It(Turner Prize) was established in 1984 to celebrate new developments in contemporary art and is given to a British artist under 50 who judges believe has put on the best exhibition of the last 12 months.
I couldn't understand the structure of relative clause,which was highligted, in the sentence. Was the word judge used as noun or verb? If it is noun I think it should/could have been written in passive voice like
" ...who is/has been believed to have put on the best exhibition of the last 12 months by judges"