Thinking Beyond Stripes in Flat Panel Displays
Daniel S. Burgess
Thomas L. Credelle, vice president of engineering at Clairvoyante Inc. in Cupertino, Calif., has spent the past few years trying to convince flat panel display manufacturers that they do not have to resort to massive investments in next-generation fabs to improve performance -- if they reconsider the geometry of the subpixels in their products.
Now, with more than a dozen display and chip manufacturers in the Far East working with his company, he is hopeful that that message has been adopted and will lead to the production of low-power, high-pixel-count liquid crystal displays (LCDs) for mobile applications such as cellular phones, digital cameras and media players.
The PenTile RGBW technology uses a geometry comprising two subpixels per pixel to improve the performance of pixilated flat panel displays, such as high-resolution liquid crystal displays in cellular phones. The replacement of some of the color subpixels with white ones doubles the brightness, enabling the manufacturer to reduce power consumption by cutting backlighting elements and/or to achieve more realistic colors by incorporating alternate materials in the color filter.
"I think we're over the hump of believing that we have to build displays with RGB [red, green, blue] stripes," Credelle said.
Clairvoyante was formed in 2000 to develop and license technology for the manufacture of flat panel displays. The resulting design, PenTile Matrix, is an outgrowth of work by Candice H. Brown Elliott, a founder of the company. It is premised on the recognition that the triads in a display -- the groups of red, green and blue subpixels -- do not yield an output optimized to the human vision system.
For example, although human color perception is trichromatic, there is a highly unequal distribution of the three photosensors in the eye. The eye has an order of magnitude fewer blue receptors than red or green ones (although each blue receptor is more sensitive than its green or red counterparts), and the worst sensitivity to short wavelengths is in the fovea, where the more numerous red- and green-sensitive cells are most dense. Similarly, Credelle noted, the resolution of spatial detail is dependent on the detection of red and green. Tying red and green subpixels, therefore, leads to lost resolution in the display.
The latest iteration of the technology, PenTile RGBW, employs a geometry of two subpixels per pixel to offer the same performance as a 480 × 640-pixel 2.4-in. RGB stripe LCD, but it needs half the LEDs for backlighting and a third fewer data drivers to do so. As a result, power consumption might be halved and the cost of electronics reduced by up to 33 percent.
The approach achieves this by replacing approximately one-fourth of the subpixels with white ones. In the case of an LCD, that involves substituting a clear subpixel for a color one in the filter. The effect is a doubling of the brightness.
Rather than simply cutting power consumption, this higher brightness may be used to enable the incorporation of alternative materials in the filter to yield more realistic colors. One reason that the displays on today's cell phones have performance inferior to LCD televisions, Credelle explained, is the result of materials choice. To minimize power consumption by reducing the number of elements in the backlight, manufacturers must employ more-transmissive materials in the color filter.
"With RGBW," he said, "we can use some of that higher brightness to let us use more saturated color subpixels without using any more power."
Although moving to the PenTile RGBW configuration for LCDs requires a modification to the display driver integrated circuit to translate the incoming RGB data, Clairvoyante suggests that minimal change is required to other existing processes, such as involving the production of thin-film transistors and color filters. As a result, Credelle said, manufacturers can use older, third- and fourth-generation fabs to produce high-resolution displays, rather than writing off these facilities.
The company is targeting portable high-resolution LCDs at this point, but Credelle noted that the approach is suitable for use with any type of color pixilated display. Plasma displays that employ it are possible, but more changes in the manufacturing process are required, he said. The method also is compatible with amorphous silicon and polycrystalline silicon technologies. Products incorporating PenTile RGBW are expected from Samsung Electronics Co. Ltd. of Seoul, South Korea, later this year.
Credelle said that Clairvoyante is investigating improvements to the technology, such as expanding perceived gray levels to increase realism. Other research and development topics include more advanced subpixel rendering software and means of applying PenTile RGBW to the entire imaging chain; for example, adding it to graphics chips to improve rendering.
"But right now, our hands are pretty full just with the displays," he remarked.
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