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HLG: Meaning & Definition in Film

HLG (Hybrid Log-Gamma) is a royalty-free High Dynamic Range (HDR) standard designed primarily for broadcast television due to its backward compatibility.

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HDR for Live Broadcasting

Hybrid Log-Gamma (HLG) is a clever and practical HDR standard developed jointly by two of the world’s leading public broadcasters, the BBC in the UK and NHK in Japan. Its primary design goal was to solve a major problem for live television broadcasting: how to send a single video signal that works on both new HDR-capable televisions and older Standard Dynamic Range (SDR) televisions without requiring separate broadcasts.

The Hybrid Signal

HLG achieves this backward compatibility through its ‘hybrid’ nature. The HLG video signal is composed of two parts:

  1. The SDR Portion: The lower part of the signal’s brightness range uses a standard ‘gamma’ curve, which is the same as any normal SDR broadcast signal. An older SDR TV will read this part of the signal and display a perfectly normal, good-looking SDR image, ignoring the rest of the data.
  2. The HDR Portion: The upper part of the signal’s brightness range uses a ‘logarithmic’ curve. This ‘log’ part of the signal contains the extra information about the brilliant highlights. An HLG-compatible HDR television recognizes this extra data and uses it to display the full, stunning High Dynamic Range image with all its additional brightness and color detail.

Because it doesn’t rely on metadata like HDR10 or Dolby Vision, HLG is much simpler to implement in a live production workflow, making it the ideal HDR solution for live sports, concerts, and other broadcast events.

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