A device for measuring the displacement of a movable object includes a stationary light source that produces an incident light beam. A target feature, attached to, or integral with, the object, reflects the incident light beam. A stationary photodetector, spaced from the object, receives the reflected light beam and, in response thereto, produces an electric signal having a characteristic (such as amplituded) which is proportional to a received location on the photodetector of the reflected light beam and which represents a position of the object. An imaging lens receives the reflected light beam and focusses an image of the light source on the photodetector. The target feature includes a curved surface that reflects the light beam such that a small, point-like or line-like image of the light source is focussed on the photodetector. The displacement sensor device is particularly useful in a disk drive servo system application.