Position measurement system, grating for a position measurement system and method

A method to determine a position of a first object relative to a second object, comprising:

• providing an encoder head on the first object,
• providing a grating on the second object, wherein the grating comprises 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 at least one first diffracted beam is used for position measurement in the first direction and wherein the at least one second diffracted beam is used for position measurement in the second direction,
• projecting by the encoder head a measurement beam on the grating, wherein the measurement beam has a power quantity, and wherein the grating is configured to distribute the power quantity unevenly over the at least one first diffracted beam and the at least one second diffracted beam,
• receiving by the encoder head the at least one first diffracted beam and the at least one second diffracted beam, and
• determining a position of the first object relative to the second object on the basis of the at least one first diffracted beam and/or the at least one second diffracted beam.

A grating comprising;

• a first array of grating lines in a first direction;
• a second array of grating lines in a second direction;
• wherein the first array of grating lines comprises a first duty cycle,
• wherein the second array of grating lines comprises a second duty cycle,
• wherein the first duty cycle and the second duty cycle are different.