Display technologies: Refinements and new entrants

Article By : Brian Dipert

Over the last decade, display developments have been largely evolutionary and pretty predictable. Recent technology newcomers, however, may turn the situation in a more revolutionary direction.

When I last wrote in detail about large-screen display technologies, LCDs were becoming increasingly dominant, at the expense of legacy CRT, plasma, and projection-based predecessors. And OLEDs were ascendant, albeit with no shortage of potential disqualifying fine print, leaving them largely restricted to use in wearables, smartphones, and other small-format and/or “highly disposable” electronics devices.

Fast-forward nearly a decade, and in one sense the largely evolutionary changes in the intermediate years have been pretty predictable. Screens are trending larger, as well has exhibiting higher pixel densities (i.e. resolutions for a given screen size), and LCDs remain dominant. LG in particular continues to push the OLED alternative hard into televisions, retail displays, and other large-screen applications, as traditional color-balance shifts (blue-spectrum in particular) over time, visibility issues in high ambient light environments, and other historical shortcomings are alleviated with technology advancements.

But somewhat surprisingly, as I noted in my coverage of this year’s Consumer Electronics Show, LG’s primary legacy OLED competitor Samsung is seemingly redirecting its large-screen efforts at two alternative technologies, QLED and microLED. They might sound similar, but they’re really not. And inherent in their implementation details, both relative to each other and to LCD and OLED alternatives, are (along with a fiscal gamble on high-profit-margin proprietary technologies instead of commodity alternatives) some clues into why Samsung is going this route. Without further ado, here are some thoughts on the current display technology market leaders along with the upstart contenders to the throne.

The fundamental strength of LCD technology is its longevity, said another way its maturity, given that it’s been under active development for many decades (or over a century, depending on when you start counting). Stealing a couple of graphics plus text from my 2010 article provides an overview review of its operation:

The conventional-LCD-pixel structure is easy to understand, but minor structure variations produce major differences in results. An RGB subpixel triplet is the most common means of delivering color to the eye, but plenty of alternative approaches, such as the PenTile arrangement on the left, are also possible.

Normally, the perpendicular polarization orientations of two parallel polarizer layers block transmission of light, leading to an array of perceived-black pixels. However, ITO (indium-tin oxide) delivers a sufficiently strong applied electric field to alter the intermediary liquid crystal’s modulation properties. This alteration translates to light transmission of varying intensity.

The active-matrix LCD relies on a matrix of TFTs (thin-film transistors), with at least one transistor devoted to each pixel, thereby allowing for precise column-line-to-pixel correlation. After the display controller activates a row line, it drives the relevant pixels’ specific voltages on the column lines. With the now-dominant TN (twisted-nematic) LCD, the liquid-crystal elements twist to varying degrees in response to a varying applied voltage, constructively or destructively interacting with the polarizing filters’ effects to pass varying amounts of light. Precise electric-field control combines with refresh-pattern-modulation techniques to enable the generation of any per-pixel gray-scale value. IPS (in-phase switching) LCDs emerged in response to display users’ requests for improved viewing angles, deeper black levels, and other enhancements. The IPS LCD horizontally aligns the liquid-crystal cells with subsequent application of the per-pixel electrical field through the crystals’ ends, thereby requiring two transistors per pixel—more costly than TN’s approach.

The varying twist and refresh-modulation techniques enable an LCD to dynamically calibrate the luminance intensity of each pixel, thereby generating pure black, pure white, and shades of gray between these range extremes. Subpixels are the keys to an LCD’s ability to generate color from a white-backlight illumination source. Each of the 307,200 pixels in a conventional VGA (video-graphics-array)-resolution panel, for example, comprises three close-proximity subpixels, each with an associated red, blue, or green filter that enables only the relevant portion of the visible-light spectrum to pass through it. Selective control of the subpixels creates the illusion of a pure-color pixel. Dithering further fools the eye and brain, thereby expanding the perceived-color palette.

In that earlier article, I also noted that the light passing through the liquid crystal elements could originate as “ambient-environment-generated light that reflects off a mirrored back panel, self-illumination by a backlight, or both.” Nowadays, nearly all LCDs leverage a backlight, initially comprised of a row of CCFLs (cold cathode fluorescent lamp) but in recent years replaced by an array of LEDs (light-emitting diodes), leading to the misleading marketing term “LED TV” (the backlight, not the inherent display element, is LED-based). More advanced LED backlights employ various esoteric techniques in attempting to expand the displays’ color gamut, contrast range, and other characteristics: clusters of RGB LED triplets instead of full-spectrum “white” LEDs, for example, along with “local dimming” selective illumination control of particular regions within the LED array.

Samsung was a primary “offender” in the “LED TV” marketing moniker debacle, presumably (IMHO) to blur the distinctions between its LCDs and the OLED displays that primary competitor LG was beginning to ramp up promotions of at the time (and in spite of the fact that, with its Galaxy S series beginning in 2010, Samsung was one of the first smartphone manufactures to incorporate OLEDs). In reality, the two technologies are quite different, beginning with OLED’s self-illumination characteristics (“emissive electroluminescence,” if you prefer the fancier term), which therefore require no separate backlight. As a result, OLED displays can be extremely thin as well as highly flexible; at CES this year, LG even showed off OLED TVs that an owner could “roll up” when not in use.

LG showed off its OLED displays at CES on a vast bank of curved screens. Photo: David Benjamin

You might also think that the lack of a backlight would result in OLEDs exhibiting lower power consumption than LCD counterparts, and you’d be right … sometimes. Again quoting from my 2010 OLED coverage, “They deliver excellent power consumption with mostly dark content. However, their battery drain when displaying mostly light material, such as the common arrangement of dark text on a light or white background, can be significantly higher than that of an LCD/backlight combination.” Thereby explaining the “dark” display modes making their way into Android, Chrome OS, iOS and the like, as well as associated applications.

[Continue reading on EDN US: QLED and microLED]

Brian Dipert is Editor-in-Chief of the Embedded Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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