There's no question that 3D televisions are the most talked about home entertainment products of the summer. Japanese and Korean consumer-electronics giants Sony, Panasonic, LG and Samsung have all rolled out new models featuring LED backlights, and Sharp and Toshiba are expected to enter the fray in the second half of the year.

In Taiwan, VEA, a subsidiary of shoemaker La New, introduced a 74-inch 3D-ready rear-projection television that costs NT$148,000 in June. Chimei plans to launch Taiwan's first LED-backlit 3D television in the fourth quarter and will be focusing on 55-inch and larger models. At less than NT$100,000 each, these Chimei televisions are expected to be among the most competitively priced 3D TVs on the market.

All of the 3D TVs mentioned above are also capable of displaying 2D images and switching between modes depending on the program content. As a consequence, interest in them is expected to extend beyond tech lovers to ordinary members of the public looking to replace older-model TVs.

The 3D TVs currently on the market make use of one of two technologies, either active-shutter or polarized lenses.

Golden Tiao, deputy general director of the Electronics and Optoelectronics Research Laboratories at the Industrial Research Technology Institute (ITRI), explains that the active-shutter approach works by "controlling time." Viewers wear glasses with liquid-crystal "shuttered" lenses that flip between transparent and opaque on alternate sides every 1/120th of a second. This results in each eye seeing a different image. Human beings can't perceive the miniscule difference in the timing of the images and the "persistence of vision" effect enables the brain to knit the two scenes together into a single 3D image.

Televisions utilizing the polarization technique take advantage of the direction of vibration of light waves. Special optical elements installed in front of the display separate the signals for the left and right eye into differently polarized images. The viewer wears polarized glasses that filter the incoming light such that each eye sees only the appropriate image. For example, the system might present the left eye with a vertically polarized image, and the right with a horizontally polarized one. The brain automatically knits these slightly different pictures together into a 3D image.

The active-shutter and polarization techniques each have advantages and disadvantages. The active-shutter glasses that go with the former require electronic drivers that darken the lenses (controlling when each eye receives an image), batteries, and a means of synchronizing the opening and closing of the shutters with the TV. They are complicated, expensive devices that cost US$100 or more per pair.

Moreover, active-shutter glasses, which look much like sunglasses, filter so much light that they cut the brightness of the image down to just 17% of its original level. Polarized glasses, on the other hand, have ordinary optical lenses, reduce image brightness to 30% of its original level, and are much cheaper, costing as little as US$10 per pair.

But TVs employing the polarization technique deliver only half the resolution to each eye, so the images they produce aren't as sharp as those of active-shutter televisions. Recognizing that consumers demand high-quality images, Japanese and Korean manufacturers have chosen to focus primarily on active-shutter technology.

Polarization technology has its advocates as well, especially in specialized markets or where cost is an important consideration. LG sells 3D TVs utilizing polarization technology in India and AU Optronics recently introduced a 65-inch polarizing model in China.

Right now, 3D televisions are the hot new thing and the constant rollout of new models suggests that manufacturers have high expectations for the new technologies. But Tiao and others predict that consumers won't widely adopt 3D televisions for their homes until autostereoscopic 3D (i.e. 3D without glasses) TVs make it to market.

"Consumers want to relax when they're at home," argues Tiao. "They find the need to wear special glasses just to watch TV inconvenient and unnatural. The 3D glasses are especially awkward and uncomfortable for those who wear corrective lenses." As a result, research teams around the world (Taiwan included) are working feverishly to develop autostereoscopic 3D, even though the current 3D TV technologies have themselves just been introduced.

Tiao reveals that ITRI has already successfully developed new kinds of autostereoscopic 3D displays with multiple viewing angles, one using a lenticular lens and the other a parallax barrier. Lenticular lenses are strips of convex lenses attached to the front of the display panel that allow light from odd and even elements of the picture to be projected at different angles. When viewers watch the screen from a specific distance and angle, their right and left eyes see different images. Parallax barriers are a kind of diffraction grating that interweaves transparent and non-transparent lines. When viewers watch through the grating, their left and right eyes see different images, creating a 3D effect.

But multiple-viewing-angle autostereoscopy requires that broadcast content be split into different files for the different fields of view, greatly complicating production.

Tiao admits that autostereoscopic technology has a long way to go: hardware and software must be improved, and production costs have to come down. Moreover, the technology can only display 3D content, i.e. consumers won't be able to use the TVs to view older 2D content. Consequently, autostereoscopic TVs are unlikely to see mass production any time soon.