RunTheTests
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Local Dimming / Black Level Test

See whether your display dims its backlight in zones, and how coarse those zones are.

In a dark room — how far does the glow around the square spread?

How this test works

A small white square is drawn on a full-black field, which is the pattern that makes a backlight reveal its structure. If the display dims in zones, only the zones under the square are lit and you see a halo around it whose width is roughly the size of one zone — so a tight halo means many zones and a wide one means few. If the whole screen lifts, the backlight is being driven as a single unit. And if there is no halo at all, that is ambiguous rather than perfect: it means either light controlled per pixel, as on OLED, or a backlight that never dims, so the tool asks a second question about the black level to tell those two opposite cases apart.

What the results mean

Tight halo
Full-array local dimming with a lot of zones. Working as designed — the halo is what buys the deep black everywhere else.
Wide halo
Zones exist but are large, so a small bright object lights a whole region. A design limit rather than a defect.
Whole screen lifts
No effective local dimming. Blacks look grey whenever anything bright is on screen.
No halo, true black
Per-pixel emission. A black pixel is simply off, so there is no backlight to leak and no zone to bloom.

Common problems and fixes

There is a halo around bright objects in films
That is blooming, and it is the expected trade for local dimming. More zones reduce it; nothing eliminates it short of per-pixel emission.
The whole screen brightens when a bright object appears
Either there is no local dimming, or dynamic contrast is on and driving the whole backlight from average picture level. Check for a dynamic contrast setting first.
No halo but the black looks grey
That is a backlight at constant output with no dimming at all. There is no zone structure to bloom and equally nothing making black look black.

Frequently asked questions

Is blooming a defect?

No. It is the visible cost of local dimming, and a display that shows none either controls light per pixel or is not dimming at all. The useful question is how large the halo is, because that is roughly one zone.

Can this tell me how many dimming zones my TV has?

No, and any page claiming to is estimating. The halo width indicates the class — many zones, few zones, or none — but the count is not readable from a web page. Manufacturer specifications and independent reviews are where that number comes from.

Why does my OLED show no halo at all?

Because there is no backlight. Each pixel emits its own light and a black pixel is off, so there is nothing to leak into neighbouring areas and no zone structure to reveal.

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