Local dimming and HDR: which DisplayHDR tiers need it
By TechCompare · Updated · How we check
VESA's note on the CTS 1.2 contrast test: DisplayHDR 400 can pass with high native contrast or 1D local dimming, 500 and 600 need 1D local dimming or better, and 1000 and 1400 need 2D local dimming or better.
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What each tier asks of the backlight
An LCD can't switch a pixel off. It blocks light from a backlight behind it, and some always leaks through. Local dimming splits the backlight into zones that dim on their own, so a bright spot lifts less of the black around it. DisplayHDR 400 doesn't ask for any, so an edge lit monitor with one backlight level can carry it.
- DisplayHDR 400: No local dimming if the LCD panel has enough contrast of its own. Without that, it needs 1D local dimming.
- DisplayHDR 1000: 2D local dimming or better, so a full array or mini LED backlight.
Step 1
No local dimming
Global dimming, usually edge lit
The whole backlight runs at one level. Most monitors like this light the panel from a row of LEDs along one edge.
A bright spot turns the backlight up for the whole screen, so every dark part turns gray. Contrast is what the LCD panel can do on its own.
The least these tiers need
- DisplayHDR 400 (left tier)
Step 2
1D local dimming
Edge lit zones
The LEDs still sit on one edge, but in groups that dim on their own. VESA's FAQ says a typical string has eight to sixteen groups, and each one lights a strip of the screen.
A bright spot lights its whole strip from edge to edge, so the dark parts in line with it glow too.
The least these tiers need
- DisplayHDR 500
- DisplayHDR 600
Step 3
2D local dimming
Full array (FALD) or mini LED
The LEDs move behind the panel in a grid of zones, and each zone dims on its own. Mini LED is the same idea with smaller LEDs and more zones.
A bright spot only lights the zones around it. Small bright things on black can still get a soft halo, called blooming.
The least these tiers need
- DisplayHDR 1000 (right tier)
- DisplayHDR 1400
Step 4
Self lit pixels
OLED, no backlight
Each pixel makes its own light, so there's no backlight and nothing to split into zones.
A black pixel switches off right next to a bright one, with no halo.
The least these tiers need
- True Black 400
- True Black 500
- True Black 600
- True Black 1000
- True Black 1400
A better backlight can pass a lower tier too, so a mini LED monitor can carry DisplayHDR 600. Sources: VESA's CTS 1.2 criteria and FAQ.
An LCD makes black by blocking a backlight, and some light always gets through. HDR makes that harder, since the backlight has to get brighter for highlights. Local dimming splits the backlight into zones that dim on their own. The four steps below go from no dimming at all to self lit pixels, with the DisplayHDR tiers that need each one.
How this is calculated
No local dimming is the cheapest way. The LEDs usually sit along one edge of the panel and run at one level for the whole picture. A bright highlight pulls that level up, and every dark part of the frame gets brighter with it. Contrast is whatever the panel manages on its own. DisplayHDR 400 allows this, since VESA's FAQ says local dimming isn't required at the 400 level.
1D local dimming keeps the LEDs on the edge but splits them into groups, typically eight to sixteen according to VESA's FAQ. Each group lights a strip of the screen, so a highlight brightens its whole strip from edge to edge. CTS 1.2 asks for 1D or better at DisplayHDR 500 and 600.
2D local dimming puts a grid of LEDs behind the panel, the way full array and mini LED backlights work. A highlight only lights the zones around it. CTS 1.2 asks for 2D or better at DisplayHDR 1000 and 1400. Zones are still far bigger than pixels, so a small bright object on black lights a whole zone and shows a glow, called blooming. The dimming algorithm decides how much: some keep zones bright to show detail, others dim them and lose some of the highlight.
Self lit pixels, like OLED, need no backlight. Each pixel switches off on its own, which is what the True Black tiers are built for.
VESA's CTS 1.2 static contrast test checks black and white in the same image. The minimums are 1,300:1 for DisplayHDR 400, 7,000:1 for 500, 8,000:1 for 600, 30,000:1 for 1000 and 50,000:1 for 1400. Its subtitle flicker test caps how much a patch may change when a subtitle appears next to it, which catches dimming that pumps.
Verdict
For HDR on an LCD, local dimming matters more than the peak number. DisplayHDR 1000 and 1400 are the tiers that need a 2D grid of zones. Within a tier, the zone count and the algorithm still vary, so look for a review that shows blooming on dark scenes before you buy.
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Frequently asked questions
What is blooming?
Are more dimming zones always better?
Can an edge lit monitor do HDR?
Should I turn local dimming off?
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