Step aside, 4K: High Dynamic Range (HDR) is the most exciting leap in picture quality since the transition to HD, and is available on more TVs than ever. But if you take your shiny new HDR TV home and find that the shows are too dark to watch, you might think something is wrong – after all, isn’t HDR all about brightness? Here’s what’s going on and what you can do to make the image brighter.
Why HDR appears dark on some TVs
The movies and shows you’ve been watching for years were mastered in what we now call standard dynamic range or SDR – and it’s actually pretty dim, mastered with peak brightness levels of only about 100 nits. However, most modern LCD TVs can emit 300 nits or more when playing that SDR content, so if you’re in a brightly lit room you can just increase the backlight, which will increase the brightness of everything in the picture . – from dark shadows to bright highlights.
HDR is different. Its main purpose, as the name suggests, is to create a higher dynamic range – that is, a wider gap between the dark parts of a scene and the bright parts. In HDR, bright highlights can be 1000 nits or more, depending on the capabilities of your TV. In HDR, a sun shining through the forest will really bang against the shady foreground, or a campfire will glow like an oasis of warmth against the dark desert night. On the right TV, this makes for an incredible picture, but it doesn’t mean the whole picture is brighter than its SDR counterpart – only those highlights are. The average brightness of the HDR scene should theoretically be comparable to that same scene in SDR (although this can vary from movie to movie depending on how it’s rated).
There’s a problem, though – many TVs use the maximum backlight and contrast levels in HDR mode by default, so you can’t turn them higher for that well-lit living room like you can with SDR content. This is not true for all of them TVs, but it’s common, and it can leave you quite acidic.
Even worse, some TVs actually darken the picture to make up for their HDR shortcomings. “The light output of many valuable 4K HDR TVs is often no different than many non-HDR TVs,” said Robert Heron, a professional TV calibrator and host of the AVExcel home theater podcast. This is most common on cheaper TVs, but it can happen on certain midrange or even high end models that reduce brightness. Combine that with HDR’s wider color palette, which many of these underperforming TVs can’t reproduce, and the TV has to do something to make up for the shortcomings.
When a TV cannot reproduce those bright highlights at the specified levels, it performs a process called tone mapping to adjust the content to its capabilities. Suppose you have a lower TV that can only display 350 nits in HDR. When it plays a scene with 1000 nits peak, it has to adjust the scene so that the peak is only 350 nits. There are two ways TV engineers approach this:
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Some TVs ‘cut’ the bright highlights, keeping the average brightness of the scene. The photo will not darken much, but the highlights may have been blown out a bit.
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Other TVs lower the average brightness of the scene, preserving the details in the highlights, but making the overall picture darker than originally controlled.