Patent ID: 7933025

Claim:
A method comprising: combining a first light beam and at least a second light beam to form a combined light beam; introducing a sinusoidal phase shift with a frequency f between a phase of the first light beam and a phase of the second light beam; recording at least one interference signal based on a modulation of the combined light beam in response to the sinusoidal phase shift, the interference signal comprising at least three different frequency components, wherein each of the at least three different frequency components has a frequency which is an integer multiple of f; for each interference signal, determining information related to the difference in optical path lengths of the first and second light beam by comparing the intensity of the at least three different frequency components of the interference signal; and outputting the information, wherein the comparing comprises assigning a respective weight to the intensity of each of the at least three different frequency components to provide a corresponding weighted intensity; and comparing the weighted intensities, the respective weights being selected to compensate an error, where the respective weights are selected so that the effect of the error on the weighted intensity corresponding to a first frequency component is compensated by the effect of the error on the weighted intensity corresponding to a second frequency component.