Are there still possibilities to improve the contrast of an LCD panel? In addition to the use of local dimming and a solid anti-reflection layer, not much has changed in LCD panels for a long time. That could change with the introduction of Dual Layer LCD technology for televisions. A new term, which we explain in detail in this article.
What is that, a Dual Layer LCD TV?
By way of introduction, the technology appears under two different names: dual layer, or dual cell. We prefer to use dual layer, because that term is clearer.
The concept of a dual layer LCD TV is comparable to the concept of local dimming . We know that an LCD panel is unable to perfectly block the light, so that the black value (and therefore the contrast) is limited. To improve this, manufacturers use local dimming: the background lighting is subdivided into different segments and the TV dims the light in those areas where the image is dark. However, if there are too few zones, there is a risk that you will see the zone boundaries. More zones is therefore better. This is possible, for example, with the help of mini-LED .
Dual layer LCD is another way to achieve the same result. If one LCD panel can create a contrast value of 1,000: 1, you can create a theoretical contrast of 1,000: 000: 1 by placing two LCD panels one behind the other. That is an idea that has been around for a long time and has already been used in some medical imaging monitors.
This is how it works. In the structure of the LCD TV, a second LCD panel is slid between the backlight and the original LCD panel. That extra panel determines how much light passes through to the original panel, it only works in grayscale. That is why it is often referred to as a light modulator or dimming panel. You can consider it as a special kind of local dimming. Each pixel of the dimming panel counts as a dimming zone. For example, a dual layer LCD TV can have millions of dimming zones.
4K or 2K dimming panel?
Theoretically, you would naturally opt for a 4K dimming panel. You actually have about 8 million dimming zones, one per pixel, and you are at the same level as OLED. But we also see that a Full HD dimming panel is chosen, which provides approximately two million dimming zones (one zone per four pixels). The reasons for this are of a different nature. It will undoubtedly be cheaper to use a 2K dimming panel instead of a 4K version. In addition, there are probably also technical reasons: for example, impact on energy consumption. And the extra benefit of a 4K panel may be too small.
We also notice that Hisense still communicated with one million dimming zones at IFA, which would indicate a dimming panel with only the half of Full HD resolution. At CES Hisense spoke of two million dimming zones.
What can you expect from a dual layer LCD TV?
Dual layer LCD TV should be a cheaper alternative to OLED, just like mini-LED. Currently there are professional grading monitors for the film studio of Sony (BVM-HX310) and Panasonic (Megacon). Towards consumers, only Hisense (HZ65U9E) comes out with this technology.
The contrast values currently being claimed vary widely, varying between 1,000,000: 1 and 150,000: 1. This variation is large but not unexpected, since a small difference in black value has a huge impact on contrast. In any case, those values are considerably better than for a traditional LCD TV that is somewhere between 1,000: 1 and 3,000: 1 (without local dimming).
As maximum brightness, 1,000 nits seems to be the guide number, which is brighter than what you can do with OLED today (which is generally around 750 nits). The color range should be close to 100% DCI-P3, the Hisense uses quantum dot technology.
Other reasons for Dual Layer LCD technology for televisions?
LCD panel manufacturers have another reason to look at dual layer solutions. So many LCD factories have been set up in China that there is considerable overcapacity. Excess capacity reduces the price, which is good for the consumer, but manufacturers prefer to reduce their output a bit. But an LCD factory can be compared to many other factories: if it is not running at full capacity, there is a risk of financial loss. A solution where you can use that overcapacity to make a better product (in this case with two LCD panels) is of course attractive.
Unfortunately there are a few important hurdles for this technology, and the most important ones seems to us the energy consumption. An LCD panel has a considerable loss because the light has to go through different optical layers. We often see a figure of around 6% light efficiency. If we place two LCD panels behind each other, that problem will of course be much worse. That energy consumption will not be underestimated, we could already more or less estimate at the show of the Panasonic Megacon at IFA last year. Even when we were standing a meter from the screen you could still feel the heat coming from the screen. This is of course no problem for a studio monitor, but for a consumer product it is different.
Hisense claims that their panel has an efficiency of 4%. That may not seem dramatically less, but still, that means that you have to generate 50% more light in the background lighting to achieve the same brightness as a traditional LCD TV. Such a TV therefore uses at least 50% more power. That could be a problem given the strict energy consumption standards that will come in 2021.
The construction of such a panel is also a challenge. After all, the two LCD panels must be perfectly aligned, or there will be a shadow effect. If the pixel grid of the dimming panel is not perfectly hidden behind the grid of the second LCD panel, it casts a shadow on the second LCD panel. This requires clarity, but also creates other visible image errors. If too many faulty panels roll off the belt (in other words, if the “yield” is low), then that is reflected in a higher price. It remains to be seen whether the result is still economically meaningful.
Finally, we all want slim televisions, but two LCD panels are obviously thicker than one panel. The manufacturer can compensate for this by working with an edge-LED backlight, but in combination with a FALD backlight, such a TV would clearly be a bit thicker, although we do not expect the step to be very large.
Of course, the possible breakthrough of this technology also revolves around cost. At the moment, the price of dual layer LCD seems to be somewhere between a classic FALD LCD TV and an OLED TV, according to Trendforce analysts (attention, indicated prices are only for the panel, not for the finished TV). They also estimate the price to be slightly lower than that of a mini LED TV.
Since Hisense anticipates the introduction of the HZ65U9E for this year, we may know more within a few months. In any case, we expect the price to be below that of a 65-inch OLED.
Dual Layer LCD technology for televisions offers interesting prospects. By sticking two LCD panels behind each other with LCD technology you can raise the contrast to OLED level and the fact that professional grading monitors use it is a clear indicator that the technology has potential. But the consumer market also imposes other requirements on a TV, in particular on energy consumption, and there remains a doubt as to whether dual layer LCD will not consume too much energy.
The technology might also find its way to monitors and laptop computers. displays. BOE (the Chinese panel manufacturer that makes the panels for Hisense) has already announced that it has also developed a 31.5-inch gaming monitor.