HDR Brightness Window HDR peak brightness test window (2%, 10%, 25%, 50%, 100% of screen area)

What a brightness window is

A TV cannot light every pixel at its maximum output at once, so HDR brightness is not one number. Measurers show a white square or rectangle on a black background, sized as a percentage of the screen area, and read the luminance at its center. A 10% window covers one tenth of the screen; a 100% window is a full-screen white field. Reviewers typically measure several sizes because a small window shows the peak a highlight can reach and the full-screen window shows the sustained output a bright scene such as snow or a sports field can hold.

Tom's Guide states the convention plainly: it reports HDR brightness in a 10% window "to keep everything kosher," and adds the full-screen figure separately, because the two differ by a factor of several on the same set.

Why brightness falls as the window grows

The limit is power and heat. Lighting a larger share of the panel at full output draws far more current and generates far more heat, so the TV's automatic brightness limiter (see automatic brightness limiter) reduces output as the average picture level rises. The size of the drop varies by panel type and by picture mode, and measured pairs make it concrete.

On OLED, the LG C6 measures 1,355 nits on a 10% HDR window and 236 nits on a 100% window (Tom's Guide), a ratio of 5.7 to 1. TechRadar measured the LG G6 at 2,471 nits at 10% and 455 nits at 100% in Filmmaker Mode, a ratio of 5.4 to 1, and the LG G5 at 2,268 nits and 331 nits, 6.9 to 1. In a separate review TechRadar logged the Samsung S95F at 2,388 nits at 10% and 465 nits full-screen. Ecoustics measured the Samsung S95H at 2,553 nits on a 10% window in Standard mode and 251 nits at 100%, a 10.2 to 1 spread, and the same set at 1,072 nits at 10% in Filmmaker Mode with 251 nits full-screen. That is the other lever: a picture mode can cut the peak by more than half without touching the sustained number.

On LCD sets with local-dimming backlights the 10% numbers run higher. Tom's Guide measured 3,903 nits on the Sony BRAVIA 9 II, 4,370 nits on the Hisense UR9, 2,713 nits on the original BRAVIA 9 and 2,077 nits on the BRAVIA 7 II, against 2,721 nits for the OLED Samsung S95H, which is above Ecoustics' Standard-mode reading because the two labs used different units and settings. I did not find a full-screen figure for those LCD sets in a source I could load, so the sustained gap is not quantified here. The LG G6 vs Samsung S95H and LG G6 vs Sony BRAVIA 9 II comparisons show how those trade-offs play out between panel types.

The windows that standards use

Three different windows appear in specs, and they answer different questions. VESA's DisplayHDR CTS 1.2, released May 7, 2024, measures peak white on an 8% center patch with a 2% APL background, then adds a full-screen flash test that represents brief usage such as an explosion in a movie or game, and a long-duration full-screen test for stable output. For the LCD tiers the minimums are: DisplayHDR 400 at 400 cd/m² patch, 400 flash and 320 sustained; DisplayHDR 600 at 600, 600 and 350; DisplayHDR 1000 at 1,000, 1,000 and 600; and DisplayHDR 1400 at 1,400, 1,400 and 900. Even the top tier asks for sustained output 36 percent below its peak.

ITU-R BT.2408-9 uses a much smaller window for a different job. It defines HDR reference white as 203 cd/m², about 58 percent on the PQ signal scale, the level for diffuse white and graphics, and it sets monitor peak luminance with an HDR reference white test patch typically 1% of screen area. That is a calibration target, not a capability test. LG Display's panel spec for its fourth-generation WOLED, 4,000 nits of white brightness, is stated on a 3% flash window per FlatpanelsHD, which is why TechRadar's 2,471 nits at 10% for the finished LG G6 sits well below the panel-level claim. See WOLED for the panel structure.

What the numbers mean for real content

Reference white at 203 cd/m² is where most of a movie frame sits, with highlights above it. Compare the full-screen readings to that anchor: the C6's 236 nits is barely 16 percent above reference white, while the G6's 455 nits is 2.2 times it. A snowfield or a white-walled interior that fills the frame is therefore a place where one OLED has headroom and another is already at its limit. Ecoustics notes that most 4K/HDR movies max out around 1,000 nits, so the 10% number is the better guide for highlights and the 100% number is the better guide for bright, flat scenes. The related nits (cd/m²) entry covers the unit, and MaxFALL is the metadata field that describes a title's own brightest average frame.

Common confusions

A "peak brightness" claim without a window size is incomplete, and the smallest window, the shortest flash and the brightest picture mode produce the biggest figure. Compare like with like: same window, same picture mode. Filmmaker Mode figures are the ones to use for movie nights because they reflect the setting a calibrated viewer runs. And a high 10% reading does not predict full-screen output, as the S95H shows: the brightest OLED Ecoustics had measured at 10% in Standard mode held only 251 nits full-screen.