Patent ID: 7227124

Claim:
A device for measuring the relative displacement between two members, the device comprising: a scale extending along a measuring axis direction; a readhead designed to be positioned at a nominal distance L from the scale to receive operable scale image light from a field of view on the scale and provide an operable image of the scale when the scale is illuminated, the readhead comprising: an imaging portion that provides the operable image of the scale, the imaging portion comprising an imaging lens; and an illumination portion that provides illumination to the field of view, the illumination portion comprising a condenser lens having a focal length f and a light source having an emitting element having a width w, wherein the emitting element is nominally located at the focal length f from condenser lens, wherein: the illumination portion is configured such that the illumination provided to the field of view is approximately uniform across the field of view and at least partially incoherent such that the operable image of the scale does not include self-imaging effects; and the device is configured to fulfill at least one design criterion from a group of design criteria comprising: (a) a design criterion wherein the field of view has a dimension FOV, the emitting element emits radiation having a wavelength λ, and the illumination portion is configured such that w/f>2.44*λ/(FOV/k), where k is a number that is at least 1, (b) a design criterion wherein the scale includes a periodic grating structure having a pitch P, the emitting element emits radiation having a wavelength λ, and the illumination portion is configured such that w/f>2.44*λ/(n*P), where n is a number that is at most 10, (c) a design criterion wherein the imaging portion comprises a telecentric imaging arrangement having a full admittance angle α and, for each respective angle that is equal to or less than α, the intensity distribution of the illumination provided to the field of view along that respective angle varies by at most 15% across the field of view, (d) a design criterion wherein the imaging portion comprises a telecentric imaging arrangement having a full admittance angle α, the field of view has a dimension FOV, the condenser lens has a clear aperture D, and the device is configured such that L<(D−FOV)/α, and (e) a design criterion wherein the illumination portion further comprises a diffuser positioned to receive light from the condenser lens and provide the illumination to the field of view.