HDR brightness limiter: why big bright areas get dimmer

The limiter goes by a few names. On OLED it's often called ABL, for automatic brightness limiter. Either way, the screen looks at how much of the picture is bright and turns the whole picture down when that's more than it can drive. It reacts to the picture, not to the room, so it's a different thing from automatic brightness.

VESA builds it into DisplayHDR. Every tier has a peak test on a small patch and a full screen test held for a long time, and the full screen minimum is lower. DisplayHDR 1000 asks for 1,000 nits on the patch and 600 nits on the full screen. True Black 400, a tier many OLED monitors carry, asks for 400 nits on the patch and 250 on the full screen.

The link below opens the test at 1,000 nits with a 25% box. Start it full screen and hide the controls, then step the box down to 2% and back up with the arrow keys while you watch the white. Your eyes adapt to what fills your view, so switch between sizes quickly instead of staring at one. On many screens the 25% and 50% boxes already look dimmer than the 2% one.

Reviews measure this with windows of set sizes, often 2%, 10%, 25%, 50% and 100%. A row of those numbers tells you more than one peak figure, since real scenes rarely put a single small highlight on black.