I used to believe this.
I “knew” that CDs were made by blasting microscopic pits into their mirror surface with a writing laser, and that later a reading laser beam would be dispersed by these pits on its way to a light sensor, creating electrical pulses from that sensor which could be converted to a stream of binary digital data. Physical pits in an aluminum surface are quite a robust way to store data. With a little care against physical abuse, such a record ought to last for a very long time – and they do.
Except it turns out that is not how computer CDs and DVDs are made or used!
My understanding belongs in that category of “A little knowledge is dangerous”.
This laser pit type of CD was how the original CD invention worked and is used for mass produced music CDs – except that a “master” is burned by laser and the mass-produced CD you have is pressed (physically stamped) from a master.
Computer CDs and DVDs use a much less powerful laser, which simply heats up a tiny spot on a layer of chemical dye on your blank CD or DVD disk. These dyes are much less reliable than physical pits. They change chemically and weaken in a rather short time and can also be affected by chemicals in the air. And different manufacturers use different dye formulas, some of which don’t last as long as others. Some are even formulated to look a certain way to consumers, in spite of the fact that this makes them more short-lived.
I read one online report about a man whose ailing mother lived with him and she required breathing oxygen. With this extra oxygen in the air of their home, his CDs became unreadable in a few months.
There isn’t room here to go into all the details, but here’s the bottom line:
If you want to burn CDs or DVDs which can be reliably read after more than 3 to 5 years, do some online research before you start archiving and get all the facts. Otherwise, you’re in for a rude awakening in the (not very distant) future.
Please feel free to comment below.
‘Til next time, happy (and long lasting) computing!
_jim coe
Tags: archive, CD, DVD, reliability
