Formats and what they carry
HDR is a bundle of three things: a brightness curve, a color space and a way to send hints about the picture. The curve most HDR video uses is PQ (SMPTE ST 2084), made by Dolby and standardized by SMPTE in 2014. It describes absolute brightness from 0 to 10,000 nits, so a value in the file means one specific brightness on screen.
The formats sit on top of that:
- HDR10: the open baseline. It uses 10-bit color, Rec. 2020 primaries, the PQ curve and one set of static metadata for the whole video.
- HDR10+: HDR10 with dynamic metadata, so the hints change scene by scene. Samsung made it, and content makers don't pay to use it.
- Dolby Vision: also dynamic metadata, and it can describe up to 10,000 nits.
- HLG: made by the BBC and NHK for broadcast. It carries no metadata and is royalty free.
Content is commonly mastered at 1000 or 4000 nits with P3 colors, then adapted to whatever screen plays it. That step is called tone mapping. A monitor that peaks at 600 nits has to squeeze the highlights that a 1000 nit master put above its limit, and how gracefully it does that decides a lot of how good its HDR looks.
What the DisplayHDR tiers test
VESA's DisplayHDR program puts a number on a monitor, and that number is the minimum peak brightness in nits: 400, 500, 600, 1000 or 1400. Peak is measured on a small bright patch, 8% of the screen in the center, so it says little about a mostly white scene. Sustained full screen brightness is tested separately and is lower: 320 nits for the 400 tier, 350 for 600 and 600 for 1000. On the 1000 tier, full screen brightness is 40% less than the peak.
The tiers also set a maximum black level, and that's where panel types start to differ. It's 0.4 nits for the 400 tier, 0.1 for 500 and 600, 0.05 for 1000 and 0.02 for 1400. From the 500 tier up, local dimming is required: 1D or better for 500 and 600, 2D for 1000 and 1400. The 400 tier asks for 1D dimming or a large boost in native contrast instead. Emissive screens like OLED have True Black tiers, with a black level of 0.0005 nits.
Every tier needs a 10-bit video signal, though the panel itself can be 8-bit with dithering.
Why the label can mislead
A peak number is a best case on a small patch, and two screens with the same one can behave very differently.
OLED screens light each pixel on their own, so contrast is excellent, but they limit total power. When a big part of the picture is bright, the screen dims itself. A small sun in a dark sky can reach full peak, while a snowfield can't. Mini LED screens hold their brightness better across the whole picture, but their backlight zones are much bigger than a pixel, so a bright object on black can get a halo, and fewer zones means a bigger halo.
A good HDR review gives three numbers: peak brightness on a small patch, brightness at full screen, and black level with the dimming on. A spec sheet that only gives the first leaves out the two numbers that are usually less flattering.
What HDR adds to the cable
HDR video is normally 10-bit, and the extra bits add to what the cable has to carry. A 4K 60 Hz signal at 8-bit needs 14.3 Gbps and fits the 14.4 Gbps data rate of HDMI 2.0. At 10-bit it needs 17.8 Gbps and doesn't. The usual workaround is chroma subsampling. At 10-bit, 4:2:2 needs 11.9 Gbps, and that fits.
HDMI 2.1 and DisplayPort 2.1 have room for full color HDR at higher refresh rates. The Bandwidth Calculator shows what any mode needs and whether a link carries it.