Lens-based phase retrieval under spatially partially coherent illumination - Part II: Shape measurements

Abstract

We experimentally demonstrate the shape measurement of micro objects by means of phase retrieval under spatially partially coherent illumination. This approach offers an extended depth of focus, vibration insensitivity, short measurement time and precise shape measurements, enables the use of spatially extended light sources such as LEDs and thus significantly improves the eye safety of the method. The employed setup is based on a telecentric 4f-imaging system with a tunable lens across the common Fourier plane. Based on the theoretical investigation presented in Part I, we verify that 4f-based phase retrieval is feasible if the radius of the point spread function of the imaging system is smaller than that of the correlation (spatial coherence) area across the object plane. In this case, a light field with a defined complex amplitude can be assigned to the measured intensity distributions. As a proof of principle towards industrial applications, the 3D shape of a cold-formed micro-cup is reconstructed by combining phase retrieval with two-wavelength contouring. A depth of focus in the range of few millimeters and the measurement time and uncertainty of 0.4 s, and 2μm, respectively, can be achieved.