Most active 3D TV systems use 'shutter glasses' technology; these have been around for a while, but did not gain popularity until the boom of 3D television. They had use in virtual reality, but they also had disadvantages compared to other virtual reality systems, as they required the user to still watch a screen, and not be able to move freely like the virtual reality HUD and other systems.
The system is called 'active' because unlike passive technologies, the glasses have to 'do work' to achieve the effect; that's why they require batteries.
The stereoscopic effect is achieved by the synchronized operation of the shutter glasses and the TV set: the TV box displays alternately images that are intended for one eye, then for the other eye; and while the screen displays an image intended for one eye, the glasses 'close' the view of the other eye. This is because the lenses of the glasses are made of LCD (liquid crystal). The property of the liquid crystals that is useful in screens is that they allow us to control the passing of light; in a way, they operate as 'gates of light'.
Since we want to see a continuous movement and continuous action in the screen, the refreshing of the screen and the alternating of images for each eye must be repeated many times in a second, so not to break apart the effect of continuous action in each eye.
The next interactive explanation displays flickering images (caution is required if you are prone to seizures); you can control the speed of the flickering with the slider at the bottom.
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Because the 'blocking' and alternating of images occur so fast and so many times every second, the action of blocking the images is not apparent at conscious level; yet the brain may perceive the flickering and may get tiring for some people after a while (in any system, if the user feels eye strain or headache, he or she should take a break; results and intensity may vary from person to person). Also, because the eye sees a mix or average of the image displayed, plus a closed lens (which is a black screen, or a 'no way for light' lens), the resulting image may be darker than normal television, but the TV set may compensate this with adequate brightness and contrast control.
A mayor inconvenient with the active systems is the requirement of special glasses. Since the operation of the TV and the glasses must be synchronized (the glasses must know which eye to shut, and when), the system requires special glasses able to synchronize to this particular TV set. Now days the companies that provide active systems based on shutter glasses are moving toward a standardization (which is not there yet), but not long ago every system required glasses specially for that set (even different models of the same brand, in some cases); glasses of this kind are normally more expensive than those required for other technologies, and are required for each user that watches the movie in the same TV screen; they also require a change of batteries every once in a while.
