Does your display actually show HDR?
Five real Rec.2100 PQ patches, each encoded at a known absolute luminance. Where you stop seeing a difference between steps is roughly where your display runs out of headroom.
This is a free online HDR display test. It renders a ladder of real Rec.2100 PQ patches encoded at known absolute luminances — 203, 600, 1,000, 1,600 and 4,000 nits — so you can see for yourself where your screen stops resolving steps. If every patch above 203 nits looks identical, your display has no HDR headroom, or HDR is switched off in the operating system, and that is the expected result rather than a fault. This matters before exporting anything: the glow depends on the viewer's display, and no file can promise a fixed brightness on every screen.
Checking your display…
The patches below will all look like ordinary white. That is the correct result for an SDR display — nothing is wrong, and the files themselves are unchanged.
Reference white203 nits
The level your interface white sits at.
8-bit PQ code 148
Subtle600 nits
About 1.6 stops above white.
8-bit PQ code 178
Glow1,000 nits
About 2.3 stops. Visible on most HDR laptops and phones.
8-bit PQ code 192
Strong1,600 nits
About 3 stops. Needs a good XDR or OLED panel.
8-bit PQ code 205
Extreme4,000 nits
Beyond nearly all consumer displays; will be tone-mapped down.
8-bit PQ code 230
These are genuine HDR image files, not CSS brightness values. A simulated ladder would tell you nothing about your own hardware.
How to read the result
- All five look identical — no HDR headroom. An ordinary SDR monitor, or HDR switched off in the OS.
- 203 and 600 differ, the rest merge — modest headroom, roughly a DisplayHDR 400-class panel. About one stop.
- You can separate 1,000 from 1,600 — a good HDR display. Recent iPhone Pro, iPad Pro, MacBook Pro or a decent OLED.
- 1,600 and 4,000 look similar even on a great display — expected. Very little consumer hardware reaches 4,000 nits, so the top of the range is tone-mapped.
Why the numbers are what they are
Ordinary interface white sits at about 203 nits — the reference level for diffuse white. Everything above it is headroom. Because perceived brightness works in stops rather than linearly, 600 nits is about 1.6 stops above white and 1,600 is about three. That is why the ladder is not evenly spaced in numbers: it is roughly even in perception.
The 8-bit PQ code beside each patch is the actual byte in the file. Under PQ those codes are absolute: 148 means 203 nits, 192 means 1,000, and 230 means 4,000 — which is also why attaching a PQ profile to ordinary sRGB pixels produces nonsense, since a value of 255 would then be asking for 10,000 nits.
Questions
All the patches look the same. Is something broken?
Almost certainly not. If your display has no HDR headroom, every patch clamps to ordinary white and they become indistinguishable. That is the expected outcome on an SDR monitor.
I am on Windows and see nothing.
Check that HDR is enabled in Display settings. Many capable monitors ship with it off, and the browser will report no headroom until it is on.
Why does the 4,000-nit patch look the same as 1,600?
Because almost no consumer display can produce 4,000 nits. Above the panel’s peak, the system tone-maps everything into what it can actually show, so the top of the ladder compresses.
Does this measure my display’s peak brightness?
No, and nothing in a browser can. The platform deliberately does not expose the display’s real headroom. This shows you where steps stop being distinguishable, which is a useful proxy but not a measurement.
Can I screenshot the result to show someone?
No — a screenshot is saved as SDR, so every patch will look identical in the capture regardless of what you saw. Send them this page instead.
What are the numbers?
Every figure below is either a published constant or a value this tool computes. None is an estimate, and each one is traceable to the standard beside it.
| Measurement | Value | Source |
|---|---|---|
| Diffuse white reference | 203 nits | ITU-R BT.2408 |
| PQ maximum | 10,000 nits | SMPTE ST 2084 |
| 8-bit PQ code for diffuse white | 148 | Computed |
| 8-bit PQ code for 1,600 nits | 205 | Computed |
| Rec.2020 white ceiling | 10,000 nits | Computed |
| Rec.2020 green ceiling | 6,780 nits | Computed |
| Rec.2020 red ceiling | 2,627 nits | Computed |
| Rec.2020 blue ceiling | 593 nits | Computed |
| Blue's share of Rec.2020 luminance | about 6% | Computed |
| LinkedIn logo size | 400 × 400 px | Measured |
| LinkedIn post and ad size | 1200 × 627 px | Measured |
| LinkedIn JPEG re-encode | quality 90, 4:4:4 | Measured |
| Price | Free, no account | This site |
Where do these figures come from?
This tool implements published standards rather than an in-house approximation of them. Each specification below defines part of what the exported files contain.
- ITU-R BT.2100
Defines the HDR system this tool encodes into — BT.2020 primaries with the PQ transfer function.
- ITU-R BT.2408
Sets diffuse white at 203 nits, which is the reference every brightness figure here is measured against.
- SMPTE ST 2084
The PQ transfer function itself: absolute luminance from 0 to 10,000 cd/m², independent of display.
- ITU-T H.273
The code points behind cICP 9 16 0 1 — colour primaries, transfer characteristics, matrix and range.
- W3C PNG Third Edition
Specifies the PNG cICP chunk, which is how an HDR PNG declares itself without an ICC profile.