With just over a month to go until Sony puts the world's first OLED (organic light emitting diode) television on sale, two rival display makers took the wraps off their latest OLED TV panel prototypes at an exhibition in Japan this week.

OLED technology is being fiercely developed by many panel makers because it offers a brighter, more vivid picture than today's LCD (liquid crystal display) panels. And because OLED pixels emit their own light a backlight isn't required, meaning OLED TVs use less power and are also much thinner -- the Sony set is just 3 millimeters thick.

Samsung Electronics was showing three prototype TVs based on 14-inch OLED panels. That's larger than the 11-inch screens that are in the new Sony TVs but a close look at the Samsung panels revealed some imperfections. In each of the three prototypes on display were pixels locked to a single color.

The problems stem from difficulties that remain in the manufacturing stage, said a Samsung representative on the company's stand at the FPD Expo in Yokohama. Samsung is one of the world's largest manufacturers of LCD panels and the OLED screens on display were designed to be manufactured using an existing TFT (thin-film transistor) LCD production process. That presents some hurdles now but could mean big cost savings in the future because Samsung won't have to invest in a new factory to make the screens.

Japan's Seiko Epson, which has been researching OLED technology for many years, was displaying five prototype 8-inch OLED panels. In contrast to the Samsung panels, the Epson ones didn't have any visible defects and offered a bright, smooth and crisp image. The screens, which are 3 millimeters thick, attracted a crowd at the show -- many more people than the company expected, a representative confided.

Neither Samsung nor Epson would say when their larger TV screens might be ready for mass production. Sony's TV is due on sale in Japan on Dec. 1. The XEL-1 comes with a price tag of US$1,745 and Sony plans to produce about 2,000 of the sets per month. There are no plans to sell the TV outside of Japan.

It's not just technology hurdles that are stopping such screens from becoming mass market items. The panels are still very expensive to manufacture and companies must simultaneously bring down the manufacturing cost while improving the technology.

At the smaller end of the display scale many companies were showing OLEDs suitable for cell phones and music players. Such panels are already available in commercial products but some development effort remains on the smaller panels.

Samsung SDI was showing a bendable 4-inch OLED. The screen was fixed in a slightly curved position and delivered a good picture but again there were imperfections visible. The company was also showing what must be one of the thinnest OLED panels in the world. At just 0.37 millimeters the 2.4-inch QVGA panel is super thin and destined for use in mobile phones.