What nits mean for HDR, from SDR white to 10,000 nits

SDR and HDR treat brightness in different ways. In SDR, a code value is a share of whatever the screen can do, so white is as bright as your brightness setting makes it. PQ, the curve HDR10 and Dolby Vision use, gives every code value a fixed brightness in nits instead. The top code means 10,000 nits on any screen, and the screen has to work out what to do with the parts it can't reach.

Most of an HDR picture isn't bright at all. ITU-R BT.2408 puts HDR reference white, the level for a white page or paper, at 203 nits. Faces, walls and skies sit at or below that. The room above it is headroom for highlights: sun on metal, a lamp, a flash of light in a game. A screen with more headroom shows those highlights with more punch, while the rest of the picture looks about the same.

That's why a peak number on a spec sheet only tells part of the story. It's measured on a small patch, and most screens can't hold it on a big bright area. DisplayHDR 1000, for example, asks for 1,000 nits on a small patch and 600 nits on a full screen held for a long time. The brightness limiter guide covers that side.

The Brightness steps test lines up SDR white, 203 nits and seven more levels up to 10,000. On most setups the first few steps get brighter and the last few look the same. Where they stop is your setup's top end.