[hdtvtest.co.uk]...4K Blu-ray vs Blu-ray Reveals HDR Is Too Dim for Daytime...
http://www.hdtvtest.co.uk/news/4k-vs-201604104279.htm
Summary
Let’s recap the problem in a nutshell: the HDR presentation in current Ultra HD Blu-ray films is not bright enough for viewing under moderate/ strong ambient lighting, because the HDR metadata forces compatible TVs to be driven at their maximum backlight capacity, leaving no room for higher light output. Of course, there exist other avenues (such as gamma adjustment or dynamic contrast trickery) to brighten the on-screen image for daytime viewing, but these are generally insubstantial compared with the most effective method of raising backlight luminance.
This issue is not confined to 4K Blu-ray – any HDR content that pushes backlight/ contrast to the limit for peak brightness while maintaining an SDR-like APL is going to suffer from the same problem. One potential solution is for manufacturers to provide the option to disable HDR mode either from the display or the Ultra HD BD player (so you’ll get 3840 x 2160 in SDR instead), but that would mean missing out on the most attractive component in the next wave of UHD (ultra high-definition) development.
Whilst it took the arrival of Ultra HD Blu-ray (so we could compare against standard Blu-ray) for us to spot the problem, it seems the video industry was already aware of this potential banana skin. A white paper titled “HDR/WCG Systems Survey: Emerging UHDTV Systems” published by leading calibration software developer SpectraCal here (email registration required to download) contains this illuminating snippet:
"For HDR, the industry is considering 5 nits (cd/m2) to be a desirable surround luminance level.
5 cd/m2 is very dark, roughly the amount of light generated by five lit candles; any switched-on room lamp is likely to exceed this figure. With its usage pretty much restricted to a dark room (not dissimilar to 3D if you think about it), 4K HDR Blu-ray is a niche format that may become even more niche, although in fairness most video enthusiasts would do their critical viewing in a dimly-lit environment anyway. It will be interesting to see if HDR broadcast can succeed in the average living room which is typically not light-controlled…
