ISO 18909-2022 Photography – Processed colour photographic film and printed matter – Methods for measuring image stability
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This document was prepared by the Technical Committee ISO/TC 42, Photography.
The second edition cancelled and replaced the first edition (ISO 18909:2006), which was a minor revision. The changes are as follows:
— incorporated corrections published in 2006, and
– Has been updated to adjust and complement the testing methods for digital printed materials.
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This document is divided into two parts. The first methods and procedures covering the prediction of the long-term dark storage stability of color photographic images; The second part describes the methods and procedures for measuring the color stability of such images exposed to light of specified intensity and spectral distribution at specified temperatures and relative humidity.
Today, most continuous tone photographs are made with color photographic materials. The length of time such photos are stored can range from a few days to hundreds of years, and the importance of image stability can be correspondingly small or large. Often, the ultimate purpose of a particular photograph may not be known in the first place. Knowing how long a color photo will last is important for many users, especially since stability requirements often vary from application to application. It is important for museums, archives, and others responsible for the care of color photographic materials to understand how they behave under various storage and display conditions if they are to be maintained in good long-term condition.
Organic cyan, magenta, and yellow dyes are dispersed in a transparent adhesive layer coated on a transparent or white opaque carrier, forming the image of most modern color photographs. Color photography dye images usually fade during storage and display; They also usually alter the color balance, as the three image dyes rarely fade at the same rate. In addition, yellowish (or occasionally other colored) stains may form, and physical degradation, such as embrittlement and cracking of the support and image layers, may occur. Fading and staining rates can vary greatly depending on the inherent stability of the color photographic material and the conditions under which the photo is stored and displayed. The quality of chemical processing is another important factor. Post-processing, such as painting, plastic laminate, and color modification, may also affect the stability of the color material.
The two main factors that affect storage behavior or dark stability are the temperature and relative humidity of the air that can enter the photo. High temperatures, especially when combined with high relative humidity, accelerate chemical reactions that lead to degradation of one or more image dyes. On the other hand, low temperature and low humidity storage can greatly extend the life of photographic color images. Other potential causes of image degradation are atmospheric pollutants (such as oxidizing and reducing gases), microorganisms, and insects.
It is mainly lighting intensity, duration of exposure to light, spectral distribution of lighting and ambient conditions that affect the stability of color photographs when displayed indoors or outdoors. (However, usually slower dark fading and staining reactions also occur during display and can result in an overall change in image quality). Ultraviolet (UV) radiation is particularly harmful to certain types of color photographs and can cause rapid fading and degradation of plastic layers, such as the tinted polyethylene layer of resin-coated (RC) paper carriers.
In practice, color photographs are stored and displayed at different temperature, relative humidity, and lighting combinations and for different lengths of time. Therefore, it is impossible to accurately predict the service life of a given type of photographic material unless the specific conditions of storage and display are known in advance. In addition, the amount of acceptable variation varies from viewer to viewer and is affected by the type of scene as well as the tone and color quality of the image.
Following extensive examination of amateur and professional colour photographs suffering from varying degrees of fading or staining, no consensus has been reached on acceptable changes to the various image quality standards. Therefore, acceptable end points for fading and color balance changes are not specified in this document. In general, however, the acceptable limit for a change in overall image density is twice that for a change in color balance. Therefore, different criteria are used in this document as examples to predict changes in image density and color balance.
Graphic tests are helpful in assessing the visual changes that occur during the chiarosculator stability tests, but are not included in this document because no single scene is representative of the variety of scenes actually encountered in photography.
In normal room temperature dark storage, the images of most modern color film and paper fade and stain too slowly to assess dark storage stability solely by measuring specimen changes over time. In this case, it would take too many years to obtain meaningful stability data. However, possible long-term fading and staining behavior at moderate low temperatures can be assessed in a relatively short time by accelerated aging tests performed at high temperatures. The effect of relative humidity can also be assessed by conducting high-temperature tests at two or more humidity levels.
Similarly, information about the optical stability of color photographs can be obtained from accelerated optical stability tests. These require special test units equipped with high intensity light sources, in which the strips can be exposed for days, weeks, months or even years to produce the desired amount of image fading (or staining). The temperature of the sample and its moisture content should be controlled throughout the test period, and the type of light source selected to produce data that is satisfactorily related to data obtained under normal service conditions.
Accelerated optical stability tests used to predict the behavior of photographic color images under normal display conditions may be complicated by reciprocity failures. When applied to light-induced fading and color image staining, reciprocity failure is the inability of many dyes to fade or form a stain when exposed to high intensity light or low intensity light, even though the total light exposure (intensity × time) remains the same by properly adjusting the exposure duration (see Reference ). Under accelerated conditions, the degree of dye fading and dye formation can be greater or smaller, depending on the photochemical reactions involved in dye degradation, the kind of dye dispersion, the nature of the binder material, and other variables. For example, in accelerated testing, the supply of oxygen that can diffuse from the surrounding atmosphere to the emulsion layer containing the image in the photo may be limited (dry gelatin is an excellent oxygen barrier). This may change the rate at which the dye fades, rather than what would occur under normal display conditions. The temperature and moisture content of the sample also affect the degree of reciprocity failure. In addition, light fading is affected by the exposure mode (continuous versus intermittent) and the light/dark cycle rate.
