Introduction
When people talk about “HDR,” they usually just mean brighter highlights. Behind true HDR, though, sits a formal standard: ITU-R BT.2100, better known as Rec.2100, which defines not just brightness but color, bit depth, and transfer functions all together. If Rec.709 built television’s SDR foundation and Rec.2020 expanded its color limits, Rec.2100 is the rulebook that turns those capabilities into real, viewable high-dynamic-range images. Here’s how it works, why it matters, and how to actually use it in an HDR workflow.
What Is Rec.2100?
Rec.2100 is the official international recommendation for High Dynamic Range Television, published in 2016 and updated in 2023. It uses Rec.2020’s wide-gamut RGB primaries, adds two new transfer functions built for HDR (PQ and HLG), supports 10- and 12-bit precision, defines system brightness up to 10,000 nits, and standardizes HDR video across capture, grading, and display. In short: Rec.2100 equals Rec.2020 color plus HDR encoding, whether that’s PQ or HLG.
Why We Needed Rec.2100
Rec.2020 only ever defined color and resolution, not how brightness itself should be encoded. Without a new transfer curve, all UHD content would have stayed capped at SDR’s 100-nit ceiling regardless of the wider color gamut. Rec.2100 solved that by introducing Perceptual Quantizer (PQ) and Hybrid Log-Gamma (HLG), curves modeled on human vision rather than old CRT physics.
Rec.2100 vs Rec.2020 vs Rec.709 Quick View
| Feature | Rec.709 | Rec.2020 | Rec.2100 |
|---|---|---|---|
| Year | 1990 | 2012 | 2016 |
| Color Primaries | HDTV | UHD (WCG) | Rec.2020 |
| Transfer Curve | Gamma 2.4 | None | PQ or HLG |
| Bit Depth | 8 bit | 10–12 bit | 10–12 bit |
| Dynamic Range | ~6–8 stops | Up to 14 | Up to 14+ |
| Peak Brightness | 100 nits | 1000–10,000 nits | 1000–10,000 nits |
| Use Case | SDR HD | UHD Base | True HDR Content |
Transfer Functions Defined
Perceptual Quantizer (PQ), developed by Dolby Laboratories and standardized as SMPTE ST 2084, encodes brightness logarithmically to match the eye’s nonlinear sensitivity, supporting values up to 10,000 nits. It preserves highlight and shadow detail with real precision and powers HDR10, HDR10+, and Dolby Vision, making it ideal for mastering and streaming. The tradeoff: it isn’t backward-compatible with SDR displays and needs metadata plus tone mapping for every screen it lands on.
Hybrid Log-Gamma (HLG), created by the BBC and NHK for live broadcast HDR, blends a gamma curve for dark values with a logarithmic curve for bright ones, encoding relative rather than absolute luminance, which means no metadata is required. It stays backward-compatible, looking fine on SDR TVs, and it’s simpler for live or broadcast workflows, adapting automatically to whatever brightness the display offers. The tradeoff: less control over absolute highlight rendering, making it less ideal for precise HDR grading.
Encoding Ranges
| Parameter | PQ | HLG |
|---|---|---|
| Peak Luminance | 10,000 nits absolute | Reference 1000 nits relative |
| Black Level | 0 nits (absolute EOTF) | Display-dependent |
| Bit Depth | 10/12 bit | 10/12 bit |
| Metadata | Required (HDR10, DV) | Optional / None |
| Compatibility | HDR only | HDR + SDR fallback |
Color Primaries (Rec.2020 Inherited)
| Primary | x | y |
|---|---|---|
| Red | 0.708 | 0.292 |
| Green | 0.170 | 0.797 |
| Blue | 0.131 | 0.046 |
| White | D65 | |
These coordinates enclose roughly 75% of the visible spectrum, far beyond what Rec.709 can reach.
Bit Depth and Precision
Rec.2100 mandates a minimum 10-bit pipeline, with 12-bit recommended for professional mastering. A wider brightness range needs smaller step sizes to avoid banding in sky gradients and HDR highlights, and that extra bit depth is what enables smoother tone mapping across different devices.
