ISO 18909 pdf download – Photography — Processed photographic colour films and paperprints-Methods for measuring image stability

ISO 18909 pdf download – Photography — Processed photographic colour films and paperprints-Methods for measuring image stability.
4.8 Effects of dye fading and stain formation on the printing quality of colour negative
images
The printing quality of colour negative images is mainly governed by three factors:
— the colour printing densities of its image dyes;
— the overall contrast and the contrast balance of these images;
— the colour printing density of the masking dyes and the stain.
Any change in these properties, whether due to dye fading, changes in dye morphology or discolouration of residual substances, has a greatly amplified effect on the final image because most colour print materials have high Inherent contrast. The most damaging change tends to be contrast balance distortions brought about by differential fading of the three image dyes. These manifest themselves as shifts in colour balance from highlights to shadows that are especially noticeable in a scale of neutrals; e.g. a shift from magenta to green due to llght’fadlng of the negative’s magenta Image dye, or from yellow to blue (colour negatives) or cyan to red (recent generations of colour negatives) due to dark- jading of the yellow or cyan dye. Generally, the acceptable deviation from essentially equal contrast values olall three dye images varies from 0,10 to 0,15. dependIng on the scene content, on the Inherent contrast values of the negative and associated positive material, and on the conditions under which the print image is viewed. For instance, the tolerance of colour shifts is much smaller with reflection prints viewed in normal surround conditions than with projected Images viewed in a dark room,
The second most consequential change is that caused by an increase in stain (discolouration of residual colour couplers and/or residual processitlg chemicals, etc.) and/or fading of masking dyes. This Is so because the combined colour density of the stain and masking dyes governs the colour and density of low-density image areas and non-image areas of the negative and, thereby, its printing filter balance. They also affect the colour balance in high-density Image areas. Changes in the dmin colour balance of colour negatives of more than 0,05 result in appreciable changes in the colour balance of prints unless compensating adjustments are made in the ratios of blue, green and red exposure of the print materials. The required compensation usually Is determined with colour analysers as part of the overall evaluation of the printing characteristics of individual or related sequences of images. The output of such colour analysis systems is used to control print exposures directly in automatic printers, and indirectly in manual or semi•automatic printers.
Depending on the magnitude of the dmin imbalance, the colour rendition in shadow areas of the print Image may vary from acceptable to unacceptable. Usually a change of 15 % in dmn balance borders on the unacceptable.
The printing densities of colour negative image dyes usually change as a result of dark fading rather than light fading, because normally only a few prints are made from original camera negatives and this involves only modest aggregate exposure to light. This is true with most still-camera negative films and is also true with most motion picture camera colour negative films, because the latter serve only as masters for the generation of a small number of intermediate colour nugauve films. It is these motion picture intermediate films, which often are used to print large numbers of positive release prints, and the release prints themselves which may receive appreciable light exposure.
The dark-fading of image dyes typically results in a proportional loss of density at all density levels, without significant changes In spectral absorption. The practical effect of such change isa loss In Image contrast. Contrast losses of up to O,1S are tolerable with many scenes, provided that reasonable grey balance is maintained throughout the tone scale.
5 Test methods — Dark stability
5.1 Introduction
Long-term dark stability is evaluated by a series of tests carried out at several elevated temperatures at a particular relative humidity. Where practical, a useful relative humidity at which to test is 50%.