a photosensitive medium , normally a diazo printing plate or photopolymer - based proof , is exposed in a series of exposures each of a small rectangular area . typically several linearly - arrayed deformable mirror devices each spatially encode a light beam in order to expose corresponding pixels within corresponding regions of the medium . between successive exposures , the several dmd &# 39 ; s are moved to a new position relative to the medium along a first , x , axis so that new exposures of new regions can be made . normally the arrayed dmd &# 39 ; s are moved relative to the imager &# 39 ; s frame by a screw drive . there is an additional , separate relative movement between the dmd &# 39 ; s and the medium along another , perpendicular , y , axis . normally the medium is moved relative to the imager &# 39 ; s frame by nip rollers . in order to provide a required precision in registering successively imaged regions of the medium along either or both axis , reference marks are exposed directly on the medium . after each movement of the medium along either or both axis , these reference marks are sensed , typically by four - quadrant photodetectors , and errors in the repositioning of the medium are determined . to compensate for these errors , the nip rollers are adjusted by a servomechanism . the medium is held by a cyclic pressure pad while it is imaged . so proceeding , the entirety of a large photosensitive medium is imaged in successive regions along two perpendicular axis while errors in image alignment and registration are substantially reduced .