Patent ID: 7812965
Filing Date: 2010-10-12
Classification: G01N,G03F

Abstract:
1. A method, comprising: conditioning a first input beam and a second input beam, wherein conditioning the first and second input beams comprises directing the first and second input beams to reflect from a measurement object; forming a conditioned output beam from the conditioned first input beam and the conditioned second input beam, the conditioned output beam comprising a first interferometric phase comprising information about an optical path difference between the conditioned first input beam and the conditioned second input beam; deriving a first measurement beam and a first reference beam from the conditioned first input beam and conditioned second input beam, respectively, using an optical assembly and directing the first measurement beam to reflect from the measurement object remote from the optical assembly, wherein the conditioning causes the first measurement beam to be normally incident on the measurement object for a range of orientation angles of the measurement object with respect to the optical assembly, combining the first measurement beam and the first reference beam to form a first output beam comprising a second interferometric phase comprising information about an optical path difference between the first reference and measurement beams; detecting the conditioned output beam and the first output beam and monitoring the first and second interferometric phases based on the detected conditioned output beam and first output beam, respectively; monitoring a degree of freedom of the measurement object based the second interferometric phase; and reducing uncertainty in the monitored degree of freedom due to variations in the optical properties of a gas between the optical assembly and the measurement object based on the information from the first and second interferometric phases.