Nikkei Electronics Asia --September 2008
Features
Omron Develops Ultra-Thin Backlight for LCD Panels

Omron Corp and Omron Precision Technology Co Ltd, both of Japan, have jointly developed a backlight only 0.59mm thick (Fig 1). Intended for use in small liquid crystal display (LCD) panels in handsets, etc, it is about only two-thirds as thick as the firm's prior model, at 0.8mm thick. Omron Precision Technology plans to begin volume production in April 2009.

Demand for thinner LCD panels for handsets sharpened from the end of 2006. As one engineer at a domestic LCD panel manufacturer said, "Our customers want them thinner by even 0.1mm." At exhibitions and other events a number of prototype LCD panels under 1mm in thickness were shown by multiple panel manufacturers. LCD panels now in volume production, though, are about 1.5mm to 2mm, making it clear that production panels are not as thin as needed yet. The biggest reason has been that it is difficult to maintain uniform brightness when making the backlight thinner. According to Omron, the newly developed backlight will "...make it possible to volume produce LCD panels about 1.3mm thick." The firm plans to use the technology to develop even thinner designs.

The developed backlight is a 3-inch design, with a surface brightness of 10,000cd/m2 typ. The light source is six white light-emitting diodes (LED), only 0.6mm thick. A source at Omron revealed that the brightness variation within the light emission plane is about 15%, which is equivalent to volume production backlights. The firm's prior model, a 0.8mm-thick design also using LEDs, has a surface brightness of 11,500cd/m2 typ, so the reduction in LCD light utilization efficiency was held to no more than 15% when the backlight was thinned out.
As the backlight is only 0.59mm thick, it is also easily bendable (photo, top). Omron says that brightness changes very little even when the backlight is bent, so that it can be utilized as a backlight for bendable LCD panels as well.

Finer Structure

The thinner backlight was possible thanks to the development of a light guide only 0.23mm thick. The light guide formerly developed by Omron Precision Technology was 0.5mm, so the new one is less than half as thick. The prism sheet, diffusion sheet, reflecting sheet and other components are identical to those used in the prior design.

Omron positioned a radial three-dimensional (3D) lens, called a radial prism, near the LEDs to reduce the thickness of the light guide to 0.23mm (Fig 2). Microfabrication on the 100um level was used to increase the utilization efficiency of light emitted by the LED. Even with the thickness of the ends of the light guide reduced to about half the thickness of the LED zone, optical utilization efficiency was maintained at no less than 90%. In conventional light guides, reducing the light guide end thickness to about half that of the LED zone meant a maximum optical utilization efficiency of about 70%. Brightness uniformity was also enhanced by forming "multi-mirrors" about 20um in diameter at the ends of the light guide.

These two structures are formed in the light guide by a technique developed by Omron, dubbed Multi-Scale Imprinting (MSI). Details have not been disclosed, but it is said to be based on nanoprinting technology and capable of forming a wide variety of microscopic shapes. Light guides are usually formed by injection molding, which can handle a diversity of shapes but (according to Omron) has trouble with thin structures. The company already has experience in using MSI technology in volume production of prism sheets, and Omron Precision Technology is confident there will be no difficulties using it in backlight volume production.

1mm Thick Panels

MSI technology can make it possible to produce even thinner light guides: in addition to the light guide scheduled for volume production, they were also successful in prototyping one only 0.095mm thick. The company commented, "There are no particular problems making light guides of 0.1mm or even thinner." The ultra-thin light guide would mean a backlight of only 0.287mm.

This backlight, in turn, would probably make it possible to produce LCD panels less than 1mm thick. A source at Omron, however, noted that the volume production date is pending, and said, "Backlights of 0.3mm or less in thickness are basically films. Many panel manufacturers are reluctant to use them, because they can't be handled on existing LCD panel assembly lines."

by Shinya Saeki

MOST POPULAR ARTICLES

24 Hours | 7 Days