Patent ID: 9074911
Filing Date: 2015-07-07
Classification: G01D

Abstract:
1. A measurement system for measuring relative movement of a first object and a second object, the first object and the second object being movable relative to each other, the measurement system comprising: a measurement grating coupled to one of the first object and the second object; and an encoder head that directs a measurement beam at the measurement grating; wherein the measurement beam impinging on the measurement grating creates a +1 order measurement beam and a −1 order measurement beam directed back at the encoder head; and wherein the +1 order measurement beam from the measurement grating is directed at an imperfect first redirector that directs a +1 order second pass beam at the measurement grating, and wherein the −1 order measurement beam from the measurement grating is directed at an imperfect second redirector that directs a −1 order second pass beam at the measurement grating; and wherein a plane of incidence of the +1 order second pass beam is not parallel to a plane of incidence of the −1 order second pass beam.