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Comment by bluSCALE4 | Hacker News Reader

What's annoying is that this tech already existed for LCD screens. It supposedly had deep black that OLED brings but at LCD prices. If the quality is similar / no better, it's frustrating to see tandem tech never made it out of China.

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Those Hisense tandem LCDs had some significant issues that they weren't able to iron out, they had trouble with poor response times, ghosting, and color banding. Flanders Scientific also used to make tandem LCDs for ultra-high-end professional monitoring, which was a market that could tolerate high prices and poor power efficiency in the name of performance, but even they have migrated their HDR monitors over to QD-OLED now so I think the consensus is that tandem LCD is a dead end.

Dual-layer LCD is best thought of as a separate technology from tandem OLED, due to being transmissive rather than emissive. In many respects, it surpasses OLED, which is why mastering monitors used in Hollywood still employ this technology. Unfortunately, the poor efficiency and excessive energy consumption/heat output have hindered its adoption in the consumer market.

> In many respects, it surpasses OLED, which is why mastering monitors used in Hollywood still employ this technology.

Flanders Scientific were the main champions of dual LCDs in that market, and even they have phased out all of their dual LCD models in favor of QD-OLED ones now. I think they just brute force through OLEDs usual brightness limits by actively cooling the panel.

And speaking of high-brightness LED panels, I can swear I got a tan from one during a conference where the panel was sitting with the back towards the panel.

Those things are hot and very bright.

Dual LCDs also have poor brightness since a lot less light gets through the two layers. That can be overcome by a much stronger backlight, which produces tons of heat that require active cooling.

Sony also makes dual-layer LCDs, they even introduced a new model in 2023: https://pro.sony/en_GB/products/broadcastpromonitors/bvm-hx3...

Good to know, but that spec sheet neatly demonstrates the efficiency challenges of dual-layer LCD. That's a 31" panel which burns up to 610 watts!

That's probably illegal to sell in the EU?

It's a professional reference monitor not a TV so maybe there's an exception.

Those rules and regulations only apply to consumer products, business/professional products are exempt. This product is obviously not a consumer product.

Also, it costs $25,000. But that might not be related to it being a dual-layer LCD, but rather other exacting tolerances for professional mastering use.

>It supposedly had deep black that OLED brings

How does stacking displays let you get pixel level light emission of OLED or better blacks in general? The issue with LCD is that there is a display wide backlight so you have to actively block out the the backlight to make black. Even if there was some tech to stack something that added more blocking (I guess, adding another liquid crystal layer? Another polarizer? I admit to not really understanding the tech beyond a surface level, just trying to understand.), it doesn't seem to be the same tech as stacking OLEDs which is to get more brightness/less burnout. And if you mean stacking more light emission as in the case of OLED, that also wouldn't seem to contribute to better blacks.

Two LCDs, front to back, provide much better contrast by blocking more of the light in the liquid crystal layers.

The rear one is greyscale.

https://www.reddit.com/r/hardware/comments/w0u2rf/did_dual_l...

https://www.rtings.com/tv/reviews/hisense/u9dg

OTOH, an LCD, at best, let's 50% of the light pass through it, so, for two stacked LCD panels, you'll need a much brighter backlight.

LCDs work on polarization... you typically have an unpolarized source, run it through a polarized filter which is where your 50% loss comes from, then the LCD changes or doesn't the polarization, and you run it through a filter again. If you stacked a second LCD, you shouldn't see a second 50% loss, the light is already polarized by the first LCD.

You are right - the 50% loss comes from the filters, but I forgot to count the loss from the filters. You could add a second monochrome LCD on top of the first one for the same result.

I must be wrong, because this sounds very trivial.

>The rear one is grayscale.

If I understand correctly, you're saying that there is a rear LCD that only either lets light in or blocks it without caring about the color? That's interesting. As for why they aren't investing in this then, it's might be at least in part due to the power usage which would make it unsuitable for battery powered devices such as the iPad mentioned in the article.

There's a diy perks video about making your own which goes into the pros and cons for the curious.

Due to the way LCDs work it’s never useful for a consumer monitor. The backlights needed to punch enough lumens through 2 layers of LCD is prohibitively large/expensive, and the cooling needed to make sure you don’t burn it out leads to a pretty thick and bulky design. Take a look at the professional monitors like this… there’s a reason why they’re so thick, and it’s not because they’re “industrial” or something. Also, power consumption is atrocious.

This sounds like a problem that PixelQi tech could solve - it used prisms instead of filters for colours, which reduced light absorption on inactive LCD pixels, so if the lower LCD is a PixelQi panel, you can add a set of filters with the second LCD panel and drive the backlight harder to gain better contrast (a problem with PixelQi) within the same power envelope of a "regular" LCD.

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