HDR Reference White

The ITU-R BT.2408 document describes a reference (i.e. diffuse) white level for HDR content of 203 nits. It might seem logical therefore to render SDR content at 203 nits peak, by simply scaling the luminance. Recent research suggests that this approach does not preserve the ‘look’ and ‘creative intent’ of SDR content. In addition, despite ITU recommendations, most theatrical and episodic HDR content does not have a diffuse white level of 203 nits. The diffuse white level in such content usually changes from scene to scene, and is far closer to the typical diffuse white level of SDR content of between 80 and 100 nits at most. 

LG Electronics OLED Pro Monitor Best Practices and Additional Usage Information

In reality, there is no such thing as reference white, any more than there is a fixed value for 18% gray or fair skin. Diffuse white can be 140 nits or less indoors or as much as 400 nits outdoors, skin tones can be + 1.7 stops brighter outdoors than indoors, and the same goes for 18% gray!

“One key new feature of HDR is that it can allow for scene to scene overall luminance changes, so that daylight scenes can feel substantially different from indoor scenes, and night scenes. Thus, constraining a system to set the diffuse white point to a constant luminance defeats some of the advantages of HDR.”

Pupillometry of HDR Video Viewing

Leaving little headroom (i.e. a higher value) means brighter diffuse whites at the expense of flatter looking specular highlights, while leaving more headroom allows for better looking highlights.

If graphics white levels are set too high, they will destroy the impact of highlights in the picture, which is why some studios insist on a maximum of 203 cd/m2 unless there is a compelling reason for doing otherwise:

“The video level for the diffuse white, that is the brightest diffuse reflecting part of the scene, is an important aspect of HDR video. The human visual system seems to adapt to this video level and interpret the image accordingly. So, for example, if the image includes graphics, it is important to set that graphics level correctly. If the level is set too high, for example near to the peak brightness of the display, this level is interpreted as the level of a diffuse reflector in the image. Consequently, with the graphic set at this level, there is little impression of speculars or highlights in the image, which looks more like a printed image. However, with the graphic set at an appropriate level, several stops below the peak display brightness, the human visual system interprets higher luminances as highlights and speculars, an effect known as “brilliance”. This leads to the qualitatively greater impact of HDR video and a feeling of being closer to the real world.”

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“When creating text and graphics, it is important to prepare them for an HDR/PQ finish further downstream in the workflow as they can produce unexpected results. For example, white text/graphics created at code value 1023 (10bit) will appear as 100% white which is around 100 nits on a calibrated SDR display. This same text/graphic will again present itself at 100% white in PQ which would translate to 10,000 nits on a PQ display”. – Dolby