A method of step height measurement within the unambiguous range of two laser wavelengths interferometer

When two laser beams of different wavelength are simultaneously applied to a Michelson interferometer, a moiré of parallel patterns appears due to the difference in optical path length between the reference beam and the measuring beam. The pattern appears with a wavelength of λaλb/|λa − λb|, and the distance between the zeroes of the envelope is also λaλb/|λa − λb|. Using the moiré of parallel patterns, unambiguous range of step height is half of λaλb/|λa − λb|. However, intensity varies with an average wavelength of λaλb/(λa + λb), and intensity variation differs between the zeroes of each envelope. The unambiguous range is several times wider than λaλb/|λa − λb|. Intensity variation over the unambiguous range is measured, and coincides with calculated intensity variation, step height can be measured.