How Do 3D TVs Work?

In order to choose between 3D glasses, it is important to understand how they work, and it is impossible to fully understand how the glasses work without understanding the basics of 3D. Humans perceive objects in three dimensions because the human brain synthesizes a single image out of two disparate ones. This works because the human vision system is based around the eyes, which are both not in the exact same place on the head, so no object is perceived at exactly the same distance with both eyes, making it easier for the brain to place them in a 3D space. This also makes it possible to judge relative distance between objects. Unfortunately, it is very difficult to reproduce this when viewing images on a screen, as every component of the image is the same distance from the viewer’s eye.

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Basics of 3D TV
Just as the human eye perceives three dimensions due to the differences in perception from one eye to the other, 3D TV and 3D movies, attempt to create the illusion of three dimensions by providing different images to each eye. While the methods may change, every system designed to create 3D images from a two-dimensional display is based around this same principle. This was how the red and blue glasses of the 1950s worked, and it is how both active and passive 3D TVs work.

How Passive 3D TV Works
Passive 3D TV works exactly the same way that 3D works in the movies, using polarized lenses to provide different images to each eye. With a passive system, the TV screen is coated so that light from alternate scan lines is polarized differently. The TV then interlaces two images on the screen, one for each eye. Meanwhile, the polarization in the lenses, which matches the coating on the screen, ensures that the proper image is delivered to each eye. It is a simple and elegant system that relies on basic optics. Passive 3D is currently only used on some LCD TV and LED LCD TV models, and it is not available on plasma TVs. Due to the fact that it requires a screen coating, it may never be available for projectors.

How Active 3D TV Works
Active 3D TV uses a much more complex system to achieve the same goals. Rather than interleaving two half-resolution images in space, it interleaves two full resolution images in time. The TV rapidly alternates showing images for the left and right eye, while the glasses use electronic shutters synchronized to the TV to ensure that each eye sees a different image. This system is more complex but allows for a higher resolution 3D image. The only real technological requirement for the TV is that it be able to refresh the screen quickly enough to supply images for both eyes. Another thing to consider is that active 3D is the only form available on projection and plasma TVs, as well as LCD and LED LCD models. Regardless of the technology the viewer prefers, there is probably an active 3D TV available for their needs.

Comparing Active and Passive 3D Glasses
One thing that should be noted before comparing active and passive 3D glasses is that the choice of which glasses to buy is as much about choosing which 3D TV implementation as it is about the glasses themselves. The following table provides a handy feature comparison for the two different types of 3D glasses.

Feature	Active	Passive
Weight	Heavy, glasses have to contain both the shutter mechanism and battery.	Light, thin plastic lenses and frames are the only components required.
Image Quality	Very good, provides full resolution high-definition image. However, some viewers may notice flickering due to the shutter mechanism, which may also lead to headaches. Lower effective frame rate can also lead to jerkiness in the image during fast pans and other movement.	Average, provides half-resolution high-definition image. This means that depending on where the viewer is sitting, they may see either black lines on the screen like some old video games, or noticeable jaggedness. This can be minimized either through the use of a smaller screen or by sitting farther away from the screen.
Price	High, active glasses are complex electronic devices with wireless communication abilities. May be cost prohibitive if many pairs are needed for a big family.	Low, passive glasses have no moving parts or electronics.
Ease of Use	Average, active glasses require connection and synchronization before use.	Easy, passive glasses only require wearing. No need for connection or synchronization.
Compatibility	Low, the majority of glasses only work with TVs from the same manufacturer. There is a plan for a universal active 3D glasses standard but not all TVs may follow it.	High, passive 3D glasses use the same system found in movie theaters. It is a common standard, and all modern passive 3D glasses are compatible with all TV sets that use the system.
Availability	High, active 3D can be used with any TV technology, including both projection and plasma as well as LCD.	Average, passive 3D only works with LCD-based TVs. It specifically does not work with projection or plasma TVs.
Brightness	Average, the shutter mechanism in active 3D glasses causes a measurable reduction in light transmission, producing a dimmer image. This is particularly noticeable when watching in brightly lit conditions.	Good, the polarizing filters in passive 3D glasses have a minimal effect on the transmission of light. While they do cut out some light, images do appear brighter than when using an active 3D system.

Both active and passive 3D glasses have advantages and disadvantages in both casual and regular use. Passive glasses require less set up but work with fewer technologies, while active glasses provide a crisper image at the cost of brightness and possible flickering.