Sharp's LED Device Expands LCD Color Gamut Without Lowering Brightness

Dec 17, 2013
Naoki Tanaka, Nikkei Electronics
A comparison between the new technology (right) and conventional technology (left)
A comparison between the new technology (right) and conventional technology (left)
[Click to enlarge image]
Another comparison between the new technology (top) and conventional technology (bottom)
Another comparison between the new technology (top) and conventional technology (bottom)
[Click to enlarge image]
The new technology (top) and conventional technology (bottom)
The new technology (top) and conventional technology (bottom)
[Click to enlarge image]
The new technology (top) and conventional technology (bottom)
The new technology (top) and conventional technology (bottom)
[Click to enlarge image]
The color gamut with the new technology
The color gamut with the new technology
[Click to enlarge image]

Sharp Corp has developed an LED device that is used for the backlight of an LCD panel and expands its color gamut by 25% without lowering its screen brightness much.

It is possible to realize an LCD panel with a 90% color gamut on NTSC (CIE1931) standards just by applying the new LED device to backlight without making any changes to liquid crystal cells.

Sharp developed four models of the device: two for small- and middle-size LCD panels (0.4 and 0.6mm in thickness) and two for large-size panels (edge- and direct-type backlights). The price of their samples is ¥40 (approx US$0.39, including tax). Sharp plans to release the samples Dec 24, 2013, and start volume production in April 2014.

For the backlights of its LCD panels, Sharp has been mainly using an LED device made by combining blue LED chips and yellow phosphors. The company's LCD panel using such a backlight has a 72% color gamut on NTSC (CIE1931) standards.

Also, for some of its wide color gamut LCD panels, Sharp used an LED device that is made by combining blue LED chips and red and green phosphors and realizes a 83% color gamut on NTSC (CIE1931) standards. However, in this case, screen brightness lowers by 20%, compared with the LCD panel using yellow phosphors.

This time, Sharp combined a blue LED chip with red and green phosphors made using totally new materials and realized the 90% color gamut on NTSC (CIE1931) standards and a high brightness. With the 0.4mm-thick model for small- and middle-size LCD panels, it is possible to ensure a screen brightness that is only 3% lower than the screen brightness of the LCD panel using yellow phosphors, the company said. And the wider color gamut enables to display video that looks stereoscopic.

Commenting on the reason why it became possible to realize a brightness higher than that of the existing wide color gamut LCD panels, Sharp said, "It is because we have developed the red and green phosphors that use totally new materials and have high conversion efficiencies (in regard to the light that comes from a blue LED chip and hits a phosphor, resulting in wavelength conversion)."

A phosphor maker developed the red phosphor for Sharp. And the green phosphor was co-developed by Sharp and a phosphor maker.

In addition to the high conversion efficiency, the new device increases the peak intensity of the spectrum of red light and decreases its peak width, realizing a spectral distribution in which the peaks of red and green lights do not overlap much. As a result, color gamut was expanded from 83% to 90% on NTSC (CIE1931) standards.