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3D Glasses Explained

3D Glasses - Active Shutter and Passive Polarized

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Modern 3D Glasses are stylish and comfortable.

Current 3D technology relies on the viewer wearing a set of glasses designed to deliver a separate image to each eye. The illusion of depth is created by sending a slightly different image to each eye. Human beings rely on stereoscopic vision for depth perception. Our two eyes see everything at a slightly different angle due to the space between them and our brains process and combine these two images into one complete picture. We can very reliably process depth out to about 20 feet because of this. If you'd like to see how important the stereoscopy is to depth perception have someone throw you a baseball, then cover one eye and have them throw again and see how much more difficult it is to catch the ball. While our brain can give us some idea of depth based on the changing focus of the lens in one eye it is not nearly as accurate as when the brain is able to process two images.

3D Glasses can be divided into two main categories, active 3D glasses and passive 3D glasses. Active glasses are powered, either by batteries or by power adapter. Passive 3D glasses use a number of different methods to deliver a different image to each eye.

Active 3D Glasses

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Shutter Glasses are no longer big and bulky.

Shutter glasses are the most commonly used Active 3D glasses, the lenses are actually small LCD screens. When voltage is applied, the lens goes dark, the "shutters" close. This behavior is synced with the screen displaying the 3D content. As each different image is flashed the eye shutters alternate in time. Each eye sees a slightly different angle of the same image creating the 3D effect. On an LCD or LED television this method of 3D effectively cuts the refresh rate in half and has been known to cause headaches in some people. 3D capable DLP televisions also use this method of displaying 3D content.

Passive 3D Glasses

The old style colored 3D glasses use a method called Complementary Color Anaglyphs. These work by using a filter to block certain colors from each eye. The most commonly seen glasses utilize a red and cyan lens to pass red to one eye and blue and green to the others. At first this could only work with a black and white image since the filters blocked colors but modern methods allow for color viewing although the colors will be muted. Although improvements have been made with this method of creating a 3D effect it is still considered inferior to polarization methods due to color loss and is not used in current 3D TV's.

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Polarization works by blocking certain wavelengths.

Polarization is another method of displaying 3D content via passive glasses. It works by using lenses that block certain wavelength of visible light. Linear polarized glasses use vertical polarization on one lens and horizontal polarization on the other. In a movie theater two images shot at slightly different angles are shown on the screen with two projectors. Each image is polarized to match one glasses lens and each image is then shown to a separate eye. This produces a 3D effect as long as the users head is kept straight. Tilting the head will break the 3D effect.

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Passive 3D glasses are lightweight and inexpensive.

Circularly polarized lenses are polarized clockwise for one eye and counter-clockwise for the other. This method of polarization will maintain the 3D effect if the head is tilted and it also only requires a single projector. A circular polarizer is placed in front of the projector and it quickly alternates between the two polarizations to create the 3D effect.

Active 3D Glasses vs. Passive 3D Glasses

With all the new 3D TVs on the market there are a lot of questions about 3D viewing. One of the most common is whether or not you will have to wear glasses to watch 3D, the answer to that one is still yes. After that people want to know what will be the best way to watch 3D content. Let's compare the different technologies and the glasses you use to watch them.

Passive 3D Glasses

Passive 3D glasses are any 3D glasses that don't require a power source to view 3D content. The major types of passive glasses used on current 3D TV's are polarized 3D glasses. One of the main benefits to passive 3D glasses is cost. Since they do not require a power source or powered lenses, passive 3D glasses can be very inexpensive.
Polarized 3D glasses come in two forms, linear polarized and circular polarized. Linear polarized glasses require the user to maintain a vertical head position, tilting the head left or right can break the 3D effect because the content relies on one eye seeing the vertically polarized image and the second eye seeing the horizontally polarized image.

When the head is tilted the polarized lenses no longer line with the polarized double image on the screen. Circular polarization does away with this problem but it requires a special projector and filter.

Another benefit to passive 3D glasses is that since the viewer is being shown both images at once it does not half the frame rate of the content like active glasses.

Active 3D Glasses

Active 3D glasses require a power source to power the lenses in the glasses, for shutter glasses a mechanism to sync the glasses to the display is also required.

Shutter glasses are the active 3D glasses most viewers will use for 3D content, they use LCD lenses that are commanded to alternately open and shut each lens to show each eye a different image. It used to be that shutter glasses were connected by a wire that provided both synchronization and power but now most shutter glasses are powered by small batteries and receive sync signals via an infrared beam or RF signal similar to a TV remote. The technology involved makes shutter glasses considerably more expensive than passive glasses, expect to pay anywhere from $50 to $150 for a pair of shutter glasses.

The major disadvantage to active 3D glasses is that the separate images for each eye are not delivered at the same time. The content will alternate the images shown to each eye at whatever the frame rate of the content and refresh rate of the display offers. For example, with 60 frame per second content each eye is being shown only 30 images per second effectively cutting in half the frame rate seen by the viewer. This is most noticeable during slow camera pans or during fast motion. The higher the frame rate the smoother each of these actions will appear so active 3D glasses can show slight to noticeable judder effects during these scenes.

However, Active 3D glasses allow full color and picture resolution information since both images are not being overlaid on one another as in Passive 3D (This overlaying cuts the effective resolution in half). This benefit outweighs the effect that active glasses have on frame rate. Active shutter 3D glasses provide the best home 3D experience if one is looking for the best overall picture quality.

 

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