Patent ID: 7826063

Claim:
A method, comprising: using an interferometry assembly to provide three different output beams, each output beam comprising an interferometric phase related to an optical path difference between a corresponding first beam and a corresponding second beam, each first beam having the same wavelength and contacting a measurement object at least once; monitoring the interferometric phases for each of the three different output beams; deriving information about variations in the optical properties of a gas in the first beam paths from the three monitored phases; wherein deriving the information comprises determining a parameter based on the monitored interferometric phases for the three different output beams and removing a contribution to the parameter due to variations in the optical properties of the gas having a particular range of frequencies; and wherein the three monitored interferometric phases are each related to a respective first, second, and third displacement of the measurement object along a corresponding axis and the parameter is a second-difference parameter, SDP, which corresponds to a weighted difference between a first parameter and a second parameter, the first parameter being the difference between the first displacement and the second displacement and the second parameter being the difference between the third displacement and the first displacement.