Patent ID: 9534934
Date: 2017-01-03
CPC Classifications: G01D,G03F,H01L

Claim:
1. A measurement system for measuring a relative movement along a first axis 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 grating that is coupled to one of the first object and the second object; and an encoder head that is coupled to the other of the first object and the second object, the encoder head including a first optical member and a second optical member, the encoder head directing a first measurement beam at the grating at a first angle, and directing a second measurement beam at the grating at a second angle; wherein an absolute value of the first angle relative to a normal of the grating is different than an absolute value of the second angle relative to the normal of the grating, and wherein the first measurement beam has a first wavelength, and the second measurement beam has a second wavelength that is different than the first wavelength; wherein the first optical member redirects a diffracted first measurement beam back at the grating; and wherein the second optical member redirects a diffracted second measurement beam back at the grating.