Patent ID: 7186969

Claim:
A device for measuring the relative displacement between two members, the device comprising: (a) a scale grating extending along a measuring axis direction and having a scale grating pitch P; and (b) a readhead positionable to receive operable scale grating image light from the scale grating and provide an operable periodic image of the scale grating when the scale grating is illuminated by an illumination source that provides an effective illumination wavelength λ, the readhead comprising: (i) a housing; (ii) a set of respective optical detectors positioned within the housing to respectively receive different phases of the operable periodic image; (iii) a first lens positioned within the housing to receive light from the scale grating, the first lens having a focal length F that defines a focal point located between the first lens and the set of respective optical detectors; and (iv) an aperture positioned approximately at the focal length F between the first lens and the set of respective optical detectors; wherein: the operable periodic image has a fundamental spatial frequency corresponding to the scale grating pitch P; and the aperture is configured with a dimension W along the measuring axis direction such that W=F*λ*(n/P), wherein n/P is a cutoff spatial frequency, and one of the following conditions is satisfied: C1) the operable scale grating image light is incoherent and n is greater than 1.2 and at most 3.5, and C2) the operable scale grating image light is coherent and n is greater than 2.0 and less than 6.0.