On the Road to 3-D

In the early 2000s, 42 in. plasma TV was new on the market with a price tag exceeding $10K. This year, LCD has overtaken plasma (Fig. 1) and 3-D TV is going into the home.

1. Plasma display panel (PDP) TVs were first introduced in 2003, but early this year, LCD TVs 40 in. and larger have taken the lead. (Source: DisplaySearch)

A part of this progression since 1993 is the U.S. Display Consortium (USDC, San Jose), an association consisting of display manufacturers and developers. The new kid on the block, 3-D, “is making a strong impact in theaters and is ready for high-definition 3-D in the home,” said Mark Hartney, CTO of USDC.

Such innovations were recently demonstrated at the 3-D Business Symposium & Exhibition in San Francisco, organized by the consortium and Insight Media (Norwalk, Conn.). More mature stereoscopic systems like Texas Instruments’ (TI, Dallas) digital light processing (DLP) projection technology (Fig. 2) have been taken to new heights, and methods like multilayer displays from PureDepth (Redwood City, Calif.) provide “real” 3-D images, stated Jin Kim, director of TFT-LCD market researchat DisplaySearch (Austin, Texas).

2. The schematic illustrates how a digital light processing (DLP) projection system uses a single digital micromirror device (DMD) to generate a sense of depth for 3-D viewing from a 2-D source. (Source: Texas Instruments)

Stereoscopic image viewing

Stereoscopic viewing is currently the technology of choice for 3-D TV and cinema. To convey the perception of depth on a 2-D screen, in stereoscopic viewing, one eye must see a left camera view and the other a right camera view. The cameras create two images set up to match the average interocular distance of a view’s eyes, so that they are aligned 64 mm apart.

In a cinema, the most common 3-D stereo approach is to place a switch in front of the projection lens to alternately polarize the left and right views at a rate fast enough so that no flicker is perceived. The switch polarizes so that one view is circularly polarized 180º out of phase with the circular polarization of the other view. And that is what reaches the screen.

The viewer, in turn, wears a pair of glasses with opposite polarity, which effectively isolates the left and right eye so that each eye sees what the respective cameras view. The brain then takes over, integrating the isolated views to recreate a depth perception of a 2-D image.

Digital micromirror device panels

3. DMDs are MEMS structures consisting of out-of-plane mirrors that tilt ±10° to deflect light in projection display systems. (Source: Texas Instruments)

3-D DLP projectors generally use three DMDs, one for each color: red, green and blue. Each color is mechanically affixed to a color-splitting prism. A light source illuminates each DMD, and the light is recombined through another prism and directed to the projection lens.

The single-DMD system is more sophisticated, using a color wheel (disk) consisting of 3-6 color segments. The wheel is made of glass with die color coatings. With a six-segment wheel housing red, green, blue, cyan, yellow and magenta filters, displayed images arebrighter with more vibrant colors, said Ken Bell, project manager, DLP TV at TI. The wheel spins in the DMD’s illumination light path and outputs a color sequence. With switching speeds of 8 µsec per mirror, a single DMD can sequentially output six different colors through the single spinning wheel in a manner synchronized to the input signal. At this speed, the eye integrates the images and sees a continuous gray scale.

Creating the 3-D effect for DLP television requires the same elements of stereoscopic cinema. But instead of placing the polarizing filters in front of the projector lens for watching TV, an active shutter is put in the eyewear, switching from left to right at 60 times per sec per eye. The TV provides a synchronization IR signal that is transmitted to the eyewear, making it possible for the user to view 3-D TV from any angle in the room.

Liquid crystal on silicon displays

Liquid crystal on silicon (LCOS) technology is technically another LCD, which controls the amount of light projected by applying an electric field to the liquid crystal gap formed between two parallel planes. However, there are two significant differences. Where LCDs sandwich the crystals between glass planes through which light is transmitted, LCOS has a silicon backplane off of which light is reflected. The second way the technologies differ is in the manner they display absolute black, a quality essential to achieving high contrast.

In typical LCD devices, black is produced when an electric field is applied across the liquid crystal cell gap. However, molecules near the surface of the glass substrate are difficult to control because of its dependence on the alignment of the crystals. Thus, when dark images are displayed, these molecules may tend to leak light, diminishing the opacity of black features.

In contrast, LCOS devices do not exhibit this phenomena because black is displayed when an electric field is not applied and surface molecules are in the correct alignment.

In LCOS-based products, such as the Sony SXRD 3-D projection unit (Fig. 4), contrast ratios as high as 1800:1 have been achieved by decreasing the cell gap to <2 µm with a propriety planarizing technology applied to the silicon backplane. Developers at Sony also eliminate “spacers,” which are typically used to maintain a constant gap between the parallel planes in the SXRD device structure. The resulting reduction in scatter and reflected light prevents degradation of the high-contrast image.

4. The schematic, a cross-section of Sony’s SXRD system, demonstrates a reflective 3-D projection system based on liquid crystal on silicon (LCOS). (Source: Sony)

Autostereoscopic viewing

Conventional stereoscopic viewing requires the use of eyewear. This has led to an increasing number of companies pursuing autostereoscopic viewing methods that enable the user to experience 3-D without the need of eyewear.

However, these are still stereoscopic viewing systems in which the left eye must necessarily see only the images intended for it; same for the right eye. To do this without glasses requires special filters such as lenticular lenses to be placed on the front of the TV screen so that the appropriate light rays are directed to the left and right eyes, respectively. Such a lens produces half resolution 3-D images because each eye sees half of the pixels on the TV.

To create 3-D viewing effectively over an entire room is very difficult and often results in a sweet spot where the 3-D experience is optimized.

The overlaying lens technologies can be placed on the front of displays, whether plasmas or LCDs. And while autostereoscopic image viewing frees the user from eyewear, the TVs are currently very expensive.

Multilayered displays

“One of the best ways to experience 3-D depth that is currently available is through a layering of image planes,” Kim said. This is the approach used by PureDepth. The concept is a simple one: Two distinct LCD panels are layered, one opaque and one transparent, and share a common backlight source. The method is not limited to LCD, and may include optically controlled birefringence (OCB) and organic light emitting diodes (OLEDs).

Each display panel receives independent control signals through the coordination of the images displayed. Patented interstitial elements located within the optical stack of display panels remove interference between the display layers, maintain compatibility and perform optical correlation. The result is a true multilayer visual display (Fig. 5). However, it comes at a price, for the cost of each panel must necessarily be compounded.

5. The image shown is based on a 3-D viewing method that stacks two LCD panels, one opaque and one transmissive, to give a sense of depth without the need for additional eyewear. (Source: PureDepth)

The interest in the 3-D experience is clearly taking hold in cinemas where film companies such as Dreamworks are targeting 3-D for all animated features, eyewear notwithstanding. Likewise, video games and other entertainment vehicles are a natural application for 3-D. What remains to be seen is whether the stereoscopic technologies the public is accustomed to will find a comfortable niche in the home or be supplanted by newer, eyewear-free autostereoscopic and multilayered methods.