Technical Articles

The Inner World of LCD TVs - Part II


A Bright Future

With OLED TVs becoming available—albeit at exorbitant cost—you might think that LCD TVs have reached the apex of their maturity. However, there are still a few advancements on the horizon that could extend the life of LCD technology for years to come.

Probably the most obvious advancement is the increase in pixel resolution from 1920x1080 to 3840x2160—a four-fold increase in the number of pixels from so-called Full HD to Ultra HD. Many manufacturers are jumping on the UHD bandwagon, offering LCD TVs with over 8 million pixels. The manufacturing process isn't that much different than current panels, so companies are churning them out and selling them at a premium. (Surprisingly, that premium is decreasing much faster than the premium charged for LCD TVs when they were first introduced to compete with plasma TVs.)

Even so, the introduction of UHDTVs is premature, since there is very little content with native UHD resolution. Also, other aspects of UHD content have not yet been finalized, including dynamic range, color gamut, bit depth, and color subsampling. As a result, many UHDTVs purchased today will likely be unable to fully reproduce the UHD content of the future.

The biggest current advantage of UHDTVs is their ability to display 3D with Full HD resolution for each eye using passive-polarized glasses or no glasses at all. Passive-glasses flat panels cut the vertical resolution available for each eye in half, which is a problem when the panel starts with 1080 lines of vertical resolution. But with 2160 lines of total resolution, each eye gets 1080 lines.

Glasses-free or autostereoscopic 3D also reduces the resolution available for each eye, so UHD TVs are prime candidates for this technology. This is great news for companies such as StreamTV Networks and Dolby, which are on the verge of releasing glasses-free 3D for large-screen LCD UHDTVs.

Another advancement is quantum-dot backlighting, which is being developed by Nanosys, QD Vision, and other companies. As particles of certain semiconductor materials shrink to a size of around 2 to 10 nanometers in diameter—larger than a water molecule, smaller than a virus—they emit light in a very narrow band of wavelengths when bombarded with blue light. Why? Because particles that small exhibit quantum-mechanical effects—in this case, they absorb the energy of blue photons and release it again at a very specific wavelength.

The wavelength emitted by a quantum dot depends mostly on its size—larger dots emit red light, smaller dots emit green light, and so on. In fact, the emitted wavelength can be fine-tuned simply by changing the size of the particles, which can be accurately controlled in the manufacturing process.

In a quantum-dot backlight system, a thin film of material containing quantum dots is placed behind the LCD panel, or thin straws of material are placed along the edges of the screen. Instead of using white LEDs, this system uses blue LEDs. The dots in the film or straws are tuned to emit red and green light when blue light from the LEDs hits them, though some of the blue light passes through the material without being absorbed. All three colors combine to form white light, which then passes through the LCD layer like any other backlight.

Fig.11: In QDEF (quantum-dot enhancement film) technology from Nanosys, red and green quantum dots are embedded within a thin film that is placed in front of an array of blue LEDs. The light from the red and green quantum dots combines with the light from the blue LEDs to form white light.

Quantum-dot backlighting can expand the color gamut of an LCD TV and make it brighter—as if LCD TVs needed to be brighter!—and more energy efficient. Sony introduced the first commercial LCD TV using quantum-dot backlighting from QD Vision at CES 2013, calling it Triluminos.

Yet another recent advancement is a new type of transistor. As we've seen, every subpixel in an LCD TV requires at least one transistor, which has traditionally been fashioned from amorphous silicon or low-temperature polysilicon. These transistors are limited in their size and transparency, which reduces the amount of light that can pass through each subpixel. They also need a certain amount of power and can't be turned off entirely, so some light always leaks through even when a subpixel is supposed to be completely black. Finally, the transistors have a limited switching time, leading to motion blur.

A new type of transistor called IGZO (indium gallium zinc oxide) could improve the performance of LCD TVs and other types of displays that rely on these semiconductor devices. IGZO transistors can be made smaller than other types with greater transparency, so more light can get through the panel.

Fig.12: IGZO transistors can be made much smaller than amorphous silicon.

Other advantages include faster switching speeds for less motion blur, lower power consumption, and lower current leakage, resulting in deeper blacks. IGZO transistors have been in development since the mid-1980s, but Sharp is the first company to master the manufacturing process, and the company is now including this type of transistor in its higher-end sets.

LCD TVs are among the most successful consumer-electronics products of all time. What began as a high-end niche product is now the de facto standard for flat-panel TVs, with models available at every price level. The inner workings are pretty geeky, but I find the whole subject fascinating, and if you've made it to the end of this 2-part article, you obviously do too. May this knowledge serve you well in your quest for ever-greater understanding of video technology.

Many thanks to Pete Putman of ROAM Consulting LLC for his help with this article.