A position measurement system to determine a position of a first object relative to a second object, includes an encoder head mounted on the first object, a grating mounted on the second object, wherein the grating includes a first array of grating lines in a first direction and a second array of grating lines in a second direction to diffract a measurement beam incident on the first and second arrays in at least one first diffracted beam in the first direction and in at least one second diffracted beam in the second direction, wherein the first diffracted beam is for position measurement in the first direction and the second diffracted beam is for position measurement in the second direction, wherein the measurement beam has a power quantity, and the grating is configured to distribute the power quantity unevenly over the first and second diffracted beams.