After the working principle of the four-quadrant photo-detector is analyzed in detailed

a novel alignment method which is used to realize the mask-plate alignment by two four-quadrant photo electric detectors is provided.The location of the lasers and the four-quadrant photo-detectors are fixed

the mask-plate is between the lasers and the four-quadrant photo electric detectors.Through the two transparent reticle prealignment marks

the laser beam is projected on the active area of the four-quadrant photo electric detectors.According to the output signals of the four-quadrant photo electric detectors

the position of mask-plate and pose errors are calculated.So the position and pose of mask-plate are adjusted by an adjustable table to realize alignment.