an extinction coefficient below about 1000 more preferably below about 100 at the desired wavelength of irradiation for photopolymerization by way of example a preferred class of ketone sensitizers has the formula where x is co or cr 1 r 2 where r 1 and r 2 can be the same or different and can be hydrogen alkyl alkaryl or aralkyl b is zero or 1 and a and b can be the same or different and can be substituted having one or more non interfering substituents or unsubstituted aryl alkyl alkaryl or aralkyl groups or together a and b can form a cyclic structure which can be a substituted or unsubstituted cycloaliphatic aromatic heteroaromatic or fused aromatic ring suitable ketones of the above formula include monoketones b 0 such as 2 2 4 4 or 2 4 dihydroxybenzophenone di 2 pyridyl ketone di 2 furanyl ketone ps di 2 thiophenyl ketone benzoin fluorenone chalcone michler 39 s ketone 2 fluoro 9 fluorenone 2 chlorothioxanthone acetophenone benzophenone 1 or 2 acetonaphthone 9 acetylanthracene 2 3 or 9 acetylphenanthrene 4 acetylbiphenyl propiophenone n butyrophenone valerophenone 2 3 or 4 acetylpyridine 3 acetylcoumarin and the like suitable diketones include aralkyldiketones such as anthraquinone phenanthrenequinone o m and p diacetylbenzene 1 3 1 4 1 5 1 6 1 7 and 1 8 diacetylnaphthalene 1 5 1 8 and 9 10 diacetylanthracene and the like suitable u03b1 diketones b 1 and x co include 2 3 butanedione 2 3 pentanedione.