Patent ID: 7636165

Claim:
A displacement measurement system comprising: first and second diffraction gratings, the displacement measurement system being configured to measure a displacement between said first and second diffraction gratings, wherein the measurement system is configured such that: a first beam of radiation input to the measurement system is divided into first and negative first order diffracted radiation beams by the first diffraction grating; said first and negative first order diffracted radiation beams are further diffracted by the second diffraction grating, reflected from the second diffraction grating in a direction towards the first diffraction grating, and subsequently recombined to form a second beam of radiation; one or more optical components configured to re-direct the second beam of radiation such that the second beam of radiation is incident on the first diffraction grating and is diffracted in turn by the first diffraction grating, the second diffraction grating and then the first diffraction grating in a same manner as the first beam of radiation in order to generate a third beam of radiation; a sensor configured to determine the relative displacement of said first and second diffraction gratings from a determination of a phase difference between a first component of said third beam, derived from said first order diffracted radiation beam, and a second component of said third beam, derived from said negative first order diffracted radiation beam; and at least one linear polarizer configured such that said first and second components of said third beam of radiation are linearly polarized and oriented in substantially mutually orthogonal directions.