Life expectancy of digital media

At the start of this chapter we saw that electronic media have, in addition to potential physical and chemical degradation, the problem of obsolescence. This has been realized for some years but the solution is neither simple nor obvious. One suggestion is that digital data should be transferred from the old storage media to the new media as it becomes available. This is a process known as migration. Depending on the amount of image data, this may not be the simple solution it appears to be. It is time-consuming and requires whole systems of data management which themselves may become obsolete. In Chapter 12 we saw that there is a potential for a similar and related problem in the very different file formats and compression techniques which may be standards now but not necessarily in the future. It is extremely unlikely that existing materials and formats will be readable in a few decades let alone 100 years or more. This poses very difficult problems for archivists who wish to preserve our visual heritage for future generations and individuals who want to pass on images to their children and grandchildren. The migration of digital data from one medium to another as it becomes the standard is an essential requirement despite the difficulties.

Modern storage media have two life expectancies: one for the physical and chemical degradations and the other for the expected obsolescence. Figure 22.8 gives an indication of these times for some commonly used electronic storage media and further reinforces the view that migration of data is essential. New storage media are being introduced and life expectancy data on these newer media for the storage of image data, such as Flash SSSDs is difficult to obtain. Electro-optical storage media for rewritable discs are also included in Figure 22.8 and have been used for more than 12 years, with a claimed life expectancy of at least 40 years.

Figure 22.8 also indicates that migration must be carried out within the time span of the medium's physical/chemical life expectancy. In the current hybrid imaging era in which much data is recorded on film and then digitized, the film serves as an additional backup, which it is wise to keep. This can be re-scanned at a later date, assuming scanners still exist at that time. In view of the many billions of photographic images that are stored worldwide and are still being produced in vast quantities, it is highly likely that scanners will be available for many decades to come. Obviously the strategy must be to provide storage media with the longest possible life


Figure 22.8 Life expectancies of electronic recording media (based on data of Rothenberg with additions)


Figure 22.8 Life expectancies of electronic recording media (based on data of Rothenberg with additions)

expectancy to make data migration less frequent. In addition, a solid state storage medium is being introduced. This has the advantage of being a compact storage medium which does not involve any moving parts for writing and reading data, which is required by most of the storage media in current use.

The current most popular storage for images is the CD, in both read-only (CD-ROM) and writable forms (CD-R). The basic structure of a CD-ROM is shown in Figure 22.9. A spiral groove is moulded in the plastic, which on writable CDs is continuous. CD-ROMs have pits impressed on one side which encode the data which interrupts a laser beam which reads the data. The laser beam is reflected back by a shiny metallic surface (the land) to the sensor, whereas when a pit is encountered the beam is not reflected. The writable CD-Rs have a slightly different structure, also shown in Figure 22.9, in which the pits are recorded in a dye layer in the guiding grove where the recording beam converts the dye to products that block reflection. Above the dye layer is a reflective gold layer and above this is a layer of protective lacquer. CD-ROMS have a characteristic reflective appearance on both sides which enables them to be distinguished from CD-Rs which usually appear a shiny green, golden-green, or silvery green on the bottom and show a golden or silvery, less shiny coloration on the top (label) surface.

CDs are subject to deterioration, and a lifetime of around 100 years is a reasonable estimate based on moderate storage conditions, which is indicated in Figure 22.8. Like other non-electronic media, they may be affected by poor storage conditions, handling, scratches etc., although they are more tolerant to scratches than most other media. This is because the reading laser beam is focused on the pits (see Figure 22.9), some distance away from the surface of the disc where the scratches are located, and they are out of focus. Apart from the more obvious physical damage, CDs may deteriorate through oxidation of the reflective metallic layer, which in early CDs was aluminium. In more recent CDs aluminium alloys are used, which are more resistant to oxidation, and gold is used in writable CDs, which does not oxidize readily. Like hard copy media, slow chemical changes will cause degradation of the data over long periods of time, such as dark and light fading of dyes in CD-R discs and the oxidation of metallic reflective layers in CD-ROMs.

Protective lacquer

Metallic reflective layer

Polycarbonate substrate

Protective lacquer Gold reflective layer

Protective lacquer Gold reflective layer

Pregroove Pit

Pit in spiral groove Land

Direction of laser beam for reading

Pregroove Pit

Direction of laser beam for reading

Figure 22.9 (a) Cross-section of a read only CD-ROM and (b) of a writable CD-R

Other problems that can occur in CDs and must be guarded against are:

• Diffusion of solvents from labels.

• Peeling off labels which may cause localized delamination.

• Diffusion of solvents from felt tip pens for marking and identifying the discs.

• Writing on discs with a ball-point pen or pencil.

• Cleaning with solvents.

• Exposure to sunlight.

• Storage at temperatures greater than 25 °C and relative humidities greater than 50 per cent.

• Sudden and rapid changes in temperature and humidity.

• Exposure to dust and dirt.

• Handling the surface.

Predictions of the life expectancy of CDs come from accelerated ageing tests very similar to those previously described for photographic and related media and extrapolation to storage conditions of 25 °C at 40 per cent RH. Life expectancy is based on readability errors in the data stored on the disc. It may be determined as block error rates (BLER). The BLER is the number of errors detected in a 10 second period and can be expressed as a BLERmax50, where max refers to the maximum BLER found on reading the disc and 50 refers to the end-life of 50 errors. This is the electronic data equivalent of a 10 per cent fade value for a hard copy material for example. However, it has been pointed out that most discs are still readable with a BLERmax50 and that this test provides a pessimistic measure of life expectancy.

Like all materials that suffer chemical degradation over long periods of time, the life expectancy of CDs can be substantially prolonged by storage at a reduced temperature of 10 °C and a relative humidity within the range of 20-50 per cent. This slows down the rate of any chemical reactions that may be taking place. Discs that are in daily use are likely to last less long than those that are back-up archival copies kept under consistent and controlled conditions.

Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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