Commercial passive millimetre wave imagers for stand-off security screening

This paper discusses the conflict between the mutually exclusive drivers of improved performance and lower cost which have had to be managed during the development and commercial exploitation of passive imaging technology operating in the 250GHz atmospheric water window. Now that the product line has been established, ongoing development continues to focus on providing more pixel elements, faster frame rate, better sensitivity. The continuous drive for improved performance invariably increases system complexity conflicting with the requirement for cost reduction. A work around has been achieved by optimising the time spent per pixel viewing the critical areas in the scene. By taking this approach it has been possible to maximize thermal sensitivity and at the same time improve parameters such as frame update rate which is an important driver for screening moving people. The requirement for a staring focal plane array has effectively been negated. This has been achieved by adopting an adaptive scanning architecture driven by external algorithms that take their input from external sensors with higher spatial and temporal resolution. The 250GHz water window has been shown to offer improved detection over lower frequencies, particularly in realistic outdoor environments. The shorter wavelength also provides increased resolution and/or range for a given system volume. This is an important consideration for mobile solutions that can be rapidly deployed. A robust product platform operating at 250GHz is now firmly established and recent attention has therefore shifted to developments that will enable the next generation of products. The heterodyne detector technology used in the Digital Barriers: ThruVision platform is readily adapted for other frequencies and so internal development activities have been directed at gaining an understanding of what other bands might offer. This has shown that a multi-frequency approach may offer improved capability. An overview of these de- elopments will be given.