Why doesn't a monitor shine as brightly as the box says?

A television or a monitor bought for being bright: 600 stood on the box, and a film on it really does dazzle — highlights are sharp, the sun in shot is not grey. An hour later you close the player and open a white page of text, without touching the brightness slider. The picture is unremarkable. That sheet glows noticeably dimmer than the same sun did a minute ago.

The explanation that suggests itself is that the screen is cutting corners somewhere. Either the maker claimed more than it should have, or brightness «sags» as the panel warms up. Nothing sags. The sticker was describing a different mode from the one you are reading in, and its number was taken off a small scrap of the frame.

One sticker, three separate measurements

The VESA DisplayHDR certification criteria call for three luminance measurements rather than one, each with its own threshold. In the summary table they carry their full names:

• «8% Center Patch Test Luminance with 2% APL Background – Minimum Luminance Level (cd/m2)» — a white patch over eight per cent of the area with the rest of the frame nearly black, the background set at 2 % APL, where APL stands for average picture level;
• «Full-screen Flash Test – Minimum Luminance Level (cd/m2)» — the whole screen flashing white for a brief moment;
• «Full-screen Long-duration Test – Minimum Luminance Level (cd/m2)» — the whole screen glowing white for a sustained period.

What the first measurement is for, VESA states directly: «an 8% center patch is used to measure the maximum luminance level achievable by the display for a relatively small segment of the screen, while also displaying a lower luminance image across the rest of the screen». That is the number carried onto the sticker. It is also the largest of the three.

Eight per cent is a fairly recent arrival: the patch appeared in revision 1.2 of the criteria, published on 7 May 2024. Every figure below comes from it.

What the DisplayHDR 600 row actually says

Thresholds by tier are gathered into one table, and what to read there is not the names of the tiers but the way three columns come apart inside a single row:

All the figures are candelas per square metre. Take the DisplayHDR 600 row: the sticker promises 600, and the monitor honestly delivers them — across eight per cent of the area. Over the whole area, sustained, it is asked for 350. Which is the white page of text, almost twice as dim as the figure on the box and wholly within the certification.

Another awkward pair of numbers hides in the third column. Tiers 400 and 500 stand equal there, at 320. On sustained full-screen output DisplayHDR 500 is not obliged to be a single candela brighter than DisplayHDR 400, and the entire difference between those two stickers sits in the first two columns.

Nor does the gap between the first and third columns widen as the tier rises. Work it out: 600 ÷ 350 = 1.71, and two steps higher 1400 ÷ 900 = 1.56. The greatest distance between the promise and the everyday belongs not to the brightest tier but to DisplayHDR 600.

That same division by area returns further down the table, now for colour. VESA measures the summed luminance of the three channels separately on a ten per cent colour patch and separately on full-screen colour, and for the 600 tier it is 600 against 350 once more. Splitting by area is no quirk of the white test; it is how the whole certification is built.

Duration is the second thing that moves the figure

Area does not account for the gap on its own. The full-screen flash is required to match the patch: in the 600 row it is 600 again. Sustained glow from that very screen is 350. Brightness that exists for an instant and brightness that lasts are governed by different numbers at identical area.

Why a short test is needed, VESA explains outright: «We also test a full-screen maximum luminance level, both in a quick flash test which represents very brief usage such as an explosion in a movie or game, and the long-term stable luminance levels in full screen usage». The long test is about the opposite case — how a screen behaves when the white field will not go away.

On OLED the flash buys nothing

DisplayHDR keeps a separate branch of tiers for screens with true black — True Black. Its first column repeats the ordinary one, while the other two behave differently:

Columns two and three coincide in every row. A brief instant gives such a screen nothing beyond its sustained output, whereas in the ordinary tiers the flash reached all the way up to the sticker. Against the first column the gap meanwhile runs to a factor of two: True Black 1000 asks for 1000 on the patch and 500 across the whole screen, True Black 1400 for 1400 and 700.

Intuition is upended separately by the top row. On sustained full-screen output True Black 400 is required to manage 250 — less than ordinary DisplayHDR 400 with its 320. Both stickers say 400.

This is a numerical account of what OLED is loved and cursed for. On a small bright detail in the middle of a dark frame it produces a striking figure. On a white sheet across the full screen, half of that. Hence the two separate branches in the certification: one set of thresholds will not cover both kinds of screen.

Find the third column in your own spec sheet

Look up the certification tier in your screen's specification — not «up to 600 nits» from a shop listing, but the word DisplayHDR with a tier beside it. Then open the VESA table and look at the third column of your row. That is the brightness you see on a light interface and in a browser, which is to say most of the time.

Compare it against what is printed on the box. The gap between those two numbers is neither a defect nor a deception: it is the difference between two measurements, of which one went into the advertising.

Seeing it takes no table at all. Open a white page full-screen, then, in the same mode, a dark frame with a small bright source in it: a window, a headlight, a muzzle flash. The distance between the first and third columns is what you have just looked at.

The frame that no test describes

The table sets a minimum for certification, not a passport for your particular unit. A monitor that passed at DisplayHDR 600 might well have shown 700 on the patch — the certificate says only that it never fell below 600.

Its main limitation lies elsewhere, in the method itself. The patch is measured against a background at 2 % APL, which is to say on an almost black frame. Real video hardly ever has such a background: it is neither solid darkness nor a solid white field.

So none of the three numbers will tell you how bright a given film turns out. They describe the bounds within which a screen operates. Which is exactly why there are three of them and not one.

Nothing in the summary of criteria says how long an «instant» lasts, either, or at which second «sustained» begins. Thresholds in frames or seconds are absent from the public table — they live in the full CTS documents, which VESA publishes as separate files. The columns on the site show that there are three measurements; how long each one runs has to be established somewhere other than the sticker.