Measuring Three-Dimensional Surface Deformations of Skin Using a Stereoscopic System and Intrinsic Features

Abstract

Measurement of three-dimensional (3D) surface deformations is an important step in characterising mechanical properties and developing computational models of skin. Compared to two-dimensional (2D) measurements, the ability to acquire 3D information involves extra levels of complexity, particularly during calibration and reconstruction. Furthermore, the addition of speckle patterns to the skin is typically required to enhance surface contrast. We have developed a method that uses a calibrated four-camera stereoscope to accurately measure 3D surface deformations of skin without requiring the addition of extrinsic surface features. To validate the method, a flat, rigid disc was reconstructed in 3D, at different orientations, and subsequently compared against its known dimensions. The root mean squared error of our method in measuring geometric features on the disc was 44 um 25 um over a 100 mmx100 mm field of view. Our method could accurately measure disc displacements, with a maximum error of 7.8 um (relative error 0.0031) at an applied translation of 2500 um. Deformations of a uniaxially stretched rubber membrane were also measured, showing close agreement with the expected values, assuming a homogenous and linear stress/strain response. Finally, we demonstrated 3D deformation measurement of unpatterned post-mortem pig skin subject to uniaxial stretch using four cameras oriented as the edges of an octahedron with 90° angles between their optical axes. The ability to measure 3D full-field deformations of unpatterned skin, at wide camera angles, enables our method to be used in various skin experiments, including tissue indentation and dissection.