Patent ID: 7505143

Claim:
A method of dynamic reference plane compensation, comprising: impinging radiation from a first radiation source onto a surface of an object; generating an uncompensated measurement signal from radiation reflected from a first location on the surface and a second location; generating a compensation signal from radiation reflected from a third location and a fourth location on the surface; and generating a compensated measurement signal using the uncompensated measurement signal and the compensation signal; wherein the compensated measurement signal comprises a corrected phase signal calculated using the formula: θ corr =θ−θ aux ( d/d aux )(λ aux /λ) where: θ corr is the corrected phase difference; θ is the phase difference measured between the measurement beams; θ aux is the phase difference measured between the compensation beams; d is the distance between the point of incidence of the first portion of the measurement beam and the point of incidence of the second portion of the measurement beam; d aux is the distance between the point of incidence of the first portion of the compensation beam and the point of incidence of the second portion of the compensation beam; λ is the wavelength of the radiation used for the measurement beam λ aux is the wavelength of the radiation used for the compensation beam.