Ask the editor is the spot where we try to answer readers’ questions.

What’s the difference between ‘active’ and ‘passive’ 3D glasses?
Sam Smart

Panasonic 3D TV

Both types of 3D glasses are significantly different from the old ‘red and green’ 3D glasses of yore, which always seemed to make the picture look fuzzy and out of focus. Modern 3D technology means that 3D images are much clearer, and don’t have the coloured fringing which the older technology suffered from.

Simply, active 3D glasses are powered by a battery which switches each of the lenses from opaque to see-through 120 times a second, synchronised with the images on the screen, which flicker between left and right eye versions of the same picture at the same rate. So when the left eye image is on screen for 120th of a second, the left lens is transparent, and vice versa with the right eye image. The changeover is much too quick to register in the brain, but is enough to give the impression of a picture seen from two slightly different perspectives, giving the impression of depth.

Passive 3D glasses are cheaper to make and are the kind used in cinemas, theme parks and pubs. The 3D film uses two images from slightly different angles shown at the same time. Each image is polarized to match one passive eyeglass lens and polarizing filters in the lenses filter the images so that the left eye only sees the left image and vice versa.