True pixel-level local dimming explained
December 10, 2019
With the TV market saturated, competition between TV makers becomes ever fiercer. Along with other properties, the contrast ratio of the display is one of the key features that relates to picture quality and therefore is a key selling point for TVs.
OLED TVs are a growing premium TV segment, with LG Display currently dominating the market. OLED TVs 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 - i.e. regardless of the state of its neighbours.
By contrast, LCD TVs have a static contrast of a few thousand to one. They can be made very bright by simply using a brighter backlight, but the ‘off’ state will correspondingly get a little less black, because the ability of the liquid crystal and polarizers to block the passage of the light is, though excellent, not perfect.
LCD has reinvented itself many times in the past two decades in response to competitive technologies. Over time the so-called dynamic contrast of LCDs has improved through the addition of more advanced backlights that can be dimmed in different zones dependent on the image being shown. This effectively gives much higher contrast, but not at the single pixel level. For example, a local dimming backlight on a display with a million pixels may have 100 zones that can be independently dimmed. This provides very good dynamic contrast for most content such as movies. But if you need some pixels to be very bright and some to be very dark within a single zone of the backlight, then the contrast within that zone is limited to the static contrast of a few thousand to one.
Recent backlight developments are producing ever more zones. For example, mini LED backlights have taken the numbers of zones from hundreds to thousands, but still far fewer than the millions that an OLED TV effectively has.
Dual Cell LCDs
Rather than adding cost to the backlight with an ever-increasing number of zones, there is another way to increase the LCD contrast that display makers are developing and marketing: Dual Cell LCDs. These comprise two LCD displays stacked on top of each other, in theory taking the contrast from thousands to millions to 1. Examples include Panasonic’s Megacon and Hisense ULED XD. In theory you can achieve pixel level dimming with this approach, but in practice the large separation between the two cells, caused by the two layers of glass in between, means that the dimming effect is spread over many tens of pixels or more. This is a still a great improvement on conventional LCD, but still not quite comparable to OLED.
This inter-cell separation is at the heart of many of the limitations of dual cell. As well as bringing true pixel-level dimming, eradicating the separation will simplify the device (lower cost) and improve the transmission. TVs are typically manufactured on 0.4mm or 0.5mm glass meaning that the inter-cell separation can easily be well over 1mm or more for a glass dual cell.
A new technology – Organic LCD (OLCD) – is built onto TAC film, which is ten times thinner that glass, meaning that the two cells can be brought together with a separation that is much smaller than the pixel pitch. This allows dual cell to enter a new regime where each pixel on the modulation cell acts as a local dimming for a “zone” of one pixel on the display cell. In this way, true pixel level dimming can be achieved with OLCD, providing OLED-like display contrast at a lower cost.
Time will tell which technology will prevail, but with so much development happening, TVs will only become better.
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