Initiator system comprising: (a) at least one substance capable of absorbing IR radiation, (b) at least one compound capable of forming free radicals, and (c) at least one 1,4-dihydropyridine derivative of the formula (I) wherein R.sup.1 is selected from a hydrogen atom, --C(O)OR.sup.7, an optionally substituted alkyl group, an optionally substituted aryl group and an optionally substituted aralkyl group, R.sup.2 and R.sup.3 are independently selected from optionally substituted alkyl groups, optionally substituted aryl groups, CN and a hydrogen atom, R.sup.4 and R.sup.5 are independently selected from --C(O)OR.sup.7, --C(O)R.sup.7, --C(O)NR.sup.8R.sup.9 and CN, or R.sup.2 and R.sup.4 together form an optionally substituted phenyl ring or a 5- to 7-membered carbocyclic or heterocyclic ring, wherein the unit is present in the carbocyclic or heterocyclic ring adjacent to position 5 of the dihydropyridine ring and wherein the carbocyclic or heterocyclic ring optionally comprises additional substituents, or both R.sup.2 and R.sup.4 as well as R.sup.3 and R.sup.5 form either optionally substituted phenyl rings or 5- to 7-membered carbocyclic or heterocyclic rings, wherein the unit is present in the carbocyclic or heterocyclic rings adjacent to positions 3 and 5 of the dihydropyridine ring and wherein the carbocyclic or heterocyclic rings optionally comprise additional substituents, or one of the pairs R.sup.2/R.sup.4 and R.sup.3/R.sup.5 forms a 5- to 7-membered carbocyclic or heterocyclic ring, wherein the unit is present in the carbocyclic or heterocyclic ring adjacent to position 5 or 3 of the dihydropyridine ring and wherein the carbocyclic or heterocyclic ring optionally comprises additional substituents and the other pair forms an optionally substituted phenyl ring, or R.sup.2 and R.sup.4 or R.sup.3 and R.sup.1 form a 5- to 7-membered heterocyclic ring which can optionally comprise one or more substituents and which, in addition to the nitrogen atom it shares with the 1,4-dihydropyridine ring, optionally comprises additional nitrogen atoms, --NR.sup.13 groups, --S-- or --O--, R.sup.6 is selected from an alkyl group optionally substituted with a halogen atom or a --C(O) group, an optionally substituted aryl group, an optionally substituted aralkyl group, an optionally substituted heterocyclic group and the group Y is an alkylene group or an arylene group, R.sup.7 is a hydrogen atom, aryl group, aralkyl group or alkyl group, wherein the alkyl group and the alkyl unit of the aralkyl group optionally comprise one or more C--C double and/or C--C triple bonds, and R.sup.8 and R.sup.9 are independently selected from a hydrogen atom, an optionally substituted alkyl group, an optionally substituted aryl group and an optionally substituted aralkyl group.