In our blog post detailing the color correction workflow for HDR10, a reader asked where reference white and 18% gray should fall on the waveform monitor, and we incorrectly answered 100 nits for reference white and in the 40s for 18% gray. While typical white levels presently used in PQ production range anywhere from 145 – 250 nits, it is recommended that HDR Reference White (diffuse white) be 203 nits or 58% of the full PQ signal (input) level, as shown in the illustration below. 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. It should be noted that 203 nits is only a recommendation for 1,000 nit peak brightness displays: that figure gets progressively higher for displays brighter than 1,000 nits.
If however, you’ve got graphics at 203 nits over a dark image, they may overpower the scene, whereas if the scene is very bright, the graphics may be difficult to see; larger areas of diffuse white may also appear brighter than a small area – which is why 203 nits is only a recommendation and not carved in stone!
“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