Light fastness

Hello Luckchai, in few words: pigment particles are chemically changed by light absorption. The more pigment particles are in a volume the longer will be characteristical light absorption.

Try to imagine the pigment particles in the ink: they are distributed in the whole volume. If a light ray enters and meets such a particle, it will be absorbed by the molecules and actuivate them chemically. Now this is chemically active and can be agressed by others. For instance oxygen. This will move the light absorption frequency an - mostly - reduce its colour for our eyes.

More pigment molecules are in the mixture, the more others are present to go on absorbing the next rays. So our light fastness is measured in a standard concentration of pigment.

By the way - scale inks are pigmented with a lower level than this standard concentration. So four colour print of a picture will never reach the light fastness of 5 as theoretically given in the inks magenta and yellow.

And- of course - low ink film thickness and even screening lowers the fastness, too.