Patent ID: 8390820

Claim:
A displacement measuring system configured to measure a displacement between first and second components, the displacement measuring system comprising: a first elongated diffraction grating formed on a first face of a prism mounted to the first component, the first elongated diffraction grating having an elongated direction orientated substantially parallel to a first direction; a second elongated diffraction grating connected to the second component, the second elongated diffraction grating having an elongated direction orientated substantially parallel to a second direction which is not parallel to the first direction; and a first sensor configured to detect a first pattern of radiation generated by the diffraction of at least one beam of radiation by the first and second elongated diffraction gratings; wherein the first face of the prism is configured to receive the diffracted radiation beam from the first elongated diffraction grating and a second face of the prism is configured to re-direct the diffracted radiation beam such that the diffracted radiation beam is incident on the first elongated diffraction grating on the first face and diffracted in turn by the first elongated diffraction grating, then the second elongated diffraction grating, and then the first elongated diffraction grating, and wherein the first pattern of radiation is indicative of the displacement of the first elongated diffraction grating relative to the second elongated diffraction grating in a third direction orientated substantially perpendicular to both the first and the second direction.