I see people publishing and using 1000nit HDR patterns for calibrating the Colour Management System (CMS) on TVs and Projectors. As I have also outlined this in The Display Calibration Guide, this is incorrect!
The reasons for this are many so let’s cover them in this article.
Issue 1: Non-Linearity of CMS Systems
The CMS on TVs and Projectors has varying levels of linearity between the darkest and brightest (most luminous) colours. Since the majority of content is in the 0 – 150nit range, it makes a lot more sense to make sure that you get an accurate calibration in that range, as opposed to focusing around the 1000nit range where we are well into highlights territory for most content.
If the CMS was linear on all displays, you could ensure that a 1000nit CMS calibration would get you accurate colours down low, but this is not really the case.
Issue 2: Tone-Mapping
A lot of TVs – and pretty much all projectors – use tone-mapping around the 1000nit range already. Tone-mapping can add an extra layer of inaccuracy to the above mentioned non-linearity issue. I would simply not risk this.
Of course, we can argue that projectors need to tone-map the whole range including 0 – 150nits, however, there is a difference between using a multiplier for a range and using further tone-mapping whereby the relative levels between luminance steps are altered.
Issue 3: White Boosting
Some display technologies will use white boosting to reach the quoted display brightness. For example:
- WRGB OLEDs use a white sub-pixel to boost brightness above around 200nits.
- Some DLP projectors will use a white segment in their colour wheels to boost brightness.
- Some LED or Laser projectors use a “boost LED” or a “boost laser” to boost brightness in the higher ranges – normally to add more blue energy.
- Greyscale on all displays can be calibrated to boost the upper range using additional green and blue energy while keeping below 60% more accurate for the majority of content. This is actually a good strategy to use if a display is not bright enough.
What white boosting will do is desaturate colours in the higher luminance ranges and will most certainly affect 1000nits.
Conclusion
As you can see, it is a much better strategy to use 100nit patterns to calibrate a display’s CMS for HDR as opposed to 1000nit patterns. This is the strategy I use and recommend others to do the same. Thankfully, then you use HCFR to do a manual calibration, the automatic pattern generator will generate the correct pattern intensity automatically. However, external patterns still seem to be an issue.
Now this doesn’t mean that you can’t use 1000nit (or even 4000nit and 10,000nit) patterns after calibration to check how a display will render highlight information. You can absolutely do that. However, it is bad economy to prioritise highlight accuracy above the accuracy of the rest of the 99% of content.
The Display Calibration Guide
If you liked this article and would like to learn more about displays, and display calibration, you can get The Display calibration Guide here.
Leave a Reply