Abstract: The invention relates to a reference mark detector for use with a metrological scale or encoder (10) having a patterned reference mark rm movable relative to the reference mark detector. The detector may comprise a photodetector array (20) for detecting the patterned reference mark, the array comprising at least two sets of detector elements A and B each set being formed as a pattern which relates to the pattern of the reference mark. The two sets may be separate rows of photodectors or different elements of the same row of photodetectors. The pattern is preferably irregular.