- stamped DVD's only - does not apply to recordable DVD -
One of the secrets to DVD's sevenfold increase in capacity is in its use of a new RED LASER with a smaller, brighter and sharper focal point. This enables finer ' engraving ' of the focal point. For instance the minimum length of the pits on a DVD was reduced from 0.834 - 0.97 micrometers ( Standard CD, left picture ) to just 0.4 micrometers. At the same time it became possible to reduce the distance between tracks from 1.6 micrometers to only 0.74 micrometers.
Dual Layer Discs incorporate two 0.6 mm thick layers which are bonded together to provide rigidity; And it's all thanks to a partially permeable reflecting layer on the laser side which gives rise to the unique dual layering process. Basically, they begin with a thin, plastic circular sheet (about the thickness of a transparency for an overhead projector), stamp it, then glue on another thin sheet of plastic and stamp it as well:
When you insert the DVD, the player begins reading the lower layer. Following complete playback of the lower layer, the laser is automatically & immediately focused on the second layer ( top layer - which is partially permeable ). An electronic buffer ensures smooth playback between the 2 layers.
Although the 2 layers are only 40 micrometers apart ( Less than half the diameter of a human hair ), there's only a slight visual hiccup during layer translation.