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. This is a permanent change, and both CD and CD-R drives can read the modified dye as a bump later on.
It turns out that the dye is fairly sensitive to light -- it has to be in order for a laser to modify it quickly. They are coated with metal and then overlaid with photosensitive organic dye.
During recording, a laser heats the metal and the dye layers in certain spots so that they change their reflectivity to resemble a pit on an aluminum-pitted CD. When a drive reads the CD, the CD appears to have the normal pit and land areas of a commercially produced CD, even though there are not actually any pits. What is multisession recording?
When you write data to a CD-R, you permanently change the areas on which it is written. Because CD-R discs were so expensive, multisession recording was devised to help take advantage of the wasted space. With multisession-capable recording software, you can reuse a partially used CD-R by creating a new session on the remaining blank space of the disc.
When you do so, however, the previous sessions become inaccessible. Only the last session on the disc can be read. You could use the same CD several times and always have access to the most recent copies. Since CD-R discs have become cheaper in recent years, the popularity of multisession recording has waned.
Although most recording software supports it, few consumers go to the trouble anymore. What do the different colors of CD-R signify? Different CD-R manufacturers have devised different combinations of metals and dyes. Any of them will work fine for almost any task, but certain colors and combinations are especially advantageous for certain applications.
It has a rated lifespan of 10 years and uses cyanine dye, which can be more forgiving of disc-write and disc-read variations than some other dyes. This results in a CD that will likely play well in any CD player. Gold dye, gold metal : The gold dye used here is phthalocyanine.
It is a more sensitive CD with less tolerance for power variations and might be less likely to work in a wide variety of drives. Manufacturers include Mitsui, Kodak, Maxell, and Ricoh. Blue dye, silver metal : The blue dye is azo. This combination has similar properties to the green-gold combo but is rated to last much longer: years. These discs are great for long-term data storage. 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. These spots can then be erased in a process known as annealing. This is accomplished by heating the material to a lower temperature, which transforms it back to its crystalline state.
Existing data can be overwritten by turning the laser on continuously to the erase power, which will erase any existing marks. Switching the laser to a higher power, one sufficient to melt the material, enables the creation of a new mark. Once an area of the disc has been written to, it cannot be erased. The recordable layer is an organic cyanine or pthalocyanine dye. Initially, the organic dye has high reflectivity. When the laser is applied in write mode, however, a chemical reaction occurs that makes that "pit" less reflective than the "land" around it.
During readout, the laser detects the difference in reflectivity between the "pits" and "lands" to read the data or music. CD-R discs are highly reflective--about 60 to 70 percent of light is reflected or bounced back to the photo detector or read laser.
Phase change technology enables this rewriteability. The recordable layer on a CD-RW disc is made up of a rare-earth metal alloy "sandwich," which includes silver, indium antimony and tellurium elements. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close.
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