Dual-Projector Polarization

Dual-projector polarized displays are used in most 3D theaters. IMAX 3D, RealD, and Dolby 3D all use a variation of this 3D display technology (there are some technical differences but for the sake of simplicity we won't go into those details here). The method is somewhat simple and elegant when compared to alternate-frame sequencing. There is no mechanical trickery required to cover one of the viewer's eyes at a time or technology needed to sit on the viewer's head in the form of expensive LCD glasses. You do need passive polarized glasses, but the cost is insignificant compared to LCD shutter glasses. Not coincidentally, this is why the method is so prevalent in theaters where patrons might accidentally damage the equipment.

This technology makes use of the polarization of light, a relatively old and commonly-used technique implemented in garden-variety sunglasses. A dual-projector polarized theater relies on two projectors to deliver video to the screen, with each projector delivering a unique perspective for a specific eye. Each projector lens is attached to its own polarized filter. The viewer has to wear glasses, but the glasses contain no electronic parts. Instead, the glasses simply employ passive polarized filters. The filter over the right eye will block out the polarized video that is intended for the left eye, and the filter over the left eye will block out the polarized video intended for the right eye. This way, each eye only sees its intended perspective, even though both perspectives are displayed on the same screen.

Dual-projector polarization is used not only in theaters, but also in some specialty monitors designed for the PC. When used in a monitor, a polarized filter is located directly on top of the monitor's screen. Two examples of this are the iZ3D and Zalman 3D monitors.

What are the advantages of dual-projector polarization? Its most significant benefit is that it is easy on the eyes, regardless of the refresh rate of the projectors. Because no active LCD shutter glasses are strobing the light that gets to the eye, it is comfortable to watch this method of stereoscopic 3D with a pair of 60 Hz projectors.

The second advantage is that polarized glasses can be purchased for less than a dollar a pair. Want to invite 20 people over for a 3D party? Less than $20 worth of glasses will take care of it.

What's the downside? The up-front costs are much greater. A dual-projector system requires two projectors, which is a significant expense. On top of that, you'll need a polarized-compatible screen and polarized filters for each of the projectors. While this might seem like a lot, keep in mind that 720p projectors can be had for under $1,000, meaning the total cost of this setup can be kept well under $3,000 with little effort. The cost is comparable to a single 46" 3D-ready TV, which will require at least one pair of expensive glasses to operate.

What are the disadvantages to a polarized setup? First, crosstalk can be an issue with dark scenes. The polarized filters are never 100% perfect at blocking all of the light from one of the projectors, so if there is a dark scene and a bright object, some light on the edge of the bright object might be seen with the wrong eye. But if it doesn't bother you in the theater, then it probably won't bother you at home. Still, the drawback is something to consider.

Example of crosstalk--notice some leaves can be seen through the gun

Example of crosstalk--notice some leaves can be seen through the gun

Another slight disadvantage is that the filters polarize the light across a plane, so the more you tilt your head, the more crosstalk you will see. Once again, this isn't a problem for most people, since they don't watch the big screen with their heads tilted sideways, but it's something else to keep in mind.

Perhaps the most notable disadvantage of the polarized method is that it remains questionable whether or not 3D Blu-ray discs will ever be compatible. No one has yet announced a dual-projector 3D Blu-ray software player. CyberLink's PowerDVD 10 looks like it might be one of the only games in town when it comes to 3D Blu-ray playback on the PC, and the company has indicated that dual-projection setups will not be supported. It intends to rely mostly on alternate-frame sequencing for 3D Blu-ray playback, although it will support a number of other display options.

In any case, this article is about recreating a dual-projector polarized 3D setup in the home. Just because we can't playback 3D Blu-ray discs with this method yet doesn't mean we can't play games in wall-shattering awesomeness and it doesn't mean we can't watch movies in 3D, either. Even though we've made clear our intention to create a dual-projector polarized 3D theater, there's still a lot more ground to cover when it comes to a wall-sized 3D display in the home, and specifically on a home-theater PC. The key is the software.