A scale in the form of a grating is read by a sensor movable in the measurement direction relative to the scale. An optical system (3) in the sensor limits the illumination of the sensing region on the scale (4) by focussing the illumination beam transversely w.r.t. the measurement direction but allowing it to extend in the measurement direction. An optical astigmatism component can be used with a cylindrical lens (3b) whose focal line is parallel to the measurement direction and a condenser (3a) which collimates the beam in the measurement direction. USE/ADVANTAGE - For length or angle measurement. Insensitive to Moire tilting around axis perpendicular to scale.
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