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DVDs pack up to 17 GB using finer reading mechanisms and multiple data layers<br><br>DVD and CD formats share the same basic optical storage technology—microscopic pits formed on the surface of the plastic disc when the material is injected into the mold, which represents information. The pitted side of the disc is then coated with a thin layer of aluminum followed in the case of the CD by a layer of protective lacquer and a label. To read the data, the player shines a small spot of laser light through the disc substrate on to the die data layer as the disc rotates. The intensity of the light reflected from the disc’s surface varies according to the presence or absence of pits along the die information track. A pit reflects much less light than the flat part of a track. A photo-detector and other components inside the player translate this variation into 0s and 1s, representing the stored information.<br><br>There are two essential physical differences between CD and DVD discs.<br><br>First, the smallest DVD pits are only 0.4 micron in diameter; the equivalent CD pits are twice as large or 0.83 micron wide. Moreover, DVD data tracks are only 0.74 micron apart, where as 1.6 microns separate CD data tracks. Therefore, although DVD is the same size as a CD, its data spiral is upward of 11 km long—more than twice the die length of a CD’s data spiral.<br>The read-out beam of a DVD must achieve a finer focus than a CD player does. In order to do this, it uses a red semiconductor laser that has a wavelength of 635 to 650 nanometer. In contrast, CD players use infrared lasers with a longer wavelength of 780 nanometer. In addition, DVD players employ a more powerful focussing length—one having a higher numerical aperture than the lens in a CD player. These differences together with the additional efficiencies of DVD format described below account for the huge 4.7gigabyte capacity of each DVD information layer.<br>
