Image storage and archiving
INTRODUCTION
In this chapter we will consider general aspects of image storage and archival properties or life expectancy of different imaging media. Prior to digital photography the term archival was used to describe materials with a long life, which is now taken to mean a minimum of 100 years. Nowadays the term archival is less commonly used and materials are described in terms of their life expectancy instead. This is because the newer electronic media related to imaging do not have proven archival properties. All materials degrade over time and differing imaging media are subject to many causes of degradation, many of which are not well understood. Steps may be taken to ensure that their life expectancy is as long as possible. As imaging systems advance and new systems evolve, the initial prime criterion is to provide a system that works and fulfils the needs of the consumer. In these initial stages of providing new products their longevity is not necessarily a high priority. This was the situation at the start of the negativeepositive photographic process when W.H.F. Talbot experimented with various fixing methods to achieve a stable image and this approach continues to this day. However, as our knowledge advances, both manufacturers and users are more aware of the need for imaging media to have reasonably long life expectancies and much research is undertaken to achieve this. Today, this is becoming a useful strategy in marketing imaging products. A relatively recent example is in colour hard-copy output using inkjet or thermal dye-transfer systems in which the early products had very poor stability to light, as did colour photographic print materials before them. This problem has been researched thoroughly; more recent products have much improved light stability and predictions about their life expectancy are very optimistic.
Longevity in all imaging storage media depends on the stability of the medium, the storage conditions and handling. With digital technologies a number of other considerations concerning the life expectancy of the digital image information have to be taken into account in addition to degradations of the storage medium itself. Of particular concern is for how long the hardware and software reading and rendering of the image information will be available. With the rapid advances in both hardware and software, long-term preservation of digital information is only achieved by transferring from obsolete to newer systems. Migration is the periodic transfer of digital files from one hardware/software configuration to another. Ensuring image longevity presents various challenges. Some are common to both silver-based and digital media, whereas others are unique to one or the other. Storing photographic films, prints and digital storage media under controlled environmental conditions, handling them with care and migrating digital image files from old to newer media are all techniques which help to prolong the lifespan of photographs. The focus of this chapter will be on three main areas: (i) life expectancy and storage of traditional photographic media; (ii) life expectancy and storage of digital prints; and (iii) digital image storage and migration of digital information.
LIFE EXPECTANCY OF TRADITIONAL PHOTOGRAPHIC MEDIA
The longevity of silver-halide-based media depends on the photographic medium itself, the chemical processing and the consequent storing conditions. Black-and-white analogue materials have a number of advantages with respect to their life expectancy over any other media, including the fact that they are viewed with the unaided eye, there is no need for hardware or software to render the image data to viewable records, and history. We know that properly processed and stored black-and-white photographic records have a life expectancy of at least 150 years.
According to work published by the Image Permanence Institute (IPI) of the Rochester Institute of Technology (RIT) in the USA there are three categories of environmentally induced types of deterioration in photographic (and other image-related) media. Biological decay involves living organisms, such as mould and bacteria that damage films and prints. Mould, once present and if left untreated, eventually destroys all pictorial information. Mechanical decay is related to changes in the structure of the photographic image, such as its size and shape. Overabsorption of moisture from photographic media found in humid environments causes swelling; equally, lack of humidity and dryness in the atmosphere cause shrinking and cracks. Finally, chemical decay changes the chemistry of the photographic image. Incorrect processing – for example, leaving residual chemicals or final prints with an unfavourable pH value – causes fading of the colour dyes in colour materials.
Nine types of decay that are major threats to photographic and digital image collections are listed in Table 18.1. Some forms may affect one only media type; others may affect other types too. Proper environmental conditions minimize the risk of decay-related damage.
On the longevity of the photographic medium itself, generally fine-grain silver images are more susceptible to degradation than larger-grain images. Prints are more susceptible than negatives or slides. Fibre-based printing papers, especially silver-enriched premium weight types, provide better image stability than resin-coated material. Colour photographic media are more at risk than black-and-white. Furthermore, photographic materials have to be properly developed, fixed and washed to avoid later yellowing and fading on storage. In Chapter 13 attention was drawn to the importance of properly fixing and washing black-and-white films and prints to avoid fading and yellowing on storage. However, even if photographic materials are properly processed they may suffer from any of several forms of degradation in the long term. A summary of the types of degradation and their causes for both black-and-white and colour materials is given in Table 18.2.
Table 18.1 The 9 types of decay, published by Peter Adelstein of the Imaging Permanence Institute in the USA, that are a major threat to photographic and digital image collections
Table 18.2 Some causes of deterioration in photographic materials
EFFECT |
CAUSE |
Overall yellow stain |
Residual chemicals (thiosulphate, silverethiosulphate complexes) |
Image fading and yellowing |
As above |
Brown spots |
Localized retention of above |
Local or general damage |
Bacterial or fungal attack on gelatin, degradation of gelatin and paper fibres by acidic atmospheric gases |
Microspots (red/yellow) |
Oxidizing atmospheric gases |
Dye fading |
Dark reactions and/or light-induced reactions of image dyes |
All of the degradations listed in Table 18.2 can be avoided or at least minimized by eliminating their known causes as far as possible. A number of international standards exist for proper storage environments, display conditions and protective treatments of photographic media which, if followed, will prolong the life expectancy, as will following the recommended processing conditions. ISO 18909:2006 is an example, relating to dark storage and the light fading of colour prints.
