Surface shape measurement by phase-shifting digital holography with dual wavelengths

A simple method for surface contouring by phase-shifting digital holography is reported. Diffusely reflected light from a coherently illuminated object is recorded by a CCD with coherent in-line superposition of a reference beam subject to phase-shifting. From three phase-shifted in-line holograms the complex amplitude of the object wave at the CCD plane is derived to reconstruct phase distributions before and after wavelength shift that is provided by a mode-hopping induced by a change of injection current of a laser diode. The difference of the reconstructed phases corresponding to each of the wavelengths is proportional to surface height from the reference plane that is normal to the incident beam. This setup permitting normal incidence introduces neither the carrier component corresponding to oblique reference plane nor the shadowing effect that arise in the fringe projection and the dual incident angle methods requiring oblique illumination. In experiments plane surfaces tilted by various angles are first measured to evaluate accuracy. Then a spherical surface and a miniature valve are employed. The results are compared with one-dimensional simulations using random number model of surface roughness to exhibit good agreement.