The XEL-1 organic light-emitting diode (OLED) TV released by Sony Corp of Japan in December 2007 has a display only 3mm thick. In addition to its innovative design, another key selling point is the display performance. We measured and evaluated the display performance with the help of a panel engineer, and came to understand the difficulties that Sony faced in overcoming the drawbacks associated with using OLED panels in TVs, and how it made the best use of the superlative image quality that OLED panels offer.
Extending Service Life
The most intriguing of the difficulties Sony faced was brightness time control. It turns out that after the TV is turned on, the brightness is automatically adjusted through five levels in only 300s (Fig 1). This appears to be a clever way of utilizing still-developing OLED panels.
For example, when the entire screen is white the brightness gradually drops from the power-on level of just under 200cd/m2 after about 20s. After 60s brightness stabilizes again, leveling off just a bit over 150cd/m2. About 150s later it again begins to gradually drop, ending up just under 100cd/m2 at the 300s mark, and then maintaining 100cd/m2 thereafter. In other words, the brightness does not drop suddenly, but rather changes gradually and in stages. The engineer assisting us with the measurements commented that this was a way to prevent most viewers from noticing any changes in brightness.
Brightness control also shows tweaks to prolong display life, reduce power consumption and otherwise improve performance. An engineer from a TV manufacturer explained, "Similar techniques were adopted in PDP TVs and other products in the initial period." OLED light-emitting materials are still being actively developed, and service life is not yet sufficiently long. Light-emitting materials degrade the more they emit, and Sony appears to have reduced brightness as far as possible to minimize this deterioration to make it possible to utilize OLEDs under tough TV conditions.
Another innovation to make it easier to utilize OLED panels in the TV is the adoption of a pseudo-impulse drive. Our analysis showed that a black screen 6ms to 7ms long is inserted into every frame (about 16.7ms).
Pseudo-impulse drive is a technique to simulate an impulse drive, like that used in cathode ray tube (CRT) TVs. It helps relieve the retinal afterimage causing blurriness in human eyes, improving apparent motion display performance. It is becoming widely used in liquid crystal display (LCD) TVs where motion display is a problem.
The OLED emits white light, so the motion display performance of the panel itself is supposed to be quite high, but as long as it's a hold type there will be problems with retinal afterimages. Apparently Sony decided that the pseudo-impulse drive was essential for use in TVs, where motion is almost constant.
While some issues were resolved through modifications to the drive, we found others that still need to be addressed - specifically, power consumption and viewing angle.
A look at power consumption showed that the self-emitting design exhibits about a 10W difference between all-white and all-black states. The average power consumption for all white is 28.4W, and for all black (no emissions) 18.3W. This is the total power consumption for the entire TV, including components other than panel drive; but even so, the TV engineer was surprised at how high it was, considering this is an 11-inch set. We also discovered a slight difference in white and green hues depending on viewing angle (Fig 2).
Sony said from the start that reducing power consumption would be the next development challenge, said Yoshito Shiraishi, general manager in the E Products & Business Development Department of the TV Business Group at Sony. One of the reasons why Toshiba Corp of Japan delayed release of its 30-inch class OLED TV originally slated for 2009 is also thought to have been high power consumption. According to president and chief executive officer (CEO) Katsuji Fujita of Toshiba Matsushita Display Technology Co Ltd, "OLEDs of 30 inches or more consume two to three times more power than LCDs. It will take a little more time to drop this to at least the level of LCDs."
The measurement and evaluation process also identified a number of points where the OLED really shines, such as the fact that the color reproduction range is 106% of National Television System Committee (NTSC) standard (Fig 3).
The OLED panel developed by Sony has a proprietary optical structure that is very advantageous in color reproduction. Reflection is repeated between positive and negative electrodes, and only light of a specific frequency is amplified by the interference effect. This specific frequency is then output through the color filters. Measurements of emission intensity show the effects of the structure clearly.
In addition to display characteristics, we also examined pixel structure. Each subpixel appears to be provided with two thin-film transistors (TFT), as show in Fig 4. This is probably the minimal structure, with one for switching and one for drive. Normally three or four TFTs are used with a compensation circuit to minimize luminance variation due to variation in TFT characteristics, so this is indeed a simple design. Fewer TFTs is effective in increasing the aperture ratio. Previously we estimated the aperture ratio at about 75%. Improvements in manufacturing technology have reduced the variation in TFT characteristics, and made it possible to cut the number of TFTs used.
by Takuya Otani
Form : http://techon.nikkeibp.co.jp/article/HONSHI/20080226/148048/