Processing conditions
Development conditions as recommended by the manufacturer should be followed. Acidic stop baths can influence the keeping properties of silver images. Excess acetic acid in the stop bath may cause some papers to become brittle on drying and subsequent storage. Also, with carbonate-buffered developers excess acetic acid may cause bubbles of carbon dioxide gas to be formed in the paper fibres of prints. Thiosulphate and silverethiosulphate complexes can become trapped in these bubbles and may not be readily washed out. This in turn can cause localized spots of silver sulphide to be formed on storage. The minimum permissible levels of thiosulphate ions for some photographic materials are specified in standards. For maximum stability of silver images it is usually recommended that all thiosulphate is removed by thorough washing or with the aid of a hypo eliminator when a short wash time is used. However, it has been suggested that for prolonged life expectancy it is better that the material contains a very small amount of thiosulphate, e.g. 0.03 g m−2 for microfilms. Archival keeping properties are greatly improved by the use of protective toning treatment (see later in this chapter), which is why it may be better to include a small residue of thiosulphate in the processed material to form a protective layer of silver sulphide on the silver grains.
Incorrect processing of colour materials, which may leave residual chemicals in the layers or provide a processed record with an unfavourable pH value, can lead to the destruction or fading of image dyes, known as dark fading as opposed to light fading.
Dark fading
The fading of dyes has two causes, categorized as dark fading and light fading. Different mechanisms for the destruction of the dyes are involved for these two types of fading. Dark fading involves chemical reactions that depend upon on the structure of the dye. These aspects will be considered separately. As with silver-based materials, the dark fading of image dyes is influenced by temperature, relative humidity (RH) and the chemistry of the environment. Common atmospheric gases that can cause dark fading of dyes are oxides of sulphur and of nitrogen, and ozone, but these are insignificant when compared with other causes. As with black-and-white images, dye images are affected by errors in processing. Dark fading is accelerated by the presence of residual thiosulphate ions. Inefficient washing of colour materials not only leads to incomplete removal of thiosulphate and other harmful chemicals, but also can leave the material with a low pH value, which has also been shown to accelerate dark fading. If correct processing procedures are adopted, the reduction in the keeping properties of materials associated with these causes is avoidable.
The main reactions responsible for dark fading in chromogenic materials are those involving hydrolysis and oxidationereduction reactions, which cause the dye to be converted into a colourless form, or into forms that are different in colour from the original dye. Hydrolysis is decomposition by water and is accelerated by extremes in pH value as well as humidity and temperature. Yellow dyes are particularly susceptible to this form of destruction, whereas cyan dyes are susceptible to reduction to a colourless form by residual thiosulphate ions, ferrous ions or other reducing agents present in the material or the environment. An understanding of the mechanisms involved has led to the present generation of chromogenic materials, which contain colour couplers that form image dyes with greater resistance to dark fading than earlier materials. The predictive tests for dark fading involve accelerated ageing tests carried out under a fixed RH (40%) at elevated temperatures for long periods of time.
This procedure is based on the classical Arrhenius equation (see ISO 18924 for more details), which is given below in a simplified form:
where k is a measure of the rate of dye fading and T is the thermodynamic temperature in kelvin. If a plot of loge k against 1/T for a number of temperatures gives a straight line, then it is possible to extrapolate the straight line to other temperatures to predict the rate of fading. Also, it has been shown for dark fading that:
where t10% is the time required to fade 10% of the dye density from an original density of 1.0. Hence, from the simplified form of the Arrhenius equation:
In practice, 1/T may be plotted against log10 t10%, as shown in the lower graph in Figure 18.1.
The application of this procedure for the dark fading of a magenta dye is shown in Figure 18.1 and its use for predicting dark fading, at a temperature at which fading would take too long to measure, is given below. Dark fading figures for temperatures ranging from 24 to 93°Cat 40% RH are shown as the full curves in the top graph of green density DG against log10 time (in days). A horizontal line is drawn at the 10% fade level criterion. Where this line crosses the actual fading curves, a vertical line is drawn to the lower graph where the temperature (1/T in kelvin) is plotted against the log of the fading time (log10 t10%). The linear relationship obtained is shown by the line joining the full circles. Extrapolation of this line to the required temperature of 24°C is shown as the broken line. Translation of this point vertically back to the top graph horizontal line labelled 10% gives the predicted value for the fading of the dye to the 10% level, from which it can be seen that this dye would fade 10% in antilog 4.1 days, i.e. 34 years. This procedure could be repeated for other fading levels and by this means a complete fading curve would be predicted.
All dyes fade and dye images cannot be considered to be of archival quality. However, manufacturers have paid considerable attention to minimizing dye fading, and modern colour photographic materials are capable of lasting for many decades if properly stored and processed. Manufacturers claim a 100-year dark storage capability for their colour print materials. However, the only really effective method for achieving extended life expectancy of colour materials is to convert them to black-and-white records by making separation positives or negatives and to process and store these under the conditions discussed in the earlier sections. These black-and-white separations can then be reconstituted as full colour images as required, using appropriate photographic or digital imaging techniques.
Figure 18.1 Arrhenius plot for dark fading of magenta dye.
Some dyes are inherently more stable than others. For example, those used in the photographic dye-transfer process, those present in silveredyeebleach materials and metal-based dyes are generally more resistant to certain types of fading than those formed in chromogenically developed materials. For example, one of the most dark-fade-resistant colour photographic materials is Kodak’s Kodachrome film.† Large studies on old photographic collections have indicated Kodachrome to be the only transparency film that remains totally free from yellow staining formation during long-term safe storage. The film’s stability is largely due to its processing, which is very complex when compared to E6 processing used for all chromogenic slides. Unless stated otherwise, considerations of dye fading which follow have been applied to chromogenic materials.
