5.6.5 Cleaning, Carrier Restoration
184.108.40.206 CDs or DVDs do not require routine cleaning if carefully handled, but any surface contamination should be removed before replay or in preparation for storage. It is important when cleaning to avoid damaging the disc surface. Particulate contamination such as dust may scratch the disc surface when cleaning, or use of harsh solvents may dissolve or affect the transparency of the polycarbonate substrate.
220.127.116.11 Use an air puffer or compressed clean air to blow off dust, or for heavier contamination the disc may be rinsed with distilled water or water based lens cleaning solutions. Care should be taken as the label dyes in many CD-Rs are water soluble. Use a soft cotton or chamois cloth for a final wipe of the disc. Never wipe the disc around the circumference, only radially from the centre to the outside of the disc - this avoids the risk of a concentric scratch damaging long sections of sequential data. Avoid using paper cleaning products or abrasive cleaners on optical discs. For severe contamination isopropyl alcohol may be used if required.
18.104.22.168 It is preferable that no repairs or polishing is undertaken on archival optical discs as these processes irreversibly alter the disc itself. However, if the disc surface (reading side) shows scratches that produce high level errors, repairs which return the disc to a playable state may be allowed for the purposes of transfer. These may include wet polishing systems providing careful testing of the effect of these restoration systems have been undertaken before being applied to important carriers. This should be undertaken by testing an expendable disc, undertaking the restoration process, and retesting to determine the effect of restoration (for further details consult ISO 18925:2002,AES 28-1997, or ANSI/NAPM IT9.21 and ISO 18927:2002/AES 38- 2000). Though some initial testing of wet polishing indicates adequate results, the removal of surface material makes sound archivists reluctant to endorse this approach. Moreover wet polishing is only effective with small scratches; discs with deep scratches deliberately inflicted with, for example a knife or scissors, will not be returned to playability by wet polishing. Damages on the label side will not benefit from any repair measures described.
22.214.171.124 Before and after cleaning and/or repairing measures and prior to the reproduction it may be advisable to first measure the CD’s or DVD’s error rates, as a minimum:
126.96.36.199.1 Frame burst errors (FBE) or Burst Error length (BERL)
188.8.131.52.2 Block error rate (BLER)
184.108.40.206.3 Correctable errors (E11, E12, E21, E22, errors before interpolation)
220.127.116.11.4 Uncorrectable errors (E32)
18.104.22.168.5 Radial noise and tracking error signals (RN)
22.214.171.124.6 High frequency signals (HF)
126.96.36.199.7 Dropouts (DO) 188.8.131.52.8 Focusing errors (PLAN)
184.108.40.206 There are a range of error measuring devices available for CD and DVD of varying sophistication, accuracy, and cost. A reliable tester is, however, a necessary part of a digital disc collection to determine if critical error thresholds are exceeded (cf 8.1.5 Optical Discs – Standards and 8.1.11 Testing Equipment). If after cleaning and repair one or more of the error rates exceed these thresholds refer to 5.6.3 “Selection of Best Copy”.