The TV displays market is a competitive space which has continued to drive the development of new HDR technologies to enhance picture quality for LCDs, the dominant display technology today. Some of these new technologies aim to deliver a contrast performance which is equivalent to OLED, but at lower cost in order to be able to address the mass consumer market.
The deeper blacks possible from HDR displays are strongly desirable in TVs (for movies, particularly in low lighting) but also for other products such as surface-integrated automotive displays, monitors and notebooks. While OLEDs offer excellent blacks, LCD can offer much brighter luminance, meaning HDR LCD has the opportunity to offer the best of both worlds for the viewer.
What is HDR?
HDR is short for high dynamic range and is used to describe TVs which offer higher contrast, vivid colors and brighter images. Apart from the picture quality, the price is the other significant difference between TVs with HDR and those without.
Let’s take 65” 4K TVs as an example — with and without HDR. At the lower end of the price spectrum today you can buy a non-HDR LCD TV model at this size for less than $500. Such low prices are the result of continued huge (over) capacity growth for TV production in the past 2-3 years. The price drops over that span have been dramatic, leading to amazing bargains for consumers, whilst display makers have been facing margin challenges.
At the other end of the price spectrum, you could opt for a 65” OLED HDR TV for closer to $4,000. These displays offer excellent contrast and are considerably more costly to manufacture than LCDs. LG Display has nearly monopolized production of these displays for the time being and can therefore command the high price.
HDR technologies — pros and cons
OLED — OLED TVs are a growing premium TV segment and are coveted for their extremely high contrast. When the pixel is “off” it emits essentially no light, meaning that the contrast ratio (the brightness of the pixel when fully “on” divided by its brightness when fully “off”) is higher than 1 million to one. Furthermore, each and every pixel can achieve this contrast independently, regardless of the state of its neighbors. OLED displays are expensive to manufacture due to high materials costs and lower yield than other technologies. They are also prone to burn in and have shorter lifetime than LCD displays. OLED TVs consequently have a high price, which remains the biggest barrier for the average consumer.
FALD and Mini LED — Recently, premium LCD HDR TVs have adopted several backlight innovations that allow LCD performance to move toward the pixel-level very-high contrast of OLED TVs.
Full array local dimming (FALD) LCD TVs have a backlight that is an array of LEDs that is divided into tens or several hundreds of zones, each of which is independently dimmable. This helps to produce deeper blacks and is a vast improvement over basic LCD panels, but can lead to some halo effects for some content (e.g. if a still image requires an abrupt change from bright to dark at a spatial frequency higher than that of the dimming zones).
Mini LED backlight is conceptually an extension of the FALD approach (with more advanced technology required), but there are many more LEDs and therefore many more, and smaller, zones. Rather than several hundred LEDs there will be several thousand or tens of thousands. This adds more cost through BOM and process complexity, but produces even better HDR performance, and many display makers are looking at this today. In such HDR TVs, the backlight can be the most expensive component in the whole unit.
Dual-cell LCD technology — Dual-cell LCD enables a dual-cell structure that allows LCDs to compete with OLED TVs on contrast, particularly levels of black. Dual Cell TVs work by inserting a second pixelated cell (‘modulation cell’) between the display cell and the backlight, which allows the backlight to be modulated down in brightness before it reaches the display cell. This increases the contrast from 1,000:1 to 1,000,000:1 allowing 12-bit depth whilst carrying over all the existing performance advantages of LCD, such as high luminance, long lifetime, no burn-in. Examples include Panasonic’s 55” MegaCon and Hisense 65SX Dual Cell TV.
But as with all new technologies there are trade-offs. The most obvious disadvantage of dual-cell displays is their thickness and weight, but optical issues also arise and require compensation. These issues include moiré/mura and reduced brightness at wide angles due to parallax, and, unlike OLED TVs, glass dual-cell models are not capable of true pixel-level dimming.
All of these performance trade-offs in dual cell are a direct result of the very large separation between the modulation cell and display cell, caused by the thickness of display glass. This inter-cell separation, a couple of millimeters, is much larger than the pixel pitch (for example, a 55” 4K TV has a pixel pitch of 0.31mm). Having an inter-cell separation of several pixels inevitably causes major parallax issues that need overcoming with a suitable compensation film (diffusion layer) between the two cells.
The addition of a compensation film spreads the light from a pixel in the modulation cell over several pixels in the display cell. This explains why the TV cannot offer true pixel-level dimming because even if the two cells are of the same resolution, one pixel illuminated on the modulation cell will illuminate several pixels on the display cell – still excellent performance and a huge improvement on FALD, but not quite the same as OLED in terms of true pixel level dimming. Secondly, the additional diffusion layer between the cells resets the polarization of the light, meaning that four polarizers are needed in total, to ‘re-set’ the polarization. This extra polarizer further reduces the transmission and therefore the brightness or power efficiency of the display.
Dual cell OLCD — A new glass-free display technology, organic LCD (OLCD), has been better known for its flexibility which allows it to conform to surfaces, making it suitable for applications in consumer electronics and automotive. Another key property of OLCD is its thinness, which is key for dual cell design.
OLCD is built onto TAC film, which is ten times thinner than glass, meaning that for dual cell use the two cells can be placed very close together, with an inter-cell separation which is much less than the pixel pitch. Indeed, the two cells can be so close together that no compensation layer is needed. This extreme proximity means that true pixel-level dimming can be achieved — where one open pixel on the modulation cell illuminates one pixel in the display cell, allowing contrast at the single pixel level of 1,000,000:1.
In addition, the absence of the diffusion layer means 3 rather than 4 polarizing layers can be used. This results in lower cost and reduced transmission losses, enabling higher brightness and lower power. The overall OLCD dual-cell stack is thinner than a single sheet of FPD glass.
All of these advantages of OLCD dual cell are realized while maintaining the ability to conform the display — key for applications such as automotive, TVs and monitors.
FlexEnable, the company behind the technology, created a first demo which achieved a contrast of a quarter of a 250,000:1, with a dual-cell thickness of <1 pixel pitch (<300µm), including the polarizers. In a further advancement, in August 2020 the company presented a paper at SID Display Week showing that it can build transistors directly onto polarizers — something that’s uniquely possible with the low temperature manufacturing of OTFT. This is exciting because it means no substrate is required, allowing the cells to be even closer together, with only a 25µm polarizer separating them — which is to say, approaching the cell gap of a single-cell display.
All these competing HDR technologies face their own specific technical and manufacturing challenges. Some of them are easier to overcome than others and it will be ultimately up to display makers and the supply chain to invest and realize the most promising technologies – manufacturing costs will play an important role as they will impact on product price, as will of course demand and market potential. Time will tell which technology will prevail, but with so much innovation happening, TVs performance is only set to improve.
Dr. Paul Cain is Strategy Director at FlexEnable, and has been working in flexible electronics industries for 20 years, in both technical and strategic management roles. Paul has taken new flexible display technologies from lab to fab to commercial product, and has 25 patents relating to processes and architectures that enable the high yield manufacture of flexible displays. Paul has a PhD in Physics from the University of Cambridge and an MBA from London Business School.
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