Dual-wavelength Shift Interferometry

Phase-shifting interferometry (PSI) as a high accuracy method is widely used in modern measurements. Traditional single-wavelength PSI has small phase measurement range. Two-Wavelength PSI (TWLPSI) is an effective method for larger phase measurement range with the advantages of: 1) larger measurement range and 2) higher accuracy of phase measurement.Several phase-shift error insensitive algorithms are discussed. Two methods are developed to obtain the effective sampling region. Combined ideas of iterative-reconstruction filtering technique and phase unwrapping method, the phase data with large noise can be unwrapped. Phase-shifter's calibration error can introduce phase shift error, which is the main error of PSI. Two methods are developed to calibrate phase-shifter. By using damped-least-square method, on-line phase-shifter's calibration is achieved, and experiments show its high precision. TWLPSI is used to solve the 2k ambiguity problem of single-wavelength PSI. An error compensation technique is developed to solve the error magnification effect of TWLPSI, which guarantees the same precision of single-wavelength PSI. Based on C++ language, application of each algorithm is presented. Experiments are performed to verify the capability of TWLPSI. Result shows that it can be a practical technique for testing steep surfaces.