For all these reasons, long-term changes in image density, color balance, and stain levels can be reasonably estimated only under conditions similar to those used in accelerated tests, or when a good correlation has been confirmed between accelerated tests and actual conditions of use.
To determine the validity of the test methods for evaluating the dark and light stability of different types of photographic color film and paper, the following product types were selected for testing:
a) Color negative film with oil soluble coupler;
b) Color negative preprinting and negative films with oil soluble coupler;
c) Color reversal film with oil-soluble coupler;
d) Color reversal film with Fischer type coupler;
e) Color reversal film with coupler in developer;
f) Silver dye bleached film and printed matter;
g) Colour prints with oil-soluble couplings;
h) colour motion picture printing film with oil-soluble coupler;
i) color dye absorption (dye transfer) printing;
j) Integral color instant printing film with dye developer;
k) Peel color instant printing film with dye developer;
l) Integral color instant print film with dye releaser.
The results of extensive testing using these materials indicate that the methods and procedures in this document can be used to obtain meaningful information about the long-term dark and light stability of color photographs made with specific products. They can also be used to compare the stability of color photographs made using different products and to access the effects of processing changes or post-processing. The accuracy of predictions based on such accelerated burn-in tests will depend largely on the actual storage or display conditions.
It should also be remembered that density changes caused by test conditions and measured during and after the test include density changes in film or paper carriers and in various auxiliary layers that may be included in a particular product. For most materials, however, the dye imaging layer changes significantly.
Stability when stored in the dark
A test to predict the stability of color photographic images in dark storage is based on an adaptation of the Arrhenius method described by Bard et al. ) and early references by Arrhenius, Steiger, et al. (see References , , and ). Although this method derives from well-known and proven rules of chemical theory, its effectiveness for predicting changes in photographic images depends on empirical confirmation. Although many chromogenic color products produced image fading and staining data in both accelerated and non-accelerated dark aging tests, which closely matched the Arrhenius relationship, other types of products did not.
Note, for example, that monolithic instant color printing materials often exhibit atypical staining at high temperatures; Processing of some color developing materials at temperatures higher than 80°C and 60%RH can lead to the loss of high boiling point solvents and abnormal image degradation. The dye of the silver dye bleached image breaks down under the combination of very high temperature and high relative humidity, resulting in abnormal changes in color balance and saturation (see reference ). In general, photographic materials tend to change dramatically at relative humidity above 60% (especially at the high temperatures used in accelerated tests) due to changes in the physical properties of gelatin.
Stability when exposed to light
The method for testing light stability in this document is based on the concept that increasing light intensity without changing the spectral distribution of the light source or the ambient temperature and relative humidity should increase proportionately the photochemical reactions that occur under typical viewing or display conditions without introducing any adverse side effects.
However, due to the reciprocity failure discussed in this introduction, this assumption does not always apply. Therefore, the accelerated optical stability test method described in this document is valid under specified accelerated test conditions, but may not reliably predict the behavior of a given product displayed over a long period of time under normal conditions.
Translucent printed materials designed for viewing reflected or transmitted light (or a combination of reflected and transmitted light) should be evaluated as transparent film or reflected prints, depending on how they are used. Data should be reported for each intended use condition.
This document does not specify which of several optical stability tests is most important for any particular product.
This document describes the test methods used to determine the long-term dark storage stability of color photographic images and the color stability of such images when exposed to certain illuminants at specified temperatures and relative humidity.
This document is applicable to color photographic images made using traditional continuous tone photographic materials, as well as images formed with dyes. These images were generated using color rendering, silver dye bleach, dye transfer, and dye diffusion transfer real-time systems. These tests have not been validated to assess the stability of color images produced using dry ink and liquid toner electrophotography, thermal dye transfer (sometimes called dye sublimation), inkjet, pigment gelatin systems, offset lithography, intaglio printing, and related color imaging systems. If these reflective printing materials (including silver halide (color rendering)) are digitally printed, refer to ISO 18936, ISO 18941, ISO 18946 and ISO 18949 for dark stability testing, and to ISO 18937 series for light stability testing.
This document does not include procedures for testing the physical stability of image, carrier or adhesive materials. However, it is recognized that in some cases, physical degradation, such as carrier embrittlement, emulsion cracking or the layering of the image layer with its carrier, rather than image stability, will determine the service life of color film or printed material.
2 Normative references
The following files are referenced in the text in such a way that some or all of the content constitutes the requirements of this document. For dated references, citation-only versions apply. For undated references, the latest version of the reference (including any revisions) applies.
ISO 5-2, Photographic and graphic techniques – Density measurement – Part 2: Geometric conditions for transmittance density
ISO 5-3, Photographic and graphic techniques – Density measurement – Part 3: Spectral conditions
ISO 5-4, Photographic and graphic techniques – Density measurement – Part 4: Geometric conditions for reflection density
ISO 18911, Imaging Materials – Processing safe photographic film – Storage practice
ISO 18913, Imaging materials — Persistence — Vocabulary
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