Method of making a relief plate using a photopolymerizable recording composition

A process for the preparation of a relief plate by imagewise exposure of a layer, applied to a support, of a photopolymerizable recording composition which consists mainly of a mixture of (a) a photoinitiator, (b) one or more monomers possessing one or more photopolymerizable olefinically unsaturated double bonds, and (c) one or more organic polymeric binders, and subsequent removal of unexposed areas of the layer of photopolymerizable recording composition, wherein the photopolymerizable recording composition contains, as the photoinitiator, an acylphosphine oxide compound of the formula ##STR8## where R.sup.1 is alkyl of 1 to 6 carbon atoms, cycloalkyl of 5 or 6 ring carbon atoms, aryl which is unsubstituted or substituted by halogen, alkyl or alkoxy, or a S-containing or N-containing five-membered or six-membered heterocyclic radical,

• R.sup.2 has the same meanings as R.sup.1, or is alkoxy, aryloxy or aralkoxy, R.sup.1 and R.sup.2 being the same or different or together form a ring, and
• R.sup.3 is tertiary alkyl of 4 to 18 carbon atoms or tertiary cycloalkyl of 5 to 6 ring carbon atoms or is a cycloalkyl, aryl or 5-membered or 6-membered heterocyclic radical which contains substituents A and B bonded at least to the two ring positions, capable of substitution, which are adjacent to the point of attachment to the carbonyl group, A and B being identical or different and each being alkyl, alkoxy, alkoxyalkyl, alkylthio, cycloalkyl, aryl or halogen.