Photoinitiators mixture comprising (i) at least one compound selected from the group consisting of alpha-hydroxy ketones, monoacylphosphine oxides, bisacylphosphine oxides, ketosulfones, benzil ketals, benzoin ether, phenylglyoxylates, borates and titanocenes; and (ii) at least one compound of the formula (I) or (I′) R1, R′1, R2 and R2′ R′9 independently of each other for example are hydrogen or C1-C12alkyl R7, R8, R9, R′8 and R′9 independently of each other for example are hydrogen, C1-C12alkyl which optionally is substituted or phenyl which optionally is substituted, exhibit excellent photoinitiating properties.