Image display device, 3D viewing device, and method for operating the same

A three-dimensional (3D) viewing device comprising:

• a left glass and a right glass;
• a switch;
• a power supply unit to be activated to supply power to each component in the 3D viewing device when the switch in the 3D viewing device is turned on;
• a wireless communication unit to transmit a pairing signal to an image display device after the 3D viewing device is activated and to receive a synchronization signal from an image display device after pairing between the 3D viewing device and the image display device is completed;
• and
• a controller to perform a control operation for alternately opening the left glass and the right glass according to a synchronization signal received from the image display device through the wireless communication unit during an image display period of a 3D image displayed on the image display device, and the controller simultaneously opening the left glass and the right glass in one of a left-eye image display period and a right-eye image display period of the 3D image displayed on the image display device, and the controller determining a synchronization signal period at intervals of the received synchronization signal, and the controller simultaneously closing the left glass and the right glass in the other one of the left-eye image display period and the right-eye image display period when the synchronization signal period is out of a predetermined range, and the controller simultaneously opening the left glass and the right glass during an image display period of a two-dimensional (2D) image displayed on the image display device,
• wherein the controller performs setting such that an opening period of the left glass and the right glass in which the left glass and the right glass are simultaneously opened is shorter than an opening period of the left glass and the right glass in which the left glass and the right glass are alternately opened.