Patent ID: 7599068

Claim:
A measuring method for detecting a deviation Δz of a distance z of at least one surface point from an objective, comprising the steps of: providing at least one light source producing spectral components comprising a first signal frequency f o and of a second signal frequency f o +Δf, where Δf is sufficiently small that a beat frequency occurs in the Inverse Fourier transformation; dividing the light from the at least one light source into a measuring beam and a reference light beam; directing the measuring beam onto a surface point and, following reflection by the surface point, combining the reference and measuring beams and directing the combined beam onto a photo receiver, whereby a length of a measuring light path traversed by the measuring light beam is changed relative to a length of a reference light path traversed by the reference light beam; from a time-variant measuring signal containing spectral components produced by the photo receiver, extracting the two spectral components comprising a first signal frequency f o and a second signal frequency f o +Δf; determining the relative phase angles φ 1 and φ 2 of the first and second signal frequencies f o and f o +Δf, respectively; and determining the deviation Δz from the difference between the respective phase angles φ 1 and φ 2 by determining a phase difference Δφ from the phases φ 1e and φ 2e , from the equation Δ ⁢ ⁢ z = Λ ⁢ ⁢ Δ ⁢ ⁢ φ 4 ⁢ π determining a first estimated value Δz syn for Δz, wherein Λ is the difference of the reverse values for the light wavelengths λ 1 and λ 2 , and determining an improved estimated value from the estimated value by using the condition  m ⁢ ⁢ λ o 2 + φ 1 ⁢ e ⁢ λ o 4 ⁢ π - Δ ⁢ ⁢ z syn  = minimum with m ε{ . . . , −1, 0, 1, 2, . . . } to determine a suitable whole-number multiple m o , based on which the searched for estimated value Δz hr is determined from Δ z hr =m o λ o /2+φ 1e I o /4π.