A problem with colour materials is that image dyes fade at different rates; this results in a change in colour balance (see example of dark fading in Figure 18.4). Also, there is no universally agreed criterion by which fading is judged or of the degree of fading that can be tolerated. Originally manufacturers adopted a criterion of a reduction in density of 10% (i.e. 0.1 from an original density of 1.0), though it has been argued that greater changes than this may be acceptable to the average observer. This quantitative measure of dye fading, rather than one concerned with quality, has been termed a just-noticeable difference. It provides a useful criterion for the comparison between different products when treated in the same way for laboratory evaluations of dye stability. However, more recent standards use a 30% density loss from a density of 1.0 as the criterion for predictive aspects of dye fading. This 30% loss in density is regarded as the density change that becomes noticeable to the consumer. There may well be some confusion as to what represents a just-noticeable difference or a minimum acceptable difference. This is a controversial area of colour appearance which is dependent on the colour content of real scenes.
Predictive tests for dye fading are being undertaken by all manufacturers, but at present there is no guarantee that these tests will predict precisely what will happen under actual conditions of storage. A further difficulty is that many claims use different test conditions and criteria for acceptability of fading. This means that published data must be interpreted and compared with care, especially if different test conditions have been used. This procedure provides a predictive technique for manufacturers and researchers, and typical fading results are shown in Figure 18.2.
Figure 18.2 Dark fading of a cyan image dye at various temperatures.
From this it can be seen that in order to reduce dye fading to a minimum a low storage temperature should be used. If refrigerated storage conditions are used to minimize dark fading, the materials will have to be enclosed in suitable sealed containers and, because the RH increases as the temperature is lowered, the photographic materials must be preconditioned at low RH (25–30% at 20°C) before they are sealed in their containers. The practical conditions for storage given earlier also apply to colour materials and are relatively easy to achieve without the use of costly refrigeration facilities. Colour materials such as slides can be duplicated every few years and good colour fidelity maintained for generations; colour negatives can be reprinted.
Light fading
Light fading of the photographic dyes is the fading that results from exposure to light and ultraviolet (UV) radiation. Predictions from light fading tests carried out in the laboratory are less easily made than those from dark fading, which use the more reliable Arrhenius method. Light fading of dyes depends on the intensity, duration and spectral energy distribution of the radiation to which they are subjected. These vary considerably with the display conditions under which the materials are viewed. The fading of dyes is a photochemical reaction in which oxygen is involved, and leads to destruction of the dye. This photochemical fading is increased by high humidity.
Two laboratory methods are used for investigating light stability, relating to the viewing and display conditions for the particular material. Slides can be subjected to hundreds of projections in a slide projector and decreases in density for neutral and colour patches measured and plotted against the number of projections. Results for a reversal film neutral patch of original density 1.0 are shown in Figure 18.3. Most reversal films follow the trends shown in this figure, but there are variations between different products, and current films show less steep curves and better light stability.
Colour print materials are usually exposed to an intense xenon source at 5.4 klux, 40–50% RH at 22–23°C and the time taken for a fixed loss in density (10%, 15% or 30% as mentioned earlier under dark fading) is used as a measure of the light stability which is obtained from graphs of density against time. Extrapolations are made from 5.4 klux to 120 lux, which is regarded as a typical light level in the home environment where the pictures may be displayed. Chromogenic colour photographic papers have made substantial improvements in their light stability over the last 50 years. Current materials show considerable resistance to light fading. Colour photographic prints are now claimed to have a useful life of 150 years in typical home-display conditions of around 120 lux at 50% RH and 23°C. Improvements are being made by all manufacturers of chromogenic print materials; modern materials are almost as stable to light as are dye-transfer prints. The latter have been regarded as a standard with which other materials are compared. Silveredyeebleach materials are particularly resistant to light fading.
Figure 18.3 Light fading of a neutral patch imaged on a colour reversal film.
As mentioned earlier, the instability of dye images is due to a combination of light absorption and dark storage effects, although the latter are often ignored when accelerated light fading is carried out. There exist many additional complicating factors which are involved in light fading. Examples of these include staining, the influence of UV radiation and UV absorbers, the presence of residual chemicals and colour couplers in chromogenic materials, anti-oxidants and stabilizers which may be included in modern materials, the influence of the substrate and environmental factors, such as temperature, humidity, pH and the presence of oxygen. Because of the possibility of reciprocity effects, all accelerated light-fading studies, which use high intensity and relatively short duration exposures, are subject to the criticism that the effects may not relate directly to longer exposures at lower intensities. It is possible that the conditions used for accelerated light fading may induce reactions that are not found for ambient conditions under which the materials are displayed. There is, however, little alternative if fading studies are to be carried out within reasonable periods of time and most investigations involve the use of high-intensity sources and an extrapolation to other conditions when predictions are being made.
An additional problem is in the choice of assumed typical ambient display conditions when predictive calculations are made for fading by a specified amount. One manufacturer has used a value of 500 lux for 12 hours per day in such calculations. This was based on the assumption that in common domestic situations sunlight varies between 1000 lux during the day, decreasing to 300 lux in the evening, and an average value of 500 lux for 12 hours per day appears to be a reasonable estimate. Whilst others have adopted 450 lux for the same time period at a temperature of 24°C and 60% RH, yet others use 120 lux, as mentioned earlier. This further illustrates the difficulties and the caution that must be exercised in making comparisons between claims for the light stability of imaging materials.
Modern chromogenic printing papers have evolved over a period of at least 50 years and much research has been carried out to improve their stability. Particular attention has been concentrated on improvements in the intrinsic stability to light of magenta dyes, which are regarded as having the most noticeable fading and were the least stable of the image dyes. In addition, UV absorbers and other minor chemical components are included in modern materials to reduce their rates of fading. These are mature products, whereas materials for hard-copy output from electronic systems have evolved over a period of less than a decade and fade more rapidly than colour photographic materials.
