Reprinted from and with the permission of Noritsu News
Image permanence for C D-R’s (Compact Disk Recordable) is an area that is not well known or understood by the general consumer or by many photo labs. Contrary to popular belief, most CD-R’s are not permanent and low priced CD-R’s may not be readable, at all, and error in as little as two years. An inexpensive CD-R is great for moving files from one PC to another, but risky if being used to archive files or images. Quality CD-R’s utilize high quality recording and reflective layers and are well sealed to reduce the harmful effects of the human environment.
The material used for the plastic substrate (polycarbonate) of the CD & CD-R is also important, as it need to be gas impermeable. Most plastics are not, but some plastics are better than others. Even more important is the quality and thickness of the top coatings used to seal layers coated on the substrate.
Prerecorded CD’s are made by “stamping” the information into the plastic substrate then an aluminum alloy coating is applied to the “bumpy” stamped surface. The laser either reflects off of this reflective layer and a “1 bit” is determined, or the laser is deflected by the “stamped” bump and a “0” bit is determined. Contrary to popular belief, the recorded layer of a CD/CD-R is not “sandwiched” within the plastic substrate. A clean lacquer protective layer is put on top of the aluminum reflective layer. A label is put on top or a thick ink coating applied. As the CD is read from the plastic side, a paper, plastic or ink label applied to the CD provides extra protection against the data surfaces from being scratched.
CD-R’s are not stamped with data; they are burned with data by the end user. However the plastic polycarbonate is stamped with splines (tracks or lines) for the laser to follow. A recording layer is also referred to as the dye layer and it is sprayed on top of these splines. Four basic chemical formulas are used for the recording layer dye:
1.Cyanine / light green/blue in color, costs less to make, most common and lowest permanence
2.Phthalocyanine / transparent with a slight green tint, Highest permanence and second most common
3.Metallize Azo / blue, Similar in quality to Phthalocyanine, costs less to make and are not common
4.Formazan / light green, combination of Cyanine and Phthalocyanine, similar in quality to Phthalocyanine, costs less to make and are not as common
CD-R Manufacturers may modify one of these dyes and create a custom, proprietary formula and/or change the traditional color of the dye. Thus CD-R color cannot accurately be used to determine the type of dye used.
CD read lasers are infrared and are not affected by the color or visible light opacity of the dye. As a result, the recording layer dye color is irrelevant to the laser as it will effortlessly pass through any dye that is not infrared opaque. The dye will become infrared opaque when burned by the write laser.
Some CD-R’s have a colored dye layer in front of the recording layer dye creating colored CD-R’s . Black CD-R’s block visible light from reaching the recording and reflective layer, thus these layers cannot be seen by the eye. Yet the type of black dye used will let the infrared laser pass. Gamers and music experts believe that black CD-R’s produce better quality CD-R’s, but this researcher could not find any scientific evidence to support this claim. However, as a black layer prevents visible light from reaching the recording layer dye (these dyes will fade over time when exposed to visible light), a black layer may increase longevity over an identical grade CD-R when both are stored in the light.
A reflective coating made of silver alloy; pure silver, or pure gold is layered on top of the recording layer. Although silver will show the true color of the recording layer dye, gold will change the eye color because of its yellowish color. The burning laser melts a “pit” into the dye which then blocks the read laser from reflecting back, and a “0 bit” is determined. A good dye burns a nice clean pit so the read laser knows for sure if it is a 0 or 1 bit. If the burn is not clean and the edge is not a clean cut, the CD-R will yield an error.
To protect the coatings lacquer is applied and high quality manufacturers make it nice and thick or will even apply a separate protective coating. Poor quality CD-R’s have very little protection on the coatings and will scratch easily, or worse, delaminate. By writing with a non-water base felt pen, the ink could make its way to the data layer and damage a poorly sealed CD-R. But be aware, even the best coated CD-R’s can be damaged over time by non-water based felt makers.
It is difficult to use CD-R color only as a guide, but as a general guideline, if the CD-R does not have a color tint; it is likely a lower quality silver alloy with a poor dye layer. Blue, Green and faint green CD-R’s will be good if they used quality silver. Unfortunately there is no way to know for sure. Gold CD-R’s are best because when the manufacturer used gold, they us a quality dye. But don’t be fooled by manufacturers that place a gold color label on the non-read side or the ones that add a gold color layer on the read side!
There are several reasons for good or poor C D-R permanence;
1: Plastic (polycarbonate substrate) is oxygen permeable. Oxygen eventually makes its way through the non-lacquered side ( as well as the lacquered side in some cases) and reaches the reflective layer. As aluminum corrodes when exposed to oxygen and silver corrodes or tarnishes when exposed to sulfides in the air, air reaching the reflective layers will cause corrosion causing a read error. This could happen in as little as two years with poor CD-R’s. Gold CD-R’s are best in this area followed by gold/silver alloy. silver/aluminum alloy is the poorest.
Equally important is the optical quality of the plastic. High optical quality CD-R’s permit the light to pass through the polycarbonate with little or no diffusion permitting a cleaner burn to the dye. The spiral grooves stamped to the CD-R vary by manufacturer. It is easier and cheaper to make a “V” shaped groove than a sharp edge “U” groove. A “V” type stamp will have a higher degree of skipping errors as the laser may not be able to track properly, much like the needle of a phonograph if it does not have enough weight on it. Additionally, the stamp will wear as it stamps CD-R after CD-R resulting in a “U” shaped groove becoming more “V” shaped over time which may lead to errors.
2: The Dyes used in the recording layer are light sensitive and will react to ambient light and fade over time. Quality CD-R’s use a dye that resists fading. To be safe, store them in the dark.
3: Humidly may seep through a poor lacquer coating. Quality CD-R’s are well-sealed and resist seepage from markers and moister. To make them last, store in low humidly and use water based markers and write on the center core.
4: A scratch on the base side can be repaired, but a scratch on the lacquer side renders the CD useless. Quality CD-R’s have a thick protective coating to resist scratches. Archiving reports vary by manufacture, but 70 years would be low for a quality CD-R with the norm being 100 years. Some manufacturers of Gold CD-R’s claim 100-200 years!
You generally get what you pay for. Don’t put those precious images on a CD-R that costs just a few nickels and dimes.