Optical Discs- (Part I of 3)
There are several types of discs used to store information that are read optically (hence the term) by a laser. Technically there are only three main types of optical discs that we will be talking about. First are the ROM discs, either CD-ROM or DVD-ROM, where the information on the discs is 'read-only memory' (ROM) and therefore cannot be changed or added to by the end user. The second type of discs are WORM or 'write once-read many' discs, better known as read-write optical discs. The most common type of worm discs are the CD-R's (recordable) discs that can be recorded on only once. The last optical discs to mention are the rewritable discs. The most common forms of the rewritable discs are CD-RW and DVD-RW which are used when information will be revised, edited or updated on a regular basis.
Optical discs are made from a polycarbonate plastic base comprised of 3, 4 or 5 layers which includes a metal layer usually made of aluminum, silver or gold. The data that is stored on the disc is created by making marks or pits using lasers. These pits or marks are then read by a laser through absorbing or reflecting the light. These attributes are most common for the ROM discs. For CD-R's a photosensitive dye layer is added between the polycarbonate and metal layers to capture and store data. The technology for the rewritable discs involves adding a phase-changing film between the polycarbonate and metal layers. This film gets heated by the laser beam which changes its light transmission properties therefore making it possible to erase and rewrite data. Check back in a few days to read part 2 on Optical Discs.