A Novel Terahertz Dark-Field Imaging Method for Contrast Enhancement

In this article, a novel terahertz (THz) dark-field imaging method is proposed to achieve the THz high-contrast imaging. The novel THz dark-field imaging is built based on a classical $4{f}$ imaging system, and the beam stop is placed in the Fourier plane acted as a spital filter. In the Fourier plane, the low-frequency content of the target is in the center of the THz beam and other areas are the high-frequency content of the target. The beam stop in the Fourier plane can block the low-frequency content of the target and only keep the high-frequency content to achieve high-contrast imaging. The size of beam stop determines the image quality, which is the key component of the system. The larger beam stop blocks more low-frequency content, and can get higher contrast image, but also cause lower signal to noise ratio (SNR) of the image. The letters with different contrasts are simulated and the resultant image shows the remarkable contrast enhancement. At the same time, the proposed THz dark-field imaging method is compared with the traditional dark-field imaging, which verifies the superiority of the imaging method that proposed in this article. Proof-of-principle experiment is carried out by our designed THz dark-field imaging system, and validates the effectiveness of target contrast enhancement on the THz images. A metal target, an HDPE piece with a scratch, and a leaf are imaged. The contrast is improved perfectly, which are extremely consistent with the THz dark-field imaging theory.