HDR, or high dynamic range, is a term often thrown about without much explanation. You know it's something good to have on your TV, even if you're less sure exactly what it refers to – a bit like the AV world's equivalent of 'organic'.
What's important is that HDR is going to change the way you watch TV.
4K displays tend to get most of the attention, as they produce four times the number of pixels of any HD TV on the market. But is more necessarily better? Do you care how many pixels there are if none of them look any good?
What HDR is promising is better pixels. With HDR, bright whites get brighter, dark blacks get darker, and 10-bit panels are finally able to display the 1 billion colors you've been wanting to see.
4K describes the quantity of pixels, but it's HDR that describes quality.
What is HDR?
So let's clear up some confusion first: The kind of high dynamic range that your TV is capable of shouldn't be confused with the HDR photography options that have recently been added to smartphone cameras.
Both result in images that have a greater contrast between light and dark, but the way they work is slightly different.
High-end cameras and recent smartphone apps utilize HDR by combining several photos taken during a single burst.
Separate photos are taken at different exposures during the process. These are called stops, and the amount of light is doubled from one to the next. So while the first stop produces an extremely dark image, the last result is exceptionally bright, lending better luminosity to the final portmanteau photo.
But that's not the same for video.
Yes, you're still getting an expanded color gamut and contrast ratio, but you're not doing it by combining several images. It all has to do with the way an image is displayed on the screen and the source content used to do it.
While it shares a name and some common points with photography, clearly HDR video is different to smartphone photography. The end result, an image with more contrast between its lightest and darkest areas is the same, but instead of combining separate images this effect is produced using better camera technology to capture footage in the first place.
The improvements you'll see with an HDR image are numerous.
Whereas on a standard display everything below a certain brightness is the same shade of black, an HDR display's range goes further, allowing you to tell the difference between something that's really really dark, and something that's just dark. (Go put on a horror movie and you'll be able to tell the difference immediately.)
So, for example, a wide shot of a campfire at night could have subtle details in the flame rather than appearing 'blown out', along with palpable definition to objects that are barely lit in the gloomy surroundings at the edge of the frame.
As Panasonic's training manager Michael Price says, "It's about control, not just extra brightness and results in more natural expressions of light and shadow."
When we caught up with Hollywood cinematographer Vanja Cernjul he was positively brimming with excitement about HDR, saying that "The whole composition is affected by the increased detail in brightness.
"You no longer have to choose whether to expose for the highlights or the shadows, so when I shoot I try and include a light source in the shot, which I can play with in the HDR grading session."
In post-production Cernjul will tinker with the levels of exposure, brightness, contrast and colour saturation, making them look as good as possible on an HDR compatible screen.
He thinks that filmmakers will, like him, be attracted to work on TV. Having just completed shooting the second season of Marco Polo in HDR, Cernjul sees streaming services as the natural home for HDR production, because of the number of original shows being commissioned by the likes of Netflix and Amazon.
HDR is bringing media closer to what the human eye sees, and by doing so, is creating more realistic images, from scenes bleached with sunlight to nighttime shots on city streets.
Here's the takeaway: HDR TVs and content will display a more realistic color range, with an expanded contrast ratio to make black parts of the image look closer to 'true' black.
So what's HDR like?
Watch some carefully crafted HDR such as Cernjul's short showcase movie, Converging Beams, made for Panasonic, and you can see the potential. You get so accustomed to simultaneously seeing shadow detail and details in the highlights that you may come to find watching standard images a bit flat and dissatisfying.
Not only that, but HDR has real extra zip to it compared with standard viewing. There's a new level of dynamism to images, something hugely evident in a film like like the Lego Movie.
When we went from standard definition to HD, screen sizes were a lot smaller and just having the extra pixels was enough to transform the picture. Now that screens are so much bigger, however, it's not enough to simply add more pixels, as there are other aspects of the image that can be improved.
