Why different gray levels matter
A single gray value doesn't tell the whole story, because different uniformity problems reveal themselves at different brightness levels. Near-black levels — 5% and 10% — are where OLED panels most commonly show faint vertical banding and near-black tinting, since that's the darkest range where the panel's individual zones or sub-pixels are doing the least work and any inconsistency between them stands out. Mid-tones — 20% to 50% — are typically where LCD clouding and Dirty Screen Effect show up most clearly, since there's enough light output for a patchy area to visibly stand apart from its surroundings without being washed out. Higher levels, up toward 80-100%, are where corner vignetting is easiest to judge, since the eye is comparing bright, evenly-lit regions against anything dimmer.
What you're actually looking for
Vignetting
A darkening specifically in the corners compared to the center, typically fading gradually rather than having a sharp edge. Common on budget monitors, usually minimal on well-made higher-end ones.
Clouding
An uneven, blotchy patch that can appear anywhere on the panel, not just the corners — often described as looking like a cloud or smudge sitting behind the gray field.
Dirty Screen Effect (DSE)
A stationary mark that stays in exactly the same spot regardless of what's on screen. It's usually invisible on a static gray field at a glance and becomes obvious during real content with camera pans — sports broadcasts and nature footage are the classic examples, where a moving background exposes the one fixed mark that doesn't move with it.
Color tinting
A gray field that leans warm (slightly pink or yellow) or cool (slightly blue or green) instead of neutral, sometimes more noticeably on one side of the screen than the other. This points to a color-filter or backlight-diffusion inconsistency rather than a brightness problem specifically.
Vertical banding
Faint vertical lines running top to bottom, most associated with OLED panels at very low gray levels, sometimes called "5% banding" for the level where it's usually most visible.
Generally not through ordinary settings, since this is a characteristic of the physical panel and its backlight rather than something a picture mode or color adjustment reaches. A handful of higher-end monitors include a "uniformity compensation" feature that deliberately dims brighter zones to match dimmer ones — it genuinely helps, but caps the whole screen's peak brightness at whatever the weakest zone can do, so it's a real trade-off rather than a free fix.
Outside of that specific feature, meaningful non-uniformity on an otherwise properly functioning monitor is something you either live with, or pursue as a warranty claim if it's severe enough — the same category as backlight bleed in that regard. Dell, LG, and Samsung all handle these claims through their own support sites.
This vs. Backlight Bleed Test
These two tools are close relatives and it's worth knowing which one you actually need. Backlight Bleed Test uses near-black and pure-black levels specifically, and focuses on light leaking in from the edges and corners — a distinctly edge-weighted problem. This tool uses a wider range of gray levels across the whole brightness scale, and looks at the panel's consistency everywhere, not just its edges — which is what catches vignetting, tinting, and Dirty Screen Effect that a pure-black test wouldn't necessarily reveal.
In practice, running both takes only a couple of minutes and gives a genuinely fuller picture than either alone.
The honest limits of this test
This is a visual screening tool, not a measurement instrument. It can tell you where to look, whether an issue is repeatable, and whether one setting or one monitor looks better than another side by side — but it can't report an actual luminance value in candelas per square meter, a calibrated color-difference number, or anything a proper colorimeter would give you. Room lighting, your browser's own color management, your viewing angle, and your monitor's current picture settings all affect what you see here, same as with any test that relies on your eyes rather than a meter.
Uniformity is one piece of overall display quality. These cover the rest.
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