Enlarging the Application Range of Digital Holographic Microscopes for Deformation of Rough Surfaces

Certain rough surfaces can raise difficulties in interferometric and holographic contour measurements when they cause a speckle pattern, which complicates phase analysis. This can occur when surfaces exhibit characteristic dimensions below the lateral resolution limit, or steep local slopes, whose reflections exceed the numerical aperture of the objective. In Digital Holographic Microscopy, the use of synthetic beat wavelengths from two laser sources can improve the situation for some of those mentioned rough surfaces. However, certain measurement applications, in particular deformation, can also be performed by a slightly modified measurement procedure of a single-wavelength setup. This way, it is similar to a speckle pattern interferometry setup and the use of a Fourier phase-shifting algorithm enables measurement of out-of-plane deformation. In combination with a correlation-based algorithm sensitive to in-plane deformation, the 3D deformation can be obtained, thus enlarging the variety of measurable rough surfaces. The results are also compared to simulation sets obtained from an enhanced Monte-Carlo model, reproducing the measurement procedure for different roughness values.