3D millimeter wave tomographic scanner for large size opaque object inspection with different refractive index contrasts A.Younus a , S. Salort b , B. Brecur c , P. Desbarats c , P. Mounaix a , J-P. Caumes* b and E. Abraham* a a CPMOH, Université de Bordeaux / CNRS b ALPhANOV, Centre Technologique Optique et Lasers c LABRI, Université de Bordeaux / CNRS 351 Cours de la Libération, 33405 Talence (France) ABSTRACT The potential of terahertz technology has been clearly demonstrated by its large applications in security and defence (remote detection of object). A flexible alternative monochromatic millimeter wave system coupled with an original infrared temperature sensor has been developed to visualize large size 3D manufactured opaque phantoms with different refractive index contrasts. The results clearly illustrate applied terahertz tomography particularities such as boundary effects, refraction and diffraction losses that must be prevented for efficient inspection and detection. Keywords: computed tomography, 3D reconstruction, filtered backprojection, terahertz radiation, pyroelectric sensor, Fresnel losses. 1. INTRODUCTION In the field of 3D imaging, X-ray Computed Tomography (CT) is a ubiquitous technique which provides cross-sectional images of an object by analyzing the radiation transmitted by the sample through different incidence angles. Then a filtered backprojection algorithm using a Radon inverse transform makes it possible to reconstruct the cross-sectional images 1 . However, X-ray CT cannot be easily applied to soft materials such as plastics, papers or paintings owing to the low absorption of the X-ray radiation. In this case, terahertz (THz) CT has been developed to visualize 3D objects with THz radiation 2-7 . Zhang et al. demonstrated that cross-sectional images can be obtained by measuring the transmitted amplitude and phase of broadband THz pulses at multiple projection angles 2 . However, it has been emphasized that several peaks in the THz waveform obtained with a time-domain spectrometer strongly complicate the signal analysis 8 . Generally, a backprojection algorithm is used to reconstruct the cross-sectional images and the 3D content of the sample 1 . Although THz CT seems powerful, few papers were published after the first demonstration in 2002. The main reasons are the strong limitations associated with THz CT. The backprojection algorithm assumes that diffraction effects and Fresnel losses can be neglected 2 . Until now, most samples that have been imaged using THz CT were made of polystyrene or similar low refractive index materials. However, as soon as the refractive index of the sample is in the order or greater than 1.5, which represents the majority of realistic samples, the THz beam is strongly refracted by the sample and the transmitted signal is very difficult to detect. In this paper, we propose a flexible alternative system based on a millimeter wave source coupled with a commercial infrared temperature sensor. For the study, large size 3D visualizations of manufactured opaque phantoms with different refractive index contrasts have been compared. This was possible due to the fast and low-cost acquisition coupled with an optimized reconstruction algorithm. As a result, the system underlines the potential of THz technology for possible * jean-pascal.caumes@alphanov.com , em.abraham@cpmoh.u-bordeaux1.fr Millimetre Wave and Terahertz Sensors and Technology III, edited by Keith A. Krapels, Neil A. Salmon, Proc. of SPIE Vol. 7837, 783709 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.867943 Proc. of SPIE Vol. 7837 783709-1 Downloaded From: http://spiedigitallibrary.org/ on 11/18/2013 Terms of Use: http://spiedl.org/terms