https://forum.beyond3d.com/posts/1956375/
Another thing that's worrying is current LG WOLED architecture. Because according to Dolby, not every colors are created equal. When BBC & NHK's HLG (Hybrid Log Gamma) broadcast HDR standard has been introduced, Dolby has wasted no time attacking it.
https://www.dolby.com/in/en/technologies/dolby-vision/color-volume-limitations-with-HLG.pdf
Colour BT.2100 PQ Y cd/m2 BT,2100 HLG Y cd/m2
{1,1,1} // Peak White 1,000.0 1,000.0
{1,0,0} // Maximum red 262.7 201.1
{0,1,0} // Maximum green 678.0 627.3
{0,0,1} // Maximum blue 59.3 33.7
Dolby has discovered in their experiment, despite both PQ EOTF and HLG OETF outputting same 1000 cd/m2 white, HLG had less luminances in 3 primaries. Assuming they are both graded in DCI-P3 gamut, HLG will have less color volume access in comparison to the Dolby PQ.
Same thing with the LG OLED. Samsung's mobile RGB OLEDs have perfect colors because they actually fully use 3 primaries that are self-emitting to the fullest extent at any luminance. This is not possible on the LG. Samsung mobile OLEDs are susceptible to burn-in, so LG has decided to use Kodak's WOLED patent instead which enables producing extremely burn-in resistant OLEDs. Instead of 3 RGB subpixels self-illuminating, LG's White OLED method uses stacked OLED color materials that when stacked, produces a white color. This white color goes through a color filter that's located between the stack and subpixels, and for each pixel, three subpixels each have red, blue, and green color filters. So this white light that goes through these color filters regains RGB primaries which can now be combined to make a color.
How does this design mitigrate burn-in? Because burn-in itself can be seen when half-life between R,G,B subpixels are uneven, and in OLED's case (assuming Phosphorus OLED materials are used) , red material has half-life of 250,000 hours, green 400,000 hours, but blue only 20,000 hours. This continues to be a serious problem for Samsung's OLED production. But LG's methods are much simpler. It only uses two colors in a stack, yellow and blue. This yellow material has wavelength between 455nm and 560nm, which means it's actually a yellow material that's veered more towards green, this material alone can replace red and green materials. Together, they produce a white light and color filters pick up just exact balanced amout of red,green,and blue light so uneven luminance balance between three subpixels simply cannot exist on LG OLED TVs. Of course, LG WOLED is not home free yet. Uneven half-life luminance balance still exists on a stack level, so with enough usage, blue will start to emit less light than yellow and this will mean white will start to look more yellow than blue. So, LG has employed another method. Real time white balance correction. There is a sensor that measures color temperature and white balance and if it's being veered in a wrong way, it's processed to change it to correct color balance. This technique has actually been used to early Sony FALDs too when they used to use RGB LEDs instead of White LEDs we use nowadays. FALDs that used RGB LEDs were also very susceptible to uneven dimming zones. The Samsung OLEDs cannot use it because there's nowhere to put processor when light is already illuminated at subpixel level output. LG WOLEDs and FALDs could use this processing because both OLED stacks and LEDs are behind subpixels, meaning more "behind the curtain" jobs can be performed.
Of course, those two aforementioned techniques will ensure balanced color output, but they will still not solve the problem of eventual luminance drop at a pixel level. 20,000 hours blue half-life applies whether it's Samsung or LG.This is where the fourth subpixel comes in, hence the name White Red Green Blue OLED. Because LG has to use color filter, the color filters simply cannot capture every light the stack produces. There has to be wastes. Samsung on the other hand, can have each RGB colors illuminates at a subpixel level, so the entire light can be utilized. So, what should be done with leftover white light? Utilize it as a white subpixel of course! Since white is composed of RGB, whenever a content material calls for a white color output, why waste precious RGB colors when white color can do the same job much better? This is basically the idea. Use an OLED color stack with color filter to mitigrate burn-in, but color filters produce wastes, so use another white subpixel to counterbalance that. A very ingenious solution indeed.
Unfortunately, it still comes with a catch. It will also mean narrower color volume for each RGB colors. Just like Dolby's PQ vs. HLG comparison has shown, Samsung's pure RGB method will contain much more color volume when producing a white color. LG's WOLED will be closer to HLG, more whites, but less RGB. This will mean color saturation will be sacrificed. LG first started out using "Yellow-Blue" stack. Once they decided to make it even more burn-in proof, they went to a three stack, but stll two colors solution: "Blue-Yellow-Blue" stack. two times 20,000 hours = total 40,000 hours blue half-life to spare. Very elegant solution indeed. It has become much more balanced to yellow too.
But it has turned out that yellow alone does not produce enough colors to truly replace red and greens. A single color can only produce so much, so now that LG is deprived of both color gamut and luminance. Since FALDs with gobs of brightness has appeared, LG had to become much more aggressive to combat them. So, now they've gone to more traditional three colors stack (RGB) for next year's model.
Sony has recently released Sony PVM-X550 OLED professional monitor. It also uses Yellow-Blue stack. But for a professional monitor, its brightness was rather low, 400 cd/m2.
Sony has explained the reason: "The panel is rated at 400 nits. Any brighter and the white subpixel starts to wash out the RGB pixels and shrinks the color gamut."
If Sony is to be believed, that means current Y/B stack WOLED LG TV can only attain perfect color volume up to 400 cd/m2 only. Brighter than that, and color volume will start to lose out.
So, going to an RGB stack will have negative impact on LG's current burn-in resistant design, but since it still uses a stack to produce white light, it's still going to be vastly superior to Samsung's RGB design. It will ensure pristine RGB color volume at least up to 1000 cd/m2. Going over to 4000 cd/m2, then it will still remain inadequate. And the brighter it gets, the less color volume the LG WOLEDs will obtain. A compromised design, but at least such compromised design has allowed me to purchase a big-sized OLED TV at a dirt cheap price.
Quantum Dots by the way is more effective solution than WOLEDs when in comes to retaining color volume at higher luminance. Same thing used to happen to LCDs too. When RGB-LEDs were both more burn-in prone and more expensive to produce, Samsung has lead the charge in producing White LED. Blue LED by its design has a very long lifetime, so it was coated with a yellow phosphor and together, they produced a white light, just like the Yellow-Blue WOLED stack. So only one color LED could be used instead of three. However, like WOLED, Y/B combination simply did not have color volume equal to previous RGB LED. Previous RGB LED owners were dissapointed with how colors simply didn't have satuation with White LED. So, QD was introduced so when used as a color filter, nets more light for its color output, improving color saturation even at bright luminance. Of course, traditional LED manufacturers did not sit idle either. Companies like Panasonic has made improvements to yellow phosphors to retain more color volume, so color gamut wise, they're actually wider than QDs.
