Patent ID: 7420686

Claim:
A method for measuring a wavelength of optical radiation, the method comprising: a) providing a first signal having a substantially periodic dependence on the wavelength; b) providing a second signal having a substantially periodic dependence on the wavelength, wherein the first and second signals have substantially the same period, and wherein the first and second signals have a phase difference substantially equal to an odd multiple of 90 degrees; c) providing first and second measured values of the first and second signals, respectively; d) determining whether or not the measured values are within a predetermined first zone, wherein sensitivity of the first signal to wavelength change is minimal in the first zone; e) determining whether or not the measured values are within a predetermined second zone, wherein sensitivity of the second signal to wavelength change is minimal in the second zone; f) if the measured values are within the first zone, determining a wavelength in part from the second measured value without making use of the first measured value; g) if the measured values are within the second zone, determining a wavelength in part from the first measured value without making use of the second measured value; h) if the measured values are not within the first zone and not within the second zone, determining a wavelength in part from both the first and second measured values.