One word, several measurements
Brightness on a spec sheet is given in nits, which are candelas per square meter, a measure of how much light a surface emits toward the viewer. The unit is clear. What is less clear is what was lit up when the number was taken, and for how long. A screen does not have a single brightness; it has a range of values that depend on how much of the image is bright, how long it stays bright and what is happening in the rest of the picture.
Two figures often get mixed up:
- Peak brightness is the highest level the screen can reach, typically in a small bright area and for a short time.
- Sustained brightness is the level the screen can hold for a long time, usually across a large portion of the image or the whole of it.
For most displays the peak is higher than the sustained level, and the gap can be large. Neither number is dishonest on its own. Trouble starts when a peak figure is read as if it described the whole screen.
Why a screen cannot light everything at peak
Producing light costs power and creates heat. A display is designed within limits on both, and it cannot necessarily give each pixel or each backlight zone its maximum output all at the same time.
On emissive screens, where each pixel makes its own light, a small bright area draws little total power, so the panel can drive it hard. As the bright area grows, the total power rises, and the display’s circuitry typically reduces the level to stay within limits. The organic materials in OLED are also sensitive to heat and current, which is one reason designers limit the brightness of large bright areas. The technique is often called automatic brightness limiting, and it is why a mostly white screen can be dimmer than a dark scene with a small bright object.
On backlit LCDs, the backlight sets how bright the white can be. A backlight with many zones can focus its output on the area that needs it, boosting a small highlight while dimming the rest. When most of the image is bright, the backlight cannot concentrate its power, and the achievable level may be lower. Backlights also have thermal and power-supply limits, and some designs run at a higher level only for brief periods, after which they step down to protect themselves. Display cooling and the way the housing handles heat have an effect here.
Both approaches can also respond to the length of time. A highlight that lasts a fraction of a second may be allowed to reach a level that would be reduced if the same frame stayed on screen for minutes.
Window sizes
To make brightness measurements comparable, testers often light a patch of white in the center of an otherwise black screen and vary its size, called the window. A small window might cover a few percent of the screen area; a large one may fill half or all of it. The results usually look like this:
- The smaller the window, the higher the measured brightness.
- As the window grows, the measured brightness falls, sometimes sharply.
- A full-screen white measurement is the lowest of the series on many designs, but it is also the one closest to the sustained level.
A hypothetical illustration: a screen might reach 1,000 nits in a 2% window but only 500 nits at full screen. Those numbers are made up to show the idea, and real screens vary widely. The point is that the single number “1,000 nits” would be accurate for one test and misleading for another. Without the window size and the duration, a figure cannot be compared across products.
Why this matters for what you see
How much the difference matters depends on the content and the room.
High dynamic range video has small, bright highlights in mostly darker scenes, such as stars, reflections, lamps or sparks. Those are well suited to a high peak, and a panel with a strong peak can make them stand out. This is why peak brightness gets so much attention in discussions of high dynamic range. By contrast, a bright, snowy scene or a white document fills much of the screen, so it runs against the sustained limit, and the screen may look less bright than the headline number suggested.
Bright rooms and outdoor use depend more on sustained brightness. If you need to read a screen in sunlight, a short flash of high brightness in a small area does not help; the whole image must stay visible. Sunlight readability and window displays illustrate how the demand on the continuous level is much greater in those settings. Reflections and coatings also affect how bright a screen appears, as with screen coatings.
Everyday office work mostly uses a modest level far below either figure. Many people run desktop screens well under the maximum for comfort, a point connected to eye comfort and ambient light adjustments.
Power, heat and lifespan
Running a display near its limit usually means more power and more heat. On some technologies, sustained high output also accelerates aging, so manufacturers design in margins and restrict how long the maximum level is held. This is part of why long-term panel aging and display lifespan are linked to how a screen is used, not just which panel it has. Phones and portable devices may also reduce brightness automatically when warm, which can appear as a sudden dimming outdoors.
Reading brightness claims
Some questions help when you meet a brightness number:
- Is it typical or peak? A typical value is often a full-screen or large-area white; a peak may be a small area.
- What was the window size and how long was it held?
- Does the figure apply in the mode you will use? Brightest modes may differ from default modes, and some HDR modes behave differently from standard ones.
- Does the screen limit brightness automatically, and under what conditions?
- How bright do you actually need it? Your room and content decide that, not the headline figure.
If a listing offers only one brightness number with no conditions, treat it as the most flattering one available. When it matters, independent measurements that show brightness at several window sizes give a fuller picture, and no single figure replaces seeing the screen in the conditions where you will use it.