8.1.8 Disc Selection  There are three basic types of dye used on write once recordable discs, phthalocyanine, cyanine, and azo. Manufacturers of phthalocyanine discs claim a longer life for their product than the competitors. Some, though not all initial testing supports this view. Some manufacturers use Azo dyes in discs that they claim are archival. Cyanine was the first dye type developed for optical disc recording, and is generally recognised by most manufacturers as having a shorter life expectancy (LE). Dye type, though significant, is only one of the factors determining the life of the media.  The variation in the amount of dye used in the dye layer, a result of the manufacturers’ race for even higher recording speeds and higher density recording, is a contributing factor in the long term failure of recordable optical media. Recording speed has increased from X1 to X52 and is still rising, as the recording density has gone from 650MB to 800MB for CD-Rs. It should be noted that discs optimised for high speed recording use less dye, which may indicate a shorter life expectancy. DVD-R uses less dye as a matter of course, as the data rate when writing to a recordable DVD is much higher than for CD-R.  It is not, however, just a matter of reducing speed; if discs with a denser dye layer, optimised for writing at lower speeds, are written at higher speeds, they deliver a worse error rate. Though manufacturers indicate the maximum recording speed, writing at that maximum speed may not achieve adequate results. There is an optimum writing speed at which the disc produced obtains the best possible technical measurement for performance. Identifying this speed is best done by trial and error measurement using a reliable disc tester. Typically, the best results will be achieved on a dense dye layer disk written at around 8 times speed.  At best, the quality of blank recordable CD and DVD media can be described as variable. The recordable CD and DVD- manufacturing industry has become a market place driven by narrow profit margins and large quantities. Recordable CD and DVD manufacturing equipment has become smaller, cheaper and more self-contained. As a consequence, the production of reliable data carriers for the quality market has largely been replaced by manufacturers of recordable CD and DVD, producing recordable CD and DVD for the low cost market.  Many discs that appear to be reputable brands may turn out to have been manufactured by a second party and repackaged for sale. A recordable CD or DVD manufacturer can manipulate the dye, reflective layer and the now expensive polycarbonate components to reduce price or control quality. As a general rule, it has often been recommended that only reliable brand recordable CD and DVD are purchased, however, testing has revealed a range of compliance with agreed standards even amongst them. Instead, it is recommended that the responsible individual or institution insist on dealing with a supplier that is open about the importer or manufacturer they deal with, and who is able to provide contact with the relevant technical personnel in the manufacturing company. Discs that fail the standard specified below should be returned.  It is quite difficult to identify the best quality media without high level analysers (Slattery et al., 2004). In most practical circumstances discs must be recorded before they can be tested. Some very high quality CD and DVD testing equipment will analyse an unrecorded disc, but most testing is carried out by recording a test signal and analysing the result. ISO 18925:2002,AES 28-1997, or ANSI/ NAPM IT9.21 is a standard test method to establish the life expectancy of compact discs, and ISO 18927:2002/AES 38-2000 is a standard for estimating method for estimating the life expectancy based on the effects of temperature and relative humidity for recordable compact disc systems. As temperature and humidity aging does not always produce clear results, other approaches have concerned themselves with the susceptibility of recordable dye based discs to light exposure with age, and some manufacturers have undertaken testing in this area. There is, however, no standard for this (Slattery et al., 2004). Summary of Disc Selection Purchase a range of best quality discs, based on market research. Purchase more than one of each type. (Though price is not necessarily an indicator, always remember that the cost of even the most expensive discs is small compared to the value of the data.) Under controlled conditions record some data on each of the discs. Test to see which discs perform best with regard to specification in this document. All discs must exceed the recommended quality standards recommended below (see Table 1, Maximum error levels in an archival CDR). Test at a number of different writing speeds. Keep disc/burner compatibility in mind: different burners may yield different results. Choose the three best discs, from at least two dye types (phthalocyanine and azo). Record identical copies of the data on the three chosen discs. Ensure that delivered supplies of chosen discs are identical with tested sample discs. Repeat tests each time a batch of discs are purchased.