Patent ID: 7002137

Claim:
An optical encoder including: A. a scale, the scale including an optical grating and an optical element; B. a sensor head, the sensor head including a light source and a detector array both of which are disposed on a substrate, the scale being disposed opposite the sensor head and being disposed for movement relative to the sensor head, a distance between the scale and a talbot imaging plane closest to the scale being equal to d, the sensor head being disposed within a region bounded by a first plane and a second plane, the first plane being separated from the scale by a distance substantially equal to n times d plus d times x, the second plane being separated from the scale by a distance substantially equal to n times d minus d times x, n being an integer and x being less than or equal to one half, the light source emitting a diverging beam of light, the diverging beam of light being directed towards the scale, light from the diverging beam of light being diffracted by the grating towards the detector array; C. a mask disposed between the scale and the sensor head, the mask defining an aperture, the mask remaining substantially fixed relative to the sensor head, the aperture being sized and positioned to substantially prevent fifth order beams diffracted from the grating from reaching the detector array.