HLG (Hybrid Log-Gamma) is a high dynamic range transfer function that encodes scene light into a video signal using a gamma curve for darker tones and a logarithmic curve for highlights. Developed jointly by the BBC and NHK and standardized in ITU-R BT.2100 and ARIB STD-B67, it is the main HDR video format for live broadcast and television production.
Its defining property is backward compatibility: an HLG signal displayed on a standard dynamic range television with Rec. 2020 color still produces a watchable picture, and it works without any metadata, which makes it well suited to live production chains.
How the hybrid log-gamma curve works
HLG is a scene-referred system. The signal describes the relative light in the scene, not an absolute brightness on a display. The opto-electrical transfer function (OETF) is split into two parts that join smoothly at one twelfth of the normalized scene light.
- Lower segment: a square-root curve, close to the conventional camera gamma, covers signal values from 0 to 0.5. This part behaves much like SDR video.
- Upper segment: a logarithmic curve covers signal values from 0.5 to 1.0 and compresses the highlights, giving roughly 12 or more stops of encoded scene range.
- System gamma: the display applies an adjustable gamma, nominally 1.2 at 1,000 nits, which is adapted to the display’s peak luminance and viewing environment.
Because the display, not the content, decides the final brightness, an HLG picture scales naturally from a 400-nit television to a 1,000-nit or brighter panel. The receiver renders the same relative image and adjusts system gamma to maintain perceived contrast.
Key HLG signal parameters
The parameters below define a typical HLG production signal. The reference values come from BT.2100 and its accompanying operational guidance; production practice may vary between broadcasters.
| Parameter | Typical value | What it affects |
|---|---|---|
| Color primaries | Rec. 2020 | Range of reproducible colors |
| Bit depth | 10-bit minimum | Smoothness of gradients, freedom from banding |
| Nominal display peak | 1,000 nits | Reference for system gamma of 1.2 |
| Reference diffuse white | 75% signal level | Placement of skin tones and graphics |
| Transition point | 50% signal level | Boundary between gamma and log segments |
| Metadata | None required | Simplifies live distribution chains |
The 75% reference white is particularly important. Graphics, captions and skin tones are placed around this level so that the remaining quarter of the signal stays free for speculars and bright highlights.
How HLG compares with PQ and other HDR formats
HLG and PQ (Perceptual Quantizer) are the two transfer functions of BT.2100, and they represent opposite design philosophies. PQ is display-referred and encodes absolute luminance up to 10,000 nits, while HLG is relative and leaves rendering to the display.
| Format | Transfer function | Metadata | Main use |
|---|---|---|---|
| HLG | Hybrid log-gamma, relative | None | Live broadcast and streaming |
| HDR10 | PQ, absolute | Static | Discs and streaming |
| HDR10+ | PQ, absolute | Dynamic | Streaming with scene-by-scene tone mapping |
| Dolby Vision | PQ or HLG base layer | Dynamic | Premium streaming and cinema releases |
The trade-off is control versus compatibility. PQ formats let a colorist specify exact luminance and guide tone mapping with metadata, whereas HLG gives up that precision in exchange for a single signal that serves both SDR and HDR screens and survives switching, mixing and editing without metadata management.
Where HLG fits in production
HLG is chosen when one signal must serve every viewer and when there is no time for per-scene grading. Live events are the clearest case, but the format is also practical for user-generated HDR and some mobile recording.
| Situation | Recommended format | Reason |
|---|---|---|
| Live sports broadcast in UHD | HLG | One feed for SDR and HDR homes |
| Scripted drama for streaming | PQ with HDR10 or Dolby Vision | Precise creative control over highlights |
| Phone recording for social media | HLG | Plays acceptably on non-HDR screens |
| Cinema-to-home release | PQ | Matches the mastering workflow and metadata |
Mixed productions often convert between HLG and PQ using the conversion defined in ITU-R BT.2408, so live material can be inserted into a PQ program and vice versa.
Limitations and common pitfalls
HLG’s compatibility is conditional, not universal. A legacy television expecting Rec. 709 color shows HLG pictures with muted, desaturated color, because backward compatibility applies fully only to SDR displays that understand Rec. 2020 primaries.
- Graphics placed too bright: inserting logos at 100% signal makes them glare on HDR screens; place them near the 75% reference.
- Lower creative precision: without metadata, highlight rendering varies more between television models than with PQ.
- Wrong system gamma on monitors: judging HLG on a monitor set to a different peak luminance misrepresents contrast.
- Assuming SDR conversion is free: a dedicated down-converted SDR feed often looks better than relying on the backward-compatible signal alone.
Stripped to the essentials
HLG is a way of recording and sending HDR video. Its main trick is that one picture works on both old and new televisions. A regular television shows it as a normal picture. An HDR television uses the extra information to make bright parts shine more realistically, such as sunlight or stadium lights.
Think of a song mixed to sound fine on a small radio and even better on a large speaker system. No extra instructions travel with the signal. Each television decides for itself how bright the picture should be.
Where it is applied
A national broadcaster covers a football tournament in UHD and wants to offer HDR to viewers with new televisions without running a second channel. The outside broadcast truck uses 10-bit cameras set to HLG with Rec. 2020 color, and the vision engineers shade the cameras on HDR monitors set to 1,000 nits while checking an SDR monitor fed by the same signal.
Score graphics are inserted at about the 75% reference level, so they look natural on both screen types. Viewers with HDR televisions see brighter floodlights and more detail in sunlit parts of the pitch, while viewers with older UHD sets receive a clean standard picture from the same feed, and the broadcaster avoids the cost of a parallel SDR production.