3D terahertz synthetic aperture imaging of objects with arbitrary boundaries

Terahertz (THz) imaging has shown promise for nondestructive evaluation (NDE) of a wide variety of manufactured products including integrated circuits and pharmaceutical tablets. Its ability to penetrate many non-polar dielectrics allows tomographic imaging of an object's 3D structure. In NDE applications, the material properties of the target(s) and background media are often well-known a priori and the objective is to identify the presence and/or 3D location of structures or defects within. The authors' earlier work demonstrated the ability to produce accurate 3D images of conductive targets embedded within a high-density polyethylene (HDPE) background. That work assumed a priori knowledge of the refractive index of the HDPE as well as the physical location of the planar air-HDPE boundary. However, many objects of interest exhibit non-planar interfaces, such as varying degrees of curvature over the extent of the surface. Such irregular boundaries introduce refraction effects and other artifacts that distort 3D tomographic images. In this work, two reconstruction techniques are applied to THz synthetic aperture tomography; a holographic reconstruction method that accurately detects the 3D location of an object's irregular boundaries, and a split-step Fourier algorithm that corrects the artifacts introduced by the surface irregularities. The methods are demonstrated with measurements from a THz time-domain imaging system.