Prism coupler thins out backlights

17 Oct 2006

Omron in Japan is developing an optical component which they say can reduce the thickness of a mobile phone's backlight by 40%.

An optical component known as a hybrid normal-reverse prism coupler (HPC) could reduce the thickness of a mobile phone's LED backlighting system by 40%, according to researchers from Omron Corporation in Japan. They believe that the HPC is a crucial component in thin, lightweight LED backlights that can provide high-quality image displays (Applied Optics 45 7273).

The team says that to date it has been difficult for backlights to be both thin and offer a high brightness, without using prism and diffusive optical sheets. However, the Omron researchers say that the HPC is the first device to eliminate these previously indispensable components.

Omron's backlight system consists of an LED, a light guide and a reflection sheet. HPCs are formed on the bottom surface of the light guide in a circular pattern. "A typical light guide has an area of 2.5 square inches and contains around 2 million HPCs," researcher Shigeru Aoyama told optics.org. "The resultant light intensity profile was symmetrical because of the radial direction focusing effect of the HPC. It was asymmetrical for a conventional LED backlight."

The HPC has two functions: controlling the light coupling efficiency between the guided and radiated light, and determining the divergence of the radiated light.

"In fabricating conventional backlight systems, it has been impossible to design a good light intensity angle distribution," added Aoyama. "The HPCs allow us to improve light efficiency by 30% compared with conventional devices by bringing more light into the forward direction."

To manufacture the HPCs, Aoyama and colleagues form a master using electron-beam lithography. "In the reproduction process, sub-micron order has been achieved using vacuum injection molding," said Aoyama. "The HPCs are fabricated from PMMA or PC."

The team, from Omron's Advanced Device Laboratory, says that the HPCs will initially be used for mobile phones. "We have no plans to expand the application area to LCD TVs," concluded Aoyama.