Defocusing Recovery Technology of Terahertz Image Based on 3-D PSF Simulations

Terahertz (THz) imaging has broad application prospects in nondestructive testing (NDT). However, compared to visible light, it has a low resolution and limited depth of field (DOF) due to its long wavelength. To address these limitations, a 3-D point spread function (PSF) based on a simulated THz time-domain spectroscopy (THz-TDS) system combined with the Lucy–Richardson method is proposed for image defocus restoration. The experimental results show that the contrast of THz images at different defocus positions is 2–5 times higher than that of the original image after restoration using the proposed method, and the resolution is improved by approximately 60%–80%. Moreover, the maximum contrast enhancement at the boundaries of tiny features can reach 1.81 times. For a hole-defect sample, the relative error of the defect area at different depths is < 5%. The proposed method solves the problem of defocus in THz images, enhances the resolution of these images, and facilitates NDT.