Screen uniformity test
Backlight bleed is looked for on black. This fault is looked for on grey, because that is where it lives: a panel that is not uniform shows it as blotches, vertical bands or a corner darker than the middle, on exactly the flat surfaces — snow, sky, a football pitch — where nothing else is there to hide it. Five levels of grey, with a faint grid to hold the eye still, and a way to tell what is the panel from what is the room.
The test opens full screen. Space — next · Arrow keys — back and forward · Esc — exit
Your browser blocked full screen. The test still works in this window, but a full-screen view is more reliable — try pressing F11.
Was the grey field even across the whole screen?
Running it
Sit where you normally sit, at the distance you normally sit, and look at the whole screen rather than at any one part of it. The fault is a difference between areas, and it is seen by comparing areas, not by staring at one.
Space moves through five greys, from dark to light. Up or G turns a faint grid on and off. Go through all five with the grid off, then again with it on.
The room matters more here than in any other screen test. A lamp behind you, a window to one side, or a screen tilted away from you all produce exactly the unevenness you are looking for, and a fault that moves when you move your head is the room, not the panel. Dim the lights, square the screen to your eyes, and if in doubt, move and look again.
What you might see
Vertical bands — columns a little lighter or darker than their neighbours, running the full height. The panel’s driving or its light guide, and permanent. Most visible at 50 to 80.
Clouding — soft, irregular blotches, usually near the edges or where the frame touches the panel. Pressure on the panel. Common on budget monitors and on laptops that have spent time under weight in a bag; sometimes improves.
A darker corner or edge — at the darkest greys this is normal falloff on an edge-lit panel and every monitor has some. At 128 and above it is a fault.
A colour tint — one side pinker or greener than the other, usually only visible at 180 and 220. The panel’s colour is not uniform. Rare, permanent, and a defect on anything sold for colour work.
Nothing — a flat field at every level. That is a good panel, and it is not the most common result.
Whether it matters
It matters for what you watch. A panel with visible banding will show it in every sports broadcast and every film with a sky in it, and once seen it stays seen. The same panel used for documents and code will never show it, because there is no flat grey in a document.
So the decision is about use rather than about the number of bands. On a new monitor bought for video or games, uniformity you can see at 128 is a reason to return it within the window, and the used-monitor checklist treats it the same way at the point of sale. On a monitor for work, it is a thing to know about and live with.
Windows 11
- Turn off HDR and any auto brightness before testing — both alter the field as it is displayed and HDR in particular changes the grey levels entirely.
- Night light tints the whole screen, which looks like a uniform colour shift rather than an uneven one, but turn it off so the tint test means something.
- There is no Windows setting for uniformity. Some monitors have a uniformity compensation mode in their own menu, on models sold for colour work; it dims the bright areas to match the dim ones and costs contrast.
macOS
- Turn off True Tone and auto-brightness in Displays settings before testing; both adjust the panel continuously and the field will drift while you look at it.
- Apple’s own panels are generally very uniform. An external monitor is what this test is usually for.
- A MacBook that shows clouding near the hinge or the bottom corners has usually been carried with something pressing on the lid, and the marks sometimes ease over days once it stops.
When it is not the panel
A window, a lamp, a light-coloured wall behind you, glasses, a screen at an angle. All of these produce unevenness, and all of them move when you do. Before concluding anything, move your head, then move the lamp. The panel’s faults stay where they are.
Common questions
What is the dirty screen effect?
A panel whose brightness is not even across its area, so that a flat colour looks patchy — like a screen that needs wiping, which is where the name comes from. It is invisible on most content, because most content has detail to hide it, and appears on the flat, slowly moving surfaces where there is nothing else: a pan across a snowfield, a sky, a pitch, a hockey rink. Once seen it is hard to un-see, which is why it is worth knowing about before those moments rather than during.
Why grey and not black or white?
Because the faults hide at both ends. On black the backlight is mostly blocked and the only thing visible is bleed, which is a different fault with its own test. On white everything is driven fully and small differences wash out. Mid-grey is where each pixel is doing half a job and every difference between one area and another is on show. Five levels because different faults appear at different levels — edge darkening and vertical banding first at 50 to 80, clouding around 128, and colour tint from 180 up.
What is the grid for?
To give your eye something to hold on to. On a completely flat field the eye drifts, and a slow drift across a field can look like the field is uneven when it is not — or hide an unevenness that is. A faint grid fixes the gaze, and the patchiness has to be seen between the lines, which is a fairer test of the panel and harder to imagine.
Is some unevenness normal?
On any LCD, yes, and the makers say so in their specifications. Edge-lit panels are lit from one side and are never perfectly even; a few percent of falloff toward the corners on a dark grey is what every monitor does. What is not normal is banding you can see in use, a blotch in the middle of the screen, or a corner that is visibly darker at a light grey. The question is not whether the panel is perfect — none is — but whether you will see it in what you watch.
Can uniformity be fixed?
Mostly not. Banding and falloff are the panel and its lighting and no setting reaches them. Clouding from frame pressure sometimes eases with time or with the monitor stood upright and left alone, and on a laptop it can come from the case being bent, which a repair shop can address. Some expensive monitors have a uniformity compensation mode that sacrifices contrast for evenness. Everything else is a return within the window or acceptance.
The corners are darker only at the darkest grey.
That is falloff, and it is normal. Edge-lit panels are brightest near the lamps and dimmest in the far corners, and a dark grey exaggerates the difference because there is so little light to begin with. If the same corners are even at 128 and 180, the panel is fine and you are looking at physics.