An exposure system includes an exposure tool configured to project grating patterns oriented in different directions onto test areas by a linearly polarized light, respectively, each of the grating patterns having a space width preventing higher order diffracted lights from an image formation, a data collector configured to collect sets of test optical intensity data on the test areas, and a polarization direction monitor configured to monitor a polarization direction of the linearly polarized light by comparing the sets of the test optical intensity data.