Frequently Asked Questions About CD-R and CD-RW Discs

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Rumors state that silver CD-R discs can damage the lasers in drives. Do blue CD-R discs with silver metallization have the same quality as gold?

Rumors of laser damage from silver discs may be from the same sources that report that Elvis is still alive. Any CD reflects only a fraction of the incident laser power, and most of that is directed to the photodetector. The amount of reflected light that returns to the laser is very small compared to the intensity that it initially generated. Neither silver, gold, nor aluminum metal layers can harm the lasers in drives. But each metal is different.

Aluminum is used as the metal reflective layer in CD-ROM discs because of its ease of application and low cost. Dyes used as CD-R recording layers are very corrosive, and rapidly destroy aluminum. Gold is an inert noble metal that is not corroded by organic dyes, and therefore was used exclusively in CD-R discs for many years. Because gold is expensive, even very thin gold layers become costly as CD-R prices rapidly decline.

Reflectivity of a thick gold layer is about 94% in the infrared part of the optical spectrum. Attenuation in the dye layer decreases the intensity of the return laser beam to about 75% of the incident beam. Thinner gold layers may reduce this to values near the 65% minimum allowed by standards. Silver is less expensive than gold, and has equal or better optical properties.

Silver has an infrared reflectance of about 96% that is higher than that of gold. After absorption by the dye layer, the intensity of the return beam is about 77% of that of the incident beam, very near that of a CD-ROM disc. At present, the thickness of the silver layer is not a significant cost factor, although this may change in the future. Thick silver may actually be superior to thin gold because of its constant reflectivity. Small variations in thickness of a thick coating have minimal effect on the reflectance, while the same variations significantly modify the optical properties of a thin coating.

All metal layers have thickness variations, but reflectance from thick layers is not affected because the penetration depth of light is only a fraction of the thickness. Thin layers also have thickness variations, but these are now comparable to penetration depth and significantly modify reflectivity. This results in variations in the intensity of the returned laser beam during rotation of the disc. Such variations have been found to affect the quality of audio discs, and may also influence data quality.

One advantage of silver is its appearance from the label surface. Although gold is highly reflective in the infrared and red, its reflectivity begins to decrease in the green, and is low at the blue end of the visible spectrum. Blue-green cyanine dyes therefore appear green when they are backed by gold. Appearance of a label is similarly influenced by gold.

Silver has high and nearly constant reflectivity throughout the visible and near infrared portion of the optical spectrum. The true color of blue cyanine dye is now observable, while the appearance of labels is similar to that of CD-ROM discs that have aluminum metallization. Labels on discs having silver metallizations may therefore have a better appearance than discs having gold metallizations.

Longevity of CD-R discs having gold metallizations has been widely examined. Comparable longevity data for silver discs has not been widely published. Manufacturers of silver discs may have evaluated the stability of the metal-dye interface. However, the extensive history that exists for gold has not yet been established for silver discs, and negative surprises are always possible.

Sulfur is a common pollutant, and is known to react with silver. Even though the silver layer is protected by a thin coating on the label surface, pollutants could pass through very small cracks or scratches in the protective layer. The result could be loss of data. Some manufacturers have evaluated the results of exposure to sulfur dioxide and may be willing to share their results, or indicate whether they use silver alloys that are not as reactive.

Users of CD-R discs that use a silver reflective layer should request longevity information from the manufacturer. Otherwise, silver discs seem to perform at quality levels that equal or exceed those of CD-R discs that use gold metallization. Recording and read lasers interact similarly with CD-R discs having gold and silver metal layers. Silver metallizations cannot damage lasers or any other drive components.