Move away, 4K: High Dynamic Range (HDR) is the most exciting leap in image quality since the transition to HD and is available on more TVs than ever before. But if you take your brand-new HDR TV home and discover that the programs are too dark to watch, you might think there is something wrong – after all, isn’t HDR just glare? See what’s going on and what you can do to brighten the image.
Why HDR looks dark on some TVs
The films and programs you’ve watched for years have been mastered in what we now call the standard dynamic range, or SDR – and it’s pretty dark, dominated with peak brightness levels of just about 100 nits. Most modern LCD TVs, however, are able to emit 300 nits or more when playing this SDR content, so if you’re in a well-lit room, you can just increase the backlight, which brightens everything up on image — From dark shadows to bright highlights.
HDR is different. Its main objective is, as its name suggests, to create a higher dynamic range, that is, a larger gap between the dark parts of a scene and the light parts. In HDR, the bright highlights can be 1,000 nits or more, depending on the capabilities of your TV. In HDR, a sun shining through the forest will actually burst against the shaded foreground, or a fire will shine like an oasis of heat against the dark desert night. On the right TV, this creates an incredible image, but it doesn’t mean that the entire image is brighter than its SDR counterpart – only the highlights are. The average brightness of the HDR scene should, in theory, be similar to the same scene in SDR (although this can vary from film to film, depending on how it was graded).
However, there is a problem: many TVs default to the maximum levels of backlight and contrast in HDR mode, so you cannot increase them further into that well-lit living room as you would with SDR content. This is not true for all TVs, but it’s common, and can get you in trouble.
Worse, some TVs actually darken the image to compensate for your HDR failures. “The light output of many valuable 4K HDR TVs is often no different than many non-HDR TVs,” says Robert Heron, a professional TV calibrator and TV host. AVExcel home theater podcast. This is more common on cheaper TVs, but it can happen with certain mid-range or even high-end models that eliminate glare. Combine that with the wider HDR color palette, which many of these low-performance TVs cannot reproduce, and the TV has to do something to make up for its 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. Let’s say you have a low-cost TV with a capacity of only 350 nits in HDR. When playing a scene with 1,000 nits highlight, he needs to adjust the scene so that the highlight is only 350 nits. There are two main ways that TV engineers approach this:
Some TVs will “cut” the bright highlights, maintaining the medium brightness of the scene you are in. The image will not darken much, but the highlights may be slightly off.
Other TVs will lower the scene’s average brightness, preserving the details in the highlights, but making the overall image darker than it was originally created.