Active 3D technology uses special battery-powered glasses that synchronise the image display with the TV so that the shutters in the glasses open and close, allowing each eye to see a different image. In this way, the full number of lines of the image is displayed for each eye, which can provide a higher quality 3D display than passive systems. However, active 3D glasses can be heavier and more expensive than their passive counterparts, and some viewers may experience eye fatigue or other discomfort with prolonged viewing.