Gray screen
A flat neutral grey field at sRGB 128, 128, 128, with a slider so you can move to any level you need.
Esc to exit · arrow keys to change · H hides the controls
Dimming changes what is sent to the panel, so this is no longer an exact colour value. Return the slider to 100% before judging anything.
What a gray screen is for
The field that exposes unevenness
A mid grey is the most demanding field a display can be asked to show, and it is what panel engineers and reviewers use when they photograph uniformity. White saturates the eye and hides subtle variation; black hides everything except light leaking in from the edges. Grey sits in between, where the panel is working hardest and every inconsistency in the backlight, the liquid crystal layer or the emissive material has somewhere to show. If a screen looks clean on a grey field it is genuinely uniform.
What 128 actually means
The value 128 is the midpoint of the 0 to 255 code range, not the midpoint of the light your screen emits. Because sRGB applies a transfer curve of roughly 2.2, a code value of 128 produces somewhere near 21 per cent of maximum luminance rather than 50 per cent. That is why mid grey looks considerably darker than you might expect, and it is worth knowing before you conclude your display is dim. If you want a field at roughly half the emitted light, you need a code value nearer 186.
Hunting the dirty screen effect
On televisions in particular, a grey field reveals vertical banding and blotches that look exactly like a screen someone has wiped with a dirty cloth. It is a panel artefact rather than dirt, and it is the thing you notice during a wide panning shot in a football match while never seeing it in ordinary content. Step back several feet: dirty screen effect is a whole-panel phenomenon and is easier to judge from a normal viewing distance than from close up.
Levels for different jobs
Use 5 to 10 per cent for checking shadow uniformity and clouding, which is where backlight problems live. Use 25 to 50 per cent for general mura and dirty screen effect, which is where most panel unevenness shows. Use 75 per cent and above for highlight uniformity and for spotting a colour cast that varies across the panel. Running several levels tells you far more than any single one, because different defects live at different brightness levels.
How to run this test
- Go full screen at the default mid grey and step back to your normal viewing distance.
- Look for vertical or horizontal banding, cloudy patches, and regions that look warmer or cooler than the rest.
- Drop the level to around 5 to 10 per cent and look again, which is where clouding and backlight problems show.
- Raise it to 75 per cent or more for highlight uniformity, then compare what you saw at each level.
What a problem looks like
- Vertical or horizontal banding
- Regular stripes of slightly different brightness running the height or width of the panel. Common on large LCD televisions and the classic signature of the dirty screen effect.
- Mura
- Irregular cloudy patches with soft edges, caused by small inconsistencies in the panel layers. Visible on grey, effectively invisible on most real content.
- Tint variation
- One region of the grey field looking warmer or cooler than another. Very common at the edges of large panels and usually not worth returning a display over unless it is severe.
What to do next
If something looks wrong, check it again in a normally lit room and from straight on, then check whether it appears on a second device. If it only happens on this screen and you are still inside the return window, that is the time to contact the retailer.
Questions people actually ask
Why does 50 per cent grey look darker than half brightness?
Because code value and emitted light are not the same thing. The sRGB transfer curve is close to a power of 2.2, so a code value of 128 emits roughly 21 per cent of maximum luminance. To get about half the light you need a code value around 186.
What grey level should I use to check uniformity?
Run several. Around 5 to 10 per cent shows clouding and backlight problems, 25 to 50 per cent shows mura and the dirty screen effect, and 75 per cent and above shows highlight unevenness and colour drift. No single level catches everything.
Is some unevenness on grey normal?
Yes. Essentially every panel shows some variation on a grey field, and large televisions show more than small monitors. What matters is whether you can see it in real content at your normal viewing distance. If you cannot, it is not a problem worth acting on.