Reference white has been standardized as 203 nits and while that may work out well for live broadcast, for the purposes of dramatic narrative work, it leaves less headroom for specular highlights and emissive light sources. Placing diffuse white at around 140 nits allows room to increase that figure in individual scenes for powerful expressive effect. As has been noted, “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.” At the same time, as the nit value of specular highlights approaches that of diffuse white, they become dull and lifeless, which is why 3X diffuse white is a good starting point and could be the reason Netflix shows with any HDR intent at all are graded to at least 600 nits (though no DP or colorist that we’re aware of has been able to articulate precisely why a speed limit of 600 nits was settled on).
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