The material that is used for the reflective layer of a blank CD or DVD is a critical factor in the failure rate of the disc. The layer reflects the laser beam back to the laser photosensor in the laser head, so if this reflection is not precise and consistent, the disc will incur errors when writing to it or reading data from it.

The reflective layer is most commonly made from aluminium or silver.  Discs with aluminium reflective layers are generally cheaper, while silver layers offer superior reflectivity which can provide improvements in read and writing performance.  What needs to be understood is the significance of these performance improvements, and whether it justifies the price difference? 

Also, in making this assessment, it is necessary to note that lasers in optical drives deteriorate over time and become less powerful.  It is therefore relevant to understand how well optical discs perform in drives that are relatively new but also to see how well they perform when used in optical drives that are older.

HIGH FAILURE RATE OF ALUMINIUM

In tests using new optical drives (under 1 year old), the failure rate of burning content onto the aluminium discs was greater than 10% for DVDR and more than 13% for CDR compared to zero failures for silver.  However, when used on the older optical drives (over 1 year old), the burning failure rate was notably higher – over 15% for DVDR and over 25% for CDR, again compared to a zero failure rate for silver.  In addition, when executing the error rate tests, the silver layer products all performed within the book specifications (DVD: PI sum8<280 and CDR: C1<220) while all of the aluminium discs were out of spec., with significantly higher errors recorded when tested on optical drives over 1 year old.

These performance differences can be attributed directly to the inferior reflectivity of the aluminium layer compared to that of silver.
 

CORROSION

Optical media is also susceptible to corrosion, particularly in transit on-board ship where it can experience high temperatures and humidity.  Aluminium oxidizes on exposure to oxygen from the environment or to moisture that has penetrated the disc which diminishes its reflectivity, making the disc unreadable by the laser - sometimes referred to as disc “rot.” Silver can also lose reflectivity with corrosion on exposure to sulfur dioxide, an environmental pollutant that can migrate through the disc with moisture.  However, the Reliability Test showed that all the silver products passed the 100 hour heat and humidity test, which checks for length of time a disc can be exposed during shipment and storage and also its archiving capabilities.  In contrast, all of the aluminium CDs and DVDs failed to reach the 100 hour mark. 

Table – Defective Rates for optical media using Aluminium or Silver Reflective Layers

Source: CMC Quality Comparison Test

CHOOSE SILVER

In purchasing blank media, the material used for the reflective layer may not be something that is ever considered, but it is clear from this analysis that it is a key attribute and should be part of the selection criteria. The lower reflectivity of aluminium results in significant writing and reading failure rates, which is further exasperated when using optical drives that are older and have weaker lasers. Aluminium discs also have increased problems with corrosion, and there is another known issue with aluminium discs in that they become hotter when in use, which can speed up the decay of the reading head of the optical drive.

So when buying optical media, although there may be a slight price difference, it’s difficult to argue against choosing silver every time.

Identifying discs with Silver or Aluminium reflective layers.
Left: Silver reflective layers are slightly see-though. Right: Aluminium reflective layer, non-see-through.

1. PI error (PIE): A byte error occurs when one or more bits in a byte have a wrong value, as compared to their original recorded value. A row of an ECC Block that has at least 1 byte in error constitutes a PI error, in any 8 consecutive ECC Blocks the total number of PI errors before correction shall not exceed the book specification of 280.
    
2. Reliability test: The results represent continuous exposure to extreme temperature/humidity levels. The error rates are not representative of discs stored in typical, normal or ideal storage conditions. The results from these tests are to demonstrate, in terms of error rates, the ability of some DVD and CD media to maintain stability given these extreme conditions.