Storage conditions
Most of the harmful effects summarized in Tables 18.1 and 18.2 are aggravated by high temperatures and high RH. In fact, subjecting processed photographic materials to such conditions is used as a test procedure for accelerated ageing techniques to find optimum processing and storage conditions and for research into the causes of deterioration. One International Standard specifies as an image stability test incubating the material for 30 days at 60 ± 2°C and an RH of 70 2%, and states that: ‘The film image shall show no degradation that would impair the film for its intended use.’ When storing photographic materials the temperature should not exceed 21°C and the RH should be kept within the range of 30–50%. For archival storage, a lower temperature and RH is recommended (10–16°C and RH of 30–45%). This represents a practical compromise for air-conditioning. Also, the materials should be protected from harmful atmospheric gases (discussed in the next section). More recent recommendations are to keep photographic materials in sealed polypropylene bags containing a visible humidity indicator, in a controlled humidity environment at temperatures of – 20°C. This provides the highest standard of conservation whilst achieving maximum chemical stability without harmful physical change. The main limitation is that rigorous warm-up procedures have to be adopted when removing material from cold storage to avoid condensation and physical changes due to thermal shock. Figure 18.5 shows the McCormick-Goodhart recommendations for storage of photographic materials in terms of the interrelationships between temperature and RH.
Figure 18.4 The pinkish cast in (b) is a simulation of the cyan dye fading faster than the magenta and yellow dyes in old photographic prints – pre 1974 – stored in the dark.
Figure 18.5 McCormick-Goodhart recommendations for storage of photographic materials.
Adapted from paper presented at 14th Annual Archive and Records Administration Preservation Conference: Alternative Archival Facilities, Washington, DC, March 1999; reproduced by permission of M.H. McCormick-Goodhart
Photographic materials should be protected from the light. Recent tests have shown that subjecting silver materials to high-intensity simulated daylight (5.4 klux or 5400 lumens per square metre) accelerates the yellowing of images in processed materials containing small amounts of thiosulphate ion.
The materials in which photographic records are stored, as well as the processing conditions and the storage environment, are also of the utmost importance. Suitable materials for the storage of processed photographic films, plates and papers are also specified in Standards. Generally, wood and wood products (plywood, chipboard, hard-board, etc.), formaldehyde-based plastics, polyvinyl and acrylic plastic containers should not be used. These materials may contain residual solvents, catalysts, plasticizers, etc. which are known to have harmful effects on photographic materials, as can many adhesives, inks and marking pens. Preferred storage containers are those made from anodized aluminium or stainless steel, or cardboard boxes made from acid-free high-alphacellulose fibres that are lignin free and sulphur free. The latter also applies to mounting boards for prints, envelopes for negatives, etc. Fortunately there are a number of specialist suppliers of archival storage materials for processed photographic records; these are able to supply ‘archival’ polypropylene or polythene sleeves and envelopes for the storage of negatives and slides, along with files and boxes of appropriate archival materials.
Atmospheric gases
There are many pollutants present in the environment which can have deleterious effects on photographic materials. These include oxidizing gases such as hydrogen peroxide, ozone, oxides of nitrogen, peroxides and peroxy radicals emitted by car exhausts, paints, plastics, acid rain and sulphur compounds. The general effects of oxidizing gases are to induce yellowing and fading of the image at the edges of the print or negative where the atmosphere has been able to penetrate most readily. This staining may be dichroic, appearing grey by reflected light and yellow by transmitted light. Low concentrations of oxidizing gases such as hydrogen peroxide can produce microspots. In prints these appear as yellow, orange or red spots depending on the size of the silver particles, and if clustered near the surface may appear as a silver mirror. The mechanism by which microspot formation occurs involves an initial oxidation by the gas, which converts silver metal to mobile silver ions. The silver ions migrate away from the silver filaments of the image and become reduced to metallic silver by the action of light, or are converted to silver sulphide by hydrogen sulphide present in the environment. This process forms microspots of approximately 60 mm or larger in diameter, comprising a concentric ring structure of particles of 5–10 nm to 0.5 μm in diameter. In laboratory experiments, using accelerated ageing techniques at 50°C and 80% RH, it has been shown that microspot formation occurs with a concentration of hydrogen peroxide of 500 ppm.
Figure 18.6 Effect of toning on image stability to hydrogen peroxide. (a) Untoned image after 3 hours, 5% hydrogen peroxide. (b) Original image, no treatment. (c) Toned image after 3 hours, 5% hydrogen peroxide.
Protection against attack by oxidizing gases can be given by selenium toning. Figure 18.6 shows the degree of protection that is possible with this toning treatment when the material is subjected to an extreme test using 5% hydrogen peroxide. From Figure 18.6 it can be seen that toning (curve c) leads to very little change in the characteristic curve after attack by hydrogen peroxide, whereas an untoned image (curve a) shows a considerable change. Acidic atmospheric gases, often present in the environment, can degrade gelatin and paper base materials and some form of protection from these in storage is necessary for archival permanence.
Toning
Certain toning processes are now known to increase the life expectancy of silver images. Originally toning was carried out to change the colour of a photographic image by means of chemical solutions. Toning is mainly applied to prints but may also be used to confer additional stability on films or plates, and most manufacturers offer proprietary toning solutions to enhance the life expectancy of their silver photographic products. The principle of obtaining enhanced stability of silver images through toning is to convert or coat the silver image into a silver compound that it is more inert, less finely divided and less soluble than the original silver image particles.
