Refractive index gradient tomography for defect detection

Optical diffraction tomography (ODT) can measure the three-dimensional refractive index (RI) distribution of a sample, but it suffers from phase wrapping issues when imaging the 3D RI of thick or defective samples. Herein, we propose and demonstrate a 3D RI gradient optical tomography (RI-GOT) approach for 3D defect detection in optical elements, such as optical lenses. RI-GOT is implemented by recording holograms as a sample is rotated and reconstructing a 3D RI gradient map by back-projection of 2D phase gradient distributions at different angles. RI-GOT was demonstrated by defect detection in a mobile phone lens, providing accurate information on both defect locations and profiles. We envisage that the proposed technique will act as an effective approach for 3D defect detection in optical elements.