Light fastness is the resistance of colours to fading, changing shade or darkening under the influence of light (independently of the direct influence of weathering).
The reasons for such modifications in the appearance of prints are mainly due to the degradation of the colouring agent used (pigment or colorant). However, light not only affects the pigments but also the binders, the substrate and even the overprint varnish.
The printed material should therefore be considered as a whole, of which the ink film is only a part, and the colouring material only one of the constituents of the ink film.
It should be noted that other factors can cause premature ageing of the printed material:
• Humidity: which ends up leaching the prints.
• Atmospheric pollution: which can cause rapid deterioration.
• The packaged product: which can migrate or release vapours that can alter the print.
The characteristics of light that have an influence on the fastness of prints are:
• Its nature: daylight or artificial light.
• Its intensity: season, latitude, reflections, etc.
Light fastness also varies with:
• The thickness of the ink film: the thinner the ink film, the lower the light fastness.
• The transparency of the ink film: the more transparent the ink film, the lower the light fastness.
• The pigmentation of the ink: the lower the pigmentation concentration, the lower the light fastness.
• A high filler or white pigment content: pastel shades have low light fastness even when they contain “light fast” pigments.
The light fastness of inks or prints is exclusively expressed in terms of an alteration in their colorimetric appearance in comparison with standardised blue wool samples.
The DIN 16519 (printing test samples), AFNOR Q64-022, DIN 16525 and ISO 2835 standards detail methods for printing test samples and measuring light fastness.
The most commonly used method is exposure to artificial light. A xenon arc lamp, whose spectrum is similar to natural daylight, is used as the light source (Suntest).
The test samples are exposed at the same time as a standard colour scale known as the blue wool scale (or simply wool scale). This colour scale is composed of strips of wool fabric, tinted with 8 blue colorants, whose light fastness increases regularly in an arithmetic scale from 1 to 8.
The light fastness test ends when a fadingon the test sample. The limit of fading of the blue zone read off at this instant on the blue scale determines the light fastness index of the ink or print run.
It is very difficult to provide an exact equivalence between the light fastness of prints and their durability. In fact, as mentioned above, different factors combined with light can alter the printed material (intensity of the sunlight, humidity, etc.).
The following table gives an indication of the average equivalent:
NB: Above index 5, if the modification falls between two indices on the wool scale, the light fastness is noted with both indices (e.g. 5-6). Furthermore, if the colour darkens, the letter N is added to the index