and compare the two. Bleed shows as bright patches along edges that stay put when you tilt your head, while IPS glow visibly shrinks or shifts with your viewing angle. That angular dependence is the defining characteristic. Extreme bleed that remains visible during normal daytime use may justify contacting the retailer or manufacturer for a replacement.³

Ghosting and pixel response time

Ghosting is the faint trail you see behind moving objects on screen, and it is the most visible symptom of a panel whose pixels cannot keep up with the refresh rate. The underlying cause is response time: the milliseconds each pixel takes to move from one shade of grey to another. A single headline GtG figure rarely tells the whole story because real panels have dozens of possible colour transitions, and some are much slower than others.

What ghosting looks like

Ghosting is the faint trail behind moving objects caused by pixels that cannot transition between states fast enough to keep pace with the frame rate. The response time spec (measured in milliseconds, GtG) quantifies how long a pixel takes to move from one shade of grey to another, but real panels have many possible transitions, so a single headline spec may not describe every worst-case colour change you will actually notice.⁵

Reading the UFO test pattern

Run the UFO test and watch the trailing edge of each orb against the background. A faint, barely visible smear behind the fastest orb is typical for mid-range panels and is not a defect. A long bright trail that takes several frames to fade is significant ghosting worth noting. A dark halo appearing in front of the orb, known as inverse ghosting, indicates the overdrive circuit is set too aggressively and is pushing pixels past their target colour.⁴

When to run these tests

Run these tests the same day the monitor arrives, before you mount it, cable-manage around it, or get used to how it looks. Spend at least five minutes working through the full suite while the panel is still in front of you in its out-of-the-box state, so any visible defect is easy to document with plenty of time left on the return window.

If you find a defect, document it while the monitor is still in the relevant test mode. Photograph the screen, note the approximate pixel position relative to the screen edges, and record whether the fault appears on dark or bright backgrounds. Contact your retailer promptly with that evidence in hand, because most return windows are short and a clear photo speeds up the replacement process considerably.

PWM backlight flicker and eye comfort

Many LED monitors dim their backlight by switching it on and off rapidly rather than by reducing the current. This technique is called pulse-width modulation, and at low brightness settings the off-periods become long enough that some viewers notice eye strain, headaches, or a visible flicker when they move their eyes across the screen. The test patterns here help you detect whether your display uses PWM and at what brightness it becomes a problem.

How PWM dimming works

Backlight flicker occurs when a monitor uses pulse-width modulation (PWM) to control brightness. Rather than varying the backlight current continuously, a PWM-dimmed panel switches its backlight on and off at high frequency. At low switching rates this flicker may be visible to some users, or it may still cause side effects such as eye strain and headaches during long sessions, particularly at lower brightness settings where the duty cycle is most extreme and the dark intervals between pulses are longest.⁶ IEEE 1789-2015 treats LED modulation frequencies as a health-risk area and recommends practices to reduce adverse effects for sensitive viewers.⁷

Detecting flicker with a phone camera

The PWM Flicker test uses a phone camera's rolling shutter to make flicker visible to you. Point your phone camera at the test pattern on the monitor and look for dark horizontal bands scrolling through the camera preview. Their presence confirms that PWM dimming is active at the current brightness.⁸ Test at 25 to 50 percent brightness, because many monitors use PWM only below a certain threshold and switch to flicker-free dimming above it. Monitors that use DC dimming vary the backlight current directly and avoid PWM-driven rolling bands, which is the preferable behaviour if flicker-related eye fatigue affects you during long work sessions.⁶