Patent ID: 7902494

Claim:
A system ( 1 ) for detecting motion of a body ( 2 ) comprising: a first reflective elongated grating strip ( 4 ) coupled to said body elongated in a single dimension, wherein a length in the direction of said elongation of said first reflective elongated grating strip ( 4 ) covers the full translation range of the body ( 2 ) and a transverse dimension of said first reflective elongated grating strip ( 4 ) is larger than one or more light beams diffracted at said first elongated grating strip ( 4 ) to detect motion of said body a separate substantially stationary second transmissive elongated grating strip ( 5 ) elongated in a single dimension and positioned above and proximal to said first reflective elongated grating strip separated by an air gap and crossing said first elongated grating strip in a transverse direction, wherein a length in the direction of elongation of said second transmissive elongated grating strip ( 5 ) covers the full translation range of the body ( 2 ) and a transverse dimension of said second transmissive elongated grating strip ( 5 ) is larger than said one or more light beams diffracted at said first elongated grating strip ( 4 ) to detect motion of said body, wherein said first ( 4 ) and second ( 5 ) grating strips allow for all orders of diffraction from said body, wherein said system further comprises optical detection means ( 6 ) arranged to receive said one or more light beams diffracted at said first and second elongated grating strips to detect motion of said body.