How does cd rw write data to a disc

Gorilla Glass is an alkali-aluminosilicate glass developed by Corning that's primarily used as cover glass for mobile devices. Because it's resilient, durable and remarkably thin, it has been made to safeguard displays and touch screens without compromising the screen or adding bulkiness to the View Full Term. By clicking sign up, you agree to receive emails from Techopedia and agree to our Terms of Use and Privacy Policy. CD-RW allows for data erasing during each rewritable session.

However, data cannot be changed during CD-RW sessions. Some CD-RW discs have a multisession feature, in which additional data may be written at a later time if extra space is available. A CD-R disc needs to allow the drive to write data onto the disc. For a CD-R disk to work, there must be a way for a laser to create a non-reflective area on the disc.

A CD-R disc therefore has an extra layer that the laser can modify. This extra layer is a greenish dye. In a normal CD, you have a plastic substrate covered with a reflective aluminum or gold layer. In a CD-R, you have a plastic substrate, a dye layer and a reflective gold layer.

On a new CD-R disc, the entire surface of the disc is reflective -- the laser can shine through the dye and reflect off the gold layer. When you write data to a CD-R, the writing laser which is much more powerful than the reading laser heats up the dye layer and changes its transparency.

The change in the dye creates the equivalent of a non-reflective bump. But newer drives tend to be able to read the various formats. Gordon Rudd of Clover Systems offers this answer: All CDs and DVDs work by virtue of marks on the disc that appear darker than the background and can thus be detected by shining a laser on them and measuring the reflected light.

In the case of molded CDs, these marks consist of "pits" molded into the surface of the disc. Destructive interference of the laser beam caused by the difference in path length between the bottom of the pit and the surrounding "land" causes the pits to appear darker than the background. These marks, too, appear dark compared with the background. Instead of dye or pits, these discs feature a layer of phase-change material.

This material can exist in two different solid states: crystalline or amorphous. Most solids have a crystalline structure in which the atoms are close packed in a rigid and organized array. But some materials can have an amorphous state in which the atoms are organized not into arrays but randomly, as in a liquid. A common example of such a material is ordinary window glass, an amorphous form of silica.

The phase-change material can change from one phase to the other when it is heated and cooled. The material used is chosen because the two solid states reflect light differently. The amorphous state reflects less light than the crystalline state does.

Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state, which will appear dark upon playback.

Heating the material with the laser beam above its melting point transforms it from crystalline to amorphous. The rapid cooling of the spot causes the material to freeze in the amorphous state.

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