The Renaissance of 3D:
How the New 3D Technologies Work
Light polarizing systems are used in commercial cinema and other high-end applications. These methods provide a high-quality 3D experience in commercial cinemas, made even better thanks to the prevalence of digital projectors. In a polarizer system, light from the projector or projectors passes through a polarizing filter which makes all the light waves oscillate in the same direction. Special filters on the viewer's glasses allow only the light meant for that eye to pass through. If you have ever seen a set of Venetian blinds, you already understand the concept--from certain angles you can see through the window clearly while from other angles your view is obscured. The use of different polarization for each eye allows for two discrete images to be projected, one for each eye, which creates the impression of depth.
There are two different polarizer-based systems currently used for commercial 3D projection. One version uses two projectors, each with its own polarizing filter, to project the left- and right-eye images. This system is used in IMAX 3D presentations. The other system, known as RealD, uses one projector and a fast-switching single polarizer to accomplish the same thing. This system flips between left-eye and right-eye images very quickly and the polarizing filter switches between clockwise and counterclockwise polarization in time with these switches. Again, polarized glasses allow the viewer's eyes to see only the information intended for them.
Advantages of Polarized 3D
Color. Compared to anaglyph systems, color is more accurate when using a polarized system. While there is some light lost because of the glasses, colors are closer to their original values. It is also easier to color-correct material used in a polarized system since the lenses have very little color to them. Flesh tones in particular look more realistic and believable when viewed on a polarized system.
Passive glasses. Like anaglyph 3D, polarized 3D uses passive glasses which are inexpensive and contain no electronic parts. Unlike anaglyph, polarized glasses' frames are usually made of plastic which makes them more durable and reusable than their paper-framed counterparts.
Cross-talk. Polarized systems have a lower incidence of cross-talk than anaglyph systems. Due to the properties of polarized light, it is almost impossible for the left eye's image to make its way into the right eye or vice versa. Systems using left-right polarization such as IMAX can lose the 3-D effect if you tilt your head too far to either side, but unless you are sleeping on your neighbor's shoulder, this should not be a problem.
Disadvantages of Polarized 3D
Light loss. In all single-projector 3D systems there is a significant reduction in luminance when compared to a 2D system. For the non-physicists in the audience, luminance refers to the light reflected from a surface (in this case, the screen) at a given angle (in this case, towards your eyes). This is not the same as illuminance, which is a measure of the light striking a surface per unit of area, typically measured in lumens per square meter and posted in all of our projector reviews.
In all systems except anaglyph, this loss is caused by the rapid switching necessary to display discrete left and right eye images. At any given moment while watching a 3D movie, one eye sees a projected image while the other eye sees nothing at all. As such, each eye only sees half of the light reflected from the screen, resulting in a luminance reduction of at least 50% right off the bat. I say "at least" because polarizers and glasses are not perfectly efficient. Polarization by its very nature only allows a portion of the projector's total light to reach the screen. There is some further light lost due to the glasses. The end result is a picture that appears much less bright than a 2D film from the same projector.
This is actually one of the chief advantages of using a two-projector system. Each eye gets the benefit of the full light output of one projector, though light loss due to polarization and glasses still apply. The end result is a much brighter picture, all else being equal. That last part is important because all else is not likely to be equal. The most widespread commercial implementation of a two-projector system is IMAX, which also uses a much larger screen than most RealD theaters. The projectors used can vary in their lumen output. Polarizing plates can vary in their efficiencies, as can glasses. There are too many variables to make any definitive judgement about which system is "better," but each has its advantages.
Expensive. While the glasses themselves are inexpensive, the rest of the system is not. It requires at least one high-end digital projector paired with a special processing device to manage synchronization, at least one polarizing filter, and a silver screen (traditional white screens cannot hold polarity). Two-projector systems obviously require two projectors and two polarizers.
|Contents:||What is 3D||Anaglyph||Polarizers||Interference Filter|