Stereoscopic 3D glasses that are used with "active 3D" computers and TVs. The glasses use liquid crystal lenses that act as shutters, alternately opening and closing the view to the screen. Also called "wireless glasses," they are designed for one type of active 3D system; however, multi-brand glasses are available that work with different brands of active 3D TV sets. Traditionally, infrared (IR) emitters have been used to transmit signals to the glasses, but starting in 2011, Samsung switched to Bluetooth for its active 3D TVs. In addition, ceiling-mounted movie projectors use radio frequencies (see RF shutter glasses). See active 3D, 3D visualization and 3D glasses.