acetophenones e alpha haloacetophenones and f trisacyl phosphine oxides g benzoin and benzoin based photoinitiators suitable radical photoinitiators are disclosed in wo 9607662 trisacyl phosphine oxides u s pat no 5 399 782 phosphine sulphides u s pat no 5 410 060 ep a 57474 ep a 73413 phosphine oxides ep a 088050 ep a 0117233 ep a 0138754 ep a 0446175 and u s pat no 4 559 371 suitable photoinitiators are disclosed for example in ep a 0003002 in the name of ciba geigy ep a 0446175 in the name of ciba geigy gb 2259704 in the name of ciba geigy alkyl bisacyl phosphine oxides u s pat no 4 792 632 bisacyl phosphine oxides u s pat no 5 554 663 in the name of ciba geigy alpha amino acetophenones u s pat no 5 767 169 alkoxy phenyl substituted bisacyl phosphine oxides and u s pat no 4 719 297 acylphosphine compounds radical photoinitiators are discussed in general in a f cunningham v desorby k dietliker r husler and d g leppard chemia 48 1994 423 426 they are discussed in h f gruber prog polym sci 17 1992 953 1044 without wishing to be bound by theory it is preferred that the radical photo initiators all react through their excited triplet state to reduce the influence of the cage effect the radical photo initiator suitably undergoes one of the reactions set out above when excited by radiation falling generally in the range 290 800 nm for example natural sunlight which comprises light in this region will be suitable for causing the radical photo initiator to undergo one of the reactions described above preferably the radical photo initiator has a maximum extinction coefficient in the ultraviolet range 290 400.