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 (I): 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 (I): 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. <EOS>