A low energy radically curable offset or letterpress printing ink having a viscosity in the range of 2.5 to 25 Pa s at 40°C and 1000 s -1 for printing a security feature on a substrate or security document, said low energy radically curable offset or letterpress printing ink comprising: i) from 10 wt.% to 80 wt.% of radically curable (meth)acrylate compounds; ii) from 1 wt.% to 20 wt.% of one or more photoinitiators of formula: wherein R 1, R 2 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and Si(R 15) 3; preferably selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; R 3, R 4, R 5, R 6 are identical or different from each other and are selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, preferably C 1 -C 4 -alkyls, and more preferably methyls; R 7, R 8, R 9, R 10, R 11, R 12, R 13, R 14 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and halogens, preferably from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; and R 15 is selected from the group consisting of C 1 -C 4 -alkyls, phenyl, C 1 -C 4 -hydroxyalkyls and C 5 -C 8 -cycloalkyls; iii) from 1 wt.% to 60 wt.% of one or more machine readable materials selected from the group consisting of luminescent materials, magnetic materials, IR absorbing materials and mixtures thereof; and iv) from 0.5 wt.% to 20 wt.% of one or more fillers and/or extenders, the weight percents being based on the total weight of the low energy radically curable offset or letterpress printing ink. i) from 10 wt.% to 80 wt.% of radically curable (meth)acrylate compounds; ii) from 1 wt.% to 20 wt.% of one or more photoinitiators of formula: wherein R 1, R 2 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and Si(R 15) 3; preferably selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; R 3, R 4, R 5, R 6 are identical or different from each other and are selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, preferably C 1 -C 4 -alkyls, and more preferably methyls; R 7, R 8, R 9, R 10, R 11, R 12, R 13, R 14 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and halogens, preferably from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; and R 15 is selected from the group consisting of C 1 -C 4 -alkyls, phenyl, C 1 -C 4 -hydroxyalkyls and C 5 -C 8 -cycloalkyls; R 1, R 2 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and Si(R 15) 3; preferably selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; R 3, R 4, R 5, R 6 are identical or different from each other and are selected from the group consisting of hydrogen and C 1 -C 4 -alkyls, preferably C 1 -C 4 -alkyls, and more preferably methyls; R 7, R 8, R 9, R 10, R 11, R 12, R 13, R 14 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls and halogens, preferably from the group consisting of hydrogen and C 1 -C 4 -alkyls, and more preferably hydrogen; and R 15 is selected from the group consisting of C 1 -C 4 -alkyls, phenyl, C 1 -C 4 -hydroxyalkyls and C 5 -C 8 -cycloalkyls; iii) from 1 wt.% to 60 wt.% of one or more machine readable materials selected from the group consisting of luminescent materials, magnetic materials, IR absorbing materials and mixtures thereof; and iv) from 0.5 wt.% to 20 wt.% of one or more fillers and/or extenders, the weight percents being based on the total weight of the low energy radically curable offset or letterpress printing ink.