The silver image may be converted into silver sulphide (or silver selenide). Silver sulphide and silver selenide have brown and purple colours, much warmer than that of the usual silver image, so these processes are often referred to as sepia toning. Silver selenide toning provides a means of obtaining very stable images and is less expensive than gold toning, which provides the most stable images. However, selenium compounds are toxic, as are sulphides, and should be handled with caution. Indeed, all toning solutions should be handled carefully and discarded with caution. Sulphide toning is probably the most widely used form of toning, and properly carried out yields images of great permanence. Also, protective gold solutions have been proposed for the archival permanence of prints. They involve immersing the thoroughly washed print material in a mixture of dilute gold chloride and sodium thiocyanate (Kodak Gold Protective Solutions GP1 and GP2). They provide print protection while changing the image tones only slightly.
LIFE EXPECTANCY OF DIGITAL PRINTS
Preservation and conservation of digital prints is one of the major contemporary concerns of the imaging community, especially because longevity of digital image files suffers from rapid changes of technologies and media obsolescence. A number of digital image preservation specialists suggest that the first and simplest way the most valuable digital images can be preserved is to make hardcopy prints and store them under optimum conditions. Technologies and media are relatively new while their evolution has been extremely rapid. Central to this evolution has been image quality and print permanence. The permanence of digital prints varies widely with printing technology, printing material and characteristics of the colorants used to produce them. It also varies with display and storage environments that should be considered separately when the longevity of the media is in question. Digital colour prints are produced in materials that differ in composition to conventional photographic materials and in their response to environmental factors that damage them. Unfortunately, at the present time there are no ANSI or ISO standards defining the longevity of digital prints. (The ISO 42 Technical Committee on Photographic Standards is – at the time of writing – working toward the development of such standards to support new technologies.) Prognostics on the life expectancy of digital media are made exclusively by using accelerated fading techniques, of which the conditions are not universally agreed. As with analogue media, it is often disputed that in accelerating ageing procedures manufacturers do not follow the same methodologies, thus the predicted fading rates are not comparable. Also, the majority of published life-expectancy predictions are based only on the results of light-exposure testing, which accounts for only one of the possible causes of print deterioration. Until universal guidelines are published, storage recommendations should be similar to those specified for conventional colour prints.
Printing technologies and media
Inkjet print technologies are the most commonly used technologies in the production of digital photographic quality prints, largely due to the fact that they allow a greater range of dyes and pigments than any other printing process. The structure of the inkjet print partially governs its future integrity. Inks may be made from dyes, similar to those used in traditional photographic prints, or pigments, which are also used in toner. Pigment-based inks provide better stability because pigments are less sensitive to water, humidity and high temperatures than dyes. From a permanence point of view pigment-based inks are better than dye-based inks in every respect. They are expected to typically achieve more than 100 years on display or 250 years in safe storage conditions; however, this depends largely on the printing surface. For many years the shortcomings of pigment-based media were related to image quality issues, including reduced colour gamut, differential gloss problems on glossy photo-papers, metamerism problems and others. Today, the image quality of pigment-based inks/media is rapidly approaching, and in some cases exceeding, that of dye-based inks. Dye-based inks, on the other hand, contain molecular colorants that penetrate the surface. Early generations of such inks had very poor display life (often less than 10 years) but, as printing technologies and dye chemistry advance rapidly, print display life has increased tenfold, while still preserving the rich colours inherent to dyes.
Inkjet prints are made on either uncoated or coated paper. Uncoated paper absorbs the ink better, but coated paper produces photographic quality prints (i.e. coating high-quality paper results in brighter, more saturated color and greater image resolution – see Chapter 16). Coated papers have similar supporting bases to traditional colour printing papers. Acid-free, lignin-free and buffered paper bases are generally recommended for longevity. Two main types are available for inkjet printing: swellable and porous coating papers. High-quality swellable papers encapsulate the inkjet dyes. They have a protective top layer, a layer that keeps the ink droplets in place and a layer that absorbs additional ink components. The paper base is placed between two polyethylene layers and backed by an anti-curl coating and an antistatic layer. Cheaper swellable papers do not always contain all these layers. Swellable polymer coated papers are better protected from degradation caused by atmospheric pollutants than porous papers with no protective polymer. The lack of a protective layer in porous papers makes the exposed colorants susceptible to pollutants, such as ozone, oxides of sulphur and nitrogen. Prints on porous papers, on the other hand, are more resistant to humidity and moisture, especially when used with pigment-based inks. Accelerated ageing tests, conducted by Wilhelm Imaging Research Inc., recently showed that dark storage stability of inkjet prints is limited by the thermal stability (yellowing) of the modern coated papers rather than the stability of the inks.
The use of fluorescent brighteners (also called UV brighteners), which are added to many inkjet papers (and to nearly all plain papers) to make them appear ‘whiter’ than they really are, is an issue regarding inkjet paper stability. Fluorescent brighteners lose activity, mainly when exposed to light and UV radiation, but also when subjected to high temperatures used in accelerated ageing techniques. With loss of brightness, the papers appear to have a slight yellow cast. It is therefore recommended that for long-term permanence papers with brighteners should be avoided.
Generally, more and more inkjet paper manufactures claim ‘archival’ properties of their papers, typically up to 200+ years. Yet, since there are no universal standard measures for inkjet print longevity, longevity claims vary with manufacturer and product. Independent research in inkjet print permanence has often shown that inkjet prints have considerably higher longevity when they are printed on the printer manufacturer’s branded print media (i.e. papers and inks).
