An active shutter 3D system (a.k.a. alternate frame sequencing, alternate image, AI, alternating field, field sequential or eclipse method) is a technique of displaying stereoscopic 3D images. It works by only presenting the image intended for the left eye while blocking the right eye's view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image. Modern active shutter 3D systems generally use liquid crystal shutter glasses (also called "LC shutter glasses" or "active shutter glasses"). Each eye's glass contains a liquid crystal layer which has the property of becoming opaque when voltage is applied, being otherwise transparent. The glasses are controlled by a timing signal that allows the glasses to alternately block one eye, and then the other, in synchronization with the refresh rate of the screen. The timing synchronization to the video equipment may be achieved via a wired signal, or wirelessly by either an infrared or radio frequency (e.g. Bluetooth, DLP link) transmitter. Historic systems also used spinning discs, for example the Teleview system. Active shutter 3D systems are used to present 3D films in some theaters, and they can be used to present 3D images on CRT, plasma, LCD, projectors and other types of video displays. Advantages and disadvantages Although virtually all ordinary unmodified video and computer systems can be used to display 3D by adding a plug-in interface and active shutter glasses, disturbing levels of flicker or ghosting may be apparent with systems or displays not designed for such use. The rate of alternation required to completely eliminate noticeable flicker depends on image brightness and other factors, but is typically well over 30 image pair cycles per second, the maximum possible with a 60 Hz display. A 120 Hz display, allowing 60 images per second per eye, is widely accepted as flicker-free. Advantages Unlike red/cyan color filter (anaglyph) 3D glasses, LC shutter glasses are color neutral, enabling 3D viewing in the full color spectrum, though the ColorCode anaglyph system does come very close to providing full color resolution. Unlike in a Polarized 3D system, where the (usually) horizontal spatial resolution is halved, the active shutter system can retain full resolution (1080p) for both the left and right images. Like any system, manufacturers of televisions may choose not to implement the full resolution