Compact disk – CD-R, CD-RW, CD-ROM — learn about digital optical data storage

Co-developed by Philips and Sony and released in 1982, a compact disk is a digital optical data storage device, basically a thin circular disk. At the time of their development Compact Disks(CD) stored more data than the hard drives of personal computers, this created a huge demand for the CDs at that time. The data in a CD is stored in the form of pits and lands. A CD player reads data with the help of these pits and lands and convert it into a digital binary system, hence it is a digital storage.

Pits – It does not reflect the laser beam and represents a ‘0’

Lands – It reflects the laser beam and is represented as ‘1’

A compact disc has a very large number of microscopic pits and lands to store the data.

Material used in a Compact disk

A compact disk has 120mm diameter and is of 1.2mm thickness. The material used in a compact disk is –

  1. Polycarbonate base layer – pits and lands are formed here
  2. Aluminium coating over the base layer
  3. A clear acrylic layer over the aluminium layer for protection

Types of Compact disks

  1. CD-R
  2. CD-RW
  3. CD-ROM

CD-R (Compact Disk Recordable)

This type of Compact disk is written once and read many times. The data is once written on CD-R and then it can be read many times, but cannot be erased or changed. Based on the quality of the disk and if handled carefully, CD-R can be read many times for a long period of time.

  • The polycarbonate layer is covered with an Organic dye,
  • This is then covered by a reflecting layer of silver, silver alloy or gold (very rarely).
  • And for protection a coating of photo polymerizable lacquer is done.

CD-R was used for recording audio songs

CD-RW (Compact Disk ReWritable)

A Compact Disk that is writable, then used many times and on need, can be erased and rewritten. For this reason, CD-RW cannot be played by simple CD-R readers, laser readers that are specially made for CD-RW are used for this purpose.

It has the same layers as that of CD-R, but differs in reflective layer. Reflective layer in CD-RW is made up of silver-indium-antimony-tellurium alloy. This allow is crystalline in normal state, but when heated by a laser beam, it gets liquefied and converts into amorphous state.

  • Amorphous state loses reflectivity and represents ‘0’
  • Where crystalline solid is present, light is reflected and it represents ‘1’

CD-RWs were used to temporarily store backup files.

CD-ROM (Compact Disk Read Only Memory)

CD-ROM has same structure as that of CD-R. But it cannot be written. It can only be read. CD-ROM was used by software companies to distribute software and also by gaming industry to distribute computer games.

Home page

You may also like to read

Click here to learn about hard disk drives

Click here to learn about Solid state drives


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s