Little information is available on the life expectancy of digital prints by other than inkjet printing technologies. In thermal transfer (referred to also as dye diffusion or dye sublimination – see Chapter 16) print heat is used to transfer the dye from a donor on to the printing surface. Such prints often have a clear protective layer that prevents the image from smearing when it is rubbed. The media present problems with respect to accelerated ageing techniques (see below). In electrophotography, toner is transferred and fused in the paper base. The paper is usually uncoated and the images are reasonably stable, because toner is composed of pigment particles that are fused to the paper with a durable polymer binder material. This technology is not often used for photographic quality printing. Digital silver halide and silveredyeebleach colour prints consist of layers with dye-forming couplers containing layers further comprising silver halide emulsions sensitive to visible light. In some cases the silver halide processes are combined with thermal development and dye transfer. The permanence characteristics of a digital silver halide colour print are the same as those of traditional photographic prints. They may last from 50 to more than 100 years in appropriate storage. The deterioration varies between manufacturer’s products, the level of illumination and UV radiation to which the print has been exposed.
Permanence factors and test for digital prints
Exposure to light when displaying prints, high temperatures and humidity, atmospheric pollutants and mould, are common factors affecting image permanence in both conventional and digital prints, although not to the same degree. Due to lack of international standards on testing print permanence, many manufacturers consider Wilhelm Imaging Research Inc. as a reliable independent standard for digital print permanence predictions. For most types of digital prints light fading is the dominant factor of their permanence. Light-fading predictions from Wilhelm calculations are based upon relatively high lighting indoor conditions of 450 lux averaged for 12 hours, which surpass the conditions that some printing manufacturers use in their tests – for example, the published Kodak method that assumes 120 lux for 12 hours per day. Tests are conducted until a just-noticeable amount of fading has occurred, while 17 failure criteria are tracked. Traditionally, methods for evaluating light fading take into account fading in cyan, magenta and yellow patches, as well as fading and colour imbalances in the neutral patches at a single density of 1.0 (and for Wilhelm Imaging at 0.6 too). These methods do not address directly the imbalance on all types of colours, including skin tones, and this is why recently colorimetry rather than densitometry-based methods are proposed.
Slow-fade tests are also carried out in darker environments. Accelerated tests for dark fading require long-term tests at elevated temperatures, such as 50–70°C. The results are extrapolated for common room-temperature predictions. Each type of print media has its own dark storage stability characteristics. Generally, inkjet prints do not suffer too much from dark fading, since inkjet dyes and pigments are very stable and typically can last 100+ years at room temperature, provided that they are laid on high-quality papers and kept relatively safe from harmful atmospheric pollutants. Thermal transfer prints cannot withstand elevated temperatures required by accelerated tests and therefore predictions on their long-term longevity are not made.
Digital prints stored in the dark suffer slow deterioration. This manifests as yellowing of the paper, image fading, changes in colour balance, cracking and delamination of the image layer. Rates of deterioration are influenced, as with photographic prints, by the levels of temperature and RH. High RH causes colorants to migrate, causing colour ‘bleeding’ and lack of edge sharpness, while it promotes microbial and fungal growth. Tests involve the exposure of prints to elevated humidity levels (e.g. 80%) for weeks and evaluation of the fastness of colour prints as a result of RH based upon changes in the colours. Electrophotographic and pigment-based inkjet prints are generally less sensitive to high temperature and humidity than traditional photographic images and dye-based inkjet prints, because they are made with pigments rather than dyes. The behaviour of inkjet materials varies widely under these conditions, depending upon whether the images are dyes or pigments and whether the paper is uncoated, swellable or porous. In humid conditions, inkjet images composed of dyes that have been printed on swellable coated paper can appear unsharp because of dye spreading. Water damage, cause by floods, etc., is a common damaging factor that affects dye-based inkjet prints more than any other print media. Rubbing the surface while it is wet worsens the condition of the print. Pigment-based prints are relatively water resistant, even when printed on plain paper.
Airborne gases and pollutants, primarily ozone, cause inkjet prints to fade. Accelerated tests involve exposure to a high level (<1 ppm)‡ of ozone until fading is noticed. The years of ozone resistance are then calculated upon indoor average data – e.g. 40 ppm-hours of ozone is equivalent to one year under usual indoor conditions. Generally, inkjet prints made with swellable papers or with pigmented inks are resistant to air pollutants and ozone for several decades without any protection. On the contrary, dye-based inkjet prints on porous photographic papers, for example ‘instant dry’ papers, can suffer from noticeable air fade within months of exposure. Thermal transfer print resistance to dark fading is less specific; it might range from one to several decades. Generally, ozone-related damage is minimized in digital prints when they are framed behind glass, laminated or stored in albums. Table 18.3 presents a summary of the life expectancy of different print technologies under display and storage that is based on data from Wilhelm Imaging Research and HP Image Permanence Laboratory. Figure 18.7 illustrates dye fading on inkjet prints due to exposure to high and to atmospheric pollutants.
Figure 18.7 (a) An inkjet print in its original state. (b) Simulation of the effects of light fading on inkjet prints: the magenta dye fades faster than the cyan and yellow dyes when exposed to light, resulting in a green cast on the print. (c) Simulation of the effects of atmospheric pollutants: fading of cyan and magenta dyes result in a yellow cast in the print.
Original Image © Pygmalion Kalimeris
DIGITAL IMAGE STORAGE AND LONGEVITY
At the start of this chapter we saw that electronic media have, in addition to potential physical and chemical degradation, the problem of obsolescence. Unfortunately, even if the best media and storage conditions are ensured, the success in securing the longevity of digital images is only partial, because it is impossible to rely upon the hardware and software used to store the digital images being available in the future. This fact has been realized for many years but the solution is neither simple nor obvious. One suggestion is that data should be transferred from the old storage media to the new media as it becomes available. This process is known as data migration. It is suggested that permanence of digital storage should be considered as a measure of the renewal period. There is no degradation in the migration process, since digital information, unlike any other form of information, has the advantage of duplication without loss. Depending on the amount of image data, migration might be very challenging. It is time-consuming and requires whole systems for data management. However, it is very unlikely that existing materials, formats and compression techniques are reliable for more than a couple of decades. As shown in Figure 18.8, digital media used as recently as 20 years ago are already incompatible with most of today’s systems. This poses problems for artists who care about the longevity of their work, archivists who wish to preserve visual heritage and individuals who want to pass their photographs on to future generations. The migration of digital data from one medium and format to a newer one is therefore an essential requirement despite the difficulties.
