Did you know that 4K can mean different things depending on whom you ask?
Strictly speaking, a 4K TV that has a resolution of 3,840 x 2,160 pixels (or 2,160p) should really be called a UHD (ultra-high-definition) TV or 4K UHD TV to avoid confusion with the DCI (Digital Cinema Initiatives) standards, the latter of which are specs defined for digital cinema production and projection systems.
Under DCI, 4K is defined as having a resolution of 4,096 x 2,160 pixels. Many high-end home theater projectors, like the Sony VPL-GTZ1 and VPL-VW1100ES, support this DCI 4K resolution. In fact, there are other 4K resolutions under the DCI spec, such as 4,096 x 1,716 (scope) and 3,996 x 2,160 (flat). Luckily for most consumers, there’s no need to know all these; because when it comes to 4K TVs (ahem, 4K UHD TVs), there’s only one resolution that matters: 3,840 x 2,160.
Standards and trade bodies like the Consumer Electronics Association (CEA) in the U.S. and Digital Europe in Europe have also gotten into the act of defining 4K for consumer products and coming out with their own standardization logos. Both organizations agree that 4K UHD has a pixel count of 3,840 (horizontally) and 2,160 (vertically), and there must be 8 million addressable pixels (3,840 x 2,160) that use an red-green-blue (RGB) sub-pixel layout.
Which brings us to the brouhaha that started a couple of years ago with regards to whether a 4K panel using a red-green-blue-white (RGBW) sub-pixel arrangement can be considered a true 4K UHD TV.
Such RGBW-based LED-LCD 4K TVs are commonly found in China, and they use panels made by Chinese panel makers, and reportedly, South Korean heavyweights LG Display and Samsung Display as well. In LG’s case, it’s called the Green Plus or G+ panel; for Samsung, the Green panel. Long story short, by adding a fourth transparent white sub-pixel, energy consumption of such green 4K panels can be reduced (vs. a traditional RGB panel) without sacrificing brightness. Component costs drop too, which means these panels cost less to make - this is why TVs using them tend to be more affordable than their RGB-based counterparts.
That said, the biggest knock against such panels is that while resolution is increased compared to 1080p panels, due to the pixel layout and the fact that each pixel isn’t made up of three colored sub-pixels, image quality may take hit as their resolution and color accuracy aren’t as good as ‘true’ 4K panels using a RGB matrix.
For those interested, we've reached out to both LG and Samsung for comment. According to a Samsung spokesperson, Samsung Display's Green panels are made for third parties and are not used in Samsung's own 4K UHD TVs. In fact, all of Samsung's UHD TVs are certified by Digital Europe, which means they follow the UHD definitions set out by the European organization.
For LG, the company says its RGBW UHD TVs satisfy standards set by International Standard Organizations such as Intertek (U.K.), TUV (Germany), UL (U.S.), CESI (China), and JEITA (Japan). Also, its RGBW UHD implementation is able to achieve RGB UHD resolution without any loss of UHD picture quality because of its panel algorithm, where nearby pixels can share among themselves a sub-pixel. This allows the RGBW UHD panel to have the same number of pixels as RGB UHD panel. In addition to the energy consumption benefit we've mentioned above, LG also says its RGBW structure allows for the same color reproducibility as a RGB structure, as well as the same picture quality as RGB UHD when upscaling Full HD to 4K. Lastly, LG thinks that the RGBW structure is an appropriate method for implementing future standards such as 8K, which cannot be done with the current RGB method.
Be it differences in definitions or marketing methods, at the end of the day, it boils down to the question of whether you can see the difference. The problem with RGBW 4K LCD panels isn’t so much the technology (cheaper 4K TVs with non-perceptible drop in image quality - that's a good thing, right?), but rather who makes them. For major panel makers, it’s safe to assume that they’d ensure a certain level of quality, either through software or hardware means. The danger is more with RGBW panels sourced from unknown suppliers, because for these panels, you'd never know the ratio between the RGB and white sub-pixels. That is, unscrupulous makers could jolly well use far fewer RGB sub-pixels and make up the number with tons of white sub-pixels to hit the 4K resolution requirement. These are the real fake 4K TVs that we should be wary of.
Now you know.
Speaking of the definition of 4K, take note that at CES 2016 the UHD Alliance, which is made up of the several giant TV manufacturers, tech companies, and Hollywood studios, has shared the specs that a device must have in order to be UHD Alliance Premium certified. For 4K TVs, these include a 10-bit color depth signal, wide color gamut support with at least 90% coverage of the DCI-P3 color space, and more than 1,000 nits peak brightness and less than 0.05 nits black level (for LCD) or more than 540 nits peak brightness and less than 0.0005 nits black level (for OLED). There are also specs for content and mastering, which you can read here.
For consumers, there's no need to remember these specs of course. The only important thing to know is the Ultra HD Premium logo shown above, which you will see being increasingly used by 4K TVs and 4K Blu-ray players/discs to promote themselves. This logo is like a badge of honor, as it signifies that the device or content has passed the UHD Alliance's stringent criteria for providing a "premium UHD experience" for the home. Again, because these require support for the latest technologies, don't expect entry 4K TVs to carry this logo. Examples of UHD Premium-certified 4K TVs include Samsung's KS series SUHD TVs, LG's G6, E6, C6, and B6 OLED TVs and Super UHD TVs, Panasonic's DX900, and Sharp's N9000.
Want to find out if a product you're interested in is UHD certified? The UHD Alliance maintains a few lists:
Update: First published on July 17, 2015, this article has been updated several times to include more info, such as HDMI 2.0a, 2.0b, 2.1, VP9, UHD Premium certification, and more. Last updated on Nov 29, 2018 to include more info about HDMI 2.1 and Ultra High Speed HDMI cables.