(i) 5 to 80 percent by weight based on the total weight of the composition of a polymerizable matrix containing polymerizable monomers (ii) a polymerization initiator system containing (a) an alpha-diketone photoinitiator compound having a light absorption maximum in the range from 300 to 500 nm (b) a coinitiator compound of the following formula (I): A-H (I) wherein A is a moiety of the following formula (II) R1 R2R3X (II) wherein X represents Si, Ge, or Sn and R1 represents a hydrogen atom, an organic moiety or a different moiety A R2 and R3 which are independent from each other, represent an organic moiety and (c) one or more compounds selected from the following group: (1) an iodonium compound of the following formula (III): R4-l+-R5 Y- (III) wherein R4 and R5 which are independent from each other represent an organic moiety, and Y- is an anion (2) a sulfonium compound of the following formula (IV): R6R7R8S+ Y- (IV) wherein R6, R7 and R8 which are independent from each other, represent an organic moiety or wherein any two of R6, R7 and R8 form a cyclic structure together with the sulfur atom to which they are bound, and Y- is an anion (3) a phosphonium compound of the following formula (V): R9R10R11 P+ Y- (V) wherein R9, R10 and R11 which are independent from each other, represent an organic moiety, and Y- is an anion and (4) a pyridinium salt.(i) 5 à 80 pour cent en poids sur la base du poids total dune composition de matrice polymérisable contenant des monomères polymérisables (ii) un système initiateur de polymérisation contenant (a) un composé photo-initiateur à base dalpha-dicétone présentant un maximum dabsorption de la lumière dans la plage de 300 à 500 nm b) un composé co-initiateur de formule (I) suivante : A-H (I) dans laquelle A représente une fraction de formule (II) suivante : R1R2R3X (II) dans laquelle X représente Si, Ge ou Sn et R1 représente un atome dhydrogène, une fraction organique ou une fraction différente de A R2 et R3 représentent ind
