Short answer
SDR (standard dynamic range) video and images describe a picture relative to the display's white, which is assumed to be around 100 cd/m². HDR (high dynamic range), defined in ITU-R BT.2100, extends the tone curve so highlights can be many times brighter than diffuse white while keeping shadow detail — using either PQ, which encodes absolute luminance up to 10,000 cd/m², or HLG, which is relative and degrades gracefully on SDR screens. HDR also uses the Rec. 2020 primaries, so it usually arrives with wider colour too.
PQ (perceptual quantiser, standardised as SMPTE ST 2084 and adopted in BT.2100) assigns each code value a specific luminance in cd/m², shaped to match the eye's contrast sensitivity from near-black to 10,000 cd/m². Because it is absolute, a PQ signal says exactly how bright a highlight should be, and a display that cannot reach it must tone-map — often guided by metadata about the content's peak. HLG (hybrid log-gamma) is relative: its lower half behaves like a conventional gamma curve and its upper half is logarithmic, and the display scales the result to its own peak. That makes HLG convenient for live broadcast, where the same signal must look acceptable on SDR sets, while PQ dominates streaming and discs.
The single most important number in HDR production is not peak brightness but reference white — the level of a diffuse white object such as paper, or of graphics and subtitles. ITU-R Report BT.2408 recommends 203 cd/m², which is 58% of the PQ signal range and 75% of the HLG signal on a 1,000 cd/m² display. Everything above that is headroom for specular highlights, sun and light sources. Content graded with its whites far above 203 cd/m² looks harsh and leaves no room for highlights; SDR graphics inserted into HDR without being mapped to this level look either dim and grey or glaring.
Operating systems composite SDR windows and HDR content on the same screen, and they must choose how bright SDR white should be. Windows, when HDR is on, treats SDR white according to a user-set 'SDR content brightness' level, and Microsoft notes that desktop users typically set it around 200 cd/m²; out-of-gamut colours are clipped during its automatic colour management. Apple's platforms handle HDR headroom on their own displays in their own way, and behaviour on external monitors varies. On the web, HDR images and video play in capable browsers, but HDR CSS colours are still at the specification stage. The same HDR photo can therefore look spectacular on one laptop and flat or clipped on another.
Because highlights can be much brighter, HDR can show saturated colours at high luminance — a bright red neon sign that SDR has to render either dimmer or paler. Mastering monitors usually cover P3 inside the Rec. 2020 container. Tone-mapping HDR for a less capable display has to reduce both brightness and chroma, and doing that badly produces hue shifts: bright saturated blues drifting towards purple, or skin highlights turning yellow. Good tone mappers work in a perceptual space such as ICtCp (defined in BT.2100) to hold hue while compressing.
| Level | Value | Where it comes from |
|---|---|---|
| SDR nominal peak white (reference display) | 100 cd/m² | BT.2035 environment, cited in BT.2408 |
| HDR reference (diffuse) white | 203 cd/m² | BT.2408 |
| PQ signal for 203 cd/m² | ≈ 58% | BT.2408 |
| HLG signal for 203 cd/m² on a 1,000 cd/m² display | 75% | BT.2408 |
| HLG nominal peak display luminance | 1,000 cd/m² | BT.2100 / BT.2408 |
| PQ encoding ceiling | 10,000 cd/m² | BT.2100 / SMPTE ST 2084 |
| Windows scRGB value 1.0 (nominal SDR white) | 80 cd/m² | Microsoft Advanced Color documentation |
Why: SDR graphics were mapped to HDR peak instead of reference white.
Fix: Map graphics white to about 203 cd/m² (or the platform's SDR white setting).
Why: SDR content is composited at a low SDR white level, or the display's HDR tone mapping is poor.
Fix: Adjust the SDR content brightness setting; use HDR mode mainly for HDR content on displays that handle it well.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
StandardStrong evidence
ITU-R BT.2100 was first approved in 07/2016 and is at edition 3 (02/2025); it specifies the PQ and HLG transfer functions for HDR television with BT.2020 primaries.
StandardStrong evidence
The PQ system can encode luminance up to 10,000 cd/m², and ITU-R Report BT.2408 recommends an HDR reference white of 203 cd/m², corresponding to about 58% PQ signal and 75% HLG signal on a 1,000 cd/m² display.
Caveat: BT.2408 is a report giving operational guidance, not a binding recommendation.
Source: Report ITU-R BT.2408: Guidance for operational practices in HDR television production
FactStrong evidence
In HDR mode, Windows lets users set the SDR reference white level, which Microsoft says is typically around 200 nits on desktop monitors, and clips colours outside the display's target gamut during automatic colour management.
Caveat: Describes Windows behaviour as documented by Microsoft; settings and defaults vary by device and version.
Source: Use DirectX with Advanced Color on high/standard dynamic range displays
Colourwise interpretationModerate evidence
Setting graphics and diffuse white too high in HDR destroys the impact of highlights, which is why reference-white guidance matters more to perceived HDR quality than a display's peak brightness.
Based on: Colourwise reading of the reference-white rationale in ITU-R Report BT.2408.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.