Patent ID: 6906315

Claim:
A device for measuring the relative displacement between two members, the device comprising: a scale having a scale grating formed along a measuring axis direction; and a readhead operable to provide an operable self-image of the scale, the readhead comprising: a light source portion comprising at least one respective light source element; and a plurality of fiber-optic receiver channels, each respective fiber-optic receiver channel comprising: a respective receiver channel spatial phase mask portion having a respective spatial phase and having its light-blocking elements arranged at a pitch that is operable for spatially filtering the operable self-image of the scale, and generally being located at a nominal spatial phase mask plane that is operable for spatially filtering the operable self-image of the scale; and at least one respective receiver channel optical fiber having an input end that receives a respective receiver channel optical signal light; wherein: the respective receiver channel optical signal light received by the at least one respective receiver channel optical fiber comprises optical signal light collected through the respective receiver channel spatial phase mask portion over a respective collected light area having a collected light area dimension along the measuring axis direction that is at least three full periods of the respective receiver channel spatial phase mask portion; when the readhead is operably positioned relative to the scale grating at least first and second respective channels of the plurality of fiber-optic receiver channels spatially filter their respective portions of the operable self-image of the scale at the nominal spatial phase mask plane to provide at least first and second respective receiver channel optical signals having at least first and second respective signal phases; and the device outputs the at least first and second respective receiver channel optical signals along respective optical fibers to provide relative displacement measurement information in the form of a plurality of respective optical output signals, the respective optical output signals produced without the use of an electronic photodetector element.