Active and passive imaging in the THz spectral region: phenomenology, dynamic range, modes, and illumination

Abstract

The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the relationship between active and passive images, especially with respect to how they interact with the illumination-and detector-mode structures of various imaging scenarios. It is concluded that the very large dynamic range that can be obtained with active single-mode systems (including focal-plane arrays) can be used in system designs to overcome the deleterious effects that result from the dominance of specular reflections in single-mode active systems as well as to strategically orient targets to obtain recognition. This will aid in the development of a much more robust and generally useful imaging technology in this spectral region.

The submillimeter: A spectroscopist’s view

“…However, it should be noted that although many technical approaches have been proposed for the imaging applications (often referred to as 'THz imaging'), those that are the most successful have chosen to use the frequency multiplication technology that has evolved along with high-resolution SMM spectroscopy [4].…”

Millimeter-Wave and Terahertz Imaging in Security Applications

“…In these scenarios the contrast in Fig. 19.1 Passive indoor imaging at (a) 94 GHz [7], (b) 350 GHz [8], (c) 600 GHz (wearing glasses) [9] and (d) broadband (0.2 −1 THz) [10]. The increase in the visibility of clothing is apparent when moving from the lower to the higher frequencies, by the change in contrast of shirt collars and seams(White-is-"hot") the image arises from the difference between the illumination from the building and that from the human body, which may be typically ∼20 • C. An example of such imagery is shown in Fig.…”

“…Brijot 9 Imaging Systems sell several product variations based on a preamplified direct detection receiver array imager operating at a center frequency of 94 GHz. The Brijot GEN2 10 system (see Fig.…”

“…The T4000 angular 9 Brijot is now MicroSemi http://www.brijot.com/ 10 GEN2 http://www.brijot.com/products/gen2 11 Thruvison has been acquired by Digital Barriers http://www.thruvision.com/ 12 Has become TS4 http://www.thruvision.com/Products/TS4_Sub_Pages/TS4_Product_Page.htm 13 TS5 http://www.thruvision.com/Products/TS5_Sub_Pages/TS5_Product_Page.htm …”

Review of Terahertz Tomography Techniques

“…Now scientists are able to propose optimized solutions of terahertz imaging for a given application with an optimal setup configuration [35] (e.g. pulsed or continuous generator, broadband versus narrowband, spectral resolution versus temporal resolution, active versus passive imaging [36], but also near field [37][38][39][40], wave guiding [41], detectors array and even more camera [42], etc. ).…”