Figure 18.8 Timeline illustrating the developments in common removable digital storage media.
Adapted from Byers (2003)
Modern storage media have two life expectancies: one refers to the physical and chemical lifetime and the other to the expected time of obsolescence. Rothenberg (1999) has summarized the problem:
There is as yet no viable long-term strategy to ensure that digital information will be readable in the future. Digital documents are vulnerable to loss via the decay and obsolescence of the media on which they are stored, and they become inaccessible and unreadable when the software needed to interpret them, or the hardware on which that software runs, becomes obsolete and is lost.
Figure 18.9 gives an indication of these times for some commonly used storage media and further reinforces the view that data migration is essential. The figure also indicates that the migration must be carried out within the life span of the medium’s chemical/physical life expectancy. Storage media have varying suitability according to the storage capacity required and preservation or access needed.
Magnetic storage
Magnetic storage media use magnetically coated surfaces to store information. They are rewritable media and information is accessed – read and written – by magnetic heads. The most common magnetic image storage devices are hard disks (fixed or portable) and magnetic tapes (commonly in cartridges and cassettes). On a hard disk, digital information is accessed randomly and thus very rapidly; in magnetic tapes it is accessed sequentially. Although the reliability of magnetic media has improved substantially in recent years, their integrity has varied considerably from manufacturer to manufacturer. The very rapid increases in disk space and data access speeds for hard disks make them very suitable as ‘working’ storage media. Magnetic tapes, in contrast, are cheap, and have large data capacity (typically 20–40 GB) but a short life span. This is why they are recommended for keeping additional copies (back-ups) in image archiving (where rapid and frequent access is not required), provided that tapes are replaced annually.
Figure 18.9 Life expectancies of common digital storage media.
Based on data from Rothenberg (1999)
The weakness of magnetic media is in the way they read and write information; thus, longevity is not one of their advantages. They suffer degradation from high-intensity magnetic fields. Hard disks require a certain range of air pressure to operate properly and very high levels of humidity can cause damage to the heads and corrosion. Normal use can eventually lead to failure of these rather fragile devices, so backing up images frequently is imperative. Magnetic tapes should be kept in cool conditions, good-quality air and at maximum 50% RH for best preservation. Binder degradation can cause uneven tape transport and layer separation. Tapes need to be handled with care because they are thin and fragile.
Optical disc storage
The most popular storage for digital images is optical disc storage, including types such as CD-ROM, CD-R, CD-RW, DVD, DVD-R and DVD-RW. Optical disc drives use laser diodes to illuminate the information engraved as pits on the disc’s reflective layer (i.e. the land). In the case of a read-only CD-ROM the laser beam is reflected back from the reflective metallic surface (aluminum or gold), which is very close to the top of the disc, to a sensor that ‘reads’ the information; when a pit is encountered the beam is not reflected back. Fast random access is the result of the continuous sequence of sectors on the spiral layout of the discs. The basic structures of a CD-ROM, a CD-R and a DVD-R are shown in Figure 18.10. The writable CD-R has a slightly different structure than the CD-ROM, as shown in Figure 18.10, in which pits are recorded in a dye layer in the guiding groove where the recording beam converts the dye to products that block reflection. Above the dye layer is a reflective metallic area and above that is a layer of protective lacquer. The types of data (i.e. read only, recordable or rewritable) and recording layers depend on the type of disc. Table 18.4 shows the relationship between the data types stored on various optical media, the metal layers and the disc type.
CD-ROM and DVD media are read-only storage media, CD-R, DVD-R and DVD+R are recordable once, and CDRW, DVD-RW and DVD+RW can be used for re-recording digital information. They are removable media, which can be duplicated and handled quite easily. CD-Rs are preferred for image archiving because of their write-once capability, which provides high security from data erasure or modification.
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 18.9. 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 18.10), 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. Similarly to 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.
Other problems that can occur in CDs and must be guarded against are:
• Diffusion of solvents from labels
• Peeling off of 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 RHs greater than 50%
• 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 described previously for photographic and related media and extrapolation to storage conditions of 25°C at 40% RH. Life expectancy is based on readability errors in the data stored on the disc. It may be determined as block error rates (BLERs). The BLER is the number of errors detected in a 10-second period and can be expressed as 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% 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.
Figure 18.10 Cross-sections of: (a) a CD-ROM; (b) a CD-R; (c) a DVD-R.
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 an RH within the range of 20–50%. 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.
The use of the ISO 9660 for recording on CD media is recommended, because it ensures access of the stored data from all current computer platforms and operating systems. The ISO 9660 is an international standard published in 1988, which defines a method of organizing computer files for CD-ROM media. An extension to ISO 9660, the Joliet system format, allows longer file names and non-ASCII character sets. DVDs may also use the ISO 9660 file system, although the Universal Disk Format (UDF), based on the ISO 13346:1995 standard, is more appropriate on DVDs. It has better support for the larger storage media and is more suitable to the needs of modern operating systems. CD and DVD writers are common and inexpensive nowadays. CD-type discs can store over 600 MB, a capacity which is rather limited for contemporary image sizes but they cost very little. DVDs are more recent optical storage media with capacity to store 4.7–18.0 GB.
