The ISO specifications require a direct link to a certified reference. This means that companies that need ISO references have to send discs to the Philips Evaluation lab, and use these discs to calibrate their measuring. Remember that only discs (all CD formats) measured by the Philips Evaluation lab are to be considered as reference discs.
Customised CD-R/CD-RW calibration discs will improve the measurement accuracy and tolerances for each individual manufacturer significantly.
Different manufacturers use different dye technologies and phase change material. The pregroove design is optimised for each process and can therefore differ considerably. The phase change materials are fairly similar, but if a CD-RW disc is calibrated with a disc using one phase change material and pregroove design, and then measure a different disc using another phase change material and pregroove design, the measurement tolerances will be affected.
For CD-R the differences can be much bigger depending on the dye technologies. The pregroove designs are optimised for each individual dye technology. As there are quite big differences between different dye technologies and pregroove designs, the measurement tolerances can be considerably affected if a tester is calibrated with a disc using a different dye technology than the tested disc. The different dye technologies have frequently been modified over the last years. Most of them will continue to be modified to achieve good results at higher recording speeds.