Display brightness is quoted as a single number that suggests the whole screen sustains it. The measurement conditions behind that number are considerably narrower.
Peak and sustained are different measurements
Peak brightness is measured with a small bright area on an otherwise dark screen, which lets the panel concentrate available power into a few pixels.
Full-screen brightness is far lower, because driving every pixel hard at once exceeds what the power supply and thermal design can sustain.
The gap between the two is largest on panels where each pixel emits its own light, since those have no shared backlight to spread the load.
How long the figure lasts
Sustained output is limited by heat, and controllers reduce brightness once the panel warms in order to protect the components and keep behaviour consistent.
Peak figures may therefore apply for seconds rather than continuously, which matters for a video highlight and matters less for reading a document.
Manufacturers rarely publish how long peak output is maintained, so the duration has to come from independent testing rather than from the specification.
Why ambient light changes everything
What a viewer perceives depends on the ratio between screen output and surrounding light, not on the screen's output alone.
A bright screen in direct sunlight can be less readable than a dimmer one with a better anti-reflective surface, because reflected light raises the black level.
This is why two displays with similar quoted brightness perform very differently outdoors, and why surface treatment deserves as much attention as the number.
What the number does not cover
Uniformity across the panel is separate, and a screen can hit its rated figure at the centre while being noticeably dimmer towards the corners.
Colour accuracy at high brightness is separate again, since many panels shift in tone as output rises and only hold accuracy in a narrower range.
Minimum brightness matters for use in dark rooms, and it is rarely published at all despite being the figure some users care about most.
Reading a brightness claim usefully
The relevant question is what the display sustains across the whole panel in the lighting where it will be used, not what it peaks at in a test pattern.
Independent measurements usually report both full-screen and window figures, and the difference between them describes the display better than either alone.
For most indoor use the practical ceiling is set by reflections rather than by output, which is why matte and glossy finishes divide opinion so sharply.