Other digital image archiving issues
Other image storage media include flash cards and solid-state disks (SSDs) that have very fast data access and are used mostly as temporary storage devices in digital cameras. The life expectancy of such media is difficult to obtain and has not being researched in any depth. Otherwise, remote storage – where digital image files are kept in large servers and their longevity is the responsibility of the provider of the service – is becoming more and more popular, especially from institutions that hold large amounts of digital information. One example of such a service is the Digital Repository Service (DRS) of the Harvard University Library in the USA, which is a preservation repository holding millions of digital objects under managed storage and intended for highly ‘curated’ digital assets.
It is best for the longevity of digital image data to store one copy of each image in a form as close as possible to the original capture. This enables the user to always be able to refer back to a ‘master’ copy, which is what the capturing device can produce best with minimum processing (i.e. using the native RGB space of the device and minimum image processing – see Chapters 14 and 23). In such a case the image might not be optimized for a specific output. Archiving only an optimized copy often produces a loss in quality, such as compression of the native colour gamut, but has the advantage of a ready-to-use image. If only one optimized version of the image file is saved, it is recommended that one of the standard RGB colour spaces be chosen for colour encoding – e.g. Adobe RGB 1998 for print and sRGB for display media.
The requirements of a file format for archiving are that it is an open standard, non-proprietary file format and preferably no compression is used – although lossless compression may be acceptable. Uncompressed Tagged Image File Format (TIFF) has been the most popular choice for archiving images. In 2004 Adobe proposed the Digital Negative (DNG) format as a non-proprietary file format for storing camera RAW files that can be used by a wide range of hardware and software vendors. Like most RAW file formats, the DNG file holds the RAW data in a TIFF-based format. DNG is quickly becoming a choice for image archiving purposes. The Portable Network Graphics (PNG) file format is an open-source image format working toward official standardization, and is considered as a possible replacement for TIFF. Otherwise, JPEG 2000, which can be lossy or lossless depending on the compression algorithm (see Chapter 29), has recently been recommended for archiving images because of several advantages, such as the storage of several resolution levels, the allowance for image metadata to be built into the image file and bit-depth support to 48 bits. Chapter 17 provides extensive information on file formats.
Digital image files intended for preservation should be accompanied by image metadata. Preservation metadata is information supporting the digital preservation process. A number of metadata categories can be saved, including descriptive, administrative (including copyright and permissions), technical (including information on the tone and colour reproduction, resolution, etc. of the capturing device) and structural. Technical metadata are probably the most important in supporting preservation. The 2006 ANSI/NISO Z39.87 Standard on Technical Metadata for Still Images lays out a set of metadata elements to facilitate interoperability among systems, services and software, as well as to support continuing access and long-term management of digital image collections.
BIBLIOGRAPHY
ANSI/NISO Z39.87, 2006. Data Dictionary – Technical Metadata for Digital Still Images.
Byers, F., 2003. Care and Handling of CDs and DVDs: A Guide for Librarians and Archivists. Council on Library and Information Resources and National Institute of Standards and Technology, Washington, DC, USA.
Image Permanence Institute, 2004. A Consumer Guide to Traditional and Digital Print Stability. Image Permanence Institute, Rochester Institute of Technology, Rochester, NY, USA.
ISO/IEC 13346-1:1995, 1995. Information Technology – Volume and File Structure of Write-Once and Rewritable Media using Non-Sequential Recording for Information Interchange, Part 1: General.
ISO 9660:1988, 1998. Information Processing – Volume and File Structure of CD-ROM for Information Interchange.
ISO 18924:2000, 2000. Imaging Materials – Test Method for Arrhenius-Type Predictions.
ISO 18909:2006, 2006. Photography – Processed Photographic Colour Films and Paper Prints – Methods for Measuring Image Stability.
Jacobson, R.E., Attridge, G.G., McDermott, D., Mitchell, C., Tong, B., 1996. Light stability of colour hardcopy materials. Journal of Photographic Science 44, 27–30.
Jacobson, R.E., Ray, S.F., Attridge, G.G., Axford, N.R., 2000. The Manual of Photography, ninth ed. Focal Press, Oxford, UK.
McCormick-Goodhart, M.H., 1996. The allowable temperature and relative humidity range for the safe use and storage of photographic materials. Journal of the Society of Archivists 17 (1).
Miller, N., 2005. How long will your digital prints will last? Shutterbug March.
Rieger, O., 2008. Preservation in the Age of Large-Scale Digitisation. Council on Library and Information Resources, Washington, DC, USA.
Rothenberg, J. 1999. Ensuring the longevity of digital information, expanded version of the article ‘Ensuring the Longevity of Digital Documents’ published in Scientific American, January 1995, Vol. 272 (1) – http://www.clir.org/pubs/archives/ensuring.pdf
Ware, M., 1994. Mechanisms of Image Deterioration in Early Photographs. Science Museum and National Museum of Photography. Film and Television, London, UK.
White, G., 2007. Nash Editions: Photography and the Art of Digital Printing. New Riders, Berkeley, CA, USA.
Wilhelm, H. 2007. A survey of print permanence in the 4 × 6 consumer digital print market in 2004–2007. Proceedings IS&T’s 2007 International Symposium on Technologies for Digital Fulfilment, Las Vegas, NV, USA.
Wilhelm, H., Brower, C., 1993. The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures. Preservation Pub. Co., USA.
Wilhelm Imaging Research, http://www.wilhelm-research.com.
† Kodak had stopped the processing of Kodakchrome by the time of the publication of this book. The film is currently processed only by a few independent laboratories in the world.
‡ The abbreviation ppm stands for parts per million, which is a way of expressing very dilute concentrations of substances in water; 1 ppm is equivalent to 1 milligram of the substance per litre of water (1 mg l−1).