Expanding the field-of-view and profile measurement of covered objects in continuous-wave terahertz reflective digital holography

Abstract

Terahertz waves have unique propagation and penetration ability, and these reasons have been widely used in nondestructive testing. Compared with other terahertz imaging approaches, terahertz digital holography can retrieve both quantitative amplitude and phase information about an object wavefront in real time. A continuous-wave terahertz reflective digital holographic method is proposed for measuring the profile of covered objects. Subpixel image registration and image fusion algorithms are presented to expand the field-ofview (FOV) of the system. The validity of the proposed method is verified by the experiment using an optical pumped far-IR gas laser and a pyroelectric array detector. The profile of a metallic bookmark covered by a polytetrafluoroethylene plate is obtained. The FOV is expanded by a factor of 1.75 compared with that of a reconstruction performed employing single hologram.

Lensless Fourier-Transform Terahertz Digital Holography for Full-Field Reflective Imaging

“…The method of least-square surface fitting is finally applied to eliminate the aberrations and to access the practical phase distribution of the object [24]. The above reconstructed algorithm, involving only a single Fourier transform, is fast enough compared with other existing methods such as the Fresnel method [11,12,14,15] and phase shifting method [18], and so on, which involve several Fourier transforms or complex multiplications and slow down the reconstruction process. For reflective object (in air n = 1), the surface topography h(x', y') can be retrieved as:…”

“…It can obtain the amplitude and phase information of the object from the numerical reconstruction of a recorded digital hologram and acquire the surface profile and depth information of the sample. Different from the transmission TDH, it has been used for imaging of the sample hidden behind the material, which is transparent to THz wave [11,12]. Reflection TDH can achieve highresolution imaging by recording holograms with double parallel recording planes; the inherent "twin image" problem is eliminated by using iterative phase restoration algorithm [13], but it needs to record several holograms.…”

THz coherent lensless imaging

“…The obtained lateral and depth resolutions were 200 µm (≈ 1. [144] and image stitching to expand the reconstructed field of view [126]. After a comparison with reference measurements with a surface profiler, they estimated a relative error of 3% and 6% in the reconstruction of a gold plated bookmark when it was not covered and covered by a Teflon plate, respectively.…”

Urban Image Segmentation in Media Integration Era Based on Improved Sparse Matrix Generation of Digital Image Processing

“…Wang et al segmented the image by using the similarity of the internal feature information of the image area. However, the disadvantage of this method is that it is easy to generate false segmentation areas when segmenting complex and changeable natural images, thus resulting in over-segmentation [9]. Huang et al proposed an image adaptive threshold selection algorithm based on wavelet analysis, which enables the feature points of image histogram to be expressed from coarse to fine by the feature points of wavelet transform, and enables the threshold to be adaptively selected [10].…”