Shutter glasses (also known as LCD or LC shutter glasses) have lenses containing liquid crystals that turn opaque when a small voltage is applied. By varying the voltage each lens can be made to independently open and close like a shutter— hence the name. When the left image is on the screen the left shutter is open and the right shutter is closed so the left image is only viewed by your left eye. When the right image is on the screen the right shutter is open and the left shutter is closed. Since this process happens so quickly, persistence of vision lets you see both images in the stereo pair at the same time and the brain fuses them into a 3D image. If the shuttering speed isn’t fast enough, you still see a stereoscopic image, but you may also see some flickering. If you take the glasses off, the screen will look like it’s displaying both images slightly offset from one another. They are each blinking alternately but our persistence of vision smooths out the display so you don’t notice the blinking.
SHOW THAT AGAIN SAM
Traditional 2D movies are projected in theaters at 24 frames per second (fps). Systems based on shutter glasses have to project at least 48 fps so each eye sees
at least 24. To reduce flicker, the fps is often boosted by projecting each frame more than once each second. For example:
- To project 24fps to each eye each frame is projected twice.
- To project 48 fps for each eye each frame is projected twice.
- To project 72 for each eyes, each frame is projected three times.
Shutter glasses are used to view stereo images on Sony TV sets or other devices using compatible technology.
When wearing shutter glasses you don’t see both images at the same time. The frames are displayed at up to 72 framesper- second for each eye instead of the usual 24. The system uses a wireless transmitter to keep the battery powered shutter glasses and image frames synchronized using Bluetooth, infrared or radio waves.
PERSISTENCE OF VISION
Persistence of vision is the result of an after image persisting on the retina for one twenty-fifth of a second or so. To avoid movie frames fading, causing the film to flicker, they are flashed on the screen so fast that the next image is seen before the previous image has entirely faded away.
Many 3D systems use the frame sequential method of displaying 3D images. The images are displayed one after another —alternately showing the left eye image and then the right. When done fast enough our persistence of vision makes it look like a stable image.
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The technology behind shutter glasses is technically referred to as alternate-frame sequencing, time-division multiplexing or field sequential.
Shutter glasses are widely used in the first generation of 3D TVs, computer monitors, Blu-ray players and 3D digital projectors. In theaters they are also used by Nvidia, XpanD and IMax—otherwise known for its large screens, stadium seating, and great sound systems. Because these glasses contain electronics, they are called active glasses and are expensive, unlike the passive glasses used to view anaglyphs and polarized displays. At home the expense of these glasses is a drawback when you have a large family or like having guests over for viewing. If guests bring their own glasses there is no guarantee they will be compatible with your system. The short term solution to this problem is using third-party universal glasses that work with all sets. The long term solution is the development of industry-wide standards that all glasses must meet.
Monster makes 3D shutter glasses that are compatible with any brand of 3D TV and use built-in radio frequency (RF) technology to sync with the display. Courtesy of www.monstercable.com
THE FIRST 3D MOVIE FLOP
The 1922 Teleview system offered the earliest form of alternate-frame sequencing for stereoscopic projection. Installed in only one theater, it showed only one movie—not exactly a commercial success. Using two coupled projectors, alternating left and right frames were projected one after another. Viewers attached to arm-rests at each seat had mechanical shutters that were synchronized to open and close so viewers saw only one of each stereo pair at a time.
Shutter glasses are not widely used in theaters because of their expense and the need to clean and charge them between uses, but a company named XpanD has their systems installed in smaller theaters around the world.
When choosing a pair of glasses for your own system, here are some things to consider.
- Are they compatible with your system?
- How many hours of use between charges?
- Are the glasses designed to fit over your existing prescription eyeglasses or reading glasses? Although all are designed to do so, you might want to try them on over your own glasses.
- How do the glasses affect the colors you see. It seems TV manufacturers tune their glasses to improve or correct colors in their own sets. This means glasses from any other vendor, especially universal glasses from third parties may not show colors as accurately.
- What is the recommended viewing distance to the screen and from what angles can it be viewed. What is the range of the transmitter that synchronizes the glasses with the display? How do the answers to these questions affect the room layout you have in mind.
A checkerboard format has been used on some 3D TVs. Instead of interleaving rows, it assigns each image to alternate pixels— reducing the resolution by half. It is a variant of alternate-frame sequencing, requiring the same kind of LCD shutter glasses.