Luminance Targets
| Reference White | Peak White | Black Level | |
|---|---|---|---|
| SDR (Rec.709) | 100 nits | 100 nits | 0.1 nits |
| HDR (PQ Typical) | 203 nits | 1000 nits | 0.001 nits |
| HDR Peak (PQ max) | — | 1000–10,000 nits | 0.0005 nits |
| HLG Reference | Diffuse white ≈ 75% signal | Display-relative | |
Dynamic Range Comparison
| System | Stops | Contrast Ratio | Peak Brightness |
|---|---|---|---|
| Rec.709 | ~8 | 1000:1 | 100 nits |
| Rec.2020 (PQ) | 12–14 | 100,000:1 | 1000–4000 nits |
| Rec.2100 (Full) | 14+ | 1,000,000:1 | 10,000 nits potential |
Practical Workflow in Rec.2100
Capture in a Log gamma like ARRI LogC, S-Log3, V-Log, or C-Log 2, or shoot RAW, to hold onto the sensor’s full dynamic range. For grading, work directly in a Rec.2100 (PQ or HLG) timeline, monitor on a real HDR display of at least 1000 nits for PQ work, and set the output transform to the Rec.2100 EOTF. For delivery: HDR10 or Dolby Vision (PQ) for streaming, HLG for broadcast, and a 12-bit Rec.2100 master paired with an SDR trim pass for archival.
Tone Mapping and Metadata
| Format | Metadata Type | Notes |
|---|---|---|
| HDR10 | Static (ST 2086 / MaxFALL / MaxCLL) | Fixed across content |
| HDR10+ | Dynamic | Scene-based adjustment |
| Dolby Vision | Dynamic | Frame-level tone mapping |
HLG carries no metadata at all; displays derive their own tone curves automatically. Tone mapping in general is what keeps HDR content fitting within a display’s real brightness range without clipping highlights or dulling contrast.
Monitoring and Calibration
| Parameter | PQ Workflow | HLG Workflow |
|---|---|---|
| Peak White | 1000–4000 nits | 1000 nits typical |
| Reference White | 203 nits | Diffuse 75% signal |
| Surround Luminance | 5 nits dark | 10 nits moderate |
| Calibration | Gamma ST 2084 | HLG curve |
| Monitor Example | Sony BVM-HX3110, Apple XDR | Panasonic HLG OLED |
Displays should always be recalibrated with proper HDR reference probes to keep PQ roll-off and color accuracy where they need to be.
Rec.2100 and ACES
ACES 1.3 and later support Rec.2100 Output Display Transforms, tone-mapping scene-linear data directly to PQ or HLG and keeping HDR consistent across different grading suites.
Common Pitfalls
Using a Rec.709 LUT inside a Rec.2100 timeline leads to a desaturated or crushed image. Exporting without metadata means HDR TVs will render the content incorrectly. Monitoring HDR on an SDR display leads to a false sense of exposure. And skipping the SDR trim pass means the SDR fallback looks wrong on legacy screens.
Real-World Applications
| Platform | HDR Type | Transfer Function |
|---|---|---|
| Netflix, Disney+, Apple TV | HDR10 / Dolby Vision | PQ |
| YouTube HDR | HLG / PQ | Both |
| BBC, NHK Broadcast | HLG | HLG |
| UHD Blu-ray | HDR10 | PQ |
Future Outlook
The HDR ecosystem keeps evolving around Rec.2100: Rec.2111 and Rec.2120 extend HDR into live IP broadcast, MicroLED and QD-OLED displays now push past 4000 nits, 16-bit floating point HDR (Dolby Vision Cinema) is under active evaluation, and AI-driven tone mapping is starting to adapt PQ content in real time based on ambient light.
Conclusion
Rec.2100 is really the modern language of HDR, blending Rec.2020’s wide color with brightness curves modeled on human vision itself. PQ delivers reference-grade precision for mastering; HLG delivers practical flexibility for broadcast. Both exist to guarantee that creative intent survives from capture to screen, whether a viewer is watching in a dark theater or a sunlit living room. Mastering Rec.2100 isn’t just adapting to new technology; it’s learning to paint with light at the scale of human perception itself.
Next in the series:
- PQ vs HLG Deep Dive: Choosing the Right HDR Curve for Your Workflow
- Dynamic Range and Human Vision Explained
- Color Volume and Tone Mapping in HDR Display