Toward high-sensitivity and high-resolution submillimeter-wave video imaging

Abstract

Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/15/2015 Terms of Use: http://spiedl.org/termsAbstract. Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350 ± 40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Passive 350 GHz Video Imaging Systems for Security Applications

“…Experimentally, a power yield of about 300 f W/K has been determined [18]. This corresponds to an overall system efficiency of 15 %.…”

“…2) of the passive submillimeter-wave stand-off video camera presented here has evolved since 2007 and has been published before [9,12,18,[20][21][22]. Table 1 gives an overview of the different camera prototypes and their main parameters.…”

“…At a frame rate of 25 Hz, it provides a smooth and stable video stream without scanning artifacts. Major issues of previous camera prototypes [18,21] therefore have been resolved. Example video frames shot with this camera are shown in Fig.…”

“…Various objectives have been designed depending on the site of operation, ranging from an off-axis wide-angle optics for short distances (2-4 m) [22] to an on-axis telescope for distances between 10 and 20 m [12,18]. In general, for all configurations, the proportions between scan distance, object plane, and aperture diameters lead to challenging design conditions regarding a uniform illumination of the detector array at the image plane.…”

“…Using this approach, high resolution passive submillimeterwave video imaging at a frame rate of up to 10 Hz was demonstrated. This system was first presented in 2010 and has been described in detail elsewhere [18,21].…”