A device for measuring the relative displacement between two members comprises a scale grating, an illumination source, a detector portion, and an imaging portion. The illumination source outputs diffracted light components (DLC) to the scale grating. The DLC comprises desired interfering light components comprising +1 and −1 order DLC and undesired interfering light components comprising diffraction orders that are not the +1 and −1 order DLC. The detector portion comprises at least a first array of periodically arranged optical detector sensing areas. The illumination source light diffraction grating is positioned proximate to the scale grating and oriented relative to the scale grating such that respective sets of interference fringes formed by different respective sets of interfering light components are differently rotated about the optical axis. The detector portion is oriented such that the optical detector sensing areas are aligned with interference fringes formed by the desired interfering light components.