Technology

• Both dual-layer and double-sided DVDs use the same technology to record and reproduce data. A laser creates tiny pits on the disc's reflective surface. During playback, the laser shines into the pits and a small optical sensor records the position of the pits based on when the laser reflects off the remaining reflective surface. Dual-layer DVDs have a semitransparent bottom layer and the DVD player's reading laser can focus on either layer to read data.
  
  

Capacity

• While a standard DVD can hold up to 4 gigabytes (GB) of data, dual-layer and double-sided discs effectively double this capacity. Dual-layer discs can hold up to 8GB of information, which equates to approximately four hours of video footage. Double-sided DVDs with a single layer on each side can hold up to 8.75GB of data, while double-sided discs with dual layers on each side can hold 15.9GB, or eight hours of video.
  
  

Form and Handling

• Dual-layer DVDs have a gold surface rather than a silver one, but are otherwise identical to single-layer DVDs and very similar to CDs. They usually feature a label or a blank surface on the opposite side. Double-sided DVDs have a reflective surface on each side and are more difficult to handle, since smudges, dust or fingerprints on either side of the disc can have an effect on playback. Double-sided DVDs usually have their title information printed on a small ring near the disc's center hole.
  
  

Uses

• Most feature film DVDs from major manufacturers use a dual-layer format. This removes the need to flip the disc over, as with a double-sided DVD. However, some film studios release movies on double-sided DVDs in order to offer both a widescreen version and a fullscreen version on the same disc. Some longer films span two dual-layer discs, or fill an entire double-sided/dual-layer disc. Both forms are also available as blanks for backing up data from a personal computer.
  
  

Alternatives

• Blu-ray discs are a more recent alternative to dual-layer and double-sided DVDs. They come in single- and dual-layer varieties and hold 25GB and 50GB, respectively. Blu-ray discs use a special blue laser to read data that is even more densely written on the disc surface. Besides Blu-ray discs, solid state drives and magnetic hard drives offer a way to back up computer data or store movie files, such as those downloaded from the Internet.