The Future of Flat-Panel TVs Page 3

As with CRT - which still offers pictures with a reference-quality black level - the intensity of the light generated by a pixel in an SED display is modulated by the drive strength of its corresponding electron emitter. Pixels meant to display black exist in an "off" state, with no electrons reaching the phosphor dot. But the cells in plasma sets need to be "primed," with a low level of current constantly flowing through them - which serves to boost black levels toward dark gray.

SED's ability to provide power only to where it's needed moment-by-moment holds the promise of lower power consumption. In theory at least, the process is more efficient than both plasma's priming technique and the permanently switched-on fluorescent backlight that drives an LCD display - which also reduces picture contrast. (Samsung has demonstrated LCD panels using energy-efficient light-emitting diodes, or LEDs, instead of a fluorescent backlight, and Sony briefly offered a 46-inch model as part of its now-defunct Qualia line.) With home energy bills rising in tandem with the skyrocketing price of crude oil, any relief a new TV could provide on electricity consumption would be welcome.

0609_futureflat_amoledOLED :: LCD's Eventual Successor While its entry into the big-screen TV arena is farther off than SED's, OLED has the potential to be the bigger player. That's because of the flexibility of OLED displays: thin (a TV can be 3 centimeters or less deep) and lightweight, they can be manufactured in multiple sizes and resolutions, or even mounted on flexible substrates that can be used to create things like electronic paper and "wearable" displays. (Imagine a digital watch permanently embedded in your shirt cuff!) Also being developed are transparent OLEDs that can create a window-like effect by letting you see through the video screen when it's not displaying images.

The version of OLED destined to make its way into your living room is called Active-Matrix Organic Light-Emitting Diode (AMOLED). In an AMOLED, a stack of organic polymers including both emissive and conductive layers is deposited on a substrate containing a thin-film transistor (TFT) array. Different techniques can be used to apply the organic material, including an ink-jet method that "prints" a pixel matrix directly on the TFT. An electrical charge passing between the bottom electrodes and an additional transparent layer on the surface of the display stimulates the emissive organic layer, which in turn creates light. Color OLED displays can be created by depositing red-, green-, and blue-dyed pixel triads via the ink-jet technique or by color-filtering an array of white-light-emitting pixels. Either way, the shade of an individual pixel varies according to the amount of current traveling through it.

Since OLED displays create their own luminosity rather than relying on a backlight, they have a very wide viewing angle - on par with both plasma and SED. And the speedy refresh rate of active-matrix displays promises to prevent the picture-smearing effects typical of LCDs when showing fast-motion video. Also, the self-luminosity of OLED's organic material, along with its efficient emissive properties, is said to keep power use well below that of LCD or plasma. (Low power consumption has already made OLED attractive for use in portable electronics.)

But display longevity could be a stumbling block. People expect any set they buy to last 10 years or longer, and right now that's a serious stretch for OLED. To fix this, developers have been working on the differential aging of the pixels. In layman's terms, that means the life span of the blue-colored pixels, which drops off in brightness at a faster rate than that of red and green pixels, needs to be extended. (I've heard of prototype OLED screens that required replacement several times in the course of one trade show because of color shifts resulting from differential aging!)

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