What HDR adds in terms of additional colours, better clarity in shadows and highlights, is subtle but significant – and it delivers a much more visually satisfying picture than simply having extra pixels. The overall effect is a punchiness that's tantamount to creating a 3D image.
But the tech still needs to catch up to the promise. Some cheaper LCD HDR screens struggle with displaying bright objects against sharply darker backgrounds, causing streaks of light to run down the screen or create halos around bright objects.
These issues are of less concern on the kind of OLED screens made by LG, Panasonic or Sony, where each pixel generates its own light and can go completely dark.
How do I get HDR?
HDR is an end-to-end technology, so every step from creation to distribution to the screen in your home needs to be HDR-compatible, which means that legacy TVs can't show it.
When you go out shopping for a TV screen you may notice an Ultra HD Premium logo on it. This means that the screen offers a level of performance that's guaranteed to get the most out of an HDR source. In terms of specification the screen must have 3840 x 2160 pixels (although this is no different to any other UHD screen) plus it must be able to display a vast number of unique color shades captured within an image.
The other key metric for a Premium-badged panel is a contrast ratio of at least 1,000 nits peak brightness and less than 0.05 nits black level.
Nits are a term that's been adopted by the TV industry to indicate the brightness of a display. 1 nit is approximately equal to the light from a single candle.
Most TV screens in use today offer between 300 and 500 nits, so that gives you a good idea of the greater luminosity required to show HDR.
The above applies to LED-lit LCD screens while for OLED screens (which have lower average brightness and much lower black levels than LCD) it's 540 nits peak brightness and less than 0.0005 nits black level.
Panasonic's Price says that edge-lit LED screens cannot produce high enough contrast and you need direct lighting (from behind the picture, a less-common method of lighting screens) to get enough dynamic range to meet Premium HDR requirements.
In fact, the vast majority of LCD screens on sale aren't equipped to deliver the level of brightness required for a Premium badge so manufacturers are tending to simply sell non-premium models.
Since manufacturers aren't obliged to reveal the number of nits (brightness) their screens achieve, or the contrast ratio, it's up to them to decide for themselves if a non-Premium screen has enough brightness and contrast to deliver a meaningful HDR image.
The consumer electronics industry has previous form when it comes to offering similar but different standards, as while it was a lot easier to understand the difference between HD-Ready and Full HD the gap between "regular" UHD and UHD Premium displays is more fluid.
But the bottom line is that the Ultra HD Premium branding is the most reliable way of ensuring you see what the creator intended.
Another issue is that it's not compulsory to use the UHD Premium logo – others are (confusingly) using their own branding.
Sony's product training specialist Gavin McCarran says: "What the Ultra HD Premium logo doesn't do is show all the range of models that can receive an HDR signal."
Maybe Sony thinks the Premium accreditation will put people off buying the non-premium models because, after all, would you buy a bottle of Moet & Chandon champagne if it were watered down to make it cheaper?
The Ultra HD Premium hallmark can also be used on Ultra HD Blu-ray players and discs as HDR is a mandatory part of the Ultra HD Blu-ray spec.
UHD screens without HDR compatibility will still show a 4K picture in SDR (standard dynamic range) from an Ultra HD Blu-ray but they won't be able to access the HDR metadata carried within the picture.
Offering bit-rates up to 100 Mb/sec (approximately five times that used for streamed movies), in terms of content, Ultra HD Blu-ray is best placed to deliver the HDR goods.
HDR Standards – another cause for confusion
All HDR screens are able to display what's called HDR10, which has the same specification as the UHD Premium standard. It's compulsory on all Ultra HD Blu-rays, plus it appears on HDR dramas streamed by Netflix and Amazon.
But there's another reason to be careful about which brand of HDR screen you buy and it's all down to the fact that there are several varieties of HDR being used by content creators.
Dolby has an HDR10 rival called Dolby Vision, which has increased color depth (it's 12-bit rather than 10) and "up to" 10,000 nits peak brightness (which is far brighter than any current TV can offer). It also offers a feature known as 'dynamic tone mapping' which adjusts the brightness and contrast of scenes depending on how bright or dark they need to be. This dynamic mastering process is all done when the film is being edited, which means all you need to do is sit back and enjoy the fruits of someone else's hard labor.
Only screens and players equipped with Dolby Vision decoding will be able to show Dolby Vision's 'improved' version of HDR. All of LG's OLED sets support it, as does Sony's flagship A1E OLED (two other Sony sets, the XE93 and XE94 are due to receive Dolby Vision support in a forthcoming firmware update).
But Dolby Vision isn't the only player in the field of advanced HDR.
There's Advanced HDR by Technicolor, a nascent HDR format from the people who brought you colored cartoons; Hybrid Log Gamma, which is the standard designed to work with broadcast television; and finally, HDR10+, a new standard that builds some of the benefits of Dolby Vision into an open-standard akin to HDR10.
Technicolor and Philips are working together on another system that creates and delivers HDR and normal versions simultaneously, so that separate streams won't have to be created and graded. The correct content will be automatically chosen to match the screen's peak brightness and dynamic range.
Finally, the BBC and NHK Japan are co-developing a version of HDR that can be used in conventional broadcasts. Called Hybrid Log Gamma, the interesting thing about the standard is that it's backwards compatible, meaning that if an SDR television receives this HDR signal, it will still be able to display an image (though obviously not an HDR one).
HDR isn't just for TVs, it's on your phone, too
While the big screen might be the best way to indulge in high dynamic range content, it's not the only way to do so.
Interestingly, LG's handsets are compatible with the more advanced Dolby Vision standard, although you'll need to use Netflix (see below) to find any compatible content.
If you want to find some YouTube HDR videos you can find a curated list of them here.
Of course, YouTube isn't the only place to get HDR on the go: Netflix has also recently released the news that it too supports mobile HDR on the Samsung Galaxy Note8, LG V30, Sony Xperia XZ1 and Sony Xperia XZ Premium as well.
What's the verdict on HDR?
It really is a landmark in home entertainment history that we can now watch movie content in our home with the same colour grading as in the cinema.
We can't wait for the time when broadcasters adopt HDR for the same reasons – imagine watching live sport broadcast from a stadium split between shadow and bright sunlight without a sudden jump in the exposure as the ball goes into the brighter section.
Cinematically it's going to have a big effect too, with the director no longer needing to choose between exposing for the shadows or the sunlight. Within a year or two it'll likely be possible to say HDR has finally put the ultra in to UHD viewing, and all the issues over different standards will have been resolved.
But unquestionably we are seeing yet another format war and it really is one that looks impossible to call.
Most hardware manufacturers are backing HDR10 (the official standard) but Dolby is a powerful force and has the content creators on its side.
It's not impossible that both standards will continue to co-exist (after all Dolby and DTS share the Blu-ray audio market) but the way to play it safe would be to choose kit with Dolby Vision because HDR10 can always be shown on it.
When will HDR be available?
Right now. If you own an Ultra HD Blu-ray player, are a member of either Amazon Prime or Netflix – or just watch the occasional video on YouTube – and own one of the UHD Alliance-certified TVs, you can watch HDR content today.
The two major streaming providers have also both pledged to bring the technology to new series going forward.
HDR has also been included as standard in the Ultra HD Blu-ray format. The amount of discs available, not to mention the number of players, is currently slim, but as more and more films get released in the format HDR is set to hit the mainstream very soon indeed.
If you're streaming, you'll need a broadband connection of at least 25Mbits to watch streamed HDR, although Netflix uses something called adaptive streaming that gives the advanced screen tech priority over resolution in the case of insufficient bandwidth.
Editor's note: Additional reporting by Dave James, Nick Pino and Jon Porter