A stage system includes a movable stage, and an encoder for measuring a position of the stage, wherein the encoder includes an emitter for emitting an encoder beam, a grating for interacting with the encoder beam, and a detector for detecting the encoder beam having interacted with the grating, the encoder beam in use propagating along an optical path; a purging cap at least partly enclosing the optical path; and a purging medium supply device for supplying a purging medium into the purging cap.