Broadcast HDR
Hybrid Log-Gamma in depth: the piecewise OETF, STD-B67 coefficients, 75% reference white, system gamma, relative display behaviour, broadcast compatibility and Resolve workflow.
HLG stores HDR as a relative signal rather than absolute nits. The display system turns that signal into light using peak luminance and system gamma. The lower curve is square-root/gamma-like and the upper curve is logarithmic.
Quick reference
Square-root section below, logarithmic section above.
≈203 cd/m² at a nominal 1000 cd/m² display peak.
Depends on nominal peak/viewing assumptions.
ARIB STD-B67 · HLG
HLG was developed by BBC/NHK for television/live production, where a single relative camera-to-display system is useful and per-shot mastering metadata is undesirable. In BT.2100, HLG is one of the two normative HDR system alternatives alongside PQ.
ARIB STD-B67 defines E′=√(3E) for 0≤E≤1/12 and E′=a·ln(12E−b)+c for 1/12<E≤1. The coefficients are a=0.17883277, b=0.28466892 and c=0.55991073. At the boundary E=1/12, both sections meet at E′=0.5.
In shadows and mid levels, the square-root section behaves closer to conventional television encoding, while the upper logarithmic section creates highlight headroom. This hybrid design helps HLG fit practical television production chains while extending dynamic range.
An HLG code value does not define one fixed luminance for every display. Output luminance depends on nominal display peak and system gamma. The same HLG signal can therefore adapt more naturally to displays with different peak capabilities than direct absolute-PQ reproduction without EETF mapping.
BT.2100 describes the HLG OOTF using a system gamma that changes with nominal peak luminance and viewing environment. For a reference 1000 cd/m² display it is around 1.2 and falls at lower peaks. This is part of HLG system design, not a creative Gamma wheel.
In ITU operational practice, a 75% nominal HLG signal corresponds to HDR Reference White. On an HLG display with a nominal 1000 cd/m² peak, that is about 203 cd/m². So 75% is not '75% of 1000 nits'; it is a specific point inside the HLG transfer/OOTF system.
Like PQ, HLG is part of the signal-transfer system. BT.2100 uses BT.2020 colorimetry, but the HLG label alone does not define matrix/range/bit depth. Those parameters still need to be coordinated for files and SDI/IP paths.
Live production requires real-time shading, graphics, vision mixing and simulcast. HLG supports a single scene-oriented HDR production signal and controlled SDR conversions without per-shot dynamic metadata. That does not make HLG automatically 'better than PQ'; mastering and broadcasting solve different problems.
An HLG signal can produce a recognisable picture on some legacy chains, but a production-grade SDR version still requires controlled conversion and verification. You should not assume any SDR display will correctly interpret a BT.2100 HLG signal and produce master quality without colour management.
SDR graphics, Rec.709 inserts and camera HLG arrive in different source states. Before mixing, they need to be brought into a common production context with a deliberate white mapping. Otherwise titles can become too bright while SDR inserts end up dim or over-compressed.
With HLG, asking 'how many nits does this code value make?' is incomplete without display peak/system gamma. Production is easier when using reference anchors such as the 50% curve split, 75% HDR Reference White and nominal 100% signal/highlight range, while final luminance is evaluated under target display assumptions.
In a managed workflow, assign sources correctly and use a BT.2100 HLG/Rec.2020 output target. In a manual CST pipeline, HLG belongs at the output stage when sources live in scene-referred camera/working spaces. Do not use HLG transfer functions as random look transforms inside the grade or mix HLG output with PQ monitoring without a deliberate conversion.
For HLG, think less in terms of 'this code value equals this many nits' and more in terms of scene-relative signal, nominal display peak and system gamma. That is the simplest way to avoid confusing HLG with PQ.
Technical definitions are anchored to standards bodies and primary specifications wherever that distinction matters.
Primary HLG specification: OETF, coefficients and signal format.
HLG OOTF/EOTF context, system gamma and BT.2020 colorimetry.
75% HLG reference white, monitoring and HDR/SDR production practice.
RCM/CST output transforms and HDR workflow.
