Why some LinkedIn images glow brighter than white
You scroll past a post and one logo appears lit from within while everything around it stays flat. It is not an animation, a filter, or a trick of contrast — the image is encoded as HDR.
Try it on your image →The short answer
Because the image is encoded as HDR. On an HDR-capable display, its brightest pixels can use luminance headroom above the interface’s normal white level. It is not a CSS glow or animation.
The longer answer
Everything you see on a screen is a set of instructions about light. In an ordinary sRGB image, those instructions are relative: a pixel value of 255 means “as bright as white gets here”, and the operating system decides what that is. On a typical display it lands around 203 nits — the reference level for diffuse white.
An HDR image works differently. Under the PQ transfer function — SMPTE ST 2084 — pixel values are absolute. A value does not mean “bright relative to white”, it means a specific luminance in nits, on a scale that runs to 10,000. So an image can ask for 1,600 nits in a way an SDR image has no vocabulary for.
The feed around that image is still being drawn at ordinary white. The logo is not. That gap is the entire effect.
See it for yourself
Both patches below are the same shade of white. The left one is ordinary interface white. The right one is a real Rec.2100 PQ image encoded at 1,600 nits.
Some LinkedIn images glow brighter than white because they are encoded as HDR, not because of any filter, animation or CSS trick. An ordinary SDR image tops out at the white the interface is drawn with. An HDR image carries absolute luminance instead, so a pixel encoded at 1,600 nits stays 1,600 nits while the feed around it renders at the 203-nit diffuse white standard. On a display with headroom that pixel simply emits more light. This guide explains the mechanism, what survives LinkedIn's re-encoding, and why roughly half of any audience sees it flat.
Why your screenshot looks normal
This is the most common source of confusion. You see the effect, you screenshot it to show someone, and the screenshot looks completely ordinary.
Screen capture on most systems writes an SDR file. The capture pipeline tone-maps whatever was on screen down into the ordinary range and saves that. The 1,600-nit highlight becomes plain white, indistinguishable from the interface around it — the luminance information is discarded at the moment of capture.
Which means the effect cannot be demonstrated in a screenshot, an article image, or a slide. It has to be seen on the actual file, on a display with headroom.
Why only some people see it
- The display needs HDR headroom. Recent iPhones, iPads, MacBook Pros and modern Android OLEDs have it; most ordinary desktop monitors do not.
- On Windows, HDR has to be enabled in the operating system. Many capable monitors ship with it switched off.
- The browser or app has to honour the file’s colour signalling. Most modern ones do.
- The platform has to preserve that signalling through its own image processing — which is the fragile link.
In practice, expect roughly half of a feed audience to see the glow and the other half to see a normal image. Both should look intentional.
Why it survives LinkedIn at all
LinkedIn re-encodes uploads to JPEG, and that re-encode is where most HDR signalling dies. A PNG cICP chunk — the modern, standards-clean way to declare BT.2100 PQ — does not make it through.
An embedded ICC profile does. So the working route is a JPEG carrying a Rec.2100 PQ ICC profile: the pixel samples are already PQ values, and the profile is what tells a colour-managed viewer how to read them.
There is a failure mode worth naming, because it is where most homegrown attempts go wrong. If you attach a PQ profile to ordinary sRGB pixels without converting them, or you convert them and then let a colour-management library convert them a second time, the result is wrong in a specific way: the white comes out dark grey. The samples and the profile have to agree about what the numbers mean.
Questions
Is it a video or an animated GIF?
No. It is a still image. Nothing moves and nothing is playing — the brightness is in the encoding, not in a sequence of frames.
Is my screen broken, or is the brightness turned up?
Neither. What you are seeing is the display using luminance headroom it always had, on the small part of the screen the image asked for it.
Why does it look normal when I save the image?
It depends how you save it. Downloading the original file preserves the HDR; screenshotting the page does not, because screenshots are captured as SDR.
Does it work on every phone?
Most recent flagship phones, yes. Older or budget devices without HDR displays show a normal image. So does any SDR desktop monitor, and Windows with HDR switched off.
Is this an exploit or a trick?
No. Rec.2100 PQ is an ITU broadcast standard and the file is an ordinary image that declares its colour encoding honestly. Platforms could choose to strip that signalling, and some already do.
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.