Why it exists
Broadcast cannot send different streams to HDR and SDR households, and it cannot rely on metadata surviving a chain of transmission equipment. HLG solves both: the curve itself is designed so that an SDR display decoding it with a normal gamma gets a watchable picture, and there is no metadata to lose.
The name describes the curve literally. Below roughly half of the signal range it follows a conventional gamma curve, matching SDR. Above that it switches to a logarithmic curve, which is where the extra highlight range lives.
Relative, not absolute
This is the property that separates HLG from PQ and it changes how you think about grading. PQ encodes absolute luminance: code value X means X nits on any display, full stop. HLG is relative, so the same signal maps to different brightness depending on the display's own peak. A 500-nit TV and a 1,500-nit TV showing the same HLG file produce genuinely different pictures, both correct.
How to spot it
ffprobe -v error -select_streams v:0 \
-show_entries stream=color_transfer -of default=nw=1 input.mp4
HLG reports color_transfer=arib-std-b67. HDR10 reports smpte2084. Both usually sit alongside color_primaries=bt2020, so the transfer field is what distinguishes them.
Where you meet it without asking
YouTube accepts and delivers HLG. iPhone HDR recording uses Dolby Vision profile 8.4, which is HLG-based, so an HLG-compatible player sees a valid HLG stream. Most broadcast HDR in Europe and Japan is HLG.
Questions people ask
Is HLG worse than HDR10?
Different rather than worse. HLG carries no metadata and is relative to the display, which makes it robust through a broadcast chain and less precise about intent. HDR10 pins absolute luminance and carries mastering metadata, which is better for a controlled delivery.
Can I convert HLG to HDR10?
Yes, and it is a real conversion rather than a retag: HLG is a relative curve and PQ is absolute, so the mapping depends on an assumed display peak. Simply relabelling the transfer field produces a file that renders wrong.
Why does my HLG file look different on two TVs?
Because that is how HLG works. The curve is relative to the display's own peak brightness, so a 500-nit and a 1,500-nit panel both render the same signal correctly and differently. PQ content does not behave this way.
Related: where HDR video actually plays.