Light fastness is evaluated in relation to the whole pigmented system, not just
the pigment. The binder imparts a varying degree of protection to the pigment, so
the same pigment will tend to have better light fastness in a polymer than it will
Pigments will nearly always have a much poorer light fastness in a printing ink
system, where there is less resin to protect the pigment, and where there is a double
effect of light passing through the pigmented layer, being reflected by the substrate
and back through the pigmented layer.
Other pigments may also influence light fastness in a pigmented system:
dioxide promotes the photodegradation of most organic pigments. Therefore,
high ratios of titanium dioxide lead to poorer levels of light fastness.
- Iron oxide can improve the light fastness of organic pigments, due to the
fact that it is an effective absorber of UV light.
When the association of two pigments gives a better light fastness, it is called
a synergistic effect.
When the light fastness obtained is lower, it is called an antagonistic effect.
Some inorganic pigments are unchanged by exposure to light, but most pigments, and
all organic pigments, are changed in some way: darkening or complete fading can
A Pigment's ability to resist light is influenced considerably by chemical constitution.
Other less significant influences are pigment concentration, the crystal modification,
and particle size distribution. Additionally, factors in the environment can dramatically
affect results, such as the presence of water and chemicals in the atmosphere or
in the paint system.
The light fastness of a pigmented system can only truly be tested in the final formulation
and application. Light fastness tests must be carried out only under carefully controlled