379.full - doi: 10.1098/rsta.2003.1323 , 379-393 362 2004...

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Unformatted text preview: doi: 10.1098/rsta.2003.1323 , 379-393 362 2004 Phil. Trans. R. Soc. Lond. A R. Appleby terahertz imaging wave imaging and how it differs from- Passive millimetre Email alerting service here right-hand corner of the article or click Receive free email alerts when new articles cite this article - sign up in the box at the top http://rsta.royalsocietypublishing.org/subscriptions go to: Phil. Trans. R. Soc. Lond. A To subscribe to on January 30, 2012 rsta.royalsocietypublishing.org Downloaded from 10.1098/rsta.2003.1323 Passive millimetre-wave imaging and how it differs from terahertz imaging By R. Appleby QinetiQ, St Andrews Rd, Great Malvern WR14 3PS, UK (rappleby@qinetiq.com) Published online 18 December 2003 It is well known that millimetre-wave systems can penetrate poor weather, dust and smoke far better than infrared or visible systems. Imaging in this band offers the opportunity to be able to navigate and perform surveillance in these conditions of poor visibility. Furthermore, the ability to penetrate dielectrics such as plastic and cloth has opened up the opportunity of detecting weapons and contraband hidden under people’s clothing. The optical properties of materials have a direct impact on the applicability of imaging systems. In the terahertz band solids have absorp- tions which can be assigned to vibrational modes. Lattice modes occur at the lowest frequencies and polythene, for example, has a lattice mode at 2.4 THz. Solids have no such absorptions in the millimetre bands (30–300 GHz) and image contrast is produced by differences in transmission, reflection and absorption. A novel, real-time, mechanically scanned, passive millimetre-wave imager has been designed. The antenna elements are based on a combination of a Schmidt camera and a conical scanner, both of which have their origins in optical systems. Polarization techniques, which were developed for operation in the centimetric band, are used to fold the optics. Both 35 GHz and 94 GHz versions have been constructed. Keywords: millimetre; imaging; terahertz; passive 1. Introduction In our everyday experience we see images of the world in the visible spectrum, which extends from 0.4 to 0.7 µ m. By imaging at different wavelengths, additional informa- tion can be obtained; for example, operating in the infrared with a thermal imager enables us to see in the dark. Exploiting different areas of the electromagnetic spec- trum brings different benefits that facilitate new applications. During World War II, active infrared systems operating in the 0.3–1.1 µ m near- infrared band were used to enhance nighttime capability. Searchlights with infrared filters were used as illuminators and the reflected radiation was detected with an image intensifier mounted on the observer’s helmet. The image intensifiers were vacuum-tube devices with S1 photocathodes. This development led to the current generation of image intensifiers. They are based on similar technology but incorporate more effective electron multipliers and photocathodes and are able to amplify natural...
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This note was uploaded on 02/04/2012 for the course ECON 101 taught by Professor Gulipektunc during the Spring '11 term at Middle East Technical University.

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379.full - doi: 10.1098/rsta.2003.1323 , 379-393 362 2004...

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