Shape measurement of diffuse and transparent objects by two wavelength contouring using phase retrieval

Abstract

Two wavelength contouring is used especially for the shape measurement and testing of steep objects. Conventional and digital holographic techniques can be utilized for this. But both these methods require the interference of the test wavefront with a known background or reference wavefront. This makes the adjustment of the two beams intensities to get high contrast fringes necessary. These methods are also prone to external noise like vibrations. Phase retrieval from intensity sampling at multiple axial planes offer an attractive alternative for whole field imaging. Diffusively reflecting objects produce volume speckle field when exposed to highly coherent radiation like laser. This volume speckle field has appreciable intensity variation both in the transverse as well as axial direction, which is the necessary condition to reconstruct the complex amplitude of the object wavefront from the intensity values. The reconstruction process uses the angular spectrum propagation approach to the scalar diffraction theory. This paper explores the use of phase retrieval in two wavelength contouring. The method can be used for the shape measurement of steep diffuse objects and the optical path length measurement of transparent objects. The method is explained using simulations. Some experimental observations are also provided to validate the simulation results.