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Primer: The Principles Of 3D Video And Blu-ray 3D

3D Displays

A 3D display must be able to display two separate video images on the same screen. There are several methods that are used to accomplish this. Each display method must be paired with the corresponding 3D glasses technology designed to assure that each eye only sees the video meant for that eye.

Polarized Displays and Polarized Glasses

Modern TVs and displays emit light from each pixel in some combination of red, green, and blue wavelengths. The light emitted by a TV or display can be filtered, such that all of the light coming from a row of pixels has the same electromagnetic orientation. Though the light travels in a straight line from the display pixel to your eye, it may be filtered so that it has one of two circular polarization states (left-hand or right-hand). 

For example, imagine that a beam of light is traveling along the center of the spiral graphic below. The arrows pointing outward from the axis of the direction of travel represent the changing direction of the orientation of the electric field of the light beam (though we don’t think of light in the same way we think of radio waves, light is another type of electromagnetic wave). If you align the thumb of your left hand along the center axis of the spiral graph below (the direction of travel of the light), you will be able to close your fingers into a fist in the direction that the electric field rotates around this beam of light. Light with this circular polarization is said to be left-handed.

The graphic below shows the direction that the electric field rotates around a beam of circularly-polarized light with right-handed orientation.

Circularly-polarized light with one orientation can pass through a polarizing filter with the same orientation, but will be blocked by a polarizing filter with the opposite orientation. In this way, half of the pixels of a 3D display can be used to display the video for one eye, while the other half display the picture for the other eye.

3D displays can be manufactured with polarization filters, which are aligned with the rows of pixels on the display. This allows half the pixels on the display to be dedicated to displaying the picture for one eye, and the other half of the pixels for the other eye. Note that the effective resolution provided by a polarized display to each eye is half of the full display resolution.

Row interleave polarized 3D display. Odd horizontal rows of pixels are used for one eye, even rows for the other eye. Red and blue are used to indicate left and right eye images.

To play back a stereoscopic 3D video program, such as a Blu-ray 3D on a polarized display, the left and right video frames are converted to interlaced video frames. The display is designed to show odd rows of pixels to one eye, and even rows of pixels to the other eye.

With polarized 3D glasses, each eye will only see the part of the image meant for that eye. In the image above, red and blue indicate the different circular polarization on the lens for each eye. Though two images appear on the display at the same time. With the 3D glasses, each eye only sees the image meant for that eye. The human visual system combines the image into a single 3D image.

Polarized displays are one of the least-expensive ways to display a 3D video, and polarized glasses are inexpensive. However, polarized displays aren’t always able to filter the light perfectly, such that 100% of the light meant for each eye has the correct orientation. Similarly, polarized 3D glasses aren’t always able to block 100% of the light that is meant for the other eye. This problem, where one signal bleeds into another signal traveling along the same transmission path is known generically as cross-talk. For 3D display systems, cross-talk leads to double images (fuzzy, unsharp images). The image quality of a 3D polarized display decreases noticeably if the viewer is not directly in front of (